Aggregation phenomena on the ternary ionic-nonionic surfactant system: didodecyldimethylammonium bromide/octyl-beta-D-glucopyranoside/water. Mixed microaggregates, vesicles, and micelles

Temperature-induced vesicle to micelle transition in cationic/cationic mixed surfactant systems

Temperature-induced vesicle to micelle transition (VMT), which has rarely been reported in cationic/cationic mixed surfactant systems, was systemically studied in a didodecyldimethylammonium bromide (DDAB)/dodecyltrimethylammonium chloride (DTAC) aqueous solution. We investigated the effect of temperature on DDAB/DTAC aqueous solutions by means of turbidity, conductivity, cryo-TEM, a UV-vis spectrophotometer, and a steady-state fluorescence spectrometer. It was found that increasing temperature could induce the transformation from the vesicle to the micelle in this cationic/cationic mixed surfactant system. The degree of transformation can be easily controlled by the operation temperature. Additionally, by adjusting the proportion of the mixed cationic/cationic systems and employing cationic surfactants with different chain-lengths, we were able to conclude that the hydrophobic tail length of the surfactant affects the aggregation behavior of cationic/cationic mixed surfactant systems as a function of temperature. It is universal to induce the transformation from the vesicle to the micelle by temperature in cationic/cationic mixed surfactant systems. A possible mechanism for the temperature-induced VMT was proposed based on the experimental results.

Interplay between the surface adsorption and solution-phase behavior in dialkyl chain cationic-nonionic surfactant mixtures

Neutron reflectivity, NR, and surface tension have been used to study the adsorption at the air-solution interface of mixtures of the dialkyl chain cationic surfactant dihexadecyl dimethyl ammonium bromide (DHDAB) and the nonionic surfactants monododecyl triethylene glycol (C12E3), monododecyl hexaethylene glycol (C12E6), and monododecyl dodecaethylene glycol (C12E12). The adsorption behavior of the surfactant mixtures with solution composition shows a marked departure from ideal mixing that is not consistent with current theories of nonideal mixing. For all three binary surfactant mixtures there is a critical composition below which the surface is totally dominated by the cationic surfactant. The onset of nonionic surfactant adsorption (expressed as a mole fraction of the nonionic surfactant) increases in composition as the ethylene oxide chain length of the nonionic cosurfactant increases from E3 to E12. Furthermore, the variation in the adsorption is strongly correlated with the variation in the phase behavior of the solution that is in equilibrium with the surface. The adsorbed amounts of DHDAB and the nonionic cosurfactants have been used to estimate the monomer concentration that is in equilibrium with the surface and are shown to be in reasonable qualitative agreement with the variation in the mixed critical aggregation concentration (cac).

Spontaneous formation of nanovesicles in mixtures of nonionic and dialkyl chain cationic surfactants studied by surface tension and SANS

Surface tension and small-angle neutron scattering have been used to characterize the surface properties and the structure of the aggregates formed in the dilute part of the ternary system didodecyldimethyl ammonium bromide, DDAB/tetraethylene glycol monododecyl ether, C12E4/D2O. For surfactant molar ratios, Rn, between 0.3 and 1 (pure DDAB), the surface tension measurements show the existence of two break points at concentrations of around 10(-5) and 10(-3) mol/L, respectively. The SANS measurements have shown that the first break point corresponds to a critical micellar concentration (cmc) and the second one corresponds to a critical vesicle concentration (cvc). In the intermediate composition range, Rn = 0.3-0.8, very small unilamellar vesicles (nanovesicles) are formed with the inner radius varying between 28 and 85 A and a bilayer thickness of approximately 23 A. At Rn = 0.8, we observed a transition from small vesicles (V) to large bilamellar or multilamellar vesicles (BLV, MLV) with a relatively large lamellar periodicity of around 1000 A. In the nonionic-rich region below Rn = 0.3, more classical surface tension behavior was observed, with only one break point corresponding to the onset of formation of small globular micelles.

