Segregation phenomena in micelles from mixtures of fluorinated and hydrogenated surfactants

Towards compartmentalized micelles: Mixed perfluorinated and hydrogenated ionic surfactants in aqueous solution

HYPOTHESIS

Aqueous solutions of mixtures of hydrogenated and perfluorinated ionic surfactants are known to display anomalous aggregation behavior due to the mutual phobicity between hydrogenated and perfluorinated chains. Despite all efforts, different experimental limitations prevented so far a definite interpretation of the existing experimental results: both intermicellar and intramicellar segregation remain acceptable possibilities.

METHOD

The potential for segregation of mixtures of fluorinated and hydrogenated ionic surfactants in water was assessed using atomistic molecular dynamics simulations. Aqueous mixtures of hydrogenated and perfluorinated ionic surfactants were studied: mixtures of anionic surfactants (sodium dodecylsulfate (SDS) + sodium perfluoro-octanoate (SPFO)) and catanionic surfactants (decyltrimethylammonium bromide (DeTAB) + SPFO) were simulated.

FINDINGS

The mixture of anionic surfactants, SDS + SPFO, exhibits clear intramicellar segregation between fluorinated and hydrogenated chains, displaying hydrogenous-rich and fluorous-rich regions. Compartmentalized micelles are thus clearly formed. The simulation results also suggest the possibility of intermicellar segregation. Conversely, catanionic mixtures of DeTAB and SPFO in water solution assemble into a large oblate structure, containing all available molecules in the simulation box, resembling a double layer membrane or a vesicle wall. In this case mixing between fluorinated and hydrogenated surfactants is dictated by charge alternation.

Surface Activity and Efficiency of Cat-Anionic Surfactant Mixtures

The surface activity of surfactant mixtures is critically analyzed. Cat-anionic systems, in which two ionic species are mixed in non-stoichiometric ratios, are considered. With respect to the solution behavior, where a substantial decrease of cmc is met compared to the pure components, a moderate effect on surface tension, γ, occurs. Compared to the pure species, the decrease of surface tension for such mixtures is not significant, and no clear dependence on the mole fraction anionic/cationic is met. The surface tension is grossly constant in the whole concentration range. Conversely, the interaction parameter for surfaces, β_surf (calculated by the regular solution theory), is more negative than that for micelle formation, β_mic. This fact suggests that the desolvation of polar heads of the two species at interfaces is largely different. Very presumably, the underlying rationale finds origin in the sizes and solvation of both polar head groups.

Synthesis and Study of a New Type of Fluorinated Polyether Demulsifier for Heavy Oil Emulsion Demulsification

To solve the problem of heavy oil demulsification difficulties in Liaohe Oilfield, phenolamine resin initiator was synthesized from p-trifluoromethyl phenol, and then FB series fluorinated polyether demulsifiers were synthesized by block polymerization using ethylene oxide (EO) and propylene oxide (PO) as raw materials. The demulsifiers were characterized by infrared spectroscopy, cloud point, hydrophilic-lipophilic balance (HLB) value, and surface tension. The demulsifying and dehydrating properties were tested by demulsifying and dehydrating experiments, the demulsification mechanism was analyzed by the microscopic demulsification process test, and the influence of demulsifier addition and demulsifying temperature on demulsifying performance was also studied. The results showed that under the condition of the optimum demulsification temperature of 60 °C and the optimum demulsifier dosage of 100 mg/L, the water removal (%) of fluorinated polyether demulsifier of FB 4 was the highest, and the overall water removal (%) of 50 mL crude oil emulsion in Liaohe Oilfield reached 90.33% within 2 h, which was better than the current demulsifier used in Liaohe crude oil.

