Hydrophobic and hydrophilic solutes

The terms hydrophobic (HcftO) and hydrophilic H(f)I) solutes have been used in the literature with a variety of meanings. 

We shall introduce here a simple definition in terms of the partition coefficients of a given solute between two solventsd Qualitatively, having two immiscible solvents, water w and some reference solvent a, we add a solute s and ask how it is distributed between the two liquids. The simplest measure of this distribution is the ratio of the number densities of s in the two phases at equilibrium. This is related to the difference in the solvation Gibbs energies of s in the two phases thus 

Clearly, the larger the ratio rjs, the larger the relative affinity of s for the two phases. Since we are interested in the relative affinities of various solutes towards water, we can choose a constant reference solvent a and measure the ratio % for various solutes. The simplest reference solvent is an ideal gas, for which AGf = 0 and rjs in (3.3.3) reduces to the Ostwald coefficient (3.2.1)  

Having various solutes, we can construct a hydrophobic-ity (or hydrophilicity) scale according to the value of ys, or equivalently, according to the value AG. Within this definition. 

For groups or radicals such as methyl, ethyl, etc., we cannot measure the partition coefficient analog to yg. We shall not need these quantities in this book. It should be emphasized, however. 

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Recently, we have examined solute permeation through hydrogel membranes in an effort to develop models which describe in detail the transport phenomena with particular emphasis on the role of water in this process. These studies have utilized p-HEMA and its copolymers, and both hydrophobic and hydrophilic solutes (7., i). It was determined that p-HEMA and its copolymers are permeable to both hydrophobic and hydrophilic solutes. 

An alternative description of a molecular solvent in contact with a solute of arbitrary shape is provided by the 3D generalization of the RfSM theory (3D-RISM) which yields the 3D correlation functions of interaction sites of solvent molecules near the solute. It was first proposed in a general form by Chandler, McCoy, and Singer [22] and recently developed by several authors for various systems by Cortis, Rossky, and Friesner [23] for a one-component dipolar molecular liquid, by Beglov and Roux [24, 25] for water and a number of organic molecules in water, and by Hirata and co-workers for water [26, 27], metal-water [26, 28] and metal oxide-water [31] interfaces, orientationally dependent potentials of mean force between molecular ions in a polar molecular solvent [29], ion pairs in aqueous electrolyte [30], and hydration of hydrophobic and hydrophilic solutes alkanes [32], polar molecule of carbon monoxide [33], simple ions [34], protein [35], amino acids and polypeptides [36, 37]. It should be noted that accurate calculation of the solvation thermodynamics for ionic and polar solutes in a polar molecular liquid requires special corrections to the 3D-RISM equations to eliminate the electrostatic artifacts of the supercell treatment employed in the 3D-RISM approach [30, 34]. 

As we noted in Chapter 3, the terms hydrophobic and hydrophilic solutes are not strictly antonyms the same is true of HipO and H[Pg.417]

A mean field theory of solvent structure has been employed by Marcelja(146) to describe the effect of solvent correlation on solute-solute interactions of both hydrophobic and hydrophilic solutes. The interactions between hydrophilic solutes in water has also been considered in a group of papers(141,147-150) where the heats of dilution and of the mixing at constant molality for various non electrolytes (alcohols, amides, sugars, urea, aminoacids and peptides) are interpreted in the framework of the McMillan-Mayer theory(151) and the enthalpy effects arising from interactions between each functional group on one molecule and every functional group on the other molecule are evaluated. 

The reasons for the introduction of the terms "lyophobic" (meaning fear of lye) and "lyophilic" (meaning love of lye) are even more obscure and appear irrelevant as they are virtually alternatives to the terms hydrophobic and hydrophilic. The terms originated in the early soap industry during the mid-to-late nineteenth century. In about 1850 soap was prepared by boiling a vegetable oil with an alkaline solution obtained from leaching wood ash with water. 

