Catherine Delgoulet
email: catherine.delgoulet@parisdescartes.fr

Corinne Gaudart
email: corinne.gaudart@cnam.fr

Karine Chassaing
email: karine.chassaing@ergi.u--bordeaux2.fr

Job entry and on-the-job skills acquisition in the construction sector

This paper presents a research project focused on induction in the construction sector in France, in which employers report difficulty in retaining newly hired employees. The objective was to identify the various types of job status of these new workers and to understand what happens after they are hired, with a particular focus on the ways in which skills are acquired and transmitted by the various protagonists. To explore these issues, 26 interviews were conducted with employees of a large construction company. An analysis of the responses showed that despite the variation in the job status of these new employees, the way in which they learn the job is relatively uniform and rudimentary. Skills are acquired opportunistically, often at the initiative of the newly hired employee, with older employees lacking the resources and the time to pass on their know-how. These findings raise issues relating to workplace safety and organization -a dual focus of the debate about conditions conducive to skills transmission.

INTRODUCTION

Entry into the workforce is a major milestone in a person's life. Many issues must be confronted by the individual taking a first job and also by the company hiring him or her [START_REF] Kergoat | De l'indocilité au travail d'une fraction des jeunesses populaires. Les apprentis et la culture ouvrière (Disobedience on the job among working-class youth. Apprentices and the worker culture)[END_REF][START_REF] Cohen-Scali | Accompagnement en entreprise et intentions liées à la carrière chez les apprentis du bâtiment (On-the-job support and career plans among construction apprentices[END_REF][START_REF] Merlino | Symptoms of Musculoskeletal Disorders among apprentice construction workers[END_REF]. Companies carry out training to address such issues as over-exposure of young and inexperienced workers to occupational accidents, retention of this group of workers and transmission of knowledge and know-how between new and experienced workers.

Against that backdrop, a study was carried out in a building and civil engineering company that complained of difficulty in retaining new workers despite a pro-active recruitment and training program. Following work on a possible ergonomic approach to worker retention issues [START_REF] Gaudart | La fidélisation de nouveaux dans une entreprise du BTP: Approche ergonomique des enjeux et des déterminants (Retaining new workers in a construction and civil engineering company: an ergonomic approach to issues and determinants) Activités[END_REF], we illustrate the various aspects of the first work experience and the diversity of ways in which people take on and learn a first job in the construction sector.

METHODOLOGY Approach

To examine this issue, we took an exploratory approach.

We had an opportunity, during in-house vocational training, to meet with employees from the various divisions of this large French company. To gain greater familiarity with the industry, we also carried out three worksite visits during which we observed and met with workers.

26 interviews were conducted, each lasting between half an hour and one hour. The goal was to define: the varying status of new workers, what happens during the initial period spent in the company and, especially, the way in which knowledge is acquired and transmitted and the prerequisites for learning. Thematic analysis of these interviews [START_REF] Braun | Using thematic analysis in psychology[END_REF] enabled us to identify the salient points and also to take on board the diversity of the situations encountered. Excerpts of verbatim statements are presented. However, to ensure the anonymity of the participants, their names have been changed. For each excerpt, we specify first name, age and current employment status.

Population

The workers interviewed ranged in age from 22 to 49, had between 2 and 33 years experience in the sector and were all employed, in a variety of worksite jobs, under unlimited-term contracts. The types of job status and their training implications are described below, together with their frequency within the population studied.

RESULTS

Entry into the sector

Some

1. Internship as part of educational curriculum. We met very few people in this category (3/26). They were performing their internship as part of a vocational curriculum: stonecutter CAP, Construction BEP and worksite management BTS; they were hired after completing their training (whether or not they earned their diploma). Simon (24, crew leader) was supervised by both an administrative mentor and a worksite mentor -the crew leader in the latter case. In contrast to the other situations we encountered, he was a supernumerary in most of the teams he worked with, i.e. he was not counted in the production ratios, his official job being to receive training.

An induction program was provided and he changed teams a number of times in order to gain an overview of the way a worksite operates. He emphasises that being a supernumerary during the learning phase makes a major difference: "On the concrete worksite, I went to see the foreman, he showed me the drawings and I walked around to see what people were doing. On the drainage worksites, I spent the day doing backfill. When you are working to be productive, you can't afford to leave your workplace to go and ask a question." 2. Unlimited-term contract. Five people (of the 26) were hired under unlimited-term contracts: three, who had substantial seniority within the company, entered the sector because of family members (father, brother and brother-in-law) and two, who were younger, were recruited after graduating from a highly-regarded specialized building and civil engineering school and were offered supervision by a mentor during the first year to round out their training. However, this offer did not materialise and initial training was virtually non-existent.

Benjamin (24, foreman) says, "I signed a contract that included a mentor for the first year. But after I started, I was never provided with one." In Antoine's case, the site manager was officially appointed his mentor, but he never saw him. They both began as jacks-of-all-trades: "It is up to us to become members of the team and prove ourselves.

We have a diploma, so we have to prove ourselves."

[Antoine]. It is clear that the diploma would be a handicap of sorts if the new employee were to flaunt it.

3. Site contract for the duration of the project. Given the itinerant nature of some projects, especially in civil engineering, the company recruits workers (7/26) in regions where the worksites are located for the duration of the project (between 1 and 3 years, approximately). Here, too, job status makes no difference in the way in which new employees are inducted; practice ranges from formal induction to virtually no induction at all.

Temporary contract. Temporary job status does not

entail training as such since temporary employment companies are assumed to provide workers having the required skills. Yet this was the initial job status of the majority (11/26) of the people interviewed. They started as temporary workers, learned on the job and asked to be hired under unlimited-term contracts. "I am 31 and I have been working in construction for about seven years. I was hired two years ago and prior to that I was a temporary worker." [Mathieu,31,skilled worker]. These interviewees were former temporary workers who had "succeeded" in being hired after a more or less lengthy period (ranging between several months and sometimes or 6 years) of temporary status. Training was not official and induction, when it existed, was sketchy. A crew leader will ask for a temporary worker because an extra hand is required to take part in production. Therefore temporary workers are assigned to tasks that apparently require no training -"wielding a shovel" -and this can last for a considerable period of time. We did, however, encounter two situations in which the temporary workers were explicitly inducted and trained by a member of the team: in one case, a person on the verge of retirement trained the temporary worker to replace him; in the other case a crew leader, working alone, recruited a first person and trained him as part of the tandem. These cases show that the training objective is clearly stated when there is an immediate need to hire a team member for the long term.

Uniform induction, a severe test

The interviews show uniform induction patterned on the induction of the temporary worker, even when a mentor is provided. This kind of orientation for newly hired employees harks back to the experience described by more experienced workers (even younger ones), for whom the idea is to prove oneself. "At the start, since you don't know anything, the foreman yells at you right away. And -the team spirit that exists among skilled workers in certain teams, which prevents people from leaving, because that would be seen as letting down their co-workers. "There's a team spirit that forms, I think. There is a kind of solidarity. I may not personally feel like working today, but I am not going to let my friends down. And that effect, alternating between people, makes it work.

