Non-photochemical Laser-induced Nucleation Research Articles

Effects of nanoparticle heating on the structure of a concentrated aqueous salt solution.

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.

Supersaturation dependence of glycine polymorphism using laser-induced nucleation, sonocrystallization and nucleation by mechanical shock.

The nucleation of glycine from aqueous supersaturated solution has been studied using non-photochemical laser-induced nucleation (NPLIN), ultrasound (sonocrystallization), and mechanical shock of sample vials. It was found that at higher supersaturation, samples were more susceptible to nucleation and produced more of the γ-glycine polymorph. The results are described in terms of a mechanism common to all three nucleation methods, involving the induction of cavitation events and pressure shockwaves. The switch in preference from α- to γ-glycine was observed to occur over a narrower range of supersaturation values for NPLIN. We attribute this to induction of cavitation events with higher energies, which result in higher localized pressures and supersaturations. Experiments on NPLIN using circularly versus linearly polarized light showed no evidence for binary polarization switching control of glycine polymorphism.

Role of Impurity Nanoparticles in Laser-Induced Nucleation of Ammonium Chloride

Results of experiments on laser-induced nucleation (LIN) in supersaturated (120%) aqueous ammonium chloride solutions are presented. Measurement of the particle-size distribution in unfiltered solutions near saturation (95%) indicates a population of nanometer-scale species with a mean hydrodynamic diameter of 750 nm, which is almost entirely removed by single-pass filtration through a poly(ether sulfone) membrane (0.2 μm pores). Analysis of filter residues reveals iron and phosphate as major impurities in the solute. Experiments show that the number of nuclei induced by LIN can be reduced substantially by preprocessing (filtering or long-term exposure to laser pulses) and that this reduction can be reversed by intentional doping with iron-oxide (Fe3O4) nanoparticles. The use of surfactant to assist dispersion of the nanoparticles was found to increase the number of laser-induced nuclei. We discuss the results with reference to mechanisms of non-photochemical laser-induced nucleation.

Non-Photochemical Laser-Induced Nucleation of Sulfathiazole in a Water/Ethanol Mixture

Non-photochemical laser-induced nucleation (NPLIN) is an original and promising nucleation method that has been applied with success to small-molecule organic compounds, proteins, and inorganic compounds, leading to almost 50 publications. After the first application of NPLIN to glycine and carbamazepine, our team has performed NPLIN experiments on sulfathiazole (STZ) for the following reasons: (i) STZ is considered as a model drug for studying polymorphism; (ii) some STZ polymorphs present a two-dimensional packing type, while others have one-dimensional packing; (iii) STZ solubility curves, crystal habits, and crystallization characterization are well documented. First, we have determined the metastable zone limit of STZ in water/ethanol (v/v 1:1) for three temperatures (15, 25, and 40 °C). STZ metastable solutions have been irradiated with a nonfocused nanosecond laser (532 nm) at 25 °C. Nucleation induction time and crystal habits have been compared with those obtained by spontaneous nucleation. Nucle...

Nonphotochemical Laser-Induced Crystal Nucleation by an Evanescent Wave

Crystal nucleation at a glass–solution interface has been demonstrated by evanescent-wave laser-induced nucleation for the first time. Nucleation was induced in supersaturated aqueous potassium chloride (KCl) solutions. The interaction volume was estimated to be <2 nL, localized within approximately 100 nm of the interface. Measurements of the power dependence of nucleation events showed a laser power threshold that was approximately 3 times higher than that in experiments in bulk solution; the average number of nucleation events above threshold was similar to that in bulk. Surfaces treated with a hydrophobic polydimethylsiloxane-based coating showed no nucleation over the coated regions. The localization offered by this method has significant potential benefit for studies of the nucleation mechanism or for patterning of crystal nuclei in two dimensions.

Laser-induced nucleation of carbon dioxide bubbles.

