Viscous Solvent Research Articles

Effect of association on rheological behaviors in the non-aqueous solutions of semi-flexible cellulose derivatives

The rheology of cellulose derivatives dissolved in highly viscous solvent was studied. Results demonstrated that a concentration-dependent transition occurred at ca. 10 vol% for ethyl cellulose, and at ca. 2.0 vol% for hydroxypropyl cellulose. Below the transition point, steady shear viscosity of solutions showed weak concentration dependency. The exponent of the power law model was about 3.7. Above the transition, the viscosity was strongly dependent on the concentration. Further studies showed that such a transition strongly depended on the association between chains. Above the transition point, their long-time dynamics were dominated by the hydrogen bonds acting as the transient cross-links between chains. While below this point, long-time dynamics were governed by the percolation statistics. As a result, their viscosities indicated different concentration dependence relationships. Our studies showed that the entanglement and association behaviors were different, for the flexible and semi-flexible chains.

A fluorescence study of the solute–solvent interactions of aminochalcones in a room-temperature ionic liquid

Unlike most other electron donor–acceptor (EDA) molecules, aminochalcones exhibit unusual solvent polarity-dependent fluorescence behavior. The photophysical behavior of two aminochalcones, namely, 4-aminochalcone (AC) and 4-dimethylaminochalcone (DMAC), has been studied in a viscous room-temperature ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], by steady-state and time-resolved fluorescence techniques. The observation of a single emission band in viscous IL, which is similar to the one observed in less viscous polar conventional solvents, suggests no twisting is necessary for the formation of the charge-transfer state from which the emission of aminochalcones originates. The fluorescence decay profiles, solvation dynamics, and excitation-wavelength-dependent emission behavior of AC are found to be quite different from those of DMAC in the IL. The observed difference is attributed to specific H-bonding interaction between AC and [bmim][PF6].

Existence of Global Weak Solutions to Implicitly Constituted Kinetic Models of Incompressible Homogeneous Dilute Polymers

We show the existence of global weak solutions to a general class of kinetic models of homogeneous incompressible dilute polymers. The main new feature of the model is the presence of a general implicit constitutive equation relating the viscous part of the Cauchy stress and the symmetric part of the velocity gradient. We consider implicit relations that generate maximal monotone (possibly multivalued) graphs, and the corresponding rate of dissipation is characterized by the sum of a Young function and its conjugate depending on and , respectively. Such a framework is very general and includes, among others, classical power-law fluids, stress power-law fluids, fluids with activation criteria of Bingham or Herschel–Bulkley type, and shear-rate dependent fluids with discontinuous viscosities as special cases. The appearance of and in all the assumptions characterizing the implicit relationship is fully symmetric. The elastic properties of the flow, characterizing the response of polymer macromolecules in the viscous solvent, are modeled by the elastic part of the Cauchy stress tensor, whose divergence appears on the right-hand side of the momentum equation, and which is defined by the Kramers expression involving the probability density function, associated with the random motion of the polymer molecules in the solvent. The probability density function satisfies a Fokker–Planck equation, which is nonlinearly coupled to the momentum equation. We establish long-time and large-data existence of weak solutions to such a system, completed by an initial condition and either a no-slip or Navier's slip boundary condition, by using properties of maximal monotone operators and Lipschitz approximations of Sobolev-space-valued Bochner functions via a weak compactness arguments based on the Div-Curl Lemma and Chacon's Biting Lemma. A key ingredient in the proof is the strong compactness in L 1 of the sequence of Galerkin approximations to the probability density function and of the associated sequence of approximations to the elastic part of the Cauchy stress tensor.

