Normal and Shear Forces between Charged Solid Surfaces Immersed

Apr 10, 2014 - Friction Mechanisms by Carboxylic Acids in Aqueous Lubricants. S. Bernat , S. Armada , N. Espallargas. Tribology Letters 2018 66 (3), ...
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Normal and Shear Forces between Charged Solid Surfaces Immersed in Cationic Surfactant Solution: The Role of the Alkyl Chain Length Gilad Silbert, Nir Kampf, and Jacob Klein* Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel

ABSTRACT: Using a surface force balance (SFB), we measured the boundary friction and the normal forces between mica surfaces immersed in a series of alkyltrimethylammonium chloride (TAC) surfactant solutions well above the critical micelle concentration (CMC). The surfactants that were usedC14TAC, C16TAC, and C18TACvaried by the length of the alkyl chain. The structures of the adsorbed layers on the mica were obtained using AFM imaging and ranged from flat bilayers to rodlike micelles. Despite the difference in alkyl chain, all the surfactant solutions reduce the friction between the two mica surfaces enormously relative to immersion in water, and have similar friction coefficients (μ ≈ 0.001). The pressure at which such lubrication breaks down is higher for the surfactants with longer chain lengths and indicates that an important role of the chain length is to provide a more robust structure of the adsorbed layers which maintains its integrity to higher pressures.

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INTRODUCTION One of the most common ways to alter surface properties is through adsorption of surfactants. Since most surfaces become charged when they are introduced or dispersed in water (by ionizing to some degree or by selectively adsorbing ions), surfactants with a charged (ionized) headgroup, that can bind to the surfaces electrostatically, are often used. Such adsorption may occur in several ways:1 ion pairing between the charged headgroup and a surface charged site, ion exchange of the charged headgroup with an already adsorbed counterion, or the surfactant can be attracted to the surface via London−van der Waals−dispersion forces and to be attracted to another adsorbed molecule through hydrophobic interaction between the tails.1 The hydrophobic interaction energy between the alkyl chains, and thus the stability of monolayers and bilayers, depends strongly on the chains’ length and is known to increase by about 1.5kT for each CH2 group.2,3 The adsorption process has been examined extensively, and different models have been proposed.1,4 The adsorption of alkylammonium halide surfactants onto negatively charged mica surfaces has been investigated by several groups.5−17 Pashley et al.11 used the SFB to measure the forces between adsorbed layers of C16TAB on mica in micellar solutions above the CMC. They assumed bilayer adsorption and estimated the surface charge density from the force profiles. According to their estimation, 78% of the adsorbed surfactants had bound counterions. This value is similar to the degree of counterions binding to C16TAB micelles in solution.5,18 Later it was found that the assumption of bilayer adsorption is not necessarily true.7,10,14,15,17 Surfactants with various chain lengths (C14TAB10,17 and C12TAB7,17) were found to form flattened cylindrical aggregates on mica. C16TAB,7 on the other © 2014 American Chemical Society

hand, when adsorbed from solution at twice the CMC, initially forms rodlike micelles that later (17−25 h) transform into a flat sheet.14 Adding salt, such as KBr or HBr, transforms the flat sheet to cylinders and then to defective cylinders (i.e., shorter cylinders or with multiple changes in the direction of their long axis) with increasing salt concentration.14 The reduction of aggregate curvature is attributed to two effects:14 the cations of the salt can occupy the negative sites on the surface, competing with the surfactant monomers, while the anions bind to the monomers allowing them to release from the surface.14 It was found that larger and more polarizable counteranions are more effective in inducing sphere to cylinder transformation of the micelles, which is presumably due to stronger interaction of the anions with the micelle surface.16 Molecular adsorption in the form of aggregates is not specific to the mica surface and has been observed also on different surfaces; e.g., C14TAB and C16TAB form spherical aggregates on silica,10,16 half-cylindrical aggregates on hydrophobic graphite (C14TAB),10 and cylindrical micelles on gold surfaces (C14TAB).19 Surfactants with longer chain lengths (i.e., C18TAB and C20TAB) as well as double-chain surfactants ((C12)2DAB) were found to form flat bilayers.7,10 Fast flat bilayer formation was observed also for C16TA+ with Cl− as counterion (C16TAC),20 even though it was shown, using a high-speed AFM, that it initially adsorbs as wormlike micelles.15 Shorter chains with chloride counterions were found to adsorb on the mica surfaces as micelles (C14TAC as rodlike micelles and C12TAC as spheres).17 Surfactant films can reduce the friction between two solid surfaces in air and aqueous media.21−27 In some cases it was Received: August 27, 2013 Published: April 10, 2014 5097

