Non-photochemical laser-induced nucleation (NPLIN): An experimental investigation of crystal nucleation

Abstract

NPLIN was studied in supersaturated solutions (S = 1.06) of potassium chloride (KCl) and bromide (KBr). The fraction of samples nucleated (f) follows a nonlinear dependence on peak power density that approaches f = 1 at higher incident powers. It is shown that a lower threshold power is required for nucleation using 532 nm laser pulses than at 1064 nm, and that a higher fraction of samples nucleate when exposed to 532 nm pulses at a given laser power. Comparison with KCl shows higher fractions of KBr samples nucleate with lower threshold values at both wavelengths. Samples of KCl of equal supersaturation at two different temperatures (23 and 33 °C) exposed to 1064 nm pulses show that those at 33 °C are significantly more labile to nucleation. The ratio of samples nucleated at 33 °C compared to those at 23 °C was 2.11 ± 0.47. A classical nucleation model based on activation of subcritical solute clusters accounts remarkably well for the experimental data and provides phenomological values of the crystal–solution interfacial tension (γ) at 23 °C for KCl and KBr of 5.283 and 4.817 mJ m-2. At 33 °C, the model yields a best-fit value of γ = 5.429 mJ m-2 for KCl. As an extension of this work the use of an evanescent wave (ew) generated by total internal reflection was investigated as a method to cause nucleation in supersaturated KCl solution. Evanescent wave NPLIN (ew-NPLIN) was shown to cause nucleation. The results showed a higher laser-power threshold required for nucleation and sample lability greater than that of bulk NPLIN. In a second approach to understanding NPLIN, the structures of concentrated solutions were probed by a series of laser scattering experiments. Evidence showing populations of particles in solution was provided by Rayleigh laser scattering (RLS) experiments. Scattering in solutions prepared to be nearly saturated (S = 0.95) was observed using a low magnification (×10) microscope objective; almost all solutions showed the presence of scattering objects moving freely in solution. For those that showed no particles, it was noted that the solutions were typically of higher solute concentration (> 11 mol% solute). Ammonium nitrate solution showed no particles using ×10 magnification, however particles were identified when higher magnification was used (×50 and ×100). Video footage of the Rayleigh scattering observed in aqueous solutions of glycine, urea and ammonium nitrate obtained using ×50 magnification were analysed using a custom nanoparticle tracking software. The results showed a population of particles in aqueous urea and glycine solutions with particle concentrations of the order 108 particles cm-3 and mean hydrodynamic diameter of approximately 267 ± 1 and 173 ± 2 nm respectively. Not enough particles were identified in ammonium nitrate solution to complete the tracking analysis; however a fluctuating background scatter suggested a population of particles with sizes below the limit of resolution of the optical system. Using aqueous urea solution as a model system the structure of the particles identified in solution was investigated using scanning microscopy. The second-harmonic scattering (SHS) signal measured in concentrated aqueous urea solution was measured as a function of solution concentration (C) over a range of saturation conditions from undersaturated (S = 0.15) to supersaturated (S = 1.86). The results show a non-linear increase in SHS signal with local maxima near S = 0.95 and 1.75 suggesting a change in solution structure near these points. Rayleigh scattering images indicate the presence of particles in nearly saturated (S = 0.95) urea solutions. Time-dependent SHS measurements indicate that signals originate from individual events encountered during scanning of the sample through the focal volume of the probe laser, consistent with second harmonic generation (SHG) from particles. SHG from aqueous dispersions of barium titanate (BaTiO3) nanoparticles with diameters < 200 nm, showed signals ~20 times larger than urea solutions. The results suggest the existence of a population of semi-ordered clusters of urea that changes with solution concentration.