At some point in a team, somebody is going to flag, and the group gives him a boost again." (Franck).

-the ability to learn on one's own as a labourer (an issue we will discuss in more depth under the following chapter).

Independent of job status: "opportunistic" learning

In the case of all the workers interviewed, learning can be called "opportunistic" in the sense that the new employees make the best of the circumstances. Newly Along with remaining on the lookout for opportunities to prove their skills, these workers took a proactive approach to acquiring new knowledge by following a combination of strategies:

-watching experienced employees up close: being assigned as a pipelayer helper provides an opportunity to closely observe the pipelayer at work -"The goal of a good labourer is to make sure the pipelayer has to do as few things as possible apart from pipelaying; it means you prepare a lot of equipment." [Benjamin].

-watching people working from a distance: being assigned to doing backfill or to operating the shovel gives a good overview of the worksite.

-looking at the construction drawing: this is a sign of trust on the part of the crew leader; it provides an opportunity to understand the entire project and to anticipate the work to be accomplished in the coming day.

-asking questions: "The people who taught me the job were welders and pipelayers, because I was interested. I asked questions, I went to see how they were working. Whenever I had five minutes to spare, I went to see the pipelayer and talked with him." [Nicolas]; "They explain when we ask them about it." [Simon]; "I learned by watching. A look back over the careers of these workers shows relative uniformity in the orientation and training they received, across all types of initial job status. The status of the newly hired worker makes very little difference in the way he starts out on the job. A person who has no experience or diploma begins as a labourer, since he cannot perform a task requiring specific technical knowledge, and a person who does have a diploma must not be given preferential treatment and must "prove himself" like everyone else. On the other hand, the jack-of-all-trades status may last less long than it does in the case of temporary workers, and a new employee with experience or a diploma is able to move on to other tasks fairly quickly. 6 to 12 months later, they now had the job status of assistant pipelayers and worked de facto as pipelayers supporting the crew leader. Their start in the business is similar to that of the temporary workers, but the opportunity to move up occurs earlier.

Orientation and transmission of knowledge and know-how

While new employees focus on learning the technical ins and outs of the job and on understanding the way the worksite is organised, the more experienced workers first ask them demonstrate their backbone, grit, respect and interest in the life of the worksite. Several statements made during the interviews show the importance of these qualities: "Some temporary workers have backbone and willingness to follow orders, and others don't. We try to tell which temporary workers are able to get things done.

After four, five or six months, we are able to identify them and recognize the ones who have grit." [Gérard,44,foreman]; "You have to get off to a good start. I started out in earthworks, doing test boring. I wanted to do that, I did my job, I did it well and I liked it. They can tell right away when a young person wants to do a good job. They are there to explain things and show you how to do it.

Afterwards, when they turn their backs, the young people keep going, and really get into it. And after awhile, they leave." (Nicolas). The first tasks entrusted to new workers are tasks that don't require any particular skill. It is not necessary to learn how to hold a shovel.

The demands of the "old hands" can be ambivalent for new workers who are asked to make a commitment to the job and to show interest (for example by asking questions) while at the same time unquestioningly following orders.

Moreover, the personal qualities called for -backbone, grit, respect -are qualities that are not specific to the job.

The "old hands" act as if showing backbone were a natural, innate quality that is independent of the job and the conditions in which it is performed. Tenacity is also called for because the trial period can last for several months or even years, during which time the new employee has little information about what is in store for him and at the same time realizes that he must take charge of his own future career path.

Meanwhile the statements made by the "old hands" suggest that their attitude has been shaped by the failures they have encountered, particularly the temporary workers for whom they made an effort and who did not stick with the job. When they talk about their own start on the job 15 or 20 years ago, they note that the older workers at the time took a pro-active approach. "The skilled workers told us how to do the job and why it had to be done that way.

They had to explain it several times. [...] We watched them do it and then tried it ourselves while they watched us to make sure we didn't make mistakes." (Gérard). They also This system result in poor communication and even mistrust between new and experienced workers. We met new workers who were disappointed because they had expected to be supervised as part of a formal learning process and who were thrown off balance by tensions in relations between workers; and the older workers tended to lack confidence in them and devote little time to them, because they had been disappointed when new workers (mainly temporary workers) repeatedly left.

DISCUSSION

Our analysis of these interviews does not claim to be comprehensive. Our objective is therefore not to draw general conclusions but rather to identify trends we encountered in the company. We would highlight several points:

-The job status of the new employee appears to have little impact on the worker's career or the content of his training. New workers are asked to perform general tasks or tasks not directly related to the worksite, in other words tasks that are unskilled but productive. For those who have previous experience with providing new workers -some in their thirties -with orientation, the focus was not on transmitting knowledge and skills but on testing the new employees to see how they manage and how committed they are (just as they were tested themselves when they started out).

-The introduction of a system to support and train new workers was mentioned on a number of occasions. But it was often an expedient used to meet a short-term need (when an operator had to

Figure 1 :

 1 Figure 1 : Initial job status distribution of the 26 employees interviewed

  afterwards, you learn as you go ... now I am the one who yells, to get the work done."[Loïc, 22, skilled worker].Today's experienced foremen started out as "gofers" or labourers, no matter what their initial job statustemporary contract, site contract or internship. "I came in as a temporary worker. That must have lasted something like six months."[Mikaël, 33, crew leader]. "We are sort of like "gofers" on the worksite. We are formwork helpers, bricklayer helpers and labourers. We go everywhere, we clean up the worksite, we hose down the shutteringwhich is being done less and less now... It was a transition, it took 6 months to a year."[Franck, 36, foreman].The job of labourer has never been easy to take and all the interviewees considered that this first worksite experience is physically and emotionally trying. "I really had a hard time on my first worksite. I was so disgusted that I nearly quit. [...] After a number of arguments with the old hands, I told myself, "I can't go on like this." [...] At the beginning I was taking physical risks because I had been in an army commando unit, but physically it is very tiring. And my morale was low because to me it was demeaning. When you spend a day shovelling sand into a container, you go home in the evening and you are physically tired because shovelling is hard, and you are even more tired in your head. I'm not an engineer but I couldn't see myself spending my life shovelling sand. It's degrading. And psychologically it got to me. I felt like I was losing ground in my life." (Mikaël). Despite early familiarity with the job, as in the case of Franck who said he "always know about the job. My father always talked about it with passion, he loved his job," the beginning can be painful. "I was scared, it was a big world -major projects one after the other. It was a metal inferno. It was in the middle of winter, it was cold. I had a fur-lined jacket and I didn't move. I was standing there like a stake.And I asked myself, "What am I doing here?" The first foreman I met said to me, "I didn't go to school, but I am not an idiot"; so that day I understood that I was not going to be in charge. [...] At the beginning, it looked like a rough world and when you get there and you are weak -18 years old and with no experience -it gets to you psychologically." These excerpts from interviews show real suffering at work, and remind us that although these people "held out" despite the experience they describe, others did not hold out and gave up. In attempting to understand what enabled the people interviewed to persevere despite the experience they described, we identified four factors: -the prospect of being financially independent without necessarily completing their studies or earning a diploma; -the prospect of being promoted, perceived as a positive return on day-to-day work: "At the end of the project, I succeeded in obtaining an internship as a crew leader; so I said to myself, if the company is offering me an internship, then I have a chance of moving up -because at the start you don't know where you stand, you don't know whether you have done a great job or not." (Mikaël);

