Søren Hvidt

Roskilde University, Science, Systems and Models, Faculty Member

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Papers by Søren Hvidt

Structure of protic ionic liquids studies by NMR, IR and DFT calculations. An isotope effect study

Micellization and Dynamics of a Block Copolymer

Gels of a triblock copolymer in water consist of overlapping spherical micelles

Rheology of crude oils containing asphaltenes and wax

Rheological studies on novel high performance polyarylethersulfones

The flow properties of three different synthesized series of random copolymers of poly(aryl ether... more The flow properties of three different synthesized series of random copolymers of poly(aryl ether sulfone) (PES), poly aryl ether ether sulfone (PEES), and poly aryl ether biphenyl ether sulfone(PEBES) have been studied. The dependence of the glass transition temperature (Tg) on the chemical structure is explored. The rheology studies are done as functions of copolymer compositions at different temperatures above Tg.

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Exo-exo synergy between Cel6A and Cel7A from Hypocrea jecorina: Role of carbohydrate binding module and the endo-lytic character of the enzymes

Biotechnology and bioengineering, Aug 28, 2017

Synergy between cellulolytic enzymes is essential in both natural and industrial breakdown of bio... more Synergy between cellulolytic enzymes is essential in both natural and industrial breakdown of biomass. In addition to synergy between endo- and exo-lytic enzymes, a lesser known but equally conspicuous synergy occurs among exo-acting, processive cellobiohydrolases (CBHs) such as Cel7A and Cel6A from Hypocrea jecorina. We studied this system using microcrystalline cellulose as substrate and found a degree of synergy between 1.3 and 2.2 depending on the experimental conditions. Synergy between enzyme variants without the carbohydrate binding module (CBM) and its linker was strongly reduced compared to the wild types. One plausible interpretation of this is that exo-exo synergy depends on the targeting role of the CBM. Many earlier works have proposed that exo-exo synergy was caused by an auxiliary endo-lytic activity of Cel6A. However, biochemical data from different assays suggested that the endo-lytic activity of both Cel6A and Cel7A were 10(3) -10(4) times lower than the common end...

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A Reinvestigation of the Ionic Liquid Diisopropylethylammonium Formate by NMR and DFT Methods

The Journal of Physical Chemistry B, 2016

The complex between diisopropylethylamine (DIPEA) and formic acid has been reinvestigated. Mixing... more The complex between diisopropylethylamine (DIPEA) and formic acid has been reinvestigated. Mixing the compounds in the ratio 1:1 leads to a phase separation in which the upper phase is DIPEA and the lower phase is the "ionic liquid" named DIPEF. A combined NMR and DFT study shows that the lower phase primarily is formic acid:formate and diisopropylammonium ions in the ratio 2:1 (acid:base) plus the formic acid dimer. Addition of more acid leads to more and more of the acid dimer. The proton transfer in the system is 65-80%. The structural picture presented in this paper is very different from that presented elsewhere. However, the present picture should be considered using acids and bases with a pK a difference less than 8. The formic acid content in the DIPEF ionic liquid causes desorption of the dye-sensitized solar cell (DSC) dye N719 from the photo anode, and DIPEF is therefore not a suitable electrolyte for DSCs.

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Towards a phenomenological definition of the term ‘gel’

Polymer Gels and Networks, 1993

The term 'gel' is used so indiscriminately that it has become ambiguous. Existing definitions are... more The term 'gel' is used so indiscriminately that it has become ambiguous. Existing definitions are reviewed, examples of unfortunate uses of the term are discussed, and important phenomenological characteristics of gels are identified. We propose that the term 'gel' should be limited to systems which fulfil the following phenomenological characteristics: (a) they consist of two or more components one of which is a liquid, present in substantial quantity and (b) they are soft, solid, or solid-like materials. We further propose a definition of the solid-like characteristics of gels in terms of the dynamic mechanical properties, viz. a storage modulus, G'(to), which exhibits a pronounced plateau extending to times at least of the order of seconds and a loss modulus, G"(to), which is considerably smaller than the storage modulus in the plateau region.

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Basic rheology for biologists

Methods in cell biology, 2007

Many cellular processes lead to changes in elastic and viscous properties of cells. Rheology is t... more Many cellular processes lead to changes in elastic and viscous properties of cells. Rheology is the science that deals with deformation and flow of materials. Fundamental rheologic concepts are explained, and some of the main techniques are discussed. Nonperturbing oscillatory techniques are especially useful for monitoring structure formation including gelation, whereas other techniques such as steady shear flow and creep are useful for determining flow properties. Sample preparation is often a major obstacle, and advantages of different deformation geometries are discussed. Simple biological samples such as purified biopolymers can be investigated with a range of rheologic techniques, and factors affecting gelation of, for example, blood or cytoskeletal proteins can be studied in detail. More complex biological systems such as intact tissues can often only be studied with more qualitative techniques and results. With proper choice of experimental setup, rheologic techniques can gi...

