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

Kinetics of adsorption of a sparingly water soluble amphiphile, 2-hydroxyet hyl laurate (2-HEL), at the air-water interface is followed by the measurem ent of surface pressure (pi) with time (t). Simultaneous change in the surf ace morphology of the adsorbed monolayers is investigated with Brewster ang le microscopy (BAM). A cusp point in each of the pi-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 transitio n. One end of the separate lines of stripes gather to a point on the circul ar peripheral line of the domain like a vertex of the boojum. However, anot her 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 ob tained by the usual solvent spreading technique and compared with the adsor bed monolayers. The critical surface pressures necessary to cause the phase transition are almost the same for both adsorbed and spread monolayers. Al though the shapes of the domains are circular for both cases, the size of t hose in the spread monolayers is considerably smaller exhibiting no interna l stripes than those in adsorbed monolayers. (C) 2000 Elsevier Science B.V. All rights reserved.