Structural interactions in polymer-stabilized magnetic nanocomposites.

Superparamagnetic iron oxide nanoparticles (SPIONs) have attracted significant attention because of their nanoscale magnetic properties. SPION aggregates may afford emergent properties, resulting from dipole-dipole interactions between neighbors. Such aggregates can display internal order, with high packing fractions (>20%), and can be stabilized with block co-polymers (BCPs), permitting design of tunable composites for potential nanomedicine, data storage, and electronic sensing applications. Despite the routine use of magnetic fields for aggregate actuation, the impact of those fields on polymer structure, SPION ordering, and magnetic properties is not fully understood. Here, we report that external magnetic fields can induce ordering in SPION aggregates that affect their structure, inter-SPION distance, magnetic properties, and composite Tg. SPION aggregates were synthesized in the presence or absence of magnetic fields or exposed to magnetic fields post-synthesis. They were characterized using transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), superconducting quantum interference device (SQUID) analysis, and differential scanning calorimetry (DSC). SPION aggregate properties depended on the timing of field application. Magnetic field application during synthesis encouraged preservation of SPION chain aggregates stabilized by polymer coatings even after removal of the field, whereas post synthesis application triggered subtle internal reordering, as indicated by increased blocking temperature (TB), that was not observed via SAXS or TEM. These results suggest that magnetic fields are a simple, yet powerful tool to tailor the structure, ordering, and magnetic properties of polymer-stabilized SPION nanocomposites.

The effects of methanol clustering on methanol-water nucleation.

The formation of subcritical methanol clusters in the vapor phase is known to complicate the analysis of nucleation measurements. Here, we investigate how this process affects the onset of binary nucleation as dilute water-methanol mixtures in nitrogen carrier gas expand in a supersonic nozzle. These are the first reported data for water-methanol nucleation in an expansion device. We start by extending an older monomer-dimer-tetramer equilibrium model to include larger clusters, relying on Helmholtz free energy differences derived from Monte Carlo simulations. The model is validated against the pressure/temperature measurements of Laksmono et al. [Phys. Chem. Chem. Phys. 13, 5855 (2011)] for dilute methanol-nitrogen mixtures expanding in a supersonic flow prior to the appearance of liquid droplets. These data are well fit when the maximum cluster size imax is 6-12. The extended equilibrium model is then used to analyze the current data. On the addition of small amounts of water, heat release prior to particle formation is essentially unchanged from that for pure methanol, but liquid formation proceeds at much higher temperatures. Once water comprises more than ∼24 mol % of the condensable vapor, droplet formation begins at temperatures too high for heat release from subcritical cluster formation to perturb the flow. Comparing the experimental results to binary nucleation theory is challenged by the need to extrapolate data to the subcooled region and by the inapplicability of explicit cluster models that require a minimum of 12 molecules in the critical cluster.

The freezing behavior of aqueous n-alcohol nanodroplets.

We generate water-rich aerosols containing 1-propanol and 1-pentanol in a supersonic nozzle to study the effects of these solutes on the freezing behavior of water. Condensation and freezing are characterized by two complementary techniques, pressure trace measurements and Fourier Transform Infrared spectroscopy. When 1-pentanol and 1-propanol are present, condensation occurs at higher temperatures because particle formation from the vapor phase is enhanced by the decrease in interfacial free energy of mixed aqueous-alcohol critical clusters relative to those of pure water. FTIR results suggest that when ∼6 nm radius droplets freeze, the tetrahedral structure of the ice is well preserved up to an overall alcohol mole fraction of 0.031 for 1-propanol and 0.043 for 1-pentanol. In this concentration range, the ice nucleation temperature decreases continuously with increasing 1-propanol concentration, whereas the onset of freezing is not significantly perturbed by 1-pentanol up to a mole fraction of 0.03. Furthermore, once freezing starts the ice nucleation rates in the aqueous-alcohol droplets are very close to those for pure water. In contrast, at the highest mole fractions of either alcohol it is not clear whether droplets freeze to form crystalline ice since the final state of the particles cannot be adequately characterized with the available experimental techniques.

Homogeneous nucleation of carbon dioxide in supersonic nozzles II: molecular dynamics simulations and properties of nucleating clusters.

