SinodosChemistry

Adsorption and absorption are two of the prevailing technologies for hazardous compounds removal or valuable compounds separation and storage either from fluid streams or solid bodies. While other choices exist these two approaches exhibit significant advantages that make them suitable for such processes.

Adsorption can be a physical or a chemical processes, depending on the interaction strength between adsorbent and adsorbates. Bond formation leads to chemical adsorption classification [or chemi sorption] whereas dispersion forces interactions [London, VdW, electrostatic] lead to a physical adsorption [or physisorption] classification. Physical adsorption is observed where interaction energies [or better, isosteric heats or enthalpies] of adsorption are found in the 0-7 kcal mol^-1 range and chemisorption is usually observed when enthalpies are higher than 25 kcal mol^-1 range. There is an intermediate region that phenomena can overlap and thus simultaneous weak and strong interactions are present.

In order to accurately predict such phenomena, one employs molecular simulation approaches. These include the model building of both adsorbent and adsorbate molecules and setting up a proper molecular simulation method. Such methods include Molecular Dynamics, Monte Carlo or just Single Point calculations [less often]. Each methodology has specific advantages; Molecular Dynamics reveal real time evolution of the physical [or chemical] process involved and thus is ideal for observation and understanding of the underlying mechanisms. They also provide time-dependent properties such as diffusivities and molecular velocities. On the other hand, Monte Carlo methods are purely stochastic; they do not provide any information for the evolution of the process, just final system configuration. Their great advantages include speed of calculations, and suitability for estimation of factors of the highest importance for adsorption processes, such as chemical potential of the adsorbed phase. Single Point calculations are mainly used for ‘fixed’ configurations of molecules, in order to confirm interaction energies as a function of intra molecular distance or set up. They are only suitable for specific simulations and thus of limited use.

We currently prefer using Molecular Dynamics simulations for adsorption simulations, using all different levels of force fields, depending on the system. When physical adsorption is to be carried out, MM+ or AMBER fields [or any Molecular Mechanics field] are excellent choices, especially as the total number of system atoms increases. If the system atoms count is less than 50, Ab initio or DFT force fields can be used in order to acquire other properties, including electronic and optical ones. When chemical adsorption is modeled, is always better to simulate a smaller part of the system in order to employ Ab Initio or DFT; MM methods cannot depict bond formation [although bond breaking can be identified as we will discuss in another post soon]. 

Molecular simulations of adsorption have proven their value for more than two decades now; they have been employed in the prediction of hydrogen, carbon dioxide, carbon monoxide, VOCs, poisonous gases, precious metals and others using a vast range of adsorbents.