Dimerization Constant Research Articles

Zinc-[7]helicenocyanine and Its Discrete π-Stacked Homochiral Dimer.

In this communication, we demonstrate a novel approach to prepare a discrete dimer of chiral phthalocyanine (Pc) by exploiting the flexible molecular geometry of helicenes, which enables structural interlocking and strong aggregation tendency of Pcs. Synthesized [7]helicene‐Pc hybrid molecular structure, zinc‐[7]helicenocyanine (Zn‐7HPc), exclusively forms a stable dimeric pair consisting of two homochiral molecules. The dimerization constants were estimated to be as high as 8.96×106 M−1 and 3.42×107 M−1 in THF and DMSO, respectively, indicating remarkable stability of dimer. In addition, Zn‐7HPc exhibited chiral self‐sorting behavior, which resulted in preferential formation of a homochiral dimer also in the racemic sample. Two phthalocyanine subunits in the dimeric form strongly communicate with each other as revealed by a large comproportionation constant and observation of an IV‐CT band for the thermodynamically stable mixed‐valence state.

Reactive Extraction of Butyric Acid from Water Using Trioctyl Amine in 1-Decanol and Green Natural Oils

In this work, butyric acid (BA) was extracted using trioctyl amine (TOA). Various temperature-dependent (298.15–318.15 K) process parameters were estimated and analyzed. To reduce the harmful effects of TOA and to avoid the detrimental effects of toxic diluents on microorganisms, green and nontoxic diluents, sunflower and soybean oils, were tested. In physical extraction, KDdil and the partition coefficient, P, decreased in the order of 1-decanol > sunflower oil > soybean oil, whereas the dimerization constant, D, showed the opposite trend. In chemical extraction, the extraction efficiencies were 85–90% for 1-decanol, 82–88% for sunflower oil, and 75–86% for soybean oil. In terms of the distribution coefficient, 1-decanol was a better diluent than sunflower oil, followed by soybean oil. The high distribution coefficients and extraction efficiencies obtained using sunflower and soybean oils were found to be encouraging for their use in biorefinery industries. (KDchem)/(KDphy) was greater than 1.0 in all of the studied cases. The best distribution coefficient model was identified. The complexation reaction stoichiometry and the equilibrium complexation reaction constant were estimated using the differential evolution technique. The order of the complex molality, [HAmSn]org, in different diluents was 1-decanol > sunflower > soybean. Different thermodynamic parameters were determined and analyzed. Variations of acid–extractant, free acid, and dimer molalities at the chemical equilibrium state were found and discussed. Analysis based on the enthalpy change due to mass transfer, (ΔH)mass tr, indicated the easiest mixing of the phases in sunflower oil and the most difficult in soybean oil. The reaction equilibrium constant, KE(m:n), had the highest value at the lowest temperature (298.15 K) and decreased with an increase in temperature and had values in the order of 1-decanol > sunflower > soybean.

Reactive extraction of cis,cis-muconic acid from aqueous solution using phosphorus-bonded extractants, tri-n-octylphosphineoxide and tri-n-butyl phosphate: Equilibrium and thermodynamic study

• The muconic acid extraction by TOPO and TBP have been done firstly in the literature. • Different solvents used to solve TOPO and TBP. • Kinetic studies of the extraction mucconic acid was studied firstly in the literature. cis,cis- Muconic acid falls under the category of carboxylic acids and finds application mostly in biodegradable polymers, agrochemicals, and the food industry. Owing to the high thermal reactivity based upon the two-terminal carboxylic groups present in their structure, the chemical industry has widely used carboxylic acids. The major obstacle encountered during the production of carboxylic acids is their recovery from aqueous solutions and fermentation media. Therefore, the recovery of HccMA by reactive extraction as a particular product capture method, using tri- n -octylphosphine oxide (TOPO) and tri- n -butyl phosphate (TBP) in organic diluents such as 1-butanol, isoamyl alcohol, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), diisobutyl ketone, isobutanol, n -hexane, dimethyl glutarate, ethyl propionate, and diethyl carbonate (DEC) was executed in this study. This method was experimentally conducted to explore the most proper extractant and diluent combination. Extraction experiments were also conducted with pure diluents alone to examine the extractant effect on the extraction system. The important extraction parameters like distribution coefficient ( K D ), extraction efficiency ( E %), loading ratio ( Z ), dimerization constant ( D ), partition coefficient ( P ), and complexation constant ( K E 11 ) were determined. Mass action law model interpreted the obtained results. For HccMA/TOPO extraction system, the maximum extraction efficiency was found 93.19% in the presence of n -hexane as a diluent. For HccMA/TBP extraction system, the maximum extraction efficiency was found 90.37% in the presence of ethyl propionate as a diluent. Thermodynamic studies at variable temperatures were also carried out to estimate ΔH, ΔS, and ΔG of the process. To investigate the feasibility of designing a counter-current liquid–liquid extraction column, the number of theoretical stages (NTS) were assessed by theodified Kremser equation and found between 3 and 5 to et the desired extraction efficiency.

