Equilibrium, water activity

Trolard and Tardy (1987) calculated an equilibrium water activity of ca. 0.6 at 5 °C, 0.78 at 15 °C, 0.88 at 25 °C and 0.9 at 40 °C. At higher temperatures, hematite should be the stable phase even at an an2o of unity. Experimental confirmation of the importance of the relative humidity on the direction of the reaction is provided by the work of Torrent et al. (1982). This study also demonstrated that at aH2o < tho transformation offerrihydrite into more stable phases is very slow. 

Originally, water activity could not be measured directly. One method was to measure the weight loss of a product held at a range of controlled relative humidities, which also has the effect of holding the product over a range of water activities. If a product is held at its own water activity it neither gains nor loses weight, and this point is described as its equilibrium water activity. 

Salt pair Equilibrium water activity Rate of water transfer Maximum temperature (°C) ... 

The pairs used are identified by a shorthand notation NaI.2/0 means a combination of NaI.2H20 and anhydrous Nal (i. e. OH2O). Equilibrium water activity values are for 25 °C. Fast water transfer indicates equilibration in a few minutes, "slow that several hours may be needed. There is only limited information on the behavior of hydrate pairs giving lower water activities, though some indication that they generally tend to equilibrate slowly. From Zacharis et al.[23]. 

The equilibrium water activity achieved depends on the choice of salt hydrate pair used and the temperature. In most cases the temperature dependence is higher than for saturated salt solutions. There is also a maximum temperature at which the higher hydrate will melt to give a liquid phase, so above this the biocatalyst will probably be seriously affected. Table 8-4 gives water activity values for some pairs that can be recommended for biocatalysis, together with an indication of the rates of transfer, and the maximum temperature. A compilation from the literature1241 gives information on temperature dependence, and notes some other hydrate pairs whose use has not been (fully) tested. 

Moreyra, R. and Peleg, M. 1981. Effect of equilibrium water activity on the bulk properties of selected food powders. J. Food Sci. 46, 1918-1922. 

Desiccation of plant tissues presents a shift of the water from the liquid to the vapour phase (Sun, 2002). Temperature influences evaporation, as well as the partial water vapour pressure in the air and the energy status of water in plant tissue, both in dry and hydrated plant tissue. An increase in temperature results in a decrease in the equilibrium water content at a given relative humidity (water activity) or an increase in the equilibrium water activity for a given tissue water content (Fig. 1). Water activity can be described as the effective water content, which is thermodynamically available for various physiological processes in cells. The temperature dependence of the isotherm shift is described by the Clausius-Clapeyron Equation ... 

HDPE high density polyethylene EVOH ethylene vinyl alcohol T temperature (°C) a, , equilibrium water activity ... 

In a ternaty hqiiid-hqiiid system, such as the acetic acid-water-MIBK system, all three components are present in both liquid phases. At equilibrium the activity A° of any component is the same in both phases by definition [Eq. (15-6)]. 

Cells are normally kept at osmotic (water activity) equilibrium by the action of the Na-pump. Inhibition of the pump with the specific Na -K -ATPase inhibitor, ouabain, causes cell swelling as does inhibition of it by hypothermia. The intracellular environment contains a high concentration of K (100 to 120 mM, in most mammalian cells), lower concentrations of Na (about 10 to 30 mM), and high... 

Further stability models based on surface area, equilibrium water-content-pressure relationships, and electric double-layer theory can successfully characterize borehole stability problems [1842]. The application of surface area, swelling pressure, and water requirements of solids can be integrated into swelling models and mud process control approaches to improve the design of water-based mud in active or older shales. 

Figure 2.4 shows the equilibrium relationships of biological materials between the water content and the water activity, at constant temperatures and pressures. These data were first published in 1971, but did not find much attention in the RM field until now. At equilibrium the water activity is related to the relative humidity cp of the surrounding atmosphere (Equation 2.3) where p is the equihbrium water vapor pressure exerted by the biological material and po the equilibriiun vapor pressure of pure water at the same temperature. 

