Structural aspects in self-assembled systems of polyoxyethylene surfactants, as studied by the spin probe technique

Typical examples of structural characterization of self-assembled systems by the spin probes technique, selected from our representative results accumulated in the last years of systematic studies, are presented. The choice of the examples has aimed at emphasizing the potentiality of this technique in the study of self-assembled systems, in general, and of those of PEO surfactants, in particular. By using specific ESR parameters (the nitrogen hyperfine splitting (hfs), a_N, the rotational correlation time, τ_c, the order parameter, S) of a variety of properly chosen nitroxides, problems such hydration degree and profile of the PEO chains, ordering and order profile along these chains, their penetrability by the oil solvent, role of the terminal OH in the micellization, as well as differences in these quantities vs. the nature of the aggregate (micelle, reverse micelle, lamellar phase, etc.), nature of the surfactants (conventional or triblock copolymer), solubilizates (water in reverse micelles or various alcohols in micelles) and temperature have been discussed.     

Electrical conductivity study of the self-association of ionic surfactants in solution in ethyleneglycol,formic acid and formamide

The self-association of several cationic surfactants has been investigated by means of solubility and electrical conductivity measurements in ethyleneglycol, formic acid and formamide. Operational critical micelle concentrations (CMC_op) have been obtained from the vs. surfactant concentrationC plots by the usual interpolation procedure. It has been found that for the surfactant-solvent systems investigated the value of CMC_op increases with the extent of theC-range investigated. This result together with others concerning the effect of temperature and surfactant chain length reveal that the CMC_op-values have an ambiguous meaning concerning the surfactant self-association. Comparison of the vs.C plots for cetyltrimethylammonium chloride and tetramethylammonium chloride, for instance, reveals that some surfactant self-association takes place, probably leading to small, highly ionized surfactant aggregates, but the systems do not show true critical micelle concentration. Solubility measurements as a function of temperature lead to the same conclusion. The results are discussed according to our present understanding of the process of micellization and the necessity of using solvents of very high cohesive energy density in order to observe, with ionic surfactants, true CMCs corresponding to a highly cooperative association process. This appears to be the case only with water and hydrazine, not with formamide and other less cohesive solvents.     

Ionomers based on copolymers of alkyl methacrylates with 2‐methacryloylamino‐3‐naphthalene carboxylic acid: Association in solutions studied by steady‐state and time‐resolved fluorescence

Methyl, butyl, and octadecyl methacrylate copolymers with 3‐methacryloylamino‐2‐naphthoic acid (MANA) carrying alkali counterions exhibit two emission bands in highly dilute dioxane, heptane, and toluene solutions. One band (α) has a maximum around 400 nm, and the other (β) has a maximum around 500 nm. The ratio of the intensity of the α band to the intensity of the β band depends on the counterion (Na⁺, Li⁺, and Cs⁺) and the solvent. Varying the MANA content of the copolymer with Na⁺ counterions increases the relative intensity of the α emission in proportion to the density of MANA residues in the ionomer chain, whereas in chains containing only 0.1% MANA residues, only a β emission can be seen. The α emission is due to associated ion pairs, and the β emission is due to unassociated ion pairs. The shorter the alkyl chain pendant is of the copolymer, the larger the fraction is of the associated ion pairs. The half‐bandwidth of the β emission is much larger than that of the α‐emission band in all cases. The half‐bandwidth decreases in the order Li⁺ > Na⁺ > Cs⁺. The fluorescence anisotropy decays for the α‐ and β‐emission bands are double‐exponential. The longer rotational correlation time and its fractional contribution to the β emission increase with an increasing content of the MANA monomer in the copolymer. The decay anisotropy data are consistent with the shell–core model of ionomers in nonpolar solvents. The fluorescence anisotropy decay of the copolymer of butyl methacrylate with a low content of the nonneutralized MANA (～ 0.1 mol %) is single‐exponential, and the rotational correlation time of the MANA fluorophore is eight times longer than that of 3‐acetyl‐2‐naphthoic acid methyl ester (0.3 ns) in the same solvent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1243–1254, 2004     