  hired workers are responsible for their own training and for working their way up. They all started out as labourers and had to prove themselves, with little if any notion of what sort of career possibilities were open to them. They therefore had to start out as labourers and figure out for themselves the link between what their peers and supervisors expected of them (i.e. courage, determination, interest, etc.) and the content of the work. They had to make a mark for themselves by proving that they were able to do the work that others were doing: -by arriving earlier to get the job done, -by taking advantage of their breaks and the absences of co-workers: "The draughtsman broke his hand and I took over. [...] I finished the job and that is why I stayed in the company [Jean, 31, skilled worker] -by taking the tools from the old hands. "By asking questions and taking the tools from the old hands. You have to take the tools from them because if you don't insist, they won't lend them to you. [...] You have to nag them to let you try, otherwise you spend the whole day just watching." [Nicolas, 34, crew leader] -by disregarding decisions taken by supervisors: "Every time I saw a machine, I climbed right on it. Sometimes I got yelled at -the foreman would come and shout 'Get down from there! You have no business being up there!' The next day I would get back on it and the guy would yell at me again. And then there were days when the machinery operator wasn't there. And the foreman would say, 'O.k., Mikaël, you like those machines, you get up there,' and after a year had gone by I was operating a planing machine. Apart from the crew leader, that was the most important job." (Mikaël).

  You could get people to explain things to you if you got along well with them." [Christophe, 38, skilled worker]. -picking and choosing, i.e. adapting what others have demonstrated to one's own work -which involves putting one's own practice into perspective, and thinking about the way one does things. "Afterwards, everyone has to pick and choose which method he wants to use. It's part of learning the ropes." [Franck].

  mention team stability and deadlines as important factors in forming a working team and transmitting skills properly: "Explaining things to new people? It depends on whether you are running late or not. And you don't know in advance how long the temporary workers are going to stay." (Jean). Risk management and prevention are officially the responsibility of foremen when they are responsible for supervising interns and temporary workers. The three foremen interviewed considered that they had nothing to teach temporary workers, including newcomers to the construction sector, with the exception of safety instructions and the pitfalls of the worksite. When supervising interns in training, they send the trainees to perform a variety of tasks so as to identify those who are versatile. They also make a point of teaching trainees to be disciplined in their work -arriving on time and taking breaks on schedule. The transmission of knowledge and know-how is only rarely mentioned. Foremen delegate this responsibility to skilled workers.

  be trained fast) or an individual initiative taken by an older worker to induct a new worker. Even when supervision was included in the employment contract, it was not always effectively provided; -Since the older workers and the supervisory staff were either not transmitting work-related knowledge and know-how at all, or at best were doing so only sporadically, new workers were often obliged to take charge of their own training by asking questions, thus demonstrating their interest in learning the job. This type of learning can be physically and psychologically demanding, especially when the prospect of a job and the duration of the trial phase remain vague. -The role of the team as a resource appears crucial. Working in a team enables the new worker to cope with an environment perceived as hostile (the newer one is to the job, the more hostile the environment seems); and it makes it possible to work together to create opportunities for learning. These first findings suggest several issues that should be further explored: -The people interviewed invariably said that work situations are often dangerous and that the safety rules are strict. That being the case, to what learn by trial and error? Up to what point can learning be left up to the initiative of the learner, since by definition he is not familiar with the dangers and cannot identify the risks he is taking for himself and others? -The time constraints inherent in the organization of worksites are strong determinants of how, what and sometimes whether knowledge is transmitted. How much time should be devoted to such transmission? How can production and training, which are in principle at cross-purposes, be reconciled? -Just-in-time management involving the recruitment of temporary workers makes knowledge transmission uncertain. What can be transmitted and how? What is "worth" transmitting when teams have variable geometry?


Anne-Laure Fameau

Jean-Paul Douliez

François Boué

Frédéric Ott

Fabrice Cousin
email: fabrice.cousin@cea.fr

Adsorption of multilamellar tubes with a temperature tunable diameter at the air/water interface

The behavior at the air/water interface of multilamellar tubes made of the ethanolamine salt of the 12-hydroxy stearic acid as a function of the temperature has been investigated using Neutron Reflectivity. Those tubes are known to exhibit a temperature tunable diameter in the bulk. We have observed multilamellar tubes adsorbed at the air/water interface by specular neutron reflectivity. Interestingly, at the interface, the adsorbed tubes exhibit the same behavior than in the bulk upon heating. There is however a peculiar behavior at around 50°C for which the increase of the diameter of the tubes at the interface yields an unfolding of those tubes into a multilamellar layer. Upon further heating, the tubes re-fold and their diameter redecrease after what they melt as observed in the bulk. All structural transitions at the interface are nevertheless shown to be quasi-completely reversible. This provides to the system a high interest for its interfacial properties because the structure at the air/water interface can be tuned easily by the temperature.

I Introduction

The main aim of the green chemistry is to use renewable biological compounds for application purposes. Thus, there has been recently a growing interest for the industrial valorization of long chains saturated or/and hydroxylated fatty acid. Such hydroxylated fatty acid are indeed very common in nature and can be extracted from agricultural resources, for example from plant biopolymers like cutin [START_REF] Heredia | Biophysical and biochemical characteristics of cutin, a plant barrier biopolymer[END_REF][START_REF] Villena | Monitoring biopolymers present in plant cuticles by FT-IR spectroscopy[END_REF] and suberin [START_REF] Graca | Glycerol and glyceryl esters of omega-hydroxyacids in cutins[END_REF] or derivated from castor oil like the 12-hydroxy-stearic acid. Their main drawback for applications comes from their lack of solubility in aqueous media. However, it has been demonstrated recently that with the help of specific counter-ions (choline [START_REF] Klein | Choline carboxylate surfactants: biocompatible and highly soluble in water[END_REF] , tetrabutylammonium hydroxide 5 , ethanolamine [START_REF] Douliez | Novel lipid system forming hollow microtubes at high yields and concentration[END_REF] , lysine [START_REF] Novales | Self-Assembly and Foaming Properties of Fatty Acid-Lysine Aqueous Dispersions[END_REF] ), the dispersion in water of long chain saturated fatty acids and their hydroxylated derivatives becomes possible, making them good candidates as new green detergents.

