Conductivity and fluorescence studies on the micellization properties of sodium cholate and sodium deoxycholate in aqueous medium at different temperatures: Effect of selected amino acids Kuldeep Kumar a , Baljeet S. Patial b , Suvarcha Chauhan a,⇑ a Department of Chemistry, Himachal Pradesh University, Summer Hill, Shimla 171005, Himachal Pradesh, India b Department of Chemistry, BTC DAV College Banikhet (Dalhousie), Chamba 176310, Himachal Pradesh, India article info Article history: Received 28 July 2014 Received in revised form 17 October 2014 Accepted 19 October 2014 Available online 30 October 2014 Keywords: Amino acid Conductivity Critical micelle concentration Fluorescence Sodium cholate Sodium deoxycholate abstract In order to add to the existing knowledge of aqueous solution behavior of bile salts in presence of amino acids, the micellization properties of sodium cholate (NaC) (1 to 20) mmol  kg 1 , and sodium deoxycho- late (NaDC) (0.5 to 10) mmol  kg 1 in 0.1 mol  kg 1 aqueous solution of glycine, leucine, methionine, and histidine have been investigated at different temperatures (293.15 to 318.15) K at intervals of T = 5 K by using conductivity and fluorescence probe studies. The critical micelle concentration (CMC) values have been determined and elucidated in terms of hydrophobicity as well as hydrophilicity of NaC and NaDC in aqueous solution of these additives. Thermodynamic parameters of micellization viz. standard Gibbs free energy (D mic G o ), standard enthalpy (D mic H o ), and standard entropy (D mic S o ) have also been calculated to extract information regarding the nature of micellization of bile salts in aqueous solutions. The (enthal- py + entropy) compensation plots have been interpreted to the contribution of chemical part towards micellization or stability of the micelle formed. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Bile salts, the important bio-surfactants of living organism, are composed of a rigid steroid backbone with a-oriented hydroxyl and b-oriented methyl groups conjugated to an anionic chain. The a-orientation of hydroxyl groups places them on the concave side of the steroid skeleton, with methyl groups positioned on the oppo- site convex side [1]. This conformation of bile salts, which places them in the category of surfactants, is distinct from that of conven- tional surfactants with hydrophilic head group and long hydropho- bic tail. Consequently, the aggregation behavior of bile salts is different from that of conventional surfactants, and they form pri- mary aggregates due to hydrophobic interactions and secondary aggregates at higher concentration due to hydrogen bonding between the hydroxyl groups [2]. This is a generally accepted model for bile salt aggregates and these aggregates are also known as facial micelles [3]. Aggregation of bile salts is responsible for their important physiological functions; for example, they often act as an emulsifier or solubilizing agents for hydrophobic dietary lipids in the intestine or as gallstone solubilizing agents in clinical medi- cine [4–7]. Therefore, different aspects of the aggregation process of bile salts in aqueous medium form a major topic of investigation by using various experimental [8–13] and computational tech- niques [14–16]. Apart from bile salts, phospholipids another impor- tant class of bio-surfactants (L-a-dimyristoylphosphatidycholine (DMPC), L-a-dipalmitoylphosphatidycholine (DPPC), and L-a-dilau- roylphosphatidylethanolamine) have also been explored in mixed monomeric and dimeric cationic surfactant solutions [17,18] with the help of surface tension and fluorescence quenching measurements. The bile salts are expected to interact with ubiquitous proteins by means of aggregate formation which is well supported by in vivo and in vitro investigations [19]. Moreover, the (bile salt + protein) inter- actions in entero-hepatic and gallbladder bile are significant exam- ples of (surfactant + protein) interactions that play a key role in bio- chemically relevant processes [20]. In spite of this, not much is known about the details of (bile salt + protein) aggregates, especially by taking amino acids as model compounds [21–23] and studies on the mono-dispersity [24] and poly-dispersity [25–28] of bile salt aggregates have remained a matter of controversy. By taking these points into consideration, in this study, we aim to investigate the micellization behavior of two bile salts, sodium cholate (NaC), and sodium deoxycholate (NaDC) in aqueous solu- tion of some of different types of amino acids, namely, glycine (Gly), leucine (Leu), methionine (Met), and histidine (His) at http://dx.doi.org/10.1016/j.jct.2014.10.014 0021-9614/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +91 177 2830803; fax: +91 177 2830775. E-mail address: chauhansuvarcha@rediffmail.com (S. Chauhan). J. Chem. Thermodynamics 82 (2015) 25–33 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct