Restructuring of Hydrophobic Surfaces Created by Surfactant

Sep 1, 2011 - Restructuring of Hydrophobic Surfaces Created by Surfactant. Adsorption to Mica Surfaces. Jhuma Das,. †. Changsun Eun,. †. Susan Per...
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LETTER pubs.acs.org/Langmuir

Restructuring of Hydrophobic Surfaces Created by Surfactant Adsorption to Mica Surfaces Jhuma Das,† Changsun Eun,† Susan Perkin,‡ and Max L. Berkowitz*,† † ‡

Department of Chemistry, University of North Carolina—Chapel Hill, Chapel Hill, North Carolina 27599, United States Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom

bS Supporting Information ABSTRACT: Hydrophobic surfaces created by the adsorption of a monolayer of surfactants, such as CTAB or DODAB, to mica display long-range mutual attraction when placed in water. Initially, this attraction was considered to be due to hydrophobic interaction, but more careful measurements using AFM showed that the surfactant monolayer undergoes rearrangements to produce charged patches on the surface; therefore, the nature of the long-range interaction is due to the electrostatic interaction between patches. The monolayer rearrangement depends on the nature of the surfactant and its counterion. To study possible monolayer rearrangements in molecular detail, we performed detailed molecular dynamics computer simulations on systems containing a monolayer of surfactants RN(CH3)3+Cl (R indicates a saturated hydrocarbon chain) adsorbed on a mica surface and immersed in water. We observe that when chain R is 18 carbons long the monolayer rearranges into a micelle but it remains a monolayer when the chain contains 24 carbons.

’ INTRODUCTION Because of its importance in a large number of fields and applications, the issue of hydrophobic interaction acting between surfaces has been studied very intensely.1 3 Considering that the size of a water molecule is only ∼0.3 nm and that only a few water layers situated next to most surfaces are perturbed,4 one would expect that the proper hydrophobic interaction, due to the disruption of the water hydrogen bonding network, would manifest itself when the distance between surfaces is just a few nanometers. Nevertheless, the literature reports the existence of long-ranged hydrophobic interactions acting over distances that are as large as ∼50 nm.5 7 It is realized now that these hydrophobic forces originate in phenomena that are distinct from those that are responsible for shorter-ranged hydrophobic interactions.7 The origin of the long-ranged hydrophobic interaction depends on the nature of the hydrophobic surface and may be due to (i) the existence of submicroscopic bubbles between surfaces,8 (ii) cavitation in the intervening fluid,9 and (iii) electrostatic interactions between charged patches of restructured hydrophobic surfaces created by surfactant adsorption to mica.6,10 12 The restructuring of surfactants adsorbed on mineral surfaces is a well-studied subject in both experiment and simulations, and it occurs in both solutions and vacuum.13 17 One of the best ways to study interactions between surfaces is to use a surface force apparatus or its modification, a surface force balance (SFB).5,6,10,18,19 Thus, the interaction between hydrophobic surfaces created by placing ionic surfactants, such as CTAB, on mica, was measured by SFB.5,10,19 Depending on the r 2011 American Chemical Society

nature of the surfactant and/or its counterion, the interaction between surfaces sometimes displayed a long-ranged hydrophobic component and sometimes it did not.5,10,19 It was proposed that when such a hydrophobic interaction was present it was due to the restructuring of the surface. As a result of the restructuring, parts of the bare mica surface were exposed to water, thus creating negatively charged patches, whereas the surfactant monolayer in some places was transformed into a bilayer, which could be considered to be a positively charged patch.5,10,19 Therefore, because of surface restructuring, the interaction between homogeneous hydrophobic surfaces was transformed into an interaction between inhomogeneous surfaces with charged patches. In this work, we check by simulation if indeed a monolayer of surfactants covering mica surfaces can undergo a restructuring and if so, how this restructuring depends on the properties of the surfactant molecules, such as, the length of the surfactant tail.

’ METHODS To study the influence of the surfactant chain length on the behavior of these molecules next to mica surfaces, we performed molecular dynamics (MD) simulations on a set of different systems. In our systems, the unit cell for simulations initially contained a layer of mica, a layer of surfactant molecules physisorbed by their headgroups to the mica Received: June 8, 2011 Revised: September 1, 2011 Published: September 01, 2011 11737

