Dynamics of hydrogen‐bonded liquids confined to mesopores: A dielectric and neutron spectroscopy study

Interplay between dynamic heterogeneity and interfacial gradients in a model polymer film

Glass-forming liquids exhibit long-lived, spatially correlated dynamical heterogeneity, in which some nm-scale regions in the fluid relax more slowly than others. In the nanoscale vicinity of an interface, glass-formers also exhibit the emergence of massive interfacial gradients in glass transition temperature Tg and relaxation time τ. Both of these forms of heterogeneity have a major impact on material properties. Nevertheless, their interplay has remained poorly understood. Here, we employ molecular dynamics simulations of polymer thin films in the isoconfigurational ensemble in order to probe how bulk dynamic heterogeneity alters and is altered by the large gradient in dynamics at the surface of a glass-forming liquid. Results indicate that the τ spectrum at the surface is broader than in the bulk despite being shifted to shorter times, and yet it is less spatially correlated. This is distinct from the bulk, where the τ distribution becomes broader and more spatially organized as the mean τ increases. We also find that surface gradients in slow dynamics extend further into the film than those in fast dynamics-a result with implications for how distinct properties are perturbed near an interface. None of these features track locally with changes in the heterogeneity of caging scale, emphasizing the local disconnect between these quantities near interfaces. These results are at odds with conceptions of the surface as reflecting simply a higher "rheological temperature" than the bulk, instead pointing to a complex interplay between bulk dynamic heterogeneity and spatially organized dynamical gradients at interfaces in glass-forming liquids.

Revisiting the glass transition temperature of water-glycerol mixtures in the bulk and confined in mesoporous silica

In this work, we revisited the glass transition temperature (Tg) behavior of bulk and confined water-glycerol solutions as a function of the mixture composition and size of the confinement media, with the aim to shed some light on some controversies found in the literature. In the case of bulk mixtures, some discrepancies are observed due to the differences in the way of calculating Tg from the DSC experiments and differences in the protocols of cooling/reheating. However, unphysical behavior observed below the eutectic composition can be due to the crystallization of water during the cooling of the mixture. We also analyzed the effect of confinement on the glass transition of glycerol aqueous solutions, with glycerol mass fraction, wG, between 0.5 and 1.0, in silica mesoporous samples with pore diameters between 2 and 58 nm. Our results show that the the Tg dependence on pore size changes with the mixture composition. For glycerol-rich samples, Tg decreases with a decreasing pore size. This tendency changes with increasing water concentration below wG ∼ 0.6 for samples with dp between 2 and 8 nm, where two glass transition temperatures appear. We hypothesize that this effect is related to the existence of two liquid phases with different densities. The Tg composition dependence in confined glycerol-water mixtures was analyzed with the Gordon-Taylor equation modified for confined mixtures, which allowed us to calculate the Tg of the pure components as a function of the pore size. This analysis shows that for pores with dp > 20 nm, and for pure water and pure glycerol, Tg decreases with the pore size, attaining an almost constant value for samples with pore sizes between 2 and 8 nm. This Tg pore size dependence is explained considering the competition of two opposite effects: a reduction in Tg with a decreasing pore size given when the length scale of dynamics is comparable to the pore size, and an increment in Tg with a decreasing pore size as a result of increasing interactions of the confined liquid with the pore walls.

On the Segmental Dynamics and the Glass Transition Behavior of Poly(2-vinylpyridine) in One- and Two-Dimensional Nanometric Confinement

Geometric nanoconfinement, in one and two dimensions, has a fundamental influence on the segmental dynamics of polymer glass-formers and can be markedly different from that observed in the bulk state. In this work, with the use of dielectric spectroscopy, we have investigated the glass transition behavior of poly(2-vinylpyridine) (P2VP) confined within alumina nanopores and prepared as a thin film supported on a silicon substrate. P2VP is known to exhibit strong, attractive interactions with confining surfaces due to the ability to form hydrogen bonds. Obtained results show no changes in the temperature evolution of the α-relaxation time in nanopores down to 20 nm size and 24 nm thin film. There is also no evidence of an out-of-equilibrium behavior observed for other glass-forming systems confined at the nanoscale. Nevertheless, in both cases, the confinement effect is seen as a substantial broadening of the α-relaxation time distribution. We discussed the results in terms of the importance of the interfacial energy between the polymer and various substrates, the sensitivity of the glass-transition temperature to density fluctuations, and the density scaling concept.

