Search

Highlights • Presence of salt influences the solute association behavior in nanoconfined solutions. • Dispersion, reversible and aggregation states are still observed. • Increase of salt concentration promotes the formation and growth of clusters. • Transformation of solute association states occurs due to ion polarization. • Tails are more in alignment along radius direction at higher salt concentrations. Abstract Using molecular dynamics simulations, we have studied the effects of salt on the association behavior of solute molecules in nanoconfined aqueous solutions. Three types of solute association states – dispersion, reversible and aggregation states – are still observed in nanoconfined aqueous solutions in the presence of salt. With the increase of ion concentration, the tendency toward the formation of larger clusters is enhanced and the solute critical aggregation number decreases. We even found the transformation of solute association state from the dispersion to the reversible state or from the reversible to the aggregation state within an intermediate range of solute number. This phenomenon is similar to the salting out process and is attributed to the decrease of free water molecules induced by the ion polarization. [ABSTRACT FROM AUTHOR]

Combining the molecular dynamics (MD) simulations and the free-energy-based thermodynamic analysis, we analyze the role of alkyl chain length on the nucleation of alcohol molecules with the mono-hydroxyl in nanoconfined solution. The free energy formulae and thermodynamic equilibrium equation are parameterized by fitting the theoretical equilibrium equation to MD simulation results. We find that alcohol molecules with the varied alkyl chain length show different nucleation states, i.e. , dispersion, reversible and aggregation states, corresponding to the total free energy G c o n f i n e (n , N) with a single minimum at n ∼ 1, two minima at n ∼ 1 and N separated by a maximum, and a local minimum at n ∼ N , respectively. The alcohol molecule with a longer alkyl chain tends to aggregate into a cluster at a lower solute number in nanoconfined solution. Besides, the free energy of cluster formation in nanoconfined solution G c l u s t e r (n) also depends on the alkyl chain length, and three regions can be identified: For the shorter alkyl chain, such as ethanol, there is no local maximum on G c l u s t e r (n), indicating that alcohol molecules tend to be dispersed in water; For the intermediate alkyl chain length, such as 1-propanol, 1-butanol, 1-pentanol, and 1-hexanol, the critical nucleation number n c and nucleation barrier G b monotonically decrease with the increase of alkyl chain length; For the longer alkyl chain, n c ∼ 1 and G b ∼ 0 k B T suggest that alcohol molecules, such as 1-heptanol, can aggregate into clusters easily. Our work deepens the understanding of the alkyl chain length effect on the nucleation behavior of alcohol molecules in nanoconfined solution. [ABSTRACT FROM AUTHOR]

We develop a new Gibbs free energy-based association model and perform series of molecular dynamics (MD) simulations to study the association behavior of Lennard-Jones (LJ) particles in nanoconfined aqueous solution. In the association model, the total Gibbs free energy is the sum of free energy cost in the nucleation of a densely packed cluster with an isotropic shape and a radius dependent interfacial tension coefficient, and the translation entropy of dispersed particles outside the cluster. Two key theoretical expressions – total Gibbs free energy formula and the thermodynamic equilibrium equation – are obtained to estimate the cluster aggregation number for a given LJ particle number and the corresponding total Gibbs free energy. Using MD simulations, we observe that the association state transforms from the stable dispersion state, through the reversible state, finally to the stable aggregation state as the LJ particle number increases. In the reversible state, the system reversibly switch in between the dispersion and aggregation states. The existence of three types of association states is also found for LJ particles with the varied well depth. According to our model, the Gibbs free energy curve shows from a single minimum, through two minima, finally to a single minimum, leading to the transformation of association states. The occurrence of reversible state transition is attributed to the free energy barrier of the order of thermal fluctuation in water between the dispersion and aggregation states. These findings deepen the understanding of nucleation/aggregation of hydrophobic gas molecules or particles in solution under nanoconfinement. • LJ particles show dispersion, reversible, aggregation states in nanoconfined solution. • Three association states still exist even if the well depth changes. • A Gibbs free energy-based association model is developed. • The Gibbs free energy profile shows two minima in the reversible state. • The free energy barrier is the order of thermal fluctuation. [ABSTRACT FROM AUTHOR]