Lyophobic substances

The adsorption isotherm for pentanol is typical for lyophobic substances, i.e., substances which do not like to stay in solution, and for weakly amphiphilic substances. They become enriched in the interface and decrease the surface tension. If water is the solvent, most organic substances show such a behaviour. The LiCl adsorption isotherm is characteristic for lyophilic substances. Most ions in water show such behaviour. 

Dispersion forces are ubiquitous and are present in all molecular interactions. They can occur in isolation, but are always present even when other types of interaction dominate. Typically, the interactions between hydrocarbons are exclusively dispersive and, because of them, hexane, at S.T.P., is a liquid boiling at 68.7°C and is not a gas. Dispersive interactions are sometimes referred to as hydrophobic or lyophobic particularly in the fields of biotechnology and biochemistry. These terms appear to have arisen because dispersive substances, e.g., the aliphatic hydrocarbons, do not dissolve readily in water. Biochemical terms for molecular interactions in relation to the physical chemical terms will be discussed later. 

The alkaline product from the wood ash was a crude solution of sodium and potassium carbonates called "lye". On boiling the vegetable oil with the lye, the soap (sodium and potassium salts of long chained fatty acids) separated from the lye due to the dispersive interactions between the of the fatty acid alkane chains and were thus, called "lyophobic". It follows that "lyophobic", from a physical chemical point of view, would be the same as "hydrophobic", and interactions between hydrophobic and lyophobic materials are dominantly dispersive. The other product of the soap making industry was glycerol which remained in the lye and was consequently, termed "lyophilic". Thus, glycerol mixes with water because of its many hydroxyl groups and is very polar and hence a "hydrophilic" or "lyophilic" substance. 

Substances like metals, metal sulphides cannot be brought into the colloidal state simply by bringing them in contact with water and, therefore, special methods are devised for the purpose. Hence, they are known as hydrophobic colloids (hydro = water phobic = hating). In case of solvent other than water, the general term lyophobic is used. Further, if these colloids are precipitated, then it is not very easy to reconvert the precipitate directly into the colloidal state. Hence, they are termed as irreversible colloids (colloidal state — precipitate), irresoluble or electrocratic colloids. 

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. 

The term lyophobic interactions is intended to generalize the expres sion hydrophobic interactions to other solvents than water. Hydro-phobic interactions have been prominently implicated in determining the native configuration of proteins in aqueous solution. These interactions are actually not of a single relatively well-defined character, as are electrostatic or hydrogen bond interactions, but are rather a set of interactions responsible for the immiscibility of nonpolar substances and water. Proteins contain a substantial proportion of amino acids such as phenylalanine, valine, leucine, etc., with nonpolar side-chain residues. These nonpolar groups should tend, therefore, other factors permitting, to cluster on the... 

The condensation method makes it possible to obtain more highly dispersed systems than the dispersion method, and true lyophobic sols are always prepared by this method. Colloidal solutions are obtained by the condensation method as a result of chemical reactions of nearly all known types. But it should be noted that sols are by no means always formed, but only in the case of certain concentrations of the original substances, order of their mixing, temperatures of interaction, and a combination of several other conditions. The main method of preparing sols of heavy hydroxides is hydrolysis of solutions of salts, which takes place more completely and more rapidly at high temperatures and in dilute solutions. 

Dispersion forces are those that occur between hydrocarbons and other substances that have either no permanent dipoles or can have no dipoles induced in them. In biotechnology and biochemistry, dispersive interactions are often referred to as hydrophobic or lyophobic interactions, apparently because dispersive substance such as the aliphatic hydrocarbons do not dissolve readily in water. To a first approximation, the interaction energy Ud) involved with dispersive forces has been deduced to be... 

Some colloidal systems such as polymer solutions and surfactant solutions containing micelles are thermodynamically stable and form spontaneously. These types of colloids are called lyophilic colloids. However, most systems encountered contain lyophobic colloids (particles insoluble in the solvent). In the preparation of such lyophobic colloidal dispersions, the presence of a stabilizing substance is essential. Because van der Waals forces usually tend to lead to agglomeration (flocculation) of the particles, stability of such colloids requires that the particles repel one another, either by carrying a net electrostatic charge or by being coated with an adsorbed layer of large molecules compatible with the solvent. 

Derjaguin (25) pointed out that the term lyophilic colloids is not really accurate, suggesting that it is better to speak of a lyophilic state of colloids , because various types of lyophobic colloids, regardless of their material composition, can be brought into the lyophilic state by adsorption of surface-active substances or, in the case of silica, by rehydroxylation. 

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. 

In solution these substances scatter light, they exhibit high viscosity and low diffusion rates, and they enter into complex structural relations with the solvent. In some of their physical properties the solutions, although they are true solutions by many criteria, simulate the sols and gels of the more truly heterogeneous systems. Owing their stability, however, to the interactions between their own molecules and the solvent, they are less subject to coagulation and precipitation. In so far as they form solutions they are called lyophilic, in contradistinction to the easily precipitable sols of inherently insoluble substances such as metals or arsenic sulphide in water, which are called lyophobic. 

Indeed at the present time there seems no longer to be any real need for. a "stability theory of hydrophilic sols, as we now regard the colloid substance as truly dissolved, and its electrical properties as caused by ionisation of ionogenic groups in the dissolved macromolecules. Flocculations or coacervations are now regarded as transgressions of solubility. Many of the facts, the above stability theory seemed to explain have received other explanations. See for instance for the opalescent sols with lyophobic character obtained by adding alcohol to the sol Chapter VIII Ic (p. 234), and for the flocculation or coacervation with alcohol + indifferent salt Chapter X 3f (p. 396). 

The partial process D —C is on the other hand just as irreversible as the flocculation of the lyophobic sols in Volume I, since in this process the solubility of the macromolecular substance is not changed. 

Direct micelles contain lyophilic component of surface-active substance, whereas the reverse micelles contain lyophobic one. The miceUes can be formed in the presence and absence of water. In the case of reverse miceUes, for instance, in the hydrocarbon medium, water is easily solubilized, forming a water pool . Its size is characterized by the ratio of the water and surfactant volumes. Thus, a limited amount of water inside the micelle determines the kinetics and thermodynamics of the nanoparticles formation in a small micro/nanoreactor volume. 

Microemulsions [191, 192] are transparent, optically isotropic and thermodynamically stable liquids. They contain dispersion of polar and nonpolar solvent, usually water or aqueous solutions and oils. Adding surfactants stabilizes droplets of 1-100 nm in size. Due to amphiphilic properties of the surface active substances containing lipophilic groups and one or two lyophobic C-H chains mainly collected at the interface of two liquid phases, they cannot be mixed under normal conditions. Unlike traditional macroemulsion, which is kinetically stabilized only by the external mechanical energy supply, nano-domains in the microemulsions are formed spontaneously. Their size depends on the microemulsion composition, temperature and elastic properties of the separating film of surfactant. In particular, in the case of water-oil microemulsions with spherical nanosized micelles of water dispersed in oil, water droplets can be used as nanoreactors and templates for the solid nanoparticles fabrication. Since the reaction is initiated by the spatially restricted water and micelle, heterogeneous nucleation and crystal growth can be controlled. 

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