Freezing transitions in mixed surfactant/alkane monolayers at the air–solution interface

Abstract

Mixed monolayers at the air–water interface of the cationic surfactant, hexadecyltrimethylammonium bromide (CTAB), with alkanes show a first-order freezing transition as a function of temperature. Sum-frequency spectra and ellipsometric measurements are consistent with a structure in which the high-temperature phase is liquid-like and the low-temperature phase has all-trans, upright chains. There are strong structural similarities between the low-temperature phase in the mixed monolayers and frozen monolayers at the alkane–air and alkane–CTAB solution interfaces. The difference between the surface freezing point T_s and the freezing point of the bulk alkane T_b ranges from 1 °C for alkane chain length m = 17, to 28 °C for m = 11. Surface freezing is more favourable in mixed monolayers at the air–water interface than at the bulk alkane–water interface for the same surfactant concentration. Long-chain alkanes do not wet water, but it is postulated that if they did, they would also show surface freezing analogously to alkanes on silica. The surfactant plays the dual role of enhancing wetting and surface freezing of the alkane on water.