Spectrophotometric determination of cationic and anionic surfactants with anionic dyes in the presence of nonionic surfactants,part I: A general aspect

The behavior of color development of anionic azo dyes, methyl orange and its analogues, was examined in aqueous media by changing the microenvironment of the dyes. The addition of alcohols, organic onium ions, anionic surfactants and nonionic surfactants brought about a decrease of the band at wavelengths near 480 nm and an increase of the band at wavelengths near 420 nm. Such a shift toward the shorter wavelengths in spectra was attributed to the change of the micro-environment around the dyes from a polar field to a less polar field; that is, the shift is caused by the change of the contribution of the following resonance forms, On the basis of the color change phenomena, the spectrophotometric methods for the determination of organic onium ions and anionic surfactants were proposed.     

A review on measurements of particle velocities and diameters by laser techniques,with emphasis on thermal plasmas

An overview of measurements of particle velocities and diameters by laser techniques, with emphasis on thermal plasmas, is given. As far as veloities are concerned, laser-Doppler velocimetry is discussed as a well-established technique. Diameter measurements are much less developed. The state of the art is described and prospective considerations are stressed.     

Enhanced fluorescence of remote functionalized diaminodicyanoquinodimethanes in the solid state and fluorescence switching in a doped polymer by solvent vapors

Remote functionalized zwitterionic diaminodicyanoquinodimethanes are found to exhibit a dramatic enhancement of light emission in the solid state and when doped in polymer films, as compared to the solution state. Crystal structure analysis of prototypical molecules reveals the role of the remote functionality in the solid state molecular organization. Semiempirical quantum chemical computations provide a viable model to explain the interesting phenomenon of fluorescence enhancement as arising from the inhibition of geometry relaxation of the vertical excited state to a nonemitting state. The reversible switching of a doped polymer film fluorescence triggered by solvent vapors is demonstrated.     

Effects of polyols and sugars on the structure of water in concentrated gelatin gels as studied by low temperature differential scanning calorimetry

 Phase-transition temperatures, glass-transition temperatures, melting temperatures, gel–sol transition temperatures in differential scanning calorimetry heating curves of gelatin solutions with and without various sugars and polyols quenched by liquid nitrogen were studied. Both sugars and polyols added to concentrated gelatin solutions shifted the glass-transition temperatures to lower temperatures, and it was attributed to the increase of unfreezable water which acts as a plasticizer. The mechanisms of the increase in unfreezable water, however, seem to be different for sugars and polyols; sugars increase unfreezable water by increasing the number of junction zones which hold unfreezable water, while polyols by themselves increase the amount of unfreezable water. Received: 9 January 1997 Accepted: 1 July 1997     

Strong effect of NaBr on self-assembly of quaternary ammonium gemini surfactants at air/water interface and in aqueous solution studied by surface tension and fluorescence techniques

The effects of NaBr on the adsorption of alkanediyl-bis-(dimethyl dodecyl- ammonium bromide) (referred to as C₁₂-s-C₁₂ 2Br) at the air/water interface and on the micellization in the solution have been investigated by surface tension and fluorescence techniques. The results showed that the addition of NaBr greatly enhances their efficiency and effectiveness in surface tension reduction as well as the ability of micellization, even induces strong premicellar aggregation before the cmc. These were attributed to the unique molecular structure of gemini surfactant, where the flexible polymethylene chain was the spacer linking the two quaternary ammonium heads. By a short spacer, the charges of the two quaternary ammonium head groups are concentrated. Even for a long spacer (s = 12), since it is bent toward the alkyl tails, the similar effect is also produced. This results in the high sensitivity of their ionic head groups to salt. Besides, the addition of salt also effectively promotes the hydrophobic interaction between the alkyl tails of gemini surfactants. The addition of NaBr strongly promotes the adsorption of quaternary ammonium gemini surfactants C₁₂-s-C₁₂ 2Br at the air/water interface and the micellization in the solution.     

