Apparent molar volume and ultrasonic studies on some bile salts in water–aprotic solvent mixtures

• Density, ultrasonic velocity and ultrasonic absorption are measured in sodium cholate and sodium deoxycholate in water and water–10% (v/v) of aprotic solvent mixtures at 303.15 K and at 2 MHz frequency, respectively.
• From the measured data, various parameters such as apparent molar volume, apparent molar compressibility, hydration number, equivalent conductivity, adiabatic compressibility and intermolecular free length are calculated.
• The negative values of apparent molar volume have been discussed in terms of structured water present around the solute molecules.
• The negative values of apparent molar compressibility have been discussed in terms of hydrogen bonding and hydration.
• The variation of ultrasonic absorption with concentration of bile salts in water and water–aprotic solvent mixtures is explained in terms of making and breaking of hydrogen bonds when the ultrasonic waves pass through the medium.
• The CMC values obtained for bile salts in water and water–aprotic solvent mixtures in the present study agrees well with the equivalent conductivity measurements.

Apparent molar volume, ultrasonic velocity and absorption studies have been carried out in aqueous solutions of some bile salts namely sodium cholate and sodium deoxycholate containing 10% (v/v) of dioxane, dimethyl sulphoxide, dimethyl formamide, tetrahydrofuran and acetonitrile. These studies are carried out in the bile salt concentration of 1–15 mM at a fixed frequency of 2 MHz and at a fixed temperature of 303.15 K. The variation of apparent molar volume and ultrasonic velocity in aqueous solutions of bile salts containing 10% (v/v) of dioxane, dimethyl sulphoxide, dimethyl formamide, tetrahydrofuran and acetonitrile with bile salt concentration exhibiting break at critical micelle concentration (CMC). The observed absorption increases with increasing bile salt concentration and also exhibiting a break at CMC similar to velocity curve. The results are discussed in terms of formation of bile salt micelles through hydrophobic interaction and hydrogen bonding. The results obtained from apparent molar volume, ultrasonic velocity and apparent molar compressibility studies are confirmed by the equivalent conductance measurements carried out in the solutions of bile salts.