Two-dimensional dendritic growth of condensed phase domains in spread monolayers of cis-unsaturated fatty acids

Monolayers of cis-unsaturated fatty acids have been investigated at the air /water interface by using surface pressure (pi)-molecular area (A) isotherm s 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 t ake expanded phases on the water surface even at near 0 degreesC. However, first-order phase transitions from expanded to condensed phases have been o bserved for erucic and nervonic monolayers in certain temperature ranges, a ccompanied by nonequilibrium growth of condensed phase domains in homogeneo us fluid phases. The shape of the emerging domains in the erucic acid monol ayers is sixfold and highly dendritic, like snowflakes. The nervonic acid f orms also sixfold but rather rounded, flowerlike domains on the water surfa ce. In contrast to the nonequilibrium patterns observed for the cis-unsatur ated fatty acid monolayers, elaidic acid monolayers exhibit growth of circu lar domains in phase transition regions during compression. Formation of th e branched structures is interpreted as a consequence of higher supersatura tion arising from the packing directivity of cis-long chain into two-dimens ional crystal aggregates. For erucic acid monolayers, the shape relaxation of dendrites after compression is stopped is followed by BAM, where the hig hly branched nonequilibrium structures gradually transform into nearly roun ded equilibrium domains with elapsed time.