Surfactant Regulation of the Solution-Mediated Phase Transformation of Nitrofurantoin Anhydrate to Monohydrate.

Abstract

Purpose: Hydration of metastable anhydrates during dissolution can eliminate their supersaturation advantage and compromise formulation performance. This study used nitrofurantoin as a model system to clarify how surfactant molecular structure regulates solution-mediated phase transformation (SMPT) from anhydrate to monohydrate and supersaturation maintenance.

Methods: Six surfactants, including SDS, DTAB, CTAB, STAB, Tween 20, and Tween 80, were evaluated by dissolution testing, solid-state characterization, crystallization kinetics, and molecular dynamics simulations.

Results: Surfactants inhibited SMPT in a structure-dependent manner, following the order Tween 80 > STAB > Tween 20 > CTAB > DTAB > SDS ≈ plain buffer. Longer hydrophobic chains more effectively delayed monohydrate nucleation and reduced crystal growth, whereas differences in head-group characteristics further modulated surface adsorption and inhibition among surfactants with comparable chain lengths. Molecular dynamics simulations suggested that stronger adsorption on monohydrate surfaces was associated with greater blockage of growth sites.

Conclusions: This study provides mechanistic insight into surfactant-regulated nitrofurantoin hydration, highlighting hydrophobic chain length and head-group characteristics as key structural factors governing surfactant adsorption and stabilization of the anhydrate during dissolution.