Colloids and Surfaces B: Biointerfaces 59 (2007) 179–183 Determination of critical micellar concentrations of cholic acid and its keto derivatives Mihalj Poˇ sa a , Slavko Kevreˇ san b,∗ , Momir Mikov c , Vera ´ Cirin-Novta d , Costel Sˆ arbu e , Ksenija Kuhajda d a Department of Pharmacy, Faculty of Medicine, University of Novi Sad, Hajduk Veljkova 3, 21000 Novi Sad, Serbia b Faculty of Agriculture, University of Novi Sad, Trg Dositeja Obradovi´ ca 8, 21000 Novi Sad, Serbia c School of Pharmacy, University of Otago, Dunedin, New Zealand d Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovi´ ca 3, 21000 Novi Sad, Serbia e Faculty of Chemistry and Chemical Engineering, Babe¸ s-Bolyai University, Arany Janos 11, 3400 Cluj-Napoca, Romania Received 2 February 2007; received in revised form 18 April 2007; accepted 9 May 2007 Available online 18 May 2007 Abstract The critical micellar concentration (CMC) values of keto derivatives of cholic acid (3,12-dihydroxy-7-oxo-5-cholanoic acid, 3,7-dihydroxy-12-oxo-5-cholanoic acid, 12-hydroxy-3,7-dioxo-5-cholanoic acid, 3-hydroxy-7,12-dioxo-5-cholanoic acid, 3,7,12- triketo-5-cholanoic acid) and cholic acid itself, were determined. Replacement of hydroxyl groups in cholic acid molecule with keto groups yields the derivatives whose CMC values increase with increase in the number of keto groups introduced. The CMCs of derivatives with the same number of keto groups but at different positions do not differ significantly. The relationship between the number of keto groups in the molecule of cholic acid keto derivatives and CMC value can be described by the following equation: CMC = 43 number of keto groups + 14.667. The effect of NaCl concentration on CMC increases with increase in the number of keto groups. © 2007 Elsevier B.V. All rights reserved. Keywords: Cholic acid; Keto derivatives of cholic acid; CMC; Water-insoluble dye solubilization 1. Introduction Bile acid salts in aqueous solutions are present in the form of monomers, surrounded with a solvation shell in which, as in a two-dimensional space, the water molecules lose one degree of freedom of translatory motion, yielding a decrease of entropy [1,2]. According to a theoretical model [3] the hydroxyl groups from the  side of the bile acid steroid nucleus form hydrogen bonds with water molecules, which compensates for the negative change of entropy, so that the overall change of free enthalpy is smaller than zero. However, no such stabilization takes place on the  side, which is of a hydrophobic character. With increase in the number of monomer molecules in the system increases also the number of water molecules in the sol- vation shell from the hydrophobic side of the steroid nucleus, which gradually leads to increasing destabilization of the sys- ∗ Corresponding author. Tel.: +381214853272; fax: +38121450857. E-mail address: kevresan@polj.ns.ac.yu (S. Kevreˇ san). tem. Because of that bile acid molecules enter the boundary layer, whereby the hydrophobic part of the molecule is oriented toward the lipophilic phase or toward the air, thus releasing molecules from the  side of the steroid nucleus. As long as the incorporation of bile acid monomers in the boundary layer decreases, the surface tension (γ ) of the solution (water) decreases too [4]. However, at a certain concentration, surface saturation is attained and no more monomers can enter the boundary layer, so that the surface tension does not change as a function of concentration. According to Tanford [5], above this concentration the monomers form aggregates by bind- ing via their hydrophobic parts, whereby water molecules are released from the hydration shell and go to the solution bulk. The monomer concentration at which such situations arises is called critical micellar concentration (CMC), which means that the formation of aggregates (micelles) is a consequence of the entropy factor and not of the existence of hydrophobic attraction forces [6–8] between  sides of the steroid nucleus. Study of the formation of micelles of bile acids, that is the determination of their CMC values, is of great importance for 0927-7765/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2007.05.008