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tips with a spring constant of 0.1 N/m (NP, Veeco Probes Nanofabrication Center). The chip and the chip holder were irradiated in a UV-ozone device prior to use. The sample was prepared by immersing freshly cleaved mica glued on a clean glass Petri dish or glass slide into the surfactant solution. Surface Force Measurements. Interaction forces between the mica surfaces across pure water and surfactant solution were manually measured using a surface force balance (SFB) according to procedures described earlier.32 Two back-side silvered mica sheets from the same original facet, of area ∼1 × 1 cm and of thickness 1−3 μm, were mounted onto hemicylindrical lenses using EPON 1004 (Shell) resin. In the SFB an intense white light is projected through the lenses, and due to the highly reflected silver coating an optical cavity is formed. From the interference pattern the absolute separation between the mica surfaces can be obtained. Calibration procedure was performed prior to each experiment in order to obtain the contact position of the bare surfaces and to test their cleanness. In this step the zero separation between the uncoated mica surfaces in air and then in pure water was measured. Surfaces were considered clean if they showed vdW attraction in air or water with no detectable lateral forces, while the top surface slid laterally past the lower one, until the surfaces jumped into adhesive contact. Normal force profiles (force, F, between the surfaces as a function of surface separation, D) were recorded at several contact points. All force values obtained were normalized by R, the mean radius of curvature of the lenses to give F/R. This makes it possible to relate the force, F(D), between two curved surfaces to the interaction energy per unit area, w(D), between flat parallel surfaces at distance D apart obeying the same force−distance law, using the Derjaguin approximation when R ≫ D: F(D)/R = 2πw(D). Friction forces are measured by moving the upper surface laterally parallel to the lower one. The upper lens is mounted on a rigid piezoelectric tube (PZT), which is divided into four sectors; by applying opposite potentials to opposite sectors, the upper surface is moved laterally (applied lateral motion: Δx0). Shear forces are transmitted to lateral springs (Ks = 300 N/m) whose bending Δx is measured via an air gap capacitor. The shear force Fs = KsΔx can then be measured.

found that the friction between monolayer-coated surfaces depends on the adhesion hysteresis between the surfaces under loading and unloading cycles.24,28 Larger friction forces were associated with larger adhesion hysteresis, and a mechanism based on chain interdigitation was proposed where the shear plane is located between the surfactant hydrophobic tails.24 In other cases of charged surfactant monolayers in an aqueous medium the adhesion hysteresis model was not sufficient to explain the reduction in the friction forces.22 It was suggested that, while in air both the shear and adhesion planes are located at the interface between the surfactants tails, in water the charged surfactant head groups become hydrated and the shear plane is shifted to the interface between the head groups and the surface.22 The reduction of the friction in water is therefore due to hydration lubrication, a mechanism that was observed in different systems that involve charged or zwitterionic groups in water,29 and arises from the fluid, yet tightly bound, hydration layers.30 A large reduction in the friction between two negatively charged surfaces was observed when the surfaces were immersed in bulk aqueous cationic surfactant solutions above the CMC.23,25 In the case of two mica surfaces immersed in a trimeric surfactant solution the corresponding friction coefficient, measured using the surface force apparatus (SFA), was below the detection limit of the experimental setup and was estimated to be equal to or less than 0.004. The authors attribute this good lubrication to the hydration lubrication mechanism.23,30 At a certain load the adsorbed layer, assumed to be adsorbed as a flat bilayer, breaks and the surfaces reach adhesive contact corresponding to a separation of two monolayers.23 Similar squeezing to monolayer−monolayer contact was observed also in force measurement between a bead and a flat surface across bilayer−bilayer contact using AFM.25 In this study we measured the normal forces and the friction force between two mica surfaces immersed in surfactant solutions above the CMC using a surface force balance. We compared the results of three different single chain ammonium surfactants varying in chain length (14, 16, and 18 carbons in the alkyl chain). The structure of the adsorbed aggregates on the mica surface and their stability over time in water were examined using AFM.

