Correlation between collective and molecular dynamics in pH-responsive cyclodextrin-based hydrogels.

UV Raman and Brillouin light scattering (BLS) experiments have been used in this study to explore the complex phase change behavior occurring in pH-responsive polysaccharide hydrogels as a function of temperature. Due to the different physical quantities measured by the two techniques, the joint analysis of Raman and BLS spectra has provided an unprecedented large-scale characterization of the molecular rearrangements and of the different kinds of hydrophilic and hydrophobic interactions that cooperate to determine the phase transformation observed in these hydrogels during the heating of the gel. As the main result, the analysis of the Raman and BLS spectra showed the existence of a correlation between the local (molecular) and collective properties of the gels during the phase transformation undergone by the system, which is markedly triggered by pH. The joint set of experimental results suggests a model according to which the mechanism of pH dependence in the hydrogels under investigation is dominated by the interactions involving the hydrophobic parts of the polymer skeleton, whereas the solvation process observed under heating of the gels is driven by the progressive distancing of the polymer domains among them, as monitored by the Brillouin sound velocity.

Molecular properties of aqueous solutions: a focus on the collective dynamics of hydration water.

When a solute is dissolved in water, their mutual interactions determine the molecular properties of the solute on one hand, and the structure and dynamics of the surrounding water particles (the so-called hydration water) on the other. The very existence of soft matter and its peculiar properties are largely due to the wide variety of possible water-solute interactions. In this context, water is not an inert medium but rather an active component, and hydration water plays a crucial role in determining the structure, stability, dynamics, and function of matter. This review focuses on the collective dynamics of hydration water in terms of retardation with respect to the bulk, and of the number of molecules whose dynamics is perturbed. Since water environments are in a dynamic equilibrium, with molecules continuously exchanging from around the solute towards the bulk and vice versa, we examine the ability of different techniques to measure the water dynamics on the basis of the explored time scales and exchange rates. Special emphasis is given to the collective dynamics probed by extended depolarized light scattering and we discuss whether and to what extent the results obtained in aqueous solutions of small molecules can be extrapolated to the case of large biomacromolecules. In fact, recent experiments performed on solutions of increasing complexity clearly indicate that a reductionist approach is not adequate to describe their collective dynamics. We conclude this review by presenting current ideas that are being developed to describe the dynamics of water interacting with macromolecules.

Aqueous solvation of amphiphilic molecules by extended depolarized light scattering: the case of trimethylamine-N-oxide.

Hydrophilic and hydrophobic interactions strongly affect the solvation dynamics of biomolecules. To understand their role, small model systems are generally employed to simplify the investigations. In this study the amphiphile trimethylamine N-oxide (TMAO) is chosen as an exemplar, and studied by means of extended frequency range depolarized light scattering (EDLS) experiments as a function of solute concentration. This technique proves to be a suitable tool for investigating different aspects of aqueous solvation, being able at the same time to provide information about relaxation processes and vibrational modes of solvent and solute. In the case study of TMAO, we find that the relaxation dynamics of hydration water is moderately retarded compared to the bulk, and the perturbation induced by the solute on surrounding water is confined to the first hydration shell. The results highlight the hydrophobic character of TMAO in its interaction with water. The number of molecules taking part in the solvation process decreases as the solute concentration increases, following a trend consistent with the hydration water-sharing model, and suggesting that aggregation between solute molecules is negligible. Finally, the analysis of the resonant modes in the THz region and the comparison with the corresponding results obtained for the isosteric molecule tert-butyl alcohol (TBA) allow us to provide new insights into the different solvating properties of these two biologically relevant molecules.

Hydrophobic Hydration in Water-tert-Butyl Alcohol Solutions by Extended Depolarized Light Scattering.

Molecular dynamics and structural properties of water-tert-butyl alcohol (TBA) mixtures are studied as a function of concentration by extended depolarized light scattering (EDLS) experiments. The wide frequency range, going from fraction to several thousand GHz, explored by EDLS allows distinguishing TBA rotational dynamics from structural relaxation of water and intermolecular vibrational and librational modes of the solution. Contributions to the water relaxation originating from two distinct populations, i.e. hydration and bulk water, are clearly identified. The dynamic retardation factor of hydration water with respect to the bulk, ξ ≈ 4, almost concentration independent, is one of the smallest found by EDLS among a variety of systems of different nature and complexity. This result, together with the small number of water molecules perturbed by the presence of TBA, supports the idea that hydrophobic simple molecules are less effective than hydrophilic and more complex molecules in perturbing the H-bond network of liquid water. At increasing TBA concentrations the average number of perturbed water molecules shows a pronounced decrease and the characteristic frequency of librational motions reduces significantly, both of which are results consistent with the occurrence of self-aggregation of TBA molecules.

Stress effects on the elastic properties of amorphous polymeric materials.

