Incorporation and structural arrangement of microemulsion droplets in cylindrical pores of mesoporous silica

Abstract

The behaviour of microemulsion (ME) droplets in mesoporous systems is highly important for understanding the immobilisation of drugs or chemical formulations, cleaning processes or enhanced oil recovery. The loading of pores as well as the structural organisation of MEs within the pores is a relevant parameter, especially for immobilisation applications. For this reason, the uptake of microemulsions in cylindrical pores of SBA-15 was investigated via adsorption and small-angle neutron scattering (SANS). Adsorption isotherms revealed an adsorption of the microemulsion droplets based on the adsorption of surfactant as a driving force. The adapted scattering model is based on the analysis of bare SBA-15 in full contrast conditions and employs microemulsions inside of SBA-15 measured at the silica contrast matching point. Accordingly, the structural arrangement of microemulsion droplets in the pores of SBA-15 was determined in good detail. Microemulsion droplets smaller than the pore size access the pores while retaining their spherical shape and become increasingly ordered for higher loading, where the droplets arrange in a dense packing of spherical droplets in a cylindrical pore. Interestingly, microemulsion droplets larger than the pore size can easily be incorporated, but become deformed upon entering and are present as elongated rod-like structures.

GRAPHICAL ABSTRACT

KEYWORDS:

• Microemulsion
• SBA-15
• small-angle neutron scattering
• mesoporous silica
• confinement

Acknowledgements

For the N₂ adsorption measurements we are grateful to Christina Eichenauer in the group of Prof. Arne Thomas at Technische Universität Berlin (TU Berlin). For the SEM measurements we would like to thank Ulrich Gernert from the Zentraleinrichtung Elektronenmikroskopie (ZELMI, TU Berlin). Sebastian Kemper and Samantha Voges (OC NMR Zentrum, TU Berlin) were gratefully acknowledged for setting up and performing the quantitative NMR measurements. Furthermore, we especially thank the Institut Laue-Langevin for financially supporting the granted beam time and the Deutsche Forschungsgemeinschaft (DFG) with the International Research Training Group Self-Assembled Soft Matter Nano-Structures at Interfaces (IRTG 1524) for financial support. Last, but not least, it should be stated that this research work reported here, was initiated and interpreted largely by the late Gerhard Findenegg. The authors want to express their gratitude to him for having enjoyed this cooperation with this outstanding scientist. He will always be remembered by us, not only as this outstanding researcher, but also in general as a very thoughtful person, not only having deep scientific insights, but also a remarkable good sense for humour with respect to all aspects of life.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1 The pD value should be given based on the preparation in D₂O. Nonetheless the given pH value is the technically measured value due to the calibration of the pH-meter. The simplest conversion of measured pH into a pD value can be done by adding 0.4 to the measured value [45].