From Water Clustering to Osmotic Coefficients

Abstract

Water activity is an important macroscopic property of aerosol particles and droplets in the atmosphere as well as aqueous solutions in many other fields of physical chemistry. This study focuses on relating water activity, described using osmotic coefficients, to the microscopic water structure in systems of atmospheric relevance, namely, aqueous solutions of each of the four electrolytes: NaCl, (NH(4))(2)SO(4), NH(4)Cl, and Na(2)SO(4). The osmotic coefficients of these compounds, as reported in literature based on thermodynamic measurements, decrease as a function of molality for dilute solutions and increase as a function of molality for concentrated solutions. At an intermediate molality, a minimum value of the osmotic coefficient is observed. We explain this behavior by describing osmotic coefficients as the product of two concentration-dependent effects: incomplete electrolyte dissociation and variations in the microphysical water structure. The degree of dissociation in electrolyte solutions can be obtained directly from literature or derived from reported pK values, and in this work the water structure is quantified using low-wavenumber Raman spectroscopy. We use the band at 180 cm(-1) in Raman spectra of aqueous electrolyte solutions, which has been assigned to the displacement of the central oxygen atom in a tetrahedral hydrogen bonding environment composed of five H(2)O units. The abundance of such translationally restricted water molecules is essential in describing the local microphysical structure of water, and the height of the band is used to estimate the amount of such translationally restricted water molecules in solution. We were able to qualitatively reproduce and explain literature values of osmotic coefficients for the four studied electrolytes. Our results indicate that the effect of electrolyte dissociation, which decreases as a function of molality, dominates in dilute solutions, whereas changes in water structure are more significant at higher concentrations.

The Debye-Hückel theory and its importance in modeling electrolyte solutions

“…One also observes a minimum in the activity coefficients of NaBr in Figure 1. Some (Frosch et al, 2010 10 ) have stated that it is due to the effect of water and more specifically the change of water structure due to ions. Others (Heyrovska, 2000 11 ) have attributed it to the fact that even strong electrolytes may (only) partially dissociate, and thus there is presence of unassociated salt molecules in equilibria with the ions.…”

Water Activity

“…A thorough understanding of the interplay between hydrogen bonded water structure and dynamics is still missing. Recently we have used low-wavenumber Raman spectroscopy in the R(ν)-representation to reveal the amount of translationally "free water" in aqueous salt solutions [15,16]. The intensity of a low-wavenumber band at 180 cm −1 was used to monitor water molecules in a tetrahedral configuration with four hydrogen bonds in aqueous solutions at various concentrations (m molal) of NaCl, NH 4 Cl, Na 2 SO 4 , and (NH 4 ) 2 SO 4 , respectively.…”

“…The intensity of a low-wavenumber band at 180 cm −1 was used to monitor water molecules in a tetrahedral configuration with four hydrogen bonds in aqueous solutions at various concentrations (m molal) of NaCl, NH 4 Cl, Na 2 SO 4 , and (NH 4 ) 2 SO 4 , respectively. Water molecules in a tetrahedral configuration were trapped in a potential minimum and consequently considered as "bound" water [15,16]. These molecules may be surrounded not only by other water molecules but also ions.…”

“…Water activities for the four electrolytes investigated were calculated from vapour pressure measurements by Low [14]. A comparison between a w obtained from [14] and b obtained from [15,16] is shown in Figure 1. For each electrolyte deviations between the two curves are observed.…”

“…Assuming that the curves b in Figure 1 describe the real content of "free" water in the solutions, the relative magnitudes of the surface tensions for the four electrolytes might explain the differences between a w and b in Figure 1. For Na 2 SO 4 solutions the water [14], and translationally "free" water, b (full lines) from [15,16] in aqueous solutions of sodium-and ammonium chloride and sodium-and ammonium sulphate. All values refer to room temperature.…”