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

该奖项是由化学部门的环境化学科学项目支持的。这项合作研究包括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（大气中非常细小的液滴）的酸度。导致大气中小颗粒形成和生长的几个关键过程是由气溶胶颗粒表面和内部的酸度控制的。这些小颗粒在大气中的形成和生长对它们对健康的影响以及它们直接和间接影响气候的潜力具有影响。大气气溶胶是直径通常小于100纳米的颗粒。典型的酸度描述不能充分反映这些粒子的酸碱化学性质，因为它们太小了。该项目旨在了解酸度测量（这里是酸解离常数（pKa））在描述亚100纳米颗粒中酸和碱的行为时的适用性限制。作为本科生长期暑期实习计划的一部分，研究人员使用AVOGADRO程序进行实时演示，该程序提供了一个易于使用的图形用户界面来创建酸碱分子系统，并执行与该项目相关的初步、低级模拟。因此，该项目提供的实时演示可能会对提高未被充分代表的少数民族对计算指导实验的认识产生重大影响，可能会导致更多的少数民族学生在环境化学科学领域寻求职业。本研究的目的是确定体积相水溶液酸碱解离常数（pKa和pKb）的适用范围，特别是在100纳米以下颗粒的酸碱化学方面。在这个尺寸范围内，在CLOUD室进行的直接组成和间接吸湿性测量表明，在过量基（胺和氨）存在的情况下，由硫酸形成的纳米颗粒比使用热力学模型预测的酸性更强。此外，酸度似乎随粒径的变化而变化，从~10纳米（最酸性）到50纳米（更中和）。了解亚100nm颗粒的酸度对于理解相对简单的酸碱化学非常重要，而且对于准确预测颗粒表面和体相的酸催化过程也很重要。该项目的具体目标如下：利用分子模型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。这项活动结合了计算量子化学和分子动力学等技术来模拟直径高达5纳米的颗粒，并进行实验室实验来评估预测的pKa值作为组成和尺寸的函数。这是通过温度和湿度控制的反应室和化学电离质谱仪来确定前驱体和纳米颗粒的组成。研究人员还将开发和应用一种参数化方法，适用于在大气气溶胶颗粒内部和表面发生的相关过程的多尺度建模。后一种研究目的可能会提高模型的预测能力。这些模式涉及在大气中形成新颗粒的机制和物种，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.