Thermodynamics of the interactions of some amino acids and peptides with dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide

Abstract

The values of apparent molar volume V2,ϕ and apparent molar adiabatic compressibility KS,2,ϕ of amino acids glycine, l-alanine, dl-α-amino-n-butyric acid, l-valine, l-leucine and peptides glycyl-glycine, glycyl-glycyl-glycine and glycyl-leucine have been determined in aqueous solutions of cationic surfactants dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) by means of density and sound velocity measurements. The heat evolved or absorbed (q) during the course of interactions of amino acids and peptides with the aqueous solutions of surfactants were determined by isothermal titration calorimetry at T=298.15K. The values of standard partial molar volume V2,m0 and standard partial molar adiabatic compressibility Ks,2,m0 at infinite dilution were calculated from the values of V2,ϕ and KS,2,ϕ. Similarly the values of limiting enthalpies of dilution (ΔdilH0) of the amino acids/peptides were calculated from heat evolved or absorbed during calorimetric experiments. The standard partial molar quantities of transfer from water to aqueous surfactant solutions have been used to identify the interactions of amino acids and peptides with surfactants in terms of ionic–ionic, ionic–hydrophobic and hydrophobic–hydrophobic group interactions.