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R Zana

Publications and source records attributed to R Zana.

At least 19 recordsLinked to original sources
PubMed2005

Nuclear magnetic resonance and small-angle neutron scattering studies of anionic surfactants with macrocounterions: tetramethylammonium dodecyl sulfate.

Micellar solutions of tetramethylammonium dodecyl sulfate have been studied to determine the degree of counterion binding. Tetramethylammonium chloride was added over a wide range of surfactant concentrations such that the total concentration of tetramethylammonium ions in solution remained constant. Small angle neutron scattering experiments showed a constancy in aggregation number across this series, consistent with the constant C(aq) concept of Bales et al. (J. Phys. Chem. B 2001, 105, 6798). Pulsed-field gradient and electrophoretic NMR experiments were used to determine the degree of counterion dissociation, alpha, which was found to be 0.33. This value is in contrast to the value from conductivity measurements (alpha = 0.2), but supports the concept of an aggregation number based definition of alpha.

Magnetic Resonance Spectroscopy↗
PubMed2002

Alkanediyl-alpha,omega-bis(dimethylalkylammonium bromide) surfactants 9. Effect of the spacer carbon number and temperature on the enthalpy of micellization.

The enthalpies of dilution of micellar solutions of several 12-s-12 dimeric surfactants of the alkanediyl-alpha,omega-bis(dodecyldi-methylammonium bromide) type, differing by the carbon number s of the alkanediyl spacer, and of dodecyltrimethylammonium bromide (DTAB) have been measured calorimetrically, in a range of concentrations extending from well below to well above the critical micelle concentration (cmc). The results permitted the determination of the enthalpy of micellization, DeltaH degrees (M), of the investigated surfactants at 25 and 35 degrees C. The values of DeltaH degrees (M) were always negative and became more negative as the temperature was increased. The plot of -DeltaH degrees (M) against s showed a shallow minimum at about s=5 and a large decrease of -DeltaH degrees (M) going from 12-2-12 to 12- 4-12. This effect has been attributed to the contribution to DeltaH degrees (M) of the hindered rotation of the dodecyl chains around the spacer C-C bond for 12-2-12. This hindrance is shown to rapidly disappear when s is increased from 2 to above 4. The specific heats of micellization, the free energies of micellization, and the entropies of micellization (DeltaS degrees (M)) have been calculated using the DeltaH degrees (M) values and the reported cmc and micelle ionization degree data for 12-s-12 surfactants and DTAB. For all surfactants the results show that TDeltaS degrees (M)>-DeltaH degrees (M), indicating an entropy-driven micellization.

Journal Article↗
PubMed2000

Photophysical Behavior of a New Gemini Surfactant in Neat Solvents and in Micellar Environments.

A novel fluorescent gemini surfactant, 1,4-bis-(2'-(N-dodecyl pyridinio-4"-yl)ethenyl)benzene dibromide, abbreviated BDPEBB, has been synthesized and its photophysical properties have been studied in different environments. BDPEBB has a limited solubility in alcohols where it is found in aggregate form at concentrations>/=1 mM. In other solvents, e.g., water, it is only found in aggregate form, even at much lower concentrations. Solvent polarity has a small and insignificant solvatochromic effect but alcohols give a specific interaction with BDPEBB, causing a significant hypsochromic shift in absorption maxima and a large increase in relative fluorescence efficiency. Pyrene fluorescence is effectively quenched by BDPEBB. Pyrene also forms associative complexes with BDPEBB in water. These complexes are partly dissociated in the presence of surfactant micelles. Triton X-100 micelles provide a favorable environment for BDPEBB solubilization well distinguished from the behavior of ionic surfactants. Small quantities of BDPEBB have a large influence on the behavior of aqueous sodium dodecylsulfate (SDS) and sodium decylsulfate (SDeS) micelles, inducing the formation of large aggregates, visible by the naked eye. These large aggregates are most probably microcrystals of BDPEBB(2+)/2DS(-) or BDPEBB(2+)/2DeS(-). The aggregation number of SDS and SDeS micelles in the absence and in the presence of BDPEBB has been calculated by exploitation of the static luminescence quenching kinetics of Ru(bpy)(3)(2+) by 9-methylanthracene, both solubilized in the micellar phase. It has been observed that Ru(bpy)(3)(2+) inhibits the precipitation of SDeS micelles in the presence of BDPEBB. Our results suggest that double-chain surfactant chromophores should be employed with particular care if they are to be used as probes of the micellar phase. Copyright 2000 Academic Press.

Journal Article↗
PubMed1999

Interactions between Quaternary Ammonium Surfactant Oligomers and Water-Soluble Modified Guars.