Intramolecular charge transfer emission of trans-2-[4′-(dimethylamino)styryl]benzimidazole: Effect of solvent and pH

Abstract The intramolecular charge transfer (ICT) exhibiting fluorophore trans -2-[4′-(dimethylamino)styryl]benzimidazole ( t -DMASBI) is synthesized and its spectral characteristics are investigated using absorption and fluorescence techniques. The solvatochromic studies reveal that the molecule is emitting from the locally excited state in non-polar and from an ICT state in polar solvents. Photoisomerization competes with fluorescence, and in viscous solvents restriction on twisting of olefinic bond increases the fluorescence. Theoretical calculations predict that t -DMASBI exists in two confomeric forms. trans -B conformer is more stable than trans -A. The cis isomer also exists as two conformers. However, the relative stability of conformer B further increased in cis isomer. The spectral characteristics of both the conformers of trans isomer are nearly same. The longest wavelength transition of trans isomer has ππ* character and is due to HOMO-LUMO single excitation. The theoretical calculated excitation and emission energies of trans conformer differ very little from each other and are in excellent agreement with the absorption and fluorescence spectral maximum in cyclohexane. The prototropic study indicates that unlike other benzazole derivatives t -DMASBI forms only one kind of monocation upon protonation at benzimidazole nitrogen in water. But at lower pH the second protonation occurs at dimethylamino nitrogen to form dication.

Two-Fluid Model for the Interpretation of Quartz Crystal Microbalance Response: Tuning Properties of Polymer Brushes with Solvent Mixtures

A new approach, involving a two-fluid model, has been developed to interpret the quartz crystal microbalance response of adsorbed viscoelastic polymers. The model utilizes the Navier–Stokes–Brinkmann equation to describe the motion of a porous, semirigid, viscoelastic polymer-brush film with a viscous solvent flowing through it. The two phases, solid (polymer brush) and liquid (solvent mixture), hydrodynamically interact with each other, as represented by means of a Darcy term with a characteristic correlation factor. The two-fluid model is used to estimate structural changes in polymer brushes consisting of the copolymers poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) or poly(l-lysine)-graft-dextran (PLL-g-dextran) adsorbed on an amorphous SiO2-coated quartz surface in aqueous solutions of glycerol, ethylene glycol (EG), and dimethyl sulfoxide (DMSO). Layer thickness, polymer volume fraction, and shear modulus of the polymer films with varying co-solvent concentration are determined with this approach. It was found that preferential hydrogen-bonding interactions of solvent mixtures with the polymers leads to variation in the structural properties of the polymer brushes upon changing the co-solvent composition. Furthermore, the conformation of polymer brushes in solvent mixtures is influenced by the solvent–solvent interactions, which can be explained in terms of the free energy of solvent mixing.

Photo-dissociation dynamics of bis(p-dimethylaminophenyl) disulfide in ionic liquids studied by ultrafast transient absorption spectroscopy

Photo-dissociation reaction of bis(p-dimethylaminophenyl) disulfide in ionic liquids was studied by ultrafast transient absorption spectroscopy. Geminate recombination occurred more efficiently in the ionic liquids than in methanol due to the strong cage effect of the ionic liquids. An analysis based on the diffusion limited reaction model was performed to elucidate the recombination dynamics in ionic liquids. The very fast solvation dynamics less than 0.5ps was observed in viscous solvents together with slow solvation dynamics on nanoseconds time scale. The relation between the solvation dynamics and geminate recombination was discussed.

A comparison of fenton oxidation and photocatalyst reaction efficiency for humic acid degradation

There have been many studies on the use of photocatalysts as an advanced oxidation process (AOP) to oxidize and degrade organic-based contaminants. This research group has previously reported a production process of Ti based spherical activated carbon and the characteristics of the product. This paper further examines the process of fenton oxidation and the process of Ti-spherical carboneous material (Ti-SCM) photolysis to evaluate their respective advantages and disadvantages. Non-biodegradable humic acid (HA) was chosen for the purpose of the study to observe the degradation. It is demonstrated that fenton oxidation is fast and efficient in a low organic viscous solvent. Ti-SCM is not greatly affected by variation of the pH level and the process is also fast and efficient. Moreover, the process's retention rate of catalysts and its reusability were greater than that of fenton oxidation, which required pH balancing and produced sludge.

