Models for interpreting surface potential measurements and their application to phospholipid monolayers

• Published in: Journal of colloid and interface science Vol. 139; no. 2; pp. 508 - 518
• Main Authors: Taylor, David Martin, De Oliveira, Osvaldo Novais, Morgan, Hywel
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 15.10.1990; Elsevier
• Subjects: Chemistry; Exact sciences and technology; Gas-liquid interface and liquid-liquid interface; General and physical chemistry; Surface physical chemistry; Gas liquid interface; Monolayer; Theoretical study; Phospholipid; Models; Dipole moment; Surface potential
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/0021-9797(90)90123-6

Three models for estimating group dipole moments from surface potential measurements on monolayers at the air/water interface are critically reviewed. The Helmholtz model favored by most workers seriously underestimates the group moments since no account is taken of the polarizability of the monolayer. The partial dipole compensation technique recently introduced by Vogel and Möbius, while in principle very sound, is shown to be flawed. A basic assumption of their model, that the conformation of the two carboxyl groups in octadecylmalonic acid is identical to that in stearic acid, is unlikely to be correct. Consequently, their estimation of the contribution of the distal methyl group to the surface potential of long chain alkanoic compounds is overestimated. On the other hand, despite its limitations, the Demchak and Fort model provides good agreement between the group dipole moments of several aliphatic compounds estimated from monolayer surface potential and the values determined for the same groups from measurements on bulk material. In view of this success, the Demchak and Fort model is applied to the surface potential obtained from four phospholipids, phosphatidylcholine, phosphatidylethanolamine, phosphatidic acid, and phosphatidylserine, and their headgroup dipole moments are estimated. It is concluded that the hydrophilic headgroups contribute significantly to the surface potential, contrary to the findings of Vogel and Möbius who suggested that the terminal CH 3 groups of the lipids were mainly responsible for the observed surface potential.