Micelle Formation and Counterion Binding of Dodecylammonium Alkanesulfonates in Water at Different Temperatures

A temperature study was performed on micelle formation of a series of homologous cationic surfactants having organic counterions (alkanesulfonates) with carbon numbers ranging from 1 to 4: dodecylammonium salts of methanesulfonate (DAMS), ethanesulfonate (DAES), propanesulfonate (DAPS), and butanesulfonate (DABS) in water. The critical micelle concentrations (CMCs) and the degree of counterion binding (β) were determined at different temperatures ranging from 5 to 50°C by means of conventional electric conductance measurements. From the temperature dependence of β as well as CMC, Gibbs energy ΔG⁰_m, enthalpy ΔH⁰_m, and entropy ΔS⁰_m, on micelle formation, were estimated for the respective surfactants. As for the temperature dependence of CMC for these surfactants, the temperature-CMC curves have a minimum around 30°C and show that the CMC at each temperature is lowered by about 3 mmol dm^-3 per methylene group in the alkyl chain of the counterions. The relationship between β and temperature suggested that the counterion of MS^- behaves most similarly to common univalent ions such as halide ions. In contrast, PS^- and BS^-, having a stronger ability to lower CMC and to promote association of surfactant ions with counterions as well as of surfactant ions themselves, behave more like those of surfactant ions, and ES^- shows the most complicated character between those of common univalent ions and organic ions. However, the temperature dependence of enthalpy change, ΔH⁰_m demonstrates that these four surfactants are divided into two groups: (1) DAMS and DAES and (2) DAPS and DABS. In addition, the entropy change ΔS⁰_m as a function of alkyl chain length gives evidence that the contribution of the entropy term to the Gibbs energy on micelle formation clearly separates between DAES (m = 2) and DAPS (m = 3). A similar discontinuity is found even in the plot of ΔG⁰_m versus carbon atom number of alkyl chain, m, and in the plot of ΔG⁰_m versus estimated hydrodynamic radius of counterions. All the results obtained have indicated that lengthening the alkyl chains initially hinders micelle formation, but the longer chains are markedly effective in lowering the CMC and probably in increasing the aggregation number, owing to enhanced hydrophobic interaction between counterion and the micellar surface and/or core.     

Statistical and Mathematical Investigations for Determining the Binding Constants of Micelle with Reactants Molecules from Kinetic Data

Binding constants between reactants molecules with micelles are considered to be important parameters particularly in micellar catalysis area. Recently, we developed a statistical method based on multiple linear regression for determining those parameters from kinetic data (Phys. Chem. Liq. 2008, 46, 34–46). In the present work, we derived further two statistical equations from the same original equation using also multiple linear regression method. A substantial difference has been found between the results of those equations and with that of the recently published one. This strongly indicates that the statistical procedures are not valid for such a purpose, that is, the available statistical and graphical methods in the literature are also not suitable for such treatment. A mathematical procedure using iterative method for evaluating the binding constants is introduced. An equation for such treatment has also been derived from the same original equation, and a computer program for this purpose has been written. Application of the developed method to the kinetic data has been found to be quite successful. It has been concluded that the presented mathematical method is simple, reliable, and accurate.     

New Method for Determining H-Bond Energy: Fluorescence from Neutral Red in Water Compared to Fluorescence from Neutral Red in Benzene

A new method is presented for determining the H-bond energy between a fluorescent solute and water. In the current example the hydrogen-bond energy between neutral red (NR) and water was measured. The H-bond energy was obtained from the temperature dependence of the ratio of the 625 nm fluorescence intensity from NR in water, which increases with increasing temperature, to that of the 530 nm intensity from NR in benzene, which decreases with increasing temperature. The H-bond energy so determined is (10050 ± 60) J-mol⁻¹, which compares favorably with an H-bond E_HB value for pure water of 10170 J-mol⁻¹. Intermolecular hydrogen-bonding between water and NR increases the nonradiative deactivation process of the excited molecule due to fast energy dissipation through the vibrations associated with the hydrogen bonds. Some of the hydrogen bonds are broken with an increase in temperature, resulting in an increase in the number of free NR molecules, thus increasing the fluorescence quantum yield.     