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Langmuir surface, and water placed above the hydrophobic surface created by the surfactant tails. To construct the mica layer, we downloaded the coordinates of the mica sheet along with the coordinates for K+ ions from Heinz’s web site (http://www.poly-eng.uakron.edu/nsl/).20 This unit of mica with K+ counterions was doubled in the vertical direction, z, and after that, another three copies of this new cell were prepared and translated periodically along the lateral x and y dimensions to obtain the mica layer. The surfactant molecules were generated using the PRODRG server (http://davapc1.bioch.dundee.ac.uk/prodrg/). Each surfactant molecule RN(CH3)3+Cl contained a positively charged N(“CH3”)3 headgroup (“CH3” was represented as a united atom), a single saturated chain tail R with n united carbon atoms, and a counterion Cl . The tails of the surfactant molecules contained n = 14, 16, 18, 20, 22, and 24 carbon atoms. A monolayer of 100 surfactant molecules was prepared using the GROMACS genconf program. To obtain a hydrophobic surface, the surfactant monolayer was placed on the top of the mica surface so that the surfactant headgroup atoms faced the mica surface (away from water) and the tails were directed away from the mica surface. Our simulations are intended to simulate systems similar to the ones studied in SFB experiments; therefore, similar to the experimental setup,5 the ratio of the number of surfactants to the negatively charged sites of the mica surface was 1.66 (the ratio was 1.5 for the experimental setup). To neutralize the total charge on the surfactant headgroups, Cl counterions were inserted between the mica surface and the surfactant monolayer, next to the headgroup atoms. In summary, each mica/ surfactant system was composed of 8 mica units, including 240 K+ ions, and 100 surfactants along with 100 of their Cl counterions. Our systems were then solvated by placing a slab of water containing 25 000 molecules on top of the surfactant monolayer, where the hydrophobic surfactant tails created a surface. The details of the force field and the MD simulation parameters are given in sections S1 and S2 and Figure S1 of the Supporting Information. Initially, each system was energy minimized using the steepest-descent algorithm so that the systems attained configurations with minimal steric hindrance. Following the minimization procedure, short 0.5 ns pre-equilibration runs were performed under higher pressure (10 bar) to ensure the absence of bad steric contacts and to remove quickly any vacuum from the system. The model system for the simulation with n = 24 carbons in the surfactant tail is depicted in Figure 1. After pre-equilibration runs, 40 ns MD simulation runs (each with different surfactants) were performed. From our simulations we observed that only monolayers containing surfactants with n = 24 carbons did not undergo restructuring. Because surfactants with 18 carbons in the tail are commonly used in experiments to study hydrophobic interactions, we chose to report here the results on systems with this surfactant and a surfactant with n = 24. The surfactant with a short tail is designated as the C18 surfactant. The long-tailed surfactant is named the C24 surfactant. To expedite further simulations, we reduced the number of water molecules for the follow-up simulations of our systems. Thus, the final structures from the 40 ns simulations were taken, and the number of water molecules in the systems was reduced to 16 508. Next, the energy of the modified systems was minimized, and another 80 ns simulation run was performed for a system with the C24 surfactant and a 100 ns run was performed for a system containing the C18 surfactant. We will refer to the 40 ns simulation runs as P1 and the follow-up simulations as P2; thus we will refer to the four simulations on C24 and C18 as C24P1, C24P2, C18P1, and C18P2. We used the P1 and P2 MD trajectories for data analysis, excluding the first 1 ns of the trajectories in all simulations.

’ RESULTS The main result of our simulation is that during the time period of our simulations we observed the restructuring of a monolayer consisting of C18 surfactant molecules, but we did

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Figure 1. Initial structure of the simulated model system with C24 surfactants. This figure was created using VMD software.16 All the components of the system are color coded: N atoms represent surfactant headgroups and are depicted with green vdW spheres, the surfactant tails are shown in a blue licorice representation, red vdW spheres represent Cl ions, and tan vdW spheres represent K+ ions. The mica sheets are shown using a magenta licorice representation, and the water box is shown as a gray CPK. This color code will be used to represent the snapshots of the final structure of our model systems after P1 and P2 production runs (C24P1, C24P2, C18P1, and C18P2).

not observe any restructuring of a monolayer consisting of C24 surfactants. The restructuring can be observed by a direct observation of snapshots from simulations. Thus, the final structures of the system containing C24 surfactants from the P1 and P2 simulations are shown in Figure 2. As the figure shows, the C24 surfactants maintained the monolayer structure during the P1 simulation and also during the P2 simulation. The final structures of the system containing C18 surfactants from the P1 and P2 simulations are shown in Figure 3. Contrary to the situation with C24 surfactants, structural changes in the system with C18 surfactants began right after the P1 simulation started. Within the first 20 ns of the P1 simulation, an 11738

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Figure 2. Snapshots of the C24 model system in the presence of Cl counterions after P1 (left panel, C24P1) and P2 (right panel, C24P2) production runs. C24 surfactants preserve the monolayer formation during production runs P1 and P2. The color code used in this figure is the same as that used in Figure 1. The water molecules are removed for clarity.