Brillouin light scattering measurements have been used to study the stress induced modification in the elastic properties of two glass forming polymers: polybutadiene and epoxy-amine resin, prototypes of linear and network polymers, respectively. Following the usual thermodynamic path to the glass transition, polybutadiene has been studied as a function of temperature from the liquid well into the glassy phase. In the epoxy resin, the experiments took advantage of the system ability to reach the glass both via the chemical vitrification route, i.e., by increasing the number of covalent bonds among the constituent molecules, as well as via the physical thermal route, i.e., by decreasing the temperature. Independently from the particular way chosen to reach the glassy phase, the measurements reveal the signature of long range tensile stresses development in the glass. The stress presence modifies both the value of the sound velocities and their mutual relationship, so as to break the generalized Cauchy-like relation. In particular, when long range stresses, by improvise sample cracking, are released, the frequency of longitudinal acoustic modes increases more than 10% in polybutadiene and ∼4% in the epoxy resin. The data analysis suggests the presence of at least two different mechanisms acting on different length scales which strongly affect the overall elastic behaviour of the systems: (i) the development of tensile stress acting as a negative pressure and (ii) the development of anisotropy which increases its importance deeper and deeper in the glassy state.

Hydration and rotational diffusion of levoglucosan in aqueous solutions.

Extended frequency range depolarized light scattering measurements of water-levoglucosan solutions are reported at different concentrations and temperatures to assess the effect of the presence and distribution of hydroxyl groups on the dynamics of hydration water. The anhydro bridge, reducing from five to three the number of hydroxyl groups with respect to glucose, considerably affects the hydration properties of levoglucosan with respect to those of mono and disaccharides. In particular, we find that the average retardation of water dynamics is ≈3-4, that is lower than ≈5-6 previously found in glucose, fructose, trehalose, and sucrose. Conversely, the average number of retarded water molecules around levoglucosan is 24, almost double that found in water-glucose mixtures. These results suggest that the ability of sugar molecules to form H-bonds through hydroxyl groups with surrounding water, while producing a more effective retardation, it drastically reduces the spatial extent of the perturbation on the H-bond network. In addition, the analysis of the concentration dependence of the hydration number reveals the aptitude of levoglucosan to produce large aggregates in solution. The analysis of shear viscosity and rotational diffusion time suggests a very short lifetime for these aggregates, typically faster than ≈20 ps.

Modeling the crossover between chemically and diffusion-controlled irreversible aggregation in a small-functionality gel-forming system.

The analysis of realistic numerical simulations of a gel-forming irreversible aggregation process provides information on the role of cluster diffusion in controlling the late stages of the aggregation kinetics. Interestingly, the crossover from chemically controlled to diffusion-controlled aggregation takes place well beyond percolation, after most of the particles have aggregated in the spanning network and only small clusters remain in the sol. The simulation data are scrutinized to gain insight into the origin of this crossover. We show that a single additional time scale (related to the average diffusion time) is sufficient to provide an accurate description of the evolution of the extent of reaction at all times.

Raman-scattering measurements of the vibrational density of states of a reactive mixture during polymerization: effect on the boson peak.

Raman-scattering measurements are used to follow the modification of the vibrational density of states in a reactive epoxy-amine mixture during isothermal polymerization. Combining them with Brillouin light and inelastic x-ray scattering measurements, we analyze the variations of the boson peak and of the Debye level while the system changes from liquid to glass upon increasing the number of covalent bonds among the constituent molecules. The shift and intensity variation of the boson peak are explained by the modification of the elastic properties throughout the reaction, and a master curve for the boson peak can therefore be obtained. Surprisingly, bond-induced modifications of the structure do not affect this master curve.

Connecting irreversible to reversible aggregation: time and temperature.

We report molecular dynamics simulations of a gel-forming mixture of ellipsoidal patchy particles with different functionality. We show that in this model, which disfavors the formation of bond-loops, elapsed time during irreversible aggregation--leading to the formation of an extended network--can be formally correlated with equilibrium temperature in reversible aggregation. We also show that it is possible to develop a parameter-free description of the self-assembly kinetics, bringing reversible and irreversible aggregation of loopless branched systems to the same level of understanding as equilibrium polymerization.

Bond-induced ergodicity breakdown in reactive mixtures.

We have studied by inelastic x-ray scattering, at wave vectors 1 nm < or =q< or =15 nm(-1), the high-frequency dynamics of epoxy-amine mixtures as the monomers irreversibly polymerize. We find that chemical bonding, while inducing molecular ordering on a mesoscopic length scale, also efficiently realizes the mechanism of dynamical arrest described by the mode-coupling theory, as manifested by a cusp singularity in the behavior of the nonergodicity factor as a function of the number of chemical bonds. These results confront positively the mode-coupling theory with a new control parameter.

Ergodic to nonergodic transition in liquids with a local order: the case of m-toluidine.