Accessibility of the gel phase in macroporous network polymers: A comparison of the fluorescence probe and inverse steric exclusion chromatography techniques

Two independent methods of analysis were employed to evaluate accessibility of the gel phase in macroporous network polymers. Fluorescence probes, covalented incorporated into the gel during polymerization, can provide a qualitative assessment of accessible surface area from an analysis of the solvatochromic shift of fluorescence emission. These findings are corroborated by inverse steric exclusion chromatography (ISEC). The data permits quantification of the polymers surface character in the solvated state. The results call attention to the inadequacy of dry surface area technique (e.g., BET analysis) in providing information of this type.     

Conjugated polyelectrolyte with pendant carboxylate groups: Synthesis,photophysics, and pH responses in the presence of surfactants

This article reports the synthesis, optical properties, and pH responses of a water‐soluble poly(phenylene ethynylene) with pendant carboxylate groups. This conjugated polyelectrolyte, PPE‐OBS, was prepared via a facile synthetic route, using 1,4‐di(ethyl 4‐oxy‐butyrate)‐2,5‐diiodo‐benzene as polymeric monomer and followed by the alkali induced saponification. PPE‐OBS absorbs strongly in the blue region and emits a moderately intense fluorescence. However, it subjects to unstable emission in neutral aqueous media, which is believed to arise because the polymer tends to become more hydrophobic via protonation and to form much heavier aggregates (clusters). Subsequent studies on the interaction and stabilization of PPE‐OBS with various surfactants indicate that its fluorescence stability can be largely improved in the presence of a small amount of surfactant. More interestingly, even at low pH, the surfactant‐stabilized polyelectrolyte with its pendant groups being ? COOH rather than ? COO^? remains water‐soluble and emits efficiently, which is expected of great significance for the covalent linkage to bio‐molecules. In addition, the sensitive pH response in emission spectrum of PPE‐OBS/surfactant complex over a relatively wide range suggests surely a potential access to the fluorescent sensing of pH variation. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3056–3065, 2009     

Advanced analytical techniques for the extraction and characterization of plant‐derived essential oils by gas chromatography with mass spectrometry

In recent years, essential oils have received a growing interest because of the positive health effects of their novel characteristics such as antibacterial, antifungal, and antioxidant activities. For the extraction of plant‐derived essential oils, there is the need of advanced analytical techniques and innovative methodologies. An exhaustive study of hydrodistillation, supercritical fluid extraction, ultrasound‐ and microwave‐assisted extraction, solid‐phase microextraction, pressurized liquid extraction, pressurized hot water extraction, liquid–liquid extraction, liquid‐phase microextraction, matrix solid‐phase dispersion, and gas chromatography (one‐ and two‐dimensional) hyphenated with mass spectrometry for the extraction through various plant species and analysis of essential oils has been provided in this review. Essential oils are composed of mainly terpenes and terpenoids with low‐molecular‐weight aromatic and aliphatic constituents that are particularly important for public health.     

Simple, rapid, and sensitive determination of beta-blockers in environmental water using dispersive liquid-liquid microextraction followed by liquid chromatography with fluorescence detection

A novel method, dispersive liquid-liquid microextraction combined with liquid chromatography-fluorescence detection is proposed for the determination of three beta-blockers (metoprolol, bisoprolol, and betaxolol) in ground water, river water, and bottled mineral water. Some important parameters, such as the kind and volume of extraction and dispersive solvents, extraction time, pH, and salt effect were investigated and optimized. In the method, a suitable mixture of extraction solvent (60 μL carbon tetrachloride) and dispersive solvent (1 mL acetonitrile) were injected into the aqueous samples (5.00 mL) and the cloudy solution was observed. After centrifugation, the enriched analytes in the bottom CCl(4) phase were determined by liquid chromatography with fluorescence detection. Under the optimum conditions, the enrichment factors (EFs) for metoprolol, bisoprolol, and betaxolol were 180, 190, and 182, and the limits of detection (LODs) were 1.8, 1.4, and 1.0 ng L(-1) , respectively. A good linear relationship between the peak area and the concentration of analytes was obtained in the range of 3-150 ng L(-1) . The relative standard deviations (RSDs) for the extraction of 10 ng L(-1) of beta-blockers were in the range of 4.6-5.7% (n = 5). Compared with other methods, dispersive liquid-liquid microextraction is a very simple, rapid, sensitive (low limit of detection), and economical (only 1.06 mL volume of organic solvent) method, which is in compliance with the requirements of green analytical methodologies.     

