NER: In-Situ Second Harmonic Generation Studies to Describe Nanoscale Phenomena at the Surfaces of Microparticles

研究与环境保护相关的纳米现象，验证新的快速和非侵入性的纳米级污染控制方法，对于提高生活质量和帮助创造更高效、更清洁的制造工艺具有极其重要的意义。如果能够在分子尺度上研究胶体颗粒界面上的相互作用，那么水的净化和质量控制技术将大大改进。本研究的目的是开发和扩展非线性光学二次谐波产生(SHG)方法研究微米级胶体粒子界面上的模型纳米尺度相互作用的能力。更具体地说，阳离子和阴离子表面活性剂在微米尺寸的带电二氧化硅和氧化铝颗粒上的吸附将被SHG研究。倍频是一个二阶非线性光学过程，已被证明是具有高度表面特异性的界面结构探针，特别是表面活性物质在表面的吸附。由于一些方法学问题和对粒子-表面活性剂相互作用性质的广泛了解，它在研究微米级粒子表面的纳米结构方面的应用还没有得到充分的发展。希望本研究能对表面活性剂在胶体颗粒表面的吸附过程有新的认识，这将有助于进一步推动倍频方法在胶体颗粒表面纳米结构研究中的应用。它将与化学系约翰·康博伊教授合作建造，约翰·康博伊教授在振动和频率产生光谱方面拥有丰富的经验，也是研究团队的高级成员。他和冶金工程系博士后Zhorro Nickolov博士将在所有研究问题上与PI密切合作。由带电粒子界面取向的水分子的倍频将被监测为溶液中表面活性剂浓度的函数。表面活性剂的吸附有望通过屏蔽微粒的表面电荷来降低界面水分子的极化。这将导致可测量和量化的倍频信号相应减少。或者，染料在基波红外波长共振吸收，因此具有强烈的倍频信号，将被吸附在颗粒表面，并用于建立恒定的高倍频信号。然后加入表面活性剂，由于染料从颗粒表面置换，倍频信号将减弱。因此，可以在这两种情况下评估表面活性剂的吸附自由能和表面复盖度。建议中的风险因素涉及区分表征颗粒界面的倍频信号和可能由散装颗粒和散装水产生的倍频信号。此外，还应考虑到表面活性剂和染料在颗粒表面吸附的竞争。该项目将对推进胶体体系中纳米尺度现象的研究以及建立对粒子界面进行高水平原位非侵入性研究的能力产生重大影响。

Studies of nanoscale phenomena relevant to environmental preservation and verification of new express and nonintrusive methods to control pollution on the nanoscale are of tremendous importance to improve quality of life and help in the creation of more efficient and clean manufacturing processes. Water purification and quality control techniques can be significantly refined if interactions at the interfaces of colloidal particles can be investigated at the molecular scale. The objective of this research is to exploit and extend the capabilities of the non-linear optical method of second harmonic generation (SHG) to study model nanoscale interactions at the interface of micron-size colloidal particles. More specifically, the adsorption of cationic and anionic surfactants at micron-size charged silica and alumina particles in water will be investigated by SHG. SHG is a second-order nonlinear optical process and has been proven to be highly surface-specific probe of interfacial structure and surfactant adsorption at surfaces in particular. Its application to study nanoscale structures at the surfaces of micron-size particles has not been developed to its full potential due to some methodological problems and lack of extensive knowledge about the nature of the particle-surfactant interactions. It is expected that this research will bring new understanding of the adsorption processes at the particle surfaces, which will help to further promote the application of the SHG method to study surfactant nanoscale structures on colloidal particles.A SHG spectrometer will be built based on an available femtosecond laser and will be employed for these studies. It will be built in collaboration with Prof. John Conboy, Department of Chemistry, who has rich experience in vibrational sum frequency generation spectroscopy and is also a senior member of the research team. He and Dr. Zhorro Nickolov, who is a Postdoctoral Fellow in the Department of Metallurgical Engineering, will be closely collaborating with the PI on all research issues. SHG of water molecules oriented by the charged particle interfaces will be monitored as a function of surfactant concentration in the solution. The adsorption of the surfactant is expected to reduce the polarization of the interfacial water molecules by screening the surface charge of the microparticles. This would lead to a corresponding decrease in the SHG signal which can than be measured and quantified. Alternatively, dyes resonantly absorbing at the fundamental infrared wavelength and therefore having a strong SHG signal, will be adsorbed at the particle surfaces and used to establish a constant high level of SHG signal. Then the surfactant will be added and as a result of displacing the dye from the particle surfaces the SHG signal will decrease. Consequently surfactant adsorption free energy and surface coverage can be evaluated in both cases. The risk elements in the proposal are connected with the necessity to discriminate between the SHG signals characterizing the particle interface and SHG signals which may be generated by the bulk of the particles, and by bulk water. Also, the competition between the surfactant and the dye for adsorption at the particle surfaces should be accounted for. The project will have a significant impact on advancing of the research in nanoscale phenomena in colloidal systems and on creating capabilities to perform high-level in-situ nonintrusive studies on particle interfaces.