The dynamic structure factor of m-toluidine has been measured by inelastic x-ray scattering in the mesoscopic Q range between 1 and 10 nm(-1), where a prepeak is revealed in the static structure factor resulting from the existence of hydrogen bonded, nanometer size clusters. Evidence is given of (i) a square-root cusp in the nonergodicity factor and of (ii) critical nonergodicity parameters which oscillate in phase with the static structure factor. These results demonstrate that local order in a liquid can coexist with the signatures of the ergodic to nonergodic transition predicted by the mode coupling theory for simple, dense liquids.

Clustering and cooperative dynamics in a reactive system.

We study the dependence of the dynamics on the size of particle clusters that grow by stepwise aggregation in a reactive epoxy-amine mixture. The data reveal the cluster property involved in the glasslike arrest and its quantitative link with the structural relaxation time. We find that the number-average cluster size xn governs the formation of a glassy phase as distinct from a gel phase, and that xn correlates to the size of the "cooperatively rearranging regions" postulated by the Adam-Gibbs model for glass forming liquids. These results suggest that the step polymerization process generates clusters that behave much like dynamical heterogeneities observed in supercooled liquids.

Slow dynamics of salol: a pressure- and temperature-dependent light scattering study.

We study the slow dynamics of salol by varying both temperature and pressure using photon correlation spectroscopy and pressure-volume-temperature measurements, and compare the behavior of the structural relaxation time with equations derived within the Adam-Gibbs entropy theory and the Cohen-Grest free volume theory. We find that pressure-dependent data are crucial to assess the validity of these model equations. Our analysis supports the entropy-based equation, and estimates the configurational entropy of salol at ambient pressure approximately 70% of the excess entropy. Finally, we investigate the evolution of the shape of the structural relaxation process, and find that a time-temperature-pressure superposition principle holds over the range investigated.

Can experiments select the configurational component of excess entropy?

Configurational entropy is frequently used to rationalize the structural dynamics of glass-forming liquids. The main problem with this concept is that it is not directly accessible to experiments. We introduce a procedure to estimate the configurational component of the excess entropy of a liquid --specifically, the configurational-entropy contribution from the structural relaxation process-- through a combined investigation of dynamic and thermodynamic properties as functions of temperature and pressure. We test our method on orthoterphenyl, salol, and glycerol, and find that the fraction of excess entropy that arises from structural configurations is about 70% for all three materials.

Effect of pressure on the dynamics of glass formers.

A description of the pressure dependence of the structural relaxation time has been derived from the Adam-Gibbs theory by writing the configurational entropy in terms of the excess heat capacity and the molar thermal expansion. This new equation was tested successfully on dielectric relaxation data for an epoxy compound over a wide range of temperature and pressure.

Pressure dependence of structural relaxation time in terms of the Adam-Gibbs model.

A new equation describing the behavior of the structural relaxation time, tau(T,P), as a function of both pressure and temperature, is discussed. This equation has been derived from the Adam-Gibbs theory by writing the configurational entropy, S(c), in terms of the excess thermal heat capacity and of the molar thermal expansion. Consequently, the parameters introduced in the expression are directly related to specific physical properties of the material, such as the thermal expansion coefficient alpha and the isothermal bulk modulus K0. At a fixed pressure, for low pressures, the found equation reduces to a Vogel-Fulcher-Tammann equation of tau versus temperature with the fragility parameter independent from pressure. The equation for tau(T,P) was successfully tested directly by fitting the dielectric relaxation time data for two isothermal and one isobaric measurements on diglycidyl ether of bisphenol-A, carried out in previous experiments. The parameters estimated by the best fit were in reasonable agreement with the values determined from the known physical properties of the material. Finally, the expression for the change versus pressure of the temperatures at which the same value of tau(max) is obtained (e.g., the change versus pressure of the glass transition temperature) agrees with several expressions previously proposed in the literature to provide a phenomenological description of the observed phenomena.

Influence of temperature and pressure on dielectric relaxation in a supercooled epoxy resin.

Isothermal and isobaric dielectric measurements of a supercooled epoxy resin have been compared. A simple scaling relates isobaric and isothermal spectra corresponding to the same frequency of the main loss peak. Thus, the main and secondary processes retain a relative weight that is the same under isothermal and isobaric conditions. It is inferred that both pressure and temperature, equivalently, are able to take effect on the relaxation processes, without changing the relaxation mechanism itself. Careful analysis of the structural relaxation time behavior revealed that the traditional free volume equation, where only the macroscopic volume controls the pressure evolution of free volume, is not a suitable description of the data, as well as a Vogel-Fulcher (VF) type pressure dependent function. Based on a derivative method, a different function for describing the bidimensional surface tau(T,P) has been proposed, which accounts for the observed behavior through a nonlinear correction of the critical temperature T0 in the VF law. The function we propose predicts pressure dependencies of the glass transition temperature and fragility which are appealing in view of a comparison with experimental results in this and many other systems. Interesting hints for interpreting the phenomenological results can be obtained within the Adam-Gibbs theory.