Effect of Surface Chemistry on the Glass-Transition Dynamics of Poly(phenyl methyl siloxane) Confined in Alumina Nanopores

Broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) are combined to study the effect of changes in the surface chemistry on the segmental dynamics of glass-forming polymer, poly(methylphenylsiloxane) (PMPS), confined in anodized aluminum oxide (AAO) nanopores. Measurements were carried for native and silanized nanopores of the same pore sizes. Nanopore surfaces are modified with the use of two silanizing agents, chlorotrimethylsilane (ClTMS) and (3-aminopropyl)trimethoxysilane (APTMOS), of much different properties. The results of the dielectric studies have demonstrated that for the studied polymer located in 55 nm pores, changes in the surface chemistry and thermal treatment allows the confinement effect seen in temperature evolution of the segmental relaxation time, τα(T) to be removed. The bulk-like evolution of the segmental relaxation time can also be restored upon long-time annealing. Interestingly, the time scale of such equilibration process was found to be independent of the surface conditions. The calorimetric measurements reveal the presence of two glass-transition events in DSC thermograms of all considered systems, implying that the changes in the interfacial interactions introduced by silanization are not strong enough to inhibit the formation of the interfacial layer. Although DSC traces confirmed the two-glass-transition scenario, there is no clear evidence that vitrification of the interfacial layer affects τα(T) for nanopore-confined polymer.

Molecular dynamics simulations of propane in slit shaped silica nano-pores: direct comparison with quasielastic neutron scattering experiments

MD simulations reveal the origin of anomalous pressure dependence of propane diffusion in silica mesopores.

The peculiar behavior of the molecular dynamics of a glass-forming liquid confined in native porous materials - the role of negative pressure

In this paper, we combine Broadband Dielectric Spectroscopy (BDS) at ambient and high pressure, and positron annihilation lifetime spectroscopy (PALS) data of 2-ethylhexanol in the bulk state and when infiltrated in native silica nanopores to elucidate the relative role of surface effects on the Debye and structural relaxation processes under 2D spatial constraints. We show that the two processes have different sensitivities to (i) the changes in density as quantified by the EV/Hp ratio and (ii) the degree of confinement. Significant enhancement of the dynamics of the confined molecules at low temperatures is related to the vitrification of the interfacial molecules (Tg,int) affecting the packing density of the core molecules. This is corroborated by the PALS measurements, which demonstrated that the effective volume for the confined samples is slightly higher and seems to be temperature invariant below Tg,int. Consequently, negative pressure systematically develops with lowering temperature reaching values of -100 and -110 MPa (depending on the pore size) at the glass transition temperature. This result offers a better understanding of the counterbalance between surface and finite size effects as well as the role of negative pressure in controlling the dynamics and the glass transition of liquids under 2D spatial restrictions.

Role of density modulation in the spatially resolved dynamics of strongly confined liquids

Confinement by walls usually produces a strong modulation in the density of dense liquids near the walls. Using molecular dynamics simulations, we examine the effects of the density modulation on the spatially resolved dynamics of a liquid confined between two parallel walls, using a resolution of a fraction of the interparticle distance in the liquid. The local dynamics is quantified by the relaxation time associated with the temporal autocorrelation function of the local density. We find that this local relaxation time varies in phase with the density modulation. The amplitude of the spatial modulation of the relaxation time can be quite large, depending on the characteristics of the wall and thermodynamic parameters of the liquid. To disentangle the effects of confinement and density modulation on the spatially resolved dynamics, we compare the dynamics of a confined liquid with that of an unconfined one in which a similar density modulation is induced by an external potential. We find several differences indicating that density modulation alone cannot account for all the features seen in the spatially resolved dynamics of confined liquids. We also examine how the dynamics near a wall depends on the separation between the two walls and show that the features seen in our simulations persist in the limit of large wall separation.