Ultrafast excited state dynamics of the green fluorescent protein chromophore and its kindling fluorescent protein analogue

Fluorescent proteins exhibit a very diverse range of photochemical behaviour, from efficient fluorescence through photochromism to photochemical reactivity. Remarkably this diverse behaviour arises from chromophores which have very similar structures. Here we describe measurements and modelling of the excited state dynamics in the chromophores of GFP (HBDI) and the kindling fluorescent protein, KFP (FHBMI). The methods are ultrafast fluorescence spectroscopy with sub 50 fs time resolution and the modelling is based on the Smoluchowski equation. The excited state decays of both chromophores are very fast, longer for their anions than for the neutral form and independent of wavelength. Detailed studies show the mean fluorescence wavelength to be independent of time. The excited state decay times are also observed to be a very weak function of solvent polarity and viscosity. These results are modelled utilising recently calculated potential energy surfaces for the ground and excited states as a function of the twist coordinates about the two bridging bonds of the chromophore. For FHBMI and the scarce data on the neutral HBDI the calculations are not successful suggesting the need for refinement of these potential energy surfaces. For HBDI in methanol the simulation is successful provided a strong dependence of the radiationless decay rate on the coordinate is assumed. Such dependence should be included in future calculations of excited state dynamics. When the simulations are extended to more viscous solvents they fail to reproduce the observed weak viscosity dependence. The implications of these results for the nature of the coordinate leading to radiationless decay in the chromophore and for the photodynamics of fluorescent proteins are discussed.

Kinetic theory and simulations of active polar liquid crystalline polymers

A kinetic model is derived for active polar liquid crystalline polymers, that is, ensembles of polar, large molecular weight, rigid rod, “swimmers” in a viscous solvent. The model couples polarity and particle-activation physics to the Doi–Hess theory for passive liquid crystalline polymers (LCP). Our model extends the polar hydrodynamic liquid crystal model of the Marchetti group to active large molecular weight rods at arbitrary equilibrium volume fractions. The Doi–Hess–Smoluchowski equation inherits contributions due to spatial inhomogeneity and translational diffusion of the rod number density, rod polarity and self-propulsion, and the hydrodynamic equations inherit additional extra stress contributions. By suppression of LCP physics, our model recovers kinetic and mesoscopic active suspension theories of athermal, polar and apolar, micron-scale swimmers. The authors' full orientation space, 2D physical space, Smoluchowski–Navier–Stokes solver is generalized to the new model and implemented on the full system of 125 nonlinear PDEs from which we explore the coupling of rotational and translational diffusion, nano-rod density gradients, polarity and self-propulsion, and flow feedback through polar and nematic stresses. For this paper, results are presented for the dilute regime where the stable passive isotropic phase is driven far from equilibrium by activation physics, the analog of non-Brownian active “pusher” suspensions. We present a rich family of excitable and stationary patterns, previously seen in Landau–de Gennes-type coarse-grained models of apolar and polar active nematics and boundary-driven passive nematics at semi-dilute concentrations where the passive isotropic state is unstable. We find fluctuating, periodic and stationary patterns, including 1D banded and diverse 2D patterns, tunable by the relative strengths of rotational and translational diffusivity with all other physical parameters held constant.

49 Nucleic acids in anhydrous media: G-quadruplex folding governed by Kramers rate theory

Structures formed by human telomere sequence (HTS) DNA are of interest due to G-quadruplex forming HTS DNA that has recently generated a tremendous interest due to its involvement in aging process and cancer. The present study examines HTS in anhydrous, exceptionally viscous deep eutectic solvent (DES), comprised of choline, chloride, and urea. Under these conditions, HTS adopts an extremely stable “parallel-propeller” form of G-quadruplex, consistent with the previously observed effects of diminished water activity. Additionally, the high solvent friction of DES slows the dynamics of HTS folding on the order of months, as opposed to milliseconds in aqueous solution, and allows the entrapment of kinetic intermediates. Moreover, analogous transition studies of the quadruplex converting from the aqueous buffer structure to the parallel form in 90% DES (w/v) and 40% PEG 200 (v/v) differ from hours to days scaling inversely with viscosity ∼ 1/ƞ1.4. This diffusion control over the HTS folding is consistent with Kramers rate theory and these findings highlight the necessity to consider the viscosity of intracellular conditions when exploring the structure dynamics of telomeres and drug binding interactions. Lastly, tuning solvent viscosity could prove useful in the future study of G-quadruplex dynamics, and applied DNA nano-technology, where time dependent structural transitions are desired.