Large scale molecular dynamics simulations of the homogeneous nucleation of carbon dioxide in an argon atmosphere were carried out at temperatures between 75 and 105 K. Extensive analyses of the nucleating clusters' structural and energetic properties were performed to quantify these details for the supersonic nozzle experiments described in the first part of this series [Dingilian et al., Phys. Chem. Chem. Phys., 2020, 22, 19282-19298]. We studied ten different combinations of temperature and vapour pressure, leading to nucleation rates of 1023-1025 cm-3 s-1. Nucleating clusters possess significant excess energy from monomer capture, and the observed cluster temperatures during nucleation - on both sides of the critical cluster size - are higher than that of the carrier gas. Despite strong undercooling with respect to the triple point, most clusters are clearly liquid-like during the nucleation stage. Only at the lowest simulation temperatures and vapour densities, clusters containing over 100 molecules are able to undergo a second phase transition to a crystalline solid. The formation free energies retrieved from the molecular dynamics simulations were used to improve the classical nucleation theory by introducing a Tolman-like term into the classical liquid-drop model expression for the formation free energy. This simulation-based theory predicts the simulated nucleation rates perfectly, and improves the prediction of the experimental rates compared to self-consistent classical nucleation theory.

Homogeneous nucleation of carbon dioxide in supersonic nozzles I: experiments and classical theories.

We studied the homogeneous nucleation of carbon dioxide in the carrier gas argon for concentrations of CO2 ranging from 2 to 39 mole percent using three experimental methods. Position-resolved pressure trace measurements (PTM) determined that the onset of nucleation occurred at temperatures between 75 and 92 K with corresponding CO2 partial pressures of 39 to 793 Pa. Small angle X-ray scattering (SAXS) measurements provided particle size distributions and aerosol number densities. Number densities of approximately 1012 cm-3, and characteristic times ranging from 6 to 13 µs, resulted in measured nucleation rates on the order of 5 × 1017 cm-3 s-1, values that are consistent with other nucleation rate measurements in supersonic nozzles. Finally, we used Fourier transform infrared (FTIR) spectroscopy to identify that the condensed CO2 particles were crystalline cubic solids with either sharp or rounded corners. Molecular dynamics simulations, however, suggest that CO2 forms liquid-like critical clusters before transitioning to the solid phase. Furthermore, the critical clusters are not in thermal equilibrium with the carrier gas. Comparisons with nucleation theories were therefore made assuming liquid-like critical clusters and incorporating non-isothermal correction factors.

Mechanism of surface freezing of alkanes.

Using molecular dynamics simulation of octane (C8) and nonadecane (C19), we probe the mechanism of n-alkane surface freezing, the appearance of a crystalline monolayer above the liquid at a temperature Tsf above the bulk freezing point Tf. Formation of a crystalline monolayer occurs robustly in these systems. When Tf > Tsf, the surface frozen phase is metastable with respect to the solid but persists for long periods for study in simulations. Surface freezing of both C8 and C19 is driven by significant energy-lowering when alkane chains become ordered along the surface normal, and we elucidate the origins of this phenomenon. The degree of configurational disorder in the surface frozen layer relative to the solid is much larger for C8 compared to C19. From the Gibbsian viewpoint, we extract the excess energy and entropy of the liquid and surface frozen phases. We also consider the surface frozen layer as an intervening third phase, the viewpoint taken in previous theoretical analyses. Here, we find significantly increased entropy of the surface frozen phase of C8 associated with configurational disorder, while the energy and entropy of the surface frozen phase of C19 are marginally different from the bulk solid. Finally, by combining our previously determined solid-vapor surface free energies of C8 and C19 with liquid-vapor surface tensions from this work, we eliminate wetting as a possible mechanism for C8 surface freezing, but it remains a possibility for C19. We analyze the molecular structure of the liquid, surface frozen, and solid surfaces and discuss its relevance to thermodynamic properties.

CO2 condensation onto alkanes: unconventional cases of heterogeneous nucleation.

