Molecular dynamics simulations of Ibuprofen release from pH-gated silica nanochannels

The iboprufen delivery process from cylindrical silica pores of diameter 3~nm, with polyamine chains anchored at the pore outlets,was investigated by means of massive molecular dynamics simulations. Effects from pH were introduced by considering polyamine chains with different degree of protonation. High, low and intermediate pH environments were investigated. The increment of the acidity of the environment leads to a significant decrease of the pore aperture,  yielding an effective diameter, for the lowest pH case, that is 3.5~times smaller than the one associated to the highest pH one. Using a biased sampling procedure, Gibbs free energy profiles for the ibuprofen delivery process were obtained. The joint analysis of the corresponding profiles, time evolution of the ibuprofen position within the channel, orientation of the molecule and instantaneous effective diameter of the gate, suggests a 3-steps mechanism for ibuprofen delivery. A complementary analysis of the translational mobility of ibuprofen along the axial direction of the channel revealed a sub-diffusive dynamics in the low and intermediate pH cases.Deviations from Brownian diffusive dynamics are discussed and compared with direct experimental results. Fil: Rodriguez, Javier. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Núcleo de Investigación en Educacion Ciencia y Tecnologia; ArgentinaFil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica; Argentin

Preferential Adsorption in Ethane/Carbon Dioxide Fluid Mixtures Confined within Silica Nanopores

Molecular dynamics simulations have been performed to investigate the structural and dynamical properties of fluid ethane confined within cylindrical silica nanopores of 3.8 and 1 nm diameters. Pure ethane and equimolar mixtures of ethane and CO2 were considered for the analysis. Computer simulations were carried out along the supercritical isotherm T = 320 K, exploring densities within the range ρ/ρc = 0.05?2.28. Density profiles along the axial and radial directions of the pore, orientational distribution functions, analysis of interfacial dynamic properties, and estimations of diffusion coefficients are presented. In agreement with experimental data, the results show the formation of a dense adsorption layer, with densities that may be up to 10 times larger than those of the bulk phase, in particular at low densities and under subnanometer confinement. The incorporation of CO2 changes the scenario, leading to preferential adsorption of CO2 over ethane species. The dynamics of ethane also change in mixtures, in agreement with experimental measurements. These changes are manifested by increments in the diffusion coefficients of confined ethane in the presence of CO2. However, in contrast with experimental data, our results for the diffusion coefficients of confined ethane showed a monotonic decreasing behavior with increasing bulk density. Plausible interpretations for this discrepancy are also discussed.Fil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; ArgentinaFil: Rodriguez, Javier. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; Argentin

Solvation of Coumarin 480 within nano-confining environments: Structure and dynamics

Equilibrium and dynamical characteristics pertaining to the solvation of the fluorescent probe Coumarin 480 within different confining environments are investigated using molecular dynamics simulations. Three kinds of confining systems are examined: (i) the cetyltrimethylammonium bromide (CTAB)/isooctane/1-hexanol/water; cationic inverse micelle (IM) (ii) a CTAB/water direct micelle (DM), and (iii) a silica-surfactant nanocomposite, comprising a cylindrical silica pore (SP) containing small amounts of water and CTAB species adsorbed at the pore walls. The solvation structures in the three environments differ at a qualitative level: an exchange between bulk- and interface-like solvation states was found in the IM, whereas in the DM, the solvation states of the probe are characterized by its embedding at the interface, trapped among the surfactant heads and tails. Within the SP structure, the coumarin exhibits alternations between internal and interfacial solvation states that occur on a ∼20 ns time scale and operate via 90° rotations of its molecular plane. The solvation responses of the environment following a vertical excitation of the probe are also investigated. Solvation times resulted between 2 and 1000 times longer than those found in bulk water, with a fast-to-slow trend IM→DM→SP, which can be interpreted in terms of the solvation structures that prevail in each case.Fil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rodriguez, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentin

Structure and dynamics of nonionic surfactants adsorbed at vacuum/ionic liquid interfaces

