纳米SiO_2与阳离子表面活性剂的相互作用及其诱导的正辛烷-水乳状液的双重相转变

Interaction Between Silica Nanoparticles and Cationic Surfactants and Thereby Induced Double Phase Inversion of n-Octane-water Emulsions

研究了3种不同结构的水溶性阳离子表面活性剂对纳米二氧化硅颗粒的原位表面活性化作用,它们分别是单头单尾的十六烷基三甲基溴化铵(CTAB)、单头双尾的双十二烷基二甲基溴化铵(di-C12DMAB)和双头双尾的Gemini型阳离子三亚甲基-二(十四酰氧乙基溴化铵)(II-14-3),并通过测定Zeta电位、吸附等温线及接触角等参数对相关机理进行了阐述.结果表明,阳离子表面活性剂吸附到颗粒/水界面形成以疏水基朝向水的单分子层,从而增强了颗粒表面的疏水性是原位表面活性化的基础.通过吸附CTAB和II-14-3,颗粒的疏水性适当增强,能吸附到正辛烷/水界面稳定O/W(1)型乳状液;而通过吸附di-C12DMAB所形成的单分子层更加致密,颗粒的疏水性进一步增强,进而使乳状液从O/W(1)型转变为W/O型;当表面活性剂浓度较高时,由于链-链相互作用,表面活性剂分子将在颗粒/水界面形成双层吸附,使颗粒表面变得亲水而失去活性,但此时体系中游离表面活性剂的浓度已增加到足以单独稳定O/W(2)型乳状液的程度.因此当采用纳米二氧化硅和di-C12DMAB的混合物作乳化剂时,通过增加di-C12DMAB的浓度即可诱导乳状液发生O/W(1)→W/O→O/W(2)双重相转变.

Silica nanoparticles are not surface active due to their strong hydrophilicity.However,the nega-tively charged silica nanoparticles in aqueous media can be in situ surface activated by adsorbing cationic surfactant molecules and thereby aggregate to oil/water interface to stabilize emulsions.In this paper,the in situ surface activation effects of three water soluble cationic surfactants of different structures,cetyltrimethylammonium bromide（CTAB） with single headgroup and single chain,didodecyldimethylammonium bromide（di-C12DMAB） with single headgroup and double chains,and trimethylene-di（tetradecacyloxyethyldimethyl ammonium bromide）（II-14-3）,a Gemini cationic surfactant with double headgroups and double chains,were examined by characterizing the formation,stability and phase inversion of n-octane-water emulsions,and the relative mechanisms were explored by measuring zeta potentials,adsorption isotherms,and contact angles etc.The results show that a monolayer formation at particle/water interface with hydrophobic tail toward water,which transits the wettability of the particle from strongly hydrophilic to partially hydrophilic and partially hydrophobic,is responsible for the in situ surface activation.By adsorbing CTAB or II-14-3 the hydrophobicity of particles is appropriately enhanced so that they can stabilize n-octane-in-water[O/W（1） ]emulsion,whereas by adsorbing di-C12DMAB a much denser monolayer can be formed which endows particles a hydrophobicity so strong that the O/W（1） emulsion can be inverted to W/O type.At high surfactant concentration double layer adsorption occurs at particle-water interface and particles are returned to hydrophilic and thus lose their surface activity,while the free surfactant concentration in aqueous phase is high enough to stabilize O/W（2） emulsion solely.Thus an O/W（1） →W/O→O/W（2） double phase inversion can be induced by simply increasing surfactant concentration for the emulsion stabilized by a mixture of silica nanoparticles and di-C12DMAB.