Time-resolved Fluorescence Quenching Research Articles

Effect of the Nature of the Counterion on the Properties of Anionic Surfactants. 3. Self-Association Behavior of Tetrabutylammonium Dodecyl Sulfate and Tetradecyl Sulfate: Clouding and Micellar Growth

The surfactants tetrabutylammonium dodecyl sulfate (TBADS) and tetradecyl sulfate (TBATS) have been synthesized by ion-exchange, and their self-association behavior has been investigated. The solutions of these surfactants show clouding and phase separation as the temperature is increased. The clouding temperature TC of TBADS solutions has been found to be about 4−5 °C higher than for TBATS solutions in the whole range of composition up to a surfactant content of 64 wt %. The critical micellization concentration (cmc) of TBATS has been determined using the electrical conductivity method. The micelle aggregation numbers (N) have been determined using the time-resolved fluorescence quenching (TRFQ) method, with pyrene/dodecylpyridinium chloride as fluorescent probe/quencher pair. For the two surfactants, N increases with the surfactant concentration and above a threshold concentration with the temperature. This latter increase as well as the phase separation observed for TBADS and TBATS are very unusual fea...

Role of Counterion Concentration in Determining Micelle Aggregation: Evaluation of the Combination of Constraints from Small-Angle Neutron Scattering, Electron Paramagnetic Resonance, and Time-Resolved Fluorescence Quenching

Small-angle neutron scattering (SANS) has been used to study a series of aqueous solutions of (i) the anionic surfactant sodium dodecyl sulfate and the simple electrolyte sodium chloride and (ii) the cationic surfactants dodecyltrimethylammonium bromide and dodecyltrimethylammonium chloride, in the presence of sodium bromide and sodium chloride, respectively. For all three systems, the surfactant spans a wide concentration range, and by suitable choices of electrolyte concentration, it is possible to engineer these solutions to have the same concentration of counterions in the aqueous phase. According to a recent hypothesis by Bales et al. (J. Phys. Chem. B 2001, 105, 6798), such a series is expected to produce micelles having the same aggregation number (Nagg), a point verified by time-resolved fluorescence quenching (TRFQ) and electron paramagnetic resonance (EPR). The SANS results presented here are in good agreement with the TRFQ and EPR studies. We therefore show unequivocally that the sizes and shapes of these surfactant micelles depend only on the free counterion concentration, a conclusion supported by conventional theories of micellization. Subsequently, for the SDS data, a comparison is made between fitting the SANS data to a model in which the aggregation number and degree of hydration are constrained and fitting them to a model in which these features are fittable parameters. In essence, both approaches yield the same conclusion. Such an approach is not possible with the cationic surfactants, as the contrast between the hydrated shell and the core is minimal.

Synthesis and Aggregation Behavior of Amphiphilic Block Copolymers in Aqueous Solution: Di- and Triblock Copolymers of Poly(ethylene oxide) and Poly(ethyl acrylate)

Di- and triblock copolymers of ethylene oxide (EO) and ethyl acrylate (EA) were synthesized by the atom transfer radical polymerization technique. The chemical compositions of the di- and triblock copolymers as determined by NMR and gel permeation chromatography are represented by [EO114EA10] and [EA10EO445EA10], respectively. The aggregation behaviors of these polymers in aqueous solutions were studied using a combination of static light scattering (SLS), dynamic light scattering, steady fluorescence, time-resolved fluorescence quenching (TRFQ), and surface tension techniques. The hydrodynamic radii (Rh) of PEO-b-PEA and PEA-b-PEO-b-PEA, where PEO is poly(ethylene oxide) and PEA, poly(ethylene acrylate), in an aqueous solution possess values of 15.3 and 25.5 nm, respectively, and they are independent of the polymer concentrations, which suggest that these micelles are formed via the closed association model. The critical micelle concentration (cmc) for the di- and triblock copolymers as determined from t...

