Gérald Perron

Enthalpies of the urea-tert-butanol-water system at 25°C

The enthalpies of solution of urea (U) in water (W)-tert-butanol (TBA) mixtures and of TBA in W-U mixtures, the enthalpies of dilution of TBA in W, and the enthalpies of mixing of U and TBA aqueous solutions were measured with a solution calorimeter and a flow microcalorimeter. Enthalpies of transfer of U and TBA to the mixed solvents were derived. Also, pair and triplet interaction parameters between the various solutes were derived from the mixing and dilution experiments. The enthalpic pair parameter hU-TBA is positive, suggesting that the main contribution to this parameter is the decrease in hydrophobic hydration of TBA in the presence of U.

Reexamination of the heat capacities obtained by flow microcalorimetry. Recommendation for the use of a chemical standard

A series of measurements with aqueous electrolyte and nonelectrolyte solutions indicates that there is a small systematic difference between the heat capacities per unit volume determined with a Picker flow microcalorimeter and the original prototype. Through various tests and comparisons, it is, concluded that the commercial instrument gives results closer to the true values. Most of the previous data obtained in our laboratory have been corrected and expressed relative to aqueous NaCl at 25°C taken as a standard.

Model systems for hydrophobic interactions: Volumes and heat capacities ofn-alkoxyethanols in water

The densities and heat capacities of the first four members of the 2-n-alkoxyethanols were measured in water over the whole mole fraction range with a flow densimeter and a flow microcalorimeter. The methoxy and n-propoxy homologs were studied at 25°C, ethoxyethanol at 19, 25, and 40C, andn-butoxyethanol at 4, 10, 25, 40, and 55°C. While methoxyethanol behaves as a fairly typical polar nonelectrolyte in water,n-butoxyethanol shows trends in the concentration dependence which resemble micellization; some pseudo-microphase transition occurs at about 0.02 mole fraction, and this transition concentration decreases with increasing temperature. There is no simple relationship between this phenomenon and the existence of a lower critical solution temperature at 49°C since the sharpness of the thermodynamic changes is maximum at the lowest temperature and at 55°C the apparent molal quantities on both sides of the two-phase region appear to fall on the same continuous curve. In the region prior to the pseudo-microphase separation the apparent and partial molal heat capacities decrease regularly but beyond approximately 0.01 mole fraction increase sharply to a maximum, suggesting some type of pre-association. The apparent molal heat capacity of water in the binary solutions is larger than the molar heat capacity of water over the whole mole fraction range. The present data seem to be consistent with a clathrate model for hydrophobic hydration and interactions with these systems.

Microheterogeneity in aqueous-organic solutions: Heat capacities, volumes and expansibilities of some alcohols, aminoalcohol and tertiary amines in water

The heat capacities per unit volume and the densities of aqueous solutions of 2-propanol, neopentanol, tert-amylalcohol, 2-amino-2-methylpropanol, triethylamine and diethylmethylamine were measured, in many cases as a function of temperature, over the whole mole fraction or solubility range. Apparent and partial molal heat capacities, volumes and expansibilities were derived. The concentration dependence of these functions suggest the existence of transitions in some of these systems, in the water-rich region, qualitatively similar to micellization. The large relaxation contribution observed with some of the thermodynamic functions of hydrophobic alcohols and amines suggests a reinforcement of hydrophobic hydration due to strong hydrogen-bonding interactions of the polar groups with water.

Complete thermodynamic properties of nonyl- and decyltrimethylammonium bromides in water

Abstract The freezing point depressions, enthalpies of dilution, sound velocities, volumetric heat capacities, and densities were measured for nonyltrimethylammonium bromide (NTABr) and decyltrimethylammonium bromide (DTABr) in water over a large concentration region. The last two properties were measured as a function of temperature. From these data the nonideal free energies and entropies and the partial molal relative enthalpies, heat capacities, volumes, and expansibilities and compressibilities were derived. With the exception of compressibilities all other functions were determined as a function of temperature. The thermodynamic functions of micellization, defined as the molar change in property at the CMC, were calculated as a function of temperature. It is important in this respect to use excess entropies S 2 − S 2 0 and not only the nonideal contribution S 2 ni . All these data are self-consistent as a function of temperature, and the experimental free energies of micellization obtained from enthalpies and entropies are equal to zero, inside the experimental uncertainty, as expected from the equilibrium situation. The properties of the monomers and of the micelles and the trends in micellization functions are all consistent with current ideas on hydrophobic solutes and micelles.

