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

N. L. Gershfeld, C. Y O Pak

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The surface viscosity ηs 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.

Original languageEnglish (US)
Pages (from-to)215-220
Number of pages6
JournalJournal of Colloid and Interface Science
Issue number2
Publication statusPublished - Feb 1967


ASJC Scopus subject areas

  • Surfaces and Interfaces
  • Physical and Theoretical Chemistry
  • Colloid and Surface Chemistry

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