光敏季铵盐Gemini表面活性剂a4-6-m在气/液界面的吸附

Adsorption of Photosensitive Quaternary Ammonium Gemini Surfactant a4-6-m at the Air/Water Interface

合成了3种不对称结构的季铵盐Gemini表面活性剂a4-6-m,分子的一根疏水链是偶氮苯为端基的4个亚甲基链,另一根是不同长度的脂肪链(m=12,14,16).研究结果表明,反式偶氮苯封端的a4-6-m在气/液界面上以直立方式排列,偶氮苯端基间的π-π相互作用导致吸附分子较为紧密地排列,但吸附层外表面含有偶氮苯成分使临界胶束浓度(cmc)时的表面张力(γcmc)较大.紫外光激发使反式结构偶氮苯变为顺式结构,这些极性较强的顺式偶氮苯夹杂在直立的烷烃链间,增强的偶极-偶极相互作用促进了分子紧密排列,使分子占据面积(Amin)略微减小.增长脂肪链长度有助于降低临界胶团浓度和C20(使水的表面张力降低20mN·m-1时所需的表面活性剂浓度),但对γcmc影响不大.

Three dissymmetric quatemary ammonium Gemini surfactants （referred to as a4-6-m） were synthesized. One of the two hydrophobic chains of a4-6-m was a butyl chain terminated by an azobenzene group and the other was a conventional aliphatic chain of different length （m=12, 14, 16）. The results showed that the trans-a4-6-m molecules adsorbed at the air/water interface with a vertical orientation of the two hydrophobic chains. The π-π interaction between azobenzene groups resulted in a denser arrangement of the surfactants. The azobenzene groups toward the airside in the adsorption layer yielded a higher γcmc （surface tension value corresponding to the critical micelle concentration （cmc）） by comparison to 12-6-12. After UV-light irradiation, the trans-azobenzene groups converted into twisted cisforms with a large dipole moment. These cis-azobenzene groups lay among the vertically oriented aliphatic chains and their relatively free location enhanced their dipole-dipole interactions, which promoted the tight packing of adsorbed molecules and slightly reduced the minimum molecular occupation area （Atom）. The increase in aliphatic chain length led to a reduction of the cmc and the C20 （concentration of surfactant required to reduce 20 mN· m^-1 surface tension of water）, but had little effect on the γcmc.