Understanding the role of viscous solvent confinement in the tribological behavior of polymer brushes: a bioinspired approach

Polymer brushes consisting of micrometer-long poly(dodecyl methacrylate) (P12MA) chains can impart very low coefficients of friction to surfaces sliding in oil. Under harsh tribological conditions, however, this behavior can only be maintained for long periods of time in the presence of highly viscous fluids, which appear to protect the brushes from wear. This behaviour is analogous to the lubricating mechanism in synovial joints, inasmuch as the confinement of pressurized synovial fluid in cartilage is partially responsible for the low wear rate of the system. Sliding induces solvent confinement within the brushes, provided that the viscosity is sufficiently high, and confinement can also be observed upon indentation at sufficient high rates. Such indentation experiments have been performed to characterize the mechanical (viscoelastic) behavior of the polymer brushes in solvents. By applying Hertzian contact theory and considering the stiffening of the polymer brushes upon compression, an effective elastic contact modulus of the P12MA brushes is obtained that is much higher than the equilibrium value. The proportion of the load carried by the fluid can also be estimated as a function of speed and viscosity. Thus, high solvent viscosity, as well as substantial brush height, can lead to a shielding of the polymer from the effects of the load, and the enhancement of the effective brush elastic modulus. This explains the observed low wear rates observed in tribological experiments in viscous liquids.

Magnetically rewritable photonic ink based on superparamagnetic nanochains

One-dimensional photonic nanochains composed of periodically arranged superparamagnetic nanoparticles have been exploited as basic color units for a new type of photonic ink. The photonic response of ink can be rapidly and repetitively switched on and off by tuning the direction of external magnetic fields. Temporary bi-stability is realized in the photonic ink by introducing highly viscous solvents as a dispersant, which suppress the rotational movement of nanochains and allow them to stay temporarily aligned after the removal of external magnetic field and to retain their photonic response for a considerably long period. Increasing the viscosity of the dispersant enhances the bi-stability of photonic inks; for instance, the ink prepared in 95% glycerol solution is able to retain its color up to ten minutes. The length of the chains also plays an important role in their temporary bi-stability. By mixing chains with different photonic properties and lengths, a display has been developed which exhibits two distinct colors in response to the application and removal of external magnetic fields.

Visualization of cation diffusion at the TiO2 interface in dye sensitized photoelectrosynthesis cells (DSPEC)

Time-resolved, UV-vis spectroscopic measurements of Li+ diffusion in mesoscopic TiO2 photoanodes were conducted in dye sensitized photoelectrosynthesis cells (DSPECs) under operating conditions. In these experiments the spectral response of TiO2 derivatized with [Ru(bpy)2(4,4′-((HO)2PO)2bpy)]2+ (RuP, where bpy is 2,2′-bipyridine, (4,4′-((HO)2PO)2bpy) is [2,2′-bipyridine]-4,4′-diphosphonic acid) arises from electric field (Stark) effects on the metal-to-ligand-charge transfer (MLCT) absorption spectrum of RuP, which is screened by cation intercalation. These results verify that Li+ diffusion is coupled to electron injection and to electron recombination/extraction at the TiO2 interface. Li+ doping levels depend on the competition between dynamics of its intercalation and electron recombination/transport. For a DSPEC operating in aqueous solution at pH 4.5, the observed rate constants for Li+ intercalation and release were 0.22 s−1 and 0.014 s−1, respectively. Both processes were considerably slower in the more viscous solvent propylene carbonate with Li+ release rate constants <2 × 10−4 s−1. Accumulation of Li+ under these conditions shifts conduction band/trap states to less negative potentials, increasing electron lifetime in TiO2.

