Evidence of Strong Guest-Host Interactions in Simvastatin Loaded in Mesoporous Silica MCM-41

A rational design of drug delivery systems requires in-depth knowledge not only of the drug itself, in terms of physical state and molecular mobility, but also of how it is distributed among a carrier and its interactions with the host matrix. In this context, this work reports the behavior of simvastatin (SIM) loaded in mesoporous silica MCM-41 matrix (average pore diameter ~3.5 nm) accessed by a set of experimental techniques, evidencing that it exists in an amorphous state (X-ray diffraction, ssNMR, ATR-FTIR, and DSC). The most significant fraction of SIM molecules corresponds to a high thermal resistant population, as shown by thermogravimetry, and which interacts strongly with the MCM silanol groups, as revealed by ATR-FTIR analysis. These findings are supported by Molecular Dynamics (MD) simulations predicting that SIM molecules anchor to the inner pore wall through multiple hydrogen bonds. This anchored molecular fraction lacks a calorimetric and dielectric signature corresponding to a dynamically rigid population. Furthermore, differential scanning calorimetry showed a weak glass transition that is shifted to lower temperatures compared to bulk amorphous SIM. This accelerated molecular population is coherent with an in-pore fraction of molecules distinct from bulklike SIM, as highlighted by MD simulations. MCM-41 loading proved to be a suitable strategy for a long-term stabilization (at least three years) of simvastatin in the amorphous form, whose unanchored population releases at a much higher rate compared to the crystalline drug dissolution. Oppositely, the surface-attached molecules are kept entrapped inside pores even after long-term release assays.

Consequences of Cylindrical Pore Geometry for Interfacial Phenomena in Reversed-Phase Liquid Chromatography

The interfacial phenomena behind analyte separation in a reversed-phase liquid chromatography column take place nearly exclusively inside the silica mesopores. Their cylindrical geometry can be expected to shape the properties of the chromatographic interface with consequences for the analyte density distribution and diffusivity. To investigate this topic through molecular dynamics simulations, we introduce a cylindrical pore inside a slit pore configuration, where the inner curved and outer planar silica surface bear the same bonded phase. The present model replicates an average-sized (9 nm) mesopore in an endcapped C₁₈ column equilibrated with a mobile phase of 70/30 (v/v) water/acetonitrile. Simulations performed for ethylbenzene and acetophenone show that the surface curvature shifts the bonded phase and analyte density toward the pore center, decreases the solvent density in the bonded-phase region, increases the acetonitrile excess in the interfacial region, and considerably enhances the surface diffusivity of both analytes. Overall, the cylindrical pore provides a more hydrophobic environment than the slit pore. Ethylbenzene density is decidedly increased in the cylindrical pore, whereas acetophenone density is nearly equally distributed between the cylindrical and slit pore. The cylindrical pore geometry thus sharpens the discrimination between the apolar and moderately polar analytes while enhancing the mass transport of both.

Confined Water as Model of Supercooled Water

Water in confined geometries has obvious relevance in biology, geology, and other areas where the material properties are strongly dependent on the amount and behavior of water in these types of materials. Another reason to restrict the size of water domains by different types of geometrical confinements has been the possibility to study the structural and dynamical behavior of water in the deeply supercooled regime (e.g., 150-230 K at ambient pressure), where bulk water immediately crystallizes to ice. In this paper we give a short review of studies with this particular goal. However, from these studies it is also clear that the interpretations of the experimental data are far from evident. Therefore, we present three main interpretations to explain the experimental data, and we discuss their advantages and disadvantages. Unfortunately, none of the proposed scenarios is able to predict all the observations for supercooled and glassy bulk water, indicating that either the structural and dynamical alterations of confined water are too severe to make predictions for bulk water or the differences in how the studied water has been prepared (applied cooling rate, resulting density of the water, etc.) are too large for direct and quantitative comparisons.

Toward a Coarse Graining/All Atoms Force Field (CG/AA) from a Multiscale Optimization Method: An Application to the MCM-41 Mesoporous Silicates

Many interesting physical phenomena occur on length and time scales that are not accessible by atomistic molecular simulations. By introducing a coarse graining of the degrees of freedom, coarse-grained (CG) models allow ther study of larger scale systems for longer times. Coarse-grained force fields have been mostly derived for large molecules, including polymeric materials and proteins. By contrast, there exist no satisfactory CG potentials for mesostructured porous solid materials in the literature. This issue has become critical among a growing number of studies on confinement effects on fluid properties, which require both long time and large scale simulations and the conservation of a sufficient level of atomistic description to account for interfacial phenomena. In this paper, we present a general multiscale procedure to derive a hybrid coarse grained/all atoms force field CG/AA model for mesoporous systems. The method is applied to mesostructured MCM-41 molecular sieves, while the parameters of the mesoscopic interaction potentials are obtained and validated from the computation of the adsorption isotherm of methanol by grand canonical molecular dynamic simulation.

