Dynamics of micelles of the triblock copolymers poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) in aqueous solution.

A number of results reported on the kinetics of exchange of triblock copolymers poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, between micelles and the intermicellar aqueous solution are reviewed and analyzed to extract the rate constants k(+) for the entry of a copolymer into a micelle and k(-) for the exit of a copolymer from a micelle. Contrary to what is generally observed for conventional surfactants, the rate constant for the entry of a copolymer into a micelle is slower to much slower than for a diffusion-controlled process and decreases as the degree of polymerization of the PO block, n(PO), increases. The effect of the degree of polymerization of the EO block, n(EO), on the two rate constants is significant only for low values of n(EO). The variation of k(-) with n(PO) strongly suggests that the free copolymer molecule adopts a conformation where the PO block is tightly coiled with little contact with water and not a fully extended and hydrated conformation, in contrast to what is commonly assumed when analyzing the dependence of the cmc on the polymerization degree of the hydrophobic block.

A reinterpretation of the hydration of micelles of dodecyltrimethylammonium bromide and chloride in aqueous solution.

The hydration of dodecyltrimethylammonium (DTAB) micelles is reinterpreted in light of the results of the companion paper (immediately preceding this paper) that showed that the location of the spin probe 16-doxylstearic acid methyl ester (16DSE) changes as a function of the aggregation number, N, of anionic micelles, i.e, that it does not conform to the zero-order model (ZOM). The ZOM requires that the NO(*) moiety diffuse throughout the Stern layer of the micelle and nowhere else. By using the ZOM as a working hypothesis, the previous interpretation (J. Phys. Chem. B 2002, 106, 1926) of 16DSE data proposed that an increasing number of alkyl chain methyl groups occupied the Stern layer as N increased. In this work, the spin probe 5-doxylstearic acid methyl ester that was found to fulfill the ZOM in anionic micelles was measured as a function of N in DTAB and was found to obey the ZOM in this cationic micelle as well. Thus, a simple model of the hydration of micelles that is successful in anionic micelles is also successful in DTAB. The previous results for 16DSE are reinterpreted here as being due to small displacements of the NO(*) moiety as a function of N.

Adsorbed layer structure of cationic gemini and corresponding monomeric surfactants on mica.

We report a comprehensive study of the adsorbed layer morphologies of cationic gemini surfactants of the type dodecanediyl-alpha,omega-bis(dimethylalkylammonium bromide) and their corresponding monomers, dimethyldodecylalkylammonium bromide, on mica using atomic force microscopy soft-contact imaging. As in the bulk, aggregate curvature of the adsorbed geminis is found to increase with increasing spacer length, but the adsorbed aggregate curvature also increases in the presence of CsCl and CsBr. The monomeric surfactants exhibit an unexpected transition from globular adsorbed aggregates to a bilayer when the alkyl side chain reaches butyl, and this transition is also sensitive to added electrolyte.

Tetrabutylammonium alkyl carboxylate surfactants in aqueous solution: self-association behavior, solution nanostructure, and comparison with tetrabutylammonium alkyl sulfate surfactants.

A series of long and ultralong chain tetrabutylammonium alkyl carboxylate (TBACm, TBA = tetrabutylammonium ion; Cm = carboxylate ion C(m-1)H(2)(m-1)CO(2)(-) of total carbon number m) surfactants have been obtained by direct neutralization of the fatty acids with m = 12, 14, 18, 22, and 24 by tetrabutylammonium hydroxide. Time-resolved fluorescence quenching has been used to determine the micelle aggregation number (N) of the surfactants with m = 12, 14, and 18 in the temperature range 10-50 degrees C and of the surfactants with m = 22 and 24 in the temperature range 25-60 degrees C. In all instances the values of N were well below those that can be calculated for the maximum spherical micelle formed by surfactants with the same alkyl chain as the investigated surfactants on the basis of the oil drop model for the micelle core. The microstructure of selected solutions of TBAC22 was examined using transmission electron microscopy at cryogenic temperature and compared to the microstructure of solutions of TBA dodecyl and tetradecyl sulfates. These observations generally confirmed the findings of TRFQ. The self-association behavior of these anionic surfactants with TBA counterions is explained on the basis of the large size and the hydrophobicity of the tetrabutylammonium ions. The important differences in behavior that have been evidenced between tetrabutylammonium alkyl carboxylates and alkyl sulfates are discussed in terms of differences in distribution of the surfactant electrical charge on the headgroup and alkyl chain predicted by quantum chemical calculations (Langmuir 1999, 15, 7546).

