Surfactant-Induced Surface Freezing at the Alkane-Water Interface

Abstract

Long-chain alkanes exhibit surface freezing at the alkane-air but not the alkane-water interface. Ellipsometry and surface tensiometry are used to show that a simple cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), can induce surface freezing at the tetradecane-water interface even when present in mole fractions as low as 0.1. The surface-freezing temperature $T_s$ is a linear function of the interfacial excess of CTAB. The excess surface entropy below $T_s$, $S^{\sigma }=-0.76\pm 0.02\mathrm{mJ}{\mathrm{K}}^{-1}{\mathrm{m}}^{-2}$, is consistent with a rotator phase. Ellipsometry provides strong evidence for a frozen monolayer in which the chains are oriented near the surface normal.