Sorption weight loss/gain

Fig. 5.12 Fluid weight gain (or weight-loss) curves for initial sorption, desorption and resorption processes, as calculated by the equivalent diffusivity model, plotted vs. /time (Weitsman and Guo 2002)...

A typical sorption experiment involves exposing a polymer sample, initially at an equilibrium penetrant concentration of c to a bathing penetrant concentration of Ci. The weight gain or loss is then measured as a function of time. The term sorption used in this context includes both absorption and desorption. The sorption is of the integral type if c° = 0 in the case of absorption or if cf = 0 in the case of desorption. Details of the experimental setup for the sorption measurement are discussed elsewhere [4],... 

In a sorption experiment, a film of a given polymer is exposed to vapor of a given penetrant substance at a given pressure and the gain or loss in weight of the film is measured as a function of time. In the present paper, the term "sorption will be used loosely for both absorption and desorption, unless otherwise specified. In the usual absorption experiment the film is initially free of penetrant, but in some cases the process of absorption is studied with a film initially equilibrated at a non-zero vapor pressure. Kokes, Long and Hoard (1952) have referred to this latter type of absorption as the "interval" type. This term, however, will not be used in the present paper. Most desorption experiments measure processes from non-zero initial concentrations to the zero final concentration, and hence are of the integral" type in the sense of Kokes et al (1952). 

Dynamic gravimetric vapour sorption involves the use of an ultra-sensitive microbalance. The solid sample is placed in the sample pan at the end of the hang-down wire while the probe molecule is vaporised in an inert carrier gas stream. Adsorption of vapour is measured as a gain of weight, desorption as a loss of weight. From these changes of weight at different vapour concentrations sorption isotherms can be determined. 

It was demonstrated by means of a parametric study that the effect of D on the predictions of weight gain/loss/regain is negligible, and the results are strictly dominated by the choice of D3. It is therefore possible to calibrate the values of the three parameters D3, l, Am, and in order to fit absorption-desoiption-re-sorption data. Of the above, the latter two correspond to the gaps between the saturation values. 

The polymer sample is suspended from a balance under a uniform concentration of vapour at a known temperature and pressure. By studying the equilibrium weight gain or loss at different vapour concentrations, a sorption isotherm may be determined. A typical isotherm observed for Fickian diffusion is shown in Figure 2. 

---