A detailed experimental study of laser-induced nucleation (LIN) of carbon dioxide (CO2) gas bubbles is presented. Water and aqueous sucrose solutions supersaturated with CO2 were exposed to single nanosecond pulses (5 ns, 532 nm, 2.4-14.5 MW cm(-2)) and femtosecond pulses (110 fs, 800 nm, 0.028-11 GW cm(-2)) of laser light. No bubbles were observed with the femtosecond pulses, even at high peak power densities (11 GW cm(-2)). For the nanosecond pulses, the number of bubbles produced per pulse showed a quadratic dependence on laser power, with a distinct power threshold below which no bubbles were observed. The number of bubbles observed increases linearly with sucrose concentration. It was found that filtering of solutions reduces the number of bubbles significantly. Although the femtosecond pulses have higher peak power densities than the nanosecond pulses, they have lower energy densities per pulse. A simple model for LIN of CO2 is presented, based on heating of nanoparticles to produce vapor bubbles that must expand to reach a critical bubble radius to continue growth. The results suggest that non-photochemical laser-induced nucleation of crystals could also be caused by heating of nanoparticles.

Structure and dynamics of potassium chloride in aqueous solution.

The structure and dynamics of potassium chloride in aqueous solution over a wide range of concentrations-and in particular beyond saturation-are studied using molecular dynamics simulations to help shed light on recent experimental studies of nonphotochemical laser-induced nucleation (NPLIN). In NPLIN experiments, the duration, t, of the laser pulse (with wavelength 1064 nm) is found to influence the occurrence of crystal nucleation in supersaturated KCl(aq): if t is less than about 5 ps, no crystal nucleation is observed; if t is greater than about 100 ps, crystal nucleation is observed, and with a known dependence on laser power. Assuming that the laser acts on spontaneously formed solute clusters, these observations suggest that there are transient structures in supersaturated solutions with relaxation times on the scale of 5-100 ps. Ion-cluster formation and ion-cluster lifetimes are calculated according to various criteria, and it is found that, in the supersaturated regime, there are indeed structures with relaxation times of up to 100 ps. In addition, the ion dynamics in this regime is found to show signs of collective behavior, as evidenced by stretched exponential decay of the self-intermediate scattering function. Although these results do not explain the phenomenon of NPLIN, they do provide insights into possible relevant dynamical factors in supersaturated aqueous solutions of potassium chloride.

A new experimental setup for high-throughput controlled non-photochemical laser-induced nucleation: application to glycine crystallization

Non-photochemical laser-induced nucleation (NPLIN) has been a growing field of study since 1996, and more than 40 compounds including organics, inorganics and proteins have now been probed under various conditions (solvents, laser types, laser beams etc.). The potential advantages of using this technique are significant, in particular polymorphic control. To realize these benefits, the objective is a carefully designed experimental setup and highly controlled parameters, for example temperature and energy density, in order to reduce the uncertainty regarding the origin of nucleation. In this paper, a new experimental setup designed to study NPLIN is reported. After a full technical description of the present setup, the different functionalities of this device will be illustrated through results on glycine. Glycine crystals obtained through NPLIN nucleate at the meniscus and exhibit different morphologies. The nucleation efficiency, as a function of the supersaturation of the solution used and the laser beam energy density, has also been established for a large number of samples, with all other parameters held constant.

Experimental Demonstration of the Carbamazepine Crystallization from Non-photochemical Laser-Induced Nucleation in Acetonitrile and Methanol

This paper reports for the first time the crystallization of the carbamazepine (CBZ) molecule in two solvents (methanol and acetonitrile) using the non-photochemical laser-induced nucleation (NPLIN) technique. The metastable zone of CBZ is first determined experimentally for different temperatures in both solvents. Then, the prepared solutions are irradiated by a 532 nm wavelength nanosecond pulsed laser and permitted to obtain CBZ crystals of phases I and III. The impact of laser power and polarization (circularly (CP) and linearly (LP)) on the CBZ crystallization efficiency in both solvents is determined through experiments. According to the results, the crystallization efficiency is significantly higher in methanol than in acetonitrile, and it is higher in solutions irradiated by CP laser than those by LP laser. Moreover, the irradiation of an acetonitrile solution by a LP laser results in CBZ phases I and III, whereas irradiation by the CP laser leads to CBZ phase III crystals. An ab initio determinat...