Properties of some nonionic fluorocarbon surfactants and their mixtures with hydrocarbon ones

The adsorption of Zonyl FSN-100 (FSN100, having an average 14 oxyethylene units and 6 -CF₂ groups) and Zonyl FSO-100 (FSO100, having an average 10 oxyethylene units and 5 -CF₂ groups) as well as of their mixtures with p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycols) having 10, 16 and 8 oxyethylene groups in molecule (TX100, TX165, TX114) and cetyltrimethylammonium bromide (CTAB) at the solution-air and polytetrafluoroethylene (PTFE)-solution and polymethyl methacrylate (PMMA)-solution interfaces as well as the composition of the surface mixed layer was discussed based on the literature data. The adsorption properties of nonionic fluorocarbon surfactants were compared to those of the classical ones on the basis of the Gibbs standard free energy of adsorption determined by different ways and the intermolecular interactions of the surfactant molecules through the water phase. The synergetic effect in the reduction of the water surface tension by the mixture of fluorocarbon and classical nonionic surfactant was shown and explained by the comparison of the composition of the mixed surface layer to those in the bulk phase. The composition of the mixed fluorocarbon and classical surfactant layer at the solution-air interface was compared to that formed at the PTFE-solution and PMMA-solution interfaces. The changes of the surface tension of the aqueous solution of the fluorocarbon surfactants and their mixtures with classical hydrocarbon ones and their adsorption were analyzed taking into account the PTFE and PMMA surface wettability. This analysis was also based on the components and parameters of the head and tail of the surfactants surface tension as well as those of PTFE and PMMA. Apart from adsorption and wetting properties the aggregation of the fluorocarbon surfactants and their mixtures was discussed. A specific attention was paid to the possibility of two CMC values in the case of nonionic fluorocarbon surfactants as well as the synergism in CMC of mixtures of nonionic fluorocarbon and hydrocarbon surfactants.

Synergistic Properties of a Mixed Micelle System Consisting of a Nonionic Fluorinated Surfactant and a Cationic Surfactant

The physicochemical properties of aqueous surfactant mixtures containing a nonionic fluorosurfactant (undecafluoro-n-pentyldecaoxyethylene ether (C5F11EO10)) and various amounts of a cationic surfactant (decyltrimethylammonium bromide (DeTAB)) were determined by surface tension and conductivity measurements. All values of the critical micelle concentrations of the mixtures turn out to be smaller than those of both pure surfactants revealing the presence of a significant synergy for all DeTAB proportions in the mixed system. The analysis of the experimental data was performed on the basis of three different well established thermodynamic models of mixed micelle formation to determine several relevant parameters, especially the micelle composition, the interaction parameters, and the free energy of micelle formation. The results indicate that the dominant interactions between DeTAB and C5F11EO10 molecules are attractive. The main reason for this behavior could be attributed to the complexation between the polyoxyethylene chain of C5F11EO10 and the quaternary ammonium group of DeTAB giving rise to stable structures.

Synthesis and properties of chemiluminescent acridinium ester labels with fluorous tags

Acridinium dimethylphenyl esters are highly sensitive chemiluminescent labels that are used in clinical diagnostics. Light emission from these labels is triggered with alkaline peroxide in the presence of the cationic surfactant cetyltrimethylammonium chloride (CTAC). CTAC compresses emission times of these labels to <5 seconds and also increases overall light yield 3-4 fold. The observed enhancement in acridinium ester chemiluminescence (light yield) is quite sensitive to the polarity of the micellar interface. In the current study, we report the synthesis of new acridinium ester labels with fluorous tags of varying fluorine content and their chemiluminescence in the presence of cationic micelles of CTAC, anionic micelles of sodium perfluorooctanoate (SPFO) as well as mixed micelles of CTAC and SPFO. These studies indicate that in the presence of the mixed micelle system of CTAC and SPFO and at low mole fractions of SPFO, polarity of the mixed micelle interface is lower than that of CTAC leading to a greater enhancement of chemiluminescence for both fluorinated acridinium esters as well as a structurally analogous but non-fluorinated acridinium ester. Chemiluminescence stability of the fluorinated acridinium esters was either comparable to or better than the stability of the non-fluorinated acridinium ester. Non-specific binding to paramagnetic microparticles was higher for fluorinated acridinium esters requiring a surfactant wash to reduce their non-specific binding to the same extent as that observed for the non-fluorinated acridinium ester.

