Neutron scattering study of pentanol solubilization in sodium octanoate micelles

We extend a previous small-angle neutron scattering study of sodium octanoate (NaC₈) micelles to the ternary system sodium octanoate/pentanol/water. The use of contrast variation through selective deuteration of individual components together with explicit computation of interference effects, permits us to deduce the location of pentanol (C₅OH) in the micelles. Our main conclusion is that, although the micelles grow as (C₅OH) is solubilised, there is no concomitant variation in the NaC₈ aggregation number. At low alcohol concentrations, the C₅OH is located near the NaC₈ polar heads, while at higher concentration the -OH groups are distributed throughout the micellar core.     

Molecular dynamics simulation of pyrene solubilized in a sodium dodecyl sulfate micelle

In the present work, the structural and dynamical aspects of the solubilization process of pyrene within a sodium dodecyl sulfate micelle were studied using molecular dynamics simulations. Our results showed that free pyrene as the fluorescence probe can be spontaneously solubilized into the micelle and prefers to be located in the hydrophobic core region. As the local concentration of pyrene increased, two molecular probes could enter into the core hydrophobic region and the excited dimer of pyrene molecules was formed, showing a stacking mode of π-π conjugation. Since the π-π stacking interaction between the two pyrene molecules was very weak, formation of the excimer was a dynamic process with the two pyrene molecules alternately separating and associating with each other. In this case, the two pyrene molecules were found to be mainly distributed in the palisade layer of the micelle due to the balance between the weak π-π stacking interaction and the hydrophobic interaction of probe molecules with the surfactant tails.     

Capillary electrophoresis study on the micellization and critical micelle concentration of sodium dodecyl sulfate. Influence of solubilized solutes

The influence of solubilized solutes on the micellization and critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) were investigated by means of capillary electrophoresis (CE). Three different structural types of test solutes, including chloropyridines. chlorophenols and cephalosporins with different binding strength to SDS micelles, were selected in this study. The variations of the effective electrophoretic mobility of these solutes as a function of SDS concentration in the premicellar and micellar regions were analyzed. Interestingly, the results indicate that, in the presence of these solubilized solutes, the micellization of SDS may occur over a range of SDS concentration, with the aggregate size increasing over this range. Depending on the nature of solubilized solutes and the extent of the interactions between solubilized solutes and SDS micelles, the CMC value of SDS may vary significantly. The incorporation of solubilized solutes into SDS micelles to form mixed micelles is proposed to interpret the migration behavior of solubilized solutes in CE.     

A molecular dynamics study of free energy of micelle formation for sodium dodecyl sulfate in water and its size distribution

Free energy of micelle formation has been evaluated for spherical sodium dodecyl sulfate (SDS) in water by a thermodynamic integration method combined with a series of large-scale molecular dynamics calculations following the chemical species model. In particular, free energy change delta mu(n+1)0 with respect to the addition of one surfactant molecule to the spherical micelle of size n was obtained as a function of n. The free energy profile showed a minimum followed by a maximum, which corresponds to a peak in the size distribution. The calculated peak size n = 57 near its critical micelle concentration is in good agreement with the experimental averaged aggregation number n = 55-75 of the SDS micelle. The distribution showed a rather sharp peak, indicating that the size is almost a monodisperse one. The size is likely to be insensitive to the total concentration of the surfactant.     

Conformational study of fosinopril sodium in solution using NMR and molecular modeling

Two conformers of fosinopril sodium in methanol were unambiguously established using 2D NMR methods and variable‐temperature NMR experiments. Differences in their conformational structure were shown to be related to the rotational energy barrier about the amide bond and hydrophobic interaction. The relationship between the 3D structure and activity is discussed. It is suggested that the trans‐conformer may be more biologically active owing to its stacking structure and strong hydrophobic interaction and the cis‐conformer could be more easily hydrolyzed because of its extended structure. Copyright © 2003 John Wiley & Sons, Ltd.     

