Effect of Unsaturation on the Adsorption and the Mechanical Behavior

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Effect of Unsaturation on the Adsorption and the Mechanical Behavior of Fatty Acid Layers Alexia Crespo, Nazario Morgado, Denis Mazuyer, and Juliette Cayer-Barrioz Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00491 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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Effect of Unsaturation on the Adsorption and the Mechanical Behavior of Fatty Acid Layers Alexia Crespo, Nazario Morgado, Denis Mazuyer and Juliette Cayer-Barrioz* Laboratoire de Tribologie et Dynamique des Systèmes, CNRS UMR5513, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, 69134 Ecully cedex, France

ABSTRACT Adsorption, self-organization and mechanical properties of different fatty acid layers under different confinement states have been investigated as a function of the presence and conformation of one unsaturation in their aliphatic chain. In-situ characterization, at the molecular level, was performed with the ATLAS molecular tribometer, in terms of rheology, forces and thickness of confined fluid. The results demonstrate that the fatty acids adsorb on the surfaces by weak interactions and form viscoelastic films with a thickness of about 15 Å on each surface. The adsorption kinetics, the packing of the self-assembled monolayers and the coverage rate depend on the molecular architecture of the fatty acids and lead to various mechanical behaviors under confinement.

ACS Paragon Plus Environment

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INTRODUCTION To reduce friction and/or to prevent damage between contacting surfaces is an important challenge in many mechanical systems. One way to achieve this goal is to use lubricious molecules that avoid direct contact between surfaces. When hydrodynamics cannot provide lift and varying the fluid viscosity is no longer sufficient to promote surface separation, Friction Modifier additives, such as fatty acids, can be used in low concentration [1]. Fatty acids are amphiphilic molecules with a hydrophilic polar group and a lipophilic chain. In order to explain the friction mechanisms, Hardy assumed that polar groups adsorb on the surface and the fatty acid molecules self-assemble and orientate vertically to form close-packed monolayers [2]. This assumption was validated by X-ray experiments on metallic surfaces [3]. The physico-chemistry as well as the topography of the surfaces is a first order parameter in the adsorption process. For instance, reactive metals covered by metal oxides promote the chemisorption of the fatty acid molecules and formation of metallic soap [4]. Physisorption may also occur through weak interactions [5-7] on less reactive surfaces. The topography of the surface itself plays a role. Even though most of the literature focused on atomically smooth mica surfaces, the impact of nanometrically rough surfaces was also analyzed [8]. Roughness seems to favor disorder and loose-packing in the molecular organization. Several studies described the formation and the molecular organization of fatty acid layers on surfaces using experimental tools (see for instance [5, 9-17] or molecular dynamic simulation [8]). Campen et al. [14] confirmed the self-assembly of monolayers of stearic acid diluted in dodecane on a mica surface using liquid cell AFM. For a concentration of 0.001 M, these authors showed that fatty acids form irregular islands, corresponding to domains of tilted single monolayers. Their mean size is


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about tens to hundreds of microns and their thickness is typically 1.6 nm. Lundgren et al. [5] measured with a surface force apparatus, a “hard-wall” film of a thickness of 5.4 nm assigned to monolayers adsorbed on each mica surface separated by a thin film of solvent and fatty acid molecules oriented mainly parallel to the surface. A molecular dynamic approach allowed Ewen et al. [8] to model adsorption on iron oxide of stearic acid molecules diluted into squalene, in hydrodynamic regime. They show that stearic acids tend to adsorb on surface forming monolayers separated by a film of solvent. In a contact, if the head group is critical for the adsorption process, the close-packed alkyl chains that interact with the cumulative short range van der Waals forces, between neighboring methyl groups, determine the ability to support a load. It is likely that the existence of an unsaturation modifies the ordering within the film. [5, 18-19] showed that the film formed on the surfaces could be thinner in the case of unsaturated fatty acids, especially for oleic acid, indicating that the presence of a double bond in a CIS configuration favors loose-packing at the surface. Koshima et al. [15] confirmed that result using Sum Frequency Generation technique. The purpose of this study was to investigate how different fatty acids, with different unsaturation degrees, form films on surfaces under different confinement states. Three fatty acids of 18 carbon atoms were studied in low concentration in dodecane. Stearic acid was chosen as saturated fatty acid and oleic and elaidic acid as two mono-unsaturated fatty acids. Unsaturated fatty acids have a double bond in the middle of the chain (ω9). This double bond is in the CIS configuration in oleic acid, which bends the molecule. It is in TRANS configuration in elaidic acid, which leads to a straight shape closer to the stearic acid shape. The effects of unsaturation were then investigated, using the ATLAS tribometer, in terms of thickness and force measurements as well as mechanical properties at the molecular scale.


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EXPERIMENTAL SECTION The ATLAS molecular tribometer The ATLAS molecular tribometer used in this study and the methodology proposed to characterize confined liquids at the molecular scale, were previously described in detail in the literature [20-21]. This apparatus allows quasi-static and dynamic displacements of a sphere in front of a plane in three directions thank to piezoelectric actuators. A drop of the fluid to analyze was placed at the interface between the plane and the sphere. Displacements in the z direction allow to squeeze the fluid. A schematic is presented in Figure 1a. The velocity of quasi-static displacement ranges between 1 Å/s and 1 µm/s. Dynamic measurement, in the range of 0.01 200 Hz, allow a simultaneous rheological characterization of the confined fluid in terms of damping and elasticity in both directions (normal and tangential). Displacements and forces are measured using capacitive sensors with a displacement resolution of 1 Å for quasi-static motions and 0.1 Å for dynamic motions and a force resolution of hundreds of nN for quasi-static motions and of tens of nN for dynamic motions.


