1,720,965 research outputs found
Thermodynamic study of aqueous micellar solutions of biologically active bisquaternary ammonium chlorides
No full textThermodynamic properties of aqueous solution of bisquaternary ammonium salts, which are derivatives of N,N-bisdimethyl-1,2-ethanediamine ( bis-Cn-BEC) , of general formula /CnH2n/1OOCCH2(CH3)2N!CH2CH2N!(CH3)2CH2COOCn-H /2Cl0(bis-Cn-BEC, where the subscript n stands for the number of carbon atoms of the alkyl chain bound to the carboxyl group) are here reported and compared with those of the corresponding monomers. Dilution enthalpies have been measured by means of a flow type microcalorimeter at 313 K, and densities have been measured by means of a vibrating tube densimeter at 298 K. Apparent and partial molar quantities have been obtained from the experimental data and expressed as a function of molality assuming the infinite dilution as standard state. The changes in enthalpy and in volume upon micellization have been evaluated by assuming the pseudo-phase transition model. From the calculated enthalpy changes at 313 K and that previously reported at 298 K, the changes in heat capacity for micellization has been evaluated. The data suggest the hypothesis that the alkyl chain in the dimers are already partially associated in solution. Moreover, the trends of the cmc as a function of the chain length for monomers and dimers suggest that the association of the chains probably leaves out the first three methylene groups of the alkyl chains bound to the carboxylic groups. The packing parameter, P, gives 0.37 for bis-C10-BEC, and the formation of rod-like micelles is therefore suggested
Solubility of inert gases and liquid hydrocarbons in water
No full textThe thermodn. statistical model based on the distribution of mol. populations among energy levels has been employed for the anal. of the soly. of hydrocarbons and other inert gases or liqs. in water at different temps. The statistical distribution is described by a convoluted partition function ZG•ζs. The product of a grand canonical partition function ZG represents the distribution of the species in the reaction while the canonical partition function ζs represents the properties of the solvent. The first deriv. of the logarithm of the partition function with respect to 1/T is the apparent enthalpy which is the result of the contributions of the sep. partition functions, {ΔHapp}T = ΔH0 + nwCp,wT, where {ΔHapp}T refers to ZG, nwCp,wT = -ΔHw to ζs and ΔH0 is the change in enthalpy of hydrocarbon-water reaction. The plot {ΔHapp}T vs. T results in a straight line with slope nw at const. Cp,w. The apparent enthalpy is obtained from the coeffs. of the polynomial fitting of the soly. data, as a function of l/T. Alternatively, the apparent enthalpy can be detd. calorimetrically. The enthalpy thus obtained is a linear function of the Kelvin temp. The values of nw range from 1.6, 1.9, 5.6 to 5.8 for helium, hydrogen, butane and hexane, resp. For fluoro compds. the range of nw is 10.1 to 11.1 indicating that nw is a function of the no. of water mols. expelled from the cage of solvent to form a cavity to host the solute mol. The anal. of several sets of calorimetric or soly. data with the present mol. thermodn. model yields values of ΔH0 and nw consistent with the size of the dissolved mols
Host-guest interactions between Beta-cyclodextrin and piroxicam.
No full textIn order to obtain experimental evidence for the complexation between beta-cyclodextrin and piroxica (Chydroxy-2-methyl-N-2-pyridyl-2H-1,2-benzothiazine-3-carboboxamidede-l,l-dioxide) and to investigate the thermodynamics of this interaction, a flow microcalorimetric study of this system has been undertaken. In fact, the mixing of beta-cyclodextrin and piroxicam in ratio 1 : 1 in given experimental conditions gives rise to a system showing better pharmacological properties than piroxicam alone. The results confirm the formation of a complex between B-cyclodextrin and piroxicam and allow the evaluation of the equilibrium constant for the process, assuming the formation of a 1 : 1 complex. It results in log K’ = 3.54, where K’ is the apparent formation constant at pH 9.00, in good agreement with literature data for similar compounds. The values of AH and AS are also reported
Protonation equilibria and hydration in disubstituted benzoic acids
No full textThe protonation constants log kapp of a series of disubstituted benzoic acids in aqueous solution at different temperatures between 5” and 55°C have been determined potentiometrically. The data of log k,, have been analyzed under the light of a statistical thermodynamic model. The curvature of the function log k,, = f( l/ T) is related to the number n, of water molecules involved in the protonation and hydration reaction. The upward concavity of the curves of dinitro compounds are steeper that those for monosubstituted acids and imply higher number of water molecules. The curves of polyalkyl-substituted benzoic acids as determined by other authors show opposite (downward concavity) curvatures corresponding to negative numbers n, of water molecules. The values of log kapp at 25°C of disubstituted and polyalkyl-substituted benzoic acids plotted against the Hammett substituent constants o&,, deviate significantly from the line of the Hammett model.
