Adsorption Free Energies of Imidazolinium-Type Surfactants in Infinite Dilution and in Micellar State on Gold Surface

Y Kurapati and S Sharma, JOURNAL OF PHYSICAL CHEMISTRY B, 122, 5933-5939 (2018).

DOI: 10.1021/acs.jpcb.8b02358

We report adsorption behavior of imidazolinium-type surfactant molecules in different aggregation states on metal water interfaces studied using all-atom molecular dynamics simulations. Surfactant molecules with two different alkyl tail lengths, a 10-carbon and a 17-carbon tail (henceforth referred to as imid-10 and imid-17, respectively), have been considered. Six layers of face-centered cubic lattice of gold atoms submerged in water represent the metal-water interface. Our simulations reveal that, in infinite dilution, both types of surfactant molecules strongly adsorb onto the metal-water interface in a configuration with their alkyl tail lying parallel to the surface. This adsorption occurs through a barrierless transition with an adsorption free energy of similar to 30 k(B)T and is found to be enthalpically driven and entropically unfavorable. Surfactant micelles, on the other hand, experience a long-range repulsion from the metal surface at distances as large as 50-60 angstrom due to the presence of a large "corona" around the micelles that comprises counterions and their solvation layer. Surfactant micelles have an adsorption free energy barrier of similar to 13-16 k(B)T, which is associated with the removal of adsorbed water from the metal surface. Micelles are thermodynamically stable in the bulk aqueous phase, and the adsorbed micellar state is only metastable.

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