Excess Micelles Research Articles

Analytical photo-centrifuge-based prediction of shelf-life and droplet packing behaviour of nanoemulsions upon removal of excess micelles

An analytical photo-centrifuge was used to develop novel experimental methods to predict the shelf-life, rheology, and inter-droplet interactions in 40 wt% canola oil-in-water nanoemulsions stabilized by 3 and 5 wt% citric acid esters of monoglyceride (Citrem). The combined influence of micellar concentration and oil droplet size on the rheology, creaming stability, and compression behaviour was studied under a series of accelerated gravitations. Nanoemulsions without excess Citrem micelles were obtained by removing them from the continuous phase using multiple cycles of ultracentrifugation. Reduction in droplet size with an increase in Citrem concentration and the removal of excess micelles resulted in a significant increase in plateau storage moduli leading to a change in phase behaviour of the nanoemulsions from liquid to viscoelastic gels. Moreover, it also showed a simultaneous drastic decrease in nanodroplets creaming velocity under accelerated gravitation. The flow behaviour of the gels was well correlated with a critical relative centrifugal force required to initiate flow under accelerated gravitation. Additionally, the colloidal forces responsible for instability was satisfactorily explained by the packing behaviour of the nanodroplets studied under a series of compression and dilatational cycles of centrifugation. We showed that the strong electrostatic repulsive force between the nanodroplets without excess micelles led to a lower packing density during compaction that did not change significantly during successive dilatational cycles. The higher packing density or compaction in the presence of excess micelles was explained by the short-range attractive depletion forces leading to weak flocculation of nanodroplets. Overall, the presence and absence of excess micelles showed a significant influence on the rheology and accelerated stability of nanoemulsions, which can be further described by the colloidal forces between the nanodroplets using the real-time advanced photo-centrifuge.

Influence of emulsifier concentration on nanoemulsion gelation.

Nanoemulsion gels are a new class of soft materials that manifest stronger elasticity even at lower dispersed phase volume fraction. In this work, gelation in 40 wt % canola oil-in-water nanoemulsions was investigated as a function of emulsifier type (anionic sodium dodecyl sulfate (SDS) or nonionic Tween 20) and concentration. It was observed that the liquid nanoemulsions transformed into viscoelastic gels at a specific concentration range of SDS, whereas no gelation was observed for Tween 20. The apparent viscosity, yield stress, and storage modulus of the nanogels increased with SDS concentration until 15 times critical micelle concentration (CMC), thereafter decreased steadily as the gelation weakened beginning 20 CMC. Three regimes of colloidal interactions in the presence of emulsifier were proposed. (1) Repulsive gelation: at low SDS concentration (0.5-2 times CMC) the repulsive charge cloud around the nanodroplets acted as interfacial shell layer that significantly increased the effective volume fraction of the dispersed phase (ϕ(eff)). When ϕ(eff) became comparable to the volume fraction required for maximal random jamming, nanoemulsions formed elastic gels. (2) Attractive gelation: as the SDS concentration increased to 5-15 times CMC, ϕ(eff) dropped due to charge screening by more counterions from SDS, but depletion attractions generated by micelles in the continuous phase led to extensive droplet aggregation which immobilized the continuous phase leading to stronger gel formation. (3) Decline in gelation due to oscillatory structural forces (OSF): at very high SDS concentration (20-30 time CMC), structural forces were manifested due to the layered-structuring of excess micelles in the interdroplet regions resulting in loss of droplet aggregation. Tween 20 nanoemulsions, on the other hand, did not show repulsive gelation due to lack of charge cloud, while weak depletion attraction and early commencement of OSF regime leading to liquid-like behavior at all concentrations. The nanogels possess great potential for use in low-fat foods, pharmaceuticals and cosmetic products.

Daunomycin Binding to Detergent Micelles: A Model System for Evaluating the Hydrophobic Contribution to Drug−DNA Interactions

