ASSESSMENT of A SEMI‐EMPIRICAL FOUR PARAMETER GENERAL MODEL FOR SIGMOID MOISTURE SORPTION ISOTHERMS¹

Abstract

ABSTRACT Published sigmoid moisture sorption isotherms (0 < a w < ∼ 0.9) were fitted by the four parameter models m = k 1 a w n1 + k 2 a w n2 where m is the moisture contents (dry basis), a w the water activity and the k and n values are constants (n 1 < 1 and n 2 > 1). Not surprisingly, the model had the same or better fit than the GAB model. In contrast with both the BET and GAB models, the proposed model is not based on the assumption that there exists a well‐defined monolayer of absorbed water. At a w < ∼ 0.4 and n 1 > ∼ 0.55, however, the model produces a practically linear a w /[m(1 – a w )] vs a w plot, of the kind used to calculate the monolayer moisture with the BET model. The proposed model can be a convenient means to catalog both sigmoid and nonsigmoid isotherms, and used to calculate the equilibrium water activity of dry mixtures with equations solving software.