Estimation of group dipole moments from surface potential measurements on Langmuir monolayers

Abstract

Pressure–area isotherms and surface potential data are presented for octadecyl methyl sulphoxide (OMS) and (+)-octadecyl p-tolyl sulphoxide (OTS). The surface potential measurements indicate very clearly that both compounds are anchored at the water surface by the SO group and that the plateau in the pressure–area isotherm of OTS is the results of significant molecular orientation. The Demachak and Fort model for relating group dipole moments to the surface potential of floating Langmuir monolayers is reviewed and found to be applicable to a number of compounds. However, the values deduced by these authors for the local permittivities are not appropriate for compounds with long aliphatic chains. By drawing on previously published work a new set of values has been deduced which seems to be more applicable to such compounds and to the sulphoxides investigated here.