Collaborative Research: Applicability Limits of Aqueous pKa Values for Bulk and Surface Nanoparticle Processes

该奖项由化学部的环境化学科学项目支持。这项合作研究涉及加州大学河滨分校的Kelley Barsanti教授和Bryan Wong教授，以及加州大学欧文分校的James Smith教授。他们与研究生和本科生一起研究大气气溶胶(大气中非常细微的水滴)的酸度。导致小颗粒在大气中形成和生长的几个关键过程受气溶胶颗粒表面和内部的酸度控制。这些小颗粒物在大气中的形成和生长对健康的影响以及直接和间接影响气候的可能性都有影响。大气气溶胶是直径通常小于100纳米的颗粒。对酸度的典型描述不能充分代表这些颗粒中的酸碱化学，因为它们太小了。这个项目试图了解酸度测量(这里是酸解离常数(PKA))在描述亚100纳米粒子中酸和碱的行为时的适用范围。作为一个长期的本科生暑期实习计划的一部分，研究人员使用Avogadro计划进行实时演示，该计划提供了一个易于使用的图形用户界面，以创建酸碱分子系统，并执行与该项目相关的初步、低水平模拟。因此，该项目提供的实时演示可能会对在未被充分代表的少数群体中提高计算指导实验的认识产生重大影响，潜在地导致更多的少数群体学生寻求环境化学科学的职业。本研究的目标是确定体相水溶液酸碱离解常数(PKA和PKB)的适用范围，特别是在亚100纳米粒子中和之上的酸碱化学。在这个尺寸范围内，在云室进行的直接组成和间接吸湿性测量表明，硫酸在过量碱(胺和氨)存在下形成的纳米颗粒的酸性比热力学模型预测的要强。此外，酸性似乎随着颗粒大小的变化而变化，范围从~10 nm(最酸性)到50 nm(更中和)。了解亚100纳米粒子的酸度对于理解相对简单的酸碱化学很重要，但对于准确预测粒子表面和体相中的酸催化过程也很重要。该项目的具体目标如下：使用分子模拟来预测无机和有机酸和碱的pKa值作为颗粒组成和大小的函数。这一活动结合了计算量子化学和分子动力学等技术，对直径最大为5纳米的粒子进行建模，并进行实验室实验，以评估预测的pKa值作为组成和尺寸的函数。这是通过使用温度和湿度控制的反应室和化学电离质谱仪来确定前体和纳米颗粒组成来完成的。研究人员还将开发和应用一种适用于大气气溶胶颗粒中和之上发生的相关过程的多尺度模拟的参数化法。后一项研究目标可能导致模型预测能力的提高。这些模型解决了大气中新颗粒形成的机制和物种，以及大气纳米颗粒对人类健康和气候的影响。

This award is supported by the Chemistry Division's Environmental Chemical Sciences Program. The collaborative study involves Professors Kelley Barsanti and Bryan Wong at the University of California, Riverside, and Prof. James Smith at the University of California, Irvine. Together with their graduate and undergraduate students, they investigate the acidity of atmospheric aerosols (very fine droplets in the atmosphere). Several key processes that cause small particles to form and grow in the atmosphere are controlled by the acidity of the surface and interior of aerosol particles. The formation and growth of these small particles in the atmosphere have implications for their effects on health as well as their potential to directly and indirectly impact climate. Atmospheric aerosols are particles that are often smaller than 100 nanometers in diameter. Typical descriptions of acidity fail to adequately represent the acid-base chemistry in these particles because they are so small. This project seeks to understand the limits of the applicability of an acidity measurements (here, acid dissociation constants (pKa)) in describing the behavior of acids and bases in sub-100 nanometer particles. As part of a long-standing summer internship program for undergraduates, the researchers give real-time demonstrations using the AVOGADRO program, which provides an easy-to-use graphical user-interface to create acid-base molecular systems and perform preliminary, low-level simulations relevant to this project. As such, the project provides real-time demonstrations may have a substantial impact in increasing awareness of computationally-guided experiments in underrepresented minority populations, potentially leading to more minority students seeking careers in the environmental chemical sciences.The goal of this research is to determine the limits of applicability for bulk-phase aqueous acid and base dissociation constants (pKa and pKb), particularly as regards acid-base chemistry in and on sub-100 nm particles. In this size range, the direct composition and indirect hygroscopicity measurements performed at the CLOUD chamber show that nanoparticles formed by sulfuric acid in the presence of excess based(amines and ammonia) are more acidic than predicted using thermodynamic models. Further, the acidity appears to vary with particle size from ~10 nm (most acidic) to 50 nm (more neutralized). Knowledge of the acidity of sub-100 nm particles is important for understanding relatively simple acid-base chemistry, but also for accurately predicting acid-catalyzed processes on particle surfaces and in the bulk phase. The specific objectives of this project are as follows: to use molecular modeling to predict pKa values for inorganic and organic acids and bases as a function of particle composition and size. This activity combines techniques such a computational quantum chemistry and molecular dynamics to model particles with diameters of up to 5 nm and to perform laboratory experiments to evaluate the predicted pKa values as a function of composition and size. This is done using a temperature- and humidity-controlled reaction chamber and chemical ionization mass spectrometers to determine precursor and nanoparticle composition. The researchers will also develop and apply a parameterization applicable for multiscale modeling of relevant processes taking place in and on atmospheric aerosol particles. This latter research aim may lead to improvements in the predictive power of models. These models address the mechanisms and species responsible for the formation of new particles in the atmosphere, and the effects of atmospheric nanoparticles on human health and climate.