Hydrophilic solutes

A drop of an aqueous solution of the mixture to be separated is now placed near the bottom of the paper strip and allowed to evaporate in the air. The strip is now again suspended in the closed cylinder, but with the bottom of the strip just immersed in the solvent. The capillary action of the paper will cause the solvent to rise steadily up the strip, and during this process the solvent, which now contains the mixture in solution, is continuously extracted by the retained water molecules in the paper. A highly hydrophobic (water-repellent) solute will move up closely behind the solvent-front, whereas a highly hydrophilic solute will barely leave the original point where the drop of the mixed solutes in solution has been dried. In an intermediate case,... 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

Fig. 15-1 Schematic representation of the change in water structure (water molecule orientation) due to the presence of a charged (hydrophilic) solute, (a) Pure water, (b) A solute forming strong bonds with water (dissolution favorable), (c) a solute forming weak bonds with water (dissolution unfavorable).

Figure 4.1. (a) A cellular automata model of hydrophilic solutes in water, (b) A cellular automata model of hydrophobic solutes in water... 

MB Charro, GI Vilas, TB Mendez, MAL Q, JP Marty, RH Guy. Delivery of a hydrophilic solute through the skin from novel microemulsion systems. Eur J Pharm Biopharm 43(l) 37-42, 1997. 

In series with a desolvation energy barrier required to disrupt aqueous solute hydrogen bonds [14], the lipid bilayer offers a practically impermeable barrier to hydrophilic solutes. It follows that significant transepithelial transport of water-soluble molecules must be conducted paracellularly or mediated by solute translocation via specific integral membrane proteins (Fig. 6). Transcellular permeability of lipophilic solutes depends on their solubility in GI membrane lipids relative to their aqueous solubility. This lumped parameter, membrane permeability,... 

Jarlier, V. and Nikaido, H. (1990). Permeability barrier to hydrophilic solutes in Mycobacterium chelonei, J. Bacteriol., 172, 1418-1423. 

Mason GR, Peters AM, Bagdades E, Myers MJ, Snooks D, Hughes JMB (2001) Evaluation of pulmonary alveolar epithelial integrity by the detection of restriction to diffusion of hydrophilic solutes of different molecular sizes. Clin Sci 100 231-236. 

Because mucin and/or cilia systems of AIC cultured epithelial cells may work as a barrier for drug transport, lower Papp values are expected in cell layers cultured in AIC than in LCC methods. However, it was interesting to note that no significant differences in Rapp values were observed between the cell layers cultured with the two methods (Table 9.1). This is in contrast to solute permeabilities reported previously for cell layers cultured with LCC versus AIC methods [76, 80], For example, Yang et al. reported that Rapp of lipophilic solutes (e.g., various /3-blockers) across the primary cultured conjunctival epithelial cell layer are about threefold lower when cultured under AIC than LCC conditions, suggesting that the permeability of AIC cultured cell layers generally better reflects that of the excised tissue than LCC counterparts. Mathias et al. [76] also reported that the permeability of hydrophilic solutes across the primary rabbit tracheal epithelial cell layer cultured under AIC conditions was only half of that observed for cell layers cultured under... 

Passive diffusion is considered to be the major pathway by which xenobiotics cross the placenta. Paracellular diffusion was shown to be the predominant pathway for transfer of hydrophilic solutes, such as chloride ions across perfused placental lobes and opioid peptides and dextrans across BeWo cells [11-13], It has been proposed that denudations in the syncytiotrophoblasts-containing fibrinoid deposits provide a possible paracellular route across the placenta [14], Transtrophoblast channels in the syncytiotrophoblasts could also be responsible for this mode of diffusion [15], For more lipophilic solutes, the transplacental route appears to be the preferred mode of passage... 