Small-Angle Neutron Scattering, Electron Paramagnetic Resonance, Electrophoretic NMR, and Time-Resolved Fluorescence Quenching Studies of Sodium Dodecyl Sulfate and Tetra(ethylene oxide) Dodecyl Ether Mixed Surfactant Micelles

Small-angle neutron scattering (SANS), time-resolved fluorescence quenching (TRFQ), electrophoretic NMR (ENMR), and electron paramagnetic resonance (EPR) have been used to study mixed micelles formed from the two dodecyl tailed surfactants, sodium dodecyl sulfate (SDS) and tetra(ethylene oxide) dodecyl ether. By combining the TRFQ and EPR techniques, one is essentially able to calculate the SANS data, lending significant weight to the micelle characteristics thus obtained. EPR reports the degree of hydration of the surfactant headgroups; TRFQ, the micelle aggregation number and inter alia the volume of the hydrophobic core. Given the physical dimensions of the surfactant, i.e., alkyl chain length, headgroup volumes, etc., it is then possible to calculate the micelle ellipticity and shell thickness. The ENMR studies provide a rather different but complementary estimate of the ionic character of the micelle. With increasing nonionic content, the aggregation number increases, the micelle becomes more elliptical, and the headgroup region of the micelle becomes less hydrated and significantly more viscous. The degree of sodium counterion dissociation shows an initial small decrease with decreasing SDS micelle mole fraction but subsequently increases, reflecting the interplay between the electrostatic character of the micelle surface and the micelle curvature.

An investigation of dynamic surface tension, critical micelle concentration, and aggregation number of three nonionic surfactants using NMR, time-resolved fluorescence quenching, and maximum bubble pressure tensiometry

Several physicochemical properties have been determined for N-dodecyllactobionamide (LABA), maltose 6′- O-dodecanoate (C 12-maltose ester), and tetra(ethylene oxide) dodecyl amide (TEDAd). The increase in the flexibility of the sugar headgroup, enabling more possible molecular conformations, reduces the minimum area/molecule at the liquid–vapor interface obtained at the critical micelle concentration (cmc). The obtained cmc's were 0.35 mM (LABA), 0.3 mM (C 12-maltose ester), and 0.5 mM (TEDAd). The monomer diffusion coefficient decreased with the molecular weight and increasing headgroup flexibility of the sugar headgroup, and values were in the range from 3.1×10 −10 to 3.6×10 −10 m 2/s. The micelle diffusion coefficients (0.46×10 −10 to 0.68×10 −10 m 2/s) indicated that the TEDAd micelles deviated most from spherical shape. The micelle aggregation numbers determined by time-resolved fluorescence quenching (TRFQ) were estimated to be 120±10 (LABA), 90±10 (C 12-maltose ester), and 130±10 (TEDAd). The dynamic surface tension measurements show that the adsorption of TEDAd onto the liquid–vapor interface at short surface lifetimes is diffusion-limited, whereas an adsorption barrier is present for the sugar surfactants. The analysis of the dynamic surface tension data above the cmc shows that the rate of demicellization is faster for TEDAd than for the two sugar-based surfactants.

Characterization of Micelles of Quaternary Ammonium Surfactants as Reaction Media I: Dodeclytrimethylammonium Bromide and Chloride

Time-resolved fluorescence quenching (TRFQ) and electron paramagnetic resonance (EPR) were employed to characterize micelles of dodecyltrimethylammonium bromide and chloride (DTAB and DTAC) as reaction media. For DTAB, the aggregation numbers, N, and the quenching rate constant of pyrene by hexadecylpyridinium chloride, kq, were measured with TRFQ. Both these aggregation numbers for DTAB and those for DTAC taken from the literature depend only on the concentration of counterions in the aqueous phase, Caq, whether these counterions are supplied by the surfactant alone or by surfactant plus added salt. Both surfactants conform to the power law N = N0(Caq/cmc0)γ where N0 is the aggregation number at the critical micelle concentration in the absence of any additives (cmc0). N0 and γ differ for the two surfactants and vary with temperature in DTAB. EPR is employed to investigate the microviscosity and the hydration of the polar shell using a spin probe. The hydration is expressed by the nonempirical polarity p...

