Globular Micelles Research Articles

Adsorbed Layer Structure of Cationic and Anionic Surfactants on Mineral Oxide Surfaces

We report on the structure of adsorbed layers of sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) on titanium dioxide (rutile) and kaolinite, and of TTAB and hexadecyltrimethylammonium bromide (CTAB) on quartz imaged by atomic force microscopy above their respective critical micelle concentrations (in the plateau adsorption region). The surfactants all form globular surface micelles on these three substrates, but under very different conditions. Adsorbed micelles of SDS are found at pH's less than the point of zero charge of rutile, whereas TTAB and CTAB adsorbed micelles are found only above the points of zero charge of rutile and quartz, respectively. Adsorbed SDS micelles are observed on the terraced edges of kaolinite particles, which are positively charged under acidic conditions. In contrast, adsorbed TTAB micelles are observed only on the anionic kaolinite basal plane.

Ionic surfactants adsorption on heterogeneous surfaces

The adsorption of surfactants from aqueous solution is a phenomenon of major importance in applications ranging from ore flotation and paint technology to enhanced oil recovery. As this paper will illustrate, the process is very complex and of high scientific interest; its results can be extended to the retention of organic compounds (humic and fulvic acids, pollutants...) on solids in the biosphere. For a good understanding of the mechanisms involved in surfactants adsorption at the hydrophilic solid–aqueous solution interface, thermodynamic models have to take into account: (i) the physical chemistry of the surfactant in aqueous solution for choosing the appropriate reference phase, (ii) the surface heterogeneity of the adsorbing solid, (iii) the intensity of normal adsorbate–adsorbent bonds responsible for adsorption, (iv) the intensity of lateral bonds that favour the formation of surface aggregates through cooperative process and finally, (v) suitable theoretical models to describe adsorption phenomena. Once this has been achieved, two systems can be discussed: systems characterised by strong normal adsorbate–adsorbent bonds, currently used in ore flotation, which lead, in the case of heterogeneous surfaces, to the formation of lamellar aggregates at monolayer concentration and bilayer formation for higher concentrations. Systems characterised by weak normal adsorbate–adsorbent bonds, currently used in enhanced oil recovery and hydrocarbon (bio)remediation, which correspond to: (i) formation of globular micelles at the solid surface near the CMC when the temperature is higher than the Krafft point, (ii) formation of bilayered lamellar aggregates in the opposite case, (iii) three-dimensional condensation on substrate (T<TKrafft) if the ionic surfactant interacts with cations in the bulk. To cite this article: J.-M. Cases et al., C. R. Geoscience 334 (2002) 675–688.

Sugar-Ester Nonionic Microemulsion: Structural Characterization

Surfactants containing sugar components and fatty acids satisfy the quality standards for food application. The food grade sugar ester in this study is a commercial sucrose monoester of stearic acid (abbreviated SES), the oil phase consists of a 1:1 mixture of n-tetradecane and l-butanol. The originally planned food grade oil, a medium chain triglyceride, is substituted by tetradecane because tetradecane is available as a fully deuterated product, which is necessary for some structural investigations. The investigated system is solid at room temperature, but liquefies and structures into a homogeneous microemulsion when heated to above 37°C. The structural characterization of such microemulsions is the aim of this work. The established methods for this purpose are scattering methods, such as small-angle scattering of X-rays and neutrons and dynamic light scattering. These scattering techniques can be used to obtain valuable information on the size, shape, and internal structure of colloids and complex fluids. We started our investigation with the pseudobinary system SES, tetradecane and l-butanol, varying the SES content. The scattering results show that the sugar ester form inverse globular micelles in the oil phase. The size of these micelles is about 6 nm. While the size is nearly constant in a wide SES concentration regime (5 up to 40% surfactant), the volume or aggregation number increases significantly with SES. This is explained by an increasing replacement of l-butanol molecules by sugar-ester molecules in the micelles formed. Moreover, it can be shown that these micelles strongly overlap. Their center-to-center distance is about 3.8 nm at 40% SES at a micellar diameter of 6 nm. The micellar overlap leads to a highly reduced diffusion of the micelles as was found with dynamic light scattering. When incorporating water in the micellar core, the micelles swell up to about 10 nm and the shape of the aggregates becomes more and more elongated with higher water content.

