Properties of insoluble monolayers and Langmuir-Blodgett multilayers of a series of isomeric alkylphenyl carboxylic acids

Abstract

The properties of insoluble monolayers and Langmuir-Blodgett multilayers of a series of isomerically pure alkylphenyl carboxylic acids have been investigated. The compounds, of general formula CH 3(CH 2) n − 1 Ph(CH 2) m COOH, where Ph is a phenyl ring, all have m + n = 18 where the number of methylene “spacer” groups between the Ph group and the acid head-group is varied between 0 and 8. The stability of floating monolayers of the isomers depends markedly on the pH of the subphase and is sensitive to the hydrophobic chain structure. The dissociation of the acid groups (measured via attenuated total reflection (ATR) IR in multilayers) with pH is, however, the same for the n = 10 and n = 17 compounds. Quantitative analysis of IR spectra (from both ATR and reflection-absorption IR spectroscopy measurements) of multilayers deposited from calcium-containing subphases has been performed. It is demonstrated that the n = 15 acid is less well ordered and has a larger tilt angle (from the vertical) than the other acids, in line with the conclusions drawn from the π- A isotherms of the floating monolayers.