Measuring the hydrophobicity of lubricated blends of pharmaceutical excipients

This paper discusses how the hydrophobicity of lubricated pharmaceutical formulations is affected by process variables such as shear rate and strain. Hydrophobicity is a critical property that affects the dissolution of powder formulations, tablets and capsules as well as the performance of tablet coating and granulation operations. In this paper, hydrophobicity is measured using a modified Washburn method. Results show that, in the absence of lubricant, the hydrophobicity of powders does not change substantially as a function of shear rate or strain. However, when magnesium stearate is present (concentrations studied here range between 0.5% and 2%), hydrophobicity increases as a function of strain, shear rate and lubricant concentration. Observed changes range over several orders of magnitude, readily explaining common “overlubrication” observations of delayed drug dissolution.

This paper presents a method to quantify the effects of process variables (strain and shear rate) and magnesium stearate concentration (hydrophobic component) on powder blend hydrophobicity. This figure shows that a blend containing magnesium stearate increases its hydrophobicity Φ (Φ is the slope of linearized Washburn curves) when it is exposed to larger shear rates (rpm of the shear cell) and to larger strain (revolutions of the shear cell).Figure optionsDownload as PowerPoint slide