Optical Properties of Large Molecules in the Frenkel Exciton Approximation

Understanding Molecular Properties, 1987

The Frenkel exciton approximation provides a means for representing the excited states of large, ... more The Frenkel exciton approximation provides a means for representing the excited states of large, weakly-interacting systems in terms of the properties of their subunits. Frenkel exciton theory is reviewed, together with its application to the optical properties of large molecules and complexes, A general method is presented for calculating the absorption and circular dichroism spectra of large systems within the Frenkel exciton picture. It is shown that for systems consisting of weakly-interacting identical subunits, the rotatory power of a transition is proportional to tr[H F], where H is the interaction Hamiltonian and F is an “optical matrix” constructed from the transition dipole moments and positions of the individual subunits. Similarly it is shown that the absorption maximum is shifted in frequency by an amount which is proportional to tr[H G]/tr[G], where G is also constructed from the subunit transition dipole moments. The general theory is illustrated by a discussion of the optical properties of helical polymers.

Mathematical model quantifies multiple daylight exposure and burial events for rock surfaces using luminescence dating

Radiation Measurements, 2015

Interest in the optically stimulated luminescence (OSL) dating of rock surfaces has increased sig... more Interest in the optically stimulated luminescence (OSL) dating of rock surfaces has increased significantly over the last few years, as the potential of the method has been explored. It has been realized that luminescence depth profiles show qualitative evidence for multiple daylight exposure and burial events. To quantify both burial and exposure events a new mathematical model is developed by expanding existing models of evolution of luminescence-depth profiles, to include repeated sequential events of burial and exposure to daylight. This new model is applied to an infrared stimulated luminescence depth profile from a feldspar-rich granite cobble from an archaeological site near Aarhus, Denmark. This profile shows qualitative evidence for multiple daylight exposure and burial events; these are quantified using the model developed here. By determining the burial ages from the surface layer of the cobble and by fitting the new model to the luminescence profile, it is concluded that the cobble was well bleached before burial. This indicates that the OSL burial age is likely to be reliable. In addition, a recent known exposure event provides an approximate calibration for older daylight exposure events. This study confirms the suggestion that rock surfaces contain a record of exposure and burial history, and that these events can be quantified. The burial age of rock surfaces can thus be dated with confidence, based on a knowledge of their pre-burial light exposure; it may also be possible to determine the length of a fossil exposure, using a known natural light exposure as calibration.

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Dynamic Mechanical Properties

Science and Technology of Rubber, 1994

Publisher Summary This chapter focuses on various dynamic mechanical properties and viscoelastic ... more Publisher Summary This chapter focuses on various dynamic mechanical properties and viscoelastic functions. Rubber is frequently used for applications in which it undergoes rapid cyclic deformations at a certain frequency or over a range of frequencies. The dynamic mechanical properties are strongly dependent on temperature, frequency, the presence of fillers, and the extent of deformation if it is large. For very small deformations, the properties are found to be independent of the magnitude of the deformation (linear viscoelasticity), and temperature–frequency superposition can be applied in many cases, a feature that greatly facilitates obtaining or predicting the dynamic response over a wide frequency range. It is important to note that the dynamic mechanical properties of different unfilled amorphous elastomers are quite similar when the proper reference state is used. Dynamic mechanical properties refer generally to responses to periodically varying strains or stresses. They are most simply defined for a small sinusoidally varying strain or stress for which the response is a small sinusoidally varying stress or strain, respectively, with the same frequency but generally out of phase.

Viscosity of dilute and semi-dilute aqueous pectin solutions

ANNUAL TRANSACTIONS-NORDIC …, 1997

... Intrinsic viscosity values were determined by the following methods of calculation: Huggins-K... more

Thermodynamic investigations of methyl tert-butyl ether and water mixtures

Phys. Chem. Chem. Phys., 2011

Interactions between methyl tert-butyl ether (MTBE) and water have been investigated by scanning ... more Interactions between methyl tert-butyl ether (MTBE) and water have been investigated by scanning calorimetry, isothermal titration calorimetry, densitometry, IR-spectroscopy, and gas chromatography. The solubilization of MTBE in water at 25 °C at infinite dilution has ΔH° = -17.0 ± 0.6 kJ mol(-1); ΔS° = -80 ± 2 J mol(-1) K(-1); ΔC(p) = +332 ± 15 J mol(-1) K(-1); ΔV° = -18 ± 2 cm(3) mol(-1). The signs of these thermodynamic functions are consistent with hydrophobic interactions. The occurrence of hydrophobic interaction is further substantiated as IR absorption spectra of MTBE-water mixtures show that MTBE strengthens the hydrogen bond network of water. Solubilization of MTBE in water is exothermic whereas solubilization of water in MTBE is endothermic with ΔH° = +5.3 ± 0.6 kJ mol(-1). The negative mixing volume is explained by a large negative contribution due to size differences between water and MTBE and by a positive contribution due to changes in the water structure around MTBE. Henry&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s law constants, K(H), were determined from vapor pressure measurements of mixtures equilibrated at different temperatures. A van&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;t Hoff analysis of K(H) gave ΔH(H)° = 50 ± 1 kJ mol(-1) and ΔS(H)° = 166 ± 5 J mol(-1) K(-1) for the solution to gas transfer. MTBE is excluded from the ice phase water upon freezing MTBE-water mixtures.

