Search

A polythermal methodology to assess the mechanisms and the kinetics of solution crystallisation is described and used in connection with a recently proposed model for the dependence of the critical undercooling for crystallisation on the cooling rate (D. Kashchiev, A. Borissova, R. B. Hammond, K. J. Roberts, J. Cryst. Growth, 312 (2010) 698–704; J. Phys. Chem. B, 114 (2010) 5441–5446). This first principles model allows determination of crystallisation parameters that could otherwise only be obtained by combined application of both the isothermal and the polythermal methods. The methodology is validated through analysis of experimental data measured for methyl stearate crystallising from kerosene solutions with concentrations from 200 to 350 g l−1. The analysis reveals a progressive heterogeneous nucleation mechanism and crystallite interfacial tension values between 1.64 and 1.79 with no obvious dependence on the solution concentration, in good agreement with values derived by isothermal methods. Sensitivity analysis leads to the conclusion that a minimum of four different cooling rates spanning at least one order of magnitude together with at least five repeats for crystallisation temperature values at each cooling rate are appropriate. Extensive supplementary material provides a mathematical description of the above authors' model, insight into the relationship between this model and the empirical Nyvlt model, and further detail concerning the results of the sensitivity analysis carried out on the experimental methodology used.

The adsorption of surfactants from dilute oil solutions on to solid surfaces is studied as a function of surface curvature and surface coverage. Coarse-grained molecular models, computer simulations, and umbrella sampling are used to compute the dependence of the free energy of adsorption on to a spherical colloid surface with radius R. It is shown that for fixed surface coverage, and with all other things being equal, the free energy of adsorption decreases with decreasing R. For fixed surface curvature, the free energy of adsorption increases with increasing surface coverage. These trends arise from the excluded-volume interactions between the surfactant tails. The dependence on surface curvature is due to the geometrical effect of there being more free volume for the surfactant tails with a smaller colloid radius. The consequences of these effects on equilibrium partitioning are examined. It is shown that for surfactants adsorbed on small-colloid and large-colloid surfaces in mutual equilibrium with a dilute solution, the surface coverage of the small particles is significantly greater. The implications for industrial applications are discussed and could be significant.