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Figure 1. Experimental (a) Schematic of the ATLAS tribometer illustrating the sphere/plane contact with a droplet of fluid. The X and Z directions are indicated, (b) Molecular structure of the fatty acids (i) Stearic acid, (ii) Oleic acid and (iii) Elaidic acid. The molecular lengths, L0, estimated from the literature [23], are also indicated for each molecule.

Solid surfaces The sphere and the plane were respectively made of fused silicate glass and silicon wafer. Both surfaces were coated by a cobalt layer of 40 nm thickness, using cathodic sputtering under low argon pressure (10-6 mbar). The cobalt surface is a model surface that mimic metallic surfaces. Cobalt coating makes the two surfaces chemically symmetrical. It also permits the use of electrical measurements of the sphere/plane capacitance to define the absolute origin of distance Z [21]. XPS analysis on these surfaces showed the presence of an oxide layer estimated at around 0.3 nm thick according to past measurement [22]. After experiment, surface topography was analyzed by interferometry which gave a RMS roughness of around 0.5 nm. This observation also confirmed the absence of surface damage during the experiment. Values of sphere radius are summarized in Table 1. Table 1. Values of sphere radius used for experiments presented in this paper. Experiment Sphere radius ± 0.05 (10-3 m)

Pure dodecane

Stearic Acid solution

Oleic Acid solution

Elaidic Acid solution

2.10

2.03

2.00

1.94

Liquids


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Solutions of fatty acids of 2 mM diluted in dodecane were prepared. Dodecane was obtained from Sigma-Aldrich with purity greater than or equal to 99.0%. Dodecane was dehydrated by adding zeolites for several days and then filtrated with a nucleopore filter of 200 nm. Dodecane viscosity is 1.5 mPa.s at 23°C. Stearic acid, oleic acid and elaidic acid were also obtained from Sigma-Aldrich with purity greater than or equal to 99.0%. A low concentration of fatty acids was chosen in order to avoid micelle formation (even though the water concentration was really minimal) and maintain the viscosity of the solution constant to that of dodecane. Molecular structures of fatty acids are presented in Figure 1b according to literature [23].

Procedure For each experiment, a set of solid samples (sphere and plane) was prepared and a droplet of liquid was deposited between the two surfaces. Experiments were performed in an air-fired glass bell under atmospheric pressure of Argon at room temperature. The temperature was kept constant during the experiment and between experiments. The average temperature was 23 ± 0.5°C. To characterize the adsorption and the self-organization of the molecules on the surfaces, the evolution of the normal force, Fz, versus the sphere/plane distance was measured and analyzed. During such a quasi-static squeeze experiment, the sphere was moved toward the plane with the velocity of 2 Å/s, in the z direction, until the normal force, Fz, reached 1 mN. According to the sphere radius and the mechanical properties of the solids, a normal force of around 1 mN leads to a contact radius of around 3 µm and a mean contact pressure of about 30 MPa. To


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simultaneously measure the mechanical properties of the adsorbed interface, dynamic motions were superimposed [21] in both directions, z and x, at a frequency of 38 Hz and 70 Hz respectively and an oscillation amplitude of ± 4.5 Å and ± 0.5 Å respectively in the z and x direction. From these dynamic measurements, the mechanical impedance of the interface was obtained. The response is divided into two additives components: the conservative part coming from the in-phase response of the interface, elastic stiffness Kz(ωz) and Kx(ωx), and the dissipative part coming from the out-of-phase response of the interface, viscous damping Az(ωz).ωz and Ax(ωx).ωx. Signals were acquired during loading and unloading and at least three squeeze experiments were performed. The values presented here are representative and standard deviations are indicated.

Contact modelling When surfaces come in contact, the force applied on the sphere can induce its deformation. To measure the real distance between surfaces, D, it is necessary to determine the deformation of the solid. The Hertz’ theory was used for non-adhesive point contact, whereas in the presence of adhesive force, DMT theory was used. Thus, contact radius, a, and deformation, δ, were deduced using equations (1) to (3).

=

a =

∗ ∗

(1)

∗ ( ) ∗

(2)


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δ = ∗ with the mean reduced radius of curvature, R∗ =

(3) "

#$ #% # $ # %

&

= . The mean Young

modulus, E*, of the coated sphere/coated plane was measured under dry conditions at 74 ± 1 GPa by sphere/plane nanoindentation with the ATLAS molecular tribometer. Thus, the sphere/plane distance, D, taking into account the deformation of the solids, can be deduced from equation (4): D=Z+δ

(4)

with Z the sphere/plane displacement. As an example, Figure 2 presents the evolution of the normal force vs the distances Z and D for dodecane.

Figure 2. Evolution of the normal force, Fz, as a function of the distances, Z the relative displacement of the sphere/plane (•), and D the separation distance between the deformed solids (o) for pure dodecane.