Stabilization of the Amorphous Ezetimibe Drug by Confining Its Dimension

The purpose of this paper is to investigate the influence of nanoconfinement on the molecular mobility, as well as on the physical stability, of amorphous ezetimibe drug. Two guest/host systems, ezetimibe-Aeroperl 300 and ezetimibe-Neusilin US2, were prepared and studied using various experimental techniques, such as X-ray diffraction (XRD), differential scanning calorimetry (DSC), and broadband dielectric spectroscopy (BDS). Our investigation has shown that the molecular mobility of the examined anticholesterol agent incorporated into nanopore matrices strongly depends on the pore size of the host system. Moreover, it was found that the amorphous ezetimibe confined in 30 nm pores of Aeroperl 300 has a tendency to recrystallize, while the drug incorporated into the smaller--5 nm--pores of Neusilin US2 is not able to crystallize. It has been shown that this significant stabilization of ezetimibe drug can be achieved by an interplay of three factors: changes in molecular dynamics of the confined amorphous drug, the immobilization effect of pore walls on a part of ezetimibe molecules, and the use of host materials with pores that are smaller than the critical size of the drug crystal nuclei.

Dynamics of aqueous binary glass-formers confined in MCM-41

Dielectric permittivity measurements were performed on water solutions of propylene glycol (PG) and propylene glycol monomethyl ether (PGME) confined in 21 Å pores of the silica matrix MCM-41 C10 in wide frequency (10(-2)-10(6) Hz) and temperature (130-250 K) ranges. The aim was to elucidate how the formation of large hydrogen bonded structural entities, found in bulk solutions of PGME, was affected by the confined geometry, and to make comparisons with the dynamic behavior of the PG-water system. For all solutions the measurements revealed four almost concentration independent relaxation processes. The intensity of the fastest process is low compared to the other relaxation processes and might be caused by both hydroxyl groups of the pore surfaces and by local motions of water and solute molecules. The second fastest process contains contributions from both the main water relaxation as well as the intrinsic β-relaxation of the solute molecules. The third fastest process is the viscosity related α-relaxation. Its concentration independency is very different compared to the findings for the corresponding bulk systems, particularly for the PGME-water system. The experimental data suggests that the surface interactions induce a micro-phase separation of the two liquids, resulting in a full molecular layer of water molecules coordinating to the hydrophilic hydroxyl groups on the surfaces of the silica pores. This, in turn, increases the geometrical confinement effect for the remaining solution even more and prevents the building up of the same type of larger structural entities in the PGME-water system as in the corresponding bulk solutions. The slowest process is mainly hidden in the high conductivity contribution at low frequencies, but its temperature dependence can be extracted for the PGME-water system. However, its origin is not fully clear, as will be discussed.

Structural origin of enhanced slow dynamics near a wall in glass-forming systems

Spatial confinement is known to induce a drastic change in the viscosity, relaxation times, and flow profile of liquids near the glass (or jamming) transition point. The essential underlying question is how a wall affects the dynamics of densely packed systems. Here we study this fundamental problem, using experiments on a driven granular hard-sphere liquid and numerical simulations of polydisperse and bidisperse colloidal liquids. The nearly hard-core nature of the particle-wall interaction provides an ideal opportunity to study purely geometrical confinement effects. We reveal that the slower dynamics near a wall is induced by wall-induced enhancement of 'glassy structural order', which is a manifestation of strong interparticle correlations. By generalizing the structure-dynamics relation for bulk systems, we find a quantitative relation between the structural relaxation time at a certain distance from a wall and the correlation length of glassy structural order there. Our finding suggests that glassy structural ordering may be the origin of the slow glassy dynamics of a supercooled liquid.

