J. Gapinski

Spectrum of fast dynamics in glass forming liquids: Does the "knee＇＇ exist?

A knee-shaped feature observed earlier in light scattering spectra of Ca0.4K0.3(NO3)1.4 (CKN) below Tc is used as a strong argument in favor of mode-coupling theory of the glass transition (MCT). Our careful measurements reveal no “knee” in the spectra of two glass forming liquids, CKN and ortho-terphenyl. Instead of the knee the spectra show nontrivial broadening and an increase of the intensity with a temperature increase. Both variations are confirmed by neutron scattering measurements on CKN and are neither expected in the asymptotic MCT predictions nor in any other model.

Diffusion and microstructural properties of solutions of charged nanosized proteins: Experiment versus theory

We have reanalyzed our former static small-angle x-ray scattering and photon correlation spectroscopy results on dense solutions of charged spherical apoferritin proteins using theories recently developed for studies of colloids. The static structure factors S(q), and the small-wave-number collective diffusion coefficient D(c) determined from those experiments are interpreted now in terms of a theoretical scheme based on a Derjaguin-Landau-Verwey-Overbeek-type continuum model of charged colloidal spheres. This scheme accounts, in an approximate way, for many-body hydrodynamic interactions. Stokesian dynamics computer simulations of the hydrodynamic function have been performed for the first time for dense charge-stabilized dispersions to assess the accuracy of the theoretical scheme. We show that the continuum model allows for a consistent description of all experimental results, and that the effective particle charge is dependent upon the protein concentration relative to the added salt concentration. In addition, we discuss the consequences of small ions dynamics for the collective protein diffusion within the framework of the coupled-mode theory.

Does fragility depend on pressure? A dynamic light scattering study of a fragile glass-former

Relaxation times of the α-process in the fragile glass-forming liquid diglycidyl ether of bisphenol-A (EPON 828) were measured in a broad pressure (1–1500 bar) and temperature (264–293 K) ranges by means of the depolarized dynamic light scattering—photon correlation spectroscopy. Based on this experimental data the fragility of the supercooled liquid was calculated in two ways: as a steepness index m of the “Angell plot” and as the DT-parameter from the Vogel–Fulcher–Tammann Law, and was studied as a function of pressure. It was found, that while the steepness index depends on pressure, the DT parameter is pressure independent. The pressure dependence of the glass transition temperature Tg in EPON 828 was found to be nonlinear. Additionally, we established a relationship between the steepness index mT, the activation volume ΔV#, and the coefficient ∂Tg/∂Pg. In this pressure dependent study we found that also for EPON 828 the nonexponentiality of the correlation function of the α-process correlates well wi...

Pressure and temperature dependence of structural relaxation in diglycidylether of bisphenol A

The structural (α-) relaxation in diglycidylether of bisphenol A (DGEBA) has been examined using three spectroscopic methods: dielectric spectroscopy (DS), dynamic light scattering–photon correlation spectroscopy (LS), and mechanical spectroscopy. The DS and LS measurements were carried out as a function of both temperature and pressure. Moreover, pressure-volumetemperature measurements were obtained for the DGEBA. These data allow an assessment of the relative contributions of thermal energy and free volume to structural relaxation in DGEBA. The results clearly show a substantial role for both thermal and free volume fluctuations in the dramatic slowing down of the dynamics. The combined temperature- and pressure-dependences of the dielectric and light scattering relaxation times were analyzed using the Avramov equation, implying that the fragility (normalized temperature dependence) is pressure independent over the studied range of pressures. The pressure dependence was the same as measured by the diffe...

Interparticle correlations due to electrostatic interactions: A small angle x-ray and dynamic light scattering study. I. Apoferritin

The structure and dynamics of the spherical protein Apoferritin in aqueous solution are studied over a wide range of protein concentrations and ionic strengths. At high ionic strength and low protein concentration, the intermolecular forces are screened and, therefore, the proteins behave like uncharged molecules. Under these conditions, the form factor of Apoferritin was measured by means of small angle x-ray scattering (SAXS) and the hydrodynamic radius was determined by means of dynamic light scattering (DLS). The sample was found to be highly monodisperse. By decreasing the content of salt added, interactions between the Apoferritin particles were initiated. These intermolecular forces lead to a pronounced maximum in the SAXS intensity. At the same time, a slow mode appears in the relaxation time distribution, additionally to the diffusive mode. The relative amplitudes and correlation times of the diffusive and the slow mode were investigated and compared with predictions of the coupled mode theory. By assuming the slow mode to be related to the correlated motion of ordered domains, the size of these domains was derived from the slow relaxation time. From the x-ray data and the Apoferritin form factor, structure factors of ordered solutions were calculated. The shape of the structure factor peaks was studied as a function of Apoferritin and salt concentration. Finally, by using the DLS information regarding the size of correlated domains, we analyzed the degree of polyelectrolyte ordering within the paracrystalline domains in Apoferritin solutions.