Multivalent binding of the partially disordered SARS-CoV-2 nucleocapsid phosphoprotein dimer to RNA

The nucleocapsid phosphoprotein N plays critical roles in multiple processes of the severe acute respiratory syndrome coronavirus 2 infection cycle: it protects and packages viral RNA in N assembly, interacts with the inner domain of spike protein, binds to structural membrane (M) protein during virion packaging and maturation, and to proteases causing replication of infective virus particle. Even with its importance, very limited biophysical studies are available on the N protein because of its high level of disorder, high propensity for aggregation, and high susceptibility for autoproteolysis. Here, we successfully prepare the N protein and a 1000-nucleotide fragment of viral RNA in large quantities and purity suitable for biophysical studies. A combination of biophysical and biochemical techniques demonstrates that the N protein is partially disordered and consists of an independently folded RNA-binding domain and a dimerization domain, flanked by disordered linkers. The protein assembles as a tight dimer with a dimerization constant of sub-micromolar but can also form transient interactions with other N proteins, facilitating larger oligomers. NMR studies on the ∼100-kDa dimeric protein identify a specific domain that binds 1–1000-nt RNA and show that the N-RNA complex remains highly disordered. Analytical ultracentrifugation, isothermal titration calorimetry, multiangle light scattering, and cross-linking experiments identify a heterogeneous mixture of complexes with a core corresponding to at least 70 dimers of N bound to 1–1000 RNA. In contrast, very weak binding is detected with a smaller construct corresponding to the RNA-binding domain using similar experiments. A model that explains the importance of the bivalent structure of N to its binding on multivalent sites of the viral RNA is presented.

CAMP is an allosteric modulator of DNA-binding specificity in the cAMP receptor protein from Mycobacterium tuberculosis

Allosteric proteins with multiple subunits and ligand-binding sites are central in regulating biological signals. The cAMP receptor protein from Mycobacterium tuberculosis (CRPMTB) is a global regulator of transcription composed of two identical subunits, each one harboring structurally conserved cAMP- and DNA-binding sites. The mechanisms by which these four binding sites are allosterically coupled in CRPMTB remain unclear. Here, we investigate the binding mechanism between CRPMTB and cAMP, and the linkage between cAMP and DNA interactions. Using calorimetric and fluorescence-based assays, we find that cAMP binding is entropically driven and displays negative cooperativity. Fluorescence anisotropy experiments show that apo-CRPMTB forms high-order CRPMTB–DNA oligomers through interactions with nonspecific DNA sequences or preformed CRPMTB–DNA complexes. Moreover, we find that cAMP prevents and reverses the formation of CRPMTB–DNA oligomers, reduces the affinity of CRPMTB for nonspecific DNA sequences, and stabilizes a 1-to-1 CRPMTB–DNA complex, but does not increase the affinity for DNA like in the canonical CRP from Escherichia coli (CRPEcoli). DNA-binding assays as a function of cAMP concentration indicate that one cAMP molecule per homodimer dissociates high-order CRPMTB–DNA oligomers into 1-to-1 complexes. These cAMP-mediated allosteric effects are lost in the double-mutant L47P/E178K found in CRP from Mycobacterium bovis Bacille Calmette-Guérin (CRPBCG). The functional behavior, thermodynamic stability, and dimerization constant of CRPBCG are not due to additive effects of L47P and E178K, indicating long-range interactions between these two sites. Altogether, we provide a previously undescribed archetype of cAMP-mediated allosteric regulation that differs from CRPEcoli, illustrating that structural homology does not imply allosteric homology.