For the 12-hydroxy-stearic acid in the presence of ethanolamine as the counter-ion, a very original supramolecular assembly is obtained: fatty acid salts are forming concentric stacked bilayers that are rolled up to form the walls of tubes of a micron size [START_REF] Douliez | Novel lipid system forming hollow microtubes at high yields and concentration[END_REF] . The system has thus a hierarchical spatial structure, as it is made at local scale of bilayers of fatty acids of ~ 3-4 nm separated by a layer of water of a few tens of nm whereas at larger scale the rolling of the lamellar phases create very long tubes, with a diameter of the order of the 0.6 µm and a length of around 10 µm. Interestingly, if the self assembly of tubes has been already widely observed using various surfactants [START_REF] Terech | Self-assembled monodisperse steroid nanotubes in water[END_REF][START_REF] Terech | Ammonium lithocholate nanotubes: stability and copper metallization[END_REF][START_REF] Pescador | Microtubes self-assembled from a cholesterol-modified nucleoside[END_REF][START_REF] Terech | Self-assembled networks of ribbons in molecular hydrogels of cationic deoxycholic acid analogues[END_REF][START_REF] Oda | Tuning bilayer twist using chiral counterions[END_REF][START_REF] Shimizu | Supramolecular nanotube architectures based on amphiphilic molecules[END_REF] , such tubes made of 12 hydroxy stearic acid exhibit a remarkable additional behavior since the outer tube diameter can be increased by a factor ~ 10 in a span of a few degrees. The diameter of the tubes has a value of around 0.6 µm from 20°C to 47°C, a temperature at which it starts to strongly increase to reach a maximum value of 5 µm at 50°C. Then, it decreases again upon a further increase of temperature [START_REF] Fameau | 12-Hydroxystearic acid lipid tubes under various experimental conditions[END_REF][START_REF] Novales | Self-assembly of fatty acids and hydroxyl derivative salts[END_REF] . Finally, at 70°C, the solution becomes isotropic and no more tubes are longer observed, a disappearance probably linked to the melting of tubes into micelles. At a local scale, the interlamellar spacing also changes with temperature but do not follow the same trends as the diameter. It slightly increases from 35 nm at 20°C to 38 nm at 45°C and drops down to ~ 27 nm at 60°C. This could be related to the thermodynamical (L β ) gel/ (L α ) fluid transition of the fatty chains, measured at 46°C by calorimetry [START_REF] Fameau | 12-Hydroxystearic acid lipid tubes under various experimental conditions[END_REF] : up to 45°C, the bilayer thickness is about 40 Å, which corresponds to twice the length of the fatty acid chain in its extended conformation, showing that the fatty acids are embedded in a gel bilayer phase (L β ) while it decreases drastically down to a value of 23 Å above 50°C, a thickness markedly lower, when the fatty acids are in fluid phase (L α ).

Besides their temperature properties, these tubes can also survive to important changes of various experimental parameters such as pH and salt [START_REF] Fameau | 12-Hydroxystearic acid lipid tubes under various experimental conditions[END_REF] . This provides them an evident potential interest in medical applications, for encapsulation and control release [START_REF] Kameta | Controllable biomolecule release from self-assembled organic nanotubes with asymmetric surfaces: pH and temperature dependence[END_REF] , or in material science for templating [START_REF] Shimizu | Supramolecular nanotube architectures based on amphiphilic molecules[END_REF][START_REF] Ji | Regulation of silica nanotube diameters: Sol-gel transcription using solvent-sensitive morphological change of peptidic lipid nanotubes as templates[END_REF] . The solutions of tubes have also enhanced interfacial properties: they exhibit better foaming and emulsifying properties when the fatty chains are assembled in tubes rather than micelles [START_REF] Novales | Self-assembly of fatty acids and hydroxyl derivative salts[END_REF] .

The understanding of such peculiar interfacial behavior is not trivial and raises the following questions: Are the tubular structures stable at a water/hydrophobic interface (e g air in the case of foams)? Or, more generally, what is the relation between the structure of the fatty chains in bulk solution and the structure of the fatty chains at a water/hydrophobic interface?

These are the questions we address in this paper by presenting an exhaustive study of the structural behavior of a solution of ethanolamine salt of 12-hydroxy-stearic acid at the air/water interface by coupling surface tension measurements and Neutron Reflectivity (NR).

Indeed, it has been demonstrated that NR is a powerful technique for the study of surfactant adsorption for various interfacial architectures such are surfactants [START_REF] Tucker | Interplay between the Surface Adsorption and Solution-Phase Behavior in Dialkyl Chain Cationic-Nonionic Surfactant Mixtures[END_REF][START_REF] Jackson | Structure of Partially Fluorinated Surfactant Monolayers at the Air-Water Interface[END_REF][START_REF] Vongsetskul | Interaction of a Cationic Gemini Surfactant with DNA and with Sodium Poly(styrene sulphonate) at the Air/Water Interface: A Neutron Reflectometry Study[END_REF][START_REF] Penfold | Neutron scattering for surface characterization[END_REF][START_REF] Lu | Surfactant layers at the air/water interface: structure and composition[END_REF][START_REF] Penfold | The Application of the Specular Reflection of Neutrons to the Study of Surfaces and Interfaces[END_REF][START_REF] Penfold | The Structure of Mixed Surfactant Monolayers at the Air-Liquid Interface, as Studied by Specular Neutron Reflection[END_REF] monolayers, lamellar phases in the vicinity of the interface [START_REF] Salamat | Undulations in salt-free charged lamellar phases detected by small angle neutron scattering and neutron reflectivity[END_REF] or multilamellar vesicles [START_REF] Mcgillivray | Ordered structures of dichain cationic surfactants at interfaces[END_REF] .

II Materials and Methods

Sample preparation

12-hydroxy stearic acid (Sigma-Aldrich, 99% purity) was weighted accurately in a test tube in which ultra pure water was added to reach a concentration of 10mg/mL (1%). Then, we incorporated to reach equivalence (12-hydroxy stearic acid/counter-ion molar ratio R=1/1), the desired volume of a 1M stock solution of the counter-ion, 2-amino-1-ethanol (Sigma-Aldrich, 99% purity). The mixture was heated at 80°C for 15 min until all components were dispersed and then vigorously vortexed. Prior to use, each sample was heated at 80°C for 15 min and cooled at room temperature.

For the specular neutron reflectivity experiments, the sample was prepared in 3 different

mixtures of H 2 O/D 2 O: 40%H 2 O/60%D 2 O, 20%H 2 O/80%D 2 O and 100% D 2 O.

Specular Neutron Reflectivity

Specular neutron reflectivity (SNR) experiments were carried out on the horizontal time-offlight reflectometer EROS at the Laboratoire Léon Brillouin (CEA Saclay, France). The horizontal collimated beam is deflected by a neutron supermirror by an angle of 0.75° on the sample to collect data at a fixed incidence angle of 1.5°. With a neutron white beam covering wavelengths from 3 to 25 Å, the accessible Q-range is 0.01 -0.1 Å -1 .

The sample was placed in a sealed cell, with two quartz windows allowing the passage of neutrons, avoiding the exchange of D 2 O with H 2 O from atmosphere. Measurements were performed at ambient pressure. The temperature of the cell was precisely fixed by a circulating circuit connected with a thermalized Lauda bath. Prior to each measurement, the sample was rest for 30 minutes to reach equilibrium. The acquisition of the SNR data was then recorded for 6 hours with slices of 1 hour.

