Sergio Cabani

Group contributions to the thermodynamic properties of non-ionic organic solutes in dilute aqueous solution

The thermodynamic properties ΔGho,ΔHho, and ΔCp,hoassociated with the transfer of non-ionic organic compounds from gas to dilute aqueous solution and the limiting partial molar properties Cpo,2 and V22 of these compounds in water are described through a simple scheme of group contributions. A distinction is made between groups made only of carbon and hydrogen, and functional groups i.e. groups containing at least one atom different from carbon and hydrogen. Each group is assigned a contribution, for each property, through a least squares procedure which utilizes only molecules containing at most one functional group. Finally, for compounds containing more than one functional group, correction parameters are evaluated as the differences between the experimental values and those calculated by means of the group contributions. The different behavior of hydrophilic compared with hydrophobic groups is discussed for the various properties. A rationale for the correction parameters, i.e. for the effects of the interactions among hydrophilic groups on the thermodynamic properties, is attempted.

Partial molal expansibilities of organic compounds in aqueous solution. I. Alcohols and ethers

The temperature dependence of limiting apparent molal volumes Φ°υ in water for some alcohols (methanol, ethanol, 1-propanol, 2-butanol, 3-pentanol, 3-hexanol, 2,5-hexanediol, cyclopentanol, cyclohexanol, cycloheptanol, and 1,4-cyclohexanediol) and ethers (trimethylene oxide, tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, 1,3-dioxepane, 1,3,5-trioxane, dimethoxymethane, 1,2-dimethoxyethane, diethoxymethane, and diethyl ether) has been studied in the temperature range 10–50°C by means of an automatic digital-readout dilatometer. Values of the thermal expansion coefficient α* = (1/Φ°υ)(∂Φ°υ/∂T)p have been obtained at several temperatures and discussed together with literature data on expansibilities of related compounds. The data show a wide spectrum of values of α* at low temperatures which is narrowed at the higher ones. The expansibilities of monofunctional alcohols increase with increasing temperature; the opposite effect is observed for polyhydric alcohols. The α*s of ethers are very slightly temperature dependent and are much higher, at low temperature, than those of alcohols having the same ratio ofno/nc. These results are discussed in terms of solutewater interactions, and a possible interpretation is put forward.

Adiabatic and isothermal apparent molal compressibilities of organic compounds in water. I. Cyclic and open-chain secondary alcohols and ethers

The limiting apparent molal adiabatic and isothermal compressibilities in aqueous solutions of alcohols and ethers at various temperatures have been determined by means of ultrasonic velocity measurements. The following compounds have been considered: secondary cyclic and open-chain alcohols (cyclopentanol, cyclohexanol, cycloheptanol, 2-butanol, 3-pentanol, 3-hexanol, 4-heptanol; cyclic ethers of the types (CH2)3On (n=1, 2, 3), (CH2)nO2 (n=3, 4, 5), and (CH2)nO (n=3, 4, 5); and the linear diethers dimethoxymethane, diethoxymethane, and 1,2-dimethoxyethane. The results indicate that both alcohols and ethers cause the water near the solute to become more resistant to pressure than the bulk. This is in agreement with the prediction of a mixture model for water.

Volumetric properties of amphionic molecules in water. Part 2.—Thermal expansibility and compressibility related to the formation of zwitterionic structures

The dependence of the partial molar volumes of a number of α-amino acids, ω-amino acids, dipeptides and diketopiperazines on temperature has been determined at various concentrations and for the temperature range 0–55 °C using an automatic dilatometer; the limiting partial molar expansibilities, Φ°E, have been calculated. Measurements of the sound velocity at 15 MHz and at various concentrations allowed us to determine the compressibility of the solutions at 25 °C and to calculate the limiting partial compressibilities, Φ°K,S, for some of the above mentioned compounds. Finally, by using the known values Φ°E and Φ°K,S together with the partial molar heat capacity of the solute and some physical properties of pure water, the limiting isothermal partial molar compressibilities, Φ°K,T, have been obtained.The values of Φ°E and Φ°K,S and the trends they show when the structure of the amphionic molecule is changed have been analysed by taking into account the features exhibited by Φ°E and Φ°K,S of related charged and non-charged compounds. The dependence on temperature and pressure of the internal and external proton-exchange reaction between a carboxyl and an amino group have been evaluated and compared.

Thermodynamic study of the partitioning of organic compounds between water and octan-1-ol. Effects of water as cosolvent in the organic phase

The effect of water, as cosolvent, in changing the affinity of octan-1-ol towards organic solutes according to their chemical nature, has been examined by determining the thermodynamic functions of solvation, ΔsolvX°(j)(X=G, H, S), in anhydrous (j= oct) and in water-saturated octan-1-ol (j= oct*) of organic compounds of the RY type (R = hydrocarbon frame, Y = H, O, CO, NH2, NH, N). From these data and the analogous thermodynamic functions of hydration, ΔsolvX°(W), the thermodynamic functions, ΔtrX°(j→j*), for the three ideal or practical processes of transfer (namely, w→oct, w→oct*, oct→oct*) have been calculated.The ΔtrX°(j→j*) functions have been correlated with the structure of the solutes by using their intrinsic volume as a unique structural parameter. The coefficients of the linear equations obtained give information about the contributions to the envisaged thermodynamic function, connected with the formation of a cavity containing the solute and to the solute–solvent interactions. In the case of the ΔtrG°(j→j*) functions, the LSERs (linear solvation energy relationships) have also been used for correlating the thermodynamic functions to the structural features of the solutes.