Comparison of the Surface Activity and Bulk Aggregation of Ferrocenyl Surfactants with Cationic and Anionic Headgroups

We report the synthesis and aqueous solution properties of a novel redox-active surfactant, Fc(CH2)11SO3-Na+, where Fc is ferrocene. Electrochemical oxidation of Fc to the ferrocenium cation (Fc+) reversibly transforms the anionic surfactant (Fc(CH2)11SO3-) into a zwitterionic species (Fc+(CH2)11SO3-). When dissolved at concentrations between 0.15 and 0.4 mM in aqueous solutions containing 10 mM Li2SO4, oxidation is accompanied by the reversible transformation of a solution of monomeric species of Fc(CH2)11SO3- into vesicle-like (Dh = 70 ± 8 nm, Rg/Rh = 1.0 ± 0.3) surfactant assemblies of Fc+(CH2)11SO3-. In contrast, at concentrations above 0.4 mM, oxidation reversibly transforms a solution of globular (Dh = 6 ± 2 nm) micelles into vesicle-like assemblies of Fc+(CH2)11SO3-. These various oxidation-induced changes in aggregation are reflected in the interfacial properties of this surfactant system, which are substantially different from those of ferrocenyl surfactants with cationic headgroups (Fc(CH2)11N+(...

Investigation of Structures in Polyelectrolyte/Surfactant Complexes by X-ray Scattering

Structural investigation in systems of anionic polyelectrolytes and dodecyl- (DPC) and cetylpyridinium chlorides (CPC) were performed at various surfactant to polyelectrolyte (S/P) ratios using synchrotron X-ray scattering. The polyelectrolytes used were sodium poly(styrenesulfonate) (NaPSS), poly(acrylate) (NaPA), and poly(methacrylate) (NaPMA). From the Bragg peaks emerging in the scattering curves, different types of organization of the surfactant in conjunction with the polyion are proposed. They depend on the surfactant chain length, on the polyelectrolyte chemistry, and on the S/P value: (1) NaPSS/DPC (all S/P values), NaPSS/CPC (S/P < 1), and NaPA(NaPMA)/DPC (S/P < 1) complexes produced a micellelike organization of the surfactant along the polyion chain. The NaPSS-induced micelle is smaller in size than the ordinary one because of the inclusion of the aromatic rings on the PSS chain into the hydrophobic interior of the micelle. The size of the ordered elements in complexes with the hydrophilic NaPA and NaPMA corresponds to the radius of an ordinary globular micelle together with the thickness of the polyelectrolyte chain that surrounds it. (2) In NaPSS/CPC precipitate (S/P ≥ 1), a hexagonal phase is observed with a unit cell parameter equal to 39.5 Å. (3) The multiple reflections in the scattering curves of NaPA(NaPMA)/DPC complexes with S/P ≥ 1 and of NaPA(NaPMA)/CPC ones for all S/P values point to some cubic structure. The cell constants of these mesophases correspond approximately to 2.5 diameters of a globular surfactant micelle. (4) In addition to a cubic phase, a well-pronounced hexagonal phase with a unit cell parameter of 40.5 Å develops in the NaPA/CPC case with S/P ≥ 1.

Sodium and Calcium Laurylamidomethylsulfate: Aqueous Micellar Phases, Their Properties, and a Precipitate of Vesicles

A new anionic surfactant (M-LAMS) that is capable of forming intermolecular hydrogen bonds was investigated. Inverse solubilities of Na and Ca salts were found. Critical micelle concentration and aggregation behavior were determined by surface tension, light scattering, electric birefringence, and SANS measurements. It is found that the Na salt forms globular micelles while the Ca salt forms rodlike micelles. The phase behavior of the micellar solutions with increasing cosurfactant concentration was also studied. It is observed that 100 mM Na-LAMS solutions in the presence of 100 mM CaCl2 undergo several phase transformations with increasing n-hexanol concentration. We found not only the expected micellar L1 phase and a lamellar phase at concentrations quite low for this kind of system, but also a novel phase: At a cosurfactant/surfactant ratio xC of 1.2 a white precipitate is formed at the bottom of the sample. With increasing ratio xC the precipitate dissolves into a liquid crystalline Lα phase that at xC=3.2 is transformed into an L3 or sponge phase. Investigation by FF-TEM, light microscopy, and SANS shows that the precipitate consists of agglomerated polydisperse multilamellar vesicles. The vesicles consist of densely packed bilayers that contain little water. The bilayer thickness is about 20 Å and independent of its composition whereas the interlamellar distance is strikingly linked to concentrations of cosurfactant (surfactant/cosurfactant ratio) and electrolyte. With increasing cosurfactant content, the bilayers become less rigid and resulting thermal undulations force the membranes apart and weaken their interactions until a common Lα phase is formed. This transition is an example of a bonding–nonbonding transition of membranes.

