Phase Transitions of Soluble Surfactants at a Liquid−Vapor Interface

Abstract

Although medium chain length insoluble amphiphiles are well known to form gaseous and liquid expanded phases on an air/water interface, the situation for the soluble case is less clear. We perform molecular dynamics simulations of model surfactant molecules dissolved in a bulk liquid solvent in coexistence with its vapor. Our results indicate a transition in both soluble and insoluble surfactants: a plateau in surface tension vs. surface coverage, whose instantaneous configurations display two phase coexistence, along with correlation functions indicating a transition to gaseous to liquid-like behavior.PACS numbers: 79.20. Rf, 64.60.Ht, 68.35.Rh Surfactants are amphiphilic molecules which contain a polar head group and a nonpolar tail, which usually consists of a chain of hydrocarbon groups. At an air/water interface surfactants arrange themselves with their polar group immersed in water, interacting by dipolar forces, while the hydrocarbon tails are displaced outward into the air. The presence of surfactants at the air/water interface lowers the free energy, and in turn the surface tension, usually as a non-increasing function of the concentration of surfactant in the liquid [1]. The degree of aggregation and molecular ordering of surfactants adsorbed at an air/solvent interface is reflected in the appearance of distinct surface phases as a function of surfactant concentration. In addition to the obvious two dimensional Gas (G) and disordered liquid phases (more precisely, the Liquid Expanded LE phase), it is possible to find condensed mesophases and semi-solid crystalline-like phases in monolayers [2,3]. This paper focuses on the determination of soluble surfactant adsorption isotherms simultaneously with their phase behavior and surface tension variation, and in particular the G/LE transition, using Molecular Dynamics (MD) simulations. In the context of insoluble surfactants, this approach has been used extensively to study the structure of monolayers restricted to the surface of a substrate [4-6], obtaining information both complementary to and in semi-quantitative agreement with experiment on Langmuir films. For soluble surfactants there is emerging experimental evidence for the existence of gaseous, liquid, and condensed surface phase transitions [7], but no accompanying theory or simulation. There are MD simulations of soluble surfactant monolayers [8], but these do not study the surface-bulk equilibrium which gives rise to the surface phases, or the transitions between them. In contrast, MD simulations in liquid-liquid systems [9] have studied the equilibrium between the liquid phase and monomeric and surfactant aggregates in the bulk, but not surface phase transitions.Our simulations model surfactant molecules placed in a solvent in equilibrium with its vapor, which then migrate to the surface to a degree controlled by the choice of interaction potential. We observe that as their initial concentration increases, the surfactant structure on the liquid/vapor interface ranges from a gas phase containing...

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