Fluorescence probing investigation of the mechanism of formation of MSU-type mesoporous silica prepared in fluoride medium.

The mechanism of formation of a MSU-type siliceous material from tetraethyl orthosilicate (TEOS) in the presence of the nonionic surfactant tergitol T-15-S-12, sulfuric acid, and sodium fluoride has been investigated using mainly fluorescence probing techniques and, to a lesser extent, dynamic light scattering (DLS) and 29Si NMR spectroscopy. The tergitol micelles present in the systems obtained by progressively generating the reaction mixture giving rise to the mesostructured material by adding to an appropriate tergitol solution sulfuric acid, TEOS, and NaF were characterized by fluorescence probing (micelle aggregation number, micropolarity, and microviscosity) and also by dynamic light scattering (apparent micelle diameter). 29Si NMR experiments were also performed on selected systems after hydrolysis of the TEOS. The fluorescence probing techniques were also used to follow the changes of micelle characteristics with time during the evolution of the full reaction mixture from a limpid solution to a system containing a minor amount of condensed siliceous material. The synthesized solid material was characterized by X-ray diffraction and nitrogen adsorption-desorption analyses. The micelle aggregation number N was found to change only little, and the micropolarity remained constant when going from the tergitol solution to the full reaction mixture. The results of DLS measurements agree with this finding. Besides, while the condensation of silica took place after addition of NaF, the N value increased only very little with time up to the point where a small amount of mesostructured material precipitated out. These results indicate that the interaction between tergitol micelles and the siliceous species formed in the system by the hydrolysis of TEOS and also between micelles and the growing siliceous species must be very weak. As in our previous studies of the mechanism of formation of MCM41-type material from sodium silicate in the presence of cetyltrimethylammonium bromide, it appears that the locus of formation of the mesostructured material is not the micelle surface but the bulk phase. Micelles only act as reservoirs of surfactant providing surfactant monomer that binds to the growing siliceous species.

Rings-on-a-string chain structure in DNA.

Using fluorescence microscopy (FM), which permits the observation of single molecules, we found that a pearling structure is generated on a single long DNA molecule upon the addition of a gemini (dimeric) surfactant. This pearling structure was further investigated by performing atomic force microscopy measurements on the same DNA molecules as observed by FM. These observations revealed that the pearling structure is composed of many rings that are interconnected by elongated coil parts along a single DNA molecule, i.e., rings-on-a-string structure. The mechanism of the formation of such an intrachain segregated structure in terms of microphase separation on a single polyelectrolyte chain is discussed.

Effect of the nature of the counterion on the properties of anionic surfactants. 4. Characterizing micelles of tetraalkylammonium dodecyl sulfate as reaction media.

Micelles formed in water from tetramethyl-, tetraethyl-, tetrapropyl- and tetrabutylammonium dodecyl sulfate (TMADS, TEADS, TPADS, and TBADS, respectively) are characterized as reaction media. All of the results are identical in the presence or absence of added salt, provided micelles of the same aggregation number, N, are compared. The microviscosity (eta) deduced from the rotational motion of the nitroxide group of a spin probe increases modestly as a function of N in TMADS and TEADS, decreases slightly in TPADS, and decreases slightly before increasing in TBADS. The activation energy for the viscosity is remarkably similar in all of the tetraalkylammonium dodecyl sulfate (TAADS) micelles and is similar to that in ethanol/water mixtures as well as other anionic and cationic micelles. The volume fraction occupied by water in the polar shell, H(t), decreases with N in TMADS, TEADS, and TPADS at 10, 25, and 45 degrees C whereas it decreases, goes through a minimum, and then increases in TBADS. H(t) also decreases with the size of the counterion. The bimolecular collision rate as deduced from fluorescence quenching of pyrene by dodecylpyridinium chloride conforms well to a hydrodynamic description, varying linearly with T/eta, where T is the absolute temperature and passing through the origin. Quenching probablities of 0.53, 0.51, 0.45, 0.39 for TMADS, TEADS, TPADS, and TBADS, respectively, are rationalized in terms of small shifts of the diffusion zones outside of the Stern layer by an average of 16% of the Stern layer thickness.

