Search

We present results from detailed three-dimensional Brownian dynamics simulations of the self-assembly process in quenched short-range attractive colloids. Clusters obtained in the simulations range from dense faceted crystals to fractal aggregates which show ramified morphology on large length scales but close-packed crystalline morphology on short length scales. For low volume fractions of the colloids, the morphology and crystal structure of a nucleating cluster are studied at various times after the quench. As the volume fraction of the colloids is increased, growth of clusters is controlled by cluster diffusion and cluster-cluster interactions. For shallower quenches and low volume fractions, clusters are compact and the growth-law exponent agrees well with Binder–Stauffer predictions and with recent experimental results. As the volume fraction is increased, clusters do not completely coalesce when they meet each other and the kinetics crosses over to diffusion-limited cluster-cluster aggregation (DLCA) limit. For deeper quenches, clusters are fractals even at low volume fractions and the growth kinetics asymptotically reaches the irreversible DLCA case. [ABSTRACT FROM AUTHOR]

We have carried out numerical simulations on a model of phase separation in a binary mixture in the presence of a third, surface-active component. Based on this model we study phase separation in a quenched solution of gold nanoparticles with dodecane thiol ligands in a mixed solvent of butanone and toluene. In our model gold nanoparticles–butanone correspond to the generic binary mixture, while toluene is modeled as a surface-active third component. Our results show that the surface-active component acts as an inhibitor for the phase separation process but its effect decreases with the increase in quench temperature. Simulation results are consistent with two major findings in recent experiments: (a) growth of gold nanoparticle clusters gets arrested at late times and (b) the saturation value of the cluster size is smaller at lower temperatures of quench. [ABSTRACT FROM AUTHOR]

We have studied the OH stretch spectrum in water to explore the effects of interoscillator coupling. Both H2O, which may have significant intra and intermolecular coupling, and dilute HOD in D2O, which is isotopically decoupled, were studied over a broad range of temperature from -33 °C to 160 °C. By adding a small amount of a calibration dopant, we obtained quantitative spectra. We found interoscillator coupling plays a large role at all temperatures. At high temperature, intramolecular coupling contributes to a downshift in the peak position for H2O as compared to HOD. Intermolecular coupling, however, still has some influence at high temperature. At low temperature, the large excess intensity below 3200 cm-1 in H2O compared to HOD we find is due to an enhanced Raman cross section due to intermolecularly coupled in-phase OH stretch oscillations. We define a degree of delocalization for coupling as the idealized number of perfectly in-phase oscillators that could cause the enhanced scattering and find N[bar_over_tilde:_approx._equal_to]2 for liquid H2O at -33 °C. An exact isosbestic crossing is not found and this can be understood given the changing line shape that the intermolecular coupling can induce. All the properties of the spectrum approach those found in amorphous solid water or ice near the supercooled water anomalous temperature, Ts[bar_over_tilde:_approx._equal_to]-45 °C. [ABSTRACT FROM AUTHOR]

We present Raman data for the OD stretch mode of 10 mol % HOD in H2O for the liquid phase from -31.5 to 160 °C. We find that an exact isosbestic does not hold, but rather the crossing of isotherms slowly but uniformly changes with temperature. We present an analysis based on Boltzmann statistics which gives evidence for a distribution of deuterium hydrogen bond strengths with minimum energy near the frequency (2440 cm-1) also found in the solid ice and amorphous solid phases. This analysis also gives evidence for a band of frequencies above 2630 cm-1 due to OD oscillators all at essentially the same high energy relative to the strongest hydrogen bonds, and we interpret this band as due to broken hydrogen bonds. This allows us to calculate hydrogen bond probabilities, and we find this probability increases with decreasing temperature and approaches a value equal to the four bonded percolation threshold near the singular temperature Ts [bar_over_tilde:_approx._equal_to]-45 °C for the anomalies of supercooled water. Peak frequency and full width at half-maximum of the OD stretch band are found to drop precipitously to the amorphous solid values as T→Ts implying the ultimate state of supercooled water is similar to the amorphous solid. [ABSTRACT FROM AUTHOR]

