From water clustering to osmotic coefficients.

Area of Science:

• Physical Chemistry
• Atmospheric Chemistry
• Solution Chemistry

Background:

• Water activity is crucial for atmospheric aerosols and aqueous solutions.
• Osmotic coefficients describe water activity but require understanding microscopic water structure.
• Atmospheric electrolytes like NaCl, (NH4)2SO4, NH4Cl, and Na2SO4 are key systems.

Purpose of the Study:

• To correlate water activity (via osmotic coefficients) with microscopic water structure in electrolyte solutions.
• To explain the observed minimum in osmotic coefficients at intermediate molality.
• To elucidate the roles of electrolyte dissociation and water structure in determining solution properties.

Main Methods:

• Literature data on osmotic coefficients from thermodynamic measurements.
• Analysis of electrolyte dissociation from literature or pK values.
• Quantification of water structure using low-wavenumber Raman spectroscopy (180 cm⁻¹ band).

Main Results:

• Osmotic coefficients decrease in dilute solutions and increase in concentrated solutions, showing a minimum.
• Electrolyte dissociation's influence is dominant in dilute solutions.
• Water structure changes become more significant in more concentrated solutions.

Conclusions:

• The interplay between electrolyte dissociation and water microphysical structure explains osmotic coefficient behavior.
• Raman spectroscopy effectively quantifies translationally restricted water molecules.
• This work provides a microscopic explanation for macroscopic solution properties.