Comparison of several classical density functional theories for the adsorption of flexible chain molecules into cylindrical nanopores

Abstract

Adsorption of flexible oligomers into narrow cylindrical pores has been studied by means of several versions of classical density functional theory (CDFT) and Monte Carlo simulation. The adsorption process is interesting to study due to the competition between the entropic depletion of oligomers from the pores and the wall-oligomer attraction. It is also challenging to describe using current CDFTs, which tend to overestimate the amount of the adsorbed fluid. From a comparison of several different CDFT approaches, we find that this is due to the assumption of ideal or freely jointed chain conformations. Moreover, it is demonstrated that it is impossible to obtain a reasonable description of the adsorption isotherms without taking into account accurate contact values in the distribution functions describing the structure of the reference monomer fluid. At low densities, more accurate result are obtained in comparison with Monte Carlo simulation data when accurate contact values are incorporated into the theory rather than the more commonly used hard-sphere contact value. However, even the CDFT with accurate contact values still overestimates the amount of the adsorbed fluid due to the ideal or freely jointed chain approximation, used for the description of chain conformations in most CDFT approaches. We find that significant improvement can achieved by employing self-consistent field theory, which samples self-avoiding chain conformations and decreases the number of possible chain conformations, and, consequently, the amount of the adsorbed fluid.

Exponential approximation for one-component Yukawa plasma

“…28,29 Nonetheless, the FMSA theory is only slightly less accurate than the full MSA theory, and in contrast to the latter is "solvable" at any thermodynamic conditions. 30 Its simplicity and robustness makes it useful for application in the inverse design approaches, 31 formulation of density functional theories, [32][33][34][35][36][37] and various other advanced approximations of the liquid-state theory. [38][39][40][41][42] In this contribution a new theory based on the exponential approximation 44 will be proposed for the description of the OCYP systems.…”

Adsorption and Phase Behavior of Pure/Mixed Alkanes in Nanoslit Graphite Pores: An iSAFT Application

“…The implementations of iSAFT or polymer-DFT, to name a few, include the interfacial tension of polymers, 32 lipid assembly and confined block copolymers, 33 tethered polymers within or without solvent, 34 surfactant in oil−water interfaces, 35 the formation of micelles in oil/water/ surfactant systems, 36 tethered polymer-modified pores, 37,38 and polymer adsorption. 39 For applications in shale gas/oil systems, other DFT approaches have also been proposed on the basis of some common bulk equations of state. For example, the adsorption and desorption behavior of hydrocarbon mixtures was studied by Li et al 40 using a Peng−Robinson equation of state (EoS) free energy functional.…”

“…The development of inhomogeneous statistical associating fluid theory (iSAFT), modified iSAFT, and polymer-DFT has largely enhanced our ability to model inhomogeneous systems of complex fluids. The implementations of iSAFT or polymer-DFT, to name a few, include the interfacial tension of polymers, lipid assembly and confined block copolymers, tethered polymers within or without solvent, surfactant in oil–water interfaces, the formation of micelles in oil/water/surfactant systems, tethered polymer-modified pores, , and polymer adsorption . For applications in shale gas/oil systems, other DFT approaches have also been proposed on the basis of some common bulk equations of state.…”