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RESULTS C14TAC: Both the force measurements and the AFM imaging under surfactant solution were carried out at a concentration of 22.5 mM, which is 5 times the CMC (CMC = 4.5 mM2). AFM Imaging. The structure of the surfactants at the solid−liquid interface was determined using AFM. On the mica surface, the surfactant organizes in the form of rodlike micelles aligned with the long axes parallel having a center-to-center distance of about 5 nm with only small variation in the direction of the axis over the scale of the image (Figures 1a,c). This value is larger than the maximal diameter possible for a micelle, which is twice the fully extended molecular length of the monomer, l. For a monomer having a chain with n carbon atoms and trimethylammonium headgroup l ≈ (0.3 + 0.15 + 0.1265n) nm, which is composed of the headgroup thickness, the vdW radius of the terminal methyl group, and the length of a fully extended chain.3 Thus, the diameter of the C14TAC micelle should be less than 4.5 nm. According to the normal force profiles, this diameter is about 3.5 nm, which is half the distance of the onset of the “hard wall” repulsion (see below). The difference between the center-to-center distance and the micelle diameter indicates that the micelles are slightly spaced rather than closely packed. These structures on the surface were found to be stable for more than 10 h at room temperature. Heating the surface to 35 °C seems to promote the fusion of the micelles to form a flat layer at several areas (Figures 1b,d). Normal Force Measurements. Force measurements were taken at room temperature during the first ∼7 h from adding

MATERIALS AND METHODS

Materials. Mica was highest grade (V1) muscovite mica from S&J Trading Inc. For SFB measurements the mica preparation was performed as described previously,31 and for AFM measurements the mica sheets were cut into ∼1 cm2 pieces and glued, using a doublesided tape or Epoxy glue, onto clean Petri dish or glass slide. Octadecyl(stearyl)trimethylammonium chloride (C18TAC) was purchased from Tokyo Chemical Industries, catalogue number S0087, batch analyzed as 99.3% purity. Hexadecyltrimethyammonium chloride (C16TAC) and tetradecyltrimethyammonium chloride (C14TAC) were purchased from Sigma-Aldrich, purity >98%. Chloride was chosen as the counterion for these surfactants due its high solubility which leads to lower Kraft temperature and allows working with micelle solutions at room temperature for all the surfactants. All surfactant solutions were freshly made soon before each experiment at concentrations corresponding to 5 or 10 times the CMC (critical micellar concentration). Pure water was taken from a Barnstead Nanopure water purification system, with total organic carbon (TOC) monitoring, resistivity of 18.1−18.2 MΩ·cm−1 and applied shear force).

Figure 7. Height AFM images of the mica surface immersed in aqueous 3.5 mM octadecyl(stearoyl)trimethylammonium chloride (C18TACl) solution showing a flat layer, implying the formation of a surfactant bilayer.

in C16TAC solution at the hard wall separations and loads. The friction coefficient measured is μ ≈ 0.003 and holds up to the critical pressure (20−30 atm), where the surfactant layer is squeezed into half of its original thickness. Above the critical pressure, the friction increases steeply and the surfaces become rigidly coupled for all shear forces, which were applied; i.e., the maximal static friction force is above 150 μN. C18TAC: All measurements for this surfactant were done in surfactant solution at concentration 3.5−3.7 mM, which is about 10 times the CMC (CMC = 0.34 mM2). AFM Imaging. Figure 7 is a representative AFM image of a mica surface exposed to surfactant solution. Unlike the previous surfactants, which adsorbed as rod micelles, the C18TAC form a flat layer. This suggests the formation of a surfactant bilayer on the mica surface. Normal Force Measurements. The normal force profiles between mica surfaces immersed in C18TAC solution are presented in Figure 8. A long-range repulsion (due to counterions osmotic pressure) is observed before a “hard wall” repulsion at a separation of 6 ± 1 nm. The forces were reproducible on reapproach after separation at the same contact area and reversible; i.e., the out-profile taken by separating the surfaces is similar to the in-profile. The fit of the osmotic repulsion with the nonlinear PB equation gives a Debye length of λD = 13.7 nm (corresponds to a 1:1 ion concentration of 0.5 mM), surface potential of ψ0 = 110 mV, and surface charge density of σ = e/15 nm2. The nonadhesive−hard wall separation remains unchanged, and no collapse was observed even at pressures greater than 80 atm. Shear Force Measurements. The friction coefficient for this surfactant was found to be as low as μ ≈ 0.001 up to pressures that range from 30 atm to more than 80 atm (Figure 9). Above these pressures the friction force increases more sharply, and the friction coefficient is μ ≈ 0.01 up to a point, where the surfaces become rigidly coupled at all the shear