Long-chain alkyl sulfonate micelle fission: a molecular dynamics study

In this study, we investigate micelle fission of long-chain alkyl sulfonate molecules using atomistic scale simulation. GROMACS software code with the united atom force field was applied. 0.5-μs parallel molecular dynamics simulation study was conducted for a surfactant/water system consisting of 192 sodium pentadecyl sulfonate and 40,553 water molecules. The large preassembled micelle was ruptured at Krafft above T?=?323-K temperature, and we track two ellipsoid-like micelles over the course of the production run. To estimate the micelle shape, we determined the principal moments of inertia and the eccentricity, which proved that the micelles have a pronounced prolate spheroid shape, which agrees well with our previous experimental data. The mechanism of micelle fission was explored in detail. The aggregation number, ionization degree, and other parameters obtained from simulation were consistent with existing experimental finding. The determined parameters in addition to simple visual inspection of trajectories revealed monomer-micelle exchange—with the estimated relaxation time τ ₁?=?10^??9s. We assume that the exchange process is conditioned by the unequal size of micelles leading to adjustment of aggregation number.     

The structure and properties of medium-chain surfactant solutions: a case study of sodium octanoate

In the paper the most characteristic properties of binary sodium octanoate solutions of variable ion strengths are discussed. Both pre- and postmicellar association is investigated with reference to available scattering, spectroscopic and thermodynamic data. The focus is then placed on references dealing with, or referring to, results including sodium octanoate. A picture is offered illustrating the sodium octanoate micelle as a rather unstable entity including micelle sizes from oligomeric pre-associates up to some 20 monomers. Due to this circumstance the molecular features obtained with different scattering and spectroscopic techniques are rather divergent. However, the structure emerging describes the micelle as consisting of a small “dry” hydro-carbon core with a radius shorter than the fully extended heptyle chain of the octanoate. Instead a “rough shell” encompassing the hydrated polar head groups, some “wet” methylene/methyle groups and the hydrated counterions separates the micelle from the surrounding solvent. On purely geometric grounds this hydrated shell represent, however, more than 50% of the micellar volume. The numerous distinct properties of medium-chain surfactants should thence not, as is frequently done, be related to those of long-chain surfactants! At high concentrations of octanoate or salt, or upon solubilization, the average degree of water contact seem to diminish due to an increased aggregation number or a lack of available “free” hydration water. The latter effect is suggested to produce observable effects at the second critical concentration as well as close to the phase boundary. The influence of solubilizates are dealth with only when the results provide significant information on the properties of the binary system.     

A study of molecular complex formation between propyl gallate and ascorbic acid in the microemulsion phase of sodium dodecyl sulfate, pentanol and water system

The association between two water-soluble antioxidants, i.e. ascorbic acid and propyl gallate have been studied by absorption spectroscopy in microemulsion formed in sodium dodecyl sulfate/pentanol/water micellar system. It has been shown that propyl gallate forms 1:1 molecular complex with ascorbic acid in every solution. Evolution of the absorption spectra during the study of molecular complex formation goes through well-defined isosbestic points. The association constants were calculated using curve-fitting procedure. The observed interactions are stronger in the less polar solvents.     

A simple spectrophotometric determination of trace level mercury using 1,5-diphenylthiocarbazone solubilized in micelle.

