An AFM study of lipid monolayers. 1. Pressure-induced phase behavior of single and mixed fatty acids

Monolayers of palmitic (C16:0) and lignoceric acid (C24:0) and their equimo lar mixture were transferred to a hydrophilic mica substrate at various sur face pressures and investigated by means of atomic force microscopy (AFM) i n contact and lateral force modes. The first-order transition of lignoceric acid gives a plateau region, representing a liquid expanded to liquid cond ensed phase transition in the pressure-area isotherm. This was visualized b y AFM as stripes of a condensed phase within the expanded phase, exhibiting a small height difference but a significant difference in friction. The co rresponding phase transition of the palmitic acid was continuous, and no ch anges of the Langmuir-Blodgett films with respect to pressure were observed with AFM. Both the surface pressure-area isotherms and the direct observat ions of domains of ir regular size and shape using the AFM showed that lign oceric and palmitic acid were immiscible. The height difference between the domains was 1.1 nm corresponding to the difference in hydrocarbon chain le ngth of the two fatty acids.