Determination of glutathione content in grape juice and wine by high-performance liquid chromatography with fluorescence detection

A modified preparation of sample was developed for the determination of glutathione content in grape juice and wine by high-performance liquid chromatography with fluorescence detection, using on-line pre-column derivatization. Ice-cold deoxygenated methanol was used to deactivate the oxidation enzymes in juices or wines and keep the glutathione stable. The optimum recovery of glutathione content in grape juice and wine was obtained when either the sample of grape juice or wine was mixed in ice-cold deoxygenated methanol in the ratio 10:90 (v:v) and further diluted in sodium acetate buffer in the ratio 1:1 (v:v). The optimized method was validated for linearity, limit of detection, limit of quantification, precision and uncertainty. According to the validation data the method is appropriate for the determination of glutathione content in grape juice and wine. Glutathione contents in grape juices made from White Muscat grapes and Sauvignon Blanc wines were analysed. The average glutathione content in 28 young Sauvignon Blanc wines was 12.5 mg L⁻¹.     

Simultaneous determination of camptothecin and 10‐hydroxycamptothecine in the Camptotheca acuminate,its medicinal preparation and in rat plasma by liquid chromatography with fluorescence detection

Camptotheca acuminata Decne is an important medicinal plant that contains various cytotoxic alkaloids, such as camptothecine (CPT) and 10‐hydroxycamptothecine (HCPT). A rapid and sensitive liquid chromatography with fluorescence detection (LC‐FLD) method for the quantification of CPT and HCPT is described. The separation was carried out on a DL‐C_l8 column (4.6 × 150 mm, 5 µm), with the mobile phase of acetonitrile–sodium dihydrogen phosphate buffer (10 mm ) using an gradient elution at the flow rate of 0.6 mL/min. The LC‐FLD method was validated for linearity, sensitivity, accuracy and precision, and then used to determine the content of the above components. The lower detection limits of CPT and HCPT were 0.4 and 0.1 ng/mL, respectively. The precision was <1.58% and the mean recovery of the analytes was 96.0–98.6%. The LC‐FLD method was successfully applied to determine CPT and HCPT in real samples including C. acuminate, HCPT injection and rat plasma. Copyright © 2015 John Wiley & Sons, Ltd.     

Instrumental modification intended to save time,and volumes of sample and reagent solutions,in the atomic fluorescence spectrometric determination of mercury

Use of small membrane pumps, instead of peristaltic pumps, to introduce sample and reagent solutions into the spectrometer has several advantages in atomic fluorescence spectrometric determination of mercury. This simple modification results in a substantial saving in the time required for the measurements and so 90% of reagent solution volumes and 95% of sample solution volumes are saved, with a consequent decrease in the volume of waste generated. The sampling frequency is almost tripled, with no deterioration in sensitivity, which is similar to that obtained by use of peristaltic pumps. The relative standard deviation for ten consecutive measurements of a 1 μg L⁻¹ mercury solution was approximately 2%. Figure Small membrane pumps for the atomic fluorescene spectro metric determination of mercury     

Spectrophotometric determination of cationic and anionic surfactants with anionic dyes in the presence of nonionic surfactants,part II: Development of batch and flow injection methods

On the basis of the changes in absorption spectra of azo dyes on the addition of an organic onium ion, spectrophotometric methods for the determination of organic onium salts and anionic surfactants were developed, and applied to flow injection method. Propyl orange (PO^–) was used for the determination of organic onium ions. Pairs of PO^– and Zeph⁺ (tetradecyldimethyl-benzylammonium ion) or PO^– and nC₁₈TMA⁺ (n-octadecyltrimethylammonium ion) were used for the determination of anionic surfactants. The determination range of organic onium ions were (0–3) × 10^–5 M by a batch method and were (0–2) × 10^–5 M by a flow injection method. The determination ranges of anionic surfactants were (0–2) × 10^–5 M by the batch method, and were (0–5) × 10^–5 M by the flow injection method, and the detection limit corresponding toS/N = 3 was 3 × 10^–7 M by the flow injection method. By the proposed flow injection method, anionic surfactants in water samples were determined.     