Viscous nonpolar liquids in confinement studied by mechanical solvation

Using surface selective triplet state solvation techniques, we study the relaxation behavior of nonpolar liquids near their glass transition in confinement to porous glass with 4 nm pore diameter. As specific liquid-surface interactions are not expected for alkanes, we ask whether a substantial frustration of the dynamics is a general phenomenon for such liquids, and how the effects change with surface silanization. We find that interfacial dynamics display relaxation times that are increased by orders of magnitude for several nonpolar molecular liquids. Silanization removes this frustration effect almost entirely. Regarding the comparison of native versus silanized pore surfaces, it is concluded that the strong adhesive forces involved in alkanes wetting silica can explain the sluggish interfacial dynamics. The findings are not consistent with finite size effects.

Inelastic neutron scattering study of a glass-forming liquid in soft confinement

We have studied the microscopic dynamics of a glass-forming liquid in the soft confinement formed by microemulsion droplets using inelastic neutron scattering. The confined liquid was propylene glycol, the outer, hydrophobic phase was decalin, and the surfactant sodium dioctylsulfosuccinate (AOT) with the same composition used before with other spectroscopic methods [L.-M. Wang, F. He and R. Richert, Phys. Rev. Lett., 2004, 92, 95701]. The inelastic neutron scattering experiments were carried out on several time-of-flight and backscattering spectrometers to cover a large dynamical range. A Fourier transform was used to combine the data in terms of the intermediate scattering function S(Q,t) on a time range from 0.1 ps to 2 ns. By using two isotopic compositions the scattering of the glass-former was separated from that of the matrix liquids. In general we found an acceleration of the glass-transition-related α relaxation in confinement combined with a moderate broadening of the relaxation time distribution. This effect is most pronounced for low temperatures (≤250 K) and fades out at about 270-300 K. In addition, inelastic scattering allowed us to observe the motion of the methyl group of propylene glycol and the vibrational dynamics in the glass. For the methyl group reorientation we also found an acceleration but a narrowing of the relaxation time distribution. The vibrational dynamics show that the glass-typical 'boson peak' of bulk propylene glycol is completely washed out in the microemulsion in contrast to all earlier studies using hard confinement, which observed a low-frequency cut-off.

Interfacial effects on vitrification of confined glass-forming liquids

Mesoporous silica phases, with uniform pores of dimensions in the 2-30 nm range, offer a uniquely well-defined environment for the study of the effects of two-dimensional spatial confinement on the properties of glass-forming liquids. We report observations by differential scanning calorimetry of the vitrification of o-terphenyl (OTP), salol, and glycerol in hexagonal mesoporous silica (MCM-41 and SBA-15) in a wide range of pore sizes from 2.6 to 26.4 nm. In agreement with previous studies, where a controlled porous glass is used as a solid matrix, the glass transition temperature for o-terphenyl diminishes with decreasing pore size. In contrast to OTP, glycerol shows a gradual increase in glass transition temperature, while in salol a slight reduction of glass transition temperature is observed, followed by an increase, which results in glass transition temperature indistinguishable from that of the bulk for the smallest pores. These results are discussed in terms of liquid-surface interactions in an interfacial layer, monitored by Fourier-transformed infrared spectroscopy in the study. The hydrogen bonding with silica surface silanols dominates the glass transition trends observed in salol and glycerol.