Experimental investigation on extractive separation of vanillic acid

Vanillic acid has received remarkable attention due to its diversified applications in cosmetics, food, flavours, tobacco, pharma, alcohols, beverages, and polymer industries. However, its separation from fermentation broths and wastewater of olive oil mills, or paper and pulp industries is still a problematic process. In this paper, the liquid – liquid extraction has been investigated for the separation of vanillic acid from aqueous streams. The assessment of equilibrium has been done by calculating the partition coefficient, dimerization constant, distribution coefficient, and extraction efficiency. The extraction ability is given in terms of extraction efficiency and distribution coefficient followed a trend:1-octanol (54.07%) > iso-butanol (51.73%) > toluene (45.23%) > xylenes mix (35.47%) > petroleum ether (31.54%) > benzene (30.44%). The obtained results were discussed with help of acid properties, type of solvent used, and their physicochemical properties such as dipole moment ( D m ), dielectric constant ( ε ), solvent polarity parameter ( E T ), solubility in water, octanol-water partition coefficient ( log K ow ), etc. This is the first attempt of study using the extractive separation method for removal or recovery of vanillic acid from aqueous solutions by the above-mentioned solvents.

Dimerization of 2-Ethylhexylphosphonic Acid Mono-2-ethylhexyl Ester (HEH[EHP]) as Determined by NMR Spectrometry

ABSTRACT The aggregation state of extractant molecules is an important consideration in characterizing or modeling solvent extraction systems used in hydrometallurgy. For example, previously reported efforts to measure organic-phase dimerization constants have required the presence of an additional phase, either aqueous phase or gas phase. NMR spectroscopy can be used to probe organic-phase inter-molecular interactions without requiring the presence of an additional phase, making it possible to study the simplest organic system, that contain only extractant and diluent. In this report, the dimerization of 2-ethylhexylphosphonic acid mono-2-ethylhexl ester (HEH[EHP]) in n-dodecane and in toluene was investigated by two different NMR-based methods: chemical shifts and Diffusion Ordered SpectroscopY (DOSY). The chemical-shift analysis requires monitoring the chemical shift of the acidic proton as the concentration of HEH[EHP] changes. DOSY is a 2D NMR technique used to probe the size of molecules. The size of diffusing species was related back to the average aggregate molecular weight via a calibration curve. Because the DOSY method had not been used in this manner before, a validation of the method using the interaction constant between HDEHP and CMPO was performed. After this validation, DOSY was applied to HEH[EHP] dimerization. DOSY results demonstrated that the chemical shift of the acidic proton was the peak most affected by the dimerization state of the HEH[EHP]. All other peaks, including that of 31P, were affected more significantly by the changing dielectric constant of the solution. The dimerization constants determined were significantly lower than those reported in prior literature, possibly due to the effect of water in the literature values.

Helicates with Ether‐Substituted Catechol Esters as Ligands

Mono‐ or biscatechol esters with ether‐type substituents or spacers form either triple lithium bridged dimeric helicates or triple stranded helicates with the ability to bind three lithium cations in their interior. Hierarchical helicates with ether or thioether substituents show in solution a monomer‐dimer equilibrium which is independent of the heteroatom in the ester substituent. However, dimerization constants are significantly lower than for corresponding alkyl derivatives. Dinuclear helicates with oligoether spacers are well obtained in the presence of lithium cations. Upon removal of the cations the helicates expand and successive addition of LiCl results in compression again.

Solvent Dependence of the Monomer-Dimer Equilibrium of Ketone-Substituted Triscatecholate Titanium(IV) Complexes.

Hierarchical helicates based on ketone‐substituted titanium(IV)triscatecholates show different monomer‐dimer behavior depending on different solvents. The dimerization constants of a whole series of differently alkyl‐substituted complexes is analyzed to show that the solvent has a very strong influence on the dimerization. Hereby, effects like solvophobicity/philicity, sterics, electronics of the substituents and weak side‐chain—side‐chain interactions seem to act in concert.