The interaction between hydroxypropylguar (HPG) and its dodecyl-modified derivative (HMHPG) and cationic surfactant oligomers has been investigated by measurements of the solution viscosity at constant shear rate, microviscosity of the aggregates (dipyrenylpropane fluorescence emission spectra), and aggregation number of the polymer hydrophobe and of the surfactant (time-resolved fluorescence quenching). The surfactants are dodecyltrimethylammonium bromide (DTAB, monomeric surfactant) and some of its dimers and trimers which differed by the carbon number s of the polymethylene spacer connecting the surfactant moieties (2 </= s </= 20). Most results refer to a polymer concentration of 1 wt%. Only a weak interaction was evidenced between HPG and these surfactants, whereas strong interactions occurred between HMHPG and the surfactant oligomers. The interaction became stronger as the degree of oligomerization of the surfactant increased. The results led us to distinguish three ranges of concentration of added surfactant. The first range corresponds to surfactant concentrations below the surfactant cmc in water. In this range mixed aggregation occurs between polymer hydrophobes and surfactant ions, and the viscosity of the HMHPG + surfactant systems goes through a maximum, as usually found for associating polymers. The second range extends from the cmc to about 10 cmc. Precipitation of a polymer/surfactant complex occurs in this range with all surfactants forming threadlike micelles. For the other surfactants the viscosity goes through a minimum. In the third range, which corresponds to surfactant concentrations above 10 cmc, resolubilization of the precipitated HMHPG/surfactant complexes occurs and a solution-to-gel transition is observed for the surfactants which form threadlike micelles or vesicles. The concentration corresponding to this transition is about the same as that for pure surfactant solutions. Some polymer hydrophobes may contribute to the formation of additional bridges between surfactant micelles. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Dynamics of Micelles of Polyethyleneoxide-Polypropyleneoxide-Polyethyleneoxide Block Copolymers in Aqueous Solutions.

The dynamics of the micelles of five triblock poly(ethyleneoxide)-poly(propyleneoxide)-poly(ethyleneoxide) copolymers, the Pluronics P104 (EO27PO61EO27), P84 (EO19PO43EO19), P65 (EO18PO29EO18), P85 (EO26PO40EO26), and P103 (EO17PO60EO17), have been investigated using two chemical relaxation methods: the temperature-jump and the ultrasonic relaxation (absorption). In the frequency range investigated (0.5-50 MHz), the ultrasonic absorption spectra (absorption vs frequency plots) consisted in tails of relaxation curves, indicating characteristic times much longer than 0.3 µs for the exchange of copolymers between micelles and intermicellar solution. Absorption measurements at a fixed frequency yielded the critical micellization temperature of the solutions. The temperature-jump results obtained in this study together with those from a previous one for the copolymers L64 (EO13PO30EO13) and PF80 (EO73PO27EO73) (B. Michels et al., Langmuir 13, 3111, 1997) showed that the relaxation time associated with the formation/breakup of micelles becomes longer upon increasing copolymer molecular weight at constant composition. This time also increased when decreasing the length of the hydrophilic block at fixed hydrophobic block length or increasing the length of the hydrophobic block at fixed hydrophilic block length, similar to conventional surfactants. The dynamics of block copolymers micelles in aqueous solution are discussed. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Study of the Effect of Solubilized Gases on the Properties of Microemulsion Droplets.

The effect of dissolved gas (nitrogen, argon) on the properties of the droplets in water-in-oil and oil-in-water microemulsions (surfactant aggregation number, microviscosity, and micropolarity) has been investigated by means of time-resolved fluorescence quenching and spectrofluorometry. This study extends a similar one on aqueous micellar solutions (R. G. Alargova et al., Langmuir 14, 1575, 1998). The selected microemulsions were characterized by droplets of fairly large size and high volume fraction, in order to minimize the effect of the curvature of the surfactant layer and maximize the amount of gas that can be solubilized in the system. Within the experimental error, the investigated properties (surfactant aggregation number, intradroplet quenching rate constant which is related to the droplet microviscosity, and fluorescent probe lifetime and micropolarity) were found to be independent on whether the system was degassed, nitrogen-saturated, or argon-saturated, in the temperature range between 10 and 35 degrees C. The results confirm the conclusion reached in the above study; i.e., the effect of solubilized gases on the hydrophobic interaction which controls the formation of surfactant assemblies is extremely small and well below the sensitivity of the fluorescence probing techniques used in this investigation. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

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 ratio I1/I3 against 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 of I1/I3 at 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%. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Adsorption Mechanism of Conventional and Dimeric Cationic Surfactants on Silica Surface: Effect of the State of the Surface.