Figure 8. Normalized force vs distance profiles Fn(D)/R between two muscovite mica surfaces immersed in aqueous 3.5 mM octadecyltrimethylammonium chloride (C18TACl) solution. Under these conditions the surfactant adsorbed as a bilayer on the mica surface (Figure 7). The forces were fitted to the solution of the by nonlinear Poisson−Boltzmann interaction together with the vdW interaction under limits of constant potential and constant charge boundary conditions (broken black and gray curves, respectively). The corresponding parameters are surface potential ψ0 = 110 mV, Debye length λD = 13.7 nm, and surface charge density σ = e/15 nm.2 Hard wall repulsion was observed at a separation of about 6 nm, which corresponds to the contact between two bilayers. No collapse was observed for this surfactant. The symbols ⊗ represent an out-profile indicating that the force profiles are reversible.

forces which were applied (which correspond to a static shear force up to around 150 μN). Here, no collapse or change in the surface separation was observed at the point where the friction coefficient changes or when the surfaces become rigidly coupled. 5101

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(so that e.g. the micelle concentrations in the different solutions would not be too different). The concentrations we used are thus to some extent a compromise: we worked at concentrations well above the CMC (so as not to be in a transition regime, as for example for the C16TAC which at 2 × CMC forms bilayers rather than surface micelles) but made some attempt to mitigate the effect of the largest CMC (that of C14TAC). We did this by working at a lower concentration relative to the C14 CMC (i.e., 5 × CMC) than for the other two surfactants (where we worked at 10 × CMC). The effect of this was to reduce, if only by a limited extent, the absolute concentration differences between the three surfactants. As a control, we determined the surface structure of the C14TAC from a bulk concentration of 10 × CMC (as for the other two surfactants) and find that it is essentially identical to that from a 5 × CMC as in Figure 1. It is also possible that changes in the surface structure occur on approach of the surfaces via the “proximal adsorption mechanism” discussed by Ducker et al.,35 though we have no way of observing this. The consistency of the DLVO fit to the Fn(D)/R profiles from large values of D down to small separations suggests such changes, if any, may be small and that the structure revealed by the AFM micrographs persists until the surfactant layers are in actual contact. The normal force profiles between mica surfaces immersed in the surfactant solutions showed long-range repulsion for all the surfactants. This is due to overcompensation of the negative charges on the mica by the positively charged surfactant adsorbed as micelles (C16TAC and C14TAC) or as bilayers (C18TAC). Fitting the profiles to the nonlinear Poissson− Boltzman interaction gives the Debye length, surface charge density, and surface potential for theses layers. For all surfactants investigated, the Debye length corresponded to the CMC or a slightly higher. This is expected since the concentration of the fully ionized free monomers, which can be regarded as free ions in the solution, is pinned to the CMC. In addition to the ionized free monomers, some portion of the monomers that forms micelles is also ionized (e.g., for C16TAC about 35% of the monomers in the micelles are ionized18) and thus releases an additional amount of counterions to the bulk solution. The difference of the CMC between the different surfactants used in this work leads to a variation in the Debye lengths and hence also in the screening of the electrostatic interaction. This results in differences in the charging of the surfactant’s layers. All the surfactant layers are highly charged: the C14TAC, which has the highest CMC, creates layers with an effective charge density of one charge every 2.5 nm2, while the C16TAC and C18TAC have effective surface charge densities of σ = e/3 nm2 and e/15 nm2, respectively. The surface interaction between all surfactant layers does not show a vdW attraction at close separation, as expected from the DLVO theory,36 but rather a strong hydration repulsion leading to a “hard wall” repulsion at separations which correspond to two bilayers (one on each surface) or two layers of close-packed micelles (which should have a similar thickness to the bilayer). The hydration repulsion probably arises from the hydration layers encapsulating the charged head groups which prevent contact of the surfaces. These hydration layers are believed to underlie the good lubrication, which we will discuss below. Other contributions to the repulsion might come from fluctuations of the hydrocarbon chains, i.e., undulation and peristaltic forces.37 For the surfactants that form micelle layers on the surface (i.e., C16TAC and C14TAC), the hard wall repulsion