A very simple, ultra-sensitive and fairly selective non-extractive spectrophotmetric method is presented for the rapid determination of mercury(II) at ultra-trace level using 1,5-diphenylthiocarbazone (dithizone) as a new micellar spectrophotometric reagent (lambdamax = 490 nm) in a slightly acidic (0.07 - 0.17 M H2SO4) aqueous solution. The presence of a micellar system avoids the previous steps of solvent extraction and reduces the cost, toxicity while enhancing the sensitivity, selectivity and the molar absorptivity. The reaction is instantaneous and the absorbance remains stable for over 24 h. The average molar absorption coefficient and Sandell's sensitivity were found to be 5.02 x 10(4) L mol(-1) cm(-1) and 10 ng cm(-2) of Hg, respectively. Linear calibration graphs were obtained for 0.05 - 10 mg L(-1) of Hg; the stoichiometric composition of the chelate is 1:2 (Hg:dithizone). The method is characterized by a detection limit of 1 microg L(-1) of Hg. Large excesses of over 60 cations, anions and complexing agents (e.g. EDTA, tartrate, oxalate, citrate, phosphate, thiourea, azide, SCN-) do not interfere in the determination. The method was successfully applied to a number of environmental water samples (potable and polluted), biological samples (human blood and urine; milk and fish) and soils; solutions contained both mercury(I) and mercury(II) as well as complex synthetic mixtures. The method has high precision and accuracy (s = +/-0.01 for 0.1 mg L(-1)).     

The phenomenon of water penetration into sodium octanoate micelles studied by molecular dynamics computer simulation

 In this publication we have studied the penetration process of water molecules into the hydrophobic core of a sodium octanoate micelle. The analysis of this phenomenon was based on a molecular dynamics computer simulation. We calculated the probability to find water molecules within a specific sphere which was adjusted in the center of the micelle. It turned out that the position of the micellar mass and geometry center was not too different, so that this reference point was well characterized. Water penetration was observed within the whole aggregate but if the radius is smaller than 300 pm, polar solvent molecules are very rarely observed. The results of our computer simulations suggest that significant water diffusion into the micelle occurs at larger distances from the micellar center with a lower threshold value of about 400 pm. In addition to these calculations, we used the Connolly algorithm in order to determine the solvent accessible surfaces of different micellar equilibrium structures. We observed large dynamical fluctuations with the formation of pores and channels. These structures are occasionally filled with water molecules. Received: 29 April 1998 Accepted: 27 May 1998     

A13C spin-lattice relaxation study of micelle formation in aqueous solutions of sodium n-hexanoate

Summary The concentration dependence of the¹³C-relaxation times of the alkyl carbons in aqueous solutions of sodium hexanoate have been measured. Below CMC the relaxation is dominated by the overall motion of the hexanoate ion. In the micelles the overall motion is slower and the internal rotations around the C-C bonds contribute to the relaxation.

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Zusammenfassung Die Konzentrationsabhängigkeit der¹³C-Relaxationszeiten von Natriumhexanoat in wässriger Lösung wurde bestimmt. Unter CMC ist die Relaxation von der Bewegung des ganzen Moleküls bestimmt. In den Mizellen ist die Bewegung des ganzen Moleküls langsamer, und die inneren Rotationen in der Alkylkette tragen zu der Relaxation bei.

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With 2 figures and 1 table     

Apparent molar quantities of sodium octanoate in aqueous solutions

Densities and sound velocities of aqueous solutions of sodium octanoate were determined in a range of molalities between 0.0352 and 0.8105 mol kg^–1 at 25, 30, 35, 40 and 45 °C. The isotherms of molality dependence of both density and sound velocity were used to determine the cmcs. Apparent molar volumes and compressibilities were determined from measurements of ultrasound velocity and density. The values of apparent molar volumes and compressibilities at infinite dilution and the apparent molar quantities in the micellar range were obtained and studied as a function of temperature. Values of the critical micelle concentration and the apparent molar quantities in the premicellar and postmicellar range are discussed and compared with the values of the corresponding fluorinated compound.An erratum to this article can be found at     

Solubilization of paraffin gases in aqueous solutions of sodium octanoate

The solubilities of methane, ethane, propane and n-butane were measured in aqueous solutions of sodium octanoate of molar concentrations between 0–0.8M. From these measurements we have computed the standard free energies, entropies and enthalpies for the process of transferring the solute molecules from the gaseous phase into the solutions. We found that propane and n-butane behave as expected at, and beyond, the cmc, namely the solubility takes a steep turn upwards. On the other hand, methane and ethane seem to cross the cmc with almost no change in their solubility. A tentative interpretation of this behavior is suggested, based on the competing effects of salting out and solubilization.     