The aim of this study was to investigate the effect of the state of the silica surface and of the surfactant molecular structure on the adsorption of cationic surfactants onto silica. Thus, the adsorption of DTAB (dodecyltrimethylammonium bromide) and of the dimeric surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) on raw silica (SiNa) and on HCl-washed silica (SiH) has been investigated under "free" system conditions. The amount of surfactant adsorbed (adsorption isotherm), the sodium ion and bromide ion concentrations and the pH in the equilibrated supernatant, and the silica particle electrophoretic mobility have been measured along the isotherms. The adsorption mechanisms of the two surfactants on the raw and washed silica are qualitatively similar. Nevertheless, important quantitative differences are observed which are all due to (i) the larger number of surface sites present at the surface of SiNa with respect to SiH and (ii) the larger ionic strength of the supernatant in SiNa/surfactant systems with respect to SiH/surfactant systems, due to the much larger amount of sodium ions released by SiNa upon surfactant binding. Thus, the amounts of surfactant adsorbed at the point of zero charge and at saturation of the silica particles, of sodium ions released by the surface and the decrease of critical micelle concentration (cmc) in the supernatant with respect to pure water are all larger for the raw silica than for the treated silica. For the four silica/surfactant systems investigated, the first adsorption step corresponds to the adsorption of individual surfactant ions on the negative sites of the silica surface. It is driven by electrostatic interactions and strongly dependent on the number of surface sites and ionic strength associated to the released ions. At the end of the first adsorption step, which is clearly seen with SiH/surfactant systems, the second adsorption step starts. This step is driven by hydrophobic interaction between surfactant alkyl chains and results in the formation of surface aggregates. The surfactant adsorption on the surface is shown to continue even after the cmc in the supernatant is reached. Copyright 1999 Academic Press.

Journal Article↗
PubMed1998

Aggregation Behavior of Sugar Surfactants in Aqueous Solutions: Effects of Temperature and the Addition of Nonionic Polymers.

The aggregation behavior, critical micelle concentration (cmc) and micelle aggregation number (N), of dodecyl maltoside (DM), octyl glucoside (OG), and Hecameg has been investigated in water and in water plus one of the three water-soluble polymers, polyoxyethylene (POE), polyoxypropylene (POP), and polyvinyl pyrrolidone (PVP), by means of florescence probing and time-resolved fluorescence quenching. The cmc of DM in water increased with temperature and showed a slight increase in the presence of POE. The aggregation number N of DM micelles was nearly independent of concentration (0.25-1 wt %) and temperature (16-60 degreesC). It remained invariant upon addition of 2 wt % POE or PVP but decreased slightly upon addition of the more hydrophobic POP. With increasing temperature, the cmc of OG decreased, went through a shallow minimum at around 35 degreesC, and increased. Addition of POE slightly increased the cmc in the whole temperature range. The aggregation number of OG micelles showed a fairly flat maximum at around 30 degreesC, and was unaffected by the presence of 2 wt % POE or PVP. However, N showed a complex dependence on temperature in the presence of POP, with lower values than in pure water below 15 degreesC, and rapidly increasing quencher-dependent values above this temperature. Hecameg was characterized by N-values nearly independent of temperature and concentration. Intermicellar exchanges of probe and/or quencher were observed with OG and Hecameg, but not with DM. The above results are compared to those for the nonionic ethoxylated surfactants. The effect of various parameters on the micelle aggregation number, the micelle polydispersity, the occurrence of sugar surfactant/nonionic polymer interactions, and the mechanisms responsible for the observed intermicellar exchanges are discussed. Copyright 1998 Academic Press.

Journal Article↗
PubMed1998

Mixed Micellization of Dimeric (Gemini) Surfactants and Conventional Surfactants. I. Mixtures of an Anionic Dimeric Surfactant and of the Nonionic Surfactants C12E5 and C12E8

The mixed micellization between the anionic dimeric surfactant Dim1 (disodium 1,11-didecyl-3,6,9-trioxaundecane 1,11-disulfate) and the two nonionic surfactants C12E5 and C12E8 (penta- and octaoxyethylene monododecyl ethers, respectively) has been investigated. The cmc of the mixtures has been measured by fluorescence probing, using the fluorescent probe pyrene. Its variation with composition revealed synergism in micelle formation. The micelle aggregation (N) numbers have been measured using time-resolved fluorescence quenching at a total surfactant concentration of about 70 mM and at 25, 40, and 55&deg;C. The differences in total aggregation number of the mixed micelles in the two systems reflect (i) the higher aggregation number of C12E5 micelles with respect to C12E8 and Dim1 micelles; (ii) the higher cloud temperature of C12E8 with respect to C12E5; and (iii) the opposite temperature dependences of N for Dim1 and nonionic surfactants micelles. The results show nonideal mixing behavior with a shallow minimum in the variation of the mixed micelle aggregation number with the mixture composition at a Dim1 mole fraction around 0.5. Copyright 1998 Academic Press. Copyright 1998Academic Press

