Dynamic inhibition of constrained crystallization by mesoscopic morphologymodifiers

We investigate the mechanisms by which two chemically diverse polymer addit ives affect crystal separation from hexacosane/decane solutions under vario us conditions. The additives are dissimilarly effective for delaying the on set of crystallization in supercooled solutions, although none, of course, ultimately prevents their reaching equilibrium. Remarkably, one polymer sig nificantly raises the dissolution temperature of the crystal aggregates gro wn in its presence, even at sub-parts-per-million additive levels. More int eresting is the finding that sufficiently large concentrations of either ad ditive are able to cap at a common minimum thickness the porous layers form ed on a cold surface swept by undersaturated solutions. The thin layers for med in the presence of crystallization inhibitors are mesoscopically more c ompact (i.e., have pore sizes about 10-fold smaller) than those produced in their absence, although all melt-when rid of solvent-as neat hexacosane at 329.5 +/- 0.5 K. We infer that inhibitors permanently limit the extent of crystal separation from undersaturated solutions subjected to fixed tempera ture constraints by blocking solute transfer across the boundary layer. Thi s lasting action is mediated by the subtle modification of mesoscopic cryst al morphology and is remotely related to the kinetics of isothermal crystal nucleation or the thermodynamics of phase separation.