Chemical reactions at interfaces

Use Cases 

Acid-base reaction at the interfaces.

Investigation of  the stabilization mechanism of pH switchable chitosan particles decorated emulsions

Reference: Ren, D.; Xu, S.; Sun, D.; Wang, Q.; Xu, Z., CO2-switchable dispersion of a natural chitosan and its application as a responsive Pickering emulsifier. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2018, 555, 507-514.

Pickering emulsion are a class of emulsion stabilized with particles. Authors investigate the impact of the pH on the stabilization of Pickering emulsion decorated with chitosan particles.

CO2 dissolved in water reduces the pH of the solution below the pKa of the –NH2/–NH3+ chitosan groups (pKa = 6.5). Protonated chitosan particles are more soluble in water and desorb from the oil/water interface to the bulk phase which destabilizes the emulsion.

The authors measure the viscoelastic modulus of oil/water interfaces under N2 or CO2 conditions. A decrease of the viscoelastic modulus was observed when CO2 was dissolved in the water phase. Measuring the viscoelastic modulus highlights the change in the interfacial composition induced by the chitosan particles departure.

 

TECLIS product: TRACKER™  automatic drop tensiometer

Key words: Pickering emulsions, chitosan, viscoelastic modulus, switchable surface properties

Sol/Gel reaction at the interfaces

Can we create nano and microcapsules

through intermolecular condensation at a dichloromethane/water interface ?

Reference: Valery G. Babak, F. B., Boris A. Izmailov, Dmitry I. Maltsev,; Gali D. Markova, V., N. Sokolovc and Valery A. Vasnev, Polycondensation in tetradecaethoxyhexasiloxane adsorption layers at the methylene chloride/water interface under alkaline conditions: application to the formulation of hollow microcapsules. Mendeleev Communications 2008, 18, 193–195.

The aim of this work was to study the kinetics of adsorption of an oligoethoxysiloxane, tetradecaethoxyhexasiloxane (TEOHS), from an organic phase (dichloromethane) at the CH2Cl2/water interface, and the consequent formation of a gel-like structure inside the adsorption layer through intermolecular condensation under alkaline conditions in view of the formulation of polysiloxane hollow nano and microcapsules. Surface tension measurements and a characterization of the viscoelastic properties of the adsorption layer consisting of Si6Et14 at a CH2Cl2/water interface at different pH values using and automatic drop tensiometer (TRACKER™  by TECLIS) showed that the increase of the pH results in a decrease of surface tension and an increase of the storage elastic modulus. This result is explained by the hydrolysis of alkoxysilane. Additional experiments showed that the increase of the storage modulus begins after a critical concentration. These information are valuable to determine the liquid to solid transition observed at the interface that is required to generate microcapsules.

TECLIS product: TRACKER™  automatic drop tensiometer

Key words: microcapsules, dilational viscoelasticity, sol-gel technic

Cross linking reaction at the interfaces

Understanding and controlling the cellular organization of the liquid to control the

properties of the cellular solid

Reference: Giustiniani, A.; Guégan, P.; Marchand, M.; Poulard, C.; Drenckhan, W., Generation of Silicone Poly‐HIPEs with Controlled Pore Sizes via Reactive Emulsion Stabilization. Macromolecular rapid communications 2016, 37 (18), 1527-1532.

The authors studied the synthesis of macrocellular polymers based on PEG droplets dispersed in PDMS matrix. They observed that above a critical crosslinker/catalysis concentration, emulsions were stable and gave macrocellular polymers.

To explain the lack of stability below the critical concentration, the authors make the hypothesis of the desorption of the HO-MHDS or PDMS- b –PEG copolymers from the interface. After the drop formation and equilibration, the drop volume was rapidly decreased. A relaxation of the surface tension, that indicates a desorption, was observed when the concentration is lower than the critical concentration. Moreover, wrinkles, a signature of a liquid-to solid transition,  were also seen at the interface for concentration values above the critical concentration.

TECLIS product: TRACKER™  automatic drop tensiometer

Key words: macrocellular polymers, cross linking reaction, very low surface tension