Hydrophobic and Hydrophilic Substances

Commonly the distinction between hydrophobic and hydrophilic substances is based on the analysis of interactions between their molecules and water as a solvent. A more precise classification of liquid and solid substances as hydrophobic and hydrophilic may be constructed basing on the apolar (LW) and polar (AB) components of their surface tensions. This three-parameter approach is of great importance for the understanding of surface behaviour [22,32,39,40]. A non-metallic substance is hydrophobic if it interacts with water by exhibiting only LW character. It has very little (or none at all) Lewis acid or Lewis base character. Typical substances at the hydrophobic end have low y surface parameters and their and components are equal to zero. Hydrophilic substances have non-zero y components of the surface tension and at least one of their y and y parameters is significant. 

The values of surface tension components that have been derived from the appropriate measurements of the Gibbs energy of adhesion for numerous liquids and solids are listed in Table6.2. The determination of a set of and y values for probe liquids is based on the choice of the first reference liquid. For this purpose, Van Oss et al. [39] assumed that y Q = y Q for water. In consequence, all of acid-base parameters in Table 6.2 are relative to those of water. 

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Table 2.2 gives examples of hydrophobic and hydrophilic substances. 

Core-multishell architectures (CMS) have been developed based on hyper-branched polymers, such as poly(ethylene imine) (PEI) and PG with an amphiphilic alkyl-PEG shell. These CMS nanocarriers can encapsulate a wide range of hydrophobic and hydrophilic substances that can be transported in both organic solvents and aqueous systems [36, 37] (Eig. 6.15). 

Since microemulsions are capable of solubilizing both hydrophobic and hydrophilic substances, it is not entirely unexpected that microemulsions can disrupt the stratum corneum and increase the penetration and transdermal drug absorption. Their use in topical formulations are therefore interesting. A drawback with this approach, however, is that there is a risk of skin irritation. It is possible that the latter effect depends on the... 

Because of the ability of o/w microemulsions (or micelles) to solubilize hydrophobic substances and that of w/o microemulsions (or reverse micelles) to solubilize hydrophilic substances, these systems can be used to extract hydrophobic and hydrophilic substances. The ability of o/w and w/o microemulsions to extract various hydrophobic and hydrophilic substances has been used to remove imdesirable flavors and nutrients (e.g., cholesterol) from foods and extract valuable food components, e.g., aromas, flavors, proteins, and hydrophobic solutes. 

The Gibbs equation relates the extent of adsorption at an interface (reversible equilibrium) to the change in interfacial tension qualitatively, Eq. (4.3) predicts that a substance which reduces the surface (interfacial) tension [(Sy/8 In aj) < 0] will be adsorbed at the surface (interface). Electrolytes have the tendency to increase (slightly) y, but most organic molecules, especially surface active substances (long chain fatty acids, detergents, surfactants) decrease the surface tension (Fig. 4.1). Amphi-pathic molecules (which contain hydrophobic and hydrophilic groups) become oriented at the interface. 

The design of artificial self-organizing systems is based on the ability of some molecules which contain simultaneously hydrophobic and hydrophilic groups to form molecular assemblies of definite structure in solution. Examples of the assemblies that can be used to suppress undesirable recombination processes are polyelectrolytes, micelles, microemulsions, planar lipid membranes covering an orifice in a film separating two aqueous solutions, unilamellar vesicles, multilamel-lar vesicles and colloids of various inorganic substances (see reviews [8-18] and references therein). 

Throughout the discussion, the terms surface active agent, surfactant, and detergent are used interchangeably to refer to amphiphilic substances which form association colloids or micelles in solution. Amphiphilic substances or amphiphiles are molecules possessing distinct regions of hydrophobic and hydrophilic character. 

The solubility of noble gases in various solutions (often aqueous-nonaqueous mixtures) gives indications of both hydrophobic and hydrophilic effects (Fig. 2.68). When substances exhibiting both effects are present, there is a maximum in the solubility of argon. Thus (Fig. 2.68, curve 1) in the system water-acetone, no hydrophilic effects are caused by the added solvent component, and the solubility increases. On the other hand, for systems in which urea is added, there are no hydrophobic effects and the solubility or the gas therefore deaeases. In curve 2 of Fig. 2.68, hydrophilic and hydrophobic effects compete (due to the properties of acetamide in water) and there is a maximum on the curve. 

The amphiphilic nature of nonionic surfactants is often expressed in terms of the balance between the hydrophobic and hydrophilic portions of the molecule. An empirical scale of ffLB (hydrophile-lipophile balance) numbers has been devised (see Chapter 7, section 7.3.2). The lower the ffLB number, the more lipophilic is the compound and vice versa. ffLB values for a series of commercial nonionic surfactants are quoted in Tables 6.7 and 6.8. The choice of surfactant for medicinal use involves a consideration of the toxicity of the substance, which may be ingested in large amounts. The following surfactants are widely used in pharmaceutical formulations. 

A surface-active agent (or surfactant) is a substance that lowers the surface or interfacial tension of the medium in which it is dissolved. Surfactants have a characteristic molecular structure consisting of hydrophobic and hydrophilic groups. This is known as an amphipathic structure, and causes not only concentration of the surfactant at the surface and reduction of the surface tension of the solvent, but also orientation of the molecule at the surface with its hydrophilic group in the aqueous phase and its hydrophobic group oriented away... 

The structure of these amphiphiles is radically different from that of conventional Langmuir film-forming amphiphiles, where segregation of the hydrophobic and hydrophilic parts of molecules at the air/water interface is a prerequisite. This is the first example of a Langmuir-Blodgett film that is fabricated with a micellelike substance. It is intriguing that amphiphilicity is still required to form a stable monolayer film of hyperbranched polymers, in spite of their fundamental structural differences from conventional amphiphiles. 

From the above three struetural variants one may infer that the value of 0.03 mole in the eellular lipids strongly indieates the required eoneentration for immobilization of tadpoles . In short, it adequately eonfers the bioactivity of a drug substance to its inherent hydrophobicity and hydrophilicity. 

Nanocapsules can be formulated from a variety of synthetic or natural monomers or polymers by using different techniques in order to fulfil the requirements of various applications. Both, hydrophobic and hydrophilic liquids are of high interest for encapsulation. So, e.g., either sensitive or volatile substances, as drugs or fragrances have to be encapsulated and protected for applications with a sustained demand of the respective compound. DNA, proteins, peptides or other active substances can be encapsulated in order to target them to specific cells. A further benefit of the polymeric shell is the possibility to control the release from the composite particles and hence the concentration in the environment. 

Both hydrophobic and hydrophilic food substances can be successfully incorporated into the organogels system."... 

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