Influence of calcium ions on rhamnolipid and rhamnolipid/anionic surfactant adsorption and self-assembly

The impact of Ca(2+) counterions on the adsorption at the air-water interface and self-assembly in aqueous solution of the rhamnolipid biosurfactant and its mixture with the anionic surfactant sodium dodecylbenzenesulfonate, LAS, has been studied using neutron reflectometry and small-angle neutron scattering. The results illustrate how rhamnolipids are calcium tolerant and how their blending with conventional anionic surfactants improves the calcium tolerance of the anionic surfactant. Ca(2+) has relatively little effect upon the adsorption and self-assembly of the monorhamnose, R1, and dirhamnose, R2, rhamnolipids, even at high pH, due to their predominantly nonionic nature. For R1/R2 mixtures the addition of Ca(2+) has little impact upon the adsorbed amount or the surface composition. For R2/LAS mixtures the addition of Ca(2+) results in an increased adsorption and a surface slightly richer in R2. The weak binding of Ca(2+) to R1 and R2 does result in a change to the degree of ionization of the micelles and especially for mixed R1/R2 micelles at R1-rich solution compositions. The stronger binding of Ca(2+) to LAS results in the addition of Ca(2+) having a much greater impact on the self-assembly of R1/LAS and R2/LAS mixtures. For R1/LAS mixtures the addition of Ca(2+) promotes the formation of more planar structures, even at low surfactant concentrations where in the absence of Ca(2+) mixed globular micelle formation dominates. In R2/LAS mixtures, where there is a greater contrast between the high and low preferred curvatures associated with R2 and LAS, the addition of Ca(2+) results in a more complex evolution in micellar aggregation and the degree of ionization of the micelles. This results in variations in Ca(2+) binding that promotes micellar structures in which a spatial segregation of the two surfactant components within the micelle occurs.

Phase behavior and self-assembly aggregation of hydrocarbon and fluorocarbon surfactant mixtures in aqueous solution

Similar to hydrogenated surfactant mixtures, the ones of hydrocarbon and fluorocarbon surfactants can also self-assemble into various aggregates, including mixed micelles and vesicles, however, completely different phase behavior and self-assembly of CH/CF surfactant mixtures in solution can be observed and exhibit novel features because of the repellence between the two hydrophobic chains. These systems of hydrocarbon and fluorocarbon surfactant mixtures can provide important considerations for both theoretical and applied interest. Several advantaged techniques, including small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), (19)F- and (1)H-NMR, cryo-TEM, and Freeze-fracture TEM (FF-TEM) have been widely employed to characterize these mixture systems. In this review, the aggregation behavior, self-assembly aggregation, and interaction of hydrocarbon and fluorocarbon surfactant mixtures in solution are described and focused three aspects, (i) immiscibility and nonideal mixing in hydrocarbon and fluorocarbon surfactant mixed micelles systems; (ii) spontaneous vesicles of hydrocarbon and fluorocarbon surfactant mixtures in aqueous solution; and (iii) self-assembled aggregates of hybrid fluorocarbon/hydrocarbon surfactants in aqueous solutions.