Most of the experiments were performed during a first series of experiments where the data were acquired by a 1-D detector. It appeared afterwards that there was an intense off-specular scattering in the system that could perturb the determination of the specular scattering. In order to circumvent this difficulty, we performed afterwards some experiments with a 2-D detector to discriminate between specular and diffuse scattering. For lack of beamtime only one sample has been measured with the 2-D detector. The intense diffuse scattering raises the question of the exact determination of the specular scattering because this latter may be contaminated if it is integrated over a too large surface of the detector [START_REF] Rauscher | Local density profiles in thin films and multilayers from diffuse x-ray and neutron scattering[END_REF] . This is illustrated in Figure SI.2 that compares 3 different areas of integration of the specular signal of a solution at 10mg/ml in pure D 2 O, with respective thicknesses of 5 mm, 10 mm and 20 mm (see Supplementary Information). While they show the same characteristic fringes, the reflectivity increases when the integration area increases, and progressively deviates from the Fresnel curve as off-specular signal it integrated in the specular window. It appears that the choice of the integration area does change neither the shape nor the intensity of the fringes. In such a representation, the additional diffuse scattering increases the level of the baseline of the whole curve above the Fresnel reference curve. When the integration window is properly defined the SNR curve follows the Fresnel decay in Q -4 . We compare also in Figure SI.2 the results obtained with the 1-D detector, which has a thickness of 10 mm. It clearly appears that it slightly integrates too much diffuse scattering (its level is of the order of the 2 nd integration on the 2-D detector). This moderate excess of scattering however does not prevent a correct qualitative interpretation of the data obtained with the 1-D detector.

The reflectivity curves corresponding to the analytical models presented within the text are calculated by the optical matrix method with a slicing of the scattering length density profiles in slabs of 10 Å. The experimental resolution of the spectrometer was taken into account in the calculation.

III.1 Results and Discussion

We present here the Specular Neutron Reflectivity (SNR) measurements performed at the air/water interface on solution of fatty acid tubes to determine the corresponding interfacial structure of the fatty acids chains. We have chosen to work at a solution concentration of 10 mg/ml (with a molar ratio of 1/1) because such concentration corresponds to the lower limit for which there are tubes with a temperature-dependent diameter in bulk solution [ref].

Prior to SNR experiments, we have performed dynamic surface tension measurements to determine the characteristic time taken by the fatty chains to reach a stable structure at equilibrium at the air/water interface. The results, shown in Figure SI.1 of the Supporting Information, shows that the system is close to equilibrium after ~ 5 seconds. The evolution of the surface tension in the early 5 seconds suggests that the structure at equilibrium is different from a simple layer of fatty acids monomers at the air/water interface. When we performed the SNR measurements, the solution of tubes was placed in the measurement cell and the samples were let to rest for 30 minutes before starting data acquisition (see Materials and Methods). The system was thus at equilibrium during the experiments (see supporting Information).

III. 1 Raw data at 20°C

The curve of Figure 1.a shows the data obtained for a solution in pure D 2 O, heated at 20°C out of beam and deposited on the preheated cell at the same temperature (see Materials and Methods section). It is compared with the Fresnel curve that corresponds to the pure air/D 2 O interface. The same results are also displayed in the Fresnel representation (R(Q)Q 4 versus f(Q)) in Figure 1, which enables to focus on the scattering coming only from the layer present at the interface by compensating the Q -4 scattering decay from the pure air/water interface.

The displayed curve is the sum of 6 spectra (6 hours). Indeed, 6 consecutive spectra (not shown) have been recorded kinetically by slices of one hour up to a total duration of 6 hours.

They were all similar, confirming that the structures adsorbed at the interfaces are stable with time. It immediately appears that there are regular Kiessig fringes Kiessig réguliers pas ? in the specular signal, reflecting that there is a large layer of fatty acid materials at the interface.

The raw 2-D spectra reveal that there is also a high diffuse scattering (see Supporting Information). This intense diffuse scattering may come from large spatial heterogeneities on the surface, from the presence of large adsorbed tubes or possibly from the fluctuations of the adsorbed architectures at the interfaces. Please note that no structural ordering at large scale was observed from these off-specular scattering. As explained in the Materials and Methods section, this diffuse scattering does not allow the exact determination of the specular scattering, in particular with the 1-D detector. It nevertheless not hampers the exploitation of all of the data presented in this paper which were obtained with the 1-D detector because we will further show that the useful information is derived from the shape and the positions of the fringes and that the extra diffuse scattering can be taken into account. observed between the data for the 3 H 2 O/D 2 O contrasts unambiguously prove that the scattering indeed arises from a surface scattering of fatty acid materials. It remains however possible that a small remaining part of the measured scattering comes from SANS.

(a) R(Q) versus f(Q); (b) Q 4 R(Q) versus f(Q).

III.2 Modeling of the SNR data at 20°C:

In order to fit our SNR data, we used various models of fatty acid arrangements at the air/water interface. We limit here this modeling to the results of the 2-D detector integrated with the lower surface area (see Materials and Methods), for which the effects of the offspecular scattering do not perturb too much the SNR data. First, since the surface scattering comes from fatty acid materials adsorbed at the air/water interface, the simplest model to be tested is the presence of a single monolayer of thickness 21Å which stands for the length of the fringes, it is not consistent with our measurements because it completely overestimates the intensity of the fringes amplitudes. The only possibility to get such weak fringes with a lamellar stacking would come from strong fluctuations of the membranes [START_REF] Salamat | Undulations in salt-free charged lamellar phases detected by small angle neutron scattering and neutron reflectivity[END_REF] . But the fluctuations in bulk are weak in our system, as proved by SANS [START_REF] Douliez | Novel lipid system forming hollow microtubes at high yields and concentration[END_REF] . Moreover, the loosening of the fringes would increase with temperature, while the reverse is observed (see Figure 3a of section III.3).

The possibility of adsorption of multilamellar vesicles can also be dismissed because there is no transition from tubes to vesicles for 12 hydroxy stearic fatty acids in bulk solution. When such vesicles-tubes transition is observed, it is associated with the (L β )/(L α ) transition of the chains [START_REF] Shimizu | Supramolecular nanotube architectures based on amphiphilic molecules[END_REF] . But 12 hydroxy stearic fatty acids, multilamellar tubes are observed whether the chains are in the L β gel phase or in the L α fluid phase [START_REF] Novales | Self-assembly of fatty acids and hydroxyl derivative salts[END_REF] .

We will then assume that the fringes come from the presence of multi-lamellar tubes adsorbed at the air/water interface, adsorbed on a fatty acid monolayer at the surface (see figure 2e).

The theoretical approach of the modeling of the SNR corresponding to such structure is presented in Appendix 1. It involves the same geometrical parameter as observed in bulk [START_REF] Novales | Self-assembly of fatty acids and hydroxyl derivative salts[END_REF] (see Figure 2.e), i.e., an interlamellar spacing, d, a bilayer thickness, e, a tube diameter, D, with a number N of bilayers per tube and a given surface density Φ S . N is assumed to lie between 3 and 5 from the analysis of TEM pictures and from the thickness of Bragg peaks observed in some SANS experiments of tubes in bulk [START_REF] Douliez | Novel lipid system forming hollow microtubes at high yields and concentration[END_REF] . The thickness, t, of the monolayer is fixed to 21 Å. In the model we do not consider any additional signal to take into account the remaining background coming from the off-specular scattering.