The Winsor II microemulsion is the configuration that has attracted most attention in solvent extraction from aqueous feeds, as it does not affect the structure of the aqueous phase the organic extracting phase, on the other hand, is now a W/0 microemulsion instead of a single phase. The main reason for the interest in W/0 microemulsions is that the presence of the aqueous microphase in the extracting phase may enhance the extraction of hydrophilic solutes by solubilizing them in the reverse micellar cores. However, this is not always the case and it seems to vary with the characteristics of the system and the type of solute. Furthermore, in many instances the mechanism of extraction enhancement is not simply solubilization into the reverse micellar cores. Four solubilization sites are possible in a reverse micelle, as illustrated in Fig. 15.6 [19]. An important point is that the term solubilization does not apply only to solute transfer into the reverse micelle cores, but also to insertion into the micellar boundary region called the palisade. The problem faced by researchers is that the exact location of the solute in the microemulsion phase is difficult to determine with most of the available analytical tools, and thus it has to be inferred. 

During the past decades, liposomes have been used for drug delivery on account of their unique solubilization characteristics for water-insoluble organic substances. A liposome encapsulates a region of aqueous solution inside a hydrophobic membrane dissolved hydrophilic solutes cannot readily pass through the lipids. Hydrophobic chemicals can be dissolved into the membrane, and this way, the liposome can carry... 

INSIGHT uses the fundamental correlation between the electrical and permeability properties of skin. Skin permeability shows a strong correlation with skin impedance, as shown in Figure 4B. Figure 4B shows 150 independent and simultaneous measurements of mannitol skin permeability and skin impedance for six different enhancer formulations. The relationship between skin impedance and permeability to hydrophilic solutes confirms that the former can be used as a surrogate measure for the later. Skin conductance is quick and easy to obtain and does not require additional sample handling and analysis. 

With alkali halide-TBA-W or alkali halide-PD-W systems, the parameters Bne are negative for volumes and heat capacities (see Figures 1-5 and 10). This sign seems to be the one usually observed for the interaction of a hydrophobic with a hydrophilic solute (6). At intermediate cosolvent concentration, AYe°(W — W + TBA) and AYe°(W — W + PD) deviate in the direction we would expect for hydrophobic association the volume increases sharply, and the heat capacity decreases further. Inorganic electrolytes lower the critical micelle concentration of surfactants by salting out the monomers, thus favoring micellization (25) in a similar way, in the co-sphere of a hydrophilic ion, the hydrophobic bonding between the cosolvent molecules may be enhanced. 

In order to see how the hydrophobic hydration breaks down when an aprotic cosolvent is added to water and how it starts to contribute when water is added to the aprotic solvent, we measured the enthalpies of solution of hydrophobic and, for comparison, hydrophilic solutes in mixed solvent systems of water and... 

FIGURE 11-28 Energy changes accompanying passage of a hydrophilic solute through the lipid bilayer of a biological membrane, (a) In... 

Glucose was recovered quantitatively in the column effluent, whether humic acid was present or absent. Presumably, all other poorly volatile, neutral, hydrophilic solutes would also be recovered in this fraction. 

To test this, we determined the partition coefficients of two different solute extremes - vitamin B-12 and norethindrone. Vitamin B-12 is a relatively large, hydrophilic solute, while norethindrone is a relatively small, hydrophobic solute. Thus we would expect size exclusion to dominate partitioning with vitamin B-12, while hydrophobic interactions should dominate with norethindrone. This behavior is in fact observed in Figs. 19 and 20. The values of K... 

Fig. 19. Experimental partition coefficients and those predicted by size exclusion theory (Eq. (4), shown as a solid line) for vitamin B12 (a relatively large, hydrophilic solute) in 10 x 4 PNIPAAm gel. Although size exclusion theory consistently underestimates the value of K, the sharp drop predicted at the transition temperature is observed. Error bars (standard deviation of three samples) not shown are smaller than the symbol. The dotted line is to guide the eye. Reprinted from the Journal of Controlled Release (1992) 18 1, by permission of the publishers, Elsevier Science Publishers BV [70]...

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