Aqueous hydrophilic binary mixtures

Note that the extensive HB network is compromised near both the hydrophilic and the hydrophobic surfaces, but differently. In the case of the hydrophilic surface, the enthalpic gain from the water-surface interaction compensates for the twin losses of enthalpy and the entropy of water arising from the molecular rearrangement imposed by the surface. However, for a hydrophobic surface, such a compensation is not present. Therefore, the chemical potential of a water molecule near a hydrophobic surface is higher than that in a bulk. 

Another crucial issue is the distance to which the effect of the surface can be felt. This is coimected to the correlation length of the unperturbed liquid. The correlation length increases with lowering temperature. Both density and orientational correlation are involved. 

Many aqueous binary mixtures owe their peculiar properties to the combined effects of hydrophobic and hydrophilic interactions. Examples are abundant in chemistry. Dimethyl sulfoxide, dioxane, ethanol and methanol, iV-ethyl acetamide, many hormones, steroids, and vitamin solutes exhibit this combined hydrophobic-hydrophilic interaction in aqueous solution. Such hydrophiUc-hydrophobic combined effects are specifically termed amphiphilic effects and are extensively exploited in cosolvent chemistry. In Chapter 16 we extensively discuss the amphiphilic effects, the corresponding properties, and applications with explicit examples. According to the nature of an aqueous binary mixture they are often called non-ionic kosmotropes or chaotropes. Let us take the example of urea (H2NCONH2). 

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Because of its hydrophilic nature even unmodified BC shows great potential to separate azeotropes such as EtOH/FbO. It adsorbs seven times more water than ethanol. This selectivity and a reasonable flux increase with growing temperature and thinning of the membrane. In addition, the BC membranes also show a high water affinity in aqueous binary mixtures of organic solvents. 

K. Soontarapa, Hydrophilic membranes for pervaporation An analytical review, Desalination, 1997, 110, 251-286 T.M. Aminabhavi, R.S. Khinnavar, S.S. Harogop-pada and U.S. Arithal, Pervaporation separation of organic-aqueous binary mixtures, J. Macromol. Sci., Macromol. Chem. Phys. 1994, C34(2), 139-204 H. Kita, T. Inoue, H. Asamura, K. Tanaka and K. Okamoto, NaY zeolite membrane for the pervaporation separation of methanol-methyl tert-butyl ether mixture, J. Chem. Soc., Chem. Commun., 1997, 45-46. 

Here the surface atoms can form strong HBs with water molecules and exert influence on the extended HB network of water. Another important class of systems is aqueous binary mixtures, where both hydrophobic and hydrophilic interactions together determine many of the unusual properties exhibited by these systems. The hydrophilic effect is partly responsible for water being such a good solvent for a large number of polar molecules. Hydrophilic interaction also finds great use in industry, as in hydrophilic chromatography. 

As these cosolvents contain both hydrophilic and hydrophobic groups, the same molecule can induce opposite effects in water. The hydrophilic part can interact with water to form strong HBs, while the hydrophobic part may induce cooperative ordering in the system by a hydrophobic hydration effect. These two effects combine together to regulate the extensive HB network of water in their aqueous binary mixtures that is reflected in strong, often anomalous non-ideal behavior in many physical properties such as viscosity, density, dielectric constant, excess mixing volume, surface tension, heat of formation, etc. 

Study of Ternary Tablets Percolation theory has been developed for binary systems, however, drug delivery systems usually contain more than two components. The existence and behavior of the percolation thresholds in ternary pharmaceutical dosage forms have been studied [39] employing mixtures of three substances with very different hydrophilicity and aqueous solubility (Polyvinylpyrrolidone (PVP) cross-linked, Eudragit RS-PM, and potassium chloride). 

Aqueous solutions of many nonionic amphiphiles at low concentration become cloudy (phase separation) upon heating at a well-defined temperature that depends on the surfactant concentration. In the temperature-concentration plane, the cloud point curve is a lower consolution curve above which the solution separates into two isotropic micellar solutions of different concentrations. The coexistence curve exhibits a minimum at a critical temperature T and a critical concentration C,. The value of Tc depends on the hydrophilic-lypophilic balance of the surfactant. A crucial point, however, is that near a cloud point transition, the properties of micellar solutions are similar to those of binary liquid mixtures in the vicinity of a critical consolution point, which are mainly governed by long-range concentration fluctuations [61]. 

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