The Transition from Vesicles to Micelles Induced by Octane in Aqueous Surfactant Two-Phase Systems

The transition from vesicles to micelles by adding octane into the dodecyl-pyridinium chloride (DPCl)/sodium laurate (SL), and dodecyltrimethylammonium bromide (DTAB)/SL aqueous two-phase systems has been studied. The two-phase systems were transformed into single-phase isotropic solutions with the addition of a certain amount of octane. The results of dynamic light scattering (DLS) demonstrate the decrease of large aggregates (vesicles) and the increase of the small aggregates (spherical micelles) upon octane addition. Such transformation of the surfactant aggregates was also corroborated by the results of time-resolved fluorescence quenching (TRFQ) and viscometry. The change of the phase behavior was discussed on the basis of the interaggregate interactions influenced by the variation of organized assemblies.

Interaction between Poly(ethylene glycol) and C12E8 Investigated by Dynamic Light Scattering, Time-Resolved Fluorescence Quenching, and Calorimetry

Dynamic light scattering (DLS), time-resolved fluorescence quenching (TRFQ), and isothermal titration microcalorimetry have been used to show that, in dilute solution, low molecular weight poly(ethylene glycol) (PEG, M-w = 12 kDa) interacts with the nonionic surfactant octaethylene glycol n-dodecyl monoether, C12E8, to form a complex. Whereas the relaxation time distributions for the binary C12E8/water and PEG/water systems are unimodal, in the ternary mixtures they may be either uni- or bimodal depending on the relative concentrations of the components. At low concentrations of PEG or surfactant, the components of the relaxation time distribution are unresolvable, but the distribution becomes bimodal at higher concentrations of either polymer or surfactant. For the ternary system in excess surfactant, we ascribe, on the basis of the changes in apparent hydrodynamic radii and the scattered intensities, the fast mode to a single micelle, the surface of which is associated with the polymer and the slow mode to a similar complex but containing two or three micelles per PEG chain. Titration microcalorimetry results show that the interaction between C12E8, and PEG is exothermic and about 1 kJ mol(-1) at concentrations higher than the CMC Of C12E8. The aggregation number, obtained by TRFQ, is roughly constant when either the PEG or the C12E8 concentration is increased at a given concentration of the second component, owing to the increasing amount of surfactant micelles inside the complex. (Less)

Size, Hydration, and Shape of SDS/Heptane Micelles Investigated by Time-Resolved Fluorescence Quenching and Electron Spin Resonance

Time-resolved fluorescence quenching (TRFQ) and electron spin resonance (ESR) are used to investigate sodium dodecyl sulfate/heptane micelles in water in the presence of varying amounts of sodium chloride (NaCl). Aggregation numbers are obtained from TRFQ and the micelle hydration from ESR. At any zero or any fixed sodium chloride concentration ≤375 mM, micelles grow with heptane while maintaining the volume fraction of water in the polar shell of the micelle, referred to as hydration index, constant. Core−shell model calculations, using simple geometry, show that some hydrocarbon density extends into the polar shell. The shell comprises about 15−17% of the core hydrocarbon in addition to the headgroups and hydration water. The rate of micelle growth with sodium chloride is smaller in the presence of heptane, and at higher salt concentrations (>375 mM), the micelles decrease in size with heptane. The rate of variation of hydration index with [NaCl] or aggregation number is indicative of micelle shape and shows that the sphere-to-rod transformation occurs at higher aggregation numbers and higher salt concentrations when heptane is present.

Characterization of Mixed Micelles of SDS and a Sugar-Based Nonionic Surfactant as a Variable Reaction Medium

