Critical surface tensions of pharmaceutical solids.

Advancing contact angles measured on compacts of several drugs by the sessile drop method and also by penetration through a column of drug granules were used to find the critical surface tension of the drugs. After liquid was delivered at a very slow rate, the contact angle of sessile drops decreased with time, but use of a consistent method of timing always led to the same value for critical surface tension. Results from penetration studies and work on compacts were in agreement, provided that the surfaces of the compacts were smooth and highly reflective. Critical surface tension of the six drugs, three analgesics and three sulfonamides, ranged from 31 to 33 dynes/cm. The critical surface tension of mixtures of phenacetin and microcrystalline cellulose was not a linear function of the relative surface fractions of the two materials. If the surface contained 25% or more of phenacetin, the critical surface tension barely differed from that of pure phenacetin.