Abstract
A Langmuir monolayer can be considered as a two-dimensional (2D) sheet at higher surface pressure which structurally deform with mechanical compression depending upon the elastic nature of the monolayer. The deformed structures formed after a certain elastic limit are called collapsed structures. To explore monolayer collapses at lower surface pressure and to see the effect of ions on such monolayer collapses, out-of-plane structures and in-plane morphologies of stearic acid Langmuir monolayers have been studied both at lower (≈6.8) and higher (≈9.5) subphase in the presence of , and ions. At lower subphase and in the presence of all cations, the stearic acid monolayer remains as a monolayer before collapse, which generally takes place at higher surface pressure . However, at higher subphase , structural changes of stearic acid monolayers occur at relatively lower surface pressure depending upon the specific dissolved ions. Among the same group elements of , and , only for ions does monolayer to multilayer transition take place from a much lower surface pressure of the monolayer, remaining, however, as a monolayer for and ions. For another same group elements of and ions, a less covered bilayer structure forms on top of the monolayer structure at lower surface pressure, which is evidenced from both x-ray reflectometry and atomic force microscopy. Fourier transform infrared spectroscopy confirms the presence of two coexisting conformations formed by the two different metal-headgroup coordinations and the monolayer to trilayer or multilayer transformation takes place when the coverage ratio of the two molecular conformations changes from the critical value of . Such ion-specific monolayer collapses are correlated with the 2D lattice percolation model.
- Received 11 November 2016
- Revised 6 January 2017
DOI:https://doi.org/10.1103/PhysRevE.95.022804
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