Sigmoid sorption isotherms

The sorption behaviour of a number of dairy products is known (Kinsella and Fox, 1986). Generally, whey powders exhibit sigmoidal sorption isotherms, although the characteristics of the isotherm are influenced by the composition and history of the sample. Examples of sorption isotherms for whey protein concentrate (WPC), dialysed WPC and its dialysate (principally lactose) are shown in Figure 7.13. At low aw values, sorption is due mainly to the proteins present. A sharp decrease is observed in the sorption isotherm of lactose at aw values between 0.35 and 0.50 (e.g. Figure 7.13). This sudden decrease in water sorption can be explained by the crystallization of amorphous lactose in the a-form, which contains one mole of water of crystallization per mole. Above aw values of about 0.6, water sorption is principally influenced by small molecular weight components (Figure 7.13). 

In milk powders, the caseins are the principal water sorbants at low and intermediate values of aw. The water sorption characteristics of the caseins are influenced by their micellar state, their tendency towards self-association, their degree of phosphorylation and their ability to swell. Sorption isotherms for casein micelles and sodium caseinate (Figure 7.14) are generally sigmoidal. However, isotherms of sodium caseinate show a marked increase at aw between 0.75 and 0.95. This has been attributed to the... 

Water sorption isotherms on low rank coals have a standard type II sigmoid shape typical of porous adsorbents (4. ... 

Sorption isotherms usually have a sigmoid shape and can be divided into three areas that correspond to different conditions of the water present in the food (Figure 1-7). The... 

Figure 7 shows three typical sorption isotherms for Douglas-flr at 90 °F (32 °C) (18). The general sigmoid shapes for all three curves is apparent, but each curve represents the isotherm for a different... 

Water Sorption. The water vapor sorption isotherm (293 K) is given in Figure 3. The general sigmoidal shape is evident, and the hysteretic phenomenon is quite pronounced, even when desorption is instigated from intermediate pressures (about 0.6 Pq), and vacuum closure... 

The relationship between water activity and moisture content for most foods at a particular temperature is a sigmoidal-shaped curve called the sorption isotherm (Figure 3.10). The term equilibrium moisture content curve is also used. Sorption... 

Moisture sorption isotherm is a graphical representation of the variation in water activity or % ERH with change in moisture content of a sample at a specified temperatme. Sorption isotherms of foods are generally nonlinear and very often sigmoid in shape (see Figure 1.4). 

Adsorption isotherms of potatoes were of sigmoid shape and were affected by drying method, tanperature, and addition of sugar [85]. The freeze-dried product absorbed more water vapor than the vacuum-dried materials. The sorption isotherm prepared from fresh and freeze-dried Thompson... 

Fig. 1. Analysis of the water sorption isotherms, (a) Typical shapes of the desorption and adsorption curves of plant tissues. The difference between these two curves shows hysteresis, indicating the irreversibility of water sorption in the tissues during dehydration and rehydration. The sigmoid shape of sorption curves is presumably due to the existence of three types of water-binding sites in tissues (strong (I), weak (II) and multilayer molecular sorption sites (III)), (b) Differential enthalpy (AH), free energy (AG) and entropy (AS) of hydration. Data from Sun (2002).

The investigations showed that sorption isotherms of water vapor by various cellulose samples are sigmoidal. Moreover, with an increase in the amorphicity degree Y from 0.25 to 1, sorption capacity of the samples is enhanced (Fig. 7.29). 

Water is bound depending on the water activity of the environment according to the sorption isotherms with a sigmoid shape and hysteresis between the ad- and desorption branches. 

The structural characteristics and moisture sorption properties of two kinds of nylon-6/clay hybrids were investigated by Murase et The moisture sorption isotherms of the hybrid samples were a typical sigmoid shape similar to that of the pure nylon-6 sample, even if the extent of moisture regain of the hybrid films was comparatively low. All the isotherms obtained were explained through quantitative analysis in terms of the BET multilayer adsorption theory and complementally with the aid of the FloryHuggins solution theory. 

Many models have been developed that deal with the sorption properties of wood in the presence of moisture these have been discussed in a number of works (e.g. Skaar, 1972 Siau, 1984). They can be approximately divided into sorption models, such as the Brunauer-Emmett-Teller (BET) model, or solution models (such as the Hailwood-Horrobin, H-H, model). The sigmoidal shapes of sorption or desorption isotherms can be deconvoluted into two components. These are often taken to represent a monomolecular water layer (associated with the primary sorption sites, OH groups), and a multilayer component where the cell wall bound water molecules are less intimately associated with the fixed cell wall OH groups. 

Another case that is less frequently encountered involves the situation in which previously sorbed molecules lead to a modification of the sorbent which favors further sorption (Fig. 9.3e). Such effects have been seen in studies involving anionic or cationic surfactants as sorbates. In some of these cases, a sigmoidal isotherm shape (Fig. 9.3/) has been observed, indicating that the sorption-promoting effect starts only after a certain loading of the sorbent. 

Lipatov798 recognized that the sorption of Methylene Blue on starch is a heterogenous neutralization reaction of neutralization in which starch acts as an acid. Adsorption isotherms of Methylene Blue on starch indicate that it is an equilibrium process and that swelling as well as other internal structural factors are responsible for the sigmoidal shape of the isotherm.799-800 It is also known that the Langmuir isotherm is affected by equilibrium between the monomer and dimer forms of Methylene Blue, both of which are capable of adsorption on starch (Table XLV).801... 

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