Density Fluctuation in Aqueous Solutions and Molecular Origin of Salting-Out Effect for CO₂

Using molecular dynamics simulation, we studied the density fluctuations and cavity formation probabilities in aqueous solutions and their effect on the hydration of CO₂. With increasing salt concentration, we report an increased probability of observing a larger than the average number of species in the probe volume. Our energetic analyses indicate that the van der Waals and electrostatic interactions between CO₂ and aqueous solutions become more favorable with increasing salt concentration, favoring the solubility of CO₂ (salting in). However, due to the decreasing number of cavities forming when salt concentration is increased, the solubility of CO₂ decreases. The formation of cavities was found to be the primary control on the dissolution of gas, and is responsible for the observed CO₂ salting-out effect. Our results provide the fundamental understanding of the density fluctuation in aqueous solutions and the molecular origin of the salting-out effect for real gas.