Aggregate Structures in a Dilute Aqueous Dispersion of a Fluorinated/Hydrogenated Surfactant System. A Cryo-Transmission Electron Microscopy Study

The structural features of vesicles, micelles, and other aggregates spontaneously formed from a fluorinated surfactant (the ammonium salt of perfluoropolyether, PFPE) and a hydrogenated surfactant (n-dodecylbetaine) in dilute water solution were characterized by means of cryo-transmission electron microscopy (cryo-TEM) at different betaine mole fractions. The size distribution of the aggregates was found to depend critically on surfactant composition. In a narrow range of betaine molar fractions (xbet = 0.76−0.79), two different populations of unilamellar and spheroidal vesicles with mean radii of 70−120 and 20−30 nm, respectively, coexisted with globular and, in some cases, threadlike micelles. In the same region, vesicles with openings in the bilayer and disk-shape fragments began to appear. Further increase in betaine molar fraction resulted in an increase of the number of globular micelles and discoid aggregates and, finally, to the complete PFPE solubilization into mixed micelles. The large vesicles ...

Molecular Theory of the Sphere-to-Rod Transition and the Second CMC in Aqueous Micellar Solutions

We present a molecular-level theory for amphiphile packing in linear micelles, focusing on the early stages of micellar elongation, i.e., on small and “intermediate-size” micelles, whose endcaps are not yet molded into a final shape. The internal free energy of a micelle of given size and shape is expressed as an integral over local molecular packing free energies in different regions of the micelle. The free energy per molecule is expressed as a sum of interfacial (“opposing forces”) and chain conformational contributions, both depending on the local geometry. The equilibrium shape and energy of the micelle is determined by functional minimization of the total free energy. For amphiphiles exhibiting strong preference for packing in the cylindrical geometry, we show that the early stages of growth involve an energetic barrier, resulting in a “gap” in the micellar size distribution. That is, at low total amphiphile concentrations only small (globular) micelles appear in solution. Their concentration reaches a well-defined saturation value, beyond which, all added amphiphiles are incorporated in long micelles, whose “non-interacting” endcaps are well separated by the cylindrical middle part. This, “second CMC” behavior is demonstrated by numerical calculations of micellar size distributions and average aggregation numbers as a function of the total concentration. The conditions necessary for the appearance of a second CMC are analyzed theoretically, with explicit reference to the underlying molecular packing characteristics. In particular, it is shown that a necessary condition for the appearance of a sharply defined second CMC is that the endcap energies (of at least some) of the small or intermediate-size micelles must be considerably lower than the asymptotic (long micelle) value of this quantity. The diameter of the minimal, spherical micelles, as well as that of the final endcaps, is found to be larger than the diameter of the cylindrical body of the very long micelles. Our results are in good qualitative agreement with recent cryo-TEM imaging studies of micellar shape and growth, as well as with previous (less direct) experiments revealing second CMC behavior.

Dynamic Viscoelasticity and Flow Behavior of a Polyoxyethylene Glycol Nonylphenyl Ether/Toluene/Water System

Several samples of a ternary nonionic surfactant/ toluene/ water system at six compositions were characterized on the basis of their rheological and phase behaviors at 25 °C. Different self-association structures were identified: a dispersion of lamellar vesicles embedded in an isotropic phase (D1), a further dispersion (D2) consisting of an aqueous phase dispersed in a continuous medium whose structure is dominated by an incipient lamellar liquid−crystalline structure, three samples located in the lamellar mesophase (Lα) region and an L2 sample. The latter exhibited Newtonian behavior and no viscoelasticity, both phenomena were attributed to the occurrence of reverse globular micelles. Linear dynamic viscoelasticity revealed that samples located well within the Lα region showed mechanical and relaxation spectra typical of highly structured materials. Similar spectra were observed for a D2 sample but with a lower plateau modulus, indicating that the basic structure was the same as for standard Lα samples...

