Dynamic Duo: Vibrational Sum Frequency Scattering Investigation of pH-Switchable Carboxylic Acid/Carboxylate Surfactants on Nanodroplet Surfaces
Dynamic Duo: Vibrational Sum Frequency Scattering Investigation of pH-Switchable Carboxylic Acid/Carboxylate Surfactants on Nanodroplet Surfaces
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DOI：
10.1021/acs.jpcb.1c05508
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发表时间：
2021-08-17
影响因子：
3.3
通讯作者：
Richmond, Geraldine L.
中科院分区：
文献类型：
--
作者：
Foster, Marc J.;Carpenter, Andrew P.;Richmond, Geraldine L.
含有pH可切换的羧酸部分的表面活性剂用于需要疏水液滴在水中的受控稳定性的各种环境、工业和生物应用中。对于纳米乳液，在水中动力学稳定的油滴，羧酸表面活性剂的阴离子形式的表面吸附使液滴稳定，而中性形式的主要表面存在导致不稳定。通过使用动态光散射，zeta电位，和振动和频散射光谱（VSFSS），我们研究这种机制和相对表面人口的中性和带电物种的pH值进行调整。我们发现，在液滴表面的两种表面活性剂的相对人口是明显不同的，比他们的体积平衡浓度。的zeta电位测量表明，带电的表面活性剂的表面浓度在整个稳定的pH范围内保持几乎恒定。相比之下，VSFSS显示中性羧酸形式随着酸度增加而越来越多地吸附到表面。头基振动模式的光谱特征证实了这种行为，并进一步揭示了其吸附的其他分子细节。一个显着的氢键相互作用之间发生的头部基团，沿着与疏水链链相互作用，有助于吸引更多的羧酸表面活性剂的界面。带电表面活性剂为这些液滴提供稳定力，而中性表面活性剂随着pH降低而向界面结构引入复杂性。结果是显着不同的平面油/水的研究中发现的界面的稳定性是不是一个因素。
Surfactants containing pH-switchable, carboxylic acid moieties are utilized in a variety of environmental, industrial, and biological applications that require controlled stability of hydrophobic droplets in water. For nanoemulsions, kinetically stable oil droplets in water, surface adsorption of the anionic form of the carboxylic acid surfactant stabilizes the droplet, whereas a dominant surface presence of the neutral form leads to destabilization. Through the use of dynamic light scattering, zeta-potential, and vibrational sum frequency scattering spectroscopy (VSFSS), we investigate this mechanism and the relative surface population of the neutral and charged species as pH is adjusted. We find that the relative population of the two surfactant species at the droplet surface is distinctly different than their bulk equilibrium concentrations. The zeta-potential measurements show that the surface concentration of the charged surfactant stays nearly constant throughout the stabilizing pH range. In contrast, VSFSS shows that the neutral carboxylic acid form increasingly adsorbs to the surface with increased acidity. The spectral features of the headgroup vibrational modes confirm this behavior and go further to reveal additional molecular details of their adsorption. A significant hydrogen-bonding interaction occurs between the headgroups that, along with hydrophobic chain-chain interactions, assists in drawing more carboxylic acid surfactant to the interface. The charged surfactant provides the stabilizing force for these droplets, while the neutral surfactant introduces complexity to the interfacial structure as the pH is lowered. The results are significantly different than what has been found for the planar oil/water studies where stabilization of the interface is not a factor.