Time-resolved fluorescence quenching (TRFQ), electron paramagnetic resonance (EPR), and small-angle neutron scattering (SANS) are employed to characterize mixed micelles of sodium dodecyl sulfate (SDS) and the nonionic sugar-based surfactant dodecylmalono-bis-N-methylglucamide (DBNMG) as a reaction medium. Interpretation of the results from the three methods are constrained to fit the classical model of a hydrocarbon core surrounded by a polar shell. As measured by TRFQ, at 45 °C, the aggregation numbers increase from 48 for pure SDS to a maximum of 63 at 36 mole percent DBNMG and decrease again to 49 for pure DBNMG. The aggregation numbers of the pure DBNMG increase from 33 at 21° Ct o 49 at 45°C. SANS results of the pure DBNMG are interpreted by fixing the aggregation number at 49 (from TRFQ) and allowing the polar shell thickness to vary as a fitting parameter, yielding a value of 5.8 A at 45 °C. EPR, utilizing a hydrophobic spin probe, is used to measure the nonempirical polarity index, H(25 °C), of the polar shell. H(25 °C) is defined to be the ratio of molar concentration of OH dipoles in a solvent or solvent mixture to that in water at 25 °C; thus, both water and the sugar headgroups contribute. By fixing the volume in the polar shell inaccessible to water due to the presence of SDS (127 A 3 ) and DBNMG (580 A 3 ), theoretical values of H(25 °C) are computed, the result depending only on the number of OH bonds in DBNMG available to interact with the spin probe. A constant average value of 7.4 OH bonds out of a maximum possible number of 10 reproduces the measured values over the full range of mixed micelle compositions. At 45 °C, the microviscosity of the polar shell, as deduced from the rotational correlation time of the spin probe, varies from 2.79 ( 0.05 cP for pure SDS to 13.1 ( 0.2 cP for pure DBNMG departing only slightly from a linear dependence on the mole fraction of DBNMG. The uncertainties in the viscosity are the standard deviations in 10 measurements and therefore represent the uncertainty in the relative values of the viscosity. The viscosity decreases from 32 ( 4c P at 21°C to 13.1 ( 0.2 cP at 45 °C for pure DBNMG micelles. These viscosities are used to show that the quenching rate constant of pyrene by dimethyl benzophenone, measured by TRFQ, follows the Stokes-Einstein-Smolukhovsky equation with a quenching probability of P ) 0.5 whether the mixed micelle composition is changed at constant temperature or if the temperature is varied in pure DBNMG. The volume of the polar shell enters into the formulation because the effective concentration of the quencher depends on that volume; however, the quenching rates are not correlated with volume alone.

Small Variations in the Composition and Properties of Triton X-100

Two samples of Triton X-100 (TX100) were compared. The “old sample,” manufactured by Rohm and Haas, has a mean degree of ethoxylation of 10.1. The “new sample,” manufactured by Union Carbide, has a mean degree of ethoxylation of 9.6. The cmc values near room temperature (23°C, old sample; 24.6°C, new sample), as determined from surface tension measurements, are similar. Dynamic light scattering measurements on the new sample gave hydrodynamic radii at different temperatures in reasonable accord with those reported a number of years ago. In contrast, time-resolved fluorescence quenching (TRFQ) experiments using a pyrene-labeled triglyceride as a probe gave aggregation numbers Nagg near room temperature that were similar to those determined previously by light scattering; however, the Nagg of the new sample was larger, and the difference was outside of experimental error to one standard deviation. More troubling was that the TRFQ experiment failed to give meaningful results on solutions exposed to air, as in the case of the older TX100 sample. To get useful data by TRFQ measurements, one had to first remove oxygen from the solution.

Deriving Intrinsic Parameters of Photoinduced Electron Transfer Reaction from the Transient Effect Probed by Picosecond Time-Resolved Fluorescence Quenching

Fluorescence quenching of a pyrylium salt (PDP2+) by toluene in acetonitrile gives rise to a nonexponential decay. This behavior is ascribed to the so-called transient effect occurring at high quencher concentrations for diffusion-controlled reactions. First, the Kalman filter was used to deconvolute the original signal from the experimental decay curve and the response function of the apparatus. This treatment led to a calculated deconvoluted decay curve which enabled the transient effect analysis to be conducted. This real decay curve was then analyzed using two models. The Smoluchowski—Collins—Kimball (SCK) model, applied to diffusion-controlled reactions, yielded the reaction radius rAD and the intrinsic rate constant kact of the bimolecular electron transfer reaction. The Marcus electron transfer/diffusion (ETD) model, which provides a powerful method to evaluate the electronic coupling Hel associate with the reaction, was also used but is more difficult to handle due to extensive computational needs. Finally, the adequacy of the two models (SCK and ETD) for analysis of the transient effect was addressed, as well as the appropriateness of the Kalman filter for fluorescence signal deconvolution.