We present the SNR curve corresponding to such a calculated profile for the following We are conscious that our fit is performed with a large number of parameters. It is by the way easily possible to get a perfect fit of the data by a minimization of the χ 2 using a free set of parameters with free variation. However, we now discuss the impact of the different parameters on the SNR curve allowing testing our model. For the sake of clarity, fits using this model obtained by varying the different parameters are shown in the supplementary information section (Figure SI.5) but we only report here the main results.

-First, the thickness of the fatty acid monolayer at the interface has been varied but does not change the overall aspect of the fit. However, we can reasonably assume it is of the order of 21 Å since it corresponds to the length of the fatty acid in its extended conformation. An accurate determination is in principle possible, even for small thicknesses, since it fixes the global envelop of the SNR curve, independently from the other parameters. However, this would imply to get a very confident fitting of the shape of the curve at large Q, at which the diffuse scattering strongly perturbs here the data.

-Second, for physical reasons, we assume that the bilayer thickness, e, must not be considered as a free parameter and we have thus imposed the value of 41 Å. This stands for the value measured by SANS for tubes in bulk since fatty acids are embedded in their gel phase (L β ) at 20°C.

-Third, the interlamellar spacing, d, is the most influent parameter because it defines the Q-position of the fringes. Then, it can be confidently determined from them and was further fixed to 270 Å.

-Then, the number of bilayers per tubes, N, influences the whole SNR fitting curve. The best fit is obtained for a mixing of tubes, 30% having 3 bilayers and 70%, only 2 bilayers.

For N up to 3, one obtains additional oscillation superimposed to the Kiessig fringes which can be assigned to Bragg peaks coming from the interlamellar distance. For higher values of N, one observes additional fringes arising from the characteristic thickness of the whole tube wall, i.e., the distance between the most outer bilayer and the most inner bilayer layer of a tube.

-Also, the exact value of the tube diameter D has a weak importance, only its range of order has a real influence on the SNR fitting curve. As long as the tube diameter is both:

(i) high enough compared to the interlamellar spacing, d, and (ii) low enough to consider that the bilayers are bent (D typically of the order of the micrometer). Indeed, for D too high, the fitting curve is that obtained for a multilamellar arrangement at the interface what has been shown to yield too intense Kiessig fringes. We took here a value of 6000 Å, as obtained from TEM pictures [START_REF] Douliez | Novel lipid system forming hollow microtubes at high yields and concentration[END_REF] , but we have checked that values in the range 5000 Å -8000 Å, within the errors bars of TEM, do not noticeably modify the SNR fitting curves.

-Finally, Φ S obviously only plays on the amplitudes of the fringes. So, we have thus imposed the value of 0.8.

Thus such a modeling of the experimental data is in accordance with the presence of the multi-lamellar tubes, of same dimensions than in bulk solution, adsorbed at the air/water interface with a high packing. This high off-specular scattering we obtain is due to the presence of such tubes at the surface. SNR appears here as a powerful technique to determine their interlamellar spacing d from the positions of the fringes. Indeed, even if the multilamellar tubes essentially contains water (Φ fc (z') is at maximum 0.05 out of the surface), the jump in neutron refractive index between a bilayer of hydrogenated fatty acid and the deuterated solvent is so large that the fringes remain visible in the SNR curve.

III.3 Evolution of the SNR data with temperature for in-cell heating

We present in this section the evolution of the SNR experimental data with temperature. First, the solution of tubes was deposited at 20°C and heated directly within the measurement cell.

The results are shown in Figure 3.a. They have been all obtained with the 1-D detector and we propose only a qualitative description based on the Q-position and on the amplitudes of the fringes.

At 40°C, the SNR curve has a very similar shape as the one obtained at 20°C. The fringes are slightly shifted towards low Q, indicating an increase of the interlamellar spacing d, similar to what was observed for the tubes in bulk solution [START_REF] Fameau | 12-Hydroxystearic acid lipid tubes under various experimental conditions[END_REF][START_REF] Novales | Self-assembly of fatty acids and hydroxyl derivative salts[END_REF] .

At 50°C, the structural behavior at the air/water interface completely changes. We recall that this temperature is the one for which, in bulk, the tube diameter increases drastically (from 0.6µm at 40°C up to 5 µm à 50°C [START_REF] Douliez | Lipid tubes with a temperature-tunable diameter[END_REF] ). At this temperature, the spectra recorded every 30 min evolved with time (during the first 5 hours) showing that the system has not reached equilibrium after 30 minutes (see figure 3.c). This behavior was not observed for the other temperatures. After 1 hour, one observed strong marked fringes on the experimental curve which then resembles the SNR curve we have obtained with a model of flat stacked bilayers at the interface (see figure 2.a) and observed on lamellar systems by others [START_REF] Salamat | Undulations in salt-free charged lamellar phases detected by small angle neutron scattering and neutron reflectivity[END_REF][START_REF] Mcgillivray | Ordered structures of dichain cationic surfactants at interfaces[END_REF] . Here a perfect fit is obviously not possible because the system has evolved during the recording, i.e the SNR provides the averaged structure of the system that was not at equilibrium. However the shape prompts us to assume such stacking. We measured on that experimental curve an interlamellar spacing deduced from the position of the Kiessig Fringes and found a value (averaged thus on

H 3 H 2 H 1 H R Q (Å -1 )
one hour) of around 300 Å. After 2 and 3 hours, the Kiessig fringes are even much marked (higher intensity) and were shifted toward larger wave vectors (Figure 3.c). For these two set of data, the interlamellar spacing has decreased down to around 260 Å. At longer times, the fringes go on shifting towards large Q, until they vanish progressively (Figure 3.c).

It is likely that the slow kinetic of the progressive decrease of the interlamellar spacing induces a large polydispersity on the spacing on the lamellar phases on the surface. The progressive disappearance of the fringes comes probably from the formation of surface domains with different interlamellar spacing on the surface and thus the experimental measured SNR curve, averaged on the whole surface, is a linear combination of different SNR curves with marked fringes. In the meantime, the critical wavector Q c is shifted towards the low Q indicating that the SLD of the semi-infinite medium beneath the surface (in practice, the first thousands of Angströms) is no longer the one of the D 2 O solvent only. A shift towards the low Q means lowering of the SLD: there must be a huge enrichment of fatty acid materials over these first thousands of Angströms beneath surface. More quantitatively, from the position of Q c (0.015 Å -1 ) we get the value of the SLD after 5 hours (4.35*10 -6 Å -2 ), corresponding to a fatty acid materials fraction around 0.29 %. It is thus likely that the tubes have unfolded and coalesced to form an ordered lamellar phase stacked below the interface, oriented parallel. This finding is coherent with our previous observations in the bulk for a concentrated solution of tubes [START_REF] Douliez | Novel lipid system forming hollow microtubes at high yields and concentration[END_REF] . Indeed, we had shown that for a 100 mg/mL concentrated solution of tubes, the diameter increases upon heating but do not reach a maximal value as observed for a lower concentration (10 mg/mL, as used here). Rather, the diameter of the tubes indeed increases but then, the tubes 'coalesce' to form a multilamellar phase.

