Interactions of some amino acids and glycine peptides with aqueous sodium dodecyl sulfate and cetyltrimethylammonium bromide at T=298.15 K: a volumetric approach

Abstract

The apparent molar volumes V φ of glycine, alanine, valine, leucine, and lysine have been determined in aqueous solutions of 0.05, 0.5, 1.0 mol · kg −1 sodium dodecyl sulfate (SDS) and 1.0 mol · kg −1 cetyltrimethylammonium bromide (CTAB) by density measurements at T=298.15 K. The apparent molar volumes have also been determined for diglycine and triglycine in 1 mol · kg −1 SDS and CTAB solutions. These data have been used to calculate the infinite dilution apparent molar volumes V 2 0 for the amino acids and peptides in aqueous SDS and CTAB and the standard partial molar volumes of transfer (Δ tr V 2, m 0) of the amino acids and peptides to these aqueous surfactant solutions. The linear correlation of V 2 0 for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups (NH 3 +, COO −), CH 2 group and other alkyl chains of the amino acids to V 2 0. The results on the partial molar volumes of transfer from water to aqueous SDS and CTAB have been interpreted in terms of ion–ion, ion–polar and hydrophobic–hydrophobic group interactions. The volume of transfer data suggests that ion–ion or ion–hydrophilic group interactions of the amino acids and peptides are stronger with SDS compared to those with CTAB. Comparison of the hydration numbers of amino acids calculated in the present studies with those in other solvents from literature shows that these numbers are almost the same at 1 mol · kg −1 level of the cosolvent/cosolute. Increasing molality of the cosolvent/cosolute beyond 1 mol · kg −1 lowers the hydration number of the amino acids due to increased interactions with the solvent and reduced electrostriction.