THE EXTENT OF THE ERRORS ASSOCIATED WITH CONTACT ANGLES OBTAINED USING LIQUID PENETRATION EXPERIMENTS

Assessment of values for the contact angles of powders is important in many pharmaceutical processes, and the use of surface energy data estimated from contact angle determinations is gaining in popularity. Liquid penetration experiments offer one approach to obtaining an estimate of wettability, and its use continues despite practical and theoretical criticisms. This paper examines the sources and magnitude of errors that are associated with the estimate of wettability that is derived by use of the Washburn model and equation, for model particles (polymer coated glass spheres). It is necessary to ascribe one liquid as perfectly wetting and to compare the behaviour of the test liquid (for which the finite wettability is to be determined) to that of this defined liquid. The following conclusions were made: (1) the major source of error is in the measurement of penetration rates through the powder bed, with surface tension and viscosity data being far more reproducible; (2) the choice of the perfectly wetting liquid is not too critical if the test liquid wets reasonably well, but is far more important if the test liquid has a low contact angle; (3) the error in cos-theta, when expressed as a percentage of the mean, was in the range of 2-6%, this being the accuracy of the experiment; (4) if the results were transformed to theta (rather than cos-theta) then the error no longer reflected the accuracy of the experiment, but rather the magnitude of the contact angle, as equal changes in cos-theta will result in unequal changes in theta (low contact angles showing more apparent error than large), thus it is more appropriate to quote accuracy in the cos-theta domain for liquid penetration results, rather than converting to theta.