Abstract
The effects of a rapidly heated nanoparticle on the structure of a concentrated aqueous salt solution are studied using molecular dynamics simulations. A diamond-like nanoparticle of radius 20 Å is immersed in a sodium-chloride solution at 20% above the experimental saturation concentration and equilibrated at T = 293 K and P = 1 atm. The nanoparticle is then rapidly heated to several thousand degrees Kelvin, and the system is held under isobaric-isoenthalpic conditions. It is observed that after 2-3 ns, the salt ions are depleted far more than water molecules from a proximal zone 15-25 Å from the nanoparticle surface. This leads to a transient reduction in molality in the proximal zone and an increase in ion clustering in the distal zone. At longer times, ions begin to diffuse back into the proximal zone. It is speculated that the formation of proximal and distal zones, and the increase in ion clustering, plays a role in the mechanism of nonphotochemical laser-induced nucleation.
Highlights
Nonphotochemical laser-induced nucleation (NPLIN) is the phenomenon whereby nanosecond laser pulses of near-infrared or visible light incident upon supersaturated solutions or melts cause nucleation and growth of solid solute.[1,2,3] Because the solutions are transparent at the laser wavelengths used, the mechanism is thought to be nonphotochemical
The first mechanism involves the optical Kerr effect (OKE), whereby the electric field of the light interacts with the anisotropy of solute molecules in clusters, causing molecules to align and the clusters to become viable as crystal nuclei
Recent experiments have shown that the effects of light polarization in NPLIN are much weaker than the original experiments suggested.[9,10]
Summary
Nonphotochemical laser-induced nucleation (NPLIN) is the phenomenon whereby nanosecond laser pulses of near-infrared or visible light incident upon supersaturated solutions or melts cause nucleation and growth of solid solute.[1,2,3] Because the solutions are transparent at the laser wavelengths used, the mechanism is thought to be nonphotochemical. The first mechanism involves the optical Kerr effect (OKE), whereby the electric field of the light interacts with the anisotropy of solute molecules in clusters, causing molecules to align and the clusters to become viable as crystal nuclei This mechanism offers an explanation for the putative effect of polymorph selection by linearly polarized and circularly polarized light.[1,2,3] It should be noted, though, that simulations suggest that the field strengths employed in experiments are too low to cause sufficient orientational bias.[8] recent experiments have shown that the effects of light polarization in NPLIN are much weaker than the original experiments suggested.[9,10] The second mechanism was inspired by NPLIN having being demonstrated in simple ionic solutions,[11–17] in which the OKE is unlikely to occur at the atomic scale.
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