Nonconventional first order transitions in insoluble monolayers of surfactants

Depending upon the ranges of attractive and repulsive interactions between molecules, the transition from the liquid expanded to the condensed phase can be either horizontal or inclined. When the attractive interaction has a much longer range than the repulsive one, thermodynamics favors the aggregation of the molecules of the condensed phase in a single large aggregate, and a first order, horizontal transition will occur. When the repulsive interaction has the much longer range, the free energy of the system becomes smaller when the new phase is dispersed as islands in the old one, instead of forming a separate bulk phase, and a nonconventional first order, inclined transition will take place. It is likely that the phospholipid monolayers belong to the latter kind, while the fatty acid monolayers to the former, when they are little dissociated (sufficiently low pH values), and to the latter, when they are dissociated (sufficiently high pH values). A thermodynamic approach is employed to explain the nonhorizontal transition from the liquid expanded to the condensed phase. A modified Clausius–Clapeyron equation is derived for this nonconventional first order transition.