In this area the change of water content, Aw.c., as a function of the change of relative equilibrium humidity, Acp, as a function of water activity (a 100 = rp), Aw.c./Acp, is at a minimum. This also minimizes the potential error in a certified value by water taken up from the surrounding area. Based on these findings, it appears absolutely necessary that during the preparation of each material, water activity as well as water content must be determined and adjusted to achieve optimal stability and thus also a long shelf hfe of the final product. 

Taylor CD, Kelly RG, Neurock M. 2007b. First-principles prediction of equilibrium potentials for water activation by a series of metals. J Electrochem Soc 154 F217-F221. 

At low temperatures, the nonenzymatic reaction is reduced to a larger extent than the enzymatic reaction. The mass transfer rate is reduced to a smaller extent. Mass transfer limitation is required for high enantiomeric excess and determines the conversion rate. Therefore, the volumetric productivity decreases at lower temperatures. The equilibrium constant is considerably higher at low temperatures, resulting in a higher extent of conversion or a lower HCN requirement. Both the volumetric productivity and the required enzyme concentration increase by increasing the reaction temperature and aqueous-phase volume while meeting the required conversion and enantiomeric excess [44]. The influence of the reaction medium (solvent and water activity) is much more difficult to rationalize and predict [45],... 

Cote, G. Jakubiak, A. Bauer, D. Szymanowski, J. Mokili, B. Poitrenaud, C. Modeling of extraction equilibrium for copper(II) extraction bypyridinecarboxylic acid esters from concentrated chloride solutions at constant water activity and constant total concentration of ionic or molecular species dissolved in the aqueous solution. Solvent Extr. 

The most fundamental manner of demonstrating the relationship between sorbed water vapor and a solid is the water sorption-desorption isotherm. The water sorption-desorption isotherm describes the relationship between the equilibrium amount of water vapor sorbed to a solid (usually expressed as amount per unit mass or per unit surface area of solid) and the thermodynamic quantity, water activity (aw), at constant temperature and pressure. At equilibrium the chemical potential of water sorbed to the solid must equal the chemical potential of water in the vapor phase. Water activity in the vapor phase is related to chemical potential by... 

For a first chemical model, we calculate the distribution of species in surface seawater, a problem first undertaken by Garrels and Thompson (1962 see also Thompson, 1992). We base our calculation on the major element composition of seawater (Table 6.2), as determined by chemical analysis. To set pH, we assume equilibrium with CO2 in the atmosphere (Table 6.3). Since the program will determine the HCOJ and water activities, setting the CO2 fugacity (about equal to partial pressure) fixes pH according to the reaction,... 

The equilibrium temperature for any water activity is the temperature at which log K satisfies this equality. To find this value when the activity of water is one, we type... 

The resulting equilibrium temperature, 43.7 °C, is the temperature at which the log K is zero. For a water activity of 0.7, the equilibrium temperature drops to 11.8°C. Typing... 

Fig. 24.8. Evolution of fluid chemistry during the simulated evaporation of seawater as an equilibrium system at 25 °C, calculated using the Harvie-Mpller-Weare activity model. Upper figures show variation in salinity, water activity (aw), and ionic strength (/) over the reaction path in Figure 24.7 bottom figure shows how the fluid s bulk composition varies.

Because the chemical potentials of water distributed in two phases (i.e., solution and vapor) must be equal, the water activity of a food can be measured by bringing the food into equilibrium with the air above it. At equilibrium, under conditions of constant temperature and pressure, the aw values of the aqueous phase of a food (aw l) and of the air (aw v) are equal and can be estimated from the ratio of the partial vapor pressure of water above the food (pv) to the vapor pressure of pure water (p") at the same temperature (Walstra, 2003) ... 

When is equilibrium reached is an important follow-up, timescale question. The water activity measurement involves two interrelated time-scales. The first timescale is related to the nonequilibrium nature of most... 

Step 1. The substrate, RH, associates with the active site of the enzyme and perturbs the spin-state equilibrium. Water is ejected from the active site and the electronic configuration shifts to favor the high-spin form in which pentaco-ordinated heme Fe3+ becomes the dominant form-binding substrate. In this coordination state, Fe3+ is puckered out and above the plane in the direction of the sixth ligand site. The change in spin state alters the redox potential of the system so that the substrate-bound enzyme is now more easily reduced. 

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