Figure 3. Final structures of C18 surfactants in the presence of counterions (Cl ) after P1 (left column, C18P1) and P2 (right column, C18P2) MD simulations. C18 surfactants aggregate into a cylindrical micelle during the P1 production run. The surfactants transform into spherical micelle from the cylindrical micelle during the P2 simulation. The upper panel shows side views of the system, and the lower panel exhibits top views of the system. The color code used in this figure is the same as that used in Figure 1. The counterions and the water molecules are removed for clarity.

assembly of C18 surfactants underwent a transition from a monolayer to a cylindrical micelle. During this transformation, water molecules from the top of the monolayer proceeded toward the mica surface and replaced the surfactants. The surfactants simultaneously reoriented to form a cylindrical micelle that was transformed into a spherical micelle during the P2 simulation.

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To understand better the dynamics of the spatial rearrangement of our systems, the number densities FN(z) of certain components were calculated as a function of the position along the normal axis with respect to mica sheets (z axis) at different moments in time. The five components we followed are (i) the K+ counterions for mica sheets, (ii) surfactant counterions, (iii) centers of mass (CM) of surfactant headgroup atoms (C24-H/ C18-H), (iv) surfactant tails (C24-T/C18-T), and (v) the CM of water molecules. The CM of the surfactant headgroup was calculated by combining the three united carbon and nitrogen atoms in the headgroup. We defined z as the relative distance between the actual z coordinate (zabsolute) of each atom (or the CM of the surfactant headgroup atoms) and the zCM of two mica sheets. Figure 4a shows the number densities for the C24P1 (left panel) and C24P2 (right panel) simulations. The number density profiles for the CMs of the headgroup atoms and the tail atoms as well as the other components in C24 systems during both (P1 and P2) simulations display small fluctuations throughout the MD simulations. Thus, the monolayer conformation and the corresponding number densities for the C24 surfactants remained almost unaltered for the entire 120 ns simulation (during simulations P1 plus P2). Notice that the density of water close to the interface with surfactant tails displays behavior typical of the water/oil interface, thus confirming the hydrophobic character of the C24 surfactant-covered surface. The FN(z) values for C18 systems during the P1 simulation are shown in the left panel of Figure 4b, and for the P2 simulation, they are shown in the right panel. As we can see, during the first 10 ns of the P1 run, the FN(z) of all of the components displays spatial distributions very similar to those observed in simulations with the C24 surfactant, implying the presence of a monolayer structure. However, after 10 ns the FN(z) of every component broadens and starts to change significantly with time. This indicates that the monolayer conformation breaks down and that the C18 surfactants and their counterions slowly diffuse away from their initial positions. Eventually they form a cylindrical micelle at the end of the P1 simulation (Figure 3), although the presence of the cylindrical structure is not trivial to infer from the density plots. Nevertheless, if observed carefully, one can see that the FN(z) profiles for the counterions and surfactant headgroup atoms are arranged fairly symmetrically along the z axis, with two peaks appearing at around zlower≈ 1.5 nm and zupper ≈ 5.5 nm and with a comparatively lower concentration in between. The densities fall off rapidly outside of this range. These arrangements correspond to the lower and upper edges of the micellar cylinder. Because the headgroups are usually surrounded by counterions, corresponding ion densities are higher at the edges. Figure 4b also shows that the two pronounced peaks, implying the cylindrical shape of the micelle, eventually disappear. At a later phase in the P2 simulation (around 60 ns), the FN(z) profiles for surfactant headgroups and their counterions display density curves with a broad peak at the center, which on average is the center of the spherical micelle. It is also typical to monitor the behavior of the system energy in simulations, especially in cases when structural changes occur. Figure S2 (Supporting Information) displays the time dependence of the total energy difference, ΔE(t) = E(t) E(t = 1 ns), for our four runs. Only for run C18P1 do we observe that the energy decreases as the simulation starts. This happens because a restructuring of a monolayer to a cylindrical micelle occurs. After that, the energy stabilizes. It is interesting that the restructuring of the cylindrical micelle into a spherical one is not accompanied by 11739

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Figure 4. Time-dependent number density (FN(z)) profiles of components in our model systems as a function of the position along the normal axis with respect to mica sheets (z axis): (a) the C24 model system during C24P1 (left) and C24P2 (right) production runs and (b) the C18 model system during C18P1 (left) and C18P2 (right) production runs. The FN(z) value is calculated for the K+ ions, Cl counterions, CM of the C24/C18 headgroup atoms (C24 H/C18-H), C24/C18 tail atoms (C24-T/C18-T), and water molecules (SOL).

a total energy change. One needs to monitor the change in free energy to understand the thermodynamic reasons for the morphological changes that occurred in the system.

