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尺度现象，验证新的快速和非侵入性的纳米尺度污染控制方法，对提高生活质量和帮助创造更高效和清洁的制造过程具有巨大的重要性。如果可以在分子尺度上研究胶体颗粒界面上的相互作用，水净化和质量控制技术可以得到显著改进。本研究的目的是开发和扩展二次谐波产生（SHG）的非线性光学方法的能力，以研究微米级胶体颗粒界面上的纳米级相互作用模型。更具体地说，阳离子和阴离子表面活性剂在水中微米级带电二氧化硅和氧化铝颗粒上的吸附将被SHG研究。SHG是一个二阶非线性光学过程，已被证明是界面结构和表面活性剂吸附的高表面特异性探针。由于一些方法上的问题和缺乏对颗粒-表面活性剂相互作用性质的广泛了解，它在微米级颗粒表面纳米级结构研究中的应用尚未充分发挥其潜力。期望本研究将对颗粒表面的吸附过程带来新的认识，有助于进一步推动SHG方法在胶体颗粒表面表面活性剂纳米级结构研究中的应用。将在现有飞秒激光器的基础上建造一个SHG光谱仪，并将用于这些研究。它将与化学系John Conboy教授合作建造，John Conboy教授在振动和频率产生光谱方面具有丰富的经验，也是研究小组的资深成员。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SHG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筛选微粒的表面电荷来减少界面水分子的极化。这将导致SHG信号的相应减少，而SHG信号是无法测量和量化的。或者，在基本红外波长共振吸收因而具有强SHG信号的染料将被吸附在颗粒表面并用于建立恒定的高水平SHG信号。然后加入表面活性剂，由于将染料从颗粒表面移走，SHG信号将减弱。因此，表面活性剂的吸附自由能和表面覆盖率可以在这两种情况下进行评估。提案中的风险因素与区分表征粒子界面的SHG信号和可能由大量粒子和大量水产生的SHG信号的必要性有关。此外，表面活性剂和染料在颗粒表面的吸附竞争也应考虑在内。该项目将对推进胶体系统中纳米级现象的研究以及创建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.