Surfactant Weight Fraction Research Articles

Microemulsions Based on Soybean Phosphatidylcholine and IsopropylmyristateEffect of Addition of Hydrophilic Surfactants

This work reports the effect of the addition of n-alkyl β-D-maltosides (C n G 2 ), sucrose monododecanoate, and sodium taurocholate on the phase behavior and microstructure of a microemulsion based on water, 1-propanol, soybean phosphatidylcholine (SbPC), and isopropylmyristate (IPM). The self-diffusion coefficients of the components were determined with the pulsed field gradient NMR technique. The spontaneous curvature (H 0 ) of the surfactant monolayer separating the aqueous and oil domains was found to increase with an increasing proportion of hydrophilic surfactant for all the hydrophilic surfactants studied. The microstructure of the microemulsion, deduced from the self-diffusion coefficients and the conductivity measurements, changed from an oil-continuous structure to oil-swollen micelles through a bicontinuous structure. The general phase behavior could be described with a simple model where H 0 was assumed to vary linearly with the changing weight fraction of hydrophilic surfactant. The molar ratio of C n G 2 /SbPC needed to obtain a balanced microemulsion ∈bal was found to increase linearly with n for n ≥ 10. The ∈bal for C 12 G 2 was 0.71. There was no observed difference in ∈bal between sucrose monododecanoate and C 12 G 2 , but sucrose monododecanoate was found to increase the flexibility of the surfactant film to a greater extent than C 12 G 2 . Sodium taurocholate increased H 0 more effectively than C 12 G 2 (∈ bal = 0.22) but did not destabilize the surfactant film to the same degree.

Delayed-die swell and sedimentation of elongated particles in wormlike micellar solutions

It has been proposed recently that the combined action of inertia and non-linear viscoelasticity may be the origin of very peculiar behaviors with dramatic changes of flow type. Two examples are the problem of delayed die swell and the orientation of elongated particles sedimenting in solutions of wormlike micelles. These solutions give rise to well defined viscoelastic properties which can be tuned precisely by changing the surfactant weight fraction. Our analysis does not rely on any constitutive equation and our results strongly support the interpretation that delayed die swell and sedimentation of long bodies in wormlike micellar solutions are ruled by a change of flow type from subcritical to supercritical.

Lamellar biogels comprising fluid membranes with a newly synthesized class of polyethylene glycol-surfactants

A series of four polymer–surfactant macromolecules, each consisting of a double-chain hydrophobic moiety attached onto a monofunctional polyethylene glycol (PEG) polymer chain, were synthesized in order to study their effect upon the fluid lamellar liquid crystalline (Lα) phase of the dimyristoylphosphatidylcholine/pentanol/water system. The main finding of this study is that the addition of these compounds induces a new lamellar gel, called Lα,g. We have determined the phase diagrams as a function of PEG–surfactant concentration, cPEG, and weight fraction water, ΦW. All phase diagrams are qualitatively similar and show the existence of the gel. Unlike more common polymer physical gels, this gel can be induced either by increasing cPEG or by adding water at constant cPEG. In particular, less polymer is required for gelation as water concentration increases. Moreover, the gel phase is attained at concentrations of PEG–surfactant far below that required for classical polymer gels and is stable at temperatures comparable to the lower critical solution temperature of free PEG–water mixtures. Small angle x-ray experiments demonstrate the lamellar structure of the gel phase, while wide angle x-ray scattering experiments prove that the structure is Lα, not Lβ′ (a common chain-ordered phase which is also a gel). The rheological behavior of the Lα,g phase demonstrates the existence of three dimensional elastic properties. Polarized light microscopy of Lα,g samples reveals that the Lα,g is induced by a proliferation of defect structures, including whispy lines, spherulitic defects, and a nematiclike Schlieren texture. We propose a model of topological defects created by the aggregation of PEG–surfactant into highly curved regions within the membranes. This model accounts for both the inverse relationship between ΦW and cPEG observed along the gel transition line and the scaling dependence of the interlayer spacing at the gel transition with the PEG molecular weight. These Lα hydrogels could serve as the matrix for membrane-anchored peptides, proteins or other drug molecules, creating a “bioactive gel” with mechanical stability deriving from the polymer–lipid minority component.

