Lyophobic parts

The idealized reverse micelle sketched in figure C2.3.1 is an aggregate of a double-tail surfactant. In such systems the solvent is more compatible with the lyophobic part of the surfactant than with the headgroup. This preference... 

Critical micelle concentrations depend almost entirely on the nature of the lyophobic part of the surfactant. If micelle structure involved some kind of crystal lattice arrangement, the nature of the lyophilic head group would also be expected to be important. 

Steric stabilisers are usually block copolymer molecules (e.g. poly (ethylene oxide) surfactants), with a lyophobic part (the anchor group) which attaches strongly to the particle surface, and a lyophilic chain which trails freely in the dispersion medium. The conditions for stabilisation are similar to those for polymer solubility outlined in the previous section. If the dispersion medium is a good solvent for the lyophilic moieties of the adsorbed polymer, interpenetration is not favoured and interparticle repulsion results but if, on the other hand, the dispersion medium is a poor solvent, interpenetration of the polymer chains is favoured and attraction results. In the latter case, the polymer chains will interpenetrate to the point where further interpenetration is prevented by elastic repulsion. 

At low concentrations, surface-active additives may form a first adsorbed layer on the sol particles with the lyophobic part orientated outwards, thus sensitising the sol. At higher concentrations a second, oppositely orientated, layer would then give protection214. 

To sum it up we can stress that the substitution of the trisiloxane lyophobic part by a trimethyl silane moiety yields both nonionic and ionic silane surfactants. Aqueous solutions of these new surfactants are extremely stable in both alkaline and acidic conditions due to the fact that the siloxane substructure is left out. The surfactant properties of these substances are to a great extent comparable to the abilities of trisiloxane wetting agents. 

By using a recently developed magnesium hydride technology, the trisiloxane lyophobic part in superspreading surfactants can be substituted by a trimethylsilane moiety. This synthetic route leads to both nonionic and ionic silane surfactants, which are hydrolytically stable even under extreme pH. Aqueous solutions of these new surfactants exhibit surface tension and wetting properties comparable to the traditional organomodified trisiloxane surfactants. The combination of hydrolytic stability and biodegradability offers chance for the widespread application of these silane based surfactants. 

Surface Activity The chemical species given the general name of surface-active agents or surfactants have a special tendency to adsorb at interfaces, or to form colloidal aggregates in solution at very low molar concentrations. A surface-active material possesses lyophobic part, which has little attraction for the solvent, and lyophilic part, which has a strong attraction for the solvent, in its chemical structure. In water-based systems, the terms hydrophobic and hydrophilic are quite frequently employed in place of lyophobic and lyophilic, respectively. 

The reason for the self-assembly tendency of amphiphilic molecules is that their lyophobic parts are poorly soluble and tend to separate from the solvent, whereas the lyophilic parts prefer to be solvated. For water as the solvent, hydrophobic interaction is the major cause of aggregation of apolar molecules and molecular fragments. A more detailed discussion of the hydrophobic effect is given in Section 4.3.2. 

Beyond adsorption saturation, the lyophobic parts of the amphiphilic molecules may associate to avoid unfavorable solute-solvent contacts. Then, upon further addition of the amphiphilic compound to the solution, the concentration of free monomeric molecules hardly increases anymore, so that, according to Equation 3.88, y remains essentially constant. Thus, the onset of the association process is marked by a discontinuity in the interfacial tension. In addition, a discontinuity is observed in other physical properties, for example, osmotic pressure, light scattering, electric conduction (in case of ionic amphiphiles), and so on. 

Moreover, the adsorption of surfactants onto lyophobic particles has a second effect. Because only the lyophobic part of the surfactant adsorbs onto the lyophobic particle, its lyophilic part is oriented towards the dispersion medium. This lyophilic part forms a protective layer by which the particles can approach each other less easily. This effect is called steric stabilisation. Surfactants usually do not adsorb... 

In practice, therefore, the objective is to achieve an intermediate form by the addition of a controlled amount of electrolyte or surfactant. When the particles strongly repel each other, an electrolyte can be added. By decreasing the zeta-potential, the repulsive forces will decrease. When the particles attract each other too strongly a surfactant can be added. As the lyophobic part of the surfactant molecule adsorbs onto the surface of lyophobic colloids its lyophilic part will be oriented into the dispersion medium. By steric stabilisation, the attraction forces are decreased. The properties of flocculated and deflocculated suspensions are summarised in Table 18.18. 

The term amphiphilic or amphipathic, as it is sometimes called, implies attraction to two different kinds of media. The surfactant structure can be described as consisting of two parts with vastly different solution characteristics a solvent-soluble lyophilic part and a solvent-insoluble lyophobic part. Conventional... 

The results obtained by Mukerjee and Handa substantiate the fundamental difference between liquid-air and liquid-liquid interfaces. The surface tension of a surfactant solution depends mainly on interactions with the solvent and adsorption at the liquid-air interface. Interactions with air or vapor are weak. In contrast, a surfactant at a liquid-liquid interface interacts with two liquid phases. The lyophobic part of a surfactant oriented away from one of the liquid phases interacts with the second liquid phase in contact. The interaction between the lyophobic film and the second liquid phase is much more significant than the interaction with air or a vapor. 