Mechanistic insights and consequences of intrapore liquids in ethene, propene, and butene dimerization on isolated Ni2+ sites grafted within aluminosilicate mesopores

The stability, selectivity, and reactivity conferred by intrapore liquids onto Ni2+ species grafted within mesoporous aluminosilicates for the dimerization of C2-C4 alkenes are reported and interpreted by the preferential stabilization of dimer desorption transition states, which inhibit further growth during the surface sojourn that forms these dimers. The marked decrease in deactivation rate constants leads to catalyst half-lives > 600 h for all alkenes and to turnover rates (per Ni) at sub-ambient temperatures (240–260 K) that exceed those at the higher temperatures previously used for Ni-based solid catalysts (350–500 K). These effects appear at the relative pressures required for alkene capillary condensation within Ni-Al-MCM-41 catalysts with different mesopore diameters and are also evident when the intrapore liquids consist of unreactive alkanes, indicative of solvation effects conferred by non-covalent interactions mediated by dispersion forces. For all alkenes, turnover rates (per Ni) are unaffected by Ni content until each acidic proton in the Al-MCM-41 is replaced by one Ni atom, consistent with the involvement of grafted (Ni-OH)+ monomers as active centers and excess Ni leading to inactive NiO species. The stable nature of these catalysts allows accurate mechanistic assessments and a demonstration of the kinetic relevance of the CC coupling elementary steps on essentially bare (Ni-OH)+ centers, leading to rates described by second-order dimerization constants (αn′) and adsorption constants (βn′) for each alkene (Cn); these parameters are influenced only slightly by intrapore liquids, because they reflect free energies of formation of surface-bound species and transition states from gaseous precursors. These αn′ and βn′ parameters increase with alkene size, with the enthalpic component of αn′ increasing from 21 to 46 kJ mol−1 and the activation entropy becoming more negative (−103 to −187 J (mol K)−1), consistent with CC coupling transition states that occur late along the reaction coordinate and resemble its bound dimer product. The low C3n/C2n ratios observed in the presence of intrapore liquids (<0.04, for n = 2–4) reflect the endothermic nature of dimer desorption events, for which product-like transition states benefit from the solvation of desorbing products by a non-polar liquid phase, thus allowing detachment from Ni centers before subsequent growth. The inhibition of such growth events accounts not only for the unique dimer selectivity observed, but also for the unprecedented stability conferred by intrapore liquids by inhibiting the formation of oligomers that bind more strongly with increasing chain size, as evident from the βn′ values reported here for alkene reactants.

3 µm Water vapor self- and foreign-continuum: New method for determination and new insights into the self-continuum

The H2O self- and foreign- in-band continua in the region 3400–3900 cm−1 were experimentally determined for 296 and 353 K from multispectrum fitting results of line parameters using the Hartmann-Tran line profile (HTP) and Rosenkranz line mixing. The continua were extracted from the baselines which were determined in the microwindow-based multispectrum fits. Continua were then obtained by simultaneous fitting of all baselines from measurements containing continuum information. The self-continuum at 296 K was determined from self-broadened measurements and agrees with that determined from air broadened measurements. The overall shape and strength of the new self-continuum agrees with the CAVIAR results between 3600 and 3800 cm-1 but differences exceed the stated uncertainties at higher and lower wavenumbers. Moreover, the new self-continuum is much smoother, has no gaps and is obtained with a high resolution of 2.4 cm−1. The self-continuum was fitted as sum of modeled bound and quasi-bound dimer spectra. From rotational constants, the bound dimer parallel and perpendicular band shapes of the near prolate symmetric top molecule were calculated and used as kernels to fit fundamental wavenumbers, relative band intensities and partitioning of parallel and perpendicular band type, while the integral of the band intensities of the four fundamentals was fixed to published experimental/theoretical data. A dimerization constant for the bound dimer of KDb = 0.026(2) atm−1 and the quasi-bound dimer of KDq = 0.044(5) atm−1 was derived from the fits.The foreign-continuum has no gaps, a spectral resolution of 6–16 cm−1, and is about 40% smaller than the MT_CKD3.2 continuum model. It has a distinctly different shape showing a pronounced P-Q-R branch structure. The foreign-continuum shape is narrower than the monomer band shape which is also true for the MT_CKD3.2 continuum model. The CAVIAR foreign-continuum is much noisier but on average is in good agreement with the new measurements.

