195 research outputs found
Hydrophobic hydration processes. General thermodynamic model by thermal equivalent dilution determinations
No full textE. Fisicaro, C. Compari , A. Braibanti
Biophysical Chemistry 151 (2010) 119–138
Hydrophobic hydration processes
General thermodynamic model by thermal equivalent dilution determinations
The “hydrophobic hydration processes” can be satisfactorily interpreted on the basis of a common molecular model for water, consisting of two types of clusters, namely WI and WII accompanied by free molecules WIII. The principle of thermal equivalent dilution (TED) is the potent tool (Ergodic Hypothesis) employed to monitor the water equilibrium and to determine the number ξw of water molecules WIII involved in each process. The hydrophobic hydration processes can be subdivided into two Classes: Class A includes those processes for which the transformation A(−ξwWI→ξwWII+ξwWIII+cavity) takes place with the formation of a cavity, by expulsion of ξw water molecules WIII whereas Class B includes those processes for which the opposite transformation B(−ξwWII−ξwWIII→ξwWI−cavity) takes place with reduction of the cavity, by condensation of ξw water molecules WIII. The number ξw depends on the size of the reactants and measures the extent of the change in volume of the cavity. Disaggregating the thermodynamic functions ΔHapp and ΔSapp as the functions of T (or lnT) and ξw has enabled the separation of the thermodynamic functions into work and thermal components. The work functions ΔGWork, ΔHWork and ΔSWork only refer specifically to the hydrophobic effects of cavity formation or cavity reduction, respectively. The constant self-consistent unitary (ξw=1) work functions obtained from both large and small molecules indicate that the same unitary reaction is taking place, independent from the reactant size. The thermal functions ΔHTh and ΔSTh refer exclusively to the passage of state of water WIII. Essential mathematical algorithms are presented in the appendices
HYDROPHOBIC HYDRATION PROCESSES. THERMAL AND CHEMICAL DENATURATION OF PROTEINS
No full textThe hydrophobic hydration processes have been analysed under the light of a mixture model of water that is assumed to be composed by clusters (W5)I, clusters (W4)II and free water molecules WIII. The hydrophobic hydration processes can be subdivided into two Classes A and B. In the processes of Class A, the transformation A(−ξwWI→ξwWII+ξwWIII+cavity) takes place, with expulsion from the bulk of ξw water molecules WIII, whereas in the processes of Class B the opposite transformation B(−ξwWIII−ξwWII→ξwWI−cavity) takes place, with condensation into the bulk of ξw water molecules WIII. The thermal equivalent dilution (TED) principle is exploited to determine the number ξw. The denaturation (unfolding) process belongs to Class A whereas folding (or renaturation) belongs to Class B. The enthalpy ΔHden and entropy ΔSden functions can be disaggregated in thermal and motive components, ΔHden=ΔHtherm+ΔHmot, and ΔSden=ΔStherm+ΔSmot, respectively. The terms ΔHtherm and ΔStherm are related to phase change of water molecules WIII, and give no contribution to free energy (ΔGtherm=0). The motive functions refer to the process of cavity formation (Class A) or cavity reduction (Class B), respectively and are the only contributors to free energy ΔGmot. The folded native protein is thermodynamically favoured (ΔGfold≡ΔGmot<0) because of the outstanding contribution of the positive entropy term for cavity reduction, ΔSred≫0. The native protein can be brought to a stable denatured state (ΔGden≡ΔGmot<0) by coupled reactions. Processes of protonation coupled to denaturation have been identified. In thermal denaturation by calorimetry, however, is the heat gradually supplied to the system that yields a change of phase of water WIII, with creation of cavity and negative entropy production, ΔSfor≪0. The negative entropy change reduces and at last neutralises the positive entropy of folding. In molecular terms, this means the gradual disruption by cavity formation of the entropy-driven hydrophobic bonds that had been keeping the chains folded in the native protein. The action of the chemical denaturants issimilar to that of heat, by modulating the equilibrium betweenWI, WII, andWIII toward cavity formation and negative entropy production. The salting-in effect produced by denaturants has been recognised as a hydrophobic hydration process belonging to Class A with cavity formation, whereas the salting-out effect produced by stabilisers belongs to Class B with cavity reduction. Some algorithms of denaturation thermodynamics are presented in the Appendices
Thermodynamics of the Equilibria Between Some 2-(Methylpyridyl)Benz-X-Azoles and Transition Metal Ions
No full textStudies of Equilibria Between Amphiphilic Derivatives Of Salycilic Acid and Transition Metal Ions
No full textThermodynamics of Aqueous Solutions of Dodecyldimethylethylammonium Bromide
