Soft Materials Laboratory, Department of Physics, IIT Madras , Chennai 600 036, India.
Solid State Physics Division, Bhabha Atomic Research Centre , Mumbai 400 085, India.
Langmuir. 2017 Nov 14;33(45):13014-13026. doi: 10.1021/acs.langmuir.7b03104. Epub 2017 Oct 31.
Elastic bending rigidity of the surfactant shell is a crucial parameter which determines the phase behavior and stability of microemulsion droplets. For water-in-oil reverse microemulsions stabilized by AOT (sodium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate) surfactant, the elastic bending rigidity is close to thermal energy at room temperature (kT) and can be modified by the presence of hydrophilic polymers. Here, we explore the influence of two polymers polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP), both having nearly same size (radius of gyration, R) but different dipole moment, on elastic bending rigidity of water-AOT-n-decane reverse microemulsions via estimating the percolation temperatures (T) and droplet radii using dielectric relaxation spectroscopy (DRS) and small-angle neutron scattering (SANS) techniques. Notably, an increase in T is observed on introducing PEG and PVP polymers and is attributed to the adsorption of polymer chains onto the surfactant monolayer. The stability of the droplet phase of microemulsion after the incorporation of PEG and PVP polymers is confirmed by contrast matching SANS experiments. An enhancement in elastic bending rigidity of AOT surfactant shell amounting to ∼46% is observed upon incorporation of PVP into the droplet core, whereas for PEG addition, a smaller increase of about 17% is recorded. We conjecture that the considerable increase in elastic bending rigidity of the surfactant monolayer upon introducing PVP is because of the strong ion-dipole interaction between anionic AOT and dipoles present along the PVP polymer chains. Scaling exponents extracted from the temperature dependent electrical conductivity measurements and the frequency dependent scaling of conductivity at percolation indicate the dynamic nature of percolation for both pure and polymer loaded reverse microemulsions. The decrease in activation energy of percolation upon incorporating PEG and PVP polymer molecules also reflects the increased stability of microemulsion droplets against thermal fluctuations.
表面活性剂壳的弹性弯曲刚度是决定微乳液液滴的相行为和稳定性的关键参数。对于由 AOT（十二烷基硫酸钠）表面活性剂稳定的油包水反相微乳液，弹性弯曲刚度接近于室温下的热能（kT），并且可以通过亲水性聚合物的存在来修饰。在这里，我们通过介电弛豫光谱（DRS）和小角中子散射（SANS）技术来评估渗透温度（T）和液滴半径，来研究两种具有几乎相同尺寸（旋转半径，R）但不同偶极矩的聚合物聚乙二醇（PEG）和聚乙烯吡咯烷酮（PVP）对水-AOT-正癸烷反相微乳液的弹性弯曲刚度的影响。值得注意的是，在引入 PEG 和 PVP 聚合物时观察到 T 的增加，这归因于聚合物链吸附到表面活性剂单层上。通过对比匹配的 SANS 实验证实了微乳液液滴相在加入 PEG 和 PVP 聚合物后的稳定性。在向液滴核心中加入 PVP 时，观察到 AOT 表面活性剂壳的弹性弯曲刚度增强了约 46％，而对于加入 PEG，则记录到约 17％的较小增加。我们推测，在引入 PVP 时，表面活性剂单层的弹性弯曲刚度的大幅增加是由于阴离子 AOT 和沿 PVP 聚合物链存在的偶极子之间的强离子偶极相互作用。从温度相关的电导率测量中提取的标度指数以及在渗透时电导率的频率相关标度表明，对于纯反相微乳液和聚合物负载的反相微乳液，渗透都是动态的。在加入 PEG 和 PVP 聚合物分子后，渗透的活化能降低也反映了微乳液液滴对热波动的稳定性增加。
