Water and ion transport in ultra-adsorbing porous magnesium carbonate studied by dielectric spectroscopy

Abstract

Porous materials are used in application areas ranging from drug and vaccine delivery, medical implants, molecular sieves and cosmetics to catalysis and humidity control. In the present work, we employed an alternative approach to gain in-depth understanding about water interaction properties in such materials by the use of dielectric spectroscopy and thereby show that it is possible to obtain information that is not accessible from the more commonly employed water interaction analysis techniques. Specifically, the complex dielectric response of Upsalite, a novel, super-hydroscopic, high-surface area, porous magnesium carbonate material was measured in isothermal frequency scans between 10−3 and 106 Hz at controlled relative humidity (RH). We found the dielectric constant of the dry material to be 1.82. The ratio of bound to free water present in Upsalite after adsorption at room temperature was found to be high irrespective of the surrounding humidity with values ranging from ∼67% to ∼90%. We further found that OH− ions are the charge carriers responsible for the electrode polarization observed in the dielectric response and that the amount of these ions that are free to move in the material corresponds to a concentration of the order of 1–10 μmol l−1 independent of RH. Finally, the OH− diffusion coefficient displayed a drastic decrease with decreasing RH, typical of transport in unsaturated conditions. The presented results provide detailed insight about water interactions in the novel water adsorbing material under study and it is foreseen that the employed analysis methods can be used to evaluate other types of moisture adsorbing materials as well as the movement of functional species in the pores of inorganic drug delivery materials and materials tailored for adsorption of harmful charged species.

Diffusion-Controlled Drug Release From the Mesoporous Magnesium Carbonate Upsalite ®

“…A reasonable explanation for the slow-release region could be that the carboxylic groups of the ibuprofen molecules interacted with hydroxyl groups on the Upsalite ® pore wall surface remaining after synthesis. 13 A similar hydrogen bond interaction between the carboxylic group on ibuprofen and silanol groups on the pore walls of mesoporous silica has been hypothesized to cause the experimentally observed plateau in ibuprofen release from mesoporous silica. The slow release of drug from that material extends over several days.…”

“…[7][8][9][10] In our previous work, the novel mesoporous and amorphous material Upsalite ® was investigated as a phase stabilizer of amorphous ibuprofen to increase its dissolution rate. [11][12][13][14] Upsalite ® consists of magnesium carbonate (MgCO 3 ) that is "generally recognized as safe" by the FDA. It has a large specific surface area (typically 300-800 cm 2 /g) and a narrow pore-size distribution (usually 6-8 nm), and these material properties have been associated with high loading of ibuprofen (30%) and suppressed crystallization.…”

Nanostructure and pore size control of template-free synthesised mesoporous magnesium carbonate

“…One may expect that such surfactant-free synthesis routes, where calcination at high temperatures is not needed, may be important for the up-scalable production of mesoporous materials. The industrial feasibility of the MMC is further emphasised by the material's moisture sorption properties, [47][48][49][50] biocompatibility 51 and drug stabilising properties. 22,23 Most recently we also showed that MMC has the ability to prevent blood clotting, 52 which opens up its potential for application where blood clotting needs to be suppressed.…”

Dielectric Spectroscopy Study of Water Behavior in Calcined Upsalite: A Mesoporous Magnesium Carbonate without Organic Surface Groups

“…As previously concluded, 9 for uncalcined Upsalite the bound proportion of the adsorbed water is high irrespective of sorption route and RH: For very low RHs ∼ 80−90% is bound to the solid and when the RH increases, a somewhat higher proportion is free. At 95% RH ∼ 67% of the adsorbed water is bound.…”

“…9 DS measures the dielectric properties of matter through its response to an applied electric field in a broad frequency range (10 −6 −10 11 Hz). A sinusoidal ac voltage is applied across the sample localized between two electrodes and the current response (amplitude and phase) is measured while the angular frequency ω = 2πf of the voltage is varied.…”