Resemblance of actin-binding protein/actin gels to covalently crosslinked networks

Nature, 1990

The maintainance of the shape of cells is often due to their surface elasticity, which arises mai... more The maintainance of the shape of cells is often due to their surface elasticity, which arises mainly from an actin-rich cytoplasmic cortex. On locomotion, phagocytosis or fission, however, these cells become partially fluid-like. The finding of proteins that can bind to actin and control the assembly of, or crosslink, actin filaments, and of intracellular messages that regulate the activities of some of these actin-binding proteins, indicates that such &amp;amp;#39;gel-sol&amp;amp;#39; transformations result from the rearrangement of cortical actin-rich networks. Alternatively, on the basis of a study of the mechanical properties of mixtures of actin filaments and an Acanthamoeba actin-binding protein, alpha-actinin, it has been proposed that these transformations can be accounted for by rapid exchange of crosslinks between actin filaments: the cortical network would be solid when the deformation rate is greater than the rate of crosslink exchange, but would deform or &amp;amp;#39;creep&amp;amp;#39; when deformation is slow enough to permit crosslinker molecules to rearrange. Here we report, however, that mixtures of actin filaments and actin-binding protein (ABP), an actin crosslinking protein of many higher eukaryotes, form gels rheologically equivalent to covalently crosslinked networks. These gels do not creep in response to applied stress on a time scale compatible with most cell-surface movements. These findings support a more complex and controlled mechanism underlying the dynamic mechanical properties of cortical cytoplasm, and can explain why cells do not collapse under the constant shear forces that often exist in tissues.

Relaxation of Concentration Fluctuations in a Shear Field

Macromolecules, 1996

We have investigated relaxations in a semidilute solution of a high molecular weight polystyrene ... more We have investigated relaxations in a semidilute solution of a high molecular weight polystyrene in the vicinity of ϑ conditions, using three different techniques: dynamic light scattering, relaxation of flow after cessation of shear recorded through the decay of the intensity of scattered light, and flow relaxation after cessation of shear recorded rheologically. We have used a solution at the ϑ temperature of polystyrene with molecular weight Mw = 4.9 × 106 in dioctyl phthalate. All three techniques yield similar values of the viscoelastic relaxation time, at various temperatures between 12 and 35 °C. Two additional modes are observed with dynamic light scattering: the well-established gel diffusion mode of the transient network of the entangled semidilute solution and an additional mode intermediate to the previous two. It is found that the shear field enhances only the longest viscoelastic relaxation mode.

Relaxation time distributions of entangled polymer solutions from dynamic light scattering and dynamic mechanical measurements

Macromolecules, 1990

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Collective vibrational effects in hydrogen bonded liquid amides and proteins studied by isotopic substitution

Journal of Molecular Structure, 2000

Raman spectroscopy is used to study the fast dynamics of simple liquid amides and proteins. Raman... more Raman spectroscopy is used to study the fast dynamics of simple liquid amides and proteins. Raman spectra in the visible region of liquid amides are obtained with a triple additive scanning monochromator, whereas FT-Raman technique is used in the near-IR region in order to avoid fluorescence from impurities in the proteins. Raman spectra are shown in the amide-I region of

Micellization of block copolymer P94 in aqueous solution

Journal of Colloid and Interface Science, 2004

P94 is a triblock copolymer of poly(oxyethylene) and poly(oxypropylene), type EPE. Eluent gel per... more P94 is a triblock copolymer of poly(oxyethylene) and poly(oxypropylene), type EPE. Eluent gel permeation chromatography has been used to demonstrate the formation of large micelles in solutions of the copolymer at temperatures immediately above the critical micelle temperature. Values of the critical micelle temperature (cmt) obtained in this way are in excellent agreement with results from differential scanning calorimetry and surface tensiometry. A van't Hoff plot was used to define the concentration dependence of the cmt for dilute solutions (c 2 wt%) and to obtain an apparent value of the standard enthalpy of micellization. Using published data, comparison is made with the thermodynamics of micellization in concentrated solution (c 20 wt%).

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Effects of Poloxamer Polydispersity on Micellization in Water

Journal of Colloid and Interface Science, 2002

The poloxamer 284 (Pluronic P94) is a triblock copolymer of poly(ethylene oxide) and poly(propyle... more The poloxamer 284 (Pluronic P94) is a triblock copolymer of poly(ethylene oxide) and poly(propylene oxide). P94 and fractions of P94 obtained after centrifugation at temperatures where solutions contain both dissolved unimers and micelles have been investigated by differential scanning calorimetry, mass spectrometry, and NMR. The results show that the P94 sample is heterogeneous with respect to both chemical composition and molar mass. The first micelles formed, when the temperature is increased, contain poloxamers with a significantly higher propylene oxide content and a higher molar mass in agreement with theoretical predictions. The characteristic temperatures of micellization, sphere-to-rod, and phase separation transitions observed in thermograms are influenced by polydispersity, which results in broader transitions.

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