Ratan K. Mishra

Thermodynamically Consistent Force Fields for the Assembly of Inorganic, Organic, and Biological Nanostructures: The INTERFACE Force Field

The complexity of the molecular recognition and assembly of biotic-abiotic interfaces on a scale of 1 to 1000 nm can be understood more effectively using simulation tools along with laboratory instrumentation. We discuss the current capabilities and limitations of atomistic force fields and explain a strategy to obtain dependable parameters for inorganic compounds that has been developed and tested over the past decade. Parameter developments include several silicates, aluminates, metals, oxides, sulfates, and apatites that are summarized in what we call the INTERFACE force field. The INTERFACE force field operates as an extension of common harmonic force fields (PCFF, COMPASS, CHARMM, AMBER, GROMACS, and OPLS-AA) by employing the same functional form and combination rules to enable simulations of inorganic-organic and inorganic-biomolecular interfaces. The parametrization builds on an in-depth understanding of physical-chemical properties on the atomic scale to assign each parameter, especially atomic charges and van der Waals constants, as well as on the validation of macroscale physical-chemical properties for each compound in comparison to measurements. The approach eliminates large discrepancies between computed and measured bulk and surface properties of up to 2 orders of magnitude using other parametrization protocols and increases the transferability of the parameters by introducing thermodynamic consistency. As a result, a wide range of properties can be computed in quantitative agreement with experiment, including densities, surface energies, solid-water interface tensions, anisotropies of interfacial energies of different crystal facets, adsorption energies of biomolecules, and thermal and mechanical properties. Applications include insight into the assembly of inorganic-organic multiphase materials, the recognition of inorganic facets by biomolecules, growth and shape preferences of nanocrystals and nanoparticles, as well as thermal transitions and nanomechanics. Limitations and opportunities for further development are also described.

A force field for tricalcium aluminate to characterize surface properties, initial hydration, and organically modified interfaces in atomic resolution

Tricalcium aluminate (C3A) is a major phase of Portland cement clinker and some dental root filling cements. An accurate all-atom force field is introduced to examine structural, surface, and hydration properties as well as organic interfaces to overcome challenges using current laboratory instrumentation. Molecular dynamics simulation demonstrates excellent agreement of computed structural, thermal, mechanical, and surface properties with available experimental data. The parameters are integrated into multiple potential energy expressions, including the PCFF, CVFF, CHARMM, AMBER, OPLS, and INTERFACE force fields. This choice enables the simulation of a wide range of inorganic-organic interfaces at the 1 to 100 nm scale at a million times lower computational cost than DFT methods. Molecular models of dry and partially hydrated surfaces are introduced to examine cleavage, agglomeration, and the role of adsorbed organic molecules. Cleavage of crystalline tricalcium aluminate requires approximately 1300 mJ m(-2) and superficial hydration introduces an amorphous calcium hydroxide surface layer that reduces the agglomeration energy from approximately 850 mJ m(-2) to 500 mJ m(-2), as well as to lower values upon surface displacement. The adsorption of several alcohols and amines was examined to understand their role as grinding aids and as hydration modifiers in cement. The molecules mitigate local electric fields through complexation of calcium ions, hydrogen bonds, and introduction of hydrophobicity upon binding. Molecularly thin layers of about 0.5 nm thickness reduce agglomeration energies to between 100 and 30 mJ m(-2). Molecule-specific trends were found to be similar for tricalcium aluminate and tricalcium silicate. The models allow quantitative predictions and are a starting point to provide fundamental understanding of the role of C3A and organic additives in cement. Extensions to impure phases and advanced hydration stages are feasible.

Understanding the Effectiveness of Polycarboxylates as Grinding Aids

Over recent years, polycarboxylate superplasticizers have found their way into grinding aids used in cement production to reduce the electrical energy consumption. The effectiveness of these large molecules challenges the pre-existing theories concerning the factors that govern the performance of grinding aids. This paper reports on molecular dynamics simulations to examine a physical property believed to control the effective¬ness of grinding aids, namely their adsorption energy. The molecules selected are TIPA (Triisopropanol amine), TEA (Triethanol amine) and glycerine. The surfaces examined are dry and hydroxylated C3S surfaces, which are believed to be more representative of reality, since some humidity is always present during the grinding. Detailed results of this part of the work show that glycerine interacts relatively more with dry as well as hydroxylated surfaces of C3S both at 25°C, ambient temperature and 110°C, grinding temperature with respect to TIPA and TEA. These result help to better understand the specific interaction of these molecules with cement surfaces. In the second part of this work oligomers of some PCE superplasticizers are examined with similar numerical tools on dry and hydroxylated surfaces of C3S. Results for different types of these oligomers, together with the previous results, shed light onto the reasons why polycarboxylate superplasticizers have found to also be effective grinding aids in cement production.

En route to multi-model scheme for clinker comminution with chemical grinding aids

We present a multimodel simulation approach, targeted at understanding the behaviour of comminution and the effect of grinding aids in industrial cement mills. On the atomistic scale, we use molecular dynamics (MD) simulations with validated force field models to quantify elastic and structural properties, cleavage energies as well as the organic interactions with mineral surfaces. Simulations based on the discrete element method (DEM) are used to integrate the information gained from MD simulations into the clinker particle behaviour at larger scales. Computed impact energy distributions from DEM mill simulations can serve as a link between large scale industrial and laboratory sized mills. They also provide the required input for particle impact fragmentation models. Such a multiscale, multimodel methodology paves the way for a structured approach to the design of chemical additives aimed at improving mill performance.We present a multimodel simulation approach, targeted at understanding the behaviour of comminution and the effect of grinding aids in industrial cement mills. On the atomistic scale, we use molecular dynamics (MD) simulations with validated force field models to quantify elastic and structural properties, cleavage energies as well as the organic interactions with mineral surfaces. Simulations based on the discrete element method (DEM) are used to integrate the information gained from MD simulations into the clinker particle behaviour at larger scales. Computed impact energy distributions from DEM mill simulations can serve as a link between large scale industrial and laboratory sized mills. They also provide the required input for particle impact fragmentation models. Such a multiscale, multimodel methodology paves the way for a structured approach to the design of chemical additives aimed at improving mill performance.

Energy-effective Grinding of Inorganic Solids Using Organic Additives

We present our research findings related to new formulations of the organic additives (grinding aids) needed for the efficient grinding of inorganic solids. Even though the size reduction phenomena of the inorganic solid particles in a ball mill is purely a physical process, the addition of grinding aids in milling media introduces a complex physicochemical process. In addition to further gain in productivity, the organic additive helps to reduce the energy needed for grinding, which in the case of cement clinker has major environmental implications worldwide. This is primarily due to the tremendous amounts of cement produced and almost 30% of the associated electrical energy is consumed for grinding. In this paper, we examine the question of how to optimize these grinding aids linking molecular insight into their working mechanisms, and also how to design chemical additives of improved performance for industrial comminution.