Lipophilicity surfactant, interactions with

Some of the amino acid side chains in proteins are hydrophobic, generally buried in the interior of the folded protein molecule but exposed if the protein is unfolded. Sometimes these hydrophobic regions are partially exposed even in the native folded protein, and they are often referred to as hydrophobic patches on the protein surface. The lipophilic parts of surfactants interact with these hydrophobic regions,... 

Figure 7.17 shows the asymmetry ratios of a series of compounds (acids, bases, and neutrals) determined at iso-pH 7.4, under the influence of sink conditions created not by pH, but by anionic surfactant added to the acceptor wells (discuss later in the chapter). The membrane barrier was constructed from 20% soy lecithin in dodecane. All molecules show an upward dependence on lipophilicity, as estimated by octanol-water apparent partition coefficients, log KdaA). The bases are extensively cationic at pH 7.4, as well as being lipophilic, and so display the highest responses to the sink condition. They are driven to interact with the surfactant by both hydrophobic and electrostatic forces. The anionic acids are largely indifferent... 

These are present in an immiscible two-phase system (0 and W denoting oil and water, respectively) containing a third-surfactant component with partial solubility in both bulk phases. Each surfactant molecule has a hydrophilic (denoted by H) and a lipophilic (denoted by L) section. Conceptually then Winsor views all the possible molecular interactions in such a system in terms of their cohesive energy (denoted by C). For such a system, there are then 10 possible cohesive molecular interactions (i.e., 10 unique combinations of the letters 0, W, H, and L). In the ideal case, the lipophile-oil and the hydrophile-water interaction will be the predominant interactions. The relative magnitude (R) of these two interactions... 

Azone (laurocapram) is used extensively as a transdermal permeation enhancer, and has also found use in buccal drug delivery. It is a lipophilic surfactant in nature (Figure 10.4). Permeation of salicylic acid was enhanced by the pre-application of an Azone emulsion in vivo in a keratinized hamster cheek pouch model [35]. Octreotide and some hydrophobic compounds absorption have also been improved by the use of Azone [36], Azone was shown to interact with the lipid domains and alter the molecular moment on the surface of the bilayers [37], In skin it has been proposed that Azone was able to form ion pairs with anionic drugs to promote their permeation [38],... 

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

Similar to the situation for quadrupole-induced relaxation, the quadrupole splitting in liquid crystals is zero if the molecular symmetry is tetrahedral or higher. Electric field gradients are zero for such symmetries so there can be no quadrupolar interaction. However, one expects to see small splittings from tetrahedral or octahedral derivatives because of structural distortions. These predominately arise from specific interactions with extraneous materials such as lipophilic headgroups in surfactant systems, as seen, for instance, in both cationic and anionic octahedral cobalt(III) species [23], Much larger splittings will be expected from other structure... 

Surface-active substances — are electroactive or elec-troinactive substances capable to concentrate at the interfacial region between two phases. Surface-active substances accumulate at the electrode-electrolyte - interface due to -> adsorption on the electrode surface (see -> electrode surface area) or due to other sorts of chemical interactions with the electrode material (see - chemisorption) [i]. Surface-active substances capable to accumulate at the interface between two immiscible electrolyte solutions are frequently termed surfactants. Their surface activity derives from the amphiphilic structure (see amphiphilic compounds) of their molecules possessing hydrophilic and lipophilic moieties [ii]. 

The effect of water soluble polymers on the phase behavior of the anionic mlcroemulslon system was studied as a function of surfactant H/L properties. The cloud point temperatures for the neat mlcroemulslons and those containing 1500 ppm HPAM, partially hydrolyzed polyacrylamide, and 1000 ppm Xanthan gum are given In figure 4. The addition of either xanthan blopolymer or HPAM results In an Increase In the cloud point temperature of the mlcreomulslon. Both polymers have similar Interactions with the mlcroemulslon. Again one observes a lipophilic shift of the mlcroemulslon system Indicative of a repulsive interaction between the polymer and these anionic surfactants. 

Figure 47.2. Pluronic block copolymers with various numbers of hydrophilic EO (n) and hydrophobic PO (m) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions at concentrations above critical micelle concentration (CMC) these copolymers self-assemble into micelles.

Hydrophilic linkers act on the water-rich side of the interface by segregating or coadsorbing with the surfactant while avoiding a strong interaction with the oil phase. Because of a limited number of potential segregation sites the concentration range where linker molecules are able to enhance the solubilisation capacity is also limited. The behaviour of co-added lipophilic and hydrophilic linkers can be interpreted in terms of an assembled surfactant resulting in a very smooth and continuous variation of the polarity from bulk water to bulk oil. Assembled surfactants have already been applied to formulate biocompatible micro emulsions [51]. 

Micelles in water are formed by the self-association of surfactants, also called surface-active agents or detergents. These amphiphiles contain both hydrophilic and hydrophobic (lipophilic) moieties, so that one part of them tends to associate with water and the other part is repelled by water, but readily interacts with apolar molecules [11,12],... 

The first three effects are readily explained by the change in R numerator and denominator. As the surfactant lipophilic tail" gets longer, the imeraciion with the oil phase Aco is increased. In a similar way, a longer polyethylene oxide chain (hydrophilic group) of a nonionic surfactant results in an increase of the interaction with water Ac . 

With an ionic system, such displacement results from an increase in salinity of the aqueous pha.se, a reduction of the oil EACN either by using a shorter alkane or by adding polar or aromatic components, the addition of some lipophilic alcohol starting with n-pcntanol up to a few vol less with hexanol or higher alcohols. Finally the surfactant may be mixed with a less hydrophilic surfactant of the sante family (e.g.. with a longer tail). If dte system contains nonionic surfactants an increase in temperature would decrea.se the hydrophilic interaction quite rapidly while the electrolyte effect would be alnntsl negUgihIc. with the exception of polyvalent cation salts. 

The complexity of MLC is much greater dmn that of conventional RPLC with aqueous-organic solvents, because of Ihe number of possible interactions with both mobile and stationary phases (Fig. 5.1). Hie solutes in the mobile phase can interact electrostatically with the charged outer-layer of ionic micelles, and hydrophobically with their lipophilic interior. The steric factor can also be important. The modification of the stationary phase by adsorption of surfactant monomers, which creates a "micelle-like" surface, gives rise to similar interactions with the solutes. The combination of these interactions cannot be duplicated by any traditional pure or mixed solvent system. While micellar solutions will never totally replace traditional aqueous-organic eluents, they offer several interesting alternatives to separation work. 

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