Lyophilic sols, coagulation

Reaction of alkali cellulose with carbon disulphide to produce a cellulose xanthate which forms a lyophilic sol with caustic soda. This may be extruded into a coagulating bath containing sulphate ions which hydrolyses the xanthate back to cellulose. This process is known as the viscose process and is that used in the manufacture of rayon. 

It is well known that lyophilic sols are coagulated by the removal of a stabilizing hydration region. In this case, conversion of a sol to a gel occurs when bound cations destroy the hydration regions about the polyanion, and solvated ion-pairs are converted into contact ion-pairs. Desolvation depends on the degree of ionization, a, of the polyacid, and the nature of the cation. Ba ions form contact ion-pairs and precipitate PAA when a is low (0-25), whereas the strongly hydrated Mg + ion disrupts the hydration region only when a > 0-60. 

COAGULATION (Hofmeister Series). A definite order of arrangement of anions and cations according lo their powers of coagulation when their salts are added in quantity lo lyophilic sols. Thus, the order of cations... 

Lyophilic sols are more stable towards electrolytes. It is due to the fact that these particles are highly solvated and the electrolyte does not penetrate easily to coagulate lyophilic sols. On the other hand, lyophilic sols are readily... 

COOH and -NH2 in proteins and -OH in hydroxides and polysaccharides can bind water molecules. Lyophilic sols are coagulated when first the solvent layer and then the charge of the ion are removed. 

The phenomenon known as coacervation frequently occurs during coagulation of lyophilic sols. The sol is stratified into two distinct liquid layers that consist of microscopic and macroscopic drops or of a continuous liquid phase. 

On the other hand gelatin sol is lyophilic in nature and gelatin particles are heavily hydrated and so water envelopes around them and prevents their coming in contact with electrolyte. Hence, it is not easily coagulated on addition of electrolyte. 

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. 

In another series of experiments, hydrophobic surfaces were used the free energy of interaction between the hydrophobic macroscopic surfaces, ViAoj, with a radius 1 mm was compared to the results obtained in the coagulation measurements of a hydrophobic Aerosil sol with particles 10 nm in size. However, this time the experiments were conducted in a hydrocarbon medium, so that the system changed from lyophilic to lyophobic. This transition was caused by the addition of ethanol or propanol, which increased the polarity of the medium. The ViAOf value is low in pure hydrocarbon (see Section 1.3) and increases with increases in the alcohol concentration. At a particular critical value, ViAoff,.), which is on the order of several mJ/m, the turbidity in the system increases due to coagulation, in complete agreement with the previous series of experiments conducted in surfactant solutions. 

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