Nonphotochemical Laser-Induced Nucleation in Levitated Supersaturated Aqueous Potassium Chloride Microdroplets

We have observed nonphotochemical laser-induced nucleation (NPLIN) in levitated 8 pL microdroplets of supersaturated aqueous potassium chloride irradiated with nanosecond pulses of 532 nm light. A much higher supersaturation ratio S of 1.20 was required to observe NPLIN in a microdroplet, compared to the value of 1.06 for bulk solutions, attributed to a 40-million-fold lower nucleation rate in a microdroplet at S = 1.06 as a result of its much smaller volume. This finding has application to high spatial and temporal resolution pump–probe studies of nucleation in a containerless environment.

Nonphotochemical Laser-Induced Nucleation of Potassium Halides: Effects of Wavelength and Temperature

Nonphotochemical laser-induced nucleation (NPLIN) of KCl and KBr has been studied using nanosecond laser pulses with wavelengths of 532 and 1064 nm. The ratio of the number of samples nucleated to the number irradiated with the laser (f) follows a nonlinear dependence on peak power density that approaches f = 1 at higher incident powers, which we fit with a Poisson function. The threshold power required to nucleate samples is lower at 532 than at 1064 nm, and we observe that a higher fraction of samples nucleate when exposed to 532 nm pulses at a given laser power. In comparison to KCl, we see higher fractions of KBr samples nucleate and lower threshold values at both wavelengths. Samples of KCl of equal supersaturation at two different temperatures (23 and 33 °C) shot with 1064 nm pulses show that the 33 °C samples 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...

Non-photochemical laser-induced nucleation of supercooled glacial acetic acid

Non-photochemical laser-induced nucleation (NPLIN) of glacial acetic acid (GAA) is demonstrated. The fraction of samples nucleated depends linearly on peak laser power density at low powers (<100 MW cm(-2)) with a threshold of (9.0 ± 4.2) MW cm(-2); at higher laser powers the fraction reaches a plateau of 0.75 ± 0.24 (2σ uncertainties). A simple model based on polarizability of pre-nucleating clusters gives a value of the solid-liquid interfacial tension γ(SL) = 15.5 mJ m(-2). It is hoped that the results will stimulate new developments in experimental and theoretical studies of cluster structure and nucleation in liquids.

Chiral hide-and-seek: Retention of enantiomorphism in laser-induced nucleation of molten sodium chlorate

We report the observation of non-photochemical laser-induced nucleation (NPLIN) of sodium chlorate from its melt using nanosecond pulses of light at 1064 nm. The fraction of samples that nucleate is shown to depend linearly on the peak power density of the laser pulses. Remarkably, we observe that most samples are nucleated by the laser back into the enantiomorph (dextrorotatory or levorotatory) of the solid prior to melting. We do not observe a significant dependence on polarization of the light, and we put forward symmetry arguments that rule out an optical Kerr effect mechanism. Our observations of retention of chirality can be explained by decomposition of small amounts of the sodium chlorate to form sodium chloride, which provide cavities for retention of clusters of sodium chlorate even 18 °C above the melting point. These clusters remain sub-critical on cooling, but can be activated by NPLIN via an isotropic polarizability mechanism. We have developed a heterogeneous model of NPLIN in cavities, which reproduces the experimental data using simple physical data available for sodium chlorate.

A simulation test of the optical Kerr mechanism for laser-induced nucleation

Recent experiments have demonstrated that intense, nanosecond laser pulses can induce crystal nucleation from supersaturated solutions that are transparent at the incident wavelengths, a phenomenon termed nonphotochemical laser-induced nucleation (NPLIN). Previous work has proposed that this effect is due to the alignment of solute molecules in solution due to the electric field of the applied laser light, promoting crystalline order. We have used simulations of NPLIN to examine how an orientational bias in solution affects nucleation with Monte Carlo simulations of a Potts lattice gas model. We examine this effect within both a classical, one-step nucleation framework as well as in the context of two-step nucleation. Our results indicate that an orientational bias can reduce the free energy barrier to nucleation within the one-step picture as well as promote the crystallization of amorphous precritical nuclei (the rate-determining step in the two-step picture). However, these effects are only present with field strengths that are much greater than those used in experiments.