Journal Article↗
PubMed1997

Aggregation and Microstructure in Aqueous Solutions of the Nonionic Surfactant C12E8

Mixtures of water and of the nonionic surfactant C12E8 (octaoxyethyleneglycol monododecylether) have been investigated by time-resolved fluorescence quenching (TRFQ) and transmission electron microscopy at cryogenic temperature (cryo-TEM) to obtain information on the size and shape of the surfactant aggregates and on the microstructure of the mixtures. The studies were performed at surfactant contents and temperatures where the micelles grew significantly and also where the systems undergo phase transitions from micellar-to-cubic upon increasing surfactant content or decreasing temperature and from cubic-to-hexagonal-to-micellar upon increasing temperature. No rapid change or discontinuity was observed in the variation of the aggregation number with the surfactant content or temperature upon crossing the micellar-to-cubic phase boundary nor at the approach of the hexagonal phase from the cubic phase. The results confirmed that the cubic phase consists of spheroidal micelles forming a three-dimensional array. The aggregation numbers at high surfactant content or temperature can be much larger than that of the minimum spherical micelle for a surfactant with a dodecyl chain, suggesting that the micelles should be anisotropic. However, cryo-TEM showed that in the micellar phase the micelles are always spheroidal. These apparently inconsistent results are explained in terms of a partial mixing of the surfactant dodecyl chains and octaoxyethylene head groups which allows for spheroidal micelles of aggregation number much larger than for a surfactant with a dodecyl chain. The results show extensive exchange of material at 40&deg;C, taking place most likely via temporary merging of micelles in micelle clusters then present in the system.

Journal Article↗
PubMed1997

Vesicle-to-Micelle Transformation in Systems Containing Dimeric Surfactants

The vesicle-to-micelle transformation has been investigated thus far in lipid + surfactant systems where the vesicle-forming lipid is chemically very different from the micelle-forming surfactant. The dimeric surfactants, alkanediyl-alpha, omega-bis(dimethyldodecylammonium bromide), are known to form vesicles when the alkanediyl spacer is long enough (for instance, spacer = eicosanediyl, referred to as 12-20-12) and spheroidal micelles for shorter spacers (spacer = decanediyl, referred to as 12-10-12). These surfactants together with the conventional surfactant dodecyltrimethylammonium bromide (DTAB) permitted us to study the transformation of the 12-20-12 vesicles into micelles on addition of the chemically similar micelle-forming DTAB and 12-10-12. Spectrophotometry (light absorbance measurements), video-enhanced light microscopy, and transmission electron microscopy at cryogenic temperatures (cryo-TEM) were used to study the transformation at different scales of aggregate size. Electrical conductivity, which probes the system at the atomic scale (free counterions), was also used. Absorbance measurements showed the transformation to occur between 1.8 and 2.8 wt% added surfactant at a constant 12-20-12 concentration of 1.4 wt%. Light microscopy showed the progressive solubilization of the larger vesicles. Cryo-TEM showed that the initial effect of DTAB addition was to reduce the size of the vesicles, whereas 12-10-12 addition resulted in the formation of multilamellar vesicles. Further additions of either surfactant reduced the size of the vesicles, then brought about the formation of spheroidal micelles until complete solubilization of the vesicles. The giant threadlike micelles seen in previous studies of vesicle-to-micelle transformation in lipid/surfactant systems were never observed with the systems investigated. The conductivity results also revealed differences in behavior on additions of DTAB and 12-10-12.

Journal Article↗
PubMed1975

Interaction between hydroxystilbamidine and DNA. II. Temperature jump relaxation study. Dynamics of nucleic acids and polynucleotides.

A temperature-jump relaxation study of the interaction of hydroxystilbamidine with DNA and synthetic polynucleotides has been performed. Two concentration dependent relaxation times tau1 and tau2 have been observed in the submillisecond range when detecting relaxation effects by means of light absorption. The longer of these two times (tau1) is also observed when using "blue" or "red" fluorescence detection. In the longer time scale the "red" fluorescence shows no other relaxation but the blue fluorescence shows two additional relaxation processes (tau3 and tau4) which correspond to an increase of fluorescence with temperature and which are independent of concentration. The experimental results clearly indicate that tau1 and tau2 are associated with the binding of the dye to strong and weak binding sites, respectively. A kinetic model is given to explain the results. It allows the determination of the four rate constants for the two binding reactions and yields equilibrium association constants in good agreement with those obtained from stoichiometric studies. The study of the effect of temperature, nature of the polymer, ionic strength and fraction of bound dye on tau3 and tau4 indicates that the dye acts only as a "blue" fluorescence probe of some processes involving the DNA or polynucleotide alone. These processes appear to be related with the dynamic structure of the polymers.

Animals↗