Adsorption of a divalent cationic surfactant onto a silica surface

This study is concerned with the adsorption of a divalent cationic surfactant onto an oppositely charged silica surface. The amount adsorbed and thickness of the adsorbed layer have been determined by means of ellipsometry. These data indicate that small micelles, most likely deformed from spherical shape, are formed on the surface at surface concentrations above that required to neutralize the surface. This conjecture is in agreement with the bulk behavior of the studied surfactant, which is dominated by the presence of small micelles up to very high concentrations. We have developed a thermodynamic model for the adsorption process. Central to this model are the bulk micellar aggregation numbers, which have been determined by the time resolved fluorescence quenching (TRFQ) technique. The model contains three crucial steps: i) the formation of a micelle in the bulk; ii) the adsorption of a single micelle to the surface; and iii) the process of increasing the surface concentration of micelles to a finite amount adsorbed. In developing this model, we have used the Poisson–Boltzmann Cell Model (PBCM), together with a Derjaguin-type approximation in the description of the interaction between the surface and the charged micelle.

Investigation of the Poly(N-isopropylacrylamide)-Sodium Dodecyl Sulfate Complexation with Viscosity, Dialysis, and Time-resolved Fluorescence-Quenching Measurements

Interaction of poly(N-isopropylacrylamide) (PNIPAM) with sodium dodecyl sulfate (SDS) has been studied by viscometry, time-resolved fluorescence-quenching (TRFQ), and dialysis measurements. The reduced viscosity of PNIPAM in SDS aqueous solutions presents an abrupt increase in the low SDS concentration region, lower than the surfactant critical micelle concentration, whereas TRFQ measurements show the formation of an increasing number of polymer−surfactant aggregates in the same SDS concentration region. The aggregation number of these aggregates, determined by combining TRFQ with dialysis data, remains constant and equal to 7−8 surfactant molecules per polymer-bound surfactant aggregate. Moreover, the values of the inverse of the intramicellar quenching rate constant, expressing the effective microviscosity of the aggregates, indicate that these microdomains, formed in the low SDS concentration region, are very rigid. After saturation of the PNIPAM chain with these aggregates, a further quantity of surfa...

Time-Resolved Fluorescence Quenching Studies in Nanocomposite Materials Made of Silica and Cetyltrimethylammonium Bromide

Time-Resolved Fluorescence Quenching Studies in Nanocomposite Materials Made of Silica and Cetyltrimethylammonium Bromide

First Simultaneous Estimates of the Water Pool Core Size and the Interfacial Thickness of a Cationic Water-in-Oil Microemulsion by Combined Use of Chemical Trapping and Time-Resolved Fluorescence Quenching

The first simultaneous experimental estimates ofthe water pool core size (R w ) and the interfacial thickness (d) of a cationic water-in-oil microemulsion, CTAB/isooctane/n-hexanol/water, was achieved by the combined use of chemical trapping and the time-resolved fluorescence quenching. Our estimated values of R w = 29.1 ± 1.5 compares reasonably well with the reported size of the water pool core (32 A) of a structurally and compositionally similar CTAB/dodecane/n-hexanol/water cationic water-in-oil microemulsion (Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 4367), and our estimated interfacial thickness (6.3 A ± 0.3) is consistent with the recently estimated thickness for the hydration layer (10 A) of the cationic CTAB/ n-hexane/n-pentanol/water, water-in-oil microemulsions (Giustini, M. et al. J. Phys. Chem. 1996, 100, 3190). To our knowledge, this is the first simultaneous experimental estimate of the size of the water pool core and interfacial thickness of a cationic water-in-oil microemulsion using a chemical reaction.

Aggregation Behavior of Tyloxapol, a Nonionic Surfactant Oligomer, in Aqueous Solution