Self-assembly in complex mixed surfactant solutions: the impact of dodecyl triethylene glycol on dihexadecyl dimethyl ammonium bromide

The impact of the nonionic surfactant, dodecyl triethyleneglycol ether (C(12)E(3)) on the solution microstructure of the dialkyl chain cationic surfactant, dihexadecyl dimethyl ammonium bromide, (DHDAB) has been investigated. The variation in solution microstructure has been studied using a combination of small angle neutron scattering, ultra small angle neutron scattering, optical texture and photon correlation spectroscopy. At low surfactant concentrations (1.5 mM) the microstructure takes the form of bilamellar vesicles (BLV) for compositions containing less than 20 mol % of added C(12)E(3). Multilamellar vesicles (MLV) are the predominant microstructure for solutions richer in composition than 20 mol % C(12)E(3). At more than 80 mol % C(12)E(3), the solution microstructure reverts to that of a lamellar phase dispersion consistent with studies on the pure nonionic surfactant. At higher concentrations (60 mM) a wide continuous L beta phase region is observed for compositions in the range 20 to 80 mol % C(12)E(3). The fine details of the phase diagram were obtained from quantitative analysis of the SANS data using a well-established lamellar membrane model. Irrespective of the nonionic content, the bilayers are in general highly rigid, consistent with those stabilized by charge interactions. Furthermore estimates of the product of membrane moduli (compressibility and bending modulus) indicate that the different phase regions have very different membrane properties, however the magnitude of the variations observed are not predicted using existing theoretical treatments.

Self-assembly in mixed dialkyl chain cationic-nonionic surfactant mixtures: dihexadecyldimethyl ammonium bromide-monododecyl hexaethylene glycol (monododecyl dodecaethylene glycol) mixtures

The self-assembly of dialkyl chain cationic surfactant dihexadecyldimethyl ammonium bromide, DHDAB, and nonionic surfactants monododecyl hexaethylene glycol, C(12)E(6), and monododecyl dodecaethylene glycol, C(12)E(12), mixtures has been studied using predominantly small-angle neutron scattering, SANS. The scattering data have been used to produce a detailed phase diagram for the two surfactant mixtures and to quantify the microstructure in the different regions of the phase diagram. For cationic-surfactant-rich compositions, the microstructure is in the form of bilamellar, blv, or multilamellar, mlv, vesicles at low surfactant concentrations and is in an L(beta) lamellar phase at higher surfactant concentrations. For nonionic-rich compositions, the microstructure is predominantly in the form of relatively small globular mixed surfactant micelles, L(1). At intermediate compositions, there is an extensive mixed (blv/mlv) L(beta)/L(1) region. Although broadly similar, in detail there are significant differences in the phase behavior of DHDAB/C(12)E(6) and DHDAB/C(12)E(12) as a result of the increasing curvature associated with C(12)E(12) aggregates compared to that of C 12E 6 aggregates. For the DHDAB/C(12)E(12) mixture, the mixed (blv/mlv) L(beta)/L(1) phase region is more extensive. Furthermore, C(12)E(12) has a greater impact upon the rigidity of the bilayer in the blv, mlv, and L(beta) regions than is the case for C(12)E(6). The general features of the phase behavior are also reminiscent of that observed in phospholipid/surfactant mixtures and other related systems.

Spectrofluorimetric characterization of mixed nanoaggregates comprising a double-chain cationic surfactant and a cationic or non-ionic single-chain surfactant

A series of mixed vesicle and pre-vesicle nanoaggregates, comprising a cationic double-chain surfactant (di-decyldimethylammonium bromide, di-C(10)DMAB, or di-dodecyldimethylammonium bromide, di-C(12)DMAB), and a cationic (dodecylethyldimethylammonium bromide, C(12)EDMAB) or non-ionic (octyl-beta-D-glucopyranoside, OBG) single-chain surfactant have been characterized by means of steady-state fluorescence spectroscopy. For that purpose, the fluorescent emission of two probes, one anionic (TNS) and the other non-ionic (PRODAN), which is known to be sensitive to the polarity, rigidity, and/or microviscosity of the environment within which the probes are housed, has been measured in the presence of the above-mentioned mixed aggregates. The results of this analysis yield interesting information about the characteristics of the vesicle surface and bilayer, as well as about the existence of clusters and/or nanoaggregates prior to the formation of vesicles.

