Cation-induced monolayer collapse at lower surface pressure follows specific headgroup percolation

Kaushik Das, Bijay Kumar Sah, and Sarathi Kundu
Phys. Rev. E 95, 022804 – Published 13 February 2017

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 pH in the presence of Mg2+,Ca2+,Zn2+,Cd2+, and Ba2+ ions. At lower subphase pH 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 (πc>50mN/m). However, at higher subphase pH, structural changes of stearic acid monolayers occur at relatively lower surface pressure depending upon the specific dissolved ions. Among the same group elements of Mg2+,Ca2+, and Ba2+, only for Ba2+ ions does monolayer to multilayer transition take place from a much lower surface pressure of the monolayer, remaining, however, as a monolayer for Mg2+ and Ca2+ ions. For another same group elements of Zn2+ and Cd2+ 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 (pc) of 0.66. Such ion-specific monolayer collapses are correlated with the 2D lattice percolation model.

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  • Received 11 November 2016
  • Revised 6 January 2017

DOI:https://doi.org/10.1103/PhysRevE.95.022804

©2017 American Physical Society

Physics Subject Headings (PhySH)

Polymers & Soft Matter

Authors & Affiliations

Kaushik Das, Bijay Kumar Sah, and Sarathi Kundu*

  • Soft Nano Laboratory, Physical Sciences Division, Institute of Advanced Study in Science and Technology, Vigyan Path, Paschim Boragaon, Garchuk, Guwahati, Assam 781035, India

  • *Corresponding author: sarathi.kundu@gmail.com

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Issue

Vol. 95, Iss. 2 — February 2017

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