Synthetic protective colloids

The protective value we propose gives the number of grams of a red gold sol which are just protected by 1 g. of the protective agent against flocculation by 1% NaCl solution. Gelatine has a protective value of 90, i.e., 1 g. of gelatine protects 90 g. of gold sol. Amphions are good protecting colloids. One of the ionic groups attaches the protecting agent to the colloidal particle, the other supplies the electrical charge.

Synthetic protective colloids were prepared by introducing acidic groups into the basic polyethylenimine molecule, and by introducing basic groups into the polyacrylic acid molecule.

With some synthetic protective colloids, the protective value can be increased as much as one hundred times by the action of heat and time. The interaction between the protecting agent and the sol to be protected requires a longer time than the 3 minutes previously recommended. We recommend that the reaction be allowed to go to completion.

Good protecting colloids form stable complexes with the coagulating metal ions.

The protective value changes strongly as the pH of the sol is varied within a narrow range. With some polyamphions the protective value, at a given sol pH, can be increased by shifting the isoelectric point (IEP). The protective values and gold numbers of some protecting agents are compared. A synthetic polyacrylic hydrazide had the highest protective value of 400.

The protecting action of natural gelatine is at a minimum at the IEP, but that of the synthetic hydrazide of polyacrylic acid is greatest at this point. In this context the VW theory of Heller is discussed, and the distance between the ionic groups on the polymer chain is postulated as a further important factor contributing to the formation of the stabilizing layers.