Critical phenomena and thermodynamics of dilute aqueous sodium chloride to 823 K

Abstract

Semiempirical equations are developed that represent the behavior of dilute solutions of NaCl in water (steam) in the range 723-823 K where ion pairing is extensive.This supplements the equations given earlier for more concentrated solutions. In this temperature range the system NaCl/H20 shows critical behavior with two phases below the critical pressure. The equations for the dilute solutions yield critical behavior. Though the equations for concentrated solutions do not yield critical behavior at the critical pressure, only a very small interpolation function is required to connect smoothly the two equations. The ion-pairing equilibrium constants are reported as well as the Gibbs energies of hydration for both ions and ion pairs.Recently, we reported (1) surprisingly simple equations that describe the behavior of aqueous NaCl over the full range of solubility from 373 to 573 K and at a mole fraction above 0.1 from 573 to 823 K. In the temperature range above the critical point of water (647 K), there is phase separation and a critical curve that rises in pressure with increase in temperature. On the dilute side of the phase separation, the "steam" phase shows strong ion pairing in contrast to the full dissociation into ions in the concentrated or "liquid" phase. Above the critical pressure, the properties are, of course, continuous; but as the concentration increases the pattern changes from isolated ion pairs to a distribution with more than one ion of opposite charge near a given ion. Thus, the more concentrated solutions are appropriately described as fully ionized. This "redissociation" phenomenon is discussed more fully elsewhere (2). remains very close to that of pure steam, while the activity of NaCl can be estimated from information on the properties of hydrated gas ions. These aspects were discussed in a recent paper (4). The present results give some guidance for estimates at pressures moderately above critical. EQUATIONSThe composition measure for the concentrated range was mole fraction on an ionized basis, and we continue to use this basis. Thus, for the NaCl, X2 = 2n2/(ni + 2n2).[1]It is convenient to extend this same system for the present range but recognizing ion pairing when necessary. With 6, the fraction of solute as ion pairs, one can define mole fractions offree ions xi (either + or -), of pairs xp, and of solvent (water) x, as follows:Here n1 and n2 are the numbers of moles of H20 and NaCl, respectively, and I, is the ionic strength on the mole fraction basis. The equation used previously for the Gibbs energy (1, 5, 6) can be generalized for ion pairing to G/RT = n1ln x, + On2ln xp + 2(1 -6)n2ln xi + 2wn2(1 -X2) - China. 1268The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Ion-Pair Association in Ultrasupercritical Aqueous Environments: Successful Interplay among Conductance Experiments, Theory, and Molecular Simulations

“…The pioneering experimental work on ion association in steam by O ¨lander and Liander, 13 Sourirajan and Kennedy, 14 Martynova et al, [15][16][17] Khaibullin and Borisov, 18 Styrikovich et al, 19 Lukashov et al, 20 and the more recent studies of Pitzer, 11,21,22 Alekhin and Vakulenko, 23 and Bischoff et al 24 among others, have provided a wealth of data for correlation purposes. Traditionally, this experimental information has been gathered from solubility measurements [13][14][15][16][17][18][19]25 and electric conductance measurements of dilute alkali halides.…”

“…The pioneering experimental work on ion association in steam by Ölander and Liander, Sourirajan and Kennedy, Martynova et al, − Khaibullin and Borisov, Styrikovich et al, Lukashov et al, and the more recent studies of Pitzer, ,, Alekhin and Vakulenko, and Bischoff et al among others, have provided a wealth of data for correlation purposes. Traditionally, this experimental information has been gathered from solubility measurements − , and electric conductance measurements of dilute alkali halides. ,− The latter provides a direct route to the association constant usually supplemented by calorimetric, , isopiestic, , and volatility measurements , to facilitate the interpretation of the observed behavior.…”

Study of the Critical Behavior of Polar Fluids by Renormalization Group Theory

“…Polar fluids are often used in industry, and among them, water is the most important fluid in the world. It is well-known that water is an associating liquid with highly effective hydrogen bonding under ambient conditions, but supercritical water above 647 K and 22.05 MPa shows much less effective hydrogen bonding, as reflected by its lower dielectric constant and dissociation constant. , In this paper, we apply White's RG method to fluids interacting via a Stockmayer potential function to calculate the thermodynamic properties of water and methanol around the critical point.…”