EFFECT OF PRESSURE AND TEMPERATURE ON THE PARTIAL MOLAL VOLUME AND COMPRESSIBILITY OF SODIUM DECANOATE MICELLES

Ultrasound measurements have been carried out under high pressure for solutions of sodium decanoate above and below the critical micelle concentration (CMC) at 25°C. From these measurements the partial molal volumes and compressibilities of sodium decanoate in singly dispersed state and in micellar state have been determined in the pressure range 1-1600 bar. It was found that the partial molal volume and compressibility in the micellar state both decreased with increasing pressure, but the corresponding quantities in the singly dispersed state increased with pressure. These results have been interpreted from changes in hydrophobic interactions and in electrostriction of water molecules with increasing pressure. From density measurements and ultrasound measurements in the temperature range 10-40°C the partial molal volumes and compressibilities of sodium decanoate above and below the CMC have been determined as a function of temperature. The partial molal volumes and compressibilities increase with temperature for sodium decanoate both in singly dispersed state and in micellar state. © 1979.