Probe dynamics constraints on theoretical models for polymer dynamics

Measurements of diffusion and driven motion by probe particles in polymer solutions constrain theoretical models of polymer solution dynamics. In this paper, motions of large, intermediate (smaller than a polymer chain, larger than a solvent molecule), and small (solvent, ion) probes through polymer solutions and viscous small-molecule solvents are analyzed. The resulting constraints limit the physical models that can plausibly be used to describe polymer motion and separately limit the mathematical structures that might be used to obtain quantitative predictions from those models. A transition in small-molecule mobility through polymer solutions, at polymer concentrations near 400 g/l, is explained in terms of the size of a solvent molecule relative to the gaps between pairs of chain segments on adjacent polymer molecules.

Interaction of Auramine O with montmorillonite clays

The spectroscopic behaviour of Auramine O (AuO) in aqueous suspensions of montmorillonite clays was studied using absorption and static and dynamic fluorescence techniques. The fluorescence of Auramine O increases immediately after mixing the dye solution with the suspension of clay due to its adsorption on the external surface of the clays, which restricts the torsional molecular motion of Auramine. At longer times, the dye molecules migrate into the interlamellar region of the clay particles. Aggregation of the dye molecules can occur in the interlayer region, leading to the decrease of the fluorescence emission. The fluorescence quantum yields (ΦF) of AuO on the natural montmorillonites SAz-1, SWy-1, Syn-1 and Laponite clays were 0.015, 0.007, 0.016 and 0.017, respectively. These values are higher than the ΦF of AuO in aqueous solution and are of the same order of magnitude of the ΦF found for viscous solvents such as n-hexanol and n-heptanol (0.014 and 0.015). Time-resolved fluorescence spectroscopy studies of adsorbed Auramine on clays revealed multi-exponential decays with components in the 25–36, 219–362 and 1300–1858ps ranges. The short-lived components can be attributed to species bound to external surface and the longer lifetime is assigned to dye molecules in interlayer spaces interacting strongly with the clay. It seems clear that the binding of Auramine to clays causes a significant reduction of the rate of internal conversion that does involve rotational diffusion, so that the clay will be locked in a conformational geometry unfavourable for internal conversion.

Dynamics of Geminate Rebinding of NO with Cytochrome c in Aqueous Solution Using Femtosecond Vibrational Spectroscopy

Using femtosecond vibrational spectroscopy, we investigated the rebinding dynamics of NO to cytochrome c (Cytc) and a model heme, microperoxidase-8 (Mp), after photodeligation of CytcNO in D(2)O solution and MpNO in an 81% glycerol/water (v/v) mixture at room temperature. Whereas the stretching mode of the NO band in MpNO was described by a Gaussian centered at 1653 cm(-1) with a full width at half-maximum (fwhm) of 41 cm(-1), that in CytcNO revealed an asymmetric structured band that peaked at 1619 cm(-1) with an fwhm of about 27 cm(-1). The structured NO band in CytcNO was well described by the sum of three Gaussians, and its shape did not evolve with time but its amplitude decayed exponentially with a time constant of 7 ± 1 ps. The transient NO band in MpNO also decayed exponentially with a time constant of 8 ± 1 ps. Rebinding of NO to Cytc was slightly faster than that of NO to Mp and was almost complete by 30 ps, which was much faster than the rebinding of NO to myoglobin (Mb). When the deligated NO was constrained near the Fe atom either by a viscous solvent or by the protein matrix, it rebound to heme Fe much faster than CO, suggesting that NO has a higher propensity for binding to heme Fe and the high reactivity governed the rebinding kinetics. Moreover, the faster ligand rebinding in Cytc than in Mb suggests that Cytc does not have a primary docking site (PDS)-like structure found in Mb that suppresses rebinding by restraining ligand motion and the PDS can also hold the deligated NO in a manner that impedes NO rebinding; however, due to higher NO reactivity with heme Fe, the impediment is not as efficient as for CO.