Structural heterogeneities at the origin of acoustic and transport anomalies in glycerol glass-former

By means of large scale molecular dynamics simulations, we explore mesoscopic properties of prototypical glycerol glass-former above and below the glass transition. The model used, in excellent agreement with various experimental techniques, permits to carefully study the structure and the vibrational dynamics. We find that a medium range order is present in glycerol glass-former and arises from hydrogen bond network extension. The characteristic size of the structural heterogeneities is related to the anomalous properties of acoustic vibrations (Rayleigh scattering, "mode softening," and Boson Peak) in the glassy state. Finally the characteristic size of these heterogeneities, nearly constant in temperature, is also connected to the cross-over between structural relaxation and diffusion in liquid glycerol.

Unravelling the anomalous dielectric permittivity of nanoconfined electrolyte solutions

The dielectric properties of sodium chloride solutions confined in a hydrophilic nanocavity were investigated by means of molecular dynamics simulations. Unlike what is observed in the bulk phase, three dielectric regimes were evidenced, namely an anomalous increase in the dielectric permittivity at low concentrations (with respect to confined pure water), a dielectric plateau at intermediate concentrations and finally a bulk-like behavior for salt concentrations higher than a critical value. It was shown that this peculiar behavior results from the competition between dielectric saturation due to the electric field generated by ions (which tends to lower the dielectric permittivity) and the ion-induced perturbation of pre-oriented water molecules inside the nanocavity which gain some rotational degrees of freedom (entropic contribution) leading to an increase in dipolar fluctuations responsible for the increase in the dielectric permittivity.

Solvent contribution to the stability of a physical gel characterized by quasi-elastic neutron scattering

The dynamics of a physical gel, namely, low-molecular-mass organic gelator methyl-4,6-O-benzylidene-α-D-mannopyranoside (α-manno) in water and toluene, are probed by neutron scattering. Using high gelator concentrations, we were able to determine, on a time scale from a few picoseconds to 1 nanosecond, the number of solvent molecules that are immobilized by the rigid network formed by the gelators. We found that only a few toluene molecules per gelator participate in the network which is formed by hydrogen bonding between the gelators' sugar moieties. In water, however, the interactions leading to the gel formations are weaker, involving dipolar, hydrophobic, or π-π interactions, and hydrogen bonds are formed between the gelators and the surrounding water. Therefore, around 10 to 14 water molecules per gelator are immobilized by the presence of the network. This study shows that neutron scattering can give valuable information about the behavior of solvent confined in a molecular gel.

Dynamics of deeply supercooled interfacial water

In this review we discuss the relaxation dynamics of glassy and deeply supercooled water in different types of systems. We compare the dynamics of such interfacial water in ordinary aqueous solutions, hard confinements and biological soft materials. In all these types of systems the dielectric relaxation time of the main water process exhibits a dynamic crossover from a high-temperature non-Arrhenius temperature dependence to a low-temperature Arrhenius behavior. Moreover, at large enough water content the low-temperature process is universal and exhibits the same temperature behavior in all types of systems. However, the physical nature of the dynamic crossover is somewhat different for the different types of systems. In ordinary aqueous solutions it is not even a proper dynamic crossover, since the water relaxation decouples from the cooperative α-relaxation of the solution slightly above the glass transition in the same way as all secondary (β) relaxations of glass-forming materials. In hard confinements, the physical origin of the dynamic crossover is not fully clear, but it seems to occur when the cooperative main relaxation of water at high temperatures reaches a temperature where the volume required for its cooperative motion exceeds the size of the geometrically-confined water cluster. Due to this confinement effect the α-like main relaxation of the confined water seems to transform to a more local β-relaxation with decreasing temperature. Since this low-temperature β-relaxation is universal for all systems at high water content it is possible that it can be considered as an intrinsic β-relaxation of supercooled water, including supercooled bulk water. This possibility, together with other findings for deeply supercooled interfacial water, suggests that the most accepted relaxation scenarios for supercooled bulk water have to be altered.

