Microscopic Perspective on the Adsorption Isotherm of a Heterogeneous Surface

Abstract

Adsorption of dissolved molecules onto solid surfaces can be extremely sensitive to the atomic-scale properties of the solute and surface, causing difficulties for the design of fluidic systems in industrial, medical and technological applications. In this communication, we show that the Langmuir isotherm for adsorption of a small molecule to a realistic, heterogeneous surface can be predicted from atomic structures of the molecule and surface through molecular dynamics (MD) simulations. We highlight the method by studying the adsorption of dimethyl-methylphosphonate (DMMP) to amorphous silica substrates and show that subtle differences in the atomic-scale surface properties can have drastic effects on the Langmuir isotherm. The sensitivity of the method presented is sufficient to permit the optimization of fluidic devices and to determine fundamental design rules for controlling adsorption at the nanoscale.

Modeling the Interface between Biological and SyntheticComponents in Hybrid Nanosystems

“…For example, continuum models work well for modeling the pressure-driven or electroosmotic fluid flow through micro-and nanofluidic devices [31,32,34], but break down in the regions of fluid near surfaces [32] or macromolecules [35], where the atomic-scale characteristics of the solvent, solutes and surfaces become important, which can be on the order of nanometers [29,36]. When the interactions within these regions become important to device function or behavior, such as when adsorption of solutes [37] can lead to clogging of the device [18], discrete methods like classical molecular dynamics (MD) become necessary.…”

“…The diffusion coefficient D can be calculated directly from MD simulations, as we have done previously for DMMP in water [37]. Here, we use the same diffusion coefficient everywhere in the system, but it is also possible to implement a position-dependent diffusion coefficient [107,108] to better describe diffusion in close proximity to the walls of the channel.…”

“…The details of the calculations are described in Appendix D and in Carr et al [37] Note that in our all-atom calculations of both W PP and W PS , the rotational degrees of freedom of the solute are averaged out as the time scale of the solute rotation is much smaller than the length of a typical MD simulation in one of the umbrella-sampling windows. Solute for which such calculations are possible are considered here as "small".…”

“…For example, continuum models work well for modeling the pressure-driven or electroosmotic fluid flow through micro-and nanofluidic devices [31,32,34], but these models break down in the regions of fluid near surfaces [32] or macromolecules [35], where the atomic-scale characteristics of the solvent, solutes, and surfaces, which can be on the order of nanometers [29,36], become important. When the interactions within these regions becomes significantly affect device function or behavior, such as the adsorption of solutes [37] that could lead to clogging of the device [18], the use of discrete methods like classical molecular dynamics (MD) becomes necessary. In my published work [38,39,29,37,40,41,42], I have used MD as a tool to describe the atomic-scale interactions driving the function of biomolecules and their interface with solid-state devices and sought to use it as a starting point to create new methods for modeling, designing, and optimizing these interactions.…”

“…When the interactions within these regions becomes significantly affect device function or behavior, such as the adsorption of solutes [37] that could lead to clogging of the device [18], the use of discrete methods like classical molecular dynamics (MD) becomes necessary. In my published work [38,39,29,37,40,41,42], I have used MD as a tool to describe the atomic-scale interactions driving the function of biomolecules and their interface with solid-state devices and sought to use it as a starting point to create new methods for modeling, designing, and optimizing these interactions.…”