Isobaric heat capacity and structure of water and heavy water in the liquid state
No full textThe isobaric heat capacity of liquid H20, Cp, as a function of temperature, decreases between 0 ° and about 35°C and then increases up to 100°C. Analogous behaviour is shown by liquid D20. A statistical thermodynamic model has been applied to the experimental heat capacity data. The behaviour is explained by assuming that an equilibrium A + B = AB is established between clusters A and AB of water of different composition. The total heat capacity is considered as the sum of three terms Cp = (1 - a)Cp.0.Aa + caCp.o.A + DCp,app. The term DCp.app depends explicitly on the reaction enthalpy. In H20, the enthalpy DH = -1.84kJ mol-1 for the dissociation reaction and the heat capacity Cp. B = 47.8J K- 1 mol- 1 for free water molecules are calculated.
Analogous calculations performed for D20 yield the enthalpy, DH = - 1.64 kJ mol 1 and the heat capacity, Cp.a = 49.18 J K- 1 mol- 1
Swelling efficiency of disintegrants: A direct evaluation through calorimetric analyses
No full textThe behavior of some disintegrants used in pharmaceutical formulations (sodium carboxymethylstarch, sodium carboxymethylcellulose, polyvinylpyrrolidone) and of a new candidate disintegrant (a polymeric derivative of β-cyclodextrin) has been investigated. Calorimetric measurements have been performed on powdered samples of these materials by water addition and allowed the interpretation of the mechanism of interaction with water in terms of two distinct contributions: heat of wetting and heat of swelling
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A software for the estimation of binding parameters of biochemical equilibria based on statistical probability model
No full textAn algorithm is proposed for the estimation of binding parameters for the interaction of biologically important macromolecules with smaller ones from electrometric titration data. The mathematical model is based on the representation of equilibria in terms of probability concepts of statistical molecular thermodynamics. The refinement of equilibrium concentrations of the components and estimation of binding parameters (log site constant and cooperativity factor) is performed using singular value decomposition, a chemometric technique which overcomes the general obstacles due to near singularity. The present software is validated with a number of biochemical systems of varying number of sites and cooperativity factors. The effect of random errors of realistic magnitude in experimental data is studied using the simulated primary data for some typical systems. The safe area within which approximate binding parameters ensure convergence has been reported for the non-self starting optimization algorithms
Calculation of site affinity, cooperativity between sites and self-association in polymer-ligand-proton complexes.
No full textThe statistical probability of state of a solution containing a reacting receptor M, a ligand A (and eventually proton H) is described by a molar partition function Z M = exp(-AGIRT) referred to M, or ZA or Zu, respectively. The partition function for one class of sites can be expressed as the function of site constants kj and cooperativity functions yj.i = exp{bj(i - 1)). Binding in a single class can be represented by a vector Jpctj whose elements correspond to single species. For more classes of sites, the joined probability is obtained as tensor product of single class vectors giving rise to complexation matrices Mpqr. There is one partition function for each component of the system. If the complexes are of type HPMQAR there are three partition functions Z u, Z, and ZA. The relationships between partition functions and total analytical amounts Tu, TM, T, respectively are given. The experimental data obtained in a potentiometric titration with electrode reversible to [H] or other free component can be reproduced as the function of site constants kj and cooperativity functions exp{bj(i - l)} for each class j. The best values of kj and bj, can be calculated following a nonlinear least squares procedure by means of a computer program that is here presented
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