Binding of molecular dioxygen to Co(II) complexes in aqueous solutions

Abstract The reaction of dioxygen addition to Co(II) complexes in aqueous solution and the possible further evolution of dioxygen adducts are taken into consideration. A short survey is given of the thermodynamic features shown by the reactions of the formation of superoxo and peroxo compounds corresponding to Co(II) complexes with simple and superstructured ligands. Two topics are then developed. In the first one, the connections between the structure of the ligand bonded to Co(II) and the type and the thermodynamic and kinetic stability of the dioxygenjadducts formed are discussed. Open-chain and cyclic tetraazotate saturated amines with nitrogens connected by ethylenic or propylenic bridges different in number and in their order are discussed. In the second, the state of the art concerning the dioxygen adducts corresponding to Co(II) complexes with binucleating ligands is summarized. Attention is focused on the behaviour towards molecular oxygen shown, in aqueous solutions, by Co(II) complexes with saturated open-chain or cyclic amines containing 9 or 10 nitrogens connected by ethylenic chains over a large range of R (= C L 0 C Co 0 ) and pH values.

Thermodynamic and kinetic aspects of the binding reaction of molecular oxygen on Co(II) complexes in aqueous solutions

A short survey is given of the main characteristics of the reaction of uptake of molecular oxygen by biological dioxygen carriers. Co(ll) complexes able to bind dioxygen are then examined in order to determine whether the type and features of the peroxocomplexes formed are related to the nature of the ligand and the solvent. The thermodynamic and kinetic data relative to the formation in aqueous solution of peroxocompounds starting from Co(ll) complexes with ligands belonging to three families (open-chain, macrocyclic and macropolytopic) of saturated polyamines are also examined in greater detail.

Free energy and enthalpy changes for the process of transfer from gas and from dilute aqueous solutions of some alkanes and monofunctional saturated organic compounds

Standard free energies, ΔsolvG°(oct), and enthalpies, ΔsolvH°(oct), of solvation in octan-1-ol of some alkanes (heptane, octane), ketones (propan-2-one, butan-2-one, hexan-2-one, heptan-4-one), ethers (dipropyl ether, dibutyl ether, tetrahydropyran), alkanols (butan-1-ol, butan-2-ol, 2-methylpropan-2-ol) and amines (pyrrolidine, N-methylpyrrolidine, and piperidine) with open-chain and cyclic structure have been determined at 298.15 K. The ΔsolvG°(oct) values were obtained from measurements of partial vapour pressures of dilute solutions and the ΔsolvH°(oct) values, by adding the heats of solution, determined by calorimetry, to known values of enthalpy of vaporization.These data are compared with the standard free energies and enthalpies of solvation in water, and the standard thermodynamic functions for the ideal transfer process of the solutes from pure water to pure octan-1-ol, δtrX°(w–oct)(X=G, H) are calculated.For the examined solutes, hydrocarbons and monofunctional saturated organic compounds, the thermodynamic functions of solvation in octanol and in water are closely correlated to the position of the functional group in the molecular skeleton. The values of the enthalpy of transfer from water to octan-1-ol are also related more to the topologic characteristics of the solute molecules than to their size or to the nature of their functional group. In contrast, the size of the molecules, as well as the presence of a functional group, are important with regard to ΔtrG°(w–oct). An increase of 1 cm3 mol–1 in the intrinsic volumes corresponds to an increase in the value of the partition coefficients of ca. 15%. The substitution of a part of the hydrocarbon surface with a polar surface produces a very large increase in the values of the partition coefficients.The values of the enthalpy of transfer from water to octan-1-ol are always positive, in contrast to the standard free energies of transfer. The entropic term operates by always favouring the transfer towards the alcoholic phase.The potential ability of anhydrous octanol to extract organic solutes of various molecular structure from water-saturated octanol and from hexadecane is also evaluated and discussed.

Thermodynamic study of organic compounds in octan-1-ol. Processes of transfer from gas and from dilute aqueous solution

Free energies and enthalpies of solvation of water and some hydrocarbons (hexane, cyclohexane), ethers (diethyl ether, tetrahydrofuran) and ketones (propanone, pentan-3-one, cyclopentanone) in octan-1-ol have been determined at 298.15 K from vapour-pressure measurements of dilute solutions and from limiting heats of solution. These solvation functions in octanol have been used together with the corresponding hydration functions in order to obtain water–octan-1-ol partition coefficients and their dependence on temperature. A comparison is made with the practical partition coefficients relative to mutually saturated solvents.

Apparent molal heat capacities of organic compounds in aqueous solution. Part 3.—ω-Amino acids and related compounds

Values of apparent molal heat capacities ΦCp in the range 23°–55°C, were determined by adiabatic calorimetry for the following amino acids: glycine, α-alanine, α-aminoisobutanoic acid, valine, serine, threonine, sarcosine, dimethylglycine, betaine, β-alanine, β- and γ-aminobutanoic acids, δ-aminopentanoic acid and Iµ-aminohexanoic acid. Some hydroxy acids, sodium and ammonium salts of carboxylic acids, methyl and ethyl esters of acetic acid and glycine methylester hydrochloride have also been studied.An attempt has been made to quantify the contribution to ΦCp associated with charge separation in amphionic molecules by comparing the experimental value (ΦexpCp) for amino acids with those for similar uncharged molecules (ΦrefCp). The ΔΦ* values (ΦexpCp–ΦrefCp) thus obtained for +H3N(CH2)m COO– compounds (m= 2, 3, 4, 5) are used in order to obtain some information about the interactions between neutral or charged amino and carboxylic groups. Effects connected with the methyl additions to N+ centres in glycine or ammonium salts are considered.