The interaction of daunomycin with sodium dodecyl sulfate and Triton X-100 micelles was investigated as a model for the hydrophobic contribution to the free energy of DNA intercalation reactions. Measurements of visible absorbance, fluorescence lifetime, steady-state fluorescence emission intensity, and fluorescence anisotropy indicate that the anthraquinone ring partitions into the hydrophobic micelle interior. Fluorescence quenching experiments using both steady-state and lifetime measurements demonstrate reduced accessibility of daunomycin in sodium dodecyl sulfate micelles to the anionic quencher iodide and to the neutral quencher acrylamide. Quenching of daunomycin fluorescence by iodide in Triton X-100 micelles was similar to that seen with free daunomycin. Studies of the energetics of the interaction of daunomycin with micelles by fluorescence and absorbance titration methods and by isothermal titration calorimetry in the presence of excess micelles revealed that association with sodium dodecyl sulfate and Triton X-100 micelles is driven by a large negative enthalpy. Association of the drug with both types of micelles also has a favorable entropic contribution, which is larger in magnitude for Triton X-100 micelles than for sodium dodecyl sulfate micelles. The thermodynamic profile for the interaction of daunomycin with both types of micelles is characteristic of the "nonclassical" hydrophobic effect. The enthalpy for the interaction of daunomycin with sodium dodecyl sulfate micelles increases nonlinearly with temperature, indicating a positive (and temperature dependent) heat capacity change. The binding isotherm for daunomycin association with sodium dodecyl sulfate micelles was cooperative, with a Hill coefficient of 1.6. The cooperative behavior and the positive heat capacity change suggest that the drug alters micelle size or imposes order on the hydrocarbon interior of the micelle.

Interactions between Hydrophobically Modified Polymers and Surfactants: A Fluorescence Study

Interactions between hydrophobically modified polymers (hm-polymers) and surfactants were investigated by steady-state fluorescence as well as rheological experiments. Hydrophobically modified hydroxyethylcellulose (hmHEC) with C16 hydrophobe grafts of 0.9% mole along with tetradecyltrimethylammonium bromide and cetyltrimethylammonium bromide were studied. The presence of C16HEC induced the aggregation of surfactant micelles in aqueous solution at concentrations about one-half times lower than the critical micelle concentration. As surfactant is added, the viscosity of hm-polymer solution first increases due to the bridging of hydrophobe clusters by surfactant micelles to form mixed micelles and later decreases due to the masking of hydrophobes individually by excess micelles. Fluorescence quenching experiments showed that the number of hydrophobes (NH) in a mixed micelle decreases steadily with increasing surfactant concentration due to hydrophobe dilution by the surfactant. NH declines from greater than...

Dispersions of Silica Particles in Surfactant Phases

Addition of surfactants to aqueous dispersions of silica may either enhance or depress the colloidal stability of the dispersions. With a nonionic surfactant that forms micelles (Triton X-100), dilute silica dispersions have enhanced stability if the silica particles are fully covered with adsorbed surfactant micelles. Concentrated dispersions are also stable provided that the particles remain separated by at least one layer of micelles. Beyond full coverage, excess micelles may cause depletion flocculation of the silica particles. With a nonionic surfactant that forms vesicles and bilayers (polyoxyethylene alkyl ether C12E4), the effects of surface coverage are the same; however, the critical separation between particles is the thickness of two bilayers. Beyond full coverage, the surfactant forms a lamellar phase which excludes the particles.

Decoration of semidilute polymer solutions with surfactant micelles

Aqueous polymer solutions can be decorated with surfactant micelles which bind to the polymer strands. The elementary process wraps a section of a macromolecule around a micelle ; if the macromolecule is long enough, it clothes many micelles and connects them by free strands. Here we consider semidilute polymer solutions, where macromolecules of poly (ethylene oxide) form an irregular web throughout the sample. Sodium dodecyl sulfate micelles are added to the solution ; because of their electrostatic repulsions they tend to form a 3-dimensional array which is coupled to the web through the binding. The structures of such decorated webs are studied through small angle neutron scattering ; depending on the composition of the solution, 3 patterns of correlations are found : adaptation of the web to the original intermicellar spacings, disruption of the intermicellar spacings by the web, spillover of excess micelles by saturated necklaces.

Effect of polyoxyethylene chain length upon mixed surfactant solutions and temperature dependence on the surface tension

The surface tensions of a series of polyethylene glycol- n-dodecyl ethers ( nED) were measured by the Wilhelmy plate method in the presence of constant 6.3 m M sodium dodecyl sulfate (SDS) at temperature of 15, 25, and 40°C, respectively. The surface tension curves for the mixed system of SDS and nonionic surfactants showed a flat portion in the range of which the mixed micelles in the solution and the two-dimensional ones on the water surface occur, respectively. The width of this flat portion decreased with shortening of the polyoxyethylene chain length, and when the unit of ethylene oxide in the chain reached 3ED, the flat portion disappeared. This limiting length in the ethylene oxide chain required for the disappearance of the flat portion, that is, the so-called critical length of the ethylene oxide chain, can be considered to be one of the critical values for the existence of mixed micelles and below the critical value only excess micelles of the nonionic surfactant exist in all ranges of the mixed concentration of the SDS and nonionic surfactants. Furthermore, the effects of temperature on the flat portion were investigated on the assumption that the rise in temperature rendered a similar effect with respect to the length of the flat portion to that of reducing the length of the polyoxyethylene chain.