Effect of the nature of the counterion on the properties of anionic surfactants. 5. Self-association behavior and micellar properties of ammonium dodecyl sulfate.

Micelles formed in water from ammonium dodecyl sulfate (AmDS) are characterized using time-resolved fluorescence quenching (TRFQ), electron paramagnetic resonance (EPR), conductivity, Krafft temperature, and density measurements. TRFQ was used to measure the aggregation number, N, and the quenching rate constant of pyrene by dodecylpyridinium chloride, k(Q). N depends only on the concentration (C(aq)) of ammonium ions in the aqueous phase whether these counterions are derived from the surfactant alone or from the surfactant plus added ammonium chloride as follows: N = N0(C(aq)/cmc0)(gamma), where N0 is the aggregation number at the critical micelle concentration in the absence of added salt, cmc0, and is equal to 77, 70, and 61 at 16, 25, and 35 degrees C, respectively. The exponent gamma = 0.22 is independent of temperature in the range 16 to 35 degrees C. The fact that N depends only on C(aq) permits the determination of the micelle ionization degree (alpha) by employing various experimental approaches to exploit a recent suggestion (J. Phys. Chem. B 2001, 105, 6798) that N depends only on C(aq). Utilizing various combinations of salt and surfactant, values of alpha were obtained by finding common curves as a function of C(aq) of the following experimental results: the Krafft temperature, N, k(Q), the microviscosity of the Stern layer determined from the rotational correlation time of a spin probe, 5-doxyl stearic acid methyl ester, and the spin-probe sensed hydration of the micelle surface. The values of alpha, determined from applying the aggregation number-based definition of alpha to all of these quantities, were within experimental uncertainty of the values alpha = 0.19, 0.20, and 0.21 derived from conductivity measurements at 16, 25, and 35 degrees C, respectively. The volume fraction of the Stern layer occupied by water decreases as N increases. For AmDS micelles, both the hydration and its decrease are predicted by a simple theory of micelle hydration by fixing the parameters of the theory for sodium dodecyl sulfate and employing no further adjustable parameters. For a given value of N, the hydration decreases as the temperature increases.

Direct visualization of mesh structures at solid/solution interfaces by atomic force microscopy.

The formation of adsorbed surfactant layers consisting of a mesh or network of branched cylindrical aggregates on muscovite mica by several surfactant systems is described. The curvature of the adsorbed aggregates is varied by a variety of mechanisms that all generate morphologies between adsorbed cylinders and bilayers, and the resulting lateral structure is imaged by "soft contact" atomic force microscopy. We compare the direct images and Fourier transforms of the adsorbed layer structures, and relate them to those formed in bulk solution.

Cloud point of aqueous solutions of tetrabutylammonium dodecyl sulfate is a function of the concentration of counterions in the aqueous phase.

The cloud point of the surfactant tetrabutylammonium dodecyl sulfate is shown to be a function of the tetrabutylammonium counterion concentration in the aqueous phase whether the counterions are provided by the surfactant or both the surfactant and added tetrabutylammonium bromide. The micellized surfactant dissociates 17% of its counterions to aqueous phase.

Partial phase behavior and micellar properties of tetrabutylammonium salts of fatty acids: unusual solubility in water and formation of unexpectedly small micelles.