Viscosity and Temperature Effects on the Rate of Oxygen Quenching of Tris-(2,2′-bipyridine)ruthenium(II)

We compare the bimolecular quenching rate constant (k2) of luminescent tris(2,2'-bipyridine)ruthenium(II) by oxygen in water, ethylene glycol and glycerol as a function of temperature and viscosity to several theoretical models. The Smoluchowski equation with experimentally determined diffusion coefficients produced calculated values that were in the best agreement with experiment. For the less viscous solvent, water, this equation produced a value that was approximately an order of magnitude larger than the experimental value. With an increase in solvent viscosity, the Smoluchowski value approached the experimental value. Using the Smoluchowski equation with calculated diffusion coefficients based on the known radii of the reacting species produced deviations an order of magnitude larger in water and a factor of two or three lower in ethylene glycol and glycerol. If an assumption is made that the radii of both molecules are equal, we have the Stokes Einstein equation, and the only parameters become temperature and viscosity. Using this relationship, the calculated values for water are about a factor of two larger and with ethylene glycol and glycerol about a factor of 6 smaller than experimental data. These results show that bimolecular quenching is a more complex process affected by many parameters such as solvent cage effects in addition to viscosity and temperature.

Nitroanilines as Quenchers of Pyrene Fluorescence

The quenching of pyrene and 1-methylpyrene fluorescence by nitroanilines (NAs), such as 2-, 3-, and 4-nitroaniline (2-NA, 3-NA, and 4-NA, respectively), 4-methyl-3-nitroaniline (4-M-3-NA), 2-methyl-4-nitroaniline (2-M-4-NA), and 4-methyl-3,5-dinitroaniline (4-M-3,5-DNA), are studied in toluene and 1,4-dioxane. Steady-state fluorescence data show the higher efficiency of the 4-NAs as quenchers and fit with a sphere-of-action model. This suggests a 4-NA tendency of being in close proximity to the fluorophore, which could be connected with their high polarity/hyperpolarizability. In addition, emission and excitation spectra evidence the formation of emissive pyrene-NA ground-state complexes in the case of the 4-NAs and, in a minor degree, in the 2-NA. Moreover, time-resolved fluorescence experiments show that increasing amounts of NA decrease the pyrene fluorescence lifetime to a degree that depends on the NA nature and is larger in the less viscous solvent (toluene). Although the NA absorption and the pyrene (Py) emission overlap, we found no evidence of dipole-dipole energy transfer from the pyrene singlet excited state ((1)Py) to the NAs; this could be due to the low NA concentration used in these experiments. Transient absorption spectra show that the formation of the pyrene triplet excited state ((3)Py) is barely affected by the presence of the NAs in spite of their efficiency in (1)Py quenching, suggesting the involvement of (1)Py-NA exciplexes which--after intersystem crossing--decay efficiently into (3)Py.

Emission polarization of random lasers in organic dye solutions

This Letter presents a polarimetric study of the emission of random lasers from organic dyes. Coherent lasing modes from samples with ethanol solvent showed a high degree of polarization and did not influence each other in polarization. The proper choice of a laser dye with asymmetric absorption momenta, a highly viscous solvent, and a linear pump polarization can cause the random lasing emission to be completely linearly polarized for all wavelengths within the amplification range.

Long-living structures of photochromic salicylaldehyde azine: polarity and viscosity effects from nanoseconds to hours

In this study, we report on the effects of solvent viscosity and polarity on the photochromic salicylaldehyde azine (SAA) molecule by examining the steady-state and UV-visible absorption results in the time scale from nanoseconds to hours, in solution and in a polymer film. For the neutral structure, the viscosity strongly affects the lifetime of the photochromic (trans-keto) tautomer by suppressing the second order quenching process, and thus increasing the photochrome lifetimes in highly viscous solvents to 500 μs in polar triacetine, and to 65 μs in non-polar squalane. Trapping SAA in a non-polar polymer film (polyethylene) results in further elongation of the photochromic lifetime (700 μs) by one order of magnitude (with respect to that in squalane), due to the retardation of the intramolecular back-isomerization. Another species, living significantly longer and absorbing more in the UV comparing to the photochrome, was identified as the syn-enol tautomer. The lifetime of this tautomer, created in a competitive mechanism to the photochrome creation, is much longer in non-polar solvents (hundreds of minutes) than in polar ones (tens of minutes), opposite to the trend observed for the photochrome. For the SAA anion, the transient living on the ns-μs time scale can be exclusively assigned to the triplet state, which is not observed for the neutral form at room temperature.