Effect of counterion on thermodynamic micellar properties of tetradecylpyridinium in aqueous solutions

Electrical conductivity of aqueous solutions of tetradecylpyridinium bromide and chloride has been measured as a function of surfactant molal concentration and temperature. From the molal dependence of conductivity, the critical micelle concentration and the micellar ionization degree were estimated. The temperature dependence of these parameters has been used for calculating the thermodynamic parameters related with the micellization process by using the classical charged pseudophase separation model. The effect of the counterion on the conventional thermodynamic potentials of micellization such as standard Gibbs free energy, enthalpy and entropy has also been a matter of study. Finally, the occurrence of the enthalpy–entropy compensation phenomenon was verified and the relevant parameters discussed.     

Acid-base equilibria of some pyridinol derivatives in binary water/organic solvent systems

Summary pK _a values of 2-hydroxy 3-pyridinol (HHP), 2-mercapto 3-pyridinol (MHP), and 2-carboxy-3-pyridinol (CHP) were determined by potentiometric titration in water/organic solvent mixtures containing 20 mole% of organic solvent at 25±0.1 °C and 0.1M ionic strength (KNO₃) applying an empiricalpH correction for mixed aqueous solvents. The influence of the organic solvents on the dissociation constants and tautomeric equilibria of the pyridinol derivatives is discussed. The effect of the molecular structure of the compounds onpK _a is also explained. Titrations of a mixture of two weak diprotic acids (HHP andCHP) in a water/dimethylsulphoxide medium containing 20 mole% organic solvent at constant ionic strength were evaluated using theGran method.

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Säure-Basen-Gleichgewichte einiger Pyridinolderivate in binären Systemen aus Wasser und organischen Lösungsmitteln

Zusammenfassung DiepK _a-Werte von 2-Hydroxy-3-pyridinol (HHP), 2-Mercapto-3-pyriodinol (MHP) und 2-Carboxy-3-pyridinol (CHP) wurden durch potentiometrische Titration in wäßrigen Systemen mit 20 mol% organischem Lösungsmittelanteil bei 25±0.1 °C und einer lonenstärke von 0.1M KNO₃ unter Anwendung einer empirischenpH-Korrektur für Lösungsmittelgemische bestimmt. Der Einfluß der organischen Lösungsmittel auf die Dissoziationskonstanten und die tautomeren Gleichgewichte der untersuchten Verbindungen und der Einfluß der molekularen Strukturen auf diepK _a-Werte werden diskutiert. Die Titration eines Gemisches von zwei schwachen zweibasigen Säuren (HHP undCHP) in Wasser/Dimethylsulfoxid bei konstanter Ionenstärke wurde mit Hilfe derGranschen Methode ausgewertet.

     

Ionization constants of benzoic acid from 25 to 250°C and to 2000 bar

The ionization constant of benzoic acid has been determined by conductivity measurements of dilute aqueous solutions and found to vary from 6.27×10^–5 at 25°C to 0.39×10^–5 at 250°C. The pressure effect to 2000 bar has been measured, and the ratio of ionization constants K₂₀₀₀/K₁ is 2.26 at 25°C and 7.3 at 250°C. V°₁, the standard partial molar volume change for the ionization at 1 bar, varies from –11.7 cm³-mol^–1 at 25°C to –60 cm³-mol^–1 at 250°C. The volume changes are smaller at higher pressures.     

The ionization of trichloroacetic acid and evidence for an unusual type of ion pairing

Osmotic and activity coefficients are reported for aqueous solutions of the lithium and potassium salts of iodic acid and trichloroacetic acid. The degree of ionization of the parent acids at various concentrations was estimated from ion exchange measurements, and these values were compared with those obtained from Raman measurements. It was concluded that one obtains comparable values of α, the degree of dissociation, of iodic acid whether these are estimated from hydrogen ion concentrations or anion concentrations. Lack of agreement between like measurements of α for trichloroacetic acid leads to the postulation of an unusual ion pair in which the proton is strongly associated with the chlorine end of the anion. A similar type of ion pairing is suggested in solutions of the aromatic sulfonic acids.     

A comprehensive study of isoskeletal analogs of dibenzo[a,c]anthracene

While reviewing the chemistry of cyclopentadienone derivatives and aza isoskeletal analogs of dibenzo[a,c]anthracene, additional molecular orbital (MO) and spectroscopic results and insights are presented. The MO tendency for coplanarity of phenyl substituents on benzenoids is demonstrated. Perpendicularly oriented phenyl substituents resulting from steric interactions strongly shield appropriately situated protons in NMR spectra. The principles of alternating polarity and parallel correspondence in conjunction with MO methods are used to relatively order aza arene isoskeletal analogs according to their chemical properties. Since less than 1.87% of the isoskeletal analogs of benzo[a,c]-anthracene have been reported, this summary work will help one to forecast the major chemical properties of those not yet synthesized.A preliminary account of this work was presented at the 23rd ACS Midwest Regional Meeting, University of Iowa, Iowa City, Nov. 17, 1988     

