Evaluating the flow-enhancing and antistatic properties of excipients in free and salt forms: A case study on stearic acid and magnesium stearate

Abstract

Powder flow characteristics are crucial for efficient processing and manufacturing in the pharmaceutical industry. The occurrence of tribo-charging, a surface phenomenon where particles acquire an electrical charge due to frictional contacts with other particles or surfaces, poses risks for reduced flowability, process inefficiencies, handling difficulties, and ultimately, for the quality of the final products.

Highlights

Comparison of flow-enhancing and antistatic properties of free acid and salt lubricant forms.

Demonstrated superior antistatic properties of magnesium stearate as dry additive.

Better performance of stearic acid in solvent-mediated lubrication systems.

Appropriate lubricant–lubrication method combination is key for the charge mitigation of pharmaceutical powders.

One strategy to mitigate this effect is the addition of antistatic agents to formulations. Notably, lubricants, known for their lubricating and flow-enhancing properties, have also demonstrated antistatic capabilities. In this study, we have investigated for the first time the performance of two common lubricants, stearic acid (SA) and its salt form, magnesium stearate (MgSt), as charge mitigation agents in both dry powder and solvent–mediated application systems. Six powder blends per lubricant type were prepared at varying concentrations and blending parameters, followed by an assessment of their flow and tribo-charging behavior. In solvent–mediated experiments, the stainless-steel surface of the charge-measuring apparatus was coated with a dispersion of the lubricant in an organic solvent prior to charge measurements.

Our work uncovered the superior antistatic properties of MgSt compared to its free form when used as a dry additive. This is likely due to reduced inter-particle friction upon delamination, the ionization of the carboxylic group as a magnesium salt, and/or the enhanced mobility of its Mg²⁺ ion in the adsorbed water layer. Instead, SA demonstrated better performance in solvent-mediated experiments due to solid-state changes upon recrystallization promoting superior antistatic capabilities. These findings suggest that both the lubricant type and lubrication method should be considered at the early stage of development.

Materials

In this study, mannitol (MAN, Emprove® Essential fine powder), MgSt (Parteck® LUB MST), and SA (95 %, FCC, FG), all sourced from Merck KGaA (Germany), were employed. MAN (characterized by a dv50 of 65  µm) served as reference material, whereas MgSt and SA were assessed for their potential as charge mitigation agents. The chemical structures of the free acid (SA) and its salt form (MgSt) are shown in Fig. 1.

M. Beretta, J.T. Pinto, S. Özer, A. Paudel, Evaluating the flow-enhancing and antistatic properties of excipients in free and salt forms: A case study on stearic acid and magnesium stearate, International Journal of Pharmaceutics, 2025, 125377, ISSN 0378-5173, https://doi.org/10.1016/j.ijpharm.2025.125377.