Spectral and Aggregative Properties of Acid Blue 113 in Aqueous and Aqueous Solutions of Urea and in Colloids of Silver Nanoparticles

Photophysical and association properties of a diazo dye (acid blue 113) in the presence of a structure breaking additive (urea) and Ag nanoparticles (AgNPs) were investigated at different dye and additive concentrations (0–3 mol·L−1 urea and 0.1 mmol·L−1 AgNPs). The ionic dye, with strong hydrophilic character, has an aromatic diazo skeleton and two sulfide functional groups. The dye concentrations ranging from 1 × 10−6 to 5 × 10−4 mol·L−1 were chosen and studied, to avoid formation of the higher-order aggregates. The dye spectra were analyzed using linear and nonlinear decomposition algorithms. Using the least squares fitting approach, the dimerization constant Kd and individual monomer and dimer spectra were determined. A reduction of the dimerization constant for AB 113 in aqueous solutions containing additives was observed. The spectral parameters, structure of the dimeric species and the interaction energy between the monomers in the dye solution were estimated based on the molecular exciton theory.

Extractive separation of levulinic acid using natural and chemical solvents

Abstract Levulinic acid is an organic compound which has been replacing petroleum products in different sectors such as biofuel and chemical industry. It can be used in the manufacture of plastics, rubber, pharmaceuticals, pesticides, etc. Levulinic acid separation from aqueous streams, different types of solvents such as natural solvents (sunflower and groundnut oil), and various chemical solvents (benzene, iso-butanol, and iso-octanol) at 298.15 K were used. Using equilibrium data various parameters such as: efficiency of extraction, distribution coefficient, dimerization constant, and partition coefficient were calculated and correlated with typical physicochemical properties, molecular weight, density, dipole moment, viscosity, dielectric constant, solubility in water, octanol-water partition coefficient, refractive index, etc. The obtained experimental extraction results for separation of levulinic acid were compared with various solvents used. The maximum value of distribution coefficient and efficiency of extraction of levulinic acid was obtained for iso-butanol (1.357, 57.276%) and minimum value for sunflower oil (0.077, 7.086%) respectively.

Super-tough and rapidly self-recoverable multi-bond network hydrogels facilitated by 2-ureido-4[1H]-pyrimidone dimers

Multi-bond network (MBN) hydrogels contain hierarchical dynamic bonds with different bond association energy as energy dissipation units, enabling super-tough mechanical properties. In this work, we copolymerize a protonated 2-ureido-4[1H]-pyrimidone (UPy)-contained monomer with acrylic acid in HCl solution. After removing excess HCl, UPy motifs are deprotonated and from dimers, thus generating an UPy-contained MBN hydrogel. The obtained MBN hydrogels (75 wt% water content) exhibit super-tough mechanical properties (0.39 MPa to 2.51 MPa tensile strength), with tremendous amount of energy (1.68 MJ/m3 to 11.1 MJ/m3) dissipated by the dissociation of UPy dimers. The introduction of ionic bonds can further improve the mechanical properties. Moreover, owing to their dynamic nature, both UPy dimers and ionic bonds can re-associate after being dissociated, resulting in excellent self-recovery ability (around 90% recovery efficiency within only 1 h). The excellent self-recovery ability mainly originates from the re-association of UPy dimers based on the high dimerization constant of UPy motifs

Investigations on Reactive Extraction of Butyric Acid Using Tri-n-Octyl Amine in Various Biodiesels

Butyric acid is a monocarboxylic acid which can be produced economically by fermentation from green feedstock. As the recovery of butyric acid from dilute fermentation broth is a challenge and hence it needs to develop a competitive downstream recovery process. Butyric acid finds a wide range of applications in pharmaceutical, food, cosmetics and power industries. Reactive extraction is one of the most promising and accessible methods which can be used as first separation step for recovery of butyric acid from dilute wastewater stream or fermentation broth. In present study, the physical and reactive extraction of butyric acid using tri-n-octylamine (TOA) in biodiesel made from sesame oil, rice bran oil, karanja oil and sunflower oil were studied at 298K and comparatively found the reactive extraction as more efficient. The Partition coefficient P, dimerization constant D and the distribution coefficient KD were obtained in the physical extraction study. In reactive extraction, the results were obtained in terms of distribution coefficient KD, loading ratio O and the degree of extraction E%.

Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution.