Apparent molal volumes and heat capacities of ionic surfactants in water at 25°C

Abstract The densities and volumetric specific heats of some cationic and anionic surfactants were measured with a flow digital densimeter and a flow microcalorimeter in water at 25°C in the concentration range 0.01-0.7 m. The systems studied were sodium, octyl, decyl and dodecyl sulfate, nonyl and decyl trimethylammonium bromide. The derived apparent molal volumes φV show a sharp increase at the critical micelle concentration, CMC, and the apparent molal heat capacity φC, a sharp decrease. The volumes and heat capacities of micellization increase in magnitude with the surfactant chain length and are consistent with a large decrease in hydrophobic character during micellization. The present measurements do not support the existence of a second CMC for surfactants in water. The sign of the charge of the ionic head does not seem to have much influence on the change of volume and heat capacity on micellization. Combining the heat capacity of micellization with known heats of micellization, the temperature dependence of the CMC of sodium dodecyl sulfate can be predicted over a wide range of temperatures.

Heat capacities and volumes of interaction between mixtures of alcohols in water at 298.15 K

Abstract The heat capacities and densities of mixtures of aqueous solutions of normal alcohols (methanol to n -butanol) and t -butanol were measured at 298.15 K at low molalities. The results were used to calculate the thermodynamic pair and triplet interaction parameters between solutes for heat capacities and volumes. The pair parameters are approximately a linear function of the total number of carbon atoms of the two solutes. The enthalpic pair and triplet interaction parameters for (ROH + H 2 O) are also reported. The temperature dependence of the pair parameters for Gibbs free energies, enthalpies, entropies, heat capacities, and volumes are discussed in terms of structural changes in the aqueous solutions.

Thermochemistry of aqueous micellar systems

Surfactants tend to aggregate above a certain concentration, called the critical micelle concentration, into large clusters or micelles. With modern calorimetric and other related techniques it is now possible to measure precisely and rapidly the thermodynamic properties of liquid systems over a wide range of concentration and temperature. An overview is given of data obtained in our laboratory for heat capacities, enthalpies, entropies and free energies of aqueous sodium decanoate, octylammonium hydrobromide, nonyl and decyltrimethylammonium bromide. The thermodynamic functions of micellization are derived and interpreted with a phase—sepa— ration model. The closely related microemulsions are also briefly dis— cussed.

Apparent molal volumes and heat capacities of some alkali halides and tetraalkylammonium bromides in aqueoustert-butanol solutions

The densities and volumetric specific heats of hydrochloric acid, alkali chlorides and bromides, and tetraalkylammonium bromides were measured in 0 to 40% by weighttert-butanol (t-BuOH) in water with a flow densimeter and a flow microcalorimeter. The effect of salt concentration was investigated in the case of NaCl. The apparent molal volumes and heat capacities and the derived transfer functions of the electrolytes from water tot-BuOH-water mixtures can be interpreted through solute-solute pair and triplet interactions by analogy with the transfer functions oft-BuOH from water to electrolyte solutions, with the salting-in and salting-out effects, and with the influence of electrolytes on the thermodynamics of micellization. At lowt-BuOH concentrations, the transfer functions seem to be reflecting primarily electrolyte-nonelectrolyte pair interactions. At intermediatet-BuOH concentration, wheret-BuOH associates, the hydrophobic bonding is enhanced by hydrophilic ions through a salting-out effect on monomers and by hydrophobic salts through triplet interaction (mixed association complexes). The Me4NBr and Et4NBr are intermediate electrolytes which do not have much effect on thet-BuOH hydrophobic bonding. At hight-BuOH concentrations the transfer functions tend to the values they would have in puret-BuOH.

The effect of temperature and urea on the apparent molal volumes and heat capacities of n-nonyltrimethylammonium bromide in water

Abstract The densities and volumetric specific heats of the cationic surfactant n-nonyltrimethylammonium bromide (NTABr) have been measured in water and in urea-water mixtures in the temperature range 5 to 50°C, using, respectively, a flow digital densimeter and a flow microcalorimeter. From the experimental data, the apparent molal volumes ϕv and heat capacities ϕc have been determined. The apparent molal volume at infinite dilution ϕv0 in water increases with temperature and the transfer function ΔVNTABr0 (H2O → 3 M urea) is positive and decreases with temperature. The apparent molal heat capacity at infinite dilution ϕC0 in water passes through a maximum when plotted against temperature, while the transfer function ΔVNTABr0 (H2O → 3 M urea) is negative and tends to zero at low temperature. The concentration dependence of ϕV and ϕC in the premicellar region varies significantly with temperature and with the presence of urea and is consistent with the behavior of typical hydrophobic solutes. The absolute values of the change in volume and heat capacity during micellization, ΔVm and ΔCm, decrease with temperature and in the presence of urea and can be used to predict the temperature and pressure dependence of the CMC.