Dendrimers can also be prepared with an inverse relationship between their hydrophobic and hydrophilic constituents, i.e. with a hydrophobic periphery and a hydrophilic interior. They can then behave as reverse micelles and are able to concentrate polar molecules from solutions of nonpolar solvents. The shape of these molecules, when dissolved in a solvent that matches the hydrophobic nature of the periphery, is spherical with chain-ends extended towards the solvent. The interior may then collapse to a minimum volume, so that unfavourable interactions that might result from penetration by solvent molecules are minimized. 

In the case of PS II membrane proteins, as discussed above, the hydrophobic and hydrophilic pairs of attached lipids can partially support the protein complex at the air-water interface, despite their large size and density. However, in the case of PS II core complex, the detergent strips the attached lipids and some extrinsic proteins. The remaining protein complex is water soluble. It is very difficult to prepare a stable monolayer of water-soluble proteins with the Langmuir method. Indeed, it is hard to directly prepare a stable monolayer of PS II core complex because of its water solubility as well as density. One possible solution is to change the density and ionic strength of the subphase [9]. 

In emulsion polymerization, a solution of monomer in one solvent forms droplets, suspended in a second, immiscible solvent. We often employ surfactants to stabilize the droplets through the formation of micelles containing pure monomer or a monomer in solution. Micelles assemble when amphiphilic surfactant molecules (containing both a hydrophobic and hydrophilic end) organize at a phase boundary so that their hydrophilic portion interacts with the hydrophilic component of the emulsion, while their hydrophobic part interacts with the hydrophobic portion of the emulsion. Figure 2.14 illustrates a micellized emulsion structure. To start the polymerization reaction, a phase-specific initiator or catalyst diffuses into the core of the droplets, starting the polymerization. 

The influence of adsorption on the structure of a -chymotrypsin is shown in Fig. 10, where the circular dichroism (CD) spectrum of the protein in solution is compared with that of the protein adsorbed on Teflon and silica. Because of absorbance in the far UV by the aromatic styrene, it is impossible to obtain reliable CD spectra of proteins adsorbed on PS and PS- (EO)8. The CD spectrum of a protein reflects its composition of secondary structural elements (a -helices, / -sheets). The spectrum of dissolved a-chymotrypsin is indicative of a low content of or-helices and a high content of //-sheets. After adsorption at the silica surface, the CD spectrum is shifted, but the shift is much more pronounced when the protein was adsorbed at the Teflon surface. The shifts are in opposite directions for the hydrophobic and hydrophilic surfaces, respectively. The spectrum of the protein on the hydrophilic surface of silica indicates a decrease in ordered secondary structure, i.e., the polypeptide chain in the protein has an increased random structure and, hence, a larger conformational entropy. Adsorption on the hydrophobic Teflon surface induces the formation of ordered structural elements, notably an increase in the content of O -helices (cfi, the discussion in Sect. 3.1.4). 

In dilute aqueous solutions, copolymers having hydrophobic and hydrophilic parts may form polymeric micelles, i.e. stable particles with a core-shell structure. The association of the hydrophobic parts of the block copoly-... 

The transition enthalpies of the s- and p-fractions obtained from the feed with a comonomer molar ratio of 85 15 were equal to 6 and 7 J/g, respectively, i.e. the values are very close. This, therefore, can be indicative of almost the same average length of oligoNVCl blocks. Moreover, as we have already stressed, the fractions also had virtually the same final comonomer composition. However, since the solution properties of these fractions are drastically different, one can draw the conclusion that this is apparently due to a specific distribution of hydrophobic and hydrophilic residues along the polymer chains. In turn, because of all the properties that are exhibited by the s-fraction, this fraction can be considered to be a protein-like copolymer [27]. 