RESULTS


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Adsorption, molecular organization and mechanical properties of stearic acid layer vs pure dodecane The behavior of fluids was analyzed during confinement. The mechanical impedance of the interface was reversible during confinement (loading and unloading). This indicates the stability of the molecular layers, undisturbed by the loading/unloading cycle. Evolution of damping and stiffness in confinement are rather similar for dodecane (Figure 3a) and solution of stearic acid (Figure 3b).

Figure 3. Evolution of the stiffness, KZ, and the damping function, ωZ.AZ, in the normal direction, during confinement for (a) dodecane solution and (b) stearic acid solution. The damping function is larger than the stiffness at large distances, exhibiting a viscous behavior. At


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short distance, the stiffness strongly increases, more characteristic of a solid-like behavior of the layer under confinement.

Two conditions were distinguished. At large distance, the damping function in the z direction, AZ.ωZ, is much higher than stiffness, KZ, and both fluids can be considered as viscous. At short distance (30 Å for dodecane and 50 Å for the stearic acid solution), the stiffness increases and becomes much higher than damping, molecular motions are highly restricted and the fluid acquires a solid-like behavior. At small separation distance (not shown in Figure 3), the stiffness, KZ, of the layers becomes of the same order of magnitude as the stiffness of a dry contact (KZ ~ 400 000 N/m): this seems to indicate a strong fluid/structure interaction dominated by the solid elasticity. This increase in KZ is associated with a stabilization of the damping function AZ.ωZ and an increase of the stiffness KX and damping AX.ωX in the tangential direction x (not shown here for sake of clarity).

At large distance, the fluids are considered viscous and the interfacial hydrodynamic can be discussed in detail. Assuming that the fluid does not slip at the solid walls, the hydrodynamic flow was described using Stokes’ law. The damping function is then defined as: A( =

)* + ,

(5)

However, when a layer of molecules is adsorbed on surfaces, flow conditions are shifted from a hydrodynamic length, LH, adsorbed on each surface, which does not participate to the flow [17, 22, 24] and the equation (5) becomes:


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A( =

)* + 2 ,-.H

(6)

Thus, the evolution of the inverse of the damping function versus sphere/plane distance during loading (or unloading), compared to equation (6) allows us to define the bulk viscosity of the fluid and the thickness LH on each surface (Figure 4).

Figure 4. Evolution of the inverse of the damping function versus the sphere/plane distance, D, during loading for the dodecane and stearic acid solution. The bulk viscosity of the fluid was extracted from the slope of the curve, 1.5 and 1.8 ± 0.1 mPa.s for dodecane and stearic acid respectively, and the intercept with the abscissa defines the hydrodynamic length LH on each surface (8 ± 0.4 Å and 15.5 ± 2 Å for dodecane and stearic acid respectively).

Experimental results allow us to measure a bulk viscosity respectively of 1.5 ± 0.1 mPa.s and 1.8 ± 0.1 mPa.s for dodecane and stearic acid solution. These values are close together and similar to values observed in literature of 1.44 ± 0.04 mPa.s at 23.5°C [25]. Hydrodynamic lengths respectively of 8 ± 0.4 Å and 15.5 ± 2 Å adsorbed on each surface are measured for dodecane and stearic acid solution. This hydrodynamic length is usually attributed to the presence of an adsorbed layer of infinite viscosity, made of molecules that do not participate to


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the flow. As the surfaces are symmetrical, one can assume that adsorbed layers on each surface are identical. According to the literature [26-27], molecular dynamic experiments about organization of dodecane on mica surfaces show that the molecules adsorb parallel to the surfaces. The estimated width of dodecane is around 4 Å and its length about 14 Å, leading to suppose that a twomolecules thick layer of dodecane molecules aligned on the surface is adsorbed on each solid. In the case of stearic acid solution, the adsorbed layer is thicker. According to the literature on fatty acids adsorption on metallic surfaces, molecules are supposed to adsorb vertically on the surface due to the interaction between the polar group and metallic oxides [14]. Physisorption can occur by weak interactions like hydrogen bonds between hydrogen atoms of carboxylic group and oxide cobalt group. Chemisorption [3] or physisorption of fatty acids may occur on metallic surfaces. However, under the experimental conditions employed in the tests (low humidity, moderate contact conditions with a contact pressure of 30 MPa, squeeze-induced shear rates inferior to 1 s-1, rather smooth surfaces), it is likely that physisorption rather than chemisorption occurs. In this framework, our results suggest that stearic acid molecules are physisorbed vertically to the surface and form monolayers. Moreover, the measured hydrodynamic length, LH, is lower than the estimated length, L0, of the stearic acid molecule expected to 21.4 Å [23], which leads to the hypotheses that molecules are tilted on surfaces or/and form incomplete monolayers. Indeed, authors observed, by liquid cell AFM, that stearic acids form irregular islands with tilted molecules on mica surfaces [14]. The following analysis of the force measurement and of the mechanical behavior will allow us to discuss and conclude on the most likely organization.


