Pl. Sheridan et al., THE EXTENT OF ERRORS ASSOCIATED WITH CONTACT ANGLES .2. FACTORS AFFECTING DATA OBTAINED USING A WILHELMY PLATE TECHNIQUE FOR POWDERS, International journal of pharmaceutics, 109(2), 1994, pp. 155-171
The reasons for error, and the magnitude of error that may be expected
have been investigated for Wilhelmy plate contact angle measurements
on compacted plates. A range of different materials have been consider
ed, including a homologous series of powders (alkyl p-hydroxybenzoates
), and alpha-lactose monohydrate (from three different suppliers). The
effect of powder and plate storage conditions, the compaction force a
nd dwell time used to prepare the plate, and the approach to defining
the buoyancy slope of the Wilhelmy plate experiment have all been cons
idered. The effect of compaction seems to be material dependent, some
materials are unaffected by large changes in applied pressure, whilst
others are particular sensitive to changes in the compaction process.
Propyl p-hydroxybenzoate was especially sensitive to changes in compac
tion, such that even when trying to produce identical compacts, variab
ility was noted for different days of plate preparation. Variability i
n measured contact angle was not associated with humidity equilibratio
n, or time after the plate was made for the materials studied. The maj
or cause of variability in the contact angle seemed to be the plate ro
ughness. There is some concern that the plate roughness will not only
affect the result by normal hysteresis effects, but also because the m
easured perimeter (external dimensions) will underestimate the true pe
rimeter of a rough plate. Having noted many causes for concern with th
is technique, it was observed that standard deviations for the contact
angles were smaller than for other contact angle techniques for powde
red samples, however, accuracy and precision should not be confused.