The surface chemistry of monooctadecyl phosphate at the air/water interface. A study of molecular aggregation in monolayers

The surface viscosity η s of monooctadecyl phosphate (m-OP) monolayers increased linearly with time, and this rate increased as the pH was lowered. For the pH range where film dissociation occurred, the monolayers contracted. Increasing the temperature from 12°C. to 22°C. also resulted in a small contraction of the monolayer. By application of a modified Clapeyron equation, the process of spreading of the m-OP film from the crystal on water was found to involve a large decrease in entropy in the range of pH 3–5.8. Maximum decrease in the entropy of spreading occurred at pH 4.7, the pH region of 50% dissociation of m-OP. Crystals of m-OP became mesomorphic in water at 22°C., forming fiber-like structures which were birefringent. The fiber-like structures were not evident at 12°C., though the crystals were still swollen by water. It is concluded from these results that the structure of m-OP monolayers on water is highly complex and involves pH-dependent phosphate-phosphate as well as phosphate-water interactions.