Department of Chemistry , University of Otago , Dunedin 9054 , New Zealand.
Langmuir. 2018 Nov 13;34(45):13481-13490. doi: 10.1021/acs.langmuir.8b01351. Epub 2018 Nov 1.
Polyethoxylated (PEO) surfactant adsorption to silica under aqueous conditions is an important physical process in a multitude of industries. Consequently, a considerable number of spectroscopic and other studies have been carried out to ascertain the molecular/structural details of the adsorbed surfactant and the kinetics of PEO surfactant adsorption. However, the use of infrared spectroscopy to probe surfactant adsorption at the silica/aqueous solution interface has been limited because of the instability of silica particle films under aqueous conditions and the opacity of silicon prisms below 1300 cm typically employed for these studies. The work presented here provides infrared spectroscopic measurements of silica particle films formed from differing suspension pH on a diamond internal reflection prism to probe silica particle film stability as a function of pH. The films formed from a suspension pH of 2.5 were found to be the most stable owing to a sol-gel transition of the colloidal suspension upon drying and the reduction in electrostatic repulsion between silica nanoparticles, creating a tightly packed nanoparticle film. Colloid probe atomic force microscopy (CP-AFM) was used to confirm the alteration of surface forces between silica nanoparticles as a function of pH. Particle films from silica suspensions of pH 2.5 were formed in situ on an attenuated total reflection infrared diamond prism and used to probe Triton X-100 adsorption from an aqueous solution. The obtained infrared spectra revealed a critical surface aggregation concentration at a solution concentration of 0.14 mmol L, Triton X-100 forms discrete micelles at the silica surface, and the PEO head group preferentially adopts a helical conformation. Most intriguingly, a breakup of the silica particle film was observed at the critical micelle concentration of the surfactant. This is due to the repulsive steric forces arising from the interactions between the PEO corona of the surfactant micelles formed at the silica surface, as confirmed by the CP-AFM measurements.
在水相条件下，聚氧乙烯（PEO）表面活性剂吸附到二氧化硅上是许多工业中一个重要的物理过程。因此，已经进行了相当数量的光谱和其他研究，以确定吸附表面活性剂的分子/结构细节和 PEO 表面活性剂吸附的动力学。然而，由于水相条件下二氧化硅颗粒膜的不稳定性和通常用于这些研究的硅棱镜的不透明度，红外光谱法在探测硅质/水溶液界面上的表面活性剂吸附方面的应用受到限制。本文提供了在金刚石内反射棱镜上形成的不同悬浮 pH 值的二氧化硅颗粒膜的红外光谱测量结果，以研究 pH 值对二氧化硅颗粒膜稳定性的影响。在悬浮 pH 值为 2.5 时形成的膜最为稳定，这是由于胶体悬浮液在干燥时发生溶胶-凝胶转变以及二氧化硅纳米颗粒之间的静电排斥减少，从而形成紧密堆积的纳米颗粒膜。胶体探针原子力显微镜（CP-AFM）用于证实 pH 值对二氧化硅纳米颗粒之间表面力的改变。将 pH 值为 2.5 的二氧化硅悬浮液原位形成在衰减全反射红外金刚石棱镜上，并用于从水溶液中探测 Triton X-100 的吸附。获得的红外光谱显示，在溶液浓度为 0.14 mmol L 时存在临界表面聚集浓度，Triton X-100 在二氧化硅表面形成离散胶束，PEO 头基优先采用螺旋构象。最有趣的是，在表面活性剂的临界胶束浓度下观察到二氧化硅颗粒膜的破裂。这是由于在二氧化硅表面形成的表面活性剂胶束的 PEO 冠之间的相互作用引起的排斥位阻力所致，这一点通过 CP-AFM 测量得到了证实。
