Phase transition in adsorbed monolayers of 2-hydroxyethyl laurate at the air-water interface

Kinetics of adsorption of a sparingly water soluble amphiphile, 2-hydroxyethyl laurate (2-HEL), at the air-water interface is followed by the measurement of surface pressure (π) with time (t). Simultaneous change in the surface morphology of the adsorbed monolayers is investigated with Brewster angle microscopy (BAM). A cusp point in each of the π–t curves indicates the start of a phase transition. Highly ordered condensed phase domains having stripes in their structures are observed to grow during the phase transition. One end of the separate lines of stripes gather to a point on the circular peripheral line of the domain like a vertex of the boojum. However, another end of the separate lines disappear at the peripheral line keeping the stripe width almost constant. After the completion of the phase transition, surface pressure starts to increase again and reaches a final equilibrium value after a long time. Spread monolayers of the same amphiphile can be obtained by the usual solvent spreading technique and compared with the adsorbed monolayers. The critical surface pressures necessary to cause the phase transition are almost the same for both adsorbed and spread monolayers. Although the shapes of the domains are circular for both cases, the size of those in the spread monolayers is considerably smaller exhibiting no internal stripes than those in adsorbed monolayers.