The aggregation behavior of Tyloxapol, a nonionic surfactant oligomer with a repeating unit close to Triton X-100 (TX100), and a maximum degree of polymerization of about 7, has been investigated in aqueous solution by means of fluorescence probing, time-resolved fluorescence quenching (TRFQ) and transmission electron microscopy at cryogenic temperature (cryo-TEM). The plot of the pyrene fluorescence intensity ratioI1/I3against the Tyloxapol concentration shows no clear evidence of a critical micelle concentration contrary to TX100. Nevertheless, the fitting of these data, assuming a partition of pyrene between Tyloxapol aggregates and water, yields cmc values in the micromolar range, i.e., about a hundred times lower than for the “monomer” TX100. The values ofI1/I3at high surfactant concentrations indicate that Tyloxapol micelles provide pyrene a less polar environment than TX100 micelles. The use of the viscosity-sensitive probe 1,3-dipyrenylpropane indicates that the microviscosity of Tyloxapol micelles is quite high, three to four times larger than that for TX100 micelles, and decreases rapidly with increasing temperature. Also the microviscosities of both TX100 and Tyloxapol micelles are larger than those for the micelles of the nonionic ethoxylated surfactant C12E9. The aggregation numbers of Tyloxapol and of TX100 micelles measured using TRFQ increase with temperature, with the Tyloxapol micelles being smaller than the TX100 micelles. Cryo-TEM shows that the Tyloxapol micelles remain spheroidal up to a concentration of about 10 wt%. At 15 wt%, some regions of ordered elongated micelles are also observed which may be the precursors of the hexagonal phase known to occur at about 35 wt%.

Phase Behavior and Characterization of Micellar and Cubic Phases in the Nonionic Surfactant C〈17〉E〈84〉/Water System. A PFG NMR, SAXS, Cryo-TEM, and Fluorescence Study

An extensive study of the diffusion behavior of the C〈17〉E〈84〉/water system is presented. The surfactant, when mixed with water, forms a micellar phase below ≈13 wt % and a cubic phase between ≈20−60 wt % (at 25 °C). In addition to the pulsed field gradient (PFG) NMR technique (used to determine the self-diffusion coefficients), the system has been investigated by small-angle X-ray scattering (SAXS), cryo-transmission electron microscopy (cryo-TEM), and time-resolved fluorescence quenching (TRFQ). The self-diffusion coefficient (D) and the transverse relaxation time (T2) of the surfactant molecules decrease significantly when going from the micellar to the cubic phase. The results of the PFG NMR, SAXS, cryo-TEM, and TRFQ experiments show that the cubic phase is composed of discrete aggregates. On the basis of the rapid transverse 1H NMR relaxation in the cubic phase, it is argued that the micellar building blocks of the cubic phase are nonspherical. Furthermore, the SAXS data for four different concentrations of the surfactant (25, 35, 45, and 55 wt %) in the cubic phase can be indexed to the space group Im3m. If the data from the NMR and the SAXS measurements are combined, a lifetime of the surfactant monomers in the micelles of 8 and 7 ms was obtained at 25 and 35 wt % C〈17〉E〈84〉, respectively.

Micelle Aggregation Numbers of Surfactants in Aqueous Solutions: A Comparison between the Results from Steady-State and Time-Resolved Fluorescence Quenching

The aggregation numbers of the surfactant aggregates in surfactant-containing systems can be measured using the steady-state fluorescence quenching (SSFQ) and the time-resolved fluorescence quenching (TRFQ) methods. From a theoretical point of view the SSFQ method involves much more restrictive assumptions than the TRFQ method. As a result, the range of aggregation numbers that can be measured with SSFQ does not extend to values as high as with TRFQ. Nevertheless, SSFQ is much used because the measurements are easy to perform and only require readily available spectrofluorometers. The purpose of this study was to compare as completely as possible the two methods. For this purpose we have performed systematic determinations of the values of the surfactant aggregation number in micellar solutions in pure water and in water + polymer and oil-in-water microemulsions, using SSFQ and TRFQ. A good agreement between the two methods is observed when the quenching rate constant of the fluorescent probe (here pyrene...

Mixed Solutions of Surfactant and Hydrophobically Modified Polymer. Controlling Viscosity with Micellar Size

The viscosity of mixtures of a hydrophobically modified cellulose polymer (HMHEC) and surfactants in aqueous solution was determined as a function of the concentration of mixed micelles. It was shown that, in solutions of low viscosity where the ratio between polymer hydrophobic tails and mixed micelles is low, a high viscosity can be recovered by increasing the surfactant aggregation number in the mixed micelles. The surfactant aggregation number of the mixed micelles was increased by adding either a screening electrolyte or an oppositely charged surfactant to solutions containing the hydrophobically modified polymer and an ionic surfactant. Time-resolved fluorescence quenching (TRFQ) was used to investigate the size of the mixed micelles.