Structural and dynamical properties related to the adsorption of nonionic surfactants at vacuum/ionic liquid interfaces were studied using molecular dynamics simulations. Specifically, the surface activity of pentaethylene glycol monododecyl ether (C12E5) was investigated at the free interface of an imidazolium-based room temperature ionic liquid (RTIL), at different surface densities. At low surface coverages, the incorporation of C12E5 does not produce meaningful changes in the vacuum/RTIL interface: the C12E5 hydrophobic tails remain entangled with those of the RTIL cation groups in the outer shell, whereas the C12E5 hydrophilic heads reside at an inner layer. At high surface coverages, the structure in the substrate—in terms of the features exhibited by the local density profiles—practically vanishes; the interface becomes wider and the surfactant molecules shift toward more external positions. Information about the local structure of the interface at high surface densities can be recovered by performing a tessellation procedure. For the sake of comparison, the surface behavior of two commonly used ionic surfactants, sodium dodecyl sulfate and dodecyl trimethyl ammonium chloride, were also studied. The modifications in the width and structure of the bare vacuum/RTIL interface due to the presence of the ionic surfactants are markedly milder than those observed for the nonionic surfactant. Moreover, the RTIL seemed to behave as a better solvent for the chloride counterions than for sodium ones; which were found to remain bound to the surfactant head groups. An analysis of the dynamics at the surface was also performed. Our results indicate that the presence of increasing amounts of nonionic surfactants leads to a gradual reduction of the mobility of the RTIL species. When ionic surfactants are adsorbed, these retardations are even more severe for the surfactant head groups, where the corresponding diffusion coefficients show reductions of practically 1 order of magnitude.Fil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rodriguez, Javier. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Ionic Mobility within Functionalized Silica Nanopores

Molecular dynamics simulations were performed to investigate the structural and dynamical features of an aprotic ionic liquid confined within two types of cylindrical silica pores (hydrophilic and hydrophobic ones) as a function of the pore filling fraction. Analysis of the local density distributions revealed the existence of a dense adsorbed layer in both pores, leading to interfacial ionic liquid densities that resulted between 2 and 3 times larger than bulk. Beyond the characteristics of the surface, it is observed that the nearest-to-the-wall-adsorbed ionic liquid cations accommodate their rings and alkyl chains parallel to the pore wall. Nevertheless, the orientation of the alkyl chain of the more distant cations in the adsorbed layer depends on the functionalization of pore walls, pointing toward the center of the pore for the case of hydrophilic surfaces or toward the pore surface when the wall is covered by hydrophobic moieties. Transport properties were also investigated. The axial translational diffusive dynamics exhibits an overall slowdown upon confinement, being more pronounced in the hydrophilic cavities at low loadings, in agreement with recent experimental results. The ionic conductivity measured in the hydrophilic pores resulted ∼50% lower than in the bulk phase. In contrast, within the hydrophobic pores, the conductivity resulted 30% larger than in hydrophilic cavities and showed weak dependence on loading. The contributions to the collective conductivity, arising from single and distinct components, were analyzed and discussed in terms of microscopic correlations and local densities.Fil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rodriguez, Javier. Universidad Nacional de San Martín; Argentina. Comisión Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones No Nucleares. Gerencia Física (CAC). Departamento de Física de la Materia Condensada; Argentin

Excess Sorption of Supercritical CO2 within Cylindrical Silica Nanopores

Using Molecular Dynamics simulations, we examine structural and dynamical properties of supercritical CO2 confined within cylindrical hydrophobic nanopores of diameters 38 and 10 Å. Computer simulations were performed along the isotherm T = 315 K, spanning CO2 densities from ρ/ρc = 2.22 down to ρ/ρc = 0.22. Radial and orientational distribution functions, analysis of interfacial dynamic properties, and estimatons for local diffusion and orientational relaxation times are presented. In agreement with previous experimental data, our simulation results reveal the presence of a dense phase adsorbed within the pores. The combination of low CO2 bulk densities and narrow pores leads to ρint/ρblk ≈ 5-fold enhancement of the global density of the confined fluid. These density increments gradually become much less marked as the external phase becomes denser. Contrasting, in that latter limit, we found that the trapped fluid may become less dense than the bulk phase. Adsorption behavior of CO2 onto hydrophilic-like and rugged pore surfaces were also exmined. In these cases, we observed a global slowdown in both translational and rotational motions for the trapped CO2, the largest retardations being those associated with spatial domains of the fluid located near the silica interface.Fil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rodriguez, Javier. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentin

Influence of Cholesterol on the Dynamics of Hydration in Phospholipid Bilayers

We investigate the dynamics of interfacial waters in dipalmitoylphosphatidylcholine (DPPC) bilayers upon the addition of cholesterol, by molecular dynamics simulations. Our data reveal that the inclusion of cholesterol modifies the membrane aqueous interfacial dynamics: waters diffuse faster, their rotational decay time is shorter, and the DPPC/water hydrogen bond dynamics relaxes faster than in the pure DPPC membrane. The observed acceleration of the translational water dynamics agrees with recent experimental results, in which, by means of NMR techniques, an increment of the surface water diffusivity is measured upon the addition of cholesterol. A microscopic analysis of the lipid/water hydrogen bond network at the interfacial region suggests that the mechanism underlying the observed water mobility enhancement is given by the rupture of a fraction of interlipid water bridge hydrogen bonds connecting two different DPPC molecules, concomitant to the formation of new lipid/solvent bonds, whose dynamics is faster than that of the former. The consideration of a simple two-state model for the decay of the hydrogen bond correlation function yielded excellent results, obtaining two well-separated characteristic time scales: a slow one (∼250 ps) associated with bonds linking two DPPC molecules, and a fast one (∼15 ps), related to DPPC/solvent bonds.Fil: Elola, Maria Dolores. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; ArgentinaFil: Rodriguez, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentin

Coaxial cross-diffusion through carbon nantoubes

We present results from nonequilibrium molecular dynamics experiments describing the relaxation of local concentrations at two reservoirs, initially filled with water (W) and acetonitrile (ACN), as they become connected through a membrane composed of (16,16) carbon nanotubes. Within the hydrophobic nanotube cavities, the equilibrium concentrations contrast sharply to those observed at the reservoirs, with a clear enhancement of ACN, in detriment of W. From the dynamical side, the relaxation involves three well-differentiated stages; the first one corresponds to the equilibration of individual concentrations within the nanotubes. An intermediate interval with Fickian characteristics follows, during which the overall transport can be cast in terms of coaxial opposite fluxes, with a central water domain segregated from an external ACN shell, in close contact with the tube walls. We also found evidence of a third, much slower, mechanism to reach equilibration, which involves structural modifications of tightly bound solvation shells, in close contact with the nanotube rims.Fil: Rodriguez, Javier. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Universidad Nacional de San Martín; ArgentinaFil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentina. Universidad Nacional de San Martín; ArgentinaFil: Laria, Daniel Hector. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentin

Polar mixtures under nanoconfinement

We present results from molecular dynamics simulations describing structural and dynamical characteristics of equimolar mixtures of water and acetonitrile, confined between two silica walls separated at interplate distances of d = 0.6, 1, and 1.5 nm. Two different environments were investigated: a first one where wall-solvent dispersion forces prevail (hydrophobic confinement) and a second one in which the terminal O atoms at the silica surface are transformed into silanol groups (hydrophilic confinement). For the former case, we found that, at the shortest interplate distance examined, the confined region is devoid of water molecules. At an interplate distance of the order of 1 nm, water moves into the confined region, although, in all cases, there is a clear enhancement of the local concentration of acetonitrile in detriment of that of water. Within hydrophilic environments, we found clear distinctions between a layer of bound water lying in close contact with the silica substrates and a minority of confined water that occupies the inner liquid slab. The bound aqueous layer is fully coordinated to the silanol groups and exhibits minimal hydrogen bonding with the second solvation layer, which exclusively includes acetonitrile molecules. Dynamical characteristics of the solvent mixture are analyzed in terms of diffusive and rotational motions in both environments. Compared to bulk mixtures, we found significant retardations in all dynamical modes, with those ascribed to water molecules bound to the hydrophilic plates being the most dramatic.Fil: Rodriguez, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: Elola, Maria Dolores. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; ArgentinaFil: Laria, Daniel Hector. Comisión Nacional de Energía Atómica; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Química Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Equilibrium and dynamical characteristics of Imidazole Langmuir monolayers on graphite sheets

Using molecular dynamics techniques, we examine structural and dynamical characteristics of liquid-like, Imidazole(Im) monolayers physisorbed onto a planar graphite sheet, at T = 384K. Our  simulations reveal that molecular orientations in the saturated monolayer exhibit a bistable distribution, characterized by an inner parallel arrangement of the molecules in close contact with the substrate and a slanted alignment, in those lying in adjacent, outer locations. Compared to the results  found in three dimensional, bulk phases, the analysis of the spatial correlations between sites participating in hydrogen bonding shows a clear enhancement of the intermolecular interactions, which also leads to stronger dipolar correlations. As a result, the gross structural features of the   monolayer can be cast in terms of mesoscopic domains, comprising units articulated via winding hydrogen bonds, that persist along  typical time intervals of a few tens of picoseconds.  On the dynamical side, a similar comparison of the characteristic decorrelation time for orientational motions shows a 4-fold increment.  Contrasting, the reduction of the system dimensionality leads to a larger diffusion constant. Possible substrate-induced anisotropies in the diffusive motions are also investigated.Fil: Rodriguez, Javier. Comisión Nacional de Energía Atómica; Argentina. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Núcleo de Investigación en Educacion Ciencia y Tecnologia; ArgentinaFil: Elola, Maria Dolores. Comisión Nacional de Energía Atómica; ArgentinaFil: Laria, Daniel Hector. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin