Correlation between CMC and chromatographic index: simple and effective evaluation of the hydrophobic/hydrophilic balance of bile acids

The discovery that bile acids are involved in the modulation of nuclear steroid receptors has prompted renewed interest in this field of research. Due to the nature of research in this field, a technique that enables simple and effective assessment of the hydrophobic/hydrophilic balance, thus improving and speeding up evaluations of the biological profiles of these compounds, is greatly needed. In this context, both CMC value determination and RP-HPLC mobility evaluation were explored as possible approaches. While the CMC was calculated using the noninvasive conductimetric method, the RP-HPLC mobility was assessed by measuring the retention factor at several mobile phase compositions and extrapolating back to the pure aqueous mobile phase. The correlation of the CMC with the derived chromatographic hydrophobic index ϕ ₀ was satisfactory. Figure Experimental versus predicted pCMC values     

Amperometric and Conductimetric Methods for Simultaneous Determination of Captopril and Bendroflumethiazide

Abstract

The mixture of captopril (antihypertensive) and bendroflumethiazide (diuretic) was assayed using conductimetric and amperometric methods. Each method was applied for the analysis of laboratory-made sample mixtures in authentic and dosage forms. For captopril, the amperometric and conductimetric methods gave mean percent recoveries of 100.2 ± 1.2 and 99.6 ± 1.2, respectively. These results have shown good agreement when compared with other methods in the literature. When applied to bendroflumethiazide the conductimetric method gave percent recovery of 98.5 ± 1.6 which agreed closely with the spectrophotometric pharmacopoeial method.

The pK₁ for captopril was determined potentiometrically at room temperature (25°C) in aqueous medium. The calculated value, based on Henderson equation was 3.78.     

Conductimetric analysis of the ion binding properties of three leaf extracts of chestnut (Castanea sativa), Eucalyptus (Eucalyptus globulus) and oak (Quercus robur)

Humic materials extracted from tree leaves of chestnut (Castanea sativa), eucalyptus (Eucalyptus globulus) and oak (Quercus robur) were analyzed by performing conductimetric titrations. Values between about 84 and 236 μS cm⁻¹ for the molar conductivity and between 0.42 and 0.74 for the charge distribution parameter were obtained when the concentrations of the extract are increased from 40 to 100 mg 1⁻¹. These variations were explained by using the counterion condensation theory, and the distance between the charged groups of the polyions, the volume of the counterion condensation and the Debye-Hückel potential were also calculated.     

Hydrogen Ion Affinity in Humic Acids: A Coordination Approach

Potentiometric titrations of humic acids (HA) with 0.250 M Ba(OH)₂/BaCl₂ titrant (0.750 M constant ionic strength), have been performed at 25 ± 0.1°C with a calibrated glass electrode for measuring p[H⁺]. The pK′_w for water in this BaCl₂ medium is 12.899 ± 0.006 and 13.712 ± 0.006 in 0.500 M NaCl. Divalent Ba²⁺ cations force the ionization of the acid groups and improve solubility. Under such conditions derivative potentiometric curves show diverse equivalent peaks of the acidic sites of HA. The presence of 0.05% of a neutral detergent such as Triton X-100 is essential for effective dispersion of HA in the working solutions and to obtain very stable potentiometric measurements. Computer programs were used in the treatment of the potentiometric data in order to solve a number of simultaneous equations to obtain overall conditional β _{i H} formation constants, which come from a coordination model of hydrogen ions to the organic matrix and permit calculation of conditional pK data. Conductimetric titrations with Ba(OH)₂ or NaOH give the total acidity. A typical result in BaCl₂ medium for a peat HA presents seven acid groups with the following pK data: pK ₁ = 3.80 ± 0.3, pK ₂ = 4.67 ± 0.02, pK ₃ = 7.57 ± 0.01, pK ₄ = 8.190 ± 0.005, pK ₅ = 8.80 ± 0.01, pK ₆ = 8.91 ± 0.02, and pK ₇ = 8.93 ± 0.01.     

Active control of surface properties and aggregation behavior in amino acid-based Gemini surfactant systems

Two types of Gemini surfactants containing a disulfide bond in the spacer, sodium dilauroyl cystine (SDLC) and sodium didecamino cystine (SDDC), were synthesized, and their surface properties and aggregation behavior in aqueous solution were studied by means of surface tension measurements, dynamic light scattering (DLS), transmission electron microscopy (TEM), and fluorescence. During the transition of the Gemini surfactants to their corresponding monomers through the reduction of disulfide bonds, the surface tensions of their aqueous solutions, as well as their aggregation behavior, changed greatly. The reduction of SDLC and SDDC led to disruption of the vesicle, and the oxidation of corresponding monomers to Gemini surfactants led to vesicle re-formation. These results demonstrated the control of surface properties and aggregation behavior by the reversible transition between the Gemini surfactant and its monomer via reduction/oxidation reactions.     

Synthesis and comparative behaviour study of fluorocarbon and hydrocarbon cationic surfactants in aqueous media

A series of perfluorinated cationic surfactants and their corresponding hydrocarbon ones whose general formula is C_nX_2n+1-C(O)NH-(CH₂)₃-N⁺Me₃, I⁻, with X = F, H and n = 9, 11, have been synthesized via two steps. Their aggregative and surface-active properties were studied in aqueous solution using tensiometry and conductimetry. The critical micelle concentrations and the molecular areas at the air/water interface of fluorinated surfactants are lower than those of their hydrocarbon homologues. Micellar aggregation numbers and geometric packing parameters have been investigated. The results indicated that fluorocarbon surfactants tend to form lamellar aggregates while the hydrocarbon ones associate into spherical aggregates.     

Indirect conductimetric detection of amino acids after liquid chromatographic separation

A liquid chromatographic method using indirect conductimetric detection is proposed for the determination of low levels of organic compounds, which does not require any special functional characteristics of the analyte. The signal detected is proportional to the molar concentration of the analyte and independent of its nature. The detector response is linearly dependent on analyte concentrations over at least three orders of magnitude. The basis of the detection is to create a conducting background, which will decrease on elution of the organic compounds. The theory of the method is discussed, with special reference to the quantitative displacement of the conducting species of the mobile phase from the column by the analyte on sample injection. The proposed method has been applied to study the chromatographic behaviour of twenty-one amino acids, where a 5 -μm Econosil CN column was used as the stationary phase with a mixture of water-acetonitrile-tetrahydrofuran (70:20:3) containing 1 mM perchloric acid or trichloroacetic acid as the mobile phase. The proposed method allows as little as 10 ng of each amino acid to be determined.