Figure 9. Shear forces as a function of normal forces between two muscovite mica surfaces immersed in aqueous octadecyltrimethylammonium chloride (C18TACl) solution taken at the hard wall repulsion region. The friction coefficient, which is the slope of the graph, is μ ≈ 0.001 up to a certain pressure where it increases to about μ ≈ 0.01 and then the surfaces become rigidly coupled at all the shear forces which were applied. The critical pressures Pc at which the friction coefficient increased varied between 30 atm up to above 80 atm.

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DISCUSSION The structure of the adsorbed surfactant layers was examined using AFM imaging in surfactant solutions which are well above the CMC. The C18TAC was found to form a flat bilayer on the mica surface (Figure 7); a similar structure was observed in the past for an analogue of this surfactant, the C18TAB, which has bromide as counterion.7 In contrast, the C14TAC and C16TAC surfactants were found to form rodlike micelles on the surface (Figures 1 and 4). While a similar structure of the C14TAC surfactant’s layer on mica was reported in the past,17 this is not the case for the C16TAC. In earlier studies, C16TAC was found to form a bilayer on top of the mica surface20 or micelles which rapidly (few minutes) transform into bilayer.15 This discrepancy might arise from the different concentration of the solutions examined; the concentration examined in the current work corresponds to 10 × CMC, while in the previous work the concentration corresponded to twice the CMC. Indeed, in initial scans using the AFM, we did not find micelles on the mica surface in the presence of 2 × CMC. The issue of how different solution concentrations would affect both the surface structure of the different surfactants and the normal, and particularly the shear interactions, is of interest, and a systematic study of this, though we believe it to be outside the scope of the present work, would complement its results and conclusions. An important issue in making such comparisons between the different surfactants is that their CMCs, to which the effective free ion concentrations are approximately pinned, differ significantly. This is expected to affect the Debye screening length and the double-layer normal interaction as well as the effective surface charge, and especially the repulsion between head groups in the surfactant layers and thus their extent of hydration, and through that the friction forces via the hydration lubrication mechanism. Indeed, in the present investigation of the effect of tails lengths on the interactions, it is not straightforward to determine the right concentration for comparison between the surfactants. On the one hand, the CMC provides a natural measure of concentration so one might think to work at a fixed multiple of the CMC. On the other hand, because the CMCs are rather different, there is a case to be made for trying to work at absolute surfactant concentrations that did not differ too much 5102

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origin of this lubrication and discrepancy between the two kinds of surfactants is still not clear but might be explained by the difference in the interaction energy between the alkyl tails. For both surfactants it seems that after the collapse of the rodlike micelles there is, initially, a disordered material between the surfaces, rather than a contact between two monolayers, as discussed above. This disordered structure leads to an increase in the static friction force. Yet, for the C14TAC the comparatively weak interaction energy between the tails allows a relative lateral motion between the two surfaces driven by the shear force applied. This relative movement might, quite rapidly, induce rearrangement of the disordered structure to a contact between partial monolayers and compressed bilayers. However, in the case of the C16TAC layers the interaction energy between the tails is higher; this contributes to the ability of the layers to bear higher loads before the collapse but also creates higher static friction force of the disordered material. Under such static force no relative motion is possible as a result of the shear forces applied (the surfaces stay “rigidly” coupled on shear), and thus the relaxation of the disordered layer to a more lubricating structure is not possible in our study. The friction coefficients for all surfactants are of the same order of magnitude, initially μ ≈ ca. (1−7) × 10−3, prior to the transition (C14) or deterioration at high pressures (C16, C18). Broadly, these low values are attributed to the hydration lubrication mechanism, though the C18TAC layers are observed to give a lower value of μ initially. We believe this arises from the difference between smooth bilayers sliding past each other (C18), relative to the other surfactant layers (C14, C16) where additional dissipation may result due to viscoelastic effects as the rodlike surface micellar structures are forced past each other.

