Restructuring of Hydrophobic Surfaces Created by Surfactant Adsorption to Mica Surfaces
Entry by Yuhang Jin, AP225 Fall 2011
Jhuma Das, Changsun Eun, Susan Perkin, and Max L. Berkowitz, Langmuir, 2011, 27, 11737.
Surfactant monolayer, hydrophobic surfaces, long-range electrostatic interaction, molecular dynamics simulation
Hydrophobic interaction acting between surfaces is important in many fields and applications. Sometimes this interaction can act over distances as large as ~50 nm, probably due to the existence of submicroscopic bubbles, cavitation in the intervening fluid, or electrostatic interactions between charged patch of restructured hydrophobic surfaces created by surfactant adsorption to mica. Using a surface force balance (SFB), one can measure the interaction between hydrophobic surfaces created by placing ionic surfactants on mica. The interaction sometimes displays a long-ranged hydrophobic component owing to the coexistence of both negatively charged bare mica patches exposed to water and positively charged patches covered by a surfactant bilayer. In this paper, computer simulations confirmed this picture and studied how this restructuring depends on the properties of the surfactant molecules.
The paper focuses specifically on the influence of the surfactant chain length on the behavior of these molecules near mica surfaces using molecular dynamics (MD) simulations. The unit cell for simulations initially contained a layer of mica, a layer of surfactant molecules physisorbed by their headgroups to the mica surface, and water placed above the hydrophobic surface created by the surfactant tails, as shown in Fig. 1. The tails of the surfactant molecules contained n = 14, 16, 18, 20, 22, and 24 carbon atoms. The systems simulated here are highly similar to those in SFB measurements. Chloride counterions are introduced between the mica surface and the surfactant monolayer for the purpose of charge neutrality. The systems were then solvated by placing a slab of water on top of the hydrophobic surface of the surfactant monolayer. Initially, each system was energy minimized using the steepest-descent algorithm. Then short 0.5 ns pre-equilibration runs were performed under higher pressure to ensure the absence of bad steric contacts and to remove quickly any vacuum from the system. After pre-equilibration runs, 40 ns MD simulation runs (each with different surfactants) were performed. 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 for a system containing the C18 surfactant. The 40 ns simulation runs will be referred to as P1 and the follow-up simulations as P2.
The main result of our simulation is that the restructuring of a monolayer consisting of C18 surfactant molecules was observed, but any restructuring of a monolayer consisting of C24 surfactants never underwent such a restructuring. The C24 surfactants maintained the monolayer structure in both P1 and P2 simulations, as depicted in Fig. 2. By contrast, cylindrical micelles formed during the P1 simulation for C18 molecules, and in the meantime water molecules from the top of the monolayer proceeded toward the mica surface and replaced the surfactants. During the P2 simulation, these cylindrical micelles were further transformed into spherical ones. The processes for C18 surfactants are illustrated in Fig. 3.
For a better understanding of the dynamics of the spatial rearrangement of the systems, the number densities of certain components were calculated as a function of the position along the normal axis with respect to mica sheets at different moments in time. The five components that were followed are potassium counterions, surfactant counterions, center of mass (CM) of surfactant headgroups, surfactant tails, and CM of water molecules. The results are summarized in Fig. 4. From Fig. 4(a), it is clear that the number density profiles for the components in C24 systems during both P1 and P2 simulations only display small fluctuations, and thus the monolayer conformation remains largely unaltered. Also, the behavior of the density of water close to the interface with surfactant tails confirms the hydrophobic character of the C24 surfactant-covered surface. On the other hand, from Fig. 4(b), one finds that after 10 ns in the simulation, the number density profiles for all the components in C18 systems 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. The densities of the counterions and surfactant headgroups display two notable peaks and fall off rapidly outside of this range, implying the lower and upper edges of the micellar cylinder. At a later phase in the P2 simulation (around 60 ns), the density 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.
In summary, the C24 surfactants preassembled into a monolayer on a mica surface and imbedded into a water bath maintained the monolayer structure during a 120 ns MD simulation, whereas a monolayer with C18 surfactant molecule C18 has undergone a transition to a cylindrical micelle and eventually to a spherical one. The 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. The reason why one sees a difference in the behavior of C18 and C24 monolayers can be accounted for from a free-energy perspective. Water needs to overcome a large barrier due to the hydrophobic layer, whose thickness relies heavily on the surfactant's chain length. For systems with C18 surfactants, which mean a relatively thinner barrier, the energy gained in the interactions of water with mica and ions is enough to compensate the loss of energy in the mica-surfactant and water-water interactions. Moreover, the change in entropy also plays an important role in some stages, for instance the transition from cylindrical and spherical micelles.