Spontaneous Vesicles Formed in Aqueous Mixtures of Two Cationic Amphiphiles

The spontaneous formation of vesicles was detected in aqueous mixtures of two cationic amphiphiles: the double-tailed didodecyldimethylammonium bromide, DDAB, and the single-tailed dodecyltrimethylammonium chloride, DTAC. These aggregates appear in a high-dilution region of the system, intermediate between those where monomers and micelles prevail, and are most easily formed at a considerable excess of the single-chained surfactant (DTAC/DDAB molar ratios ≈ 2−20). Vesicles were characterized at 25 °C by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light-scattering (DLS) measurements: they present a well-defined contour, are mostly spherical and unilamellar, but show a large size polydispersity, with the more frequent population distributions of diameters from ≈40−50 to ≈500−600 nm. Apart from intact vesicles (and in most cases coexisting with them), vesicles with ruptured membranes, small bilayer disks (probably discoidal micelles, rarely found), and globular micelles were also visualized by cryo-TEM. Ruptured vesicles and disks were assigned to intermediate structures between intact vesicles and globular micelles. We propose that the main factor which drives the appearance of vesicles in this bicationic system is the difference in the spontaneous curvature (or packing parameter) of the two long-chained surfactant ions.

Structure of Conjugated Bile Salt−Fatty Acid−Monoglyceride Mixed Colloids: Studies by Small-Angle Neutron Scattering

The structures of particles found in isotropic phases of mixed surfactant systems consisting of conjugated bile salts and fatty lipids were assessed using small-angle neutron scattering. The conjugated bile salts were either cholylglycine or chenodeoxycholylglycine. The fatty lipids were mixtures of oleate and oleic acid either alone or with monoolein. The scattering data suggested that both particle interactions and polydispersity must be modeled in these systems. Particle interactions were modeled using the reduced mean spherical approximation and the decoupling approximation. Maximum entropy was used to characterize the polydispersity. A self-consistent analysis of the scattering was arrived at by making an initial estimate of particle size and shape using derivative−log and Guinier analysis and refining the estimates by analyzing the particle interactions and polydispersity and iterating. The scattering at high total lipid concentrations was consistent with globular mixed micelles with repulsive electrostatic interactions. The globular mixed micelles in these systems were similar in size and shape to those observed previously in conjugated bile salt mixtures with either egg yolk phosphatidylcholine or monoolein. Solutions of cholylglycine with monoolein and oleate/oleic acid underwent a transition to vesicles at lower concentrations. This behavior was similar to those observed in conjugated bile salts with either egg yolk phosphatidylcholine or monoolein. Cholylglycine mixtures with oleate/oleic showed somewhat different behavior at lower concentrations, since there was also evidence for coexistence of elongated and tabletlike micelles. Despite these differences, there were sufficient similarities in the particle morphologies of these and other conjugated bile salt−fatty lipid systems to suggest a common mode of self-assembly. These solutions are models for bile in the bilary system and intestine content during triglyceride digestion; the common themes of self-assembly have implications for the physiology of lipid solubilization in bile as well as intestinal absorption of dietary lipids.

