Restructuring of Hydrophobic Surfaces Created by Surfactant Adsorption to Mica Surfaces

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Entry by Yuhang Jin, AP225 Fall 2011

Reference

Jhuma Das, Changsun Eun, Susan Perkin, and Max L. Berkowitz, Langmuir, 2011, 27, 11737.

Keywords

Surfactant monolayer, hydrophobic surfaces, long-range electrostatic interaction, molecular dynamics simulation

Introduction

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.

Models

Fig.1 Initial structure of the simulated model with n = 24. Green spheres: surfactant headgroups; blue licorice: surfactant tails; red spheres: chloride ions; tan spheres: potassium ions; magenta licorice: mica sheets; gray CPK: water box.
Fig.2 C24 surfactants preserve the monolayer formation during simulation runs P1 (left) and P2 (right).
Fig.3 Final structures of C18 surfactants after P1 (left) and P2 (right) MD simulations. Upper panels: side views; lower panels: top views.

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.

Results

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.