ABSTRACT
Critical micelle concentrations (CMCs) of the penicillins cloxacillin and dicloxacillin in water were determined by conductivity measurements over the temperature range 288.15 to 313.15 K. Both penicillins showed minimum CMCs at temperatures close to 298.15 K. Thermodynamic parameters of aggregate formation were derived from the variation of the CMC with temperature using a modified form of the mass action model applicable to systems of low aggregation number. Values for the enthalpy of aggregate formation, DeltaH(0)(m), calculated by this method showed that the aggregation of both cloxacillin and dicloxacillin became increasingly exothermic with increase in temperature. The predicted DeltaH(0)(m) at 298.15 K was in good agreement with the value determined experimentally by calorimetry for each drug. Copyright 2000 Academic Press.
ABSTRACT
The self-association of propranolol hydrochloride in aqueous solution has been studied as a function of temperature. The critical concentration (C*) and the degree of ionization (alpha) were determined by conductivity measurements at temperatures over the range 298.15 to 313.15 K. The enthalpy change on aggregation in water was measured by microcalorimetry. To calculate changes in the thermodynamic properties of aggregation the mass action model for high and low aggregation numbers was applied, the latter model giving better agreement between experimental and theoretical enthalpy changes. Copyright 1999 Academic Press.
ABSTRACT
The self-association of n-hexyltrimethylammonium bromide (C6TAB) in aqueous solution has been examined as a function of temperature and electrolyte concentration. The critical micelle concentration (CMC) and the degree of counterion binding (beta) were determined by conductivity measurement at temperatures over the range 288.15-318.15 K. Ultrasound velocity measurements were used to obtain the CMC in water and in a range of concentrations of electrolyte (0.1 to 0.6 mol kg-1 NaBr) and static light scattering to obtain the aggregation number and the degree of counterion binding in water at 298.15 K. The enthalpy change on micellization in water was measured by microcalorimetry. Apparent adiabatic compressibilities were calculated from a combination of density and ultrasound velocity measurements. Changes in the thermodynamic properties on micellization were determined by applying the mass action model; good agreement was found between experimental and theoretical enthalpy changes. From comparison with the properties of other n-alkyltrimethylammonium bromides it has been shown that the CMC of C6TAB in water is lower than that predicted from the linear relationships between CMC and the number of carbon atoms in the alkyl chain. Similarly, the standard Gibbs energy of micellization is less negative than predicted, and the degree of counterion binding is much lower than for other CnTABs. It is suggested that the anomalous behavior of C6TAB is a consequence of the more highly organized core of the aggregates of very low aggregation number (3-4) and the high degree of exposure of the micellar components to the aqueous environment. Copyright 1998 Academic Press.