Plate type packing to separate the phases is discussed by Carlsson et al. (1983) and by Hatziantoniu etal. (1986). De Vos et al. (1982,1986) describe use of a monolithic porous catalyst with vertical and horizontal channels. The liquid phase flows downward through an array of parallel channels in the monolith, while gas moves in cross flow through a separate set of channels. Another approach treats the catalyst to make part of the surface hydrophobic or lyophobic (Berruti et aL, 1984). The gas phase has direct access to the surface on these unwetted portions of the surface, resulting in partial, spatial segregation of the phases. 

Organosilicones are favourable in a broad variety of industrial applications primarily because of their unique smface active properties. Especially in the case of aqueous applications the hydrophilically substituted trisiloxane derivatives are good wetting agents. Usually, the molecule consists of a lyophobic trisiloxane moiety attached directly to an alkyl spacer group via a silicon-carbon bond. This spacer group carries on the other side a nonionic or an ionic hydrophilic part. 

Surfactants are molecules that have at least one lyophobic moiety, that is, solvent hating part, and a lyophilic moiety, that is, solvent loving part. In case of water as a solvent, lyophobicity means hydrophobicity and lyophilicity translates to hydrophilicity. The molecular structure of a representative surfactant, Triton X-100, is shown in Fig. 7.10. 

The sum of these AF , S AF , taken over all the nonpolar residues found in typical protein molecules, can attain very large negative values. If the native conformation of a protein molecule in aqueous solution is indeed in considerable part stabilized by lyophobic interactions, it follows that this stabiUzation should be substantially if not completely lost on transferring the protein molecule to almost any pure nonaqueous solvent. This destabilization might be expected to be less extensive in those few weakly protic nonaqueous solvents with which hydrocarbons are only partially miscible, such as glycerol, ethylene and propylene glycols, and formamide, than in the other solvents with which hydrocarbons are completely miscible. Furthermore the latter solvents should be very little differentiated under these circumstances, since AFt is so similar for most of them. As is demonstrated subsequently, these expectations are closely realized in fact. 

The DLVO theory, which was developed independently by Derjaguin and Landau and by Verwey and Overbeek to analyze quantitatively the influence of electrostatic forces on the stability of lyophobic colloidal particles, has been adapted to describe the influence of similar forces on the flocculation and stability of simple model emulsions stabilized by ionic emulsifiers. The charge on the surface of emulsion droplets arises from ionization of the hydrophilic part of the adsorbed surfactant and gives rise to electrical double layers. Theoretical equations, which were originally developed to deal with monodispersed inorganic solids of diameters less than 1 pm, have to be extensively modified when applied to even the simplest of emulsions, because the adsorbed emulsifier is of finite thickness and droplets, unlike solids, can deform and coalesce. Washington has pointed out that in lipid emulsions, an additional repulsive force not considered by the theory due to the solvent at close distances is also important. 

W. Actually, the surfactant does not like to be in water nor in oil because one part of the molecule is always lyophobic, which is why micelles are formed to hide it away from the solvent. Hence, it may be said that in type I phase behaviour the surfactant dislikes more oil than water, and in type II it dislikes more water than oil. Then, in type III phase behaviour, the surfactant equally dislikes both phases and would seek a third alternative, e.g. forming a bicontinuous microemulsion. In thermodynamic terms, it simply means that the chemical potential of the surfactant in such a microemulsion phase is lower than when it is adsorbed at the curved interface of a drop. 

Another feature of two-phase colloidal systems (lyophobic colloids) is their sensitivity to flocculation by small amounts of added electrolyte. The electrolyte causes the diffuse part of the double layer to compress. When the double layer is reduced in thickness, the colloid floes because the particles approach close enough for London forces to take over. 

OVERBEEK, J.TH. G. 1972. Colloid and Surface Chemistry. A Self-Study Course. Part 2. Lyophobic Colloids. Cambridge, Mass. M.I.T. Center for Advanced Engg. Study. 

Colloid and Surface Chemistry, Part 2 Lyophobic Colloids, A. Virj and R. G. Donnelly (M.I.T. Press)... 

Surface active substances or surfactants are amphiphilic compounds having a lyophilic, in particular hydrophilic, part (polar group) and a lyophobic, in particular hydrophobic, part (often hydrocarbon chain). The amphiphilic structure of surfactants is responsible for their tendency to concentrate at interfaces and to aggregate in solutions into various supramolecular structures, such as micelles and bilayers. According to the nature of the polar group, surfactants can be classified into nonionics and ionic, which may be of anionic, cationic, and amphoteric or zwitterionic nature. 

Surfactants in Aqueous Solution A very important component that is usually present in the lyophobic colloids is the surfactant. These molecules are amphiphilic, that is, a part of the molecule is much more polar than the other part. On the basis of the nature of the polar groups in the surfactant molecule, they are classified as ionic (anionic or cationic) and nonionic. When ionic-type surfactants are adsorbed onto polymer particles, they provide stabilization by electrostatic repulsion between them and when the nonionic type are adsorbed instead the mode of stabilization is by steric repulsion. Electrosteric stabilization is provided by polyelectrolyte chains that give place to both modes of repulsion electrostatic and steric. 

Ions or groups on a molecule. In aqueous or other polar solvents, the lyophobic group will be non-polar. For example, the hydrocarbon group on a soap molecule is the lyophobic (hydrophobic) part. 

E.J.W. Verwey and J.T.G. Overbeek, Theory of the Stability of Lyophobic Colloids. Elsevier, Amsterdam, 1948. Part II of this classic monograph should be read by the mathematically minded as a comparison to Sec. 6.2. 

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