ABSTRACT
In an attempt to identify the nature of the adhesion forces between lactose particles and gelatin capsule surfaces under the conditions of this study, direct adhesion measurements were performed with several lactose particles of different sizes. Figure 5 shows that the adhesion force appears to be directly proportional to the size (nominal diameter) of the lactose particle for gelatin capsule 72601. The observed linear trend is consistent with the view that the dominant component of the total adhesion force, on the basis of theoretical considerations for the interaction between a sphere and flat surface, arises from van der Waals forces [45]. However, when one considers the surface chemistry of the component materials in pharmaceutical products, it seems more appropriate that the principal forces contributing to particle-surface adhesion can be classified as Lifshitz-van der Waals forces [46, 47] and acid-base interactions [47-49]. The results of the adhesion force measurements can be explained by an increase in the molecular contact area with increasing particle size, since the number of real contacts will usually be proportional to the size of a rough particle [45]. On the other hand, if electrostatic forces of interaction were to predominate in the lactose particle-gelatin capsule system, the measured adhesion force likely would not exhibit a linear variation with particle size [45]. Furthermore, capillary interactions are not believed
to play a significant role since the RH was below 65% [41]. This is supported by preliminary experiments of the effect of RH on the adhesion measurements of individual lactose particles to thin gelatin films which show that the adhesion force at 50% RH does not differ appreciably from that obtained under 'dry' conditions.
