Relaxation supersaturated solutions

The time t that must elapse before a supersaturated solution reaches a quasi-stationary concentration In dilute aqueous solutions, t is typically around 10 seconds, but for some solutes this period has proven to be much greater. For example, Dunning found that the relaxation period for concentrated sucrose solutions is about 100 hours. See Biomineralization... 

Figure 11 Contributions (not drawn to scale) to the total measured glass transition and ice-melting endotherm for a frozen aqueous solution crosses represent contribution from the heat of dilution of the supersaturated solution, circles denote the enthalpic relaxation contribution and the broken line is the glass transition. The drawn out curve represents the measured overall heat flow, and the area under that scan corresponds to the total enthalpy change.

Examples of an initially metastable phase that relaxes to a more stable phase are to be found in solutions, in suspensions, and in some solid dosage forms [31]. In solutions, the metastable form will have a higher solubility than its more stable form (and a higher free energy), so a solution of the less stable form may be supersaturated with respect to the more stable form, which then crystallizes during processing. The stable form can be a solvate or an anhydrous form. Cortisone acetate suspensions represent a well-known example where a transition to a... 

Second, we take into account the of suspended solids (relaxing assumption (3)). Large values of the suspension density Mt means the availability of a large surface area onto which solutes may be deposited from the solution. Potentially this is equivalent to having a higher level of supersaturation, (pi -Pisat) alternatively, this means a lower supersaturation is needed to achieve a given Mr- To study its effect, consider... 

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