Nucleation of Surfactant-Alkane Mixed Solid Monolayer and Bilayer Domains at the Air-Water Interface.

Hiroki Matsubara,Rikako Mori,Eisuke Ohtomi

doi:10.3390/ma15020485

Abstract

We investigated the wetting transitions of tetradecane and hexadecane droplets in dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB), and hexadecyltrimethylammonium bromide (C16TAB) aqueous solutions. By varying the surfactant concentration, the formation of mixed monolayers of a surfactant and an alkane was observed at the air–water interface. Depending on the combination of surfactant and alkane, these wetting monolayers underwent another thermal phase transition upon cooling either to a frozen mixed monolayer (S1) or a bilayer structure composed of a solid monolayer of a pure alkane rested on a liquid-like mixed monolayer (S2). Based on the phase diagrams determined by phase modulation ellipsometry, the difference in the morphology of the nucleated S1 and S2 phase domains was also investigated using Brewster angle microscopy. Domains of the S1 phase were relatively small and highly branched, whereas those of the S2 phase were large and circular. The difference in domain morphology was explained by the competition of the domain line tension and electrostatic dipole interactions between surfactant molecules in the domains.

Highlights

It is well known that the 2D analogue of surface tension, called line tension, arises at the boundary between two surface phases
Line tension has been discussed in its relation to the lipid raft formation in cellular membranes [1,2,3,4]
The extent of height mismatch between the L, S1, and S2 phases can be readily controlled for surfactant–alkane mixed adsorbed films, and the difference in electric charge density can be obtained by the analysis of surface tension data through the Gibbs adsorption isotherm

Summary

Introduction

It is well known that the 2D analogue of surface tension, called line tension, arises at the boundary between two surface phases. Deutsch et al [11] studied the wetting transitions of n-alkanes of various chain lengths in aqueous hexadecyltrimethylammonium bromide (C16TAB) and showed that an S2 film formed for octadecane and longer alkanes, while for alkanes with chain lengths similar to or shorter than C16TAB, S1 film formation dominated They applied X-ray reflectometry to determine the molecular-level structures of these surface frozen films. The extent of height mismatch between the L, S1, and S2 phases can be readily controlled for surfactant–alkane mixed adsorbed films, and the difference in electric charge density (i.e., surface density of cationic surfactants) can be obtained by the analysis of surface tension data through the Gibbs adsorption isotherm. The present experiments propose a qualitative physicochemical understanding of the nucleation of ordered domains from the comparison between domain morphologies

Materials and Methods

Ellipsometry

Brewster Angle Microscopy

BAM images of nucleated domains the

Discussion