石英表面阳离子吸附膜结构的AFM研究

为了研究阳离子表面活性剂对固体表面的润湿性的影响,了解表面活性剂在石英表面的吸附机制,利用原子力显微镜(AFM)技术,考察了不同结构阳离子改性石英片与不同基团改性AFM探针间的拉脱力,从分子角度探索石英表面双层膜的结构特性。研究发现:直链烷基季铵盐C16PC改性石英片与阴离子改性探针间的拉脱力随浓度增大在临界胶束浓度附近通过一个约200 pN的极小值;而支链烷基季铵盐C16GPC的拉脱力随浓度增大而升高,在临界胶束浓度附近达到约500 pN的平台值,说明C16PC通过分子插入的方式形成双层膜,而C16GPC则通过尾对尾的方式形成双层膜;EO基团引入后,C16(EO)3PC和C16G(EO)3PC改性石英片与阴离子改性探针间的拉脱力降低至100 pN以下,说明C16(EO)3PC和C16G(EO)3PC分子以接近平躺的方式形成松散的吸附单层,双层膜的外层中烷基所占比例高于阳离子基团。上述阳离子表面活性剂改性石英片与烷基改性探针的实验结果证实了存在双层膜结构。

Study on structure of adsorbed cationic surfactant film on quartz surface by AFM

In order to study the effect of cationic surfactants on the wettability of solid surfaces,and to analyze the adsorption mechanism of surfactants on quartz surfaces,the pull-out forces between the modified quartz surfaces by cationic surfactants with different structure and the modified AFM probes with different groups are studied.The characteristics of cationic surfactant bilayers on quartz surfaces are explored from molecular mechanics.On the basis of experimental results,the pull-out force between anionic modified probe and quartz modified by linear quaternary ammonium salt C16PC passes through a minimum value about 200 pN near its critical micelle concentration with the increasing concentration.As the concentration increases,the pull-out force of branched chain C16GPC increases up to a plateau value about 500 pN at concentration close to its CMC.This indicates that the bi-layer was formed by cationic surfactant C16PC and C16GPC with insertion mode and tail-to-tail mode on quartz,respectively.When ethylene oxide groups CH2CH2O were incorporated in the molecular structure,the pull-out force were decreased to less than 100 pN for C16(EO)3PC and C16G(EO)3PC.At this point,the surfactant molecules mostly formed a loose monolayer on quartz surfaces in the way of lying flat,and the proportion of alkyl groups were higher than that of cationic groups in the outer of bi-layer.The proposed bi-layer structures were proved by the pull-out forces between alkyl modified probe and cationic surfactant modified quartz surfaces.