Pressure effects on the α and α' relaxations in polymethylphenylsiloxane

In some polymers, in addition to the usual structural α relaxation, a slower α′ relaxation is observed with a non-Arrhenius temperature dependence. In order to understand better the molecular origin of this α′ relaxation in poly(methylphenylsiloxane) (PMPS) we have studied, for the first time, the pressure dependence of its relaxation time, together with the usual temperature dependence, by means of dynamic light scattering (DLS). For the same material the α relaxation was also studied by means of DLS and dielectric spectroscopy (DS) in broad temperature and pressure ranges. We find that the temperature dependence of both α and α′ relaxation times, at all pressures studied, can be described by a double Vogel-Fulcher-Tammann (VFT) law. The pressure dependence of the characteristic temperatures Tg (glass transition temperature) and T0 (Vogel temperature) as well as the activation volumes for both α and α′ processes are very similar, indicating, that both relaxation processes originate from similar local mol...

Effect of electrostatic interactions on the structure and dynamics of a model polyelectrolyte. II. Intermolecular correlations

The peak in the small angle x-ray scattering and the dynamic light-scattering slow mode for a 20 base-pair duplex oligonucleotide (“B-DNA”) are studied as functions of oligonucleotide and added-salt (NaCl) concentrations. Both the x-ray peak intensity and the relative intensity of the slow mode decrease as the added-salt concentration is increased. The hydrodynamic radius of the slow mode increases as the added-salt concentration is decreased. The x-ray peak gradually disappears with increasing salt while the slow mode decreases in intensity, but still has some residual intensity at the highest added-salt concentration studied. There is no abrupt change in either the peak or the slow mode with increasing salt. The existence and behavior of both the x-ray peak and the slow mode indicate local ordering in the solution due to electrostatic forces. The x-ray peak position for the oligonucleotide is correlated with the static light-scattering peak seen by other workers for dilute solutions of larger polyions. ...

Effect of electrostatic interactions on the structure and dynamics of a model polyelectrolyte. I. Diffusion

The dynamics of a 20 base pair oligonucleotide is studied by dynamic light scattering-photon correlation spectroscopy and depolarized Fabry–Perot interferometry. The 20 base pair oligonucleotide is a well-defined, albeit short, rigid rod molecule that serves as a model for polyelectrolyte solution dynamics. The effects of added salt on the solution rotational and translational dynamics are examined in detail as functions of the 20-mer concentration. Coupled mode theory together with counterion condensation theory gives good predictions for the effects of salt on the translational diffusion of the 20-mer at the relatively low oligonucleotide concentrations studied. Comparison of the experimental results with these theories shows that the effective charge density of the polyion in solution is approximately equal to the reciprocal of the product of the Bjerrum length and the counterion charge, νeff≅1/NλB. Calculation shows that the numerical solution of the coupled mode theory matrix gives a better fit of ou...

Collective diffusion in charge-stabilized suspensions: Concentration and salt effects

The authors present a joint experimental-theoretical study of collective diffusion properties in aqueous suspensions of charge-stabilized fluorinated latex spheres. Small-angle x-ray scattering and x-ray photon correlation spectroscopy have been used to explore the concentration and ionic-strength dependence of the static and short-time dynamic properties including the hydrodynamic function H(q), the wave-number-dependent collective diffusion coefficient D(q), and the intermediate scattering function over the entire accessible range. They show that all experimental data can be quantitatively described and explained by means of a recently developed accelerated Stokesian dynamics simulation method, in combination with a modified hydrodynamic many-body theory. In particular, the behavior of H(q) for de-ionized and dense suspensions can be attributed to the influence of many-body hydrodynamics, without any need for postulating hydrodynamic screening to be present, as it was done in earlier work. Upper and lower boundaries are provided for the peak height of the hydrodynamic function and for the short-time self-diffusion coefficient over the entire range of added salt concentrations.

Effect of electrostatic interactions on the dynamics of semiflexible monodisperse DNA fragments

The dynamics of three monodisperse linear duplex DNA fragments—a 2311 base pair restriction fragment and 1500 and 1100 base pair polymerase chain reaction fragments—in dilute solution are studied as functions of added salt (NaCl) concentration by dynamic light scattering-photon correlation spectroscopy. Translational diffusion coefficients and intramolecular relaxation times are extracted from the measured light scattering intensity time autocorrelation functions as the added salt concentration is reduced from 100 mM to approximately 0.1 mM. The relaxation times of the first intramolecular mode increase as the added salt concentration is lowered. The dependence of the translational diffusion coefficient D on the added salt concentration is not very large, although it exhibits a maximum for all three fragments. The maximum is interpreted as the consequence of two opposing effects—the stiffening of the molecule that produces an increase of the size (decrease of D) as the added salt concentration is lowered,...