Surfactant-Induced Surface Freezing at the Alkane-Water Interface

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^σ=-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.