Evaluation of Association Constants from Fluorescence Data: A Corrected Method*

An equation is proposed for the evaluation of partition or association constants from fluorescence properties (such as spectrum shape or anisotropy) whose values are not linearly related to the solute distribution. The proposed procedure is applied to the binding of pyrene to β-cyclodextrin (employing the I₁/I₃ relationship and fluorescence anisotropy) and to the association of pyrene to dioctadecyldimethylammonium small unilamellar vesicles (sonicated) and large unilamellar vesicles (injected). The results obtained in the latter system indicate that the pyrene partition constant between the vesicles and the aqueous medium is nearly 10 times larger in the small vesicles.     

Tyrosine‐Based Ionic Liquid Crystals: Switching from a Smectic A to a Columnar Mesophase by Exchange of the Spherical Counterion

Synthetic strategies were developed to prepare l ‐tyrosine‐based ionic liquid crystals with structural variations at the carboxylic and phenolic OH groups as well as the amino functionality. Salt metathesis additionally led to counterion variation. The liquid‐crystalline properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X‐ray diffraction (WAXS, SAXS). The symmetrical ILC chlorides bearing the same alkyl chain at both the ester and ether but either an acyclic or cyclic guanidinium group displayed enantiotropic SmA₂ mesophases with phase widths of 31–88 K irrespective of the head group. It was particularly the replacement of chloride in the acyclic guanidinium ILC by hexafluorophosphate that induced a phase change from SmA₂ to Col_r. This phase change was attributed to a higher curvature of the interface due to the larger anion, which increased the effective head group cross‐sectional area of the amphiphilic ILC. The unsymmetrical acyclic guanidinium chlorides, bearing a constant C₁₄ ester and variable alkyl chains on the phenolic position, formed enantiotropic SmA₂ phases. The derivative with the largest difference in chain lengths, however, displayed a Col_r phase, resulting from discoid aggregates of the cone‐shaped guanidinium chloride. The results are discussed in terms of the packing parameters, which indicate that the phase behaviour of the thermotropic tyrosine‐based ILCs shows analogies to those of lyotropic liquid crystals.     

Kinetic Spectroscopy of Erythrosin Phosphorescence and Delayed Fluorescence in Aqueous Solution at Room Temperature*

The photophysics and polarization of the phosphorescence and delayed fluorescence of erythrosin in conditions compatible with the current biological applications of the dye (aqueous buffers at pH 7.4 at ambient temperatures) and in ethanol have been studied as a function of dye concentration (10 ^?7-10^?5M) and temperature (245–333K). The emission decay is strictly single exponential and the detailed kinetic analysis of all the rate processes connected with the emitting T₁ state showed that (1) the lowering of the emission lifetime at the higher temperatures is due to a very efficient self-quenching process, (2) the back intersystem crossing rate T_x S₁ is temperature dependent (δE_TS7 kcal mol^?1) but the T₁S₀ is not (E_a0.1 kcal mol^?1) and (3) both intersystem crossing processes are very sensitive to solvent polarity, which accounts for the solvent dependence of the phosphorescence yield and lifetime. The high value of the phosphorescence anisotropy (r₀= 0.25 lt 0.006) is independent of the excitation and emission wavelengths, and its evolution in time accurately reflects the rotational restrictions in solid solutions. The relevance of these findings to studies with protein-dye conjugates is also outlined to facilitate the design and interpretation of phosphorescence depolarization experiments that probe the (μs-ms dynamics of biomolecules and supramolecular systems.     