Thus, in summary, self diffusion measurements by Lindman et a (29-34) have clearly indicated that the structure of microemulsions depends to a large extent on the chain length of the oosurfactant (alcohol), the surfactant and the type of system. With short chain alcohols (hydrophilic domains and the structure is best described by a bicontinuous solution with easily deformable and flexible interfaces. This picture is consistent with the percolative behaviour observed when the conductivity is measured as a function of water volume fraction (see above). With long chain alcohols (> Cg) on the other hand, well defined "cores" may be distinguished with a more pronounced separation into hydrophobic and hydrophilic regions. 

Interfacial tension studies are particularly important because they can provide useful information on the interfacial concentration of the extractant. The simultaneous hydrophobic-hydrophilic nature of extracting reagents has the resulting effect of maximizing the reagent affinity for the interfacial zone, at which both the hydrophobic and hydrophilic parts of the molecules can minimize their free energy of solution. Moreover, as previously mentioned, a preferential orientation of the extractant groups takes place at the interface. Conse-... 

Notable in the tricolorin A (106) solid state is the presence of 18 water molecules in the unit cell, and an anisotropic repartitioning of the hydrophobic and hydrophilic sections. The water molecules form a dense network that creates a dividing layer between the hydrophilic faces (see Fig. 6). The high water content indicates that the conformation in the solid state is not dominated by intermolecular forces and could be indicative of a similar conformation in both solution and supermolecular... 

Complementing the equilibrium measurements will be a series of time resolved studies. Dynamics experiments will measure solvent relaxation rates around chromophores adsorbed to different solid-liquid interfaces. Interfacial solvation dynamics will be compared to their bulk solution limits, and efforts to correlate the polar order found at liquid surfaces with interfacial mobility will be made. Experiments will test existing theories about surface solvation at hydrophobic and hydrophilic boundaries as well as recent models of dielectric friction at interfaces. Of particular interest is whether or not strong dipole-dipole forces at surfaces induce solid-like structure in an adjacent solvent. If so, then these interactions will have profound effects on interpretations of interfacial surface chemistry and relaxation. 

Fig. 11 Plots of the measured lower critical solution temperature (LCST) as a function of the theoretical average number of OEGMA475 units per chain for a series of P(Me02MA-co-OEGMA475) copolymers of various composition. Hydrophobic and hydrophilic molecular regions on the copolymer are indicated in red and blue, respectively. (Reprinted with permission from [76]. Copyright (2008) John Wiley Sons, Inc.)...

Triton SP-Series surfactants use both a hydrophobe and an ethoxylate chain hydrophile. The surfactants are characterized by nonionic surfactant features such as good detergency, surface activity, and wetting. When the pH of an aqueous solution that contains a Triton SP-Series surfactant is reduced, the bond between the surfactant hydrophobe and hydrophile is permanently destroyed, thus eliminating surfactancy. This product was launched commercially in December 1996 and is currently available. The surfactants cannot be used in highly acidic environments. Other compounds that might be found in the contaminated waste, snch as phosphate, may interfere with the oil/water separation after snrfactant deactivation. All information is from the vendor and has not been independently verified. 

Partitioning selectivity of proteins can be achieved in RME based on the hydrophobic nature of proteins due to the fact that RMs provide both hydrophobic and hydrophilic environments to solutes simultaneously. 

In this paper we apply basic solution thermodynamics to both the adsorption of single surfactants and the competitive adsorption of two surfactants on a latex surface. The surfactant system chosen in this model study is sodium dodecyl sulfate (SDS) and nonylphenol deca (oxyethylene glycol) monoether (NP-EO o) These two surfactants have very different erne s, i.e. the balance between their hydrophobic and hydrophilic properties are very different while both are still highly soluble in water. 

It depends on the solution properties, or the relative balance between the hydrophobic and hydrophilic properties of the surfactants at this temperature. This term reflects the relative gain in the free energy when the hydrocarbon part of the surfactants is transferred from the aqueous solution to a hydrocarbon environment (either the micelle interior or the surface layer). The reason for this free energy gain is that the surfactant molecules are oriented at the... 

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