Entropy/enthalpy compensation: hydrophobic effect, micelles and protein complexes

Molecular interpretations are here presented of the hydrophobic effect, which is the cause of the low solubility of apolar substances in water. The solubilization process of substances such as the noble gases consists in the formation of a cavity in the solvent with expulsion of nw water molecules. The process is associated to an entropy/enthalpy (S/H) compensation linearly dependent upon the temperature. The observed enthalpy Happ, either determined calorimetrically or by van’t Hoff equation, shows Cp,app≠0 and positive. We set Cp,app= nwCp,w where Cp,w is the isobaric heat capacity of water. The number nw (nw>0) of relaxed water molecules is proportional to the size of the solute molecule and hence of the cavity. The term nwCp,wT is actually an entropy term, which compensates for part of the reaction enthalpy (H00) of water molecules. The presence of polar groups and/or charges in the solute molecule, on the other hand, exerts on the water molecules the same action as that produced by micellisation (nw<0)

Apparent and Partial Molar enthalpies of some Potassium p-n-Alkyl-Benzoate Aqueous Solutions

Dilution heats at 298 K of aqueous solutions of potassium salts of p-n-hexylbenzoic acid (KHB), p-n-heptylbenzoic acid (KHEB), p-n-octylbenzoic acid (KOB) in KOH 0.1 m have been measured as a function of concentration by using the flow mixing cell of a Thermometric TAM microcalorimeter. From the experimental data, apparent and partial molar enthalpies versus concentration have been obtained. The group contribution of the -CH2- group in the plateau region results -1.6 kJ mol-1 per - CH2-. This value is comparable with that obtained in the case of cationic surfactants. The enthalpy changes upon micellization (DHm) have been obtained by using a pseudo phase transition approach and assuming as cmc the abscissa of the first inflection point of the curves of FL versus m. Micellization enthalpies result additive with a group contribution for the methylene group of -1.5 kJ mol-1 per group, comparable with that obtained for similar anionic compounds in the same experimental conditions. The behavior of the compounds under study is compared with that of the potassium salts of 4-((alkylcarbonyl)amino) benzoic acids in order to understand the role played by the amido group bridging the benzoic moiety and the alkyl chain. The amido group appears to behave as a part of the polar head, strongly increasing the hydrophilicity of the molecule

Protonation and hydration equilibria in carboxylic acids. Mono-substituted benzoic acids

The 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