No full textThe thermodynamic properties of the aqueous solutions of dodecyldimethylethylammonium bromide (DEDAB) were determined as a function of concentration by means of direct methods. Dilution enthalpies at 298 and 313 K, densities and sound velocities at 298 K were measured, allowing the determination of apparent and partial molar enthalpies, volumes, heat capacities and compressibilities. Changes in thermodynamic quantities upon micellization were derived using a pseudo-phase transition approach. These data allow for the determination of the effect of the –CH2– group, when added to the polar head of alkyltrimethylammonium bromides. The properties mainly affected by this addition are the enthalpies and, as a consequence, the entropies. The lowering of the charge density on the quaternary nitrogen due to the inductive effect of the ethyl group, greater than that of the methyl one, raises the plateau value of apparent and molar enthalpy by a quantity similar to that due to the removing of a methylene group from the hydrophobic chain. This effect does not play a great role in the value of the cmc (i.e. on the free energy of micelle formation), since the small decrease in cmc of DEDAB compared to DTAB reflects the increase in the overall hydrophobicity of the molecule. Volumes of DEDAB are greater than those of DTAB by about 15 cm3 mol−1, both at infinite dilution and at micellar phase, a value in agreement with that generally accepted for a methylene group. The trends of apparent molar heat capacities and compressibilities vs m are the same as for DTAB: in fact, these quantities are related to the number of water molecules involved in the hydrophobic processes in solution, not very greatly affected by the substitution of a methyl group by an ethyl one on the polar head. In summary, this substitution affects to a significant extent the first derivatives of the free energy, but does not affect the second derivatives
Protonation and hydration equilibria in carboxylic acids. Mono-substituted benzoic acids
No full textThe protonation consts. of a series of mono-substituted benzoic acids in aq. soln. in the temp. range 5-55°C has been detd. The dependence of the consts. upon temp. has been analyzed under the light of a statistical thermodn. model assuming the existence in the system of discrete enthalpy levels. Each level is assocd. with one protonated species AHi. The true equil. const. k° is bound to the apparent const. by kapp = k°[W]-nw where [W]nw represents the solvent. The values of nw for monosubstituted benzoic acids either calcd. from the protonation consts. here detd. or from those reported by others range from 1.4 to 2.6 with an av. value nw(av) = 2.1 very close to that obsd. in aliph. acids
Molecular thermodynamic model for equilibria in solution. II. Statistical microscopic properties of ensembles
No full textA statistical thermodynamic model for the interpretation of the equilibria in solution is based on the principle that the
representative statistical ensembles can be characterized by two types of molecular distribution, one for non-reacting systems and another for reacting ones, respectively. Non-reacting and reacting ensembles correspond at the molecular level to one or a couple of potential curves, respectively. The properties of the thermodynamic model for solutions can be set up following some rules. These concern the statistical extension of the microscopic model to the whole ensemble and the successive averaging to get a mean partition function. The mean partition function is linked to the experimental domain of concentrations, dilutions and equilibrium constants (probability space) and to that of calorimetry, chemical work, and potentiometry (thermodynamics space). The formal connection between probability and thermodynamic space and the conformity of thermal equivalent dilution with the formulations of statistical thermodynamics are also shown
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
Gemini Pyridinium Surfactants: Synthesis and Conductimetric Study of a Novel Class of Amphiphiles
No full textA new series of pyridinium cationic gemini surfactants was prepared by quaternization of the 2,2'- (a,w alkanediyl)bispyridines with N-alkylating agents, whose reactivity is briefly discussed. Particularly useful was the use of long-chain alkyl triflates (trifluoromethanesulfonates) for both overcoming the sterical hindrance in the pyridines and obtaining higher synthetic yields. Wellknown 4,4' (a,w-alkanediyl)bis(1-alkylpyridinium) structures showed narrow temperature ranges for practical applications, due to their high Krafft points, while the new 2,2'-(a,w-alkanediyl)bis-
(1-alkylpyridinium) series, accounted for good surface active properties. Due to the Krafft points below 0 °C, they could be exploited as solutions in water at any temperature. The characterization of the behavior of the series was performed by conductivity measurements. Some of the proposed
structures exhibited unusual surface active behavior, which was interpreted in terms of particular conformational arrangements
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.
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