Structural and dynamical properties of guest molecules confined in mesoporous silica materials revealed by NMR

In the last fifteen years several novel porous silica materials, which are periodically structured on the mesoscopic length scale, have been synthesized. They are of broad interest for fundamental studies of surface-substrate interactions, for studies of the dynamics of guest molecules in confinement and for studies of the effect of confinement on the structural and thermophysical properties of fluids. Examples of such confinement effects include the change of the freezing and melting points or glass transitions of the confined liquids. These effects are studied by combinations of several NMR techniques, such as (15)N- and (2)H-solid-state NMR line shape analysis, MAS NMR and NMR diffusometry with physico-chemical characterization techniques such as nitrogen adsorption and small angle diffraction of neutrons or X-rays. This combination does not require crystalline samples or special clean and well defined surfaces such as conventional surface science techniques, but can work with typical ill-defined real world systems. The review discusses, after a short introduction, the salient features of these materials and the applied NMR experiments to give the reader a basic knowledge of the systems and the experiments. The rest of the review then focuses on the structural and dynamical properties of guest molecules confined in the mesoporous silica. It is shown that the confinement into the pores leads to fascinating new features of the guests, which are often not known for their bulk phases. These features depend strongly on the interplay of the their interactions with the silica surface and their mutual interactions.

2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

Solid state deuterium NMR has been used to study the molecular motion of d(6)-isobutyric acid (d(6)-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I-solid II transition temperature is not affected by confinement, a significant lowering of the liquid-solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I-liquid) of d(6)-iBA in the pores occurs at a temperature 20-45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.

Correlations between isobaric and isochoric fragilities and thermodynamical scaling exponent for glass-forming liquids

Two correlations concerning the isobaric and isochoric fragilities, m(P) and m(V), as well as the scaling exponent gamma reported by Casalini and Roland [Phys. Rev. E 72, 031503 (2005)] have been examined for several van der Waals and hydrogen-bonded glass formers. It has been pointed out that the correlations lead to some serious inconsistency with the exponent gamma, which is expected to be a constant dependent only on material, but varies also on pressure if experimentally found pressure dependences of m(P) are taken into account. This problem could be solved in the case of van der Waals liquids, but then at least one of the correlations becomes dependent on thermodynamic conditions, and consequently, loses its universality. However, some H-bonded systems, due to properties of hydrogen bonding, have been well argued not to be included to determine the correlations independently of thermodynamic conditions. Furthermore, it has been noticed that another correlation concerning the fragility, between m(P) and the structural relaxation peak breadth, yields discrepancies in comparison with results of experiments under elevated pressure.

A comparative molecular simulation study of the glass former ortho-terphenyl in bulk and freestanding films

We performed molecular dynamics simulations of the low-molecular weight organic glass former ortho-terphenyl in bulk and freestanding films. The main motivation is to provide molecular insight into the confinement effect without explicit interfaces. Based on earlier models of ortho-terphenyl we developed an atomistic model for bulk simulations. The model reproduces literature data both from simulations and experiments starting from specific volume and diffusivity to mean square displacement and radial distribution functions. After characterizing the bulk model we form freestanding films by the elongation and expansion method. These films give us the opportunity to study the dynamical heterogeneity near the glass transition through in-plane mobility and reorientation dynamics. We finally compare the model in bulk and under confinement. We found qualitatively a lower glass transition temperature for the freestanding film compared to the bulk.

Dielectric relaxation processes in water mixtures of tripropylene glycol

Broadband dielectric measurements for anhydrous tripropylene glycol (3PG) and 96, 92, 84, 80, 74, 71, and 68 wt % 3PG-water mixtures are performed in the frequency range of 10(-2)-10(7) Hz and in the temperature range of 123-243 K. We examined the effect of adding water into anhydrous 3PG on relaxation dynamics. Apart from the two well-known relaxation processes, i.e., alpha and beta for anhydrous 3PG we observed new relaxation peak (beta') for all aqueous mixtures of 3PG. In addition we found the critical mole fraction of water x(w)=0.67 in which relaxation dynamics changes its behavior. According to the Sudo approach [S. Sudo et al., J. Non-Cryst. Solids 307-310, 356 (2002)], the behavior of relaxation processes was interpreted assuming the existence of three kinds of cooperative domains (CDs): containing only 3PG molecules, including only water molecules, and including both 3PG and water molecules, which molecules of each kind CD are bound by hydrogen bonds.