MALDI-TOF mass spectrometry of a combinatorial peptide library: effect of matrix composition on signal suppression

The effect of matrix composition on signal suppression caused by a dominant compound under MALDI ionization was studied using the combinatorial TQTXT pentapeptide library as a model system. The peptide library is composed of 19 components with all proteinogenic amino acids except cysteine in position X. From these compounds, only the Arg peptide (TQTRT) was detected with sufficient intensity in the MALDI-TOF mass spectrum under typical MALDI conditions (CCA matrix). The analysis of a set of compounds utilized as different matrix components, additives and a cationizing agent revealed that the composition of the matrix is a critical point in signal suppression. Highly improved ion yields were achieved by using a CCA/DHB mixture as a matrix. The addition of K(+) as a cationizing agent to the CCA matrix resulted in MALDI-TOF mass spectra with relative ion intensities very similar to those obtained by electrospray ionization.     

Ionization and protonation of NaC3: a theoretical study

A theoretical study of the NaC₃⁺ and NaC₃H⁺ systems has been carried out. Predictions have been made for some of the molecular properties, which could help in their possible experimental detection. The predicted global minimum for NaC₃⁺ is the linear isomer 1s (¹Σ). The lowest-lying triplet state is a three-membered ring 3t (³B₂), lying about 27.1 kcal/mol higher in energy than the predicted global minimum at the G2(P) level. In the case of NaC₃H⁺, there are two isomers that lie close in energy: a linear species, 1d (²Π), and a three-membered ring, 4d (²A′). The most reliable levels of theory employed predict that 1d (²Π) is the global minimum, whereas 4d (²A′) is predicted to lie 5.3 kcal/mol higher in energy at the G2(P) level. In any case it seems that both structures could be accessible to experimental detection. Low ionization potential and high proton affinities are obtained for the most stable NaC₃ isomers. Therefore, if present in the interstellar medium, NaC₃ should be easily ionized and would react quite easily to give the protonated species.     

Profile characterization and biological activities of cold pressed Garden Cress (Lepidium sativum) seed oil

Lepidium sativum is cultivated mainly for the edible oil from its seeds, and considered as an unutilized and neglected crop despite its important properties. Its oil fraction is used to produce soap and stabilize linseed oil when it is mixed with wild mustard seed oil. Once converted into fatty acid methyl esters, it represents a good substitute for imported petroleum diesel after alkaline transesterification reaction. In the current study, Lepidium sativum seeds cultivated in Tunisia and the physicochemical properties and nutrient profile of its cold pressed seed oil were investigated. The antioxidant, antibacterial, and anti-inflammatory activities of the above oil were also assessed. Lepidium sativum seed oil was abundant in both linolenic (35.59 ± 1.9%) and oleic (21.14 ± 0.63%) acids, and high amounts of β-sitosterol (42.57 ± 2.96 mg/100 g), campesterol (20.04 ± 1.4 mg/100 g) and Δ 5,24 stigmastadienol (11.82 ± 0.45 mg/100 g) were detected. The total tocopherol content of Lepidium sativum seed oil reached 136.83 ± 7.6 mg/100 g with a predominance of γ-tocopherol (86.23%). Its seed oil exhibited an IC₅₀ of 10.33 ± 0.05 mg/mL and a radical scavenging activity of 415.6 ± 40 Trolox Equivalent Antioxidant Capacity (TEAC) for the DPPH and the ABTS assays, respectively. While the thermal analysis proved a high thermal stability of Lepidium sativum seed oil, that of eight bacteria and one fungal strain showed no noticeable bacterial or antifungal effects. It was also revealed that Lepidium sativum seed oil held a remarkable anti-inflammatory activity. Hence, the obtained results evidenced remarkable chemical, antioxidant and anti-inflammatory properties of Lepidium sativum seed oil, which might potentially be promising for enhancing human health and preventing age-related diseases.     

Ionization equilibrium in the electron-proton gas

Using Fefferman's analysis of the quantum electron-proton gas, we give a rigorous proof of ionization equilibrium in this system. Ionization equilibrium phases are obtained as low-density and low-temperature limits, letting the chemical potential(T) approach the ground-state energy of the hydrogen atom as the temperatureT tends to zero. The rate of ionization is determined by the slope of(T) atT=0 and is correctly given by the Saha formula. We also discuss a simpler model where a single quantum particle interacts with a classical gas of hard spheres.     

Tunable feedback loop for suppression of relaxation oscillations in a diode-pumped Nd: laser

An optoelectronic noise suppression circuit connected to the RF input of a commercial diode laser mount is presented. Adjustable phase shift and amplitude control enable efficient suppression of the relaxation oscillations of a diode laser-pumped Nd:YVO₄ laser at variable frequencies between a few hundred kHz and 1.8 MHz. At least 25 dB reduction of the relaxation oscillations at 500 kHz and 1.5 MHz noise peaks is demonstrated.