Showing papers on "Standard molar entropy published in 1968"

Adsorption at the rutile-solution interface: i. thermodynamic and experimental study.

Abstract: A detailed thermodynamic analysis of the double layer at the interface, simple inorganic oxide-aqueous solution, is presented with special emphasis on the assumptions normally made in relating experimental observation to theoretical treatment. An experimental study of the adsorption of potential-determining ions (H+, OH−) at the rutile (TiO2)/solution interface is reported. The results obtained by potentiometric titration of suspensions of the oxide indicate that equilibration is a two-step process with the initial fast step yielding the desired information on the surface behavior of the system. The point of zero charge (pzc) for TiO2 at room temperature was observed to lie at pH 6.0, independently of its crystalline modification, but was lowered to below pH 4 by heat treatment of the oxide prior to adsorption measurement. The pzc was also observed to approach the neutral point (12 pKw) with increasing temperature in the temperature range, 25°C to 95°C. A thermodynamic consideration of the transfer of cationic and anionic potential-determining species from the aqueous solution to the surface at the pzc allowed the difference in the standard entropy of transfer to be estimated from this information. The entropy difference (∼2.0 cal/deg mole) for TiO2 is compared with the much larger difference of ∼42 cal/deg-mole obtained from more extensive measurements on AgI. The striking difference between the two double layer systems is to be sought in differences in the structure and composition of the interphasal region separating the bulk phases which, in turn, must be related to the variation in interaction of the polar solvent with the solids, TiO2 and AgI.

Adsorption of n-alkanols at the Air/Aqueous Solution Interface

Abstract: Adsorption at the air/aqueous solution interface of n -alkanols with hydrophobic alkyl chains containing 1–8 carbon atoms has been studied by the pendant drop method of surface tension measurement. Standard thermodynamic quantities of transfer of n -alkanols to this interface from bulk solution, liquid alkanol, and alkanol vapor have been calculated using thermodyamic data for solution and vaporization of liquid n -alkanols. Standard heats of adsorption of n -alkanols from methanol to pentanol from the pure liquid state to the air/solution interface show little dependence on chain length. It is inferred that methylene groups of the adsorbed molecules are in contact with the water, whereas the standard enthalphy change (≈ −2.6 kcal/mole) is a measure of the difference in hydrogen bonding energy for a hydroxyl group in the liquid alkanol and water. The standard entropy of adsorption from the vapor phase is about twice the theoretical entropy change accompanying the loss of one degree of translational freedom due to the strong interaction of the hydroxyl group with the aqueous substrate.

Entropy of vaporisation and the structure of liquids

Abstract: It has been found that the molar entropy of vaporisation of globular-shaped molecules or those with effective spherical field symmetry can be uniquely expressed as a function of molecular number (sum of atomic numbers) only. The higher values for non-globular molecules are plausibly related (through an aspherical factor, θ, which has certain additive properties) to molecular shape and consequent molecular parallelism in the liquid state. The entropies of vaporisation of flexible molecules involve an additional term of magnitude proportional to the number of flexible links present. Expressions are developed, the use of which permits (a) estimation of latent heats of a great variety of materials with a reliability of better than 1% and (b) a reliable diagnosis of molecular association.

Studies of Poly-2-vinylpyridine. V. Temperature Dependence of the Intrinsic Viscosity and Osmotic Pressure of an Ethanol Solution

Abstract: The viscosity of a series of 10 fractions of poly-2-vinylpyridine and the osmotic pressure of a fraction of the same polymer in ethanol were measured at several temperature. The temperature coefficients of the intrinsic viscosities of all the fractions measured were found to be completely negative. The Mark-Houwink equations for molecular-weight-homogeneous samples were obtained in ethanol at 10, 25, 40, and 65°C. The Flory constant, K=[η]θ⁄M1⁄2, obtained by the method of Fixman was found to be 13.0×10−4 at 25°C; this value is considerably larger than those estimated from the intrinsic viscosity data of this polymer in non-acidic solvents. The K value lowered to an ordinary level when it contained 2.7 N of dry ammonia in ethanol at 25°C. The values of the thermodynamic interaction parameters for dilution, ψ1 and θ, were obtained from the viscosity data; the former was negative, but nearly zero (−0.02), while the latter was unusually large (397°C). The excess molar entropy of dilution obtained from the mea...