Interestingly, upon a further heating, the tubes re-folded and their diameter decreased again.

One can assume that the concentration of tubes in our present experiment at the interface is high enough to reproduce this phenomenon and that the tubes also coalesce at the interface to form flat stacked multilamellar bilayers. The decrease of the interlamellar spacing as a function of time corresponds to a dehydration between the bilayers and then, an enrichment of fatty acids at the interface. There are at least several tens of bilayers in this lamellar phase, contrary to the case of the tubes in which only 3 to 5 bilayers are involved.

At 60°C, the behavior in bulk solution is known to be close to the one observed at 40°C, i.e., the tube diameter has decreased back, probably to its initial value [START_REF] Douliez | Lipid tubes with a temperature-tunable diameter[END_REF] . At the interface, one recovers a SNR curve exhibiting all the features observed at 20°C. This suggests that multilamellar tubes are again adsorbed at the interface and that the overall behavior as a function of the temperature is similar at the interface or in the bulk. The lamellar phases at the surface observed at 50°C have thus re-swollen and form again tubes adsorbed at the interface.

The fringes are largely shifted towards the large Q compared to the results at 20°C and 40°C, indicating that the interlamellar spacing d has decreased, exactly as observed for the tubes in bulk solution [START_REF] Fameau | 12-Hydroxystearic acid lipid tubes under various experimental conditions[END_REF][START_REF] Novales | Self-assembly of fatty acids and hydroxyl derivative salts[END_REF] . This behavior as a function of the temperature supports our model in which tubes are indeed adsorbed at the interface.

Finally, at 70°C, the fringes completely vanish. At this temperature, it has been shown that tubes no longer survive in bulk solution [START_REF] Douliez | Lipid tubes with a temperature-tunable diameter[END_REF] , suggesting their melting into micelles. The disappearance of the fringes would thus come from the melting of the tubes at the air/water interface, letting just of single monolayer of fatty acids. To test this hypothesis, we have checked by SANS that micelles are indeed formed at 70°C. The results are presented in Supporting Information SI5. Since there are no longer tubes at the surface, the diffuse scattering has disappeared. The experimental SNR data can then be perfectly modeled by a fatty chains single monolayer, and the expected thickness of 21Å is obtained (Figure 3.d).

III.4 Reversibility of the structural transitions with temperature

In order to test the reversibility of the observed variations, the temperature has been decreased from 60°C down to 40°C, and then heated again up to 70°C, with one measurement every 10°C. Results are presented in Figure 4. For a given temperature, there are only slight differences between the results corresponding to the first increase of temperature, the decrease of temperature and the second increase of temperature. All structural transitions at the air/water interface are then quasi-completely reversible! It is not easy to attribute the small differences between the curves to a real change in structural behavior, rather than structural changes induced by the experimental protocol chosen. In particular, the sample has been left for more than 70 hours within the cell (6 hours per measurement at a given temperature and 30 minutes for a temperature change and its equilibrium), mainly at rather high temperature, which results in the formation of a lot of D 2 O vapor within the cell. As D 2 O is a strong neutron scatterer, this vapor may alter a bit the quality of the data.

III.5 Influence of the thermal history of the sample

The results of the previous section have shown that the structural behavior of the tubes at the air/water interface is very similar to the one in bulk solution, except the specific behavior at 50°C. A series of experiment was done with the sample thermalized ex-situ out of the measurement cell at the desired temperature. Then, this heated sample has been deposited in the pre-heated measurement cell allowing testing the influence of the thermal history of the sample on the arrangements of fatty acids at the surface. The results are gathered figure 5 on which are superimposed the data of the previous section. the structure of the tubes at the air/water interface are completely driven by the structural behavior of the tubes in bulk, except for the specific behavior at the interface at 50°C.

III.6 Surface behavior versus bulk behavior

All the results from SNR unambiguously prove that the multi-lamellar tubes of fatty acid chains are present at the air/water interface, whereas one could have expected an 'unfolding' of the tubes induced by the hydrophobic planar air/water interface. The lack of historydependence of the structures at the air/water interface, and the fact that the structure in bulk and at the air/water interface follows the same trends, suggest that the surface structure is driven by the bulk structure. Remarkably, the interlamellar spacing d has exactly the same values, within the experimental errors, in bulk solution [START_REF] Douliez | Novel lipid system forming hollow microtubes at high yields and concentration[END_REF][START_REF] Fameau | 12-Hydroxystearic acid lipid tubes under various experimental conditions[END_REF][START_REF] Novales | Self-assembly of fatty acids and hydroxyl derivative salts[END_REF] and at the air/water interface, as shown by Figure 6. The values of d reported for the air/water interface have been obtained from simulations of pure lamellar phases at the interface, which gives exactly the same interlamellar spacing as in tubes when [START_REF] Douliez | Lipid tubes with a temperature-tunable diameter[END_REF][START_REF] Douliez | Novel lipid system forming hollow microtubes at high yields and concentration[END_REF][START_REF] Fameau | 12-Hydroxystearic acid lipid tubes under various experimental conditions[END_REF][START_REF] Novales | Self-assembly of fatty acids and hydroxyl derivative salts[END_REF] .

The interlamellar spacing follows the same trends in bulk and at the air/water interface, confirming from a quantitative point of view the narrow link between surface and bulk.

Conclusion

We have shown here by specular neutron reflectivity that multilamellar tubes of ethanolamine salt of the 12-hydroxy stearic acid adsorb at the air/water interface. Similarly to what observed in bulk solution, the tubes have a temperature tunable diameter at the interface.

Moreover, they have almost the same temperature dependence than in bulk. We have pointed a peculiar behavior around 50°C at which the tubes unfold and coalesce to form a multilamellar phase at the interface. It is also remarkable that this set of structural transitions at the interface is also quasi-completely reversible at surface as in bulk, whether the temperature is increased or decreased. The complete structural behavior of the tubes at the interface and in bulk solution is summarized in scheme 1. Finally, let us remark that the tuning of the structure of the multilamellar tubes at the interface by the temperature provides to the system exceptional potential for its interfacial properties (foams and emulsions).