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1H NMR dimerization constants for the monomer–dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π–π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n‐alkyls, n‐Ω‐alkenyls or n‐Ω‐alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

Optical purity, enantiomeric excess and the Horeau effect

The optical purity of an enantiomeric mixture deduced from specific rotation measurements was found by Horeau to be different from its enantiomeric excess, which came to be known as the Horeau effect. This observation had important implications in the practical use of specific rotations and has led to investigations on homochiral and heterochiral aggregation processes. In this review, dedicated to the Horeau principle, the theoretical basis for the observance of the Horeau effect and a survey of the specific rotation studies investigating the Horeau effect are provided, and possible future investigations are suggested.

Spectrophotometric-chemometrics study of the effect of solvent composition and temperature on the spectral shape and shift of copper and nickel phthalocyanines in different aqueous-nonaqueous mixed solvents.

Phthalocyanines (Pcs) are green-blue colored aromatic macrocyclic compounds used extensively in the dyeing industry. The assembly phenomenon for dye molecules is directly traceable by most of the spectroscopic methods. In this investigation, the monomer-dimer equilibria of copper and nickel phthalocyanines tetra sulfonic acid tetrasodium salts (CuPcTS, NiPcTS) have been investigated by spectrophotometric and chemometrics methods in binary mixtures of H2O-DMSO and H2O-CH3CN. The dimerization constants, (KD), enthalpies and entropies of CuPcTS and NiPcTS have been calculated by studying the UV-Vis spectra at different concentrations of dyes (10-6 to 10-4molL) and in the temperature range 298-343 K and in some samples up to 353 K by multivariate curve resolution (mcr) methods. By increasing the temperature, the value ofKD decreases. The inverse temperature dependence of KD(van't Hoff equation) was used for determination of ΔH0 and ΔS0and following that ΔG0of the dimerization reactions. As a result, upon aggregation, an increase in the intensity of the new shoulder at (~600 nm) and the Q-band at (662-670 nm) and concomitant decrease of the dimer band at (630-624 nm) are observed for all of the samples in different solvents composition. Therefore, the H-dimer type of these pigments was notable, in studied binary solvents. The effect of the solvent composition, concentration dye, and temperature on the spectral responses, the exciton parameters and concentration distribution diagrams of the two pigments were studied and discussed. The density functional theory (DFT) calculations were done in the aqueous and gas phase that they were included the HOMO-LUMO energies and the simulated UV-Vis spectrum. These calculations were beneficial for studying the UV-Vis spectrum of it in the aqueous phase for checking experimental data.

Quantitative interpretation of isopiestic measurements on aqueous solutions: Urea revisited

This investigation amends the analysis of isopiestic measurements of solvent thermodynamic activity by taking into account the fact that the solvent activity, traditionally expressed in mole-fraction terms, is a molal parameter because of the constraints (constant temperature and pressure) under which the measurements are made. Application of the revised procedure to published isopiestic measurements on aqueous urea solutions at 25 °C yields a dimerization constant of 0.066 molal−1, which is two-fold larger than an earlier published estimate based on an incorrect definition of the solute activity coefficient. Despite amendments to the quantitative detail, the present study confirms the existence of a large negative entropic contribution that largely counters its enthalpic counterpart arising from the hydrogen bonding responsible for dimer formation. This evidence of enthalpy-entropy compensation is entirely consistent with quantum-mechanical predictions of the adverse effect of water on urea dimerization. Changes in water structure thus contribute significantly to the energetics of urea dimerization in aqueous solution.

Photophysical properties of methylene blue in water and in aqueous solutions of dimethylsulfoxide.

A detailed study of the effect of aprotic polar solvents such as dimethylsulfoxide on methylene blue (MB) was carried out through a combination of UV-vis absorption and steady-state fluorescence spectroscopy techniques at 293.15 K. In aqueous solutions MB tends to exhibit strong tendency to aggregate. The dimerization behavior of MB in water was analyzed in terms of monomer-dimer equilibrium. The addition of dimethylsulfoxide prevents dimerization of dye molecules. From absorption spectra the dimerization constants and changes of standard Gibbs energy were calculated. From the steady-state fluorescence spectra the quantum yields and Stokes shifts were determined. To explain the effect of dimethylsulfoxide properly the other polar aprotic solvents such as N, N-dimethylformamide, acetonitrile and acetone were used. It is suggested that water structural effect is the major factor in aggregation phenomenon. Moreover the quantum yield increases drastically in dimethylsulfoxide compared with water assuming that MB may be characterized as viscosity probe.