Abstract

Lipid monolayers at the air-water interface serve as essential model systems for studying the biophysical properties of cellular membranes. This study investigates the relationship between the boundary potential ($\Delta V$) and the energy of compression ($W$) for various lipid monolayers, including DOPC, POPC, DMPC, DPPC, DPhPC, and DMPS, specifically within the liquid expanded (LE) state. Compression diagrams were analyzed to characterize the elastic properties of the monolayers, modeling lipid molecules as an incompressible area surrounded by a soft, exponentially pressure-dependent shell. A key finding is that the change in the interfacial Volta potential ($\Delta V$) exhibits a linear dependence on the effective work applied to compress the monolayer ($W$) in the LE phase for all tested lipids. The slope of this linear relationship is proposed as a sensitive parameter for identifying and characterizing the effects of membrane-active compounds. Furthermore, the study explores the influence of the aqueous subphase's pH and ionic composition (KCl, CaCl\textsubscript{2}, BeCl\textsubscript{2}), as well as the adsorption of polylysine (PLL) and chlorpromazine (CPZ). Results show that the $\Delta V$ vs. $W$ slope is largely independent of pH and ionic composition but decreases upon PLL adsorption. Notably, the adsorption and incorporation of the positively charged CPZ molecules cause a deviation from linearity, which is quantitatively described by a model that approximates the effect of CPZ on lateral pressure using a Boltzmann-like relation. These findings provide a refined biophysical framework for understanding the interplay between mechanical stress and electrostatic properties in model membranes, offering a valuable tool for drug-membrane interaction studies.