Water, generally anomalous behavior

In some cases the Fickian model does not accurately represent moisture uptake in adhesives. This is illustrated in O Fig. 31.12a, which shows the uptake plot for an epoxide immersed in water at SO C. The experimental data appears to indicate Fickian diffusion however, the best fit of the Fickian diffusion equation to the data indicates equilibrium is reached more slowly than predicted by Fickian diffusion. This type of behavior is sometimes termed pseudo-Fickian behavior. In general, anomalous behavior is seen at high temperatures and humidity. 

The ion selectivity data clearly indicate that veiy specific ion-solvent interactions take place in the vicinal water and that these bear little resemblance to expectations from bulk-phase observations. This, in turn, means that such quantities as the classical standard ion activity coefficients are not applicable, and hence osmotic coefficients must also differ from the expected values. In fact, the use of the generally accepted osmotic coefficients is simply inappropriate, and unusual osmotic behavior must be anticipated. Likewise, if the activity coefficients display anomalous behavior so must cell membrane potentials. In other words, ion distribution and the osmotic behavior of cells must be influenced by vicinal water, and models of cell volume regulation must anticipate and take into account this aspect (see also Wiggins, 1979). 

We conclude this section with a general comment on interstitial models. The study of such models is useful and quite rewarding in providing us insight into the possible mechanism by which water exhibits its anomalous behavior. One should be careful not to conclude that the numerical results obtained from the model are an indication of the extent of the reality of the model. It is possible, by a judicious choice of the molecular parameters, to obtain thermodynamic results which are in agreement with experimental values measured for real water. Such agreement can be achieved by quite different models. The important point is not the quantitative results of the model but the qualitative explanation that the model offers for the various properties of water. We shall use the same model in Sec. 3.6 to explain some aspects of aqueous solutions of simple solutes. 

In all of these models, the hydrogen bonds, or the structure of liquid water, were traditionally emphasized as the main molecular reasons for the anomalous behavior exhibited by liquid water. However, underlying this relatively ill-defined concept of structure (which was much later defined in statistical mechanical terms see Sec. 2.7) lies a more fundamental principle which can be defined in molecular terms, and which does not explicitly mention the concept of structure yet is responsible for the unusual properties of liquid water. This principle was first formulated in terms of generalized molecular distribution functions in 1973. It states that there exists a range of temperatures and pressures at which the water-water interactions produce a unique correlation between high local density and a weak binding energy. Clearly, this principle does not mention structure. As will be demonstrated in this section, it is this principle, not the structure per se, which is responsible for the unique properties of water as well as of aqueous solutions. ... 

Another general comment regarding the two approaches is their ability to explain the anomalous behavior of liquid water and its solutions. We... 

Water-soluble binders generally contain organic solvent (< 10-15 wt%) that originates from their production via polycondensation or polymerization reactions in an organic medium. They can, however, still be dissolved or diluted with water after neutralization. Anomalous viscosity behavior may, however, arise it is characterized by a viscosity increase on dilution with water due to association of the binder molecules in water (Fig. 3.1) [3.42]. More modern binders are supplied as preneutralized solutions in water and cosolvent. 

In general, the reduction of the liquid film thickness to fewer than 4-6 molecular layers can induce lateral ordering and lead to freezing. It has been demonstrated that water confined to nanospaces exhibits anomalous phase behaviors that are typically illustrated experimentally or via MD simula-tion. " Moreover, there is evidence that a possible liquid-solid phase transition occurs for ionic liquids in confined systems. We also reported the first simulation results of a liquid-solid freezing transition of an 1,3-dimethylimidazolium chloride ([Dmim][Cl]) ionic liquid between two parallel graphite walls. " This result is important to understand the microstructure and freezing processes of ILs in confined systems, such as lubrication, adhesion, and IL/nanomaterial composites. 

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