Surface Equation of State for Insoluble Surfactant Monolayers at the Air/Water Interface

Abstract

A surface equation of state is derived and used to interpret the liquid-expanded (LE)−liquid-condensed (LC) phase transition. The monolayer is treated as a two-dimensional mixture consisting of LC domains, disordered molecules in the LE state, and free sites. The free sites are assumed to have two-dimensional size only and to be located at the internal surface between the headgroups and the tails of the surfactant. They are introduced to account for the translational freedom of the domains and of the disordered molecules on the surface. The model can explain the nonhorizontal LE/LC phase transition and its dependence on temperature. In addition, the equilibrium radius and equilibrium area fraction of the domains are calculated as a function of the average molecular surface area (A). In the absence of domains, the surface equation of state reduces to a modified two-dimensional van der Waals equation. The dependence of the surface cohesion pressure on the molecular surface area A is calculated by including three contributions: the dipole−dipole interactions, the chain van der Waals interactions, and the chain conformation. The theoretical model is compared with the experimental π− A isotherms for six phospholipid surfactants, and good agreement is obtained.

Influence of interaction energy in fluid-fluid phase transitions on Langmuir monolayers

“…Good agreement with experimental data were also obtained with the generalized Volmer equation [13,14]. By taking into account chain conformation and dipole-dipole interactions Ruckenstein and Li [15] were able to reproduce some characteristic features of isotherms of phospholipids.…”

Equilibrium and dynamic surface properties of trisiloxane aqueous solutions. Part 2. Theory and comparison with experiment

“…(5) and (9) that at such surface pressures Γ 1 >>Γ 2 . Hence, in this range we can neglect the adsorption in the state 2 and investigate adsorption behaviour of trisiloxanes using already know earlier theoretical models of aggregation processes [10][11][12][13][14][15][16][17][18][19][20][21][22][23].…”

Interaction of lipophilic gemcitabine prodrugs with biomembrane models studied by Langmuir–Blodgett technique

“…1A). The DMPC isotherm shows characteristic features: a gaseous phase for areas larger than 100 Å 2 , a LE phase at 100 < Å 2 < 70, a LE to LC lipid phase transition between 70 and 45 Å 2 and a LC phase for areas lower than 45 Å 2 [23,24]. Gemcitabine does not form a monolayer and even at high concentrations no isotherm is observable.…”