The quantification of small degrees of disorder in lactose using solution calorimetry

There is a realisation that small quantities of amorphous material can have a significant impact on the properties of crystalline solids. Consequently there is a growing interest in quantifying the amount of amorphous material that is present in "crystalline powders". Success has been reported when using isothermal microcalorimetry and vapour sorption techniques, however, the use of solution calorimetry has largely been ignored. In this study the enthalpies of solution of mixtures of amorphous and crystalline lactose are reported concentrating on the range 0-10% w/w amorphous content. It was found that there was a possible error due to water vapour penetration into the ampoule, resulting in crystallisation of the amorphous content, however this was overcome by double sealing the ampoules with wax. Subsequently there was a good correlation between the enthalpy of solution and the amorphous content, which was not adversely affected by stirring rate used during the experiment. Over the range from 0 to 10% amorphous content, quantification of the amorphous content of an unknown would be good to +/-0.5%. The effects of residual moisture retained within a sample were also investigated. Storage at 33% or 43% RH resulted in a much reduced wetting (exothermic) response compared with that seen for completely dry samples, which in turn led to a higher net enthalpy of solution.