Phenol and phenolate sorption differently affect the bilayer properties of cationic DODAC vesicles

• Sorption of neutral phenol and anionic phenolate by cationic vesicles was quantified.
• Electrostatic attraction highly favoured sorption of (more polar) phenolate.
• Phenol sorption followed a partitioning model.
• A sorption maximum of less than one phenolate anion per DODAC cation was found.
• Whereas phenol increases the bilayer fluidity, phenolate has the opposite effect.

Interactions between bilayer membranes and solute molecules are important in various scientific disciplines, such as biochemistry, pharmacology and product formulation. In this contribution, the interaction between phenol (either in the neutral or the anionic phenolate form) and cationic DODAC vesicles was investigated as a function of pH and temperature.Whereas only a minor fraction (i.e. 25–40%) of the dissolved phenol was sorbed by DODAC vesicles at neutral pH, nearly complete sorption was obtained at pH conditions whereby phenol became dissociated into the phenolate anion provided that a molar exces of DODAC was applied. Interestingly, this observed pH effect was reversible and hence most sorbed phenolate was set free upon subsequent acidification. Whereas the surface charge density of the cationic DODAC vesicles was decreased by phenolate sorption at alkaline pH, still neutralization or charge reversal could not be obtained, which was in line with the extrapolated molar sorption maximum from Langmuir isotherm fitting.Moreover, the sorption efficiency was found to increase with temperature, with a maximum around the gel to liquid crystalline phase transition temperature. Diffusion NMR data revealed that this pronounced temperature effect was much more pronounced for extruded, as compared to sonicated DODAC vesicles.Whereas phenol incorporation at neutral pH decreased the phase transition temperature, the opposite effect was observed at alkaline pH. Apparently, the reduced repulsion by phenolate binding to the cationic headgroups overruled the effect of phenol or phenolate intercalation in between the octadecyl chains.Overall, these data clearly show that the sorption of an ionizable solute by DODAC vesicles is not only affected by the surface charge and fluidity of the bilayer, but that in return the latter characteristics are also affected by solute sorption. Hereby, the effect was strongly dependent on the ionic form of the solute: whereas the neutral phenol made the membranes less rigid, the opposite was observed upon sorption of the anionic phenolate.

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