We present Raman spectroscopic measurements in the OH stretch region of water (2900 to 3800 cm-1) for the temperature range 80 to -33 °C. This latter temperature represents the homogeneous limit of nucleation for our bulk samples. We find that the temperature dependence of both our intensity and depolarization ratio measurements are well described by a two-state model in which both states have frequency dependent depolarization ratios. We argue that the hydrogen bonded mode of the spectrum begins to develop a collective nature at both its low and high frequency ends. This collective nature can explain a breakdown in isosbestic behavior at low temperature. It is suggested that these collective modes may be due to in- and out-of-phase motions of OH oscillators. Their intensity approaches those seen in amorphous solid water as the liquid is cooled to the apparent singular temperature Ts[bar_over_tilde:_approx._equal_to]-45 °C, for a variety of bulk properties. We use our Raman data to make an estimate of hydrogen bond probability and show that four bonded molecules would percolate near the singularity. We then argue that the collective mode grows because clusters of four bonded molecules grow in extent until at Ts percolation occurs to yield an infinite cluster with the amorphous solid’s collective spectrum. [ABSTRACT FROM AUTHOR]

We have performed depolarized dynamic light scattering (photon correlation) measurements on the ionic glass forming mixture 2Ca(NO3)2·3KNO3 above its glass transition in the temperature range 68–86 °C. We find the relaxation function is a stretched exponential. The average relaxation time follows an Arrhenius temperature dependence and can be related to motions on a molecular scale. The width parameter β does not vary with temperature and is β=0.40±0.05. Combination of our β data with other literature values shows that β evolves with temperature over a larger range. At high temperature, β crosses over from β=1 to β<1 at a temperature which matches well with where the viscosity first deviates from a power law behavior. β then evolves downward with lowering temperature and then settles at 0.4 roughly 30 °C above the glass transition Tg. This behavior corresponds well with recent theory relating the glass transition to the ramification of phase space but it implies that the percolation of phase space occurs well above Tg. [ABSTRACT FROM AUTHOR]

We present static turbidity and dynamic light-scattering measurements near the critical double point of a mixture of secondary butyl alcohol, tertiary butyl alcohol, and water. At the critical double point of this mixture an upper two-phase miscibility loop merges with a lower two-phase region. When temperature is used as a field variable to describe the critical behavior of the turbidity and correlation length of the mixture, nonuniversal behavior is seen in both the critical exponents and amplitudes. If, however, the tBA concentration difference from the critical curve is used, universality is found in both exponents and amplitudes. We thus term this concentration difference to be the relevant parameter for proper description of the critical behavior and, following Griffiths and Wheeler [Phys. Rev. A 2, 1047 (1970)], show this relevant parameter was determined by it never having tangential paths of approach to the critical curve. [ABSTRACT FROM AUTHOR]

We have measured the density of supercooled water (H2O) in the range -33.41≤T≤-5.23 °C. Our samples were held in glass capillary tubing with an approximate inside diameter of 0.3 mm=300 μ. These samples were prepared by Mossop’s method and could be cooled to their homogeneous nucleation limit before freezing. We compare our density data to other measurements using capillaries and demonstrate what appears to be an excess density in smaller capillaries which is inversely proportional to the capillary inside diameter. The origins of this excess density are unknown, but we show its effect is insignificant on our measurement. The thermal expansivities derived from our data are fit to a power law in temperature relative to a singular temperature. These results are inconclusive due to a poor knowledge of the background expansivity. [ABSTRACT FROM AUTHOR]

We have measured the densities of supercooled H2O to -34.2 °C, and D2O to -19.2 °C in 25 μ i.d. glass capillaries. Because of the small size of our capillaries, some surface energy effects appear to be present in our data. Despite this we found that reasonably accurate values of the thermal expansivity could be obtained from our data. Our expansivity results support the recent contention of Leyendekkers and Hunter that previous estimates of the anomalous expansivity of supercooled water have been overestimated. The results of a power-law analysis of our expansivity were ambiguous due to the poorly known background expansivity. [ABSTRACT FROM AUTHOR]

We have performed viscosity and dynamic light scattering measurements on the system 3-methylpyridine, water, and heavy water. This system displays a closed-loop coexistence curve which shrinks with increasing H2O/D2O ratio until the upper and lower consolute points merge at a double critical point. Our measurements were performed to study critical phenomena in this mixture in both the phase-separating and non-phase-separating regimes. We have found that the correlation length derived from our measurements diverges in both regimes with a power-law dependence on temperature relative to the double critical point temperature. The power-law exponent was double that usually found in ising-like liquid systems when the system was asymptotically far from the double critical point. Implications of this work for other non-phase-separating yet nonideal systems is discussed. [ABSTRACT FROM AUTHOR]