The relationship between "BET"- and "Free volume"-derived parameters for water vapor absorption into amorphous solids

Water vapor absorption isotherms for amorphous solids with the same chemica l composition but differing in molecular weight (i.e., PVP-90, PVP-30, and PVP-12), and for glucose, trehalose, and two molecular weight grades of dex tran were obtained at 30 degrees C and analyzed using the Brunauer-Emmett-T eller (BET) equation to obtain the parameters, W-m and C-B. Similar analyse s were carried out for the same molecule (e.g., glucose or fructose) at -10 and 40 degrees C. Within each chemical group, W-m, the apparent BET-like p arameter that is generally referred to as the "monolayer-limit of absorptio n", changed very little. In contrast, C-B, a measure of the free energy of absorption, significantly increased with increasing molecular weight or dec reasing temperature, leading to a shift from a Type III to a Type II isothe rm. The shift in isotherm shape correlates directly with the glass transiti on temperature, T-g, of the dry sample relative to the operating temperatur e, T (i.e., Type III when T > T-g and Type II when T < T-g. These results a re shown to be consistent with the combined Flory-Huggins solution model an d Vrentas structural relaxation model; wherein Type II isotherm behavior, o bserved for T < T-g, reflects nonideal volumetric contributions to the over all free energy of absorption due to plasticization by water, as described by Vrentas, whereas Type III behavior only reflects the Flory-Huggins solut ion model. These volumetric free energy changes within each chemical group are shown to be correlated to the values of the "BET" parameter C-B. (C)200 0 Wiley-Liss, Inc. and the American Pharmaceutical Association.