Nanosecond pulse width dependence of nonphotochemical laser-induced nucleation of potassium chloride

The effect of laser pulse width on the efficiency of nonphotochemical laser-induced nucleation (NPLIN) of supersaturated aqueous solutions of KCl has been studied. At equivalent peak-power densities it was observed that the same fractions of samples were nucleated for both short (6 ns) and long (200 ns) near-infrared laser pulses. The results show that the efficiency depends on the peak-power density and not on the duration, or total energy, of the laser pulses. It is concluded that the mechanism is unlikely to be based solely on diffusion of solute to a sub-critical cluster.

Spatial Control of Crystal Nucleation in Agarose Gel

Spatial and temporal control of crystal nucleation is demonstrated by nonphotochemical laser-induced nucleation of an aqueous agarose gel prepared with supersaturated potassium chloride. The location of nucleation was controlled by means of an optical mask; crystals were only observed in the area exposed to near-infrared laser radiation. The dependence of nucleation on laser power was measured, and the results suggest that the agarose gel reduces the effective supersaturation of the aqueous potassium chloride.

Nonphotochemical laser-induced nucleation of nematic phase and alignment of nematic director from a supercooled thermotropic liquid crystal

A nonphotochemical laser-induced phase transition was studied in a supercooled 4;{'}-n -pentyl-4-cyanobiphenyl (5CB, also referred to as PCB and K15) liquid crystal, using linearly polarized 45 ps light pulses at a wavelength of 532 nm. The laser induced nucleation from the metastable supercooled isotropic phase to the nematic phase during slow cooling (0.001 degrees C/min) and high light intensity (3.9 MW/cm{2}) . The resulting nematic director tended to be aligned along the direction of the plane of polarization of the light. At the intensities used, there is no observable laser-induced realignment of the director once the sample is in the nematic phase, nor any permanent laser-induced ordering when the sample is illuminated only in the stable isotropic phase during slow cooling. These experimental results are consistent with a mechanism based on optical Kerr alignment.

Single Pulse, Single Crystal Laser-Induced Nucleation of Potassium Chloride

The nonphotochemical laser-induced nucleation of aqueous supersaturated solutions of potassium chloride is demonstrated. We have observed that a single, 7 ns pulse of near-infrared (1064 nm) laser light can be used to grow a single crystal of potassium chloride. The experimental results are analyzed using a model in which nucleation is enhanced through the isotropic electronic polarization of subcritical crystal nuclei by laser radiation and the associated reduction in free energy of the nuclei. Classical nucleation theory is used to calculate the fraction of subcritical nuclei, initially in zero field, which become supercritical in the laser field; this fraction is correlated with the crystallization yield and is shown to successfully describe the dependences of the experimentally observed yields upon laser power and supersaturation. The experimental results are analyzed to obtain a phenomenological value of the crystal−solution interfacial tension, γ = 2.19 ± 0.03 mJ m−2.

Nonphotochemical Laser Induced Nucleation of Hen Egg White Lysozyme Crystals

Small droplets of supersaturated hen egg white lysozyme (HEWL) solution were exposed to intense linearly polarized laser pulses with different wavelengths, intensities, and pulse durations. Laser irradiation under some conditions significantly increased the number of droplets in which crystals were observed in a given time period, compared with nonirradiated samples. As a general rule, nonphotochemical laser induced nucleation (NPLIN) in lysozyme solutions was more effective with shorter aging time, 532-nm wavelength, higher peak intensity, and shorter pulse duration. Bovine pancreatic trypsin (BPT) was also examined using NPLIN, showing the potential application of NPLIN to other proteins.

Polarization Switching of Crystal Structure in the Nonphotochemical Laser-Induced Nucleation of Supersaturated Aqueous l-Histidine

Polarization switching of polymorphs has been observed in the nonphotochemical laser-induced nucleation of aqueous l-histidine in the supersaturation range 1.40–1.60. Exposure of solutions to 0.24 GW/cm2, 7 ns pulses of 532 nm light induced the nucleation of different polymorphs of l-histidine depending on polarization state of the light. Circularly polarized laser pulses tended to nucleate the orthorhombic A polymorph, whereas linearly polarized pulses tended to nucleate a mixture of the orthorhombic A and monoclinic B polymorphs. Higher supersaturation also favors the formation of mixed polymorphs. These observations support the hypothesis that the laser is organizing hydrogen-bonded groups of histidine molecules through an optical Kerr alignment.