A pressure and temperature study on solubility and micelle formation of sodium perfluorodecanoate in aqueous solution

Both the critical solution temperature (CST, or the Krafft temperature) and the critical solution pressure (CSP, or the Tanaka pressure) were determined for sodium perfluorodecanoate (NaPFDe) in water, and the result shows that the Krafft temperature is raised with the increase in the Tanaka pressure. A thermodynamic analysis has been made on the data for the critical micellization concentration (cmc) and of the solubility at various temperatures and pressures. The estimated change in the partial molal volume, resulting from micelle formation from the singly dispersed state and from the hydrated solid state, was found to be conspicuously higher for NaPFDe compared to hydrocarbon surfactants. This has been ascribed to the more pronounced role of carbon chain-water interactions and water structure effects of the fluorocarbon surfactants.     

Enthalpies of solution in sodium octanoate + water + alcohol mixtures

Enthalpies of solution of sodium octanoate in water, 1-propanol and aqueous mixtures of 1-propanol, 1-butanol, 1-pentanol and 1-hexanol, and of the alcohols in aqueous solutions of sodium octanoate at various concentrations were determined calorimetrically at 35 °C. MostH(soln) values are exothermic and strongly dependent on the solute concentration. The main energetic factor governing the process of dissolution of the surfactant is associated with changes in the water structure caused by the presence of alcohol. That governing the process of the alcohol dissolution in surfactant solutions is due to the effect alcohols have on the CMC of the octanoate. There is no indication of the alcohol being either solubilized in the interior of the aqueous micelle, or becoming part of the micellar film.The solubility at 35 °C of sodium octanoate in water, 1-propanol and their mixtures has also been determined.     

Oxazolidine-2-thiones: a molecular modeling study

Two oxazolidine-2-thiones, thio-analogs of linezolid, were synthesized and their antibacterial properties evaluated. Unlike oxazolidinones, the thio-analogs did not inhibit the growth of Gram positive bacteria. A molecular modeling study has been carried out to aid understanding of this unexpected finding.     

13C- and Na-NMR studies of sodium octanoate in reversed micelles

The dynamic behavior of sodium octanoate (NaO), especially that of the polar headgroups of NaO, in1-hexanol in the presence of cosolubilized water was studied by Na-NMR and¹³C-NMR at 24 and 25 MHz. Na-NMR data have indicated that the mobility of the sodium ions of NaO shows the lowest value at 1.2 M of NaO at a given water content, which is related to the maximum amount of water cosolubilized into the system and the change in the structure of polar headgroups of NaO.¹³C-NMR data have shown that the mobility of methylene carbons, No. 5 and 6 of NaO, is higher than that of methylene carbons, No. 4 and 7, and that water molecules entered among the polar headgroups of NaO affect the mobility of methylene carbons of NaO, No. 5 and 6. The hydroxyl groups of1-hexanol were found to be contact with water molecules entered among the polar headgroups of NaO, and to fill the space among the hydrocarbon chains of NaO. By assuming spherical geometry the size of water pools and the average aggregation number were calculated, and the results were discussed on the basis of¹³C-NMR and Na-NMR data.     

A molecular dynamics study of sodium chenodeoxycholate in an aqueous solution

Hydration structure and dynamics of sodium chenodeoxycholate (CDC) in water are studied by a long-time molecular dynamics calculation. Strong hydration shell around the hydrophobic region of this large solute and strong hydrogen bonds of water with both hydroxyl and carboxyl oxygen atoms have been identified. The rotation of CDC around its longitudinal axis is found to be particularly active in comparison with that around other axes of the molecule. The diffusion coefficient of CDC calculated from the slope of the mean-square displacement, 0.95 × 10⁻⁹ m²/s, is only 1/6 of that for water in the solution, 5.4 × 10⁻⁹ m²/s.