Two-dimensional dendritic growth of condensed phase domains in spread monolayers of cis-unsaturated fatty acids
Langmuir, 17 (15):
4602--4609 (July 2001)455LX
Times Cited:17
Cited References Count:36.DOI: Doi 10.1021/La010236w
Abstract
Monolayers of cis-unsaturated fatty acids have been investigated at the air/water interface by using surface pressure (pi)-molecular area (A) isotherms and Brewster angle microscopic (BAM) observation The film materials used are oleic, gondoic, erucic, and nervonic acid. Elaidic acid, a trans-isomer of the oleic acid, is also employed for comparison. The measurements have been performed in a wide temperature range. Oleic and gondoic acid always take expanded phases on the water surface even at near 0 degreesC. However, first-order phase transitions from expanded to condensed phases have been observed for erucic and nervonic monolayers in certain temperature ranges, accompanied by nonequilibrium growth of condensed phase domains in homogeneous fluid phases. The shape of the emerging domains in the erucic acid monolayers is sixfold and highly dendritic, like snowflakes. The nervonic acid forms also sixfold but rather rounded, flowerlike domains on the water surface. In contrast to the nonequilibrium patterns observed for the cis-unsaturated fatty acid monolayers, elaidic acid monolayers exhibit growth of circular domains in phase transition regions during compression. Formation of the branched structures is interpreted as a consequence of higher supersaturation arising from the packing directivity of cis-long chain into two-dimensional crystal aggregates. For erucic acid monolayers, the shape relaxation of dendrites after compression is stopped is followed by BAM, where the highly branched nonequilibrium structures gradually transform into nearly rounded equilibrium domains with elapsed time.