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At short distance, the evolution of the normal force Fz during the motion of the sphere toward the plane is presented in Figure 5. For dodecane experiment, the evolution of the normal force shows an attractive phase, followed by a repulsive phase. The attractive phase is adjusted by the theoretical van der Waals force for an interface cobalt/dodecane/cobalt with a Hamaker constant, H, of 1.5x10-19 J according to the equation (7):

F123 = −

5

(7)

) ,

where the theoretical Hamaker constant, H, can be calculated from (8) [28] for dodecane assuming that the thickness of the cobalt layer (here 40 nm) is large compared to the separation distance: 6 = 7 = 7 + 7 − 2. 7 ≈

(8)

with A22 the Hamaker constant of the cobalt surfaces equal to 3x10-19 J [29] and A33 the Hamaker constant of dodecane, equal to 5x10-20 J according to [28]. Equation (8) gives a theoretical value of Co/dodecane/Co = 1x10-19 J. This value is also consistent with another value at 2x10-19 J measured in the literature [22]. For stearic acid solution, no attractive force was observed, meaning that stearic acid molecules adsorbed on surfaces screen adhesion between cobalt surfaces. When surfaces come closer, a repulsive force appears from a length called 2L. The latter is 31 Å and 66 Å for dodecane and stearic acid solution respectively. The gradient of this repulsive force depends on the liquid confined between the surfaces. At large distance, it results from the combination of steric effects mainly from entropic origin, related to the different configurations


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taken by the molecules at the vicinity of the surfaces. At short distance, it results from the elastic deformation of the interface, related to the mechanical properties of the confined layers and solids. In the case of stearic acid, this distance of first repulsion, 2L, is three times the length of the molecule. Then, from a certain distance called 2LC, normal force increases at almost constant distance D. This thickness characterizes the confined layer. The inward and outward force measurement follow the same curve, indicating a confined elastic wall. This wall is characterized by a thickness, 2Lc, of 24 Å for dodecane solution and 38 Å for stearic acid solution. These characteristic lengths are reported in Table 2.

Figure 5. Evolution of the normal force as a function of the separation distance for dodecane


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and stearic acid solution and zoom of the onset of normal force variation. The characteristic length, 2Lc and 2L, are indicated as well as the theoretical van der Waals force in continuous line for the attractive part of the normal force measured for dodecane with an Hamaker constant of 1.5x10-19 J.

Using dynamic measurements in the tangential direction, the stiffness, KX, of the adsorbed layer was obtained as a function of the confinement. The tangential stiffness, KX, increases for short separation distances, close to 2LC, to reach few thousands of N/m. When the separation distance reaches 2LC, the stiffness stabilizes at mean values of 120 000 N/m for dodecane and 43 000 N/m for stearic acid respectively. These measurements seem to indicate that at 2LC, the tangential stiffness, KX, is the signature of the interfacial elasticity. The shear elastic modulus, G, of the confined layer was then calculated assuming two hypotheses: • the confined layer is elastic and much less stiff than the substrate, • the oscillation amplitude was small enough to prevent slipping. If the interface is homogeneously sheared through its thickness, D, the elastic shear stress � induced by a displacement ∆x, is given by the Hooke’s law as: ?=@

AB

(9)

This stress τ can also be written as:

?=

CD E

=

FD AB E

(10)


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Comparing equations (9) and (10) leads to

@=

FD

(11)

E

where S is the contact area. For a normal force of 1 mN and a separation distance D = 2LC, the shear elastic modulus deduced from (11) is 20.0 ± 5.0 MPa for pure dodecane and 6.3 ± 1.2 MPa for stearic acid solution. Another model, developed by Gacoin et al. [30], allows one to estimate the tangential stiffness of a thin layer taking into account the elasticity of the substrates. The values obtained for G are 23.3 ± 6.9 MPa for pure dodecane and 6.6 ± 1.3 MPa for stearic acid solution, in good agreement with the previously calculated values. These values of shear elastic modulus G are of the same order of magnitude as the ones found in [31] for a thicker antioxidant adsorbed layer of 40 Å on each surface using a complete mechanical modeling of the confined interface that takes into account the compressibility of the layer and the deformation of the substrate.

Table 2. Summary of the characteristic lengths for stearic, oleic (after 5h) and elaidic acids. LH corresponds to the thickness of the immobile layer, L is the first repulsion length and LC is the thickness of the confined layer on each surface. Values of shear elastic modulus, G, are also indicated. Pure dodecane

Stearic Acid solution

Oleic Acid solution

Elaidic Acid solution

LH (Å)

8 ± 0.4

15.5 ± 2

13 ± 1

14.5 ± 1

L (Å)

15.5 ± 0.5

33 ± 2

22.5 ± 1.5

20.5 ± 1.5

Experiment


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Lc (at 1 mN) (Å)

12 ± 1

16 ± 1

16 ± 1

15.5 ± 1

LH/L0

Not relevant

0.7

0.7

0.7

L/L0

Not relevant

1.5

1.2

1.0

LC/L0 mN)

(at

1

Not relevant

0.9

0.8

0.7

LH/LC mN)

(at

1

0.6

0.8

0.8

0.9

20 ± 5.0

6.3 ± 1.2

1.6 ± 0.6

0.4 ± 0.3

G (at 1 mN) (MPa)

