Surface tension and UV–visible investigations of aggregation and adsorption behavior of NaC and NaDC in water–amino acid mixtures

Abstract

Aqueous micellar solutions of bile salts, sodium cholate (NaC) (1–30mmolkg−1) and sodium deoxycholate (NaDC) (1–17mmolkg−1) in the absence and presence of amino acids viz. glycine, leucine, methionine, and histidine (0.1molkg−1) have been examined by surface tension and UV–visible measurements at different temperatures (293.15–318.15K). From the surface tension study, critical micelle concentration (CMC) values of bile salts have been calculated and reviewed in terms of effect of amino acids on the hydrophobic character of bile salt–amino acid complex. Further, the adsorption behavior of NaC and NaDC in aqueous solution of amino acids has been analyzed with the help of interfacial parameters like maximum surface excess concentration (Γmax), minimum area per surfactant molecule (Amin), surface pressure at CMC (ΠCMC), efficiency in reducing surface tension (pC20), and adsorption at the air/water interface relation to micellization (CMC/C20). The thermodynamic parameters, standard free energy of micellization (ΔGm°), standard Gibb’s free energy of adsorption (ΔGad°), and standard free energy of transfer (ΔGtr°) have also been calculated. UV–visible studies, by using pyrene as a probe, have also been carried out in order to substantiate the CMC values derived from surface tension studies.