Effect of the hydroxy group position on the monolayer characteristics of hydroxypalmitic acids

The effect of the OH-group position on the monolayer characteristics of hydroxypalmitic acids is studied by coupling the results of surface pressure–area (π–A) isotherm measurements and Brewster angle microscopy (BAM). In the present work, the phase behaviour and the domain morphology of 2-, 3- and 9-hydroxypalmitic acid monolayers are investigated. They deviate considerably but in different way from the adequate monolayer characteristics of the usual non-substituted amphiphiles. General differences exist also between the monolayer characteristics of palmitic acid OH-substituted near the COOH-group (in 2- or 3-position) and those when the OH-substitution is in or near the mid-position of the alkyl chain (in 9-position). This is demonstrated by differences in the temperature dependence of the phase transition pressure. The values of enthalpy and entropy change for the main phase transition are discussed and suggest in the case of 9-hyroxypalmitic acid, a structure improvement in the fluid monolayer phase at compression to the phase transition point because of reorientation into a perpendicular position. e two types in the position of the OH-substitution, large differences in the morphology of the condensed phase domains indicate adequate differences in the long-range orientational order. In the case of OH-substitution near the COOH-head-group, irregular domain growth occurs while at OH-substitution in or near the mid-position of the alkyl chain, regular patterning of the domains indicate a high ordering. The reasons for the differences in the ordering are discussed.