Mixed monolayers of straight and branched chain fatty acids Part 1: Mixed monolayers of n-tridecanoic, n-tetradecanoic and n-pentadecanoic acids with the iso-fatty acids 13-methyltetradecanoic acid, 14-methylpentadecanoic acid, 15-METHYLHEXADECANOIC ACID (cas 1603-03-8), 16-methylheptadecanoic acid and 17-methyloctadecanoic acid

Add time:07/17/2019    Source:sciencedirect.com

A study has been initiated to investigate the behavior of mixed straight-chain/iso-branched-chain fatty acid monolayer systems. In this publication we examine the effects of changing the iso-branched chain length while maintaining the chain-length of the straight-chain fatty acid constant. Three straight-chain fatty acids were selected: n-tridecanoic, n-tetradecanoic and n-pentadecanoic acid. The second component, iso-branched fatty acids selected were 13-methyltetradecanoic, 14-methylpentadecanoic, 15-methylhexadecanoic, 16-methylheptadecanoic and 17-methyloctadecanoic. The behavior of these systems is characterized primarily in terms of the shift of the liquid expanded/liquid condensed phase transition of the straight-chain acid produced by the addition of the iso-branched acids. The shifts are compared in terms of a relative chain-length number which is defined as the number of carbon atoms by which the pentadecanoic acid exceeds, or falls short of the branching carbon atom of the iso-branched acid. The shifts of the straight-chain acid phase change were found to range from large positive to smaller negative surface pressure changes as the relative chain-length number changed from +2 to −2. The two phase region width showed a corresponding dependency. Finally, it was found that, in spite of compensatory factors, the internal energies of the liquid expanded/liquid condensed phase change for iso-branched acids are much smaller than those of the corresponding straight-chain acids and for the mixed systems the (negative) values of the internal energy change increase as the iso-branched chain increases in length. This latter observation shows that the effects of chain branching decrease with an increase in the overall branched chain length or a decrease in the relative chain-length number.

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