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The measurement of nitrogen adsorption isotherms by volumetric technique is a standard way to characterize mesoporous materials. However, this technique does not allow for the continuous measurement of the amount $m$ of condensed fluid as a function of the surrounding gas pressure $P$, a capability that has been shown to provide detailed insights into the cavitation process in porous materials [Bossert 2021, Bossert 2023]. In this previous work, we primarily used thin monolithic porous alumina or porous silicon samples where $m$ could be determined by measuring continuously the effective optical index of the porous material and converting this index into $m$ using simple effective medium models [Casanova 2008, Bossert 2020].
Recently, we have been investigating cavitation in ordered porous silica materials, such as SBA-16, which are synthesized as powders [See Cavitation in confined Fluid, E. Rolley el al., this conférence]. To enable continuous measurement of the isotherms, we have designed a simple setup: the sample is placed between the electrodes of a planar capacitor, monitored by a high resolution capacitance bridge operating in the kHz range. As an initial test, we measured the capacitance value $C_0$ at zero pressure and its value $C_{sat}$ when the capacitor is fully filled with liquid, for various quantities $m_{Si}$ of SBA-16. Both $C_0$ and $C_{sat}$ dependence on $m_{Si}$ are in agreement with effective medium model.
In a second step, we have measured capacitive isotherms $C(P)$ for various porous silica samples. When converted into conventional volumetric isotherms using effective medium models, these isotherms exhibit shapes that differ significantly from those measured directly by volumetric techniques. For most samples, the capacitance response in the pressure range correponding to the adsorption in mesopores is lower than expected. This could be due to changes in the orientational polarisability of silanols at the surface of the silica structure [Guermeur 1991], or changes in the polarisability of the adsorbate [Keller 2005]. This effect complicates the detailed interpretation of $C(P)$. However, for simple mesoporous materials with a well-defined pore size, our capacitive technique provides an accurate determination of the pressure where condensation or evaporation occurs.
| References | [Bossert 2020] Bossert et al., Langmuir 2020, 36, 11054−11060, [Bossert 2021] Bossert et al., Langmuir 2021 37 (49), 14419-14428, [Bossert 2023] Bossert et al., PNAS 2023 120(15), e2300499120 [Casavona 2008] Casanova et al., Nanotechnology 2008, 19, 315709, [Guermeur 1991] Guermeur et al., Surface Science 255 (1991) 157.[Keller 2005] Gas adsorption equilibria: experimental methods and adsorption isotherms (chap. 6), Springer 2005. |
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| Country | France |
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