Dynamics of propylene glycol and its oligomers confined to a single molecular layer

The dynamics of propylene glycol (PG) and its oligomers 7-PG and poly-propylene glycol (PPG), with M(w) = 4000 (approximately 70 monomers), confined in a Na-vermiculite clay have been investigated by quasielastic neutron scattering. The liquids are confined to single molecular layers between clay platelets, giving a true two-dimensional liquid. Data from three different spectrometers of different resolutions were Fourier transformed to S(Q,t) and combined to give an extended dynamical time range of 0.3-2000 ps. An attempt was made to distinguish the diffusive motion from the methyl group rotation and a fast local motion of hydrogen in the polymer backbone. The results show that the average relaxation time tau(d) of this diffusive process is, as expected, larger than the relaxation time tau averaged over all dynamical processes observed in the experimental time window. More interesting, it is evident that the severe confinement has a relatively small effect on tau(d) at T = 300 K, this holds particularly for the longest oligomer, PPG. The most significant difference is that the chain-length dependence of tau(d) is weaker for the confined liquids, although the slowing down in bulk PG due to the formation of a three-dimensional network of OH-bonded end groups reduces this difference. The estimated average relaxation time tau at Q = 0.92 Angstroms(-1) for all the observed processes is in excellent agreement with the previously reported dielectric alpha relaxation time in the studied temperature range of 260-380 K. The average relaxation time tau (as well as the dielectric alpha relaxation time) is also almost unaffected by the confinement to a single molecular layer, suggesting that the interaction with the clay surfaces is weak and that the reduced dimensionality has only a weak influence on the time scale of all the dynamical processes observed in this study.

Influence of nanosized confinements on 4-n-decyl-4'-cyanobiphenyl (10CB): a broadband dielectric study

Real (epsilon') and imaginary (epsilon") parts of the complex dielectric permittivity (epsilon*) of the liquid crystal (LC) 4-n-decyl-4'-cyanobiphenyl (10CB) embedded in Anopore membranes and Vycor porous glass, as well as dispersed with hydrophilic aerosils, have been studied by means of broadband dielectric spectroscopy in the frequency range from 10(-2) Hz to 1 GHz . In bulk 10CB, which has a direct transition from an isotropic to a smectic- A phase, there exists one main relaxation process for the parallel orientation of the director with respect to the probing field and a faster one for the perpendicular orientation. All molecular relaxation processes in 10CB are of Debye type and have Arrhenius like temperature dependence. For 10CB embedded in untreated and surface treated cylindrical pores of Anopore membranes the dielectric spectra are similar to the bulk with the exception that both the rotation around the short axis and the libration motion are faster in the pores. In the case of 10CB dispersed with two different concentrations of hydrophilic aerosils an emergence of a slow relaxation process, which is stronger for the higher concentration, is additionally observed along with the bulklike processes. The slow process in the LC-hydrophilic aerosil system is attributed to the relaxation of the molecules that are homeotropically aligned close to the surfaces of the aerosil particles. This process also has an Arrhenius type of temperature dependence. For 10CB embedded in narrow channels of Vycor porous glass three relaxation processes are observed. Two of these processes are bulklike and are due to the librational motion of molecules and the rotation of molecules around their short axes. The slowest process seems to be a surface process, similar in origin to that observed for 10CB dispersed with hydrophilic aerosils, and is prominent amongst all. The material in the Vycor porous glass could be supercooled by at least 185 degrees below bulk crystallization temperature. The slow process has a Vogel-Fulcher-Tamman (VFT) type of temperature dependence typical for glass formers in this wide temperature range. In addition, the bulklike processes are found to be strongly modified and also have a VFT like temperature dependence from measurements done in a wide temperature range. This behavior is in contrast to other reports of glassy behavior in confined LC, where the glassy behavior as concluded from a slow relaxation process observed in a narrow temperature range.