The classical picture invoked for heterogeneous nucleation is frequently that of a liquid condensing onto an immiscible solid particle. Here, we examine heterogeneous nucleation of CO2 onto particles comprised of n-pentane or n-hexane under conditions where CO2 should be a solid and the seed particles may be liquid or solid. Although CO2 condensed under all but one of the six conditions investigated, these experiments do not easily fit into the framework of standard heterogeneous nucleation experiments. Rather they explore unconventional regimes of heterogeneous nucleation in which the state of the seed particle may both affect whether deposition can proceed, and, in turn, be influenced by the presence of the condensing species. The work complements the earlier work of Tanimura et al. [RSC Adv., 2015, 5, 105537-105550] that investigated CO2 condensation onto ice nanoparticles, by using seed particles comprised of non-polar compounds that form and freeze under conditions where CO2 is already supersaturated with respect to the solid ice. In some cases, the conditions for seed formation approach the limit of homogeneous CO2 nucleation. Vibrational spectroscopy measurements help pinpoint where CO2 starts to condense. Furthermore, these IR measurements suggest that the n-alkanes never freeze in the presence of CO2, even if the temperatures are well below those required for them to freeze when CO2 is absent. Over the temperature range 65 < T/K < 140, the conditions corresponding to the onset of CO2 heterogeneous nucleation on pre-existing seed particle almost all lie very close to the extrapolated vapor-liquid equilibrium line of CO2 for a broad range of seed materials.

Vapor-phase nucleation of n-pentane, n-hexane, and n-heptane: Critical cluster properties.

The first and second nucleation theorems provide a way to determine the molecular content and excess internal energies of critical clusters, which rely solely on experimental nucleation rates measured at constant temperatures and supersaturations, respectively. Here, we report the size n* and excess internal energy Ex(n*) of n-pentane, n-hexane, and n-heptane critical clusters when particles form under the highly supersaturated conditions present in supersonic expansions. In summary, critical clusters contain from ∼2 to ∼11 molecules and exhibit the expected increase in the critical cluster size with increasing temperature and decreasing supersaturation. Surprisingly, the n* values for all three alkanes appear to lie along a single line when plotted as a function of supersaturation. Within the framework of the capillarity approximation, the excess internal energies determined for the n-heptane critical clusters formed under the low temperature (∼150 K) conditions in our supersonic nozzle are reasonably consistent with those determined under higher temperature (∼250 K) conditions in the thermal diffusion cloud chamber by Rudek et al. [J. Chem. Phys. 105, 4707 (1996)].

Nanoparticle packing within block copolymer micelles prepared by the interfacial instability method.

The interfacial instability method has emerged as a viable approach for encapsulating high concentrations of nanoparticles (NPs) within morphologically diverse micelles. In this method, transient interfacial instabilities at the surface of an emulsion droplet guide self-assembly of block co-polymers and NP encapsulants. Although used by many groups, there are no systematic investigations exploring the relationship between NP properties and micelle morphology. Here, the effect of quantum dot (QD) and superparamagnetic iron oxide NP (SPION) concentration on the shape, size, and surface deformation of initially spherical poly(styrene-b-ethylene oxide) (PS-b-PEO) micelles was examined. Multi-NP encapsulation and uniform dispersion within micelles was obtained even at low NP concentrations. Increasing NP concentration initially resulted in larger numbers of elongated micelles and cylinders with tightly-controlled diameters smaller than those of spherical micelles. Beyond a critical NP concentration, micelle formation was suppressed; the dominant morphology became densely-loaded NP structures that were coated with polymer and exhibited increased polydispersity. Transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) revealed that NPs in densely-loaded structures can be well-ordered, with packing volume fractions of up to 24%. These effects were enhanced in magnetic composites, possibly by dipole interactions. Mechanisms governing phase transitions triggered by NP loading in the interfacial instability process were proposed. The current study helps establish and elucidate the active role played by NPs in directing block copolymer assembly in the interfacial instability process, and provides important guiding principles for the use of this approach in generating NP-loaded block copolymer composites.

Ice nucleation rates near ∼225 K.

