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 Ts is a linear function of the interfacial excess of CTAB. The excess surface entropy below Ts, S = –0.76±0.02 mJ K–1 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.