Equilibrium, Kinetics, Diffusion and Self-Association of Proteins at Membrane Surfaces: Measurement by Total Internal Reflection Fluorescence Microscopy

Abstract— The equilibrium, kinetics, diffusion and self-association of proteins at membrane/solution interfaces may deviate substantially from these processes in bulk solution. A set of methods for examining these phenomena combines substrate-supported planar model membranes and the use of evanescent illumination with laser-based, quantitative fluorescence microscopy. Measurement of the steady-state, surface-associated fluorescence can be used to examine the thermodynamic properties of proteins at membranes. When combined with fluorescence photobleaching recovery, this technique provides information about membrane-binding kinetics; and when combined with fluorescence pattern photobleaching recovery, measurement of the translational diffusion coefficients of proteins weakly bound to membranes is possible. The use of polarized evanescent illumination can provide information about the orientation distributions of adsorbed fluorophores. Fluorescence correlation spectroscopy provides information about the self-association ( e.g. dimerization) of membrane-associated proteins.     

Safety Analysis: Relative Risks of Ultraviolet Exposure from Fluorescence Spectroscopy and Colposcopy Are Comparable*

Fluorescence spectroscopy is a promising tool for use in the diagnosis of disease in human tissue. However, few published reports have evaluated the safety of this technique, despite the fact that many spectroscopic systems use UV illumination. This study determined the relative risk associated with light exposure from spectroscopic systems compared with the traditional light sources that are used to illuminate tissue and direct biopsies. We compared spectroscopic detection systems for the cervix to the colposcope, a low-power microscope routinely used to illuminate the cervix, which does not cause any known photochemical damage. We measured the average spectral irradiance (W/[cm²nm]) and the average tissue exposure time during a diagnostic colposcopy examination. To quantify the relative risks, we multiplied illumination spectra by several action spectra from the literature and compared the areas under the curves corresponding to each procedure. The risk associated with the average power colposcope served as our basis for comparison. We conclude that the risks of illumination using spectroscopic systems are lower than or comparable to those already encountered in routine diagnostic procedures such as colposcopy with an average power colposcope. Spectroscopic examination can be associated with a somewhat higher risk than a colposcopy with the lowest power colposcope or a shorter than average colposcopy. The analysis presented can be repeated to estimate the magnitude of risks associated with other spectroscopic diagnostic devices.     

ETD in a Traveling Wave Ion Guide at Tuned Z-Spray Ion Source Conditions Allows for Site-Specific Hydrogen/Deuterium Exchange Measurements

The recent application of electron transfer dissociation (ETD) to measure the hydrogen exchange of proteins in solution at single-residue resolution (HX-ETD) paves the way for mass spectrometry-based analyses of biomolecular structure at an unprecedented level of detail. The approach requires that activation of polypeptide ions prior to ETD is minimal so as to prevent undesirable gas-phase randomization of the deuterium label from solution (i.e., hydrogen scrambling). Here we explore the use of ETD in a traveling wave ion guide of a quadrupole-time-of-flight (Q-TOF) mass spectrometer with a “Z-spray” type ion source, to measure the deuterium content of individual residues in peptides. We systematically identify key parameters of the Z-spray ion source that contribute to collisional activation and define conditions that allow ETD experiments to be performed in the traveling wave ion guide without gas-phase hydrogen scrambling. We show that ETD and supplemental collisional activation in a subsequent traveling wave ion guide allows for improved extraction of residue-specific deuterium contents in peptides with low charge. Our results demonstrate the feasibility, and illustrate the advantages of performing HX-ETD experiments on a high-resolution Q-TOF instrument equipped with traveling wave ion guides. Determination of parameters of the Z-spray ion source that contribute to ion heating are similarly pertinent to a growing number of MS applications that also rely on an energetically gentle transfer of ions into the gas-phase, such as the analysis of biomolecular structure by native mass spectrometry in combination with gas-phase ion-ion/ion-neutral reactions or ion mobility spectrometry.     

Infrared Spectroscopic Study on Adsorption of Acetonitrile from Solution onto Several Oxide Surfaces at High Pressure

Adsorption of acetonitrile from toluene solution at a liquid-solid interface under pressures of up to 300 MPa was investigated by IR spectroscopy. The CN stretching vibration bands (v_CN) of adsorbed acetonitrile were observed at higher frequencies than those of the same species in the liquid phase. The shift on alumina-pillared montmorillonite (ALPM) was the largest (ca. 8 cm^-1) for the adsorbents studied. The v_CN intensities of adsorbed acetonitrile on ALPM and on alumina (Al₂O₃) considerably increased with increased pressure, indicating an increase acetonitrile adsorption due to compression. It was concluded that the total volume of the system was reduced by adsorption, and that the reduction was brought about not only by the formation of an adsorption bond but also by the change in the solvation of the adsorbate in the adsorbent pore.     

Fluorescence quenching of dyes by tryptophan: interactions at atomic detail from combination of experiment and computer simulation

Fluorescence spectroscopy and molecular dynamics (MD) simulation are combined to characterize the interaction of two organic fluorescent dyes, rhodamine 6G (R6G) and an oxazine derivative (MR121), with the amino acid tryptophan in aqueous solution. Steady-state and time-resolved fluorescence quenching experiments reveal the formation of essentially nonfluorescent ground-state dye/Trp complexes. The MD simulations are used to elucidate the molecular interaction geometries involved. The MD-derived probability distribution of the distance r between the centers of geometry of the dye and quencher ring systems, P(r), extends to higher distances for R6G than for MR121 due to population in the R6G/Trp system of fluorescent interaction geometries between Trp and the phenyl ring and ester group of the dye. The consequence of this is the experimental finding that under the conditions used in the simulations about 25% of the R6G dye is fluorescent in comparison with 10% of the MR121. Combining the above findings allows determination of the "quenching distance", r, above which no quenching occurs. r is found to be very similar (approximately 5.5 A) for both dye/Trp systems, corresponding to close to van der Waals contact. Both experimental dynamic Stern-Volmer analysis and the MD trajectories demonstrate that the main determinant of the fluorescence intensity is static quenching. The approach presented is likely to be useful in the structural interpretation of data obtained from fluorescent conjugates commonly used for monitoring the binding and dynamics of biomolecular systems.     

Determining the point of origin of bitumen coats on ceramic tableware excavated from ancient settlement at Menteshtepe, Azerbaijan

An analysis of bitumen coatings on surfaces of ceramic tableware found during excavations at the Menteshtepe settlement (Tovuz region, Azerbaijan) has been performed. Biomarker analysis has been conducted, hydrocarbon and elemental composition of the bitumen have been determined using highly sensitive equipment, and the point of origin of the bitumen is discovered.     

Measurements recovery evaluation from the analysis of independent reference materials: analysis of different samples with native quantity spiked at different levels

Accreditation and Quality Assurance - Measurement uncertainty evaluation involves combining uncertainty components reflecting all relevant random and systematic effects: the precision and trueness...     

Measurements of the viscosity of carbon dioxide at temperatures from (253.15 to 473.15) K with pressures up to 1.2 MPa

The viscosity of carbon dioxide was measured over the temperature range T = (253.15 to 473.15) K with pressures up to 1.2 MPa utilizing a new rotating-body viscometer. The relative expanded combined uncertainty (k = 2) in viscosity (including uncertainties of temperature and pressure) was (0.20 to 0.41)%. The instrument was specifically designed for measurements at low gas densities and enables measurements of the dynamic viscosity at temperatures between T = 253.15 K and T = 473.15 K with pressures up to 2 MPa. For carbon dioxide, the fluid specific measuring range with regard to pressure was limited to 1.2 MPa due to the formation of disturbing vortices inside the measuring cell at higher pressures. The model function for the viscosity measurement was extended in such a way that the dynamic viscosity was measured relative to helium. Therefore, the influence of the geometry of the concentric cylindrical system inside the measuring cell became almost negligible. Moreover, a systematic offset resulting from a small but inevitable eccentricity of the cylindrical system was compensated for. The residual damping, usually measured in vacuum, was calibrated in the entire temperature range using viscosity values of helium, neon and argon calculated ab initio; at T = 298.15 K recommended reference values were used. A viscosity dependent offset of the measured viscosities, which was observed in previously published data, did not occur when using the calibrated residual damping. The new carbon dioxide results were compared to other experimental literature data and to the correlation, which is currently considered the reference for viscosities of carbon dioxide.