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 result of significant molecular orientation. The Demchak 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.

Molecular dipoles at substrate/film interfaces influencing surface energy of Langmuir–Blodgett films

“…The ε 2 value for the head groups lies between 3.9 and 5 for the different amphiphiles in this study. These values agree reasonably well with previously reported values [26]. However, for the -OCH 3 -substituted amphiphiles the value is almost around 16, and such a high value should lead to a large polarity, which is indicated by the measured γ p S values of the compound.…”

Molecular Interactions in Monolayers οf Azo Dye/Liquid Crystal Mixtures at Interfaces

“…According to the Demchak-Fort model [20], µ ⊥ is composed of three independent contributions arising from (i) orientation of water dipoles in the vicinity of the headgroups, (ii) the headgroup dipoles, and (iii) the hydrophobic part dipoles. Moreover, in our calculations we assumed, as it is made customary [19,21], that ε r = 1, although it is known that dipoles in a monolayer may be embedded in media with distinct, nonunity dielectric constant values, which depends on whether these dipoles are at the monolayer-water or monolayer-air interface [22,23].…”

Surface properties of mixed monolayers of sulfobetaines and ionic surfactants

“…In addition the monolayers of the pure zwitterionic components have also positive surface potential. This fact was also observed in zwitterionic insoluble monolayers [33], and is due to the contribution of the tail group moments [34,35].…”