Werawatganone Pornpen, Wurster Dale Eric
Division of Pharmaceutics, College of Pharmacy, and Obermann Center for Advanced Studies, University of Iowa, Iowa City, Iowa 52242, USA.
J Pharm Sci. 2007 Feb;96(2):448-58. doi: 10.1002/jps.20800.
The change in the hexadecyltrimethylammonium bromide (CTAB) critical aggregation concentration (CAC) was studied in the presence of various concentrations and grades of hydroxypropylmethyl cellulose (HPMC) using surface tension measurement (duNoüy ring and Wilhelmy plate) and oil red O solubilization. According to the surface tension methods, the CAC was higher than the CTAB critical micelle concentration (CMC). CAC and CMC were not different when the solubilization method was used. Micellar solutions of CTAB have been found to accelerate the hydrolysis of alpha-naphthyl acetate (alpha-NA) by o-iodosobenzoic acid (IBA), a strong nucleophile. Pseudo-first-order kinetics were utilized for rate constant determination. The observed rate constants for the degradation of alpha-NA in the presence of varying CTAB concentrations with and without HPMC were analyzed according to the pseudophase model. The micellar rate constants and the micellar binding constants for the substrates were obtained. The presence of HPMC retarded the reaction rate, and the rate constant decreased as the polymer concentration increased. However, there was no obvious difference in the observed rate constants among the different grades of HPMC (Methocel E5, Methocel E15, Methocel E50). The decrease in the rate constant was likely due to the polymer-micelle interaction interfering with substrate binding to the CTAB micelles.
使用表面张力测量法（杜诺伊环法和威廉米板法）和油红O增溶法，研究了在不同浓度和等级的羟丙基甲基纤维素（HPMC）存在下十六烷基三甲基溴化铵（CTAB）临界聚集浓度（CAC）的变化。根据表面张力方法，CAC高于CTAB临界胶束浓度（CMC）。使用增溶法时，CAC和CMC没有差异。已发现CTAB胶束溶液可加速强亲核试剂邻碘苯甲酸（IBA）对α-萘乙酸（α-NA）的水解。采用伪一级动力学来测定速率常数。根据假相模型分析了在有和没有HPMC的情况下，不同CTAB浓度存在时α-NA降解的观测速率常数。获得了底物的胶束速率常数和胶束结合常数。HPMC的存在阻碍了反应速率，且速率常数随聚合物浓度的增加而降低。然而，不同等级的HPMC（美多秀E5、美多秀E15、美多秀E50）之间观测到的速率常数没有明显差异。速率常数的降低可能是由于聚合物-胶束相互作用干扰了底物与CTAB胶束的结合。