On the localization of water-soluble porphyrins in micellar systems evaluated by static and time-resolved frequency-domain fluorescence techniques

Fluorescence quenching of meso-tetrakis-4-sulfonatophenyl (TPPS(4)) and meso-tetrakis-4-N-methylpyridil (TMPyP) porphyrins is studied in aqueous solution and upon addition of micelles of sodium dodecylsulfate (SDS), cetyltrimethylammonium chloride (CTAC), N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and t-octylphenoxypolyethoxyethanol (Triton X-100). Potassium iodide (KI) was used as quencher. Steady-state Stern-Volmer plots were best fitted by a quadratic equation, including dynamic (K(D)) and static (K(S)) quenching. K(S) was significantly smaller than K(D). Frequency-domain fluorescence lifetimes allowed estimating bimolecular quenching constants, k(q). At 25 degrees C, in aqueous solution, TMPyP shows k(q) values a factor of 2-3 higher than the diffusional limit. TPPS(4) shows collisional quenching with pH dependent k(q) values. For TMPyP quenching results are consistent with reported binding constants: a significant reduction of quenching takes place for SDS, a moderate reduction is observed for HPS and almost no change is seen for Triton X-100. Similar data were obtained at 50 degrees C. For CTAC-TPPS(4) system an enhancement of quenching was observed as compared to pure buffer. This is probably associated to accumulation of iodide at the cationic micellar interface. The attraction between CTAC headgroups and I(-), and repulsion between SDS and I(-), enhances and reduces the fluorescence quenching, respectively, of porphyrins located at the micellar interface. The small quenching of TPPS(4) in Triton X-100 is consistent with strong binding as reported in the literature.     

Intermicellar material exchange in reverse micelles formed by ionic AOT and nonionic Igepal surfactants studied by means of pulse radiolysis. Influence of the temperature

The recombination of thiocyanate anion radicals, (SCN) ₂ ⁻ , formed pulse radiolytically within the water pools of reverse micelles stabilized with anionic AOT and nonionic Igepal surfactants, was proved as an indicator reaction to study intermicellar exchange. It was found that the exchange process is slower inIgepal than in AOT reverse micelles with the same water to surfactant ratio. The apparent activation enthalpy and entropy of the exchange process were determined in different alkanes. For the AOT and Igepal reverse micelles the activation parameters increase with the droplet size, but for the AOT systems they do not significantly change with the increase of droplet concentration. For non-percolated systems the activation parameters for Igepal reverse micelles approach those for AOT reverse micelles. This result supports existing suggestions that the mechanism of intermicellar exchange does not differ in principle between reverse micelles stabilized with ionic and nonionic surfactants.     

Destabilization of collagen in hide and leather by anionic surfactants. II. Calorimetry of the reaction of collagen with sulfates

Leather, a textile based on collagen, usually requires the addition of sulfated oils that have been recently found to cause instability when heated in critical manufacturing processes. Here reactions between collagen and sodium dodecyl sulfate (SDS), sulfated castor oil, or a synthetic sulfated oil are studied calorimetrically. Sodium lauryl sulfate below its critical micelle concentration (cmc) displayed an immediate exotherm due to equilibrium binding of the reagents with stoichiometry n = 12.6 ± 0.2, K = (2.02 ± 0.8) × 10⁷ M⁻¹, and enthalpy ΔH = 62 ± 2 Kcal/mol; and a delayed endotherm due to denaturation of collagen. The endotherms accompanying the reactions with sulfated oils with longer chains were smaller, with no apparent denaturation of collagen. The micellar nature of these surfactants was apparent from very large n for sulfated castor oil, 4082 ± 11 and a very small value of ΔH, 0.77 ± 0.01 cal/mol. The binding of sulfated castor oil at the polar bands of collagen crystallites, comprising extended molecules arranged side-by-side, was shown directly by electron microscopy. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 805–813, 1998