We have measured the ice nucleation rates, Jice, in supercooled nano-droplets with radii ranging from 6.6 nm to 10 nm and droplet temperatures, Td, ranging from 225 K to 204 K. The initial temperature of the 10 nm water droplets is ∼250 K, i.e., well above the homogeneous nucleation temperature for micron sized water droplets, TH ∼235 K. The nucleation rates increase systematically from ∼1021 cm-3 s-1 to ∼1022 cm-3 s-1 in this temperature range, overlap with the nucleation rates of Manka et al. [Phys. Chem. Chem. Phys. 14, 4505 (2012)], and suggest that experiments with larger droplets would extrapolate smoothly the rates of Hagen et al. [J. Atmos. Sci. 38, 1236 (1981)]. The sharp corner in the rate data as temperature drops is, however, difficult to match with available theory even if we correct classical nucleation theory and the physical properties of water for the high internal pressure of the nanodroplets.

Vapor phase nucleation of the short-chain n-alkanes (n-pentane, n-hexane and n-heptane): Experiments and Monte Carlo simulations.

We measured the nucleation rates of n-pentane through n-heptane in a supersonic nozzle at temperatures ranging from ca. 109 K to 168 K. For n-pentane and n-hexane, these are the first nucleation rate measurements that have been made, and the trends in the current data agree well with those in the earlier work of Ghosh et al. [J. Chem. Phys. 132, 024307 (2010)] for longer chain alkanes. Complementary Monte Carlo simulations, using the transferable potentials for phase equilibria-united atom potentials, suggest that despite the high degree of supercooling, the critical clusters remain liquid like under experimental conditions for n-pentane through n-heptane, but adopt more ordered structures for n-octane and n-nonane. For all three alkanes, the experimental and simulated nucleation rates are offset by ∼3 orders of magnitude when plotted as a function of ln S/(Tc/T - 1)1.5. Explicitly accounting for the surface tension difference between the real and model substances, or alternatively using the Hale [Phys. Rev. A 33, 4156 (1986); Metall. Mater. Trans. A 23, 1863 (1992)] scaling parameter, Ω, consistent with the model potential, increases the offset to ∼6 orders of magnitude.

Freezing of supercooled n-decane nanodroplets: from surface driven to frustrated crystallization.

Whether crystallization starts at the liquid-vapor interface or randomly throughout the bulk has been the subject of intense debate. In our earlier work, we investigated the freezing of supercooled nanodroplets of short chain (C8, C9) n-alkanes formed by homogeneous condensation in a supersonic nozzle. The rate at which the solid appeared suggested freezing starts at the droplet surface well before the rest of the droplet freezes. Experiments were, however, limited to a single condition for each compound and it was not clear whether freezing of n-alkanes always occurs in this two step manner. Here, we expand our work to include freezing of a third n-alkane, n-decane, and, furthermore, we vary the temperatures at which droplets are formed and freeze. The phase transitions are again characterized using three experimental techniques - pressure trace measurements (PTM), Fourier Transform Infrared Spectroscopy (FTIR), and Small Angle X-ray Scattering (SAXS). We also use Wide Angle X-ray Scattering (WAXS) to confirm, for the first time, the crystalline nature of our frozen n-alkane nanodroplets. As the temperature at which the droplets form and freeze decreases, the kinetics of the phase transition changes. At higher temperatures, the phase transition occurs in two steps characterized by different rates, whereas at lower temperatures we observe only a single step. Finally, in the lowest temperature experiment, where droplets start to form and freeze ∼50 K below the bulk melting temperature, we found that the particles develop a fractal structure and appear locked in a "frustrated" crystalline state.

Overview: Homogeneous nucleation from the vapor phase-The experimental science.

Homogeneous nucleation from the vapor phase has been a well-defined area of research for ∼120 yr. In this paper, we present an overview of the key experimental and theoretical developments that have made it possible to address some of the fundamental questions first delineated and investigated in C. T. R. Wilson's pioneering paper of 1897 [C. T. R. Wilson, Philos. Trans. R. Soc., A 189, 265-307 (1897)]. We review the principles behind the standard experimental techniques currently used to measure isothermal nucleation rates, and discuss the molecular level information that can be extracted from these measurements. We then highlight recent approaches that interrogate the vapor and intermediate clusters leading to particle formation, more directly.

On the determination of the crystal-vapor surface free energy, and why a Gaussian expression can be accurate for a system far from Gaussian.

