Water sorption absorption

Another major drawback of polysaccharides is their hydrophilic nature leading to low degrees of adhesion between fiber and matrix [11]. Moisture absorption takes place by three types of mechanisms namely diffusion, capillarity, and transport via micro cracks [2]. Among the three, diffusion is considered to be the major mechanism. Water absorption largely depends on the water-soluble or hygroscopic components embedded in the matrix, which acts as a semipermeable membrane. While, fiber/matrix adhesion and fiber architecture also affect the moisture absorption. The results of the water sorption experiment showed an interesting trend. The extent of water uptake was not very significant and also did not increase linearly with amount of filler (Table-2). 

This preservative is comparatively new to ophthalmic preparations and is a polymeric quaternary ammonium germicide. Its advantage over other quaternary ammonium seems to be its inability to penetrate ocular tissues, especially the cornea. It has been used at concentrations of 0.001-0.01% in contact lens solutions as well as dry eye products. At clinically effective levels of preservative, POLYQUAD is approximately 10 times less toxic than benzalkonium chloride [87,137], Various in vitro tests and in vivo evaluations substantiate the safety of this compound [137,141,142], This preservative has been extremely useful for soft contact lens solutions because it has the least propensity to adsorb onto or absorb into these lenses, and it has a practically nonexistent potential for sensitization. Its ad-sorption/absorption with high water and high ionic lenses can be resolved by carefully balancing formulation components [143],... 

The addition of a filler changes the kinetics of the water absorption by an epoxy binder, water absorption becoming a multistage process (Fig. 12). Crank and Park150) have given the equation for the kinetics of water sorption by a thin plate, as well as a solution of the Fickian diffusion differential Equation as ... 

Most of these aspects of water-sorption equilibrium correspond to the equality of chemical potentials of water in the medium and in the polymer. The consequences of this principle are illustrated by the experiment of Fig. 14.2, where an interface is created between water and a nonmiscible liquid (oil, hydrocarbon, etc.), and a polymer sample is immersed into the organic liquid. It can be observed that, despite the hydrophobic character of the surrounding medium, the sample reaches the same level of water saturation as in direct water immersion or in a saturated atmosphere. What controls the water concentration in the polymer is the ratio C/Cs of water concentrations in the organic phase, where Cs is the equilibrium concentration, which can be very low but not zero. In other words, hydrophobic surface treatments can delay the time to reach sorption equilibrium but they cannot avoid the water absorption by the substrate. 

According to the results shown for this polymer, it is possible to confirm the hypothesis of Riggs et al. [129] concerning the existence of two mechanisms of water absorption. On the other hand it is interesting to note that the dielectric analysis of these polymers allow to know the importance of water sorption in this kind of polymers what is very important from technological an medical point of view. Therefore dielectric measurements on these kind of polymers result in a powerfull tool to analyze the effect of water absorption on the polymeric matrix and then to applications of these materials. 

Since water vapor dissolves in the solid during absorption, several models based on solution theory, proposing that the sorbate is taken up into the solid as a solid solution, have been derived and used to describe water sorption on polymers (e.g., Flory-Huggins, Hallwood-Horrobin ). More recently, Vrentas et developed a solution-based model... 

Another approach is to proceed from the chemical composition. This may work for some fairly dry foods, although it implicitly assumes an absorption mechanism, which is that certain chemical groups bind certain amounts of water, and by determining the concentration of these groups, the water sorption can be calculated. It concerns especially ionized groups (a few water molecules per group) and dipoles, such as a peptide bond (<1 water molecule per group). This method works reasonably well for proteins around aw = 0.5. 

Sorption processes are very effective and include adsorption/desorption (reversible binding at the solid-water interface), absorption (diffusion of pollutants into the solid matrix), precipitation and coprecipitation (incorporation into a freshly formed solid), and occlusion (sequestration of adsorbed pollutants during mineral growth). The most important factors for retention processes are pollutant concentration, the composition of the solid matrix, solution composition (e.g., complexing agents) and E/pH conditions (Brady and Boms 1997). 

Water sorption In blomaterlals Is very Important to the functioning of some polymers, such as hydrogels In soft contact lenses. Water uptake may also lead to absorption of Ions and other molecules, as enzymes, which can cause biodegradation of the polymer, especially If It contains susceptible bonds. 

Hysteresis phenomena in the water sorption by high polymers [125] and by other proteins such as wool fiber [125] and casein [126] have also been described. Smith [125] suggests that hysteresis is a result of differences in the ratio of bound to free water in the substrate, with a larger amount of bound water present on desorption than on absorption. 

The extent of absorption and rate of transport are important properties of a membrane. Physicochemical characteristics such as electrical conductivity and ion selectivity are closely related to the amount of water sorption. On the other hand, mass transport in solutions next to membranes depends on the buildup of the hydrodynamic boundary layers, especially at high current densities. Therefore, the maximum current density is determined by the diffusion of chemical species through the boundary layers outside the membrane and not so much by processes that occur inside the membrane. The limiting current density, therefore, is not strongly affected by the water content of the membrane or by the processes of water transport through it. 

The sorption of water in a polymer can be explained by the difference between the solubility parameter (6) of the water (48 MPa° ) [62] and that of the polymer when this difference increases, the water sorption decreases. For PLA, 6 values of 19-20.5 MPa° have been reported that are similar to the 8 values of 19 and 16MPa° for PS and PET, respectively [62]. Auras et al. [54] found that the water absorption of PLA films (94% and 98% L-lactide) was difficult to measure below 100 ppm, even after these films were exposed to different temperatures and to between 0.11 and 0.94 for more than one week. Therefore, these authors concluded that the PLA films did not absorb measurable amounts of water. In contrast, although PS and PET have 8 values similar to those of PLA, water absorption values of 320 and 60 ppm for PS and PET, respectively, have been measured at 25 °C and of 0.5 [63]. 

Figure 4. Water sorption isotherms obtained for birch xylan during absorption at different temperatures.

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