Molecular Weight Of Substances In Solutions : Colloidal State

All experimental data show that the transition from crystalloids to colloids is gradual, depending on the size of the particles; suspensions of gold may be prepared which range from microscopically heterogeneous, through colloidal solutions (ultramicroscopically heterogeneous), to true solutions, with increasing fineness of the particles from 10^-5 cm. to 10^-8 cm.

The sharp differentiation between crystalloids and colloids made by Graham has not been confirmed. Albumin may be obtained in a crystalline form, and crystalline substances such as common salt may be prepared in the form of colloidal solutions by precipitation in liquids (e.g., ether) in which they do not form true solutions. The real factor determining whether a substance forms a colloidal solution or a true solution is the size of the dispersed particles; it is more correct to speak of the colloidal state of matter than of "colloidal substances."

Even carefully filtered solutions of cane-sugar show a slight Tyndall effect with a beam of light, although this is very much less than that obtained with colloidal solutions, which contain larger particles. Lord Rayleigh showed that the blue colour of the sky, which was formerly attributed to the scattering of light by suspended dust, could be accounted for by the scattering effect of the gaseous molecules of the atmosphere.

The idea that colloidal particles are necessarily amorphous is incorrect. The X-ray examination of many colloids, such as colloidal gold with particles 1.86 x 10^-7 cm. diameter, colloidal silicic acid, iron oxide, cellulose, etc., shows that they contain very small crystals of the substances. Colloidal solutions are generally called sols, and the solid forms, which are frequently gelatinous, are called gels. A distinction was made between suspensoids, in which the colloid particles are solid, and emulsoids, in which they are liquid.

There are occasional exceptions to this description of the dispersed phase, and a more recent classification is into lyophobic (solvent-repelling) and lyophilic (solvent-attracting) colloids, respectively, Lyophobic colloids (e.g., arsenious sulphide, ferric oxide, gold) are readily precipitated by electrolytes and have viscosities differing only slightly from that of the solvent; lyophilic colloids (gelatin, albumin) are not easily precipitated by electrolytes and are much more viscous than the solvent. Cataphoresis shows that colloid particles are often charged; they are precipitated by ions of opposite sign, the ion being adsorbed. Ions of higher valency (Al⁺⁺⁺) are much more effective than those of lower valency (K⁺). A positively charged sol (ferric oxide) and a negatively charged sol (arsenious sulphide) mutually precipitate each other on mixing.

Protein crystallography
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Molecular Weight Of Substances In Solutions
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