MCM-41

Two mesoporous materials were obtained using the soft templating route. The synthesis of the two mesoporous silica materials used the same surfactant—namely cetyltrimethylammonium bromide (CTAB)—which, under the basic conditions, aims to assemble into cylindrical micelles, which are further organized into a hexagonal or cubic structure. Around these structures, the silica network is formed by classical hydrolysis and condensation reactions. The template removal by calcination represents the final step.
The mesoporous materials were obtained as follows: 0.5 g CTAB was dissolved in 96 mL distilled water and sonicated until the solution became clear. Subsequently, 34 mL of ethanol and 10 mL of an ammonia solution were added and stirring was continued until the solution became homogeneous. After mixing, 2 mL of TEOS was added and stirred for an additional three hours at the same speed of rotation and a translucent precipitate was obtained. The precipitate thus obtained was filtered and washed with distilled water and ethanol. The last step of the synthesis consisted of purification/washing with ethanol (20 mL) and water (three times with 20 mL) and drying for 12 h at 100 °C. The final product was annealed for nine hours at 550 °C according to the following program: 0–300 °C at 80 °C /min and 300–550 °C at 20 °C /min. This mesoporous material was labelled MCM-41. Similarly, but using 2 g of CTAB, another mesoporous silica material was obtained and labelled MCM-48.

The mixed matrix membranes (PSF/MCM-41) were obtained from polysulfone (PSF) pellets (Sigma-Aldrich - Steinheim, Germany) via the solution casting method (Zornoza et al., 2009 (link); Murali et al., 2014 (link)). Prior to the membrane synthesis, the PSF was conditioned at 105°C for 3 h, under vacuum, in order to remove the adsorbed water, and then three main steps were performed to accomplish the membrane fabrication.
The first step consisted of the dispersion of various quantities of MCM-41 in chloroform (Sigma-Aldrich - Steinheim, Germany) using an ultrasonic bath (Elma S60H - Elma Schmidbauer GmbH, Singen, Germany) for 20 min, in order to obtain mixed matrix membranes with 5, 10, and 20 wt.% MCM-41 as filler. In the second step, the PSF was added to the obtained solution, and the mixture was magnetically stirred for 24 h in order to obtain a homogeneous membrane. During the stirring process, five sonication intervals were performed to enable the penetration of the MCM-41 pores by the polymeric chain.
In the last step, the mixtures were cast in Petri glass dishes and left overnight, at room temperature, partially closed to slow the natural evaporation of solvent. The membranes were removed by flushing the plates with ultrapure water and then were dried in the vacuum oven at 110°C and 80 mbar for 24 h.
Neat membranes, formed only from PSF, were prepared using the same recipe in order to compare them with the mixed membranes.
The thicknesses of the obtained membranes were determined by using a digital micrometer (Schut Geometrische Meettechniek - Groningen, Netherlands) with ± 0.001 mm accuracy for the 0–25 mm measurement interval. A membrane with a 5 cm² diameter was cut from the obtained material.

Simulated body fluid (SBF) was prepared by dissolving reagent grade 8.035 g·L⁻¹ NaCl, 0.355 g·L⁻¹ NaHCO₃, 0.225 g·L⁻¹ KCl, 0.231 g·L⁻¹ K₂HPO₄·(3H₂O), 0.311 g·L⁻¹ MgCl₂ (6H₂O), 0.292 g·L⁻¹ CaCl₂, and 0.072 g·L⁻¹ Na₂SO₄ in deionized water and buffering at pH 7.4 at 36.5 °C with 6.118 g·L⁻¹ tris(hydroxymethyl) aminomethane ((CH₂OH)₃CNH₂) and 1M HCl, as previously reported by Kokubo and Takadama [30 (link)]. Ca_MCM-41 particles were immersed in SBF at a 1.5 g·L⁻¹ ratio [31 (link),32 (link)]. The specimens were kept in a polypropylene container at 37 °C in an incubator on an oscillating tray for up to seven days. The solution was not renewed and a falcon tube containing SBF as a control was also used for the entire period of the experiment, in order to control over time the stability of the testing solution. At each time point, the particles were centrifuged and washed with deionized water and dried at 60 °C overnight. The microstructural changes were investigated by means of scanning electron microscopy (SEM) equipped with an Energy Dispersive X-ray Spectrometry (EDS) detector (Auriga 0750 from ZEISS, Jena, Germany). The ordered mesoporous structure was checked by high resolution transmission electron microscopy (HR-TEM), in a Tecnai G2F30 S-Twin microscope (FEI, Eindhoven, The Netherlands) with 0.2 nm point resolution, operated at 300 kV and equipped with a HAADF Fischione detector (0.16 nm point resolution, Fischione Instrument, Pittsburgh, PA, USA). For the TEM observation, particles were homogeneously dispersed in ethanol by ultrasound and dropped on a carbon film. The pore diameter analysis of the ordered mesoporous particles was conducted on HRTEM images with ImageJ2 [33 (link)] analysis software.