Testing Molecular Mechanisms for Growth and Dissolution Reactions on Calcite Surfaces

智力优势：矿物溶解和生长的动力学模型受到阻碍，我们无法精确地定义在分子尺度上的相关反应的机制。许多矿物表面过程的净反应是已知的，但它们发生的途径和机制没有得到很好的约束。方解石是一种特别重要的矿物，因为它是：土壤和地下水中酸性的主要缓冲剂，用于了解晶体生长的几种材料之一，并广泛研究以了解生物矿化过程。在方解石的非生物溶解和生长过程中，已经提出了特定的、分子尺度的限制反应机制，但尚未得到验证。同样，在生物矿化过程中，已知某些分子和蛋白质（如碳酸盐）会影响生长形态，但对其动力学效应的定量估计仍然难以捉摸。本提案的主要目标是测试方解石溶解和生长的反应机制，通过直接比较实验测量的步骤运动的形成和活化能，从第一原理模拟的反应。经典晶体生长理论将形成框架，将实验测量的量与模拟的化学机制联系起来。将被测试的反应是扭结网站形成的机制，被认为是控制步骤的运动在环中性pH值，大气pCO 2，饱和度接近平衡，以及生长抑制的模型化合物。拟议的研究将在三年内进行，并将解决三个主要的假设，将形成一个博士学位的大部分。* 假设1：通过AFM观察到的方解石表面上的台阶运动可以通过相同的生长和溶解模型来拟合，其中扭结位点的形成和传播是有限的。假设2：精确的扭结位置形成和活化能可以通过第一原理计算和模拟来模拟。假设三：天冬氨酸的羧酸官能团从钙离子上的台阶边缘的分离控制其能力，中毒growth.The激活和扭结网站的形成能将估计从原子力显微镜测量的一步速度作为饱和度和温度的函数。这些将进行比较，直接观察扭结网站的浓度在平衡的解决方案，使用高分辨率原子力显微镜。在确保最佳拟合现有的晶体截断棒数据的水结构在方解石界面后，将进行模拟。然后，将测试的反应机制，通过模拟扭结网站的形成和活化能使用从头算密度泛函理论和分子动力学伞形采样。水的解离在反应中的可能的重要性将探讨通过质子化表面碳酸盐和羟基化表面钙离子。最后，将在天冬氨酸的存在下进行相同的AFM测量和模拟技术，并且将实验估计的生长中毒的活化能与天冬氨酸从台阶边缘上的钙脱离的模拟活化能进行比较。拟议的研究将以一种新的方式将联合收割机实验和计算技术结合起来，明确测试我们对方解石生长和溶解反应的理解。这一信息可用于随后在较大规模的速率测量的物理意义分配到测量的速率常数是模糊的。更广泛的影响：这个项目将形成一个研究生的博士学位的大部分。论文此外，在第二年，一名K-12教师将获得格鲁吉亚实习教师奖学金（GIFT），并在PI实验室工作六周。我们的目标是为教师提供经验，使他或她可以更好地将现代环境科学纳入他们的K-12地球科学类。这项工作的结果也可能给科学带来一些长期的好处。这些包括增强我们预测环境中矿物表面反应动力学的能力，设计新的生长抑制剂以使结晶技术适应工业应用，创建纳米器件和改进的多相催化技术。在这些领域，由于目前缺乏了解，很难事先预测添加的组件将对系统产生什么影响。这反过来又使得为给定的系统或应用设计新的生长抑制剂或改性剂变得困难，并且矿物表面反应速率的定量预测通常是遥不可及的。

Intellectual Merit: Kinetic models of mineral dissolution and growth are hampered by our inability to precisely define the mechanisms of relevant reactions at the molecular scale. The net reaction for many mineral surface processes are known, but the pathway(s) and mechanism(s) through which they occur are not well constrained. Calcite is a particularly important mineral in that it is: a dominant buffer of acidity in soils and groundwater, one of several materials used to understand crystal growth generally, and studied widely to understand biomineralization processes. In the abiological dissolution and growth of calcite, specific, molecular-scale, limiting reaction mechanisms have been proposed, but have not been tested.Similarly, during biomineralization it is known that certain molecules and proteins such as aspartates can influence growth morphology, but quantitative estimates of their kinetic effects remain elusive. The main goal of this proposal is to test proposed reaction mechanisms for calcite dissolution and growth by directly comparing formation and activation energies for step movement measured experimentally to reactions simulated from first principles. Classical crystal growth theories will form the framework to relate the experimentally measured quantities to the simulated chemical mechanisms. The reactions that will be tested are the mechanisms of kink site formation thought to control the movement of steps at circum-neutral pH, atmospheric pCO2, and saturations near equilibrium as well as growth inhibition by a model compound.The proposed research will take place over three years and will address three main hypotheses that will form the bulk of a Ph.D. dissertation:*Hypothesis 1: Step movement on calcite surfaces observed by AFM can be fit by the same model for both growth and dissolution, where kink site formation and propagation are limiting.*Hypothesis 2: Accurate kink site formation and activation energies can be simulated from first principles calculations and simulations.*Hypothesis 3: Detachment of the carboxylic acid functional group of aspartate from calcium ions on the step edge controls its ability to poison growth.The activation and formation energies for kink sites will be estimated from atomic force microscopy measurements of step velocity as a function of saturation and temperature. These will be compared to direct observations of kink site concentrations in equilibrium solutions using high resolution AFM. Simulations will conducted after ensuring the best fit to existing crystal truncation rod date of water structure at calcite interfaces. The mechanism of the reactions will be then be tested by simulating kink site formation and activation energies using ab initio density functional theory and molecular dynamics umbrella sampling. The possible importance of the dissociation of water in the reaction will be explored by protonating surface carbonate and hydroxylating surface calcium ions. Finally, the same AFM measurements and simulation techniques will be made in the presence of aspartate and the experimentally estimated activation energy for growth poisoning will be compared to the simulated activation energy for detachment of aspartate from a calcium on a step edge. The proposed research will combine experimental and computational techniques in a novel way to explicitly test our understanding of growth and dissolution reactions on calcite. This information could be used subsequently in larger-scale rate measurements where assignation of physical meaning to measured rate constants is ambiguous.Broader Impacts: This project will form the bulk of a graduate student's Ph.D. dissertation. Additionally, in year two, a K-12 teacher will be awarded a Georgia Intern-Fellowship for Teachers (GIFT) and work in the laboratory of the PI for six weeks. The goal is to provide experience for the teacher so that he or she can better incorporate modern environmental science into their K-12 Earth Science class. The results from this work could result in some long-term benefits to science as well. These include an enhancement of our ability to predict the kinetics of mineral surface reactions in environmental settings, the design of new growth inhibitors to tailor crystallization techniques to industrial applications, creation of nano-devices and improved heterogeneous catalysis techniques. These are areas where the current lack of understanding makes it difficult to predict a priori what the effect added components will have on a system. This in turn makes design of new growth inhibitors or modifiers for a given system or application difficult, and the quantitative prediction of the rates of mineral surface reactions is often beyond reach.This project is supported jointly with the Ceramics Program in the Division of Materials Research.