For Table of Contents Use Only

Adsorption of multilamellar tubes with a temperature tunable diameter at the air/water interface Anne-Laure Fameau

Figure 1 :

 1 Figure 1: Specular Neutron Reflectivy results of a solution of fatty acids (10 mg/ml) at 20°C

A

  calculation of the thickness of the fatty chain layer from the d-spacing of the Kiessig fringes (d ~ 2π/∆Q) gives d ~ 300 Å, which is of the order of the interlamellar spacing as measured in the bulk solution (340Å from SANS experiments 6 ). Then, the SNR signal may arise from the surface scattering of fatty acid materials and/or from the intense SANS scattering of the tubes in solution below the interface. In order to discriminate between these two possibilities we have performed the same experiments at 20°C with 3 different H 2 O/D 2 O compositions of the aqueous solvent. The SNR curves are compared in Figure S.I 4. Their shapes are very similar, displaying some fringes that have the same d-spacing and the same amplitude. However, from one sample to another, the fringes are all shifted by a value equal to the critical wave vector Q c corresponding to the water composition. If the signal was due to bulk scattering, the Q-position of the fringes would have been constant with the water composition, only the amplitude of the fringes would have changed. Then, the difference 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0

the 12 -

 12 hydroxy stearic acid in its extended conformation (Figure 2.c). Clearly, Figures 2.a and 2.b show that the fit with such a small thickness does not reproduce the fringes experimentally observed.Second, another possible model of arrangement at the interface is the stacking of lamellar bilayers in the plane perpendicular to the interface. This behavior has already been observed in the literature on lamellar phases of C 10 SO 3 Na/C 12 E 5 mixtures[START_REF] Salamat | Undulations in salt-free charged lamellar phases detected by small angle neutron scattering and neutron reflectivity[END_REF] or on multilamellar vesicles of DDAB[START_REF] Mcgillivray | Ordered structures of dichain cationic surfactants at interfaces[END_REF] . Here, it could occur from the unfolding of the tubes induced by the planar surface into such an 'opened' structure. This would result in a stacking of lamellar bilayers, with an interlamellar spacing close to the one observed for tubes in bulk, i.e., 270 Å (Figure2.d). Such a layered model give rises to nice regular very large Kiessig fringes, due to the high change of neutron refractive index between the hydrogenated bilayer and D 2 O, as observed by Salamat et al 25 and Mc Gillivray et al 26 or shown by the simulations of Figure 2.a and Figure 2.b (d = 270 Å). Though such model would correctly model the Q-position of

  Figure 2.b). We attribute this feature to the perturbative additional background arising from

Figure 2 :

 2 Figure 2: Comparison of the SNR results of a solution of fatty acids at 20°C with a monolayer

Figure 3 :

 3 Figure 3: Evolution with temperature of the SNR curve of a solution of fatty acids at 10mg/ml

Figure 4 :

 4 Figure 4: Comparison between the evolutions of the SNR curves of a solution of fatty acids at

Figure 5 :

 5 Figure 5: Evolution of the SNR curve of a solution of fatty acids at 10mg/ml in pure D 2 O if

Figure 6 :

 6 Figure 6: Comparison between the evolution with temperature of the interlamellar spacing of

  their minima match the ones of the fringes (see Figure 2.b), as most experiments have been performed with the 1-D detector that do not allow an unambiguous modeling with tubes. The value at 50°C corresponds to the one of the planar stacked lamellar phase measured during the first hour of measurement for such a temperature. The bulk values of d are obtained from the positions of the Bragg peaks of the SANS experiments

(

  *) To whom correspondence should be addressed: Fabrice.Cousin@cea.fr 1) 3 different areas of integration of the specular signal of the value of the interlayer spacing. (c) effect of the given surface density of tubes Φ S_tubes . (d) Effect of the constant.

  [START_REF] Heredia | Biophysical and biochemical characteristics of cutin, a plant barrier biopolymer[END_REF][START_REF] Villena | Monitoring biopolymers present in plant cuticles by FT-IR spectroscopy[END_REF] , Jean-Paul Douliez[START_REF] Villena | Monitoring biopolymers present in plant cuticles by FT-IR spectroscopy[END_REF] , François Boué 1 , Frédéric Ott[START_REF] Heredia | Biophysical and biochemical characteristics of cutin, a plant barrier biopolymer[END_REF] and FabriceAdsorption of multilamellar tubes with a temperature tunable diameter at the air/water interface: a process driven by the bulk propertiesA-L.Fameau1,2 , J-P. Douliez 2 , F. Boué 1 , F.Ott 1 , B. Novales 2 , and F. Cousin 1,* 1 Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif sur Yvette Cedex, France 2 UR1268, INRA Nantes, Biopolymères Interactions Assemblages, rue de la Géraudière, 44316 Nantes, France

	Cousin 1,*
	Supporting Information

Appendix 1 Theoretical Scattering Length Density profile of multilamellar tubes adsorbed on a planar interface.

We define the zero position of the z-axis perpendicular to the surface at the ordinate corresponding to the contact between the monolayer and the tubes. Φ S is defined as the surface fraction occupied by tubes of diameter D at z = D, i.e Φ S is 1 if all tubes are in contact. The volume fraction of fatty chains only due coming to the sole tubes Φ fc_tubes (z) writes:

where A n (z) and B n (z) correspond respectively to the outer position and inner position of the n th bilayer in the tubes, starting from the surface.

We have thus

Thus if we define z' by z -t (z' = 0 at the air/monolayer), the volume fraction of fatty chains Φ fc (z') in the plane perpendicular to the interface becomes :

(2)

And the SLD profile becomes N b (z') :

)

where N b_fc and N b_solv are respectively the scattering length density of the fatty acid and of the solvent.

Aknowledgements:

We thank Alain Menelle (CEA, Laboratoire Léon Brillouin, France) for stimulating discussions on the NR experiments. We gratefully acknowledge Laurence Navailles and Frédéric Nallet (CNRS, Centre de Recherche Paul Pascal) for fruitful discussions on fatty acids tubes.

We show in Figure 1, the 3 different areas of integration of the specular signal with the 2detector corresponding to the reflectivity curves shown in the paper. We clearly observe a very intense diffuse scattering. Thus, if the integration area is too large on the 2-detector, we integrate some of the off-specular signal, interfering with the exact determination of the specular scattering. In this paper, we model the experimental SNR curve with tubes adsorbed on the monolayer at the interface with same geometrical parameter as observed in bulk. We show here the impact of the different parameters on the SNR curve to demonstrate that we can get a real confidence in our physical description of the data as a model of tubes adsorbed on the surface. We realized different calculations of tubes adsorbed at the interface where each parameters is varied independently from the others in order to show the influence of a variation of each parameter on the resulting SNR curve. In the figure 2a., we show the effect of the number of bilayer by tubes N. Clearly, it appears that from N=3, we have no longer tiny Kiessig fringes.

In the figure 2b, we show the effect of the interlamellar spacing d. This parameter fixes unambiguously the q-position of the minima fringes. Then, we studied the effect of the given surface density of tubes Φ S_tubes . As you can see on the figure 2c, this parameter only plays a role on the amplitude of the fringes. In the figure 2d, we show the model of tube with or Zone 3 without a constant. In fact, the model of tubes matches very well the shape of the experimental SNR curve but it remains below the level of the experimental SNR curve because of the perturbative background arising from the diffuse scattering. The intensity of such scattering as a function of Q follows the same trends as the specular intensity and must decay thus like Q -4 following the Fresnel decay. Thus if one adds to the calculated SNR curve a kQ -4 term, where k is a variable constant to roughly take into account the diffuse scattering, one recovers the level of SNR of the experimental curve. However, we can see that this rude additional treatment of background alters the shape of the calculated SNR curve because it has a more important weight for the minima of the fringes than for the maxima.