Eutectic transformation in mixed monolayers of long normal chain fatty acids with 12-hydroxyoctadecanoic acid

Abstract

Surface pressure of mixed monolayers composed of fatty acids and DL-12-hydroxyoctadecanoic acid (12HOA) was measured at various compositions and temperatures. Two-dimensional phase diagrams were constructed. Tridecanoic acid (C 13)-12HOA, tetradecanoic acid (C 14)-12HOA, pentadecanoic acid (C 15)-12HOA, hexadecanoic acid (C 16)-12HOA, and octadecanoic acid (C 18)-12HOA systems showed eutectic transformations in mixed monolayers. Surface pressure of eutectic point decreased with increase in the carbon numbers of fatty acid. The mole fraction of 12HOA at the eutectic point increased with increase in the carbon numbers. However, eicosanoic acid (C 20)-12HOA and docosanoic acid (C 22)-12HOA systems were not of eutectic type; they were immiscible with each other in the mixed monolayer. Apparent molar entropy and energy changes of C 14-12HOA and C 15-12HOA systems were evaluated by previously mentioned thermodynamic treatment. The entropy and energy changes of C 14 and C 15 decreased slightly and steeply with increase in the mole fraction of 12HOA, χ 2 π , respectively. the values of 12HOA increased gradually with increase in χ 2 π .