succeeds in resisting the pressures applied up to a certain critical pressure. The critical pressure is higher for the C16TAC than for the C14TAC (20−30 and 10−15 atm, respectively), which indicates a role of the alkyl chain length in this pressure resistance. Above the critical pressure the layers are suddenly squeezed to half of their original thickness, which fits the thickness of a contact between two monolayers. At this new contact position the surfaces become adhesive with similar surface tension of γ = 6− 7 mN/m. This value is much lower than the value expected between two hydrophobic monolayers in contact (∼50 mN/ m2); thus, the squeezing is definitely not that of a contact between perfect monolayers and many of the monomers are probably flipped. The flipped monomers might come from the outer layer of the bilayer, which squeezed into the lower one. This behavior seems reasonable since the contact diameter is too large (∼30 μm) to allow rapid diffusion of all excess material out to the bulk under strong pressure. (Some material probably diffuses out due to the gradient of the pressure from the center of the contact area toward its edges.) The moment of collapse is also accompanied by a striking change in the friction force and in the lubrication. In the case of the C18TAC, where a bilayer forms on the surface on initial adsorption, no change in the separation was observed at the “hard wall” even under pressures of above 80 atm, at which the efficient lubrication failed. All the layers exhibited very efficient lubrication at the hard wall repulsion region. The friction coefficient is in the same order of magnitude for all the surfactants (μ ≈ 10−3). The difference between the surfactants lies in the critical pressure at which this efficient lubrication fails. We attribute the very low friction coefficient to the hydration lubrication mechanism, which was observed in our previous studies on different charged systems29 such as trapped ions, polyzwitterionic brushes, polyelectrolyte brushes, charged surfactant monolayers, and charged and zwitterionic liposomes.29 The hydration lubrication arises from the hydration layers that enclose the charges; in this case these are the charged headgroups of the monomers, which create a highly positively charged layer as discussed before. These hydration layers lower the free energy of the charges and thus strongly resist being removed; this resistance can support large pressures. At the same time, the water molecules in the hydration layers can rapidly exchange with nonhydration water molecules, which renders them fluid and leads to low friction.29,30,38 The lubrication holds up to high pressures, which correlates with the surfactants’ alkyl chain length. The C18TAC, which forms a bilayer structure on the surface, seems to be the most stable under pressure. The gradual loss of efficient lubrication for this surfactant is not accompanied by collapse of the layer. The relative stability of this layer under pressure might come from stronger interaction between the alkyl tails (less curvature than in micelles and thus less tension) and from better distribution of the loads (less local pressures) on the smooth bilayer compared to the local pressures on rod micelles. The other two surfactants, the C16TAC and C14TAC, form micelles layers on the surface, which sustain less pressure than the bilayer. C16TAC can sustain higher pressures than C14TAC probably due to stronger interaction between the monomers. Above the critical pressure the C14TAC layers collapse to a new equilibrium and to adhesive contact, in which these layers behave again as good lubricants. This was not observed for the C16TAC. The friction coefficient is similar and even lower than before squeezing. The

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CONCLUSION The normal and friction forces between mica surfaces immersed in a surfactant solution having different chain lengths were measured and are interpreted in light of their structure on the surface. The C18TAC surfactant was found to form a bilayer on the mica surface, and the C16TAC and C14TAC surfactants were found to form rod micelles layers on the mica surface, which were stable throughout the time in which the force measurements were taken. The charging of the surfaces was estimated from the force profiles and friction coefficients between the surfaces were measured. All layers exhibit very good lubrication with friction coefficients in the order of μ ≈ 10−3 up to some critical pressure Pc. This good lubrication is correlated with the stability of the layer structure under pressure. The bilayer formed by C18TAC is the most stable (highest Pc), while the micelles formed by the C16TAC are more stable and lubricate well to higher Pc than those formed by C14TAC.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the European Research Council (Advanced Grant HydrationLube) and the Israel Science Foundation for support of this work. 5103

