Some substances such as sulfur and hydrocortisone are both insoluble in water and poorly wetted by it. When preparing simple aqueous dispersions, it is difficult to disperse clumps and the foam produced on shaking is slow to subside because it is stabilized by the film of unwettable solid at the liquid–air interface. To ensure satisfactory wetting, the interfacial energy between the solid and liquid must be reduced. This may be achieved by adding a suitable wetting agent, which is adsorbed at the solid–liquid interface in such a way that the affinity of the particles for the surrounding medium is increased while the interparticular forces are decreased.

For example, sulfur lotion–quillaia tincture is used as a wetting agent. The saponins in quillaia extract have been used to suspend sulfur in lotion.

Suspensions Produced by Chemical Reaction

Very occasionally, the insoluble active constituent of a lotion is produced by a chemical reaction. A finer precipitate is obtained if dilute solutions of the reactants are mixed. Hence, the reacting substance should be dissolved separately in approximately half volumes of the vehicle and the two parts mixed. When prepared in this manner, the precipitate is diffusible and no suspending agent is required. An example is magnesium hydroxide mixture.

 

Example: Rx	Aspirin
	Oxyphenbutazone
	Simple syrup
	Water

Sulfurated potash is a mixture of potassium polysulfides and other sulfur-containing compounds. It should be freshly prepared, as its solubility decreases on storage. It reacts with zinc sulfate and produces a precipitate of zinc sulfide, which is diffusible, provided sulfurated potash is added to zinc sulfate and not vice versa.

The precipitate of zinc sulfide agglomerates quickly and clays badly. Hence, the suspension should be freshly prepared and quickly used. Protection from light is necessary to reduce the oxidation of the sulfide to sulfite or sulfate.

(Note: If sulfurated potash is added to zinc sulfate, the precipitate obtained is diffusible,

If zinc sulfate is added to sulfurated potash, the precipitate obtained is indiffusible.)