Adsorption of Esters of p-Hydroxybenzoic Acid by Filter Membranes: Mechanism and Effect of Formulation and Processing Parameters

Abstract

The adsorption of esters of p-hydroxybenzoic acid (parabens) by filter membranes was studied by a flow-through technique. The hydrophobic effect was the major mechanism of adsorption. Propylparaben was more extensively adsorbed by all the membranes than was methylparaben. Hydrophobic membranes exhibited the greatest degree of adsorption, whereas adsorption was minimal for hydrophilic membranes. The charge of the filter membrane did not affect paraben adsorption.

Formulation factors studied included the concentration of paraben, the presence of a tonicity-modifying agent (sodium chloride, mannitol, glycerin), and the presence of a chelating agent (edetate sodium). Paraben adsorption was directly related to paraben concentration. The presence of a tonicity modifier or chelating agent did not alter paraben adsorption to membrane filters. Processing parameters studied included flow rate, temperature, autoclaving, flow interruption, and filter membrane presaturation. Presaturation of the filter membranes for up to 1 hr reduced but did not eliminate paraben adsorption during simulated use. Interrupting the flow of the paraben solution through the filter membrane allowed for additional paraben adsorption and caused the concentration of paraben in the restarted filtrate to be less than 100% of theory.

Keywords:

• Adsorption
• Effect of formulation factors
• Effect of processing factors
• Esters of p-hydroxybenzoic acid
• Filter membranes