Interfacial properties of the antifungal iturins on various electrolyte solutions

Abstract We have studied the interfacial behavior of antifungal iturins (principally bacillomycin F, mycosubtilin, and iturin A) in solution and spread as monomolecular films on aqueous phases containing various chloride salts. The presence of electrolytes lowers the interfacial concentration (surface excess) of iturin solutions. The compression isotherm curves are not superimposable, and for all the compounds the stability of the monolayer is higher on KCl as attested by the transition pressure (for the liquid-expanded part) and by the collapse pressure values. The reduction of the molecular free energy of spreading Δ G s , that measures the affinity of the polar cycle of the lipopeptide for the aqueous phase components, is maximum on KCl in the case of mycosubtilin. The energy of compression Δ G c , that measures the intermolecular forces between the film forming molecules, is generally higher on KCl. The compression leads to more ordered structures when mycosubtilin and iturin A are spread on KCl. When spread on CaCl 2 , iturin A and bacillomycin F molecules can adopt only two kinds of arrangements, as shown by the convergence of the LE parts of their isotherms. These data are interpreted as interactions between iturins and ions and, more precisely, as discriminating interactions between iturins and the various cations with a special role for K + .