Thermodynamics of Adsorption of L-Amino Acids at Oil–Water Interfaces

Abstract

Lowering of the interfacial tension of heptane–water, benzene–water, and nitrobenzene–water interfaces due to addition of 20 different amino acids to the aqueous phase has been measured. From the plot of surface pressure against molar concentration of amino acids, the initial slope and the surface excess Γ₂ ¹ for different amino acids have been calculated using the Gibbs adsorption equation. Γ₂ ¹ for most amino acids at benzene–water and heptane–water interfaces was found to be positive, with only a few being negative. At the nitrobenzene–water interface, both positive and negative Γ₂ ¹ values were observed. The area per adsorbed molecule at surface saturation A _m was found to vary widely, indicating different orientations of amino acid molecules at the interfaces. Using the integrated form of the Gibbs adsorption equation, the standard Gibbs energy change ΔG ^o in kJ-m² of the adsorbed surface have been calculated for various interfaces. ΔG ^o was found to vary linearly with the Γ₂ ¹ of different amino acids and the slope of the line, designated as −ΔG _B ⁰ was found to be 22 kJ-mol⁻¹ for heptane–water, 23.2 kJ-mol⁻¹ for benzene–water, and 19.3 kJ-mol⁻¹ for nitrobenzene–water interfaces, irrespective of the nature of the amino acid. The origin of the linear scale of the Gibbs energy for heptane–water, benzene–water and nitrobenzene–water interfaces has been discussed in terms of hydrophobic and other interactions.