Surface Polyfluorinated Micelles

Three novel polyfluorinated cationic surfactants and the corresponding three hydrocarbon analogues were synthesized. All of the surfactants formed globular micelles in water. The micellar formation was monitored by five independent methods. The results of the NMR measurements and the other four methods using the anionic dye Rose Bengal (RB) and anilinonaphthalenesulfonic acid as probe molecules agreed well with each other. The polyfluorinated micelles had rather low cmc values, on the order of 10-5 M. All of the micelles had similar aggregation numbers around 50−60. Using a detailed NMR relaxation study following addition of paramagnetic Mn2+ ions to the bulk water phase, the polyfluorinated micelles were shown to have a very unique microstructure with the perfluoroalkyl chain extending straightforwardly to the bulk water phase. The micropolarity around the ammonium group and the microviscosity in the core of the micelles were estimated using the probe molecules RB and perylene. The local oxygen concentration in the micelles was estimated from the NMR longitudinal relaxation time T1. The microstructure of water distributed within the micelles is discussed with relation to the observed local oxygen concentration. The rigidity of the micelle is also discussed using the mobility of each nuclei estimated from the T1 data. The unique protecting effect of the micelles against attack by hydroxide radicals generated in the bulk water phase using Fenton's method was examined using a bleaching reaction of RB as a probe molecule. The surface polyfluorinated micelles exhibited a greater protective effect than either CTAB or the hydrocarbon analogue surfactant micelles. This protective effect is discussed in relation to the unique microstructure of the surface polyfluorinated micelles.

Phase Behavior and Microstructure of Aqueous Mixtures of Cetyltrimethylammonium Bromide and Sodium Perfluorohexanoate

The phase behavior and microstructure of aqueous mixtures of cetyltrimethylammonium bromide (C16TAB) and sodium perfluorohexanoate (FC5) were investigated as a function of total surfactant concentration, mixing ratio, and temperature. The aggregation behavior of both the pure components and several mixtures was studied via surface tension experiments, and analysis of the results using the regular solution model yielded a β value of −19.4, indicating highly nonideal mixing. The evolution of phase behavior from the FC5−water binary axis with the addition of C16TAB was of particular interest. Below the critical micelle concentration or cmc of FC5 (5.70 wt %), the addition of small amounts of C16TAB leads to the formation of large structures, including vesicles, between surfactant concentrations of 2 and 4 wt %. Above the cmc, FC5 forms small globular micelles in solution. The addition of C16TAB to the FC5 micelles leads to an increase in viscosity and eventually phase separation into two rodlike micellar sol...

Aggregation Behavior of Cationic Fluorosurfactants in Water and Salt Solutions. A CryoTEM Survey

The aggregation behavior of the cationic fluorocarbon surfactants 1,1,2,2-tetrahydroperfluoroalkylpyridinium chloride and 2-hydroxy-1,1,2,3,3-pentahydroperfluoroalkyldiethylammonium chloride in aqueous solution have been studied using cryo-transmission electron microscopy as the main technique. The effects on aggregate structure of factors such as surfactant and salt concentration, counterion type, and alkyl chain length were investigated. Similar to hydrocarbon surfactants the fluorocarbon surfactants self-assemble into various aggregates such as micelles, threadlike micelles, vesicles, and other lamellar aggregates. A distinctive property of the fluorocarbon surfactants is their tendency to form structures with little curvature, such as cylindrical micelles and bilayer structures. Even a very small reduction of the repulsion between the headgroups is sufficient to accomplish a sphere to rod transition, so that a solution of globular micelles in water is turned into a highly viscous, sometimes viscoelastic solution on the addition of low concentrations of a simple salt. The differences between the fluorocarbon and hydrocarbon surfactants are discussed.

Nanopattern of Diblock Copolymers Selectively Adsorbed on a Plane Surface

Surface interaction controlled microphase separation leading to the formation of chemically heterogeneous surface nanopatterns in dry ultrathin films of A−B diblock copolymers is studied experimentally and theoretically in the strong segregation limit. On a planar surface one of the blocks (A block) is strongly adsorbed, forming a tightly bound monomolecular layer (two-dimensional melt). The nonadsorbed B blocks can aggregate due to incompatibility with the A-block layer on the substrate and with the air. As a result, a chemically heterogeneous surface pattern can emerge. Depending on the block length ratio and the interaction parameters, the dewetting B blocks can assemble to either globular surface micelles or wormlike surface aggregates, that is, a point or a striped surface pattern. The region of stability of these morphologies and the main parameters, such as aggregation number, size, and periodicity, of the surface micelles and wormlike surface aggregates have been determined as functions of the len...

