Relative humidity effects thermodynamics

Effect of High Relative Humidities on Orange Spotting Charges Based on Vanadium Oxytri-chloride , DR EV-TN-1077/73, ARDE, Canada (1973) 13) C.W. Beckett, Thermodynamics... 

The adsorption isotherms for hydrocarbons on wet cellulose approach BET type III (1 0 in form as the relative humidity is increased (9.), and the isosteric heats approach the heat of condensation for all surface coverages of hydrocarbon (11 ). Thus, water lowers the affinity of cellulose for hydrocarbons. This effect is independent of any changes in surface area with relative humidity, and is also observed on regenerated cellulose film (12 ). However, the thermodynamic data indicate that the surface does not behave as expected for pure water, even at very high water contents (ll). In fact, the GC method has also been used to study adsorption of hydrocarbons on liquid water (15, l6). In this case, the interactions are very weak and the adsorption isotherm does not fit the BET theory, so that surface areas cannot be estimated directly (16). 

A book by Laugh in [76] is a very valuable reference on the aqueous phase behavior of surfactants. It covers this vast area of science from the viewpoints of the role of phase science within physical science, physical chemistry (thermodynamics of immiscibility, phase diagrams, the phase rule, characteristic features of surfactant phase behavior, kinetic and mechanistic aspects of surfactant phase behavior, relative humidity), structures and properties of surfactant phases, molecular correlations (surfactant and nonsurfactant behavior in amphiphilic molecules, hydrophilicity, lipophilicity, proximate and remote substituent effects, influence of third components on aqueous surfactant phase behavior), the relationship of the physical science of surfactants to their utility, and the history of surfactant phase science. 

From a thermodynamic point of view, one might expect that polysaccharide crystallites would display distinct hydrates and not show continuous variation in water content as a function of relative humidity. So far, the continuous variation in unit-cell parameters as a function of relative humidity seems to be the rule. However, the variation of cell parameters with relative humidity seems to follow the shape of the moisture sorption curve.(] ) In all probability, the fine structure factor introduces localized strain effects which prevents detection of a unique hydrate at a given relative humidity. 

The effect of the kaolinite surface on the structure of water beyond monolayer coverage has not been ascertained conclusively. Multilayers of water are known to form on the clay mineral at relative humidities greater than 20 per cent, and about 10 molecular layers are thought to exist at relative humidities near 98 per cent. Thus an absolute upper limit for the dimension of the region of adsorbed water in a kaolinite suspension should be around 3 nm. Data on the thermodynamic properties (heat of immersion, partial specific entropy, isosteric heat of ackorption) of Li-saturated kaolinite-water systems, which do not show hysteresis, indicate consistently that differences between bulk liquid water and water on the clay are detectable up to about three molecular layers of coverage. Perhaps the lower limit for the dimension of the adsorbed water region is then about 1 nm. 

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 ... 

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