Adsorption of Semifluorinated Alkanes at Hydrocarbon-Air Surfaces

Abstract

Hydrocarbon and fluorocarbon chains tend to demix. Because ofthis antipathy, semifluorinated alkanes (SFAs) of the general diblock structure F(CF 2 ) n (CH 2 ) m H (abbreviated as F n H m ) are expected to show surfactant properties (adsorption and aggregation) in hydrocarbon and fluorocarbon solvents. We have used surface tensiometry to investigate the adsorption of a range of F n H m materials at hydrocarbon-air surfaces. Maximum surface pressures are found to be 0-10 mN/m. The surface pressures generally increase with increasing SFA concentrations up to concentrations very close to the solubility limit in the hydrocarbon solvents investigated. This suggests that micelle aggregate formation is either absent or occurs only over a very narrow concentration range. The extent of adsorption increases with increasing F chain length of the SFA, increasing chain length of the alkane solvent, and decreasing temperature. Weakly adsorbing systems form expanded monolayers with a minimum area per SFA molecule of a few square nanometers. Strongly adsorbing systems form condensed monolayers of minimum area per SFA of 0.26 nm 2 , corresponding to a fluorocarbon chain density equal to that found in condensed bulk phases of perfluoroalkanes. The transition from weak to strong adsorption behavior occurs in an abrupt manner and can be induced either by changing the temperature or by changing the hydrocarbon solvent composition.