Surface miscibility of Gemini surfactants and DOPE in binary mixed monolayers

Surface pressure (π)–molecular area (A) isotherms were gathered to characterize the packing of binary mixed Langmuir monolayers of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) and one of two Gemini surfactants (GS), N,N-bis(dimethyloctadecyl)-1,7-nonanediammonium dibromide (18-7-18) or 1,9-bis(octadecyl)-1,1,9,9-tetramethyl-5-amino-1,9-nonanediammonium dibromide (18-7NH-18) of varying molar fractions. Information about miscibility behavior was derived from the π–A curves by examining the excess free energy of mixing (ΔG_exc) that was calculated through the surface area additivity rule. Surface compressibility modulus (C_s⁻¹) was also used to characterize intermolecular interactions. Mutual interactions between GS and DOPE were analyzed in terms of excess Gibbs energy of mixing and the value of this parameter depended strongly on the composition of the mixed film. GS and DOPE are generally miscible as DOPE reduces intermolecular repulsion between highly charged GS molecules leading to the formation of more densely packed mixed monolayers. However, GS and DOPE are immiscible in equimolar mixtures due to tail group packing mismatch between saturated and unsaturated alkyl chains. The prevalence of attractive synergistic interactions in the monolayers studied differs from a previous finding of antagonistic mixing behavior in GS/DOPE micelles. These results contribute to the understanding of GS-lipid interactions and packing that are critical to the in vitro and in vivo stability of liposomes composed of these molecules used for non-viral gene therapy applications.