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 c an be either horizontal or inclined. When the attractive interaction has a much longer range than the repulsive one, thermodynamics favors the aggrega tion of the molecules of the condensed phase in a single large aggregate, a nd a first order, horizontal transition will occur. When the repulsive inte raction has the much longer range, the free energy of the system becomes sm aller 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, w hen they are little dissociated (sufficiently low pH values), and to the la tter, when they are dissociated (sufficiently high pH values). A thermodyna mic approach is employed to explain the nonhorizontal transition from the l iquid expanded to the condensed phase. A modified Clausius-Clapeyron equati on is derived for this nonconventional first order transition. (C) 2001 Els evier Science B.V. All rights reserved.