Relationship between monolayer structure and dynamic surface properties of alkyl dimethyl phosphine oxides. BAM studies

Abstract

A conventional LB trough combined with a Brewster angle microscope (BAM) are used for the study of monolayers of the non-ionic surfactants alkyl dimethyl phosphine oxide, eikosyl dimethyl phosphine oxide (EDPO) and octadecyl dimethyl phosphine oxide (ODPO). The monolayers were spread from benzene, hexane or chloroform at c=10 −3 M on an aqueous subphase. The two homologues show significantly different monolayer behaviour although the alkyl chain of EDPO and ODPO differ by only two –CH 2– groups. While EDPO forms well-visible domains, ODPO does not form any domains. Experiments on the relaxation behaviour have been performed using an oscillating barrier technique. The discussion of the viscoelasticity of the monolayers is based on the existence of domains. A comparison of the viscoelastic behaviour of the monolayer of the two surfactants can be used to understand the contribution of domains to the non-equilibrium monolayer properties.