Dynamic vapor sorption

Dynamic vapor sorption (DVS) is a gravimetric technique that measures how quickly and how much of a solvent is absorbed by a sample: such as a dry powder absorbing water. It does this by varying the vapor concentration surrounding the sample and measuring the change in mass which this produces. Water vapor is most commonly used, but it is also possible to use a wide range of organic solvents.

Dr Daryl Williams, founder of Surface Measurement Systems Ltd, invented Dynamic Vapor Sorption in 1991 and the first instrument was delivered to Pfizer UK in 1992. DVS was originally developed to replace the time and labor-intensive desiccators and saturated salt solutions to measure water vapor sorption isotherms.

The main application of DVS is to measure water sorption isotherms. In general, a vapor sorption isotherm shows the amount of vapor sorbed as a function of steady state relative vapor pressure at a constant temperature. For water sorption isotherms, water relative vapor pressure is more commonly expressed as relative humidity. In a DVS experiment this is accomplished by exposing a sample to a series of step changes in relative humidity and monitoring the mass change as a function of time. The sample mass must be allowed to reach gravimetric equilibrium at each step change in humidity before progressing to the next humidity level. Then, the equilibrium mass values at each relative humidity step are used to generate the isotherm. Isotherms are typically divided into two components: sorption for increasing humidity steps and desorption for decreasing humidity steps. Sorption can be further divided into adsorption (sorbate located on the surface) and absorption (sorbate penetrates the bulk).

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