Moisture hysteresis

Kapsalis, J.G. 1987. Influence of hysteresis and temperature on moisture sorption isotherms. In Water Activity Theory and Application to Foods (L.B. Rockland and L.R. Beuchat, eds), pp. 173-213. Dekker, New York. 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

Figure 7.11 Hysteresis of a moisture sorption isotherm (from Fennema, 1985).

During the last lew years, important research has gone into the addition of multiple humectants and water to food systems. Studies have shown that a hysteresis effect may occur with certain humectants. i.e, a different rate of moisture absorption than the rate for moisture desorption. Multiple humectants tend to compensate these hysteresis effects, giving uniform rates in both directions. 

Figure 10- Sheet resistance data illustrating the hysteresis in aged aluminum oxide moisture sensor. Reproduced with permission from reference 18. Copyright 1982 Institute of Electrical and Electronics Engineers.

Aluminum oxide charge-flow transistor, transfer function, 171,173,174f Aluminum oxide moisture sensor aging effects, 174f charge-flow transistor, 172f hysteresis effects, 175f... 

Sposito, G. and K.L. Babcock (1966). Equilibrium theory of kaolinite-water systems at low moisture contents with some remarks concerning adsorption hysteresis. Clays Clay Min., 14 133-147. 

Hygro-themo-chemo-mechanical behaviour of concrete is of great practical importance in many fields of civil engineering. Modelling these phenomena, especially in fresh concrete structures or concrete elements exposed to fire, is a complex problem. Several non-linear phenomena, like heat and mass sources associated with hydration or dehydration processes, phase changes, hysteresis of sorption isotherms, material properties dependent on moisture content, tem-... 

Hygroscopic Hysteresis hydroxyl content of 1.0 g of polyol. A material that absorbs moisture readily. The ability of polyurethane to absorb and dissipate energy due to successive deformation and relaxation. A measurement of the area between the deformation and relaxation stress-strain curves. 

Studies in the physical properties of soil. V. The hysteresis effect in capillary properties, and the modes of moisture distribution associated therewith. Ibid., 20 97-116. 

Sorption and desorption of water by starch exhibits hysteresis behavior. In maize starch, the sorption branch of the hysteresis loop corresponding to moisture contents above 16% displays exponential kinetics386 ... 

Additives to starch exert varying effects on the kinetics of water sorption. For example, lipids do not significantly affect the content of adsorbed water. The mode of starch defatting can also influence moisture sorption by molecules of the defatting solvent occupying active centers of sorption.389 The addition of either sucrose or lipids to starch has the same effect on both branches of the hysteresis curve.386,398 Some additives, such as dimethyl sulfoxide or ammonium rhodanide, induce selectivity of the adsorption and solvation of starch 411 Sulfur dioxide accelerates water sorption regardless of the temperature.412 Pretreatment of starch with sulfur dioxide usually increases the water sorption.413 Studies on the sorption of components of water-alcohol mixtures are discussed in Section IV. 

At equilibrium and at a particular RVP, the amount of adsorbed water held by a cellulose generally will be greater if it has been obtained following desorption from a higher RVP and not by adsorption from a lower RVP. The cause of this hysteresis is not fully established [303]. One explanation is based on the internal forces generated when dry cellulose swells, limiting the amount of moisture adsorbed whereas when swollen cellulose shrinks, stress relaxation occurs since the cellulose is plastic and permits a higher uptake of moisture. 

York, P. Analysis of moisture sorption hysteresis in hard gelatin capsules, maize starch, and maize starch drug powder mixtures. J. Pharm. Sci. 1981, 33, 269-273. 

All of the equations presented thus far are singlevalued relationships between the amount adsorbed and the pressure of water in the atmosphere. However, hysteresis is frequently observed. When a sample that has reached a specific equilibrium moisture content is exposed to lower relative humidity, desorption occurs. In many cases, adsorption and desorption isotherms are not superimposable. 

Most individuals recognize hysteresis in terms of a substance having different moisture contents at a specified relative humidity (CD in Fig. 3). The implications of this situation are obvious the moisture content of a material depends on its history of exposure to different relative humidity conditions. Controlling relative humidity can only insure that the moisture content lies within the range specified by the isotherm. The other aspect of hysteresis is that substances with equal moisture contents can have adsorbed water with different chemical activity (AB in Fig. 3). 

Starch has been classified as a moderately hygroscopic material.f Water sorption studies have been conducted using static methods (saturated salt solutions in closed chambers) modified inverse frontal gas chromatography, and automated moisture balance systems. " The isotherms are typically type II curves exhibiting hysteresis, as shown in Fig. 5. Hysteresis in starch samples has been attributed to intra- and... 

Moisture Sorption Isotherms. Green wood loses moisture to the atmosphere and approaches a moisture content designated as the equilibrium moisture content (EMC) for the particular atmospheric conditions. The EMC is a function of relative humidity, temperature, previous exposure history (hysteresis), species, and other miscellaneous factors. 

Sorption hysteresis in wood is beneficial from the viewpoint of wood utilization. This is because wood exposed to cyclic humidity conditions shows smaller changes in moisture content for given humidity changes than would be the case if there were no hysteresis (2J). Sorption hysteresis reduces the effective slope dM/dH of the sorption isotherm and the dimensional changes associated with humidity changes. 

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