The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsg _xx andg _xxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of theh _xx andh _xxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects. Preliminary data were presented at the International Conferences on Chemical Thermodynamics at Baden (1973) and Montpellier (1975). The experimental part was carried out at the Istituto Chimico of the University of Trieste. To whom correspondence should be addressed.     

Lattice Dynamics in LaTa2-2xNb2xVO9-δ (x=0–0.1) Solid Solutions

The oxygen nonstoichiometry (x) of LaTa_2-2xNb_2xVO₉- (x = 0–0.1) solid solutions was studied using Xray phase analysis, vibrational spectroscopy, and radiospectroscopy. A correlation was found between (x) and the unit cell volume V(x) of the solid solutions. It was shown that the infrared spectra of LaTa₂VO₉- change in passing from = 0 to 0. The structural position of the oxygen vacancy in LaTa_2-2xNb_2xVO₉- is discussed.     

Water in melts: Raman spectral investigation of the molten Ca(NO3)2·4H2O−KNO3 system

The Raman spectra of molten mixtures of Ca(NO₃)₂\4H₂O–KNO₃ have been examined, covering the concentration range of 0–70 mole% KNO₃. The frequencies in the spectra of the mixtures have been found to change slightly with concentration. Striking variations in the band shapes have been observed in the regions corresponding to the O–H stretching mode (2850–3850 cm^–1) and the v₄-NO ₃ ^– mode (700–750 cm^–). The results are discussed in terms of perturbed quasi-lattice structure for the melt, in which there could be a displacement of water molecules in the first coordination sphere around Ca²⁺ by the NO ₃ ^– ion.     

Sequential injection spectrophotometric determination of phenylephrine hydrochloride in pharmaceutical preparations

A fully automated sequential injection spectrophotometric method for the determination of phenylephrine hydrochloride in pharmaceutical preparations is reported. The method is based on the condensation reaction of the analyte with 4-aminoantipyrine in the presence of potassium ferricyanide. The absorbance of the condensation product was monitored at 503 nm. A linear relationship between the relative peak height and concentration was obtained in the range 0.5-17.5 mg l⁻¹. The detection limit (as 3σ value) was 0.09 mg l⁻¹ and repeatability was 0.8 and 0.6% at 2.5 and 5 mg l⁻¹, respectively. Results obtained by this method agreed very well with those obtained by the AOAC official method.     

Solubility of Triethylamine in Tetraethylammonium Chloride Aqueous Solutions from 20 to 35°C

Solubilities of triethylamine in aqueous tetraethylammonium chloride solutions were measured at 20, 25, and 35°C. The molalities in Et₄NCl of the aqueous solvents ranged from 0.03 to 1 mol-kg^–1. The data were evaluated from density measurements using a vibrating-tube densimeter. At each temperature, least-squares method was used to fit experimental density data points to double polynomial equations of various degrees. Triethylamine molalities of the saturated aqueous phases were estimated by extrapolation from those equations. Experimental data were interpreted in terms of hydrophobic and electrostatic perturbed domains in the hydration shells of the noneleceory and of the cation of the salt, as a function of temperature and salt concentration. The conclusions obtained are consistent with previous volumetric studies.     

Solubility and thermodynamics of solution of argon in aqueous solutions of sodium octanoate and sodium dodecylsulfate

The ostwald absorption coefficient of argon was measured in aqueous solutions of two surfactants; sodium n-octanoate and in sodium dodecylsulfate, at several concentrations and at a few temperatures between 10–25°C. The free energies, entropies and enthalpies of solution were computed. A tentative interpretation of the results is given, based on a competition between the solubilization and the salting-out effects of the surfactants.     

Anomalies of the partition coefficient and properties of crystals in the region of low concentrations of an isomorphous admixture. 1. Unusual decomposition of solid solutions of the NaCl—CdCl2 system

Solid solutions NaCl—CdCl₂ were studied in an interval of CdCl₂ concentrations of 0.05—3 mol.% by Raman spectroscopy. The molecular form Na₂CdCl₄ decomposes: Na₂CdCl₄ 2Na⁺ + Cd²⁺ + 2Cl₂ ^– + 2e^–; 2Na + 2e^– 2Na⁰. Free sodium atoms form color centers of crystal (F centers) in the region of jumpwise changing the partition coefficient of CdCl₂ in NaCl (K jump).     

Multiple Jacobi Elliptic Function Solutions to Integrable Higher Order Broer-Kaup Equation in (2+1)-Dimensional Spaces

In this paper, we improve the method for deriving Jacobi elliptic function solutions of nonlinear evolution equations given in Ref. [12] and apply it to the integrable higher-order Broer-Kaup system in (2 1)-dimensional spaces.Some new elliptic function solutions are obtained.     

On Physically Unacceptable Numerical Solutions Yielding Strong Chaotic Signals

Physically unacceptable chaotic numerical solutions of nonlinear circuits and systems are discussed in this paper. First, as an introduction, a simple example of a wrong choice of a numerical solver to deal with a second-order linear ordinary differential equation is presented. Then, the main result follows with the analysis of an ill-designed numerical approach to solve and analyze a particular nonlinear memristive circuit. The obtained trajectory of the numerical solution is unphysical (not acceptable), as it violates the presence of an invariant plane in the continuous systems. Such a poor outcome is then turned around, as we look at the unphysical numerical solution as a source of strong chaotic sequences. The 0–1 test for chaos and bifurcation diagrams are applied to prove that the unacceptable (from the continuous system point of view) numerical solutions are, in fact, useful chaotic sequences with possible applications in cryptography and the secure transmission of data.     

Monte carlo studies of phase transitions in polymer blends and block copolymer melts

Summary The unmixing transition of both symmetrical polymer blends AB (i.e. chain lengthsN _A=N _B=N) and asymmetrical ones (N _B/N _A=2,3) is studied by large-scale Monte Carlo simulations of the bond fluctuation model. Combination of semi-grand-canonical simulation techniques, ?histogram reweighting? and finitesize scaling allows an accurate location of the coexistence curve in the critical region. The variation of the critical temperature with chain length (N) is studied and compared to theoretical predictions. For the symmetrical case, use of chain lengths up toN=512 allows a rough estimation of crossover scaling functions for the crossover from Ising to mean-field exponents. The order-disorder transitions in melts of both symmetric (compositionf=N _A/(N _A+N _B)=1/2) and asymmetric (f=3/4) block copolymers is studied for very short chains (16≤N≤60). The interplay between structure and chain configuration is emphasized. Qualitative evidence for ?dumbell formation? of chains and vacancy enrichment in A-B-interfaces and near hard walls is presented. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.