IMPROVED MSA THEORY FOR CONCENTRATED ELECTROLYTE-SOLUTIONS BASED ON MONTE-CARLO SIMULATION AT HIGH IONIC STRENGTHS

Current search for valid ionic solution properties correlations at hig h concentrations has focused on the mean spherical approximation (MSA) because of its ability to treat ion size effects. We propose a fundam entally sound modification of the MSA based on comparisons with new Mo nte Carlo simulations for aqueous salt solutions. The osmotic coeffici ent phi=P-osm/rho k(B)T in this study is decomposed into three parts i n line with the primitive-model pair potentials. The more familiar lon g-range (Coulombic), and the contact hard-core parts are well reproduc ed by most approximate ionic theories. The short-range electrostatic ( SRE) contribution is the part that poses as a challenge to many theore tical efforts. In order to characterize the SRE behavior, Monte Carlo simulation with Ewald sum is carried out for 1-1 electrolytes up to 15 molar. The short-range electrostatic phi'(SRE), is a non-negligible q uantity that can reach 30% of the value of the long-range phi'(LR). An y ionic solution theory that ignores phi'(SRE) will incur inaccuracies . An interesting behavior of the SRE phi is a unimodal maximum as the molarity M varies from 0 to 15. This takes place an similar to 10 M fo r aqueous LiCl solutions, well beyond the ranges investigated by most existing simulation data. The new MC data cover 1.01, 2.2, 3.4, 4.3, 5 .3 and 6.4 molar for NaCl, and in addition, 7.5, 10, 12.5 and 15 molar for LiCl solutions. The diameter ratio sigma(--)/sigma(++) ranges fro m 1 to 3. To interpret this behavior, we examine a number of prevailin g theories including the mean spherical approximation (MSA), hypernett ed-chain (HNC) equation, hypervertex equations, and a number of modifi ed MSA equations (such as LIN, LIN+SQ and EXP). In addition to 1-1 ele ctrolytes, comparisons are also made with molten salt (at Bjerrum leng ths similar to 30) and 2-2 electrolyte data. Some of our observations are consistent with previous studies on primitive model systems based on less extensive simulation data sets. However, none of the analytica l theories (HNC presently excluded) are able to reproduce MC results f or wide ranges. We propose two analytical formulations: MIXL and MIXQ based on the MSA solutions from Blum and Hoye for this purpose. These have been tested: MIXQ is good for concentrated electrolyte solutions (up to 15 M), and MIXL is valid for high Bjerrum length molten salts.