RESUMO
Although the freezing of aqueous solutions is important for nature and different branches of science and freeze-applications, our understanding of the freezing process is not complete. For example, numerous measurements of micrometer-scaled (NH(4))(2)SO(4)/H(2)O droplets report one freezing event below the eutectic point. However, measurements of larger millimeter-scaled droplets reveal two freezing events: the freezing out of ice and subsequent freezing of a residual freeze-concentrated solution. To resolve this apparent contradiction we performed numerous calorimetric measurements which indicate that the freezing of a residual solution of millimeter-scaled 5-38 wt% (NH(4))(2)SO(4) droplets occurs mainly between â¼ 210 and 225 K. We also find that micrometer-scaled droplets produce one freezing event which is within or in the vicinity of the â¼ 210-225 K region. This fact and the analysis of thermograms suggest that the residual solution of micrometer-scaled droplets may partly crystallize simultaneously with ice and partly transform to glass at T(g)≈172 K. Our results suggest for the first time that the size of (NH(4))(2)SO(4)/H(2)O droplets may affect the number of freezing events below the eutectic point.
Assuntos
Sulfato de Amônio/química , Congelamento , Água/química , Varredura Diferencial de Calorimetria , Temperatura de TransiçãoRESUMO
When adsorbed on the strongly anisotropic Pt(110) surface Br forms a sequence of (n × 1) structures. In the present study we investigate the (4 × 1) structure by scanning tunneling microscopy, quantitative low-energy electron diffraction and density-functional calculations. We show that the optimal structural model contains essentially the same adsorption sites as the (3 × 1) structure, but with a different preference. The positions of the substrate atom are consistent with a frozen surface phonon of fourfold periodicity, suggesting that the phase diagram can be understood on the basis of a tunable charge density wave (Swamy et al 2001 Phys. Rev. B 86 1299). The structure could also be explained by assuming short-range interactions only, but evidence is presented that adsorbate-adsorbate interactions mediated by quasi-one-dimensional surface resonances play a major role in both cases.
RESUMO
First-order phase transitions typically exhibit a significant hysteresis resulting for instance in boiling retardation and supercooling. The hysteresis arises, because nucleation of the new phase is activated. The free-energy change is positive until the nucleus reaches a critical size beyond which further growth is downhill. In practice, the barrier is often circumvented by the presence of heterogeneous nucleation centres, e.g. at vessel walls or seed crystals. Recently, it has been proposed that the homogeneous melting of ice proceeds via separation of defect pairs with a substantially smaller barrier as compared to the mere aggregation of defects. Here we report the observation of an analogous mechanism catalysing a two-dimensional homogeneous phase transition. A similar process is believed to occur in spin systems. This suggests that separation of defect pairs is a common trigger for phase transitions. Partially circumventing the activation barrier it reduces the hysteresis and may promote fluctuations within a temperature range increasing with decreasing dimensionality.