Repulsion - Steric(entropic)
"When a sol of gelatin, for instance, is added to a gold sol prepared by the reduction of a gold salt in an alkaline medium, it appears that the gold sol is strongly protected against the flocculating action of electrolytes."
H.R. Kruyt, Colloids: A textbook; H.S. van Klooster*, Translator; John Wiley & Sons: London; 1927; p. 87. (*Who I met in the late 1960's)
It had been known for a long time that electrolytes would flocculate many sols; gold sols were a common example. These were call lyophobic colloids. The colloids insensitive to electrolyte were, in hindsight, polymeric. They were call lyophilic colloids. Kruyt reports here that some combinations of the lyophilic colloids could "protect" the lyophobic colloids from salt addition. This lyophilic colloids were also called "protective" colloids.
We now know this mechanism to be polymer adsorption; and in the present context, examples of steric stabilization.
From the very beginning, the stability of polymer-stabilized sols has been understood primarily in terms of the solution solubility behavior of the polymer. Polymer-coated sols are stable when the polymer is both adsorbed and soluble; and unstable even when the polymer is adsorbed if it is no longer soluble.