Determination of the Dimerization Constant in Self-Associating Systems

The self-association of Methylene Blue (MB), a cholic acid derivative, chlorpromazine hydrochloride (CPZ), and octaethylene glycol decyl ether has been investigated by frontal gel filtration chromatography (GFC) and spectrophotometry at 25 °C. The stepwise aggregation constant ki is estimated as a function of aggregation number i from the centroid volume Vc of the GFC profile by using appropriate association models. The dimerization constants k2 for MB and CPZ determined by spectrophotometry are close to those from Vc values. The dimerization constants are also estimated from the concentration dependence of the peak volume Vp of the derivative GFC profile. The k2 values determined from the Vp values are slightly larger than those from the Vc values. This discrepancy is ascribed to the approximation of the Vp approach. The minimum multimerization concentration (mmc), above which multimers higher than trimer are formed, is determined from each of Vc, Vp, and absorption spectral measurements. The relation between mmc and the critical micelle concentration (cmc) depends on the degree of criticalness of cmc. This degree of criticalness increases as the aggregation number at the maximum ki value increases.