Cantin S, Fauré MC, Perrot F, Goldmann M. Structure and kinetics of fatty acid Langmuir monolayers on zinc salt solutions.
J Phys Chem B 2013;
117:16275-82. [PMID:
24283749 DOI:
10.1021/jp407047a]
[Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The adsorption of zinc cations under behenic acid Langmuir monolayers was investigated by means of isotherm measurements, grazing incidence X-ray diffraction and Brewster angle microscopy. The structure of the films was characterized as a function of Zn(2+) concentration, for three different counterions (chloride, iodide, bromide) and at two subphase pHs (5.5 and 7.5). At pH 5.5 and in the studied concentration range, Zn(2+) adsorption leads to a condensation of the fatty acid monolayer with the same phase transitions as over pure water. In contrast, at higher pH the organic X-phase is evidenced immediately above a concentration threshold without any ion organization. Even though Cu(2+) and Zn(2+)cations induce both the fatty acid X-phase, the kinetics of its formation appears strongly different. Indeed, as for Mg(2+) and Cd(2+), the intermediate new I-structure is evidenced in the course of Zn(2+) adsorption although superstructures are observed only for Mg(2+) and Cd(2+). However, for Zn(2+), the I-phase evolves to the final state through a new structure called X' and a continuous X'-X transition. Finally, any effect of the counterion is evidenced neither during the kinetic process nor in the final state.
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