Characterization of Tablet Surfaces by Their Critical Surface-Tension Values

Abstract

Experiments were conducted to determine the critical surface-tension (γe) values of a range of acetylsalicylic acid tablets in combination with various adjuvants.A telemicroscope was used to measure the contact angle (θ) as test liquids with known surface tensions (γL,) advanced across the tablet surface.The cosine of the contact angles (cos θ) were plotted as a function of the γL, of the test liquids.Straight-line extrapolation to cos θ = 1 resulted in γ evalues for each particular type of tablet.These values were determined for each group of tablets, using different sets of test liquids.The γevalues obtained by this method indicated that pure acetylsalicylic acid tablets presented a surface with intermediate activity, having a γevalue of about 31 dynes/cm.The addition of a lubricant like magnesium stearate presented a surface richer in —CH3and —CH2— groups, resulting in a lower value, whereas the addition of adjuvants such as starch, cellulose, and talc resulted in surfaces richer in =O and —OH, causing an increase in the γevalues.Increased γevalues result in increased wetting by the coating solution and in an increased bonding force between the tablet surface and the polymer film coating after the solvent has evaporated.Characterization of the tablet surface should eliminate some of the guesswork involved in developing an adequate film coating for a particular type of medicinal tablet.