Cationic Prevesicle and Vesicle Nanoaggregates: An Experimental and Theoretical Study

The formation of spontaneous mixed prevesicles and vesicles consisting of a cationic double-chain surfactant, didecyldimethylammonium bromide (di-C(10)DMAB), and a cationic single-chain alkyltrimethylammonium bromide with 10 and/or 14 carbon atoms (decyltrimethylammonium bromide, C(10)TAB, and/or tetradecyltrimethylammonium bromide, C(14)TAB) has been investigated by means of a series of (i) highly precise experimental techniques, such as conductometry, transmission electronic microscopies (TEM and cryo-TEM), laser Doppler electrophoresis (LDE), and steady-state fluorescence spectroscopy and (ii) theoretical models, such as the DLVO theory and two of its main further modifications, Inoues's and Sogami's models. Two new potentials, based on the combination of DLVO or Inoue potentials with that of Sogami, have been proposed and checked. This theoretical analysis has been carried out not only for the aggregates studied in this work but also for other di-C(m)DMAB + C(n)TAB (m = 10, 12 and n = 10, 12, 14) systems previously reported by us. In respect to the experimental study, special emphasis has been devoted to the prevesicle domain. We have confirmed the existence of two critical aggregation concentrations in the very diluted concentration domain, where the conductivity plot shows a zigzag pattern: the so-called mixed critical aggregate concentration, CAC* and the mixed critical vesicle concentration, CVC*. Contrarily, only CVC* is detected. The pre-CAC* nanoaggregates, with a variety of sizes and shapes, do not show a clear aggregation pattern, but even at such low concentrations a small number of nanoaggregates with a clear and ordered aggregation pattern has been visualized. In the postvesicle domain, the aggregates (vesicles) are unilamellar and spherical with a medium polidispersity and a net averaged surface density charge of around 14 x 10(-3) (pure vesicles) and 24 x 10(-3) C m(-2) (mixed vesicles). The hydrophobicities of the lipidic bilayer and the surface of the vesicles resemble those of media with dielectric constants of around 30 and 75, respectively. Finally, theoretical predictions confirm the stability of the pure and mixed vesicles studied in this work and in other works previously reported.

Electrochemical, microscopic, and spectroscopic characterization of prevesicle nanostructures and vesicles on mixed cationic surfactant systems

Several experimental techniques (conductivity, zeta potential, transmission electronic microscopy, and steady-state fluorescence spectroscopy) have been used to study the formation of mixed colloidal aggregates consisting of a cationic double-chain surfactant, di-dodecyldimethylammonium bromide (di-C12DMAB), and a single-chain alkyltrimethylammonium bromide with 10 and/or 14 carbon atoms (decyltrimethylammonium bromide, C10TAB, and/or tetradecyltrimethylammonium bromide, C14TAB). Special interest has been devoted to the prevesicle domain, within which the formation of aggregated nanostructures was first reported in our laboratory. For that purpose, studies have been carried out on the very dilute region by means of conductivity experiments, confirming the existence of two critical aggregation concentrations in that concentration domain: the so-called mixed critical aggregate concentration, CAC, and the mixed critical vesicle concentration, CVC. By carrying out TEM experiments on negatively stained samples, we were surprised to find a number of aggregates without a clear aggregation pattern and with a variety of sizes and shapes at concentrations below CAC, where only monomers were expected. However, the nanoaggregates found at concentrations between CAC and CVC, also by TEM microscopy, show a clear and ordered "fingerprint"-like aggregation pattern similar to the liquid-crystalline phases reported for DNA-liposome complexes and/or DNA packed with viral capsids. Finally, at total surfactant concentrations above CVC, the aggregates were confirmed, by means of cryo-TEM micrographs and zeta potential measurements, to be essentially unilamellar spherical vesicles with a medium polydispersity and a net-averaged surface density charge of around 12 x 10(-3) C m(-2). The fluorescence emission of two probes, TNS (anionic) and PRODAN (nonionic), allows for the analysis of the micropolarity and microviscosity of the different microenvironments present in aqueous surfactant solutions where the above-mentioned vesicle and prevesicle aggregates are present.

