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The electrokinetic properties of cationic surfactants adsorbed on a hydrophobic substrate: effect of chain length and concentration
Microfluidics and Nanofluidics ( IF 2.8 ) Pub Date : 2019-11-14 , DOI: 10.1007/s10404-019-2288-9
Adam Snider , Francis R. Cui , Glareh Azadi , Anubhav Tripathi

Electrokinetic (EK) properties, such as the electro-osmotic flow (EOF), are influenced by surfactant adsorption at the solid–liquid interface. With the growing popularity of poly(methyl methacrylate) (PMMA) as the substrate for polymeric-based microfluidics, it is important to understand the effect of surfactants on EOF in these devices. Here, we investigate the effect of surfactant chain length and concentration on the electro-osmotic (EO) mobility induced by three cationic surfactants cetyl trimethylammonium bromide (CTAB), trimethylammonium bromide (TTAB), dodecyl trimethylammonium bromide (DTAB) in PMMA microcapillaries. The EO mobility curve as a function of concentration shows three regimes. First, at very low concentrations below 0.002 mM, the mobility is constant and approximately equal to the value obtained with the surfactant-free electrolyte (1 mM KCl). Next, the EOF reverses and mobility increases linearly with surfactant concentration. Finally, the mobility reaches a plateau at a concentration well below surfactant CMC (0.2-mM CTAB, 0.5-mM TTAB and 2-mM DTAB) and decreases at the vicinity of CMC. Our results show that the rate of change in mobility with respect to concentration is a linear function of chain length and increases with longer-chain surfactants. In addition, we deduce the magnitude of Van der Waals or cohesive energy between the adsorbed alkyl chains from the EO mobility values. For the alkyl trimethylammonium surfactants adsorbed on the hydrophobic surface of PMMA, this energy was found to be 0.114 kT smaller than the reported value for ionic surfactants adsorbed on a hydrophilic surface.



中文翻译:

阳离子表面活性剂吸附在疏水性基质上的电动特性:链长和浓度的影响

电动(EK)特性,例如电渗流(EOF),受表面活性剂在固液界面的吸附影响。随着聚(甲基丙烯酸甲酯)(PMMA)作为基于聚合物的微流体的底物的日益普及,重要的是要了解表面活性剂对这些设备中EOF的影响。在这里,我们研究了表面活性剂链长和浓度对PMMA微毛细管中三种阳离子表面活性剂溴化十六烷基三甲基溴化铵(CTAB),三甲基溴化铵(TTAB),十二烷基三甲基溴化铵(DTAB)诱导的电渗透(EO)迁移率的影响。EO迁移率曲线随浓度的变化显示出三种状态。首先,在低于0.002 mM的极低浓度下,迁移率是恒定的,并且大约等于使用不含表面活性剂的电解质(1 mM KCl)所获得的值。接下来,EOF反转,迁移率随表面活性剂浓度线性增加。最后,迁移率在远低于表面活性剂CMC(0.2-mM CTAB,0.5-mM TTAB和2-mM DTAB)的浓度达到平稳状态,并在CMC附近降低。我们的结果表明,迁移率相对于浓度的变化率是链长的线性函数,并随着长链表面活性剂的增加而增加。此外,我们从EO迁移率值推导了范德华力或吸附的烷基链之间的内聚能的大小。对于吸附在PMMA疏水表面上的烷基三甲基铵表面活性剂,发现该能量为0。

更新日期:2019-11-14
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