Influence of surface modification on wettability and surface energy characteristics of pharmaceutical excipient powders.

Influence of surface modification on wettability and surface energy characteristics of three micron size pharmaceutical excipient powders was studied using hydrophilic and hydrophobic grades of nano-silica. The wetting behavior assessed from contact angle measurements using sessile drop and liquid penetration (Washburn) methods revealed that both techniques showed similar wettability characteristics for all powders depending on the hydrophilic or hydrophobic nature of nano-coating achieved. The polar (γs(p)) and dispersive (γs(d)) components of surface energies determined using extended Fowke's equation with contact angle data from sessile drop method and inverse gas chromatography (IGC) at infinite dilution suggested a general trend of decrease in γs(d) for all the surface modified powders due to passivation of most active sites on the surface. However, depending on the nature of the functional groups present in nano-silica, γs(p) was found to be either higher or lower for hydrophilic or hydrophobic coating respectively. Results show that wettability increases with increasing γs(p). Both the techniques of surface energy determination provided comparable and similar trends in γs(p) and γs(d) components of surface energies for all excipients. The study also successfully demonstrated that surface wettability and energetics of powders can be modified by varying the level of surface coating.