Aggregation behavior in mixed system of double-chained anionic surfactant with single-chained nonionic surfactant in aqueous solution

The aggregation behavior of mixed systems of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) or sodium bis(4-phenylbutyl) sulfosuccinate (SBPBS) with nonionic surfactant pentaethylene glycol mono-n-dodecyl ether (C12E5) have been studied by means of steady-state fluorescence, electrical conductivity, dynamic light scattering, transmission electron microscopy, electrophoretic light scattering and pyrene solubilization measurements. The critical concentrations for aggregation, micropolarity, mobility, solubilization capacity and morphology of aggregates are characterized. Two critical concentrations for aggregation are observed in the mixed surfactants, which may correspond to the formation of different kinds of aggregates. Moreover, it is more favorable for AOT-C12E5 to form mixed vesicles compared to SBPBS-C12E5 at higher mole fraction of C12E5. In addition, it is revealed that SBPBS-C12E5 mixture has larger solubilization capacity for pyrene than AOT-C12E5 system.

Spontaneous formation of vesicles from mixed amphiphiles with dispersed molecular weight: Monte Carlo simulation

The spontaneous formation of vesicles from amphiphiles with dispersed molecular weight (MW) as well as with mono-MW has been studied by a lattice Monte Carlo simulation. Both pure and mixed amphiphiles were self-assembled into vesicles under appropriate conditions. When mixed amphiphiles were examined, the amphiphiles with longer hydrophilic blocks preferred to segregate into the outer monolayer of the resultant vesicles, which is consistent with the experimental observations in recent literature. The kinetic study reveals that the increase of vesicle size is mainly caused by the mechanism of vesicle fusion at the early stage, and the evaporation-condensation mechanism cannot be neglected at the late stage. The fusion of vesicles is accompanied by translocation of chains from the outer monolayer to the inner monolayer. For mixed amphiphiles, the degree of segregation exhibits a size dependence of the vesicle. Compared to the chains with shorter hydrophilic blocks, those with longer hydrophilic blocks exhibit stronger trends to translocate from the outer monolayer to the inner one in vesicle self-adjustment, which leads to the quasi-equilibrium asymmetric distribution of the hydrophilic blocks in the post-fused vesicles.

Self-organization of the ternary didecyldimethylammonium bromide/octyl-beta-D-glucopyranoside/water system

The spontaneous and thermodynamically stable mixed vesicles constituted by a double-chain cationic surfactant with 10 carbon atoms hydrophobic tail, didecyldimethylammonium bromide (di-C(10)DMAB), and a nonionic single-chain surfactant, octyl-beta-d-glucopyranoside (OBG), have been characterized in aqueous media by means of a series of experimental techniques, as well as a theoretical approach. Conductivity data allow for the determination of the concentrations at which the monomer-to-vesicle (CVC) and/or vesicle-to-micelle (CMC) transitions occur. Electrophoretic mobilities, obtained from laser-doppler-electrophoresis experiments, permit the determination of zeta-potentials and, from them, the surface charge density of the vesicle aggregates. Cryogenic transmission electron microscopy (cryo-TEM) provides pictures of the vesicles, their size and shape being, thus, determined. Finally, the sensitivity of the emission spectra of some fluorescent probes, such as the cationic TNS and the nonionic PRODAN, to the polarity of the environment, allow for a complete study of different pre- and post-vesicle microdomains, of variable rigidity and micropolarity. This, in turn, yield interesting information about the vesicle surface and bilayer, as well as, about the existence of clusters and/or nanoaggregates prior to the formation of vesicles, as was proposed by us in a previous paper.