Effect of molecular architecture: the unsaturation The effect of molecular architecture was studied by changing the nature of the additive in dodecane solution. The other fatty acids differ from the stearic acid by the existence of one unsaturation, a double bond either in CIS configuration (oleic acid) or TRANS configuration (elaidic acid). The double bond in CIS configuration results in a bent shape for the alkyl chain of oleic acid molecule (see Figure 1) compared to stearic acid and elaidic acid, which alkyl chains are both straight. The viscosity of the three fatty acid solutions, calculated from the damping function, was close to that of dodecane, confirming that the concentration was low enough to prevent from micelle formation. However, different adsorption kinetics were observed, depending on the molecule. Results for stearic acid and elaidic acid showed reproducible characteristic lengths for every confinement experiment performed three hours after drop deposition (and beyond). For oleic acid, a time effect was clearly demonstrated: three hours after drop deposition, the characteristic lengths were still thin and similar to values obtained for the dodecane solution (Figure 6). From


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five hours after drop deposition and subsequent confinements, results showed reproducible and thicker characteristic lengths close to measurements obtained for other fatty acid solutions. This indicates that the presence of the CIS-double bond slows down the adsorption kinetics. Figure 6 illustrates this time effect on adsorption via the evolution of the normal force vs the separation distance.

Figure 6. Evolution of the normal force as a function of the separation distance for oleic acid solution. The characteristic length of the repulsive wall formed by the adsorbed layers, 2LC increases with the adsorption time and stabilizes 5 hours after droplet deposition.

In addition, with oleic acid and elaidic acid, an attractive force was detected at short separation distance, consistent with an Hamaker constant of 1.4x10-19 J, confirming the van der Waals origin of this attraction. The distance of first repulsion, 2L, corresponds to twice the estimated length of the unsaturated molecules. The characteristic lengths measured for all the fatty acids molecules are summarized in Table 2.


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The stabilized values of tangential stiffness, KX, at 1 mN, are respectively 11 500 N/m and 3 400 N/m for oleic acid and elaidic acid. Values of shear elastic modulus, of respectively 1.6 ± 0.6 and 0.4 ± 0.3 MPa are also reported in Table 2.


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DISCUSSION For large separation distances, the three fatty acid solutions behave as bulk viscous fluids. At the vicinity of the surfaces, a structuration occurs and fatty acids adsorb onto the surface, modifying the flow condition at the wall. Values of hydrodynamic length, LH, showed that the thickness of the hydrodynamic layer is similar for the three molecules. However, it is lower than the estimated length of the molecule [23]. This lead us to propose a priori three organization models: complete monolayers of molecules tilted from an angle of about 50°, incomplete monolayers of more or less tilted molecules, incomplete bilayers of molecules more or less tilted on the surface (see Figure 7).

Figure 7. Schematic of the different hypotheses likely to describe the molecular organization of stearic acid molecules on the surface. These hypotheses were established from the dynamic measurements giving LH < L0. a) complete monolayer of tilted molecules, b) incomplete monolayer and c) less probable distribution of bilayers and incomplete monolayers (with large values of L).


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The first repulsion length, L, and the confined length, LC, are used to characterize the confinement process of fatty acid layers. In the case of stearic acid, the first repulsion length is larger than for oleic and elaidic acids. The confined length is also slightly higher for stearic acid. However, the thickness 2Lc corresponds to two monolayers of fatty acid on each surface, confined under 1 mN, without dodecane at the interface. The shear elastic modulus of the confined film is also higher for the saturated stearic acid than for unsaturated fatty acids.

Results from the literature (see [13-14, 17] for instance) suggest that the molecules are likely to be tilted on the surfaces. However, the angles estimated from 2LC and L0 would give a (too) large value of 45°. Values of LH could then result from a coupling between tilt and partial surface coverage. In order to calculate the coverage ratio, values of LH and LC were compared: according to [17, 24], the coverage ratio is given from LH/LC, independently of the shape and height of the molecules. This model, based on the viscous damping of a fluid that can flow through an heterogeneous immobile layer, during squeeze, allowed us to estimate a coverage ratio of 82% for the stearic and oleic acids and 94% for elaidic acid. Kipling and Wright [32] used BET technique to measure the area occupied by a stearic acid 2

molecule, at 20.5x10-20 m for a perpendicularly adsorbed molecule on alumina. Harkins and Gans [33] measured an apparent diameter of 20 Å for oleic acid from benzene solution on TiO2. A molecular model of Prentice-Hall was also used [34] to estimate the iron oxide surface area 2

covered by fatty acids: order of magnitude of 20x10-20 m was obtained for stearic acid although two values were calculated for oleic acid, according to the configuration taken by two adjacent


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2

bent molecules, either 25x10-20 m if the molecules are laterally close enough to interact, or 2

43x10-20 m if the molecules do not interact laterally. For elaidic acid, one may expect a similar area as the one occupied by stearic acid. In our case, the coverage ratio can be converted in terms of surface occupied by one molecule, 2

2

giving 25x10-20 m for the stearic acid for a coverage ratio of 82%, 22x10-20 m for the elaidic 2

acid for a 94% coverage ratio and between 30x10-20 and 52x10-20 m for oleic acid for a 82% coverage ratio. These observations confirm the lower coverage ratio of oleic acid. The steric effect may also explain the slower adsorption kinetics. If this argumentation explains well the difference observed in coverage ratio between oleic and elaidic acids, it does not justify the similar values obtained for stearic and oleic acids. The model that we used to define the coverage ratio is based on the hypothesis of rigid blocks covering the surface: in our case, it is likely that the islands of molecules do not form rigid blocks of infinite viscosity. The existence or absence of double bond in the aliphatic chain may introduce a bias in the measurement of the hydrodynamic length: for rigid molecules, such as oleic and elaidic acids, the islands are likely to exhibit an infinite rigidity; for more compliant molecules, such as stearic acids, the block viscosity may be lower leading to underestimated values of LH and of the coverage ratio.