Effect of confinement on electric field induced orientation of a nematic liquid crystal

We report the effect of confinement on the electric field induced orientation of a nematic liquid crystal, 4-cyano-4'-hexylbiphenyl (6CB), between mica surfaces. The resonance shear measurement was employed for monitoring changes in the viscosity of 6CB at various surface separation distances (D) with and without an applied electric field. The viscosity depended on the surface separations, and the behaviour for D < ca. 20 nm was quite different from that at D > ca. 20 nm. For D > ca. 20 nm, the viscosity values obtained in the presence of the electric field (ac 1 kHz, 1.87 kV mm(-1), homeotropic orientation) were ca. 2 times higher than the values obtained without the electric field (0 kV mm(-1), planar orientation) due to the difference in the molecular orientation, and were nearly constant. At D < ca. 20 nm, the viscosity of 6CB both with and without the electric field sharply increased and they merged into an identical value at D = 12.5 ± 1.3 nm (Dc), then exhibited a plateau up to D = 6 nm. With the decreasing distance below 6 nm, the viscosity of confined 6CB both with and without the electric field further increased up to more than 100 N s m(-1) at the hard wall thickness of D = ca. 4.0 nm. These results indicated that 6CB molecules both with and without the electric field had the same orientation at D < Dc. The most likely orientation of 6CB was parallel to the surfaces because 6CB was originally in a planar orientation on the mica surface. These results demonstrated for the first time that the effect of confinement exceeded the electric field, thus 6CB molecules could not change their orientation under the electric field at the surface separation below Dc.

Ultraconfinement of aqueous electrolytic solutions within hydrophilic nanotubes

By means of molecular simulations we shed light on the interplay of surface, confinement and salt effects on the structure and dynamics of water and ions highly confined within a hydrophilic silica nanotube.

Polarizable empirical force field for acyclic polyalcohols based on the classical Drude oscillator

A polarizable empirical force field for acyclic polyalcohols based on the classical Drude oscillator is presented. The model is optimized with an emphasis on the transferability of the developed parameters among molecules of different sizes in this series and on the condensed-phase properties validated against experimental data. The importance of the explicit treatment of electronic polarizability in empirical force fields is demonstrated in the cases of this series of molecules with vicinal hydroxyl groups that can form cooperative intra- and intermolecular hydrogen bonds. Compared to the CHARMM additive force field, improved treatment of the electrostatic interactions avoids overestimation of the gas-phase dipole moments resulting in significant improvement in the treatment of the conformational energies and leads to the correct balance of intra- and intermolecular hydrogen bonding of glycerol as evidenced by calculated heat of vaporization being in excellent agreement with experiment. Computed condensed phase data, including crystal lattice parameters and volumes and densities of aqueous solutions are in better agreement with experimental data as compared to the corresponding additive model. Such improvements are anticipated to significantly improve the treatment of polymers in general, including biological macromolecules.

The non-Gaussian dynamics of glycerol

We have combined incoherent quasielastic neutron scattering experiments and atomistic molecular simulations to investigate the microscopic dynamics of glycerol moving away from the hydrodynamic limit. We relate changes in the momentum transfer (Q) dependence of the relaxation time to distinct changes of the single-particle dynamics. Going from small to large values of Q, a first crossover at about 0.5 Å(-1) is related to the coupling of the translational diffusion dynamics to the non-Debye structural relaxation, while the second crossover at a Q-value near the main diffraction peak is associated with the Gaussian to non-Gaussian crossover of the short-time molecular dynamics, related to the decaging processes. We offer an unprecedented extension of previous studies on polymeric systems towards the case of the typical low-molecular-weight glass-forming system glycerol.

Molecular dynamics simulation study of glycerol-water liquid mixtures

To study the effects of water on conformational dynamics of polyalcohols, Molecular Dynamics simulations of glycerol-water liquid mixtures have been carried out at different concentrations: 42.9 and 60.0 wt % of glycerol, respectively. On the basis of the analysis of backbone conformer distributions, it is found that the surrounding water molecules have a large impact on the populations of the glycerol conformers. While the local structure of water in the liquid mixture is surprisingly close to that in pure liquid water, the behavior of glycerols can be divided into three different categories where roughly 25% of them occur in a structure similar to that in pure liquid of glycerol, ca. 25% of them exist as monomers, solvated by water, and the remaining 50% of glycerols in the mixture form H-bonded strings as remains of the glycerol H-bond network. The typical glycerol H-bond network still exists even at the lower concentration of 40 wt % of glycerol. The microheterogeneity of water-glycerol mixtures is analyzed using time-averaged distributions of the sizes of the water aggregates. At 40 wt % of glycerol, the cluster sizes from 3 to 10 water molecules are observed. The increase of glycerol content causes a depletion of clusters leading to smaller 3-5 molecule clusters domination. Translational diffusion coefficients have been calculated to study the dynamical behavior of both glycerol and water molecules. Rotational-reorientational motion is studied both in overall and in selected substructures on the basis of time correlation functions. Characteristic time scales for different motional modes are deduced on the basis of the calculated correlation times. The general conclusion is that the presence of water increases the overall mobility of glycerol, while glycerol slows the mobility of water.