A neutron reflection study of surface enrichment in nematic liquid crystals

The interfacial adsorption properties of several different dopants in cyanobiphenyl liquid crystals have been measured using specular neutron reflection. It was found that a partly fluorinated analogue of 11OCB, called F17, adsorbed strongly at the interface between 5CB and air but it was not adsorbed at the interface between 5CB and a solid substrate treated with cetyl trimethyl ammonium bromide (CTAB). The concentration dependence of the adsorption at the air interface was well described by the Brunauer, Emmett and Teller (BET) model, adapted for solutions rather than the gas phase. The isotherms are determined by two equilibrium constants: K(S) for adsorption of the dopant directly at the interface and K(L) for adsorption onto previously adsorbed dopant. The temperature dependence of K(S) indicated that the adsorption enthalpy is not influenced by the phase of the 5CB and its value of -29 kJmol(-1) is consistent with physical adsorption. The value of K(L) is zero in the isotropic phase but increases rapidly on cooling in the nematic phase suggesting that the F17 is less compatible with nematic than isotropic 5CB. The smallest layer thicknesses (~18 Å) suggest that the F17 molecules are approximately perpendicular to the surface. The other dopants studied were components of the E7 mixture: 8OCB and 5CT. No adsorption was found for 8OCB but 5CT showed adsorption at a CTAB treated solid interface when present in 5CB at the 10% level. In this case, the value of K(S) was much smaller than for F17 but the value of K(L) was such that an exponential concentration profile (predicted by the BET model) was observed with characteristic thickness of ~200 Å. The results demonstrate the potential for very precise control of surface properties in liquid crystal devices by using appropriate dopants.

Aggregation behavior of Brij-35/perfluorononanoic acid mixtures

The mixed system of a nonionic hydrocarbon surfactant, polyoxyethylene (23) lauryl ether (Brij-35), and a perfluorinated surfactant, perfluorononanoic acid, was investigated by a combination of methods. The critical micelle concentrations (cmc's) have been determined over a wide range of sample compositions by fluorescence and UV-visible spectrophotometry using pyrene and N-(4-nitrophenyl) perfluorononanamide, respectively, as molecular probes. The values of the cmc's obtained were considerably different with the two techniques employed. Measurements of the (19)F nuclear magnetic resonance chemical shift of the same mixtures showed two breaks in the plots of Δδ(f) versus molar fraction of the perfluorinated surfactant. Conductivity and surface tension measurements also showed two breaks. The behavior is attributed to the formation of mixed micelles that change their composition when the fraction of the fluorinated compound increases and some segregation of the fluorinated compound takes place at a high total surfactant concentration.

Nonideal mixed micelles of fluorinated and hydrogenous surfactants in aqueous solution. NMR and SANS studies of anionic and nonionic systems

Contrast variation SANS and (19)F chemical shifts were measured for three mixed equimolar micelle systems: sodium perfluorooctanoate (SPFO) and sodiumdecylsulfate (SDeS) in 200 mM NaCl, lithium perfluorononanate (LiPFN) and lithium dodecylsulfate (LiDS) in 200 mM LiCl, and a nonionic system C(8)F(17)C(2)H(4)(OC(2)H(4))(9) and C(12)H(25)(OC(2)H(4))(8) in water, all at 25 degrees C. The chemical shift measurements allow the calculation of the average fraction of nearest neighbors of each kind around the reporter group (the trifluoromethyl group). A preference for like neighbors were found in all systems, smallest in the SDeS/SPFO system and largest in the nonionic system, but in all cases substantially smaller than expected at critical conditions. From the SANS measurements the width of the micelle composition distribution was obtained. For the ionic systems similar values were obtained, showing a broadening compared to ideal mixtures, but not broad enough for demixing or clearly bimodal distributions. In the nonionic system the width was estimated as sigma = 0.18 and 0.22 using two different evaluation methods. These values suggest that the system is close to critical conditions. The lower value refers to a direct modeling of the system, assuming an ellipsoidal shape and a Gaussian composition distribution. The modeling showed the nonionic mixed micelles to be prolate ellipsoids with axial ratio 2.2 and an aggregation number larger than 100, whereas the two ionic systems fitted best to oblate shapes (axial ratios 0.8 and 0.65 for SDeS/SPFO and LiDS/LiPFN, respectively) and aggregation numbers of 60 for both.