Dynamic Vapour Sorption

Describes the amount of amorphous versus crystalline material for a particular sample. In amorphous materials, moisture can significantly alter the glass transition temperature, and initiate spontaneous transformation to the crystalline form. 

The DVS instruments can be used to observe a change in a material from an amorphous state to a crystalline state by exposing the material to different levels of relative humidity or organic vapor concentration. The amorphous contents can be determined using this method via measurement of vapor uptake before and after a crystallization event with a limit of detection as low as 0.5% w/w. It can also identify if the transition from amorphous to crystalline state is reversible or irreversible. 

While some amorphous content may be desirable to improve the solubility of a drug, it can also be generated unknowingly during processing via milling, wet granulation or spray drying. It has been well established that less than 10% amorphous content can drastically alter the properties of a powder, which necessitates the use of sensitive techniques to measure these small amorphous quantities.

 In hydrates/solvates, the water or solvent molecules are not the same as sorbed vapor on the surface or in pores of a material. The behaviour of pharmaceutical hydrates/solvates in response to changes in the environment can have a serious effect on the drug development and performance. Water facilitates hydrolysis and induces drug degradation. Water content is routinely used in determining the dry and solvent-free assay value of drug substances. The physicochemical stability of solvates can be concerning, because during desolvation they may convert to an amorphous form or become chemically labile. 

The DVS allows the fast and accurate determination of solvation/desolvation kinetics for a range of solid-vapor systems.

 Sorption isotherms describe the amount of vapor adsorbed or desorbed at different equilibrium concentrations (partial pressures) at constant temperature. The shape of a sorption isotherm depends on the interaction between the vapor molecules and the solid sample and allows for conclusions regarding the sorption mechanism. Adsorption isotherms are typically plotted with the change in mass (%) against partial pressure. Desorption is also measured and plotted in the same way resulting in an isotherm observing hysteresis. 

As a result, there are 5 classifications of isotherms. Type I isotherms are rarely observed unless the material is strongly microporous or chemisorption occurs. Type II and IV form when a material exhibits a monolayer mechanism and type III and V are observed in materials with a cluster mechanism of sorption.

Vapour diffusion rates are of interest in a wide range of applications including: polymers, pharmaceuticals, flavours, and foods. Compared to static measurements, the DVS allows continuous measurement of the sorption kinetics which can be used to determine diffusion coefficients for vapour. Diffusion constants can be calculated on powder samples, thin films and liquids. The DVS can use sample pans, hanging sample or a bespoke Payne diffusion cell for measurements.