Monte Carlo Simulations of Salt Solutions: Exploring the Validity of Primitive Models

Abstract

An extensive series of Monte Carlo (MC) simulations were performed in order to explore the validity of simple primitive models of electrolyte solutions and in particular the effect of ion size asymmetry on the bulk thermodynamic properties of real salt solutions. Ionic activity and osmotic coefficients were calculated for 1:1, 2:1, and 3:1 electrolytes by using the unrestricted primitive model (UPM); i.e., ions are considered as charged hard spheres of different sizes dissolved in a dielectric continuum. Mean ionic activity and osmotic coefficients calculated by the MC simulations were fitted simultaneously to the experimental data by adjusting only the cation radius while keeping the anion radius fixed at its crystallographic value. Ionic radii were further optimized by systematically varying the cation and anion radii at a fixed sum of ionic radii. The success of this approach is found to be highly salt specific. For example, experimental data (mean ionic activity and osmotic coefficients) of salts which are usually considered as dissociated such as HCl, HBr, LiCl, LiBr, LiClO(4), and KOH were successfully fitted up to 1.9, 2.5, 1.9, 3, 2.5, and 4.5 M concentrations, respectively. In the case of partially dissociated salts such as NaCl, the successful fits were only obtained in a more restricted concentration range. Consistent sets of the best fitted cation radii were obtained for acids, alkali, and alkaline earth halides. A list of recommended ionic radii is also provided. The reliability of the optimized ionic radii was further tested in simulations of the osmotic coefficients of LiCl-NaCl-KCl salt mixtures. A very good agreement between the simulated and experimental data was obtained up to ionic strength of 4.5 M.

Energy variational analysis of ions in water and channels: Field theory for primitive models of complex ionic fluids

“…Ion concentrations inside cells are controlled by biological ͑nano͒ valves called ion channels 120 and controlled and maintained by ion transporters ͑"pumps" 118,128 ͒ as ions move in pores within proteins across otherwise impermeable lipid membranes. It is surprising, but true, that many complex properties of channels selective for calcium or sodium ions can be understood [14][15][16][17][18][19][49][50][51][52]58,60,67,68,77,[109][110][111][112][113][114][115][129][130][131][132][133] using adaptations of the primitive model of ions in bulk solutions [35][36][37][38]90,98,99 fulfilling the early dreams of biophysical chemists, to some extent.…”

“…The ionic phase is a primitive model of ionic solutions. [35][36][37][38]90,[97][98][99] It is a compressible plasma made of charged solid ͑nearly hard͒ spheres. The ionic "primitive phase" is itself a composite of two scales, a macroscopic compressible fluid and an atomic scale plasma of solid spheres in a frictional dielectric.…”

Electrochemical migration technique to accelerate ageing of cementitious materials

“…High alkalinity in catholyte ensue migration of hydroxide ions to the specimen leading to lower rate in decalcification process. The anolyte solution is 2M lithium hydroxide solution providing a non-corrosive alkaline environment around the anode as well as enabling migration of calcium ions from the specimen's pore solution replaced by lithium ions with low diffusive characteristics due to their effective ionic radius [13]. The ionic migration of ions in the system is presented in Figure 3.…”

Comment on “The nonmonotonic concentration dependence of the mean activity coefficient of electrolytes is a result of a balance between solvation and ion–ion correlations” [J. Chem. Phys. 133, 154507 (2010)]

“…The very fact that a fit of a model (such as SiS) with experiment is done by adjusting ISPs does not necessarily mean that the model is empirical. For example, a MC simulation of the UPM has been reported recently, 6 which uses optimized ISPs; this does not render the PM empirical. Interestingly, not only are the optimized cation-cation ISPs of the UPM simulation usually much larger than crystallographic values (to provide a stronger core effect, see above), but the MC fit with experiment does not appear to be better than the SiS's.…”