During confinement, significant differences were observed as a result of the unsaturation: L and Lc were smaller for unsaturated molecules. Two hypotheses can be discussed. The first hypothesis consists in the formation of incomplete bilayers on the surfaces. The existence of a diffuse secondary layer was predicted for palmitic acid [13] and it would explain


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the repulsion at large distance and the expulsion, under confinement, of the molecules of the secondary layer. When totally confined, the interface would result from the interaction of two monolayers. However, several studies do not confirm the existence of this secondary layer [1415]. In addition, the reversibility that we measured during loading and unloading is not in favor of this hypothesis. The second hypothesis consists in the formation of different monolayers on the surface, depending on the saturation/unsaturation, resulting in a different structuration of the solvent (here dodecane) at their vicinity. According to Koshima [15], stearic acid molecules orientate more perpendicular and more closely-packed on the surfaces than oleic acids. One may also expect elaidic acid to form more disordered layers due to the presence of the double bond. These organization models are also in agreement with the melting temperature of the three fatty acids, respectively 68.8°C, 13.4°C and 43°C for the stearic, oleic and elaidic acids. This indicates that stearic acid films are the most ordered and that oleic acid films are the least ordered. Then, stearic acid monolayers could induce a different structuration of dodecane at their surface, slowing down their expulsion during confinement. The difference 2L-2LC is equal to 28 Å for stearic acids, which also corresponds to the distance at which the reciprocal of the damping obtained for dodecane diverges from its linear slope (see Figure 4).

A schematic representation of the effect of confinement on the molecular organization for the stearic acid solution at various separation distances (from 100 Å to 2 LC = 38 Å) is shown in Figure 8. The structuration of dodecane, parallel to the surface, is exhibited at a distance corresponding to 2L = 66 Å.


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Figure 8. Tentative description of the effect of confinement on molecular organization for stearic acid at various separation distances (a) 100 Å, (b) 2 L = 66 Å and (c) 2 LC = 38 Å.

The molecular organization for 1 mN, that is to say separation distance of 2LC, is schematically presented in Figure 9 at the scale 1:1. It shows the organization of stearic acids (a), oleic acids (b) and elaidic acids (c) on nanometrically smooth surfaces using values of L0, an area of 20 Å2 for stearic and elaidic acids, and areas of 43 and 25 Å2 (in an arbitrary ratio of 1) for oleic acid.

Figure 9. Schematic of the molecular organization of (a) stearic acid, (b) oleic acid and (c)


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elaidic acid, at the scale 1:1, for separation distance 2Lc equal to 38, 32 and 31 Å respectively.

The existence of the very small roughness on the surface does not seem to significantly modify the adsorption mechanisms and the molecular organization. However, one may expect that the existence of this nanoroughness promotes the formation of islands when antagonist surfaces are in contact.

Finally, no tangential stiffness was observed until the two antagonist surfaces were very close. This is consistent with the proposed schematic of nearly complete monolayers on each surface. Assuming that the onset of increase of the tangential stiffness correspond to the onset of interpenetration of opposite monolayers, it is possible to estimate an average interpenetration thickness of the alkyl chains during confinement: the increase in KX occurs for distance close to 2LC, resulting in a weak interpenetration, of the order of magnitude of the methyl group. This value of interpenetration distance is similar to the one calculated by [35].

CONCLUSION By using the ATLAS molecular tribometer, the structuration of different fatty acids diluted in low concentration in dodecane was studied under confinement. All these fatty acids were composed of a polar group, carboxylic acid, and a hydrophobic tail, an aliphatic chain. Stearic acid is a saturated fatty acid composed of eighteen carbon atoms whereas oleic acid and elaidic acid are also composed of eighteen carbon atoms but with one unsaturation. The double bond in


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unsaturated fatty acid is located at the same position but in CIS configuration in oleic acid and in TRANS configuration in elaidic acid. This double bond gives oleic acid molecule a bent shape and elaidic acid a linear shape, close to that of stearic acid. Using static and dynamic measurements, we were able to define characteristic lengths for all the considered organizations: -

The hydrodynamic length, LH, is indicative of the existence of an adsorbed layer at the surface and of the flow boundary condition,

-

The length of first repulsion, L, is characteristic of the structuration of the solvent molecules at the vicinity of the ‘wall’, consisting either in substrate for pure dodecane, or adsorbed monolayers for fatty acid solutions,

-

The confined length, LC, corresponds to the elastic wall. It also corresponds to the distance from which the tangential stiffness increases.