The crystal-vapor surface free energy γ is an important physical parameter governing physical processes, such as wetting and adhesion. We explore exact and approximate routes to calculate γ based on cleaving an intact crystal into non-interacting sub-systems with crystal-vapor interfaces. We do this by turning off the interactions, ΔV, between the sub-systems. Using the soft-core scheme for turning off ΔV, we find that the free energy varies smoothly with the coupling parameter λ, and a single thermodynamic integration yields the exact γ. We generate another exact method, and a cumulant expansion for γ by expressing the surface free energy in terms of an average of e(-ßΔV) in the intact crystal. The second cumulant, or Gaussian approximation for γ is surprisingly accurate in most situations, even though we find that the underlying probability distribution for ΔV is clearly not Gaussian. We account for this fact by developing a non-Gaussian theory for γ and find that the difference between the non-Gaussian and Gaussian expressions for γ consist of terms that are negligible in many situations. Exact and approximate methods are applied to the (111) surface of a Lennard-Jones crystal and are also tested for more complex molecular solids, the surface of octane and nonadecane. Alkane surfaces were chosen for study because their crystal-vapor surface free energy has been of particular interest for understanding surface freezing in these systems.

Scalable, semicontinuous production of micelles encapsulating nanoparticles via electrospray.

Nanoparticle encapsulation within micelles has been demonstrated as a versatile platform for creating water-soluble nanocomposites. However, in contrast to typical micelle encapsulants, such as small molecule drugs and proteins, nanoparticles are substantially larger, which creates significant challenges in micelle synthesis, especially at large scale. Here, we describe a new nanocomposite synthesis method that combines electrospray, a top-down, continuous manufacturing technology currently used for polymer microparticle fabrication, with bottom-up micellar self-assembly to yield a scalable, semicontinuous micelle synthesis method: i.e., micellar electrospray. Empty micelles and micellar nanocomposites containing quantum dots (QDs), superparamagnetic iron oxide nanoparticles (SPIONs), and their combination were produced using micellar electrospray with a 30-fold increase in yield by weight over batch methods. Particles were characterized using dynamic light scattering, transmission electron microscopy, and scanning mobility particle sizing, with remarkable agreement between methods, which indicated size distributions with variations of as little as ~5%. In addition, new methodologies for qualitatively evaluating nanoparticle loading in the resultant micelles are presented. Micellar electrospray is a broad, scalable nanomanufacturing approach that should be easily adapted to virtually any hydrophobic molecule or nanoparticle with a diameter smaller than the micelle core, potentially enabling synthesis of a vast array of nanocomposites and self-assembled nanostructures.

Co-condensation of nonane and D2O in a supersonic nozzle.

We study the unary and binary nucleation and growth of nonane-D2O nanodroplets in a supersonic nozzle. Fourier Transform Infrared spectroscopy measurements provide the overall composition of the droplets and Small Angle X-ray Scattering experiments measure the size and number density of the droplets. The unary nucleation rates Jmax of nonane, 9.4 × 10(15) < Jmax /cm(-3) s(-1) < 2.0 × 10(16), and those of D2O, 2.4 × 10(17) < Jmax /cm(-3) s(-1) < 4.1 × 10(17), measured here agree well with previous results. In most of the binary condensation experiments new particle formation is dominated by D2O, but the observed nucleation rates are decreased by up to a factor of 6 relative to the rates measured for pure D2O, an effect that can be partly explained by non-isothermal nucleation theory. The subsequent condensation of D2O is inhibited both by the increased temperature of the binary droplets relative to the pure D2O droplets, and because the binary droplet surface is expected to be comprised largely of nonane. For the one case where nonane appears to initiate condensation, we find that the nucleation rate is about 50% higher than that observed for pure nonane at comparable pv0, consistent with significant particle formation driven by D2O.

Experimental evidence for surface freezing in supercooled n-alkane nanodroplets.

Intermediate chain length (16 ≤i≤ 50) n-alkanes are known to surface freeze at temperatures that are up to three degrees higher than the equilibrium melting point [B. M. Ocko et al., Phys. Rev. E, 1997, 55, 3164-3182]. Our recent experimental results suggest that highly supercooled nanodroplets of n-octane and n-nonane also surface freeze, and subsequently bulk crystallization occurs. The data yield surface and bulk nucleation rates on the order of ~10(15) cm(-2) s(-1) and ~10(22) cm(-3) s(-1), respectively, at temperatures between 180 K and 200 K. Molecular dynamics simulations at the united atom level were used to follow the freezing of a supercooled n-octane drop and show that an ordered monolayer develops on the surface of the droplet almost immediately, and the rest of the droplet then freezes in a layer-by-layer manner.