Polymers Grafted onto Strongly Adsorbing Surfaces in Poor Solvents: Stretching, Fission, Phase Separation, and Globular Micelles in 2D

Using both analytic theory and computer simulation we study the stretching along the surface of a strongly adsorbed polymer chain. In marked contrast to the case of nonadsorbed chains we show that the polymer does not undergo a Rayleigh instability. Instead it deforms steadily into a lens shape. Our results have dramatic consequences for submonolayer films formed from grafted chains. We show that strongly adsorbed chains do not form ``octopus'' or pinned micelles. Instead they fuse into compact islands, which in the limit of high grafting density can form a continuous network.

Phase Behavior and Aggregate Structure in Aqueous Mixtures of Sodium Cholate and Glycerol Monooleate

The phase behavior of the glycerol monooleate (GMO)–sodium cholate–water (or 0.9 wt% NaCl) system has been examined in the solvent-rich part, using small-angle X-ray scattering and conventional methods. Addition of cholate up to 7% of the total amphiphile swells the cubic phase of the binary GMO–water system so that it takes up almost 70% of water in the salt-free case and 55% in salt. With more bile salt the lamellar phase also appears highly swollen (up to 85% in water, 75% in brine). In the salt solution a small isotropic L3-phase region replaces the lamellar phase at a solvent content of about 79%. The lamellar phase can accept only about 0.2 cholate molecule per GMO, in both water and brine, and a phase with globular micelles (L1) follows and dominates the diagram. No threadlike micelles appear in this system. Investigation of the particle structures with cryo-transmission electron microscopy (TEM) in dilute systems (99% solvent) show globular micelles and coexisting vesicles and globular micelles. In the presence of salt, dilution of the L3 phase results in dispersed globular particles with an irregular internal morphology that suggests they are a dispersed L3 phase. These particles coexist with faceted particles having an inner structure giving a hexagonal pattern in projection, suggested to derive from the cubic phase. The cubic phase in the salt-free systems did not give dispersions stable enough for cryo-TEM examination.

PH-Induced Structural Changes of a Polymer Micelle Composed of Amphiphilic Polyallylamine Containing Hydrophilic Poly(L-glutamic acid) in the Side Chain

An amphiphilic polymer composed of polyallylamine containing 7 mol% hydrophobic palmitoyl groups and 92 mol% hydrophilic poly(L-glutamic acid) grafted chains whose molecular weight was 4090 in the side chains was prepared. The amphiphilic polymers formed a stable and globular micelle in aqueous solution at pH above 4.0. We investigated pH-induced changes in the micellar structure of the amphiphilic polymer in the aqueous solution. The grafted poly(L-glutamic acid) chains of the amphiphilic polymer showed pH-dependence conformational transition (β sheet → α-helix and random coil → random coil). Conformational changes of the grafted polypeptides chains regulated the surface polarity of the micelle composed of amphiphilic polymer.

Interactions between Nonionic Surfactant Micelles Introduced by a Telechelic Polymer. A Small Angle Neutron Scattering Study

The nonionic surfactant hexa(ethylene glycol) mono-n-dodecyl ether (C12EO6) self-assembles in aqueous solutions to form small globular micelles. Progressive substitution of C12EO6 by a telechelic polymer (a water soluble poly(ethylene oxide) (PEO) chain grafted at both extremities with hydrophobic C12H 25 chains (PEO-2m)) is shown to preserve the initial C12 hydrophobic cores, at least up to 10 polymers per hydrophobic core, but to introduce additional interactions between the micelles. At low micellar concentrations (∼ <0.03 M) these interactions are attractive, leading to a phase separation even with a minute substitution of C12EO6 by the polymer while at higher concentrations even a minute substitution leads to an effective repulsive interaction. These observations are discussed in terms of a competition between bridging and decoration of the micelles.

Small-angle neutron scattering studies on the size distribution of mixed micelles

diC 6PC and diC 7PC mixed micelles are investigated by small-angle neutron scattering. The size distribution of the mixed micelles is determined from the SANS data by using the indirect transform method and is found to have only a single distribution peak. This indicates that the diC 6PC and diC 7PC do not segregate to form two size groups of micelles. The size distribution peak of these mixed micelles is also found to change from broad to narrow as the percentage of diC 6PC is increased, indicative of changing from polydisperse rod-like micelles to globular micelles.