From this analysis, we concluded on the role of the unsaturation in the molecular organization and associated mechanical properties of the fatty acid layers. Even if the polar head has a leading role in the interaction between fatty acid and surfaces, we showed that the configuration of the hydrocarbon chain leads to a different steric hindrance that plays a fundamental role, as well in the adsorption process as in the molecular organization of the adsorbed fatty acid layers. We showed and discussed a clear difference in organization and ordering between stearic and elaidic acid layers in one hand, and oleic acid layer in the other hand. This also resulted in significant variation in terms of shear elastic modulus of the films.


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ACKNOWLEDGMENT This work was supported by the Agence Nationale de la Recherche via the project Confluence ANR-13-JS09-0016-01 and by the LABEX Manutech-Sise (ANR-10-LABX-0075) of Université de Lyon, within the program "Investissements d'Avenir" (ANR-11-IDEX-0007) operated by the French National Research Agency (ANR), via the project DysCo.

ABBREVIATIONS 2L, distance of first repulsion; aDMT, contact radius using DMT theory; aHertz, contact radius using Hertz’ theory; AX, viscous damping in the x direction; AZ, viscous damping in the z direction; D, sphere/plane distance taking into account the elastic deformation of the solids; E*, equivalent mean reduced Young modulus; FVdW, van der Waals force; FZ, normal force; G, shear elastic modulus of the confined layer; H, Hamaker constant; KX, elastic stiffness in the x direction; KZ, elastic stiffness in the z direction; L0, estimated molecular length; LC, thickness of the confined elastic layer on each surface; LH, thickness of the immobile layer on each surface; R, radius of the sphere; R*, equivalent mean reduced radius of curvature; S, the contact area; η, bulk viscosity of the fluid.

REFERENCES (1)

Wells, H. M.; Southcombe, J. E.; The Theory and Practice of Lubrication: The “germ” Process. S. of Chemical Industry (Great Britain). Central House, 1920.


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Hardy, W. B.; Doubleday, I.; Boundary Lubrication – The Paraffin Series. Proc. R. Soc. Lond. A 1922, 100, 550-574.

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Bowden, F. P.; Tabor, D.; The Friction and Lubrication of Solids. Oxford at the Clarendon press, 1950.

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Bowden, F. P.; Gregory, J. N.; Tabor, D.; Lubrication of Metal Surfaces by Fatty Acids. Nature 1945, 156, 97–101.

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Lundgren, S. M.; Ruths, M.; Danerlöv, K.; Persson, K.; Effects of unsaturation on film structure and friction of fatty acids in a model base oil. J. Colloid Interface Sci. 2008, 326(2), 530–536.

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Simič, R.; Kalin, M.; Adsorption mechanisms for fatty acids on DLC and steel studied by AFM and tribological experiments. Appl. Surf. Sci. 2013, 283, 460–470.

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Campen, S.; Green, J.; Lamb, G.; Atkinson, D.; Spikes, H.; On the Increase in Boundary Friction with Sliding Speed. Tribol. Lett. 2012, 48(2), 237–248.

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Ewen, J. P.; Echeverri Restrepo, S.; Morgan, N.; Dini, D.; Nonequilibrium molecular dynamics simulations of stearic acid adsorbed on iron surfaces with nanoscale roughness Tribol. Int. 2017, 107, 264–273.

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Levine, O.; Zisman, W. A.; Physical Properties of Monolayers Adsorbed at the Solid–Air Interface. I. Friction and Wettability of Aliphatic Polar Compounds and Effect of Halogenation. J. Phys. Chem. 1957, 61(8), 1068–1077.

(10) Mathieson, R. T.; Electron Microscopy of Oleophobic Monolayers. Nature 1959, 183(4678), 1803–1804.


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(11) Jahanmir, S.; Chain length effects in boundary lubrication. Wear 1985, 102(4), 331–349. (12) Georges, J-M.; Tonck, A.; Mazuyer, D.; Interfacial Friction of wetted monolayers. Wear 1994, 175, 59-62. (13) Campana, M.; Teichert, A.; Clarke, S.; Steitz, R.; Webster, J. R. P.; Zarbakhsh, A.; Surfactant Adsorption at the Metal–Oil Interface. Langmuir 2011, 27(10), 6085–6090. (14) Campen, S.; Green, J. H.; Lamb, G. D.; Spikes, H. A.; In Situ Study of Model Organic Friction Modifiers Using Liquid Cell AFM; Saturated and Mono-unsaturated Carboxylic Acids. Tribol. Lett. 2015, 57(2), 1–20. (15) Koshima, H.; Iyotani, Y.; Peng, Q.; Ye, S.; Study of Friction-Reduction Properties of Fatty Acids and Adsorption Structures of their Langmuir Blodgett Monolayers using Sum-Frequency Generation Spectroscopy and Atomic Force Microscopy. Tribol. Lett. 2016, 64(3), 1–8. (16) Tadokoro, C.; Araya, S.; Okubo, H.; Nakano, K.; Sasaki, S.; Polarization Observations of Adsorption Behavior of Fatty Acids Using Optical Anisotropy of Liquid Crystal. Tribol. Lett. 2016, 64: 30. (17) Mazuyer, D.; Tonck, A.; Cayer-Barrioz, J.; Friction Control at The Molecular Level: From Superlubricity to Stick-Slip. A. Erdemir and J.-M. Martin, Eds. Amsterdam: Elsevier Science B.V., 2007, 397–426. (18) Lundgren, S.; Persson, K.; Mueller, G.; Kronberg, B.; Clarke, J.; Chtaib, M.; Claesson, P. M.; Unsaturated Fatty Acids in Alkane Solution: Adsorption to Steel Surfaces. Langmuir 2007, 23(21), 10598–10602.