Freezing of heavy water (D2O) nanodroplets.

We follow the freezing of heavy water (D2O) nanodroplets formed in a supersonic nozzle apparatus using position resolved pressure trace measurements, Fourier transform infrared spectroscopy, and small-angle X-ray scattering. For these 3-9 nm radii droplets, freezing starts between 223 and 225 K, at volume based ice nucleation rates Jice,V on the order of 10(23) cm(-3) s(-1) or surface based ice nucleation rates Jice,S on the order of 10(16) cm(-2) s(-1). The temperatures corresponding to the onset of D2O ice nucleation are higher than those reported for H2O by Manka et al. [Manka, A.; Pathak, H.; Tanimura, S.; Wölk, J.; Strey, R.; Wyslouzil, B. E. Phys. Chem. Chem. Phys.2012, 14, 4505]. Although the values of Jice,S scale somewhat better with droplet size than values of Jice,V, the data are not accurate enough to state that nucleation is surface initiated. Finally, using current estimates of the thermophysical properties of D2O and the theoretical framework presented by Murray et al. [Murray, B. J.; Broadley, S. L.; Wilson, T. W.; Bull, S. J.; Wills, R. H.; Christenson, H. K.; Murray, E. J. Phys. Chem. Chem. Phys.2010, 12, 10380], we find that the theoretical ice nucleation rates are within 3 orders of magnitude of the measured rates over an ∼15 K temperature range.

Binary nucleation rates for ethanol/water mixtures in supersonic Laval nozzles: analyses by the first and second nucleation theorems.

We performed pressure trace measurements and small angle x-ray scattering measurements to determine the vapor-liquid nucleation rates of EtOH/H2O mixtures including pure EtOH and pure H2O in two supersonic Laval nozzles with different expansion rates. The nucleation rates varied from 0.9 × 10(17) to 16 × 10(17) cm(-3) s(-1) over the temperature range of 210 K to 230 K, EtOH activity range of 0 to 11.6, and H2O activity range of 0 to 124. The first and second nucleation theorems were applied to the nucleation rates to estimate the sizes, compositions, and excess energies of the critical clusters. The critical clusters contained from 4 to 15 molecules for pure H2O and EtOH/H2O clusters, and from 16 to 23 molecules for pure EtOH clusters. Comparing the excess energies of the pure H2O critical clusters with the results of a quantum-chemistry calculation suggested that the pre-factor of the theoretical nucleation rate is almost constant regardless of the monomer concentration. One possible explanation for this result is that cooling of the critical clusters limits the nucleation rate under the highly supersaturated conditions. The results of the analyses also yielded the relation between the surface energy and the composition of the critical clusters, where the latter are predicted to consist only of surface molecules. Applying this relationship to the EtOH/H2O bulk liquid mixtures, we estimated the EtOH mole fraction in the surface layer and found it is higher than that derived from the surface tension based on the Gibbs adsorption equation when the EtOH mole fraction in the liquid is higher than about 0.2 mol/mol. This discrepancy was attributed to the existence of the EtOH depletion layer just below the surface layer of the liquid.

Freezing water in no-man's land.

We report homogeneous ice nucleation rates between 202 K and 215 K, thereby reducing the measurement gap that previously existed between 203 K and 228 K. These temperatures are significantly below the homogenous freezing limit, T(H)≈ 235 K for bulk water, and well within no-man's land. The ice nucleation rates are determined by characterizing nanodroplets with radii between 3.2 and 5.8 nm produced in a supersonic nozzle using three techniques: (1) pressure trace measurements to determine the properties of the flow as well as the temperature and velocity of the droplets, (2) small angle X-ray scattering (SAXS) to measure the size and number density of the droplets, and (3) Fourier Transform Infrared (FTIR) spectroscopy to follow the liquid to solid phase transition. Assuming that nucleation occurs throughout the droplet volume, the measured ice nucleation rates J(ice,V) are on the order of 10(23) cm(-3) s(-1), and agree well with published values near 203 K.