(671c) Wettability of Pharmaceutical Powders of Different Particle Size By Droplet Penetration Technique

We have recently developed a droplet penetration method to characterize the wettability of pharmaceutical powders. This method relies on two key observations during the penetration of a drop deposited on a powder bed: 1) the capillary pressure inside the porous powder is the dominant driving force for the imbibition process and 2) the contact area between the penetrating drop and the powder bed remains constant during most of the process. Under these conditions, we have shown that the penetration process in non-dimensional variables is universal. Hence, using a reference liquid it is possible to decouple the properties of the powder bed from the assessment of the contact angle of the test liquid under consideration. In addition, measuring the penetration times is sufficient to measure the contact angle, without having to solve the penetration problem.

In this work, we use the droplet penetration method to investigate the effect of particle size distribution on the wetting behavior of pharmaceutical powders. We consider two powder materials, an excipient and an active pharmaceutical ingredient (API). The excipient is lactose monohydrate, and was sieved to obtain particles in the following size ranges: 38-45, 45-53, 53-63, 63-75, 75-90, 90-106 mm. The API used is caffeine anhydrous with particle size distribution characterized by d50 = 15, 19 and 30 mm, obtained by jet milling. In all cases, the test liquid is deionized water and the reference liquid used is silicone oil. Lactose powders show a relatively constant contact angle, except for the largest particles. In the case of caffeine, we observe that larger particles yield smaller contact angles. The observed differences could be attributed to differences between milling and sieving processes. Other possible mechanisms leading to the observed dependence of the contact angle on particle size will be discussed.