Yezer Benjamin A, Khair Aditya S, Sides Paul J, Prieve Dennis C
Center for Complex Fluids Engineering, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, United States.
Center for Complex Fluids Engineering, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, United States.
J Colloid Interface Sci. 2016 May 1;469:325-337. doi: 10.1016/j.jcis.2016.02.014. Epub 2016 Feb 4.
The impedance of dodecane doped with sorbitan trioleate (Span 85), sorbitan monooleate (Span 80) and sorbitan monolaurate (Span 20) was measured as a function of frequency using a 10 mV amplitude sinusoidal voltage applied across a parallel plate cell with a 10 μm spacing. The tested solutions varied in concentration from 1 mM to 100 mM and the frequency range was 10(-2)-10(4) Hz. Nyquist plots of all three surfactants showed the high frequency semicircle characteristic of parallel resistance and capacitance but often exhibited a second semicircle at low frequencies which was attributed to charge adsorption and desorption. The electrical conductivity of each surfactant was proportional to surfactant concentration for concentrations above 10mM. Fitting the data to models for charge migration, differential capacitance, and adsorption allowed extraction of both charge concentration and two kinetic parameters that characterize the rate of adsorption and desorption. Above 10 mM the ratio of charge carriers per surfactant molecule was 22 ppm for Span 20, 3 ppm for Span 80, and 0.2 ppm for Span 85. A higher number of charge carriers per molecule of surfactant was associated with larger micelles. The adsorption rate constants were independent of surfactant concentration while the desorption rate constants were proportional to the surfactant concentration. This dependence indicated that uncharged surfactant, whether in micelles or not, participated in the desorption of charge. Predictions of the adsorption/desorption model for large constant electric fields agreed qualitatively with data from the literature (Karvar et al., 2014).
使用一个幅值为10 mV的正弦电压施加在间距为10 μm的平行板电池上，测量了掺杂有失水山梨醇三油酸酯（司盘85）、失水山梨醇单油酸酯（司盘80）和失水山梨醇单月桂酸酯（司盘20）的十二烷的阻抗随频率的变化。测试溶液的浓度范围为1 mM至100 mM，频率范围为10⁻² - 10⁴ Hz。所有三种表面活性剂的奈奎斯特图都显示出并联电阻和电容的高频半圆特性，但在低频处经常出现第二个半圆，这归因于电荷吸附和解吸。对于浓度高于10 mM的每种表面活性剂，其电导率与表面活性剂浓度成正比。将数据拟合到电荷迁移、微分电容和吸附模型中，可以提取电荷浓度以及表征吸附和解吸速率的两个动力学参数。在10 mM以上，司盘20每个表面活性剂分子的电荷载流子比率为22 ppm，司盘80为3 ppm，司盘85为0.2 ppm。每个表面活性剂分子中电荷载流子数量越多，与更大的胶束相关。吸附速率常数与表面活性剂浓度无关，而解吸速率常数与表面活性剂浓度成正比。这种依赖性表明，不带电的表面活性剂，无论是否在胶束中，都参与了电荷的解吸。对于大恒定电场的吸附/解吸模型的预测与文献数据（Karvar等人，2014年）在定性上一致。
