The hydrolysis of C12 primary alkyl sulfates in concentrated aqueous solutions. Part 2. Influence of alkyl structure on hydrolytic reactivity in concentrated aqueous mixtures of sodium primary alkyl sulfates: 1-benzoyl-3-phenyl-1,2,4-triazole as a probe of water activity
作者:Donald Bethell、Roger E. Fessey、Jan B. F. N. Engberts、David W. Roberts
DOI:10.1039/b102958b
日期:——
The kinetics of the hydrolysis of aqueous solutions of three sodium C12-alkyl sulfates (SXS), sodium 2-methylundecyl sulfate (SMS), sodium cycloundecylmethyl sulfate (SCS) and sodium 2-pentylheptyl sulfate (SPS), has been investigated at concentrations up to 70% and compared with the behaviour of sodium dodecyl sulfate (SDS). The same kinetic form as previously described for SDS was observed, namely, autocatalysis by protons generated via hydrogen sulfate ion, but there were substantial variations in the reactivity as the alkyl structure changed; β-branching reduced the reactivity, particularly for SMS which was the least reactive of the surfactants studied. The patterns of reactivity by the uncatalysed and hydrogen-ion catalysed pathways for the different SXS were rather similar, but it is argued that the results are consistent with an SN2 mechanism for uncatalysed hydrolysis and the concerted SO3 cleavage (or transfer to a pre-associated water molecule)/proton transfer mechanism for the catalytic route, as previously proposed for SDS. Changes in the microenvironment of the sulfate group in aggregates formed from the different SXS are seen as being responsible for much of the rate variation. Attempts have been made to establish the dependence of observed rate constants in dilute solutions of SXS above the c.m.c. on the water activity as indicated empirically by the rate of pH-independent hydrolysis of 1-benzoyl-3-phenyl-1,2,4-triazole (BPT) in the same solutions. It appears, however, that BPT hydrolysis is not a useful guide to water activity in SXS solutions and values of d(ln k)/d(ln [H2O]) are generally much larger than expected on the basis of simple ideas of transition state composition. The effects of surfactant aggregation on the microenvironment in which chemical reactions take place are suggested to be the dominant kinetic influence both on SXS and BPT hydrolysis.
研究了浓度高达 70% 的三种 C12 烷基硫酸钠(SXS)(2-甲基十一烷基硫酸钠(SMS)、环十一烷基甲基硫酸钠(SCS)和 2-戊基庚基硫酸钠(SPS))水溶液的水解动力学,并将其与十二烷基硫酸钠(SDS)的行为进行了比较。观察到的动力学形式与之前描述的 SDS 相同,即通过硫酸氢离子产生的质子自动催化,但随着烷基结构的变化,反应性也有很大的不同;β 支化降低了反应性,尤其是 SMS,它是所研究的表面活性剂中反应性最低的一种。对于不同的 SXS,非催化和氢离子催化途径的反应性模式相当相似,但有观点认为,结果与非催化水解的 SN2 机制和催化途径的协同 SO3 裂解(或转移到预先结合的水分子)/质子转移机制一致,正如之前针对 SDS 提出的那样。不同 SXS 形成的聚集体中硫酸基团微环境的变化被认为是导致速率变化的主要原因。我们尝试确定在高于 c.m.c. 的 SXS 稀释溶液中观察到的速率常数与水活性的关系,这是在相同溶液中 1-苯甲酰基-3-苯基-1,2,4-三唑(BPT)的水解速率与 pH 值无关的经验值。然而,BPT 的水解似乎并不能有效地指导 SXS 溶液中的水活性,d(ln k)/d(ln [H2O])值通常比根据过渡态组成的简单想法所预期的要大得多。表面活性剂聚集对发生化学反应的微环境的影响被认为是 SXS 和 BPT 水解的主要动力学影响因素。