Sostaric J Z, Riesz P
Radiation Biology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA.
J Am Chem Soc. 2001 Nov 7;123(44):11010-9. doi: 10.1021/ja010857b.
The surfactant properties of solutes play an important role in the sonochemistry and sonoluminescence of aqueous solutions. Recently, it has been shown, for relatively low molecular weight surfactants, that these effects can be correlated with the Gibbs surface excess of the solute. In the present study we investigate whether this correlation is valid for relatively high molecular weight surfactants and the mechanisms of surfactant decomposition during sonolysis. Sonolysis of argon-saturated aqueous solutions of nonvolatile surfactants [n-alkanesulfonates, n-alkyl sulfates, n-alkylammoniopropanesulfonates (APS), and poly(oxyethylenes) (POE)] was investigated by EPR and spin-trapping with 3,5-dibromo-4-nitrosobenzenesulfonate. Secondary carbon radicals (-.CH-), formed by abstraction reactions, were observed for all surfactants that were sonicated. The yield of primary carbon (-.CH(2)) and methyl (.CH(3)) radicals that are formed by pyrolysis is greatest for the zwitterionic (i.e., APS) and nonionic surfactants (i.e., POE). The yield of (-.CH-) radicals was measured following sonolysis of n-alkyl anionic surfactants [sodium pentanesulfonate (SPSo), sodium octanesulfonate (SOSo), sodium octyl sulfate (SOS), and sodium dodecyl sulfate (SDS)]. In the concentration range of 0-0.3 mM, the -.CH- radical yield increases in the order SDS approximately equal to SOS approximately equal to SOSo > SPSo. At higher concentrations, a plateau in the maximum (-.CH-) radical yield is reached for each surfactant, which follows the order SPSo > SOS approximately equal to SOSo > SDS; i.e., the radical scavenging efficiency increases with decreasing n-alkyl chain length. A similar trend was observed for the .CH(3) yield following sonolysis of a homologous series of n-alkyl APS surfactants. The results show that the Gibbs surface excess of certain nonvolatile surfactants does not correlate with the extent of decomposition following sonolysis in aqueous solutions. Instead, the decomposition of surfactants depends on their chemical structure and their ability to equilibrate between the bulk solution and the gas/solution interface of cavitation bubbles.
溶质的表面活性剂性质在水溶液的声化学和声致发光中起着重要作用。最近研究表明，对于相对低分子量的表面活性剂，这些效应可以与溶质的吉布斯表面过剩相关联。在本研究中，我们探究这种相关性对于相对高分子量的表面活性剂是否有效，以及声解过程中表面活性剂分解的机制。通过电子顺磁共振（EPR）以及用3,5 - 二溴 - 4 - 亚硝基苯磺酸盐进行自旋捕获，研究了非挥发性表面活性剂[正烷磺酸盐、正烷基硫酸盐、正烷基氨丙烷磺酸盐（APS）和聚（氧乙烯）（POE）]的氩饱和水溶液的声解。对于所有经超声处理的表面活性剂，均观察到通过夺氢反应形成的仲碳自由基（-.$\dot{C}$H-）。由热解形成的伯碳（-.$\dot{C}$H₂）和甲基（.$\dot{C}$H₃）自由基的产率对于两性离子（即APS）和非离子表面活性剂（即POE）而言最高。在正烷基阴离子表面活性剂[戊烷磺酸钠（SPSo）、辛烷磺酸钠（SOSo）、辛基硫酸钠（SOS）和十二烷基硫酸钠（SDS）]声解后，测量了（-.$\dot{C}$H-）自由基的产率。在0 - 0.3 mM的浓度范围内，-.$\dot{C}$H-自由基产率按SDS≈SOS≈SOSo > SPSo的顺序增加。在较高浓度下，每种表面活性剂的最大（-.$\dot{C}$H-）自由基产率达到平稳状态，其顺序为SPSo > SOS≈SOSo > SDS；即自由基清除效率随着正烷基链长度的减小而增加。对于一系列同系正烷基APS表面活性剂声解后的.$\dot{C}$H₃产率，也观察到了类似的趋势。结果表明，某些非挥发性表面活性剂的吉布斯表面过剩与水溶液中声解后的分解程度不相关。相反，表面活性剂的分解取决于它们的化学结构以及它们在本体溶液和空化气泡的气/液界面之间达到平衡的能力。
