Adsorption from binary liquid solutions into mesoporous silica: a capacitance isotherm on 5CB nematogen/cyclohexane mixtures

ABSTRACT

We present a capacitance method to measure the adsorption of rod-like nematogens (4-cyano-$4^{\prime }$-pentylbiphenyl, 5CB) from a binary liquid 5CB/cyclohexane solution into a monolithic mesoporous silica membrane traversed by tubular pores with radii of 5.4 nm at room temperature. The resulting adsorption isotherm is reminiscent of classical type II isotherms of gas adsorption in mesoporous media. Its analysis by a model for adsorption from binary solutions, as inspired by the Brunauer–Emmett–Teller (BET) approach for gas adsorption on solid surfaces, indicates that the first adsorbed monolayer consists of flat-lying (homogeneously anchored) 5CB molecules at the pore walls. An underestimation of the adsorbed 5CB amount by the adsorption model compared to the measured isotherm for high 5CB concentrations hint towards a capillary filling transition in the mesopores similar to capillary condensation, i.e. film-growth at the pore walls is replaced by filling of the pore centres by the liquid crystal. The experimental method and thermodynamic analysis presented here can easily be adapted to other binary liquid solutions and thus allows a controlled filling of mesoporous materials with non-volatile molecular systems.

GRAPHICAL ABSTRACT

Keywords:

• Adsorption
• binary solution
• capacitance
• isotherm
• liquid crystal

Acknowledgments

G.Y.G. and P.H. dedicate this work to Professor Gerhard Findenegg (Technical University Berlin), who performed seminal work in the field of molecular adsorption in porous materials and self-assembly in confinement. Funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Projektnummer 192346071, SFB 986 ‘Tailor-Made Multi-Scale Materials Systems’ and the DFG Graduate School GRK 2462 ‘Processes in natural and technical Particle-Fluid-Systems (PintPFS)’ (Project No. 390794421) is gratefully acknowledged. The research is also supported by the Hamburg Centre for Integrated Multiscale Materials Systems CIMMS. The presented results are part of a project that has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 778156. Support from resources for science in years 2018–2022 granted for the realisation of international co-financed project Nr W13/H2020/2018 (Dec. MNiSW 3871/H2020/2018/2) is also acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) Projektnummer 192346071, SFB 986 ‘Tailor-Made Multi-Scale Materials Systems’ and the DFG Graduate School GRK 2462 ‘Processes in natural and technical Particle-Fluid-Systems (PintPFS)’ (Project No. 390794421) is gratefully acknowledged. The research is also supported by the Hamburg Centre for Integrated Multiscale Materials Systems CIMMS. The presented results are part of a project that has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 778156. Support from the Polish Ministry of Science and Higher Eduction in the years 2018–2022 granted for the realisation of the international co-financed project Nr W13/H2020/2018 (Dec. MNiSW 3871/H2020/2018/2) is also acknowledged.