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(19) Ruths, M.; Lundgren, S.; Danerlöv, K.; Persson, K.; Friction of Fatty Acids in Nanometer-Sized Contacts of Different Adhesive Strength. Langmuir 2008, 24(4), 1509– 1516. (20) Tonck, A.; Bec, S.; Mazuyer, D.; Georges, J-M.; Lubrecht, A. A.; The École Centrale de Lyon surface force apparatus: An application overview. Proc. Inst. Mech. Eng. Part J J. Eng. Tribol. 1999, 213(5), 353–361. (21) Crespo, A.; Mazuyer, D.; Morgado, N.; Tonck, A.; Georges, J-M.; Cayer-Barrioz, J.; Methodology to Characterize Rheology, Surface Forces and Friction of Confined Liquids at the Molecular Scale Using the ATLAS Apparatus. Tribol. Lett. 2017, 65, 138. (22) Georges, J-M.; Millot, S.; Loubet, J-L.; Tonck, A.; Drainage of thin liquid films between relatively smooth surfaces. J. Chem. Phys. 1993, 98(9), 7345–7360. (23) Askwith, T. C.; Cameron, A.; Crouch, R. F.; Chain Length of Additives in Relation to Lubricants in Thin Film and Boundary Lubrication. Proc. R. Soc. London A Math. Phys. Eng. Sci. 1966, 291(1427), 500–519. (24) Mazuyer, D.; Cayer-Barrioz, J.; Tonck, A.; Jarnias, F.; Friction Dynamics of Confined Weakly Adhering Boundary Layers. Langmuir 2008, 24(8), 3857–3866. (25) Tonck, A.; Georges, J-M.; Loubet, J-L.; Measurements of Intermolecular Forces and the Rheology of Dodecane between Alumina Surfaces. J. Colloid Interface Sci. 1988, 126(1), 150–163. (26) Jabbarzadeh, A.; Harrowell, P.; Tanner, R. I.; Crystal Bridge Formation Marks the Transition to Rigidity in a Thin Lubrication Film. Phys. Rev. Lett. 2006, 96(20), 206102.


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(27) Cui, S. T.; Cummings, P. T.; Cochran, H. D.; Molecular simulation of the transition from liquidlike to solidlike behavior in complex fluids confined to nanoscale gaps. J. Chem. Phys. 2001, 114, 7189-7195. (28) Israelachvili, J. N. ; Intermolecular and Surface Forces. Elsevier 3rd edition, 2011. (29) Georges. E.; Dispersion et forces de surfaces dans les hydrocarbures. PhD Thesis, Ecole Centrale de Lyon (1996). (30) Gacoin, E.; Fretigny, C.; Chateauminois, A.; Perriot, A.; Barthel, E.; Measurement of the mechanical properties of thin films mechanically confined within contacts. Tribol. Lett. 2006, 21(3), 245-252. (31) Trifa. M.; Nanorhéologie du contact sphere/plan avec couche mince interfaciale. PhD Thesis, Ecole Centrale de Lyon (1999). (32) Kipling, J. J.; Wright, E. H. M.; The adsorption of stearic acid from solution by oxide adsorbents. J. Chem. Soc. 1964, 0, 3535–3540. (33) Harkins, W. D.; Gans, D. M.; An adsorption method for the determination of the area of a powder. J. Am. Chem. Soc. 1931, 53(7), 2804–2806. (34) Wheeler, D. H.; Potente, D. Wittcoff, H.; Adsorption of Dimer, Trimer, Stearic, Oleic, Linoleic, Nonanoic and Azelaic Acids on Ferric Oxide. J. Am. Oil Chem. Soc. 1971, 48(3), 125–128. (35) Briscoe, B. J.; Evans, D. C. B.; The shear properties of Langmuir—Blodgett layers. Proc. Roy. Soc. Lond. A. 1982, 380(1779), 389-407.

AUTHOR INFORMATION


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Corresponding Author * Author to whom correspondence should be addressed. Electronic mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally.

Funding Sources This work was supported by the French National Research Agency (ANR) via the project Confluence ANR-13-JS09-0016-01 and by the LABEX MANUTECH-SISE (ANR-10-LABX0075) of Université de Lyon, within the program "Investissements d'Avenir" (ANR-11-IDEX0007) also operated by the ANR.


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Abstract - graphic


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ACS Paragon Plus Environment 1 2

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Normal force, Fz (μN)

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Elastic deformation of the solids 1000

1 2 3 600 4 5 6 200 7 8 9 -200-20 10 11

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Kz and Az.ωz (104 N/m)

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ACS Paragon Plus Environment Distance D (Å)

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1/Az.ωz (10-4 m/N)

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Fz/R (10-3 N/m)

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Distance D (Å)

Theoretical van der Waals force


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Fz/R (10-3 N/m)

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Effect of Unsaturation on the Adsorption and the Mechanical Behavior

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