Phase behavior of Langmuir monolayers with ionic molecular heads: Molecular simulations.

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We carried out Monte Carlo simulations in the N,Π,T ensemble of a Langmuir monolayer coarse-grained molecular model. Considering that the hydrophilic groups can be ionized by modulating acid-base interactions, here we study the phase behavior of a model that incorporates the short-range steric and long-range ionic interactions. The simulations were carried out in the reduced temperature range 0.1≤T*<4.0, where there is a competition of these interactions. Different order parameters were calculated and analyzed for several values of the reduced surface pressure in the interval, 1≤Π*≤40. For most of the surface pressures two directions of molecular tilt were found: (i) towards the nearest neighbor (NN) at low temperatures, T*<0.7, and most of the values of Π* and (ii) towards next-nearest neighbors (NNN) in the temperature interval 0.7≤T*<1.1 for Π*<25. We also found the coexistence of the NN and NNN at intermediate temperatures and Π*>25. A low-temperature reentrant disorder-order-disorder transition in the positions of the molecular heads and in the collective tilt of the tails was found for all the surface pressure values. It was also found that the molecular tails arranged forming "rotating patterns" in the temperature interval, 0.5<T*<1.5, at intermediate surface pressures. We estimated the monolayer's surface pressure versus temperature and the temperature versus area per molecule phase diagrams. It was found that the LE↔LC phase transition shifts to smaller temperatures when the molecular heads carry an ion in qualitative agreement with experimental observations of fatty acid monolayers with ionic head groups. Two surface pressure versus area per molecule isotherms were also calculated. At low temperatures near the LC-NN ↔ LC-NNN transitions and at higher temperatures close to the LE ↔ LC transitions. From these isotherms the monolayer's area compression modulus was obtained and its variation ranges in the LE and LC phases were found to be consistent with the experimental values.