The tetrabutylammonium (TBA) salts of fatty acids, from dodecanoic acid (C12) to octacosanoic acid (C28), have been prepared by direct neutralization of the fatty acid by TBA hydroxide. Unexpectedly, all of these surfactants have been found to be soluble in water under the form of micelles at a sufficiently high temperature. For instance, the solubility of TBA octacosanoate in water is of about 7 wt % at 46 degrees C. Starting from TBA docosanoate, the aqueous solutions of the surfactants gelled below a certain temperature. The gelling temperature increased linearly with the fatty acid carbon number. Upon increasing temperature, the TBA octocosanoate showed a relatively complex phase behavior that has been investigated. The micellar solutions of these surfactants did not cloud at high temperatures, up to 98 degrees C, contrary to TBA alkylsulfates. The aggregation numbers of micelles of the various TBA alkylcarboxylates have been measured and found to be smaller than those for the maximum spherical micelle that a surfactant with the same alkyl chain length can form. The micelle micropolarity and microviscosity (as sensed by fluorescent probes) decreased and increased, respectively, with the fatty acid carbon number.

Effect of the nature of the counterion on the interaction between cesium and tetraalkylammonium dodecylsulfates and poly(ethylene oxide) or poly(vinylpyrolidone).

The interaction between poly(ethylene oxide) or poly(vinylpyrrolidone) and cesium and tetraalkylammonium (tetramethyl to tetrabutyl ammonium) dodecylsulfate has been investigated by means of electrical conductivity measurements to determine the critical aggregation concentration (cac) of the surfactants in the presence of polymer. The cac values were compared to the values of the critical micellization concentration (cmc) of the surfactants in the absence of polymer. The value of the cac/cmc ratio increased with the radius of the counterion in the sequence: Na(+)

Microstructures in the aqueous solutions of a hybrid anionic fluorocarbon/hydrocarbon surfactant.

The aqueous solutions of the anionic hybrid fluorocarbon/hydrocarbon surfactant sodium 1-oxo-1[4-(tridecafluorohexyl)phenyl]-2-hexanesulfate (FC6HC4) shows peculiar rheological behavior. At 25 degrees C the viscosity vs concentration curve goes successively through a maximum and a minimum, while the viscosity vs temperature curve of the 10 wt% aqueous FC6HC4 solution goes through a marked maximum at 36 degrees C [Tobita et al., Langmuir 13 (1997) 5054]. In an attempt to explain these properties the microstructure of aqueous solutions of FC6HC4 has been investigated by means of digital light microscopy, transmission electron microscopy at cryogenic temperature (cryo-TEM), rheology, and self-diffusion NMR. At 20 degrees C, the increase of the FC6HC4 concentration was found to result in a progressive change of structure of the surfactant assemblies from mainly spherical micelles at 0.5 wt% to mainly cylindrical micelles at 10 wt%. At intermediate concentrations small disk-like micelles and small complete and incomplete vesicles coexisting with cylindrical micelles were visualized. The occurrence of stretched cylindrical micelles is responsible for the effect of the surfactant concentration on the solution viscosity. Cryo-TEM, rheology, and self-diffusion NMR all suggest that an increase of the temperature brings about a growth of the assemblies present in the 10 wt% solution of FC6HC4. The structure of the assemblies present at the temperature where the viscosity is a maximum could not be elucidated by cryo-TEM because of the probable occurrence of an on-the-grid phase transformation, the result of blotting during specimen preparation. Nevertheless, the results show that the observed large assemblies break up at higher temperature to give rise to a more labile bicontinuous structure that consists of multi-connected disordered lamellae, with many folds and creases, and that may well be the L3 phase.

Dimeric and oligomeric surfactants. Behavior at interfaces and in aqueous solution: a review.

