含有酰胺基或酯基的可降解阳离子Gemini表面活性剂在水溶液中的 聚集行为

Aggregation of Biodegradable Cationic Gemini Surfactants with Amide or Ester Groups

建立高效的可降解表面活性剂是降低表面活性剂对环境污染的重要途径。本工作合成了一系列疏水链或联接基团上含有酰胺基或酯基的可生物降解阳离子Gemini表面活性剂、酰胺基在疏水链的单链表面活性剂以及酰胺基同时在疏水链和联接基团上的Gemini表面活性剂,系统研究了降解基团结构和位置对Gemini表面活性剂聚集行为的影响及Gemini表面活性剂和单体表面活性剂在表面活性和聚集行为方面的差异。结果表明,疏水链带有酰胺键的Gemini表面活性剂C12A-Cn-AC12的临界胶束浓度(cmc)比对应的单链表面活性剂低一个数量级,位于疏水链上的酰胺基可有效促进Gemini表面活性剂C12A-Cn-AC12的氢键形成和自组装。核磁共振氢谱(1H NMR)表明,短联接基团和氢键的共同作用导致C12A-C2-AC12在单体与聚集体间呈现慢交换。对于连接基团含有酰胺键的Gemini表面活性剂(C12-ACnA-C12),酰胺基可明显增加连接基团的有效长度,C12-AC12A-C12在浓度高于cmc时呈现“U”形结构,成为表面活性剂的疏水部分。与只有疏水链或者只有连接基团上含有酰胺键的Gemini表面活性剂相比,连接基团和烷基链上均含有酰胺基的Gemini表面活性剂呈现出更小的cmc和胶束微极性值、更放热的胶束化焓。含酯基的Gemini表面活性剂分子存在醇-酯平衡,且可通过水分子形成的桥连作用形成氢键,比酰胺基间的直接氢键弱,因此,含有酯基的Gemini表面活性剂C12E-C6-EC12和C12-EC6E-C12具有较低的表面活性、较高的胶束离子化度、较大的微极性、聚集过程放热更少。

In the last thirty years,Gemini surfactants with various structures have been designed,synthesized,and demonstrated to show superior physicochemical properties.However,the utilization of non-degradable surfactants,including these Gemini surfactants,poses a threat to the environment;hence,degradable Gemini surfactants are desirable.Herein,biodegradable cationic Gemini surfactants with amide or ester groups in the hydrophobic chains or the spacer were synthesized.A monomeric surfactant containing an amide group and a Gemini surfactant with amide groups both in the hydrophobic chains and the spacer were synthesized for comparison.The effects of amide group location on the aggregation behavior of Gemini surfactants were studied systematically.The differences between the Gemini surfactants with amide groups and Gemini surfactants with ester groups were evaluated by comparing their aggregation behavior and hydrogen bonding formation.The Gemini surfactants with amide groups(C12A-Cn-AC12)in the chains showed much larger exothermic ΔHmic and more negative ΔGmic values than those of the corresponding monomeric surfactant C12A;besides,their critical micelle concentration(cmc)was more than one order of magnitude lower than that of C12A.The amide groups located in the hydrophobic alkyl chains promoted hydrogen bonding formation and self-assembly of the Gemini surfactants C12A-Cn-AC12.Moreover,1H NMR spectra revealed that the coeffect of a short spacer and hydrogen bonding leads to slow exchange of the C12A-C2-AC12 molecules between the monomer and the aggregate.For the Gemini surfactant series C12-ACnA-C12,the amide groups notably increased the spacer length,and largest cmc value and smallest exothermic ΔHmic value were observed for C12-AC2A-C12 instead of C12-AC6A-C12.In C12-AC12A-C12,the spacer was long and sufficiently flexible to adopt a“U”-shaped conformation above the cmc,and it acted as the hydrophobic part of the surfactant,as confirmed by 1H NMR spectra.Among the Gemini surfactant with amide groups in both the spacer and the hydrophobic alkyl chains,C12A-AC6A-AC12 had a smaller cmc and I1/I3 ratio as well as more exothermic ΔHmic values than those of C12A-C6-AC12 and C12-AC6A-C12.1H NMR spectra indicated that an ester-alcohol structural equilibrium exists during aggregation for the Gemini surfactants with ester groups.In addition,the Gemini surfactants with ester groups formed water-mediated hydrogen bonds in the aggregates.This water-mediated hydrogen bonding between ester groups was weaker than the direct hydrogen bonding between amide groups.Therefore,the Gemini surfactants with ester groups,C12E-C6-EC12 and C12-EC6E-C12,exhibited lower surface activity,a larger micelle ionization degree,higher micropolarity,and smaller exothermic ΔHmic and less negative ΔGmic values than their counterparts with amide groups,C12A-C6-AC12 and C12-AC6A-C12.