’ DISCUSSION AND CONCLUSIONS In summary, the long-tailed surfactants (C24) preassembled into a monolayer that is physisorbed to a mica surface and imbedded into a water bath maintained the monolayer structure during a 120 ns MD simulation, whereas a monolayer with shorter-tailed surfactant molecule C18 has undergone a transition to a cylindrical micelle and eventually to a spherical one. Thus, our detailed simulations show that a restructuring of the surfactant monolayer on a mica surface indeed can take place and that the tendency to do so depends on a surfactant’s chain length. Why do we see such a difference in the behavior of the C18 and C24 monolayers? It is clear that water wants to solvate a negatively charged mica surface but has to overcome a barrier due to a hydrophobic layer containing the tails of surfactant molecules. Such a layer is larger for longer-tailed surfactants. That

water plays a very important role in the restructuring phenomena is also hinted at in some of the energy calculations that we performed. From these calculations, we observed that most of the energy change in the system with C18 surfactants, during the transition from the monolayer to a cylindrical micelle, was due to the gain in the interactions of water with mica and water with ions and that this gain compensated for the loss in mica surfactant and water water interaction energies. At the same time, it is important to understand the change in entropy during the restructuring because for some of its stages entropy plays a major role (e.g., the transition from cylindrical to spherical micelles for the C18 surfactant). Therefore, a full answer to the question posed above can be provided only by performing free-energy calculations, which will also answer the questions related to the thermodynamic stability of the structures. At this stage, we cannot perform a direct comparison between our results obtained from simulations and the experimental data obtained from the SFB measurements. In the experiment, the surfaces have large macroscopic dimensions, but our simulations are done on the small nanometer-sized part of the surface. The 11740

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experiments measure the interactions between surfaces, but we do not. However, both experiments and simulations show that hydrophobic surfaces due to surfactant monolayers can undergo restructuring. To study the long-ranged interactions between such surfaces, one needs to use coarse-grained force fields. We are in the process of performing such coarse-grained simulations.

’ ASSOCIATED CONTENT

bS

Supporting Information. Simulation details and energy analysis figures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT Financial support via grant N000141010096 from the Office of Naval Research is gratefully acknowledged. ’ REFERENCES (1) Chandler, D. Nature 2005, 437, 640. (2) Berne, B. J.; Weeks, J. D.; Zhou, R. H. Annu. Rev. Phys. Chem. 2009, 60, 85. (3) Ball, P. Chem. Rev. 2008, 108, 74. (4) Berkowitz, M. L.; Bostick, D. L.; Pandit, S. Chem. Rev. 2006, 106, 1527. (5) Silbert, G.; Klein, J.; Perkin, S. Faraday Discuss. 2010, 146, 309. (6) Meyer, E. E.; Lin, Q.; Hassenkam, T.; Oroudjev, E.; Israelachvili, J. N. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 6839. (7) Christenson, H. K.; Claesson, P. M. Adv. Colloid Interface Sci. 2001, 91, 391. (8) Attard, P. Langmuir 1996, 12, 1693. (9) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (10) Perkin, S.; Kampf, N.; Klein, J. Phys. Rev. Lett. 2006, 96, 038301. (11) Podgornik, R.; Parsegian, V. A. Chem. Phys. 1991, 154, 477. (12) Jho, Y. S.; Brewster, R.; Safran, S. A.; Pincus, P. A. Langmuir 2011, 27, 4439. (13) Heinz, H.; Vaia, R. A.; Krishnamoorti, R.; Farmer, B. L. Chem. Mater. 2007, 19, 59. (14) Heinz, H.; Vaia, R. A.; Farmer, B. L. J. Chem. Phys. 2006, 124, 224713. (15) Heinz, H.; Suter, U. W. Angew. Chem., Int. Ed. 2004, 43, 2239. (16) Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913. (17) Naik, V. V.; Chalasani, R.; Vasudevan, S. Langmuir 2011, 27, 2308. (18) Meyer, E. E.; Rosenberg, K. J.; Israelachvili, J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15739. (19) Perkin, S.; Kampf, N.; Klein, J. J. Phys. Chem. B 2005, 109, 3832. (20) Heinz, H.; Koerner, H.; Anderson, K. L.; Vaia, R. A.; Farmer, B. L. Chem. Mater. 2005, 17, 5658.

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