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(23) Drummond, C.; Israelachvili, J.; Richetti, P. Friction between two weakly adhering boundary lubricated surfaces in water. Phys. Rev. E 2003, 67 (6), 066110. (24) Yoshizawa, H.; Chen, Y. L.; Israelachvili, J. Fundamental mechanisms of interfacial friction. 1. Relation between adhesion and friction. J. Phys. Chem. 1993, 97 (16), 4128−4140. (25) Vakarelski, I. U.; Brown, S. C.; Rabinovich, Y. I.; Moudgil, B. M. Lateral force microscopy investigation of surfactant-mediated lubrication from aqueous solution. Langmuir 2004, 20 (5), 1724− 1731. (26) Boschkova, K.; Kronberg, B.; Stålgren, J. J. R.; Persson, K.; Salagean, M. R. Lubrication in aqueous solutions using cationic surfactants - a study of static and dynamic forces. Langmuir 2002, 18 (5), 1680−1687. (27) Richards, S. C.; Roberts, A. D. Boundary lubrication of rubber by aqueous surfactant. J. Phys. D: Appl. Phys. 1992, 25 (1A), A76. (28) Yamada, S.; Israelachvili, J. Friction and adhesion hysteresis of fluorocarbon surfactant monolayer-coated surfaces measured with the surface forces apparatus. J. Phys. Chem. B 1998, 102 (1), 234−244. (29) Gaisinskaya, A.; Ma, L. R.; Silbert, G.; Sorkin, R.; Tairy, O.; Goldberg, R.; Kampf, N.; Klein, J. Hydration lubrication: exploring a new paradigm. Faraday Discuss. 2012, 156, 217−233. (30) Raviv, U.; Klein, J. Fluidity of bound hydration layers. Science 2002, 297 (5586), 1540−1543. (31) Perkin, S.; Chai, L.; Kampf, N.; Raviv, U.; Briscoe, W.; Dunlop, I.; Titmuss, S.; Seo, M.; Kumacheva, E.; Klein, J. Forces between mica surfaces, prepared in different ways, across aqueous and nonaqueous liquids confined to molecularly thin films. Langmuir 2006, 22 (14), 6142−6152. (32) Klein, J.; Kumacheva, E. Simple liquids confined to molecularly thin layers. I. Confinement-induced liquid-to-solid phase transitions. J. Chem. Phys. 1998, 108 (16), 6996−7009. (33) Andelman, D. Introduction to electrostatics in soft and biological matter. In Handbook of Physics of Biological Systems; Lipowsky, R. S. E., Ed.; Elsevier Science: Amsterdam, 1995; Vol. 1. (34) Johnson, K. L.; Kendall, K.; Roberts, A. D. Surface energy and contact of elastic solids. Proc. R. Soc. London, A 1971, 324 (1558), 301. (35) Subramanian, V.; Ducker, W. Proximal adsorption of cationic surfactant on silica at equilibrium. J. Phys. Chem. B 2001, 105 (7), 1389−1402. (36) Derjaguin, B. V.; Churaev, N. V.; Muller, V. M. Surface Forces; Plenum Publishing Corporation: New York, 1987. (37) Israelachvili, J. N.; Wennerstroem, H. Entropic forces between amphiphilic surfaces in liquids. J. Phys. Chem. 1992, 96 (2), 520−531. (38) Klein, J.; Raviv, U.; Perkin, S.; Kampf, N.; Chai, L.; Giasson, S. Fluidity of water and of hydrated ions confined between solid surfaces to molecularly thin films. J. Phys.: Condens. Matter 2004, 16 (45), S5437−S5448.

This research was made possible by the historic generosity of the Harold Perlman family.

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dx.doi.org/10.1021/la501315v | Langmuir 2014, 30, 5097−5104