Dimeric and oligomeric surfactants are novel surfactants that are presently attracting considerable interest in the academic and industrial communities working on surfactants. This paper first presents a number of chemical structures that have been reported for ionic, amphoteric and nonionic dimeric and oligomeric surfactants. The following aspects of these surfactants are then successively reviewed the state of dimeric and oligomeric surfactants in aqueous solutions at concentration below the critical micellization concentration (cmc); their behavior at the air/solution and solid/solution interfaces; their solubility in water, cmc and thermodynamics of micellization; the properties of the aqueous micelles of dimeric and oligomeric surfactants (ionization degree, size, shape, micropolarity and microviscosity, solution microstructure, solution rheology, micelle dynamics, micellar solubilization, interaction between dimeric surfactants and water-soluble polymers); the mixed micellization of dimeric surfactants with various conventional surfactants; the phase behavior of dimeric surfactants and the applications of these novel surfactants.

Alkanediyl-alpha,omega-bis(dimethylalkylammonium bromide) surfactants 10. Behavior in aqueous solution at concentrations below the critical micellization concentration: an electrical conductivity study.

Ion pairing and premicellar association have been often invoked to explain results obtained in studies of aqueous solutions of ionic dimeric surfactants (gemini surfactants), mainly by means of surface tension and electrical conductivity, at concentrations below the critical micellization concentration (cmc). The present work was undertaken in an attempt to find out under which conditions these effects come into play. For this purpose the electrical conductivity of solutions of many dimeric surfactants of the type spacer-alpha,omega-bis(alkyldimethylammonium bromide) have been measured. The alkyl chain contained m=10-18 carbon atoms. The spacer group was either an alkanediyl with s carbon atoms (m-s-m surfactants) or a xylylene m-xylyl-m surfactants). The results show that ion pairing occurs in solutions of m-s-m dimers with m< or =10, mostly as a result of their high cmc values. The results for 12-s-12 dimers with s< or =10 and for 12-xylyl-12 showed no evidence of either ion pairing or premicellar association. Premicellar association was present for 12-s-12 dimers with s> or =12, for m-8-m dimers with m> or =14, and for 16-xylyl-16. It showed through a positive curvature of the specific conductivity versus concentration plot and the presence of a maximum in the equivalent conductivity vs (concentration)(0.5) plot at concentrations below the cmc. The free energy associated with the premicellar association of m-8-m dimers has been estimated from the available cmc and micelle ionization degree data.

Dimeric (gemini) surfactants: effect of the spacer group on the association behavior in aqueous solution.

Dimeric (gemini) surfactants are made up of two amphiphilic moieties connected at the level of, or very close to, the head groups by a spacer group of varying nature: hydrophilic or hydrophobic, rigid or flexible. These surfactants represent a new class of surfactants that is finding its way into surfactant-based formulations. The nature of the spacer group (length, flexibility, chemical structure) has been shown to be of the utmost importance in determining the solution properties of aqueous dimeric surfactants. This paper reviews the effect of the nature of the spacer on some of these properties. The behavior of dimeric surfactants in the submicellar range of concentration, at interfaces, in dilute solution (solubility in water, Krafft temperature, critical micellization concentration, thermodynamics of micelle formation, micelle ionization degree, size, polydispersity, micropolarity and microviscosity, microstructure and rheology of the solutions, solubilization, micelle dynamics, and interaction with polymers) and in concentrated solution (phase behavior) are successively reviewed. Selected results concerning trimeric and tetrameric surfactants are also reviewed.

Alkanediyl-alpha,omega-bis(dimethylalkylammonium bromide) surfactants: II. Krafft temperature and melting temperature.

The melting temperature T(M) of two series of dimeric (gemini) surfactants, the alkanediyl-alpha,omega-bis(dodecyl and hexadecyl dimethylammonium bromide), referred to as 12-s-12 and 16-s-16, respectively (s = carbon number of the alkanediyl spacer), and the Krafft temperature T(K) of 1 wt% aqueous solutions of these surfactants have been measured. The melting temperature of the solid surfactant increases with the carbon number m of the alkyl chain. For each surfactant series, T(M) goes through a maximum at s close to 5, irrespective of the value of m. For the 12-s-12 series, T(M) goes through a minimum at s = 10 to 12. At a constant value of s, the value of T(K) increases with m. The variations of T(M) and T(K) with s show some correlation, with T(K) decreasing when T(M) increases and vice versa. The results are discussed in relation to the solution properties of the investigated surfactants.