Phase Transition between Microemulsion and Lamellar Liquid Crystal

A phase transition from a lamellar liquid crystal to a middle-phase microemulsion (or surfactant phase) takes place with decreasing surfactant content in the water/sucrose monododecanoate/hexanol/decane system. Considering the monomeric solubility of hexanol in oil, the phase transition from the lamellar liquid crystal to a microemulsion was investigated at equal weight ratio of water and decane by a phase behavior study, small-angle X-ray scattering, and electron spin resonance spin-probe methods. In this system, the interlayer spacing of the lamellar liquid crystal, d, gradually increases upon addition of water and oil. The hexanol molecules are distributed between the palisade layers and the oil core in the lamellar liquid crystal. The bilayer becomes more flexible upon dilution and, eventually, the lamellar liquid crystal changes to a bicontinuous microemulsion. The surfactant weight fraction at the microinterface inside the bicontinuous microemulsion is also discussed.

Studies of cyclohexane/TX 100 + ethylpropionate/water system: effect of ethylpropionate as cosurfactant

Pseudoternary phase diagram of cyclohexane/TX 100-ethylpropionate/water system was determined at 30 °C. One phase microemulsion was obtained over a small area. The surfactant/ cosurfactant ratio was both kept at 1:1 and 1:2 w/w. Viscosity, conductance, adiabatic compressibility values at various temperatures show the expected trend. The addition of NaCl changes one phase microemulsion to Winsor II system and no Winsor III system was obtained. Contact angle data, conductance, etc., show oil continuous system at a constant surfactant weight fraction. The microstructure of Winsor IV microemulsion seems to be W/O.

Monte Carlo simulations of a polyoxyethylene C12E2 lamellar bilayer in water

The polyoxyethylene C12E2/ water lamellar phase has been modelled as an aqueous region sandwiched by two bilayers. The bilayers were represented as ethoxy chains attached to a hydrocarbon continuum. Initially, the bilayer separation distance was fixed at 24 A, which is a typical experimental value for the surfactant weight fraction of 71%. Both canonical and Gibbs ensemble simulations have been performed on the system at different temperatures ranging from 298⋅15 K to 353⋅15 K. In the Gibbs simulations the bilayer separation was allowed to readjust to an equilibrium value. The results are compared and contrasted with previous studies in which single chains (Kong, Y. C., Nicholson, D., Parsonage, N. G., and Thompson, L., 1994, J. chem Soc. Faraday Trans, 90, 2375) and charged OH groups (Cracknell, R. F., Nicholson, D., and Parsonage, N. G., 1992, Molec. Phys., 75, 1023), attached to a hydrocarbon substrate, were simulated. As in the previous work with single chains, there is evidence of water bridging whic...

The Effect of Mixing of Surfactants on Solubilization in a Microemulsion System

The effect of mixing homogeneous polyethylene glycol alkyl ethers (R m EO n ) on their maximum solubilizing power was investigated at the hydrophile-lipophile balance (HLB) temperature at which a microemulsion (surfactant) containing equal weights of water and oil is one phase of a three-phase body. The solubilization was compared at a fixed temperature equal to the HLB temperature for a single R 12EO 5 system. It was found that the minimum weight fraction of surfactant required to solubilize equal weights of water and oil into a single phase ( X b ) is decreased by mixing R 12EO 2 and R 12EO 8 or R 14EO 5and R 10EO 5. However, X b for the R 12OH-R 12EO 8 system is larger than for the R 12EO 2-R 12EO 8 system. Namely, the solubilization apparently reaches its maximum in a mixed surfactant system in which the hydrophilic-lipophilic properties of the surfactants are not very different. Nonionic surfactant is mainly distributed between oil microdomains and the interface between oil and water microdomains inside the microemulsion phase. Assuming that the monomeric solubility of each surfactant component in oil is the same as in the oil microdomain of the microemulsion, the weight fraction of each surfactant at the interface in the microemulsion ( C or C 1 ) was obtained. C or C 1 + C 2 indicates the real solubilizing power, and it is dramatically decreased when surfactants are mixed. The more the HLBs of the surfactants are separated, the more the real solubilizing power is increased. It was also found that the mixing ratio of surfactants at the water-oil interface inside the microemulsion phase is directly related to the HLB temperature.

Structural evolution within the one-phase region of a three-component microemulsion system: Water–n-decane–sodium-bis-ethylhexylsulfosuccinate (AOT)

The continuous inversion from a water-in-oil (w/o) microemulsion at low temperatures to an oil-in-water (o/w) microemulsion at higher temperatures within the one-phase channel of water (0.6% NaCl)–n-decane–AOT microemulsion system is investigated by small angle neutron scattering (SANS). At constant AOT (surfactant) weight fraction γ of 12%, the structural evolution as a function of temperature takes place in different forms as the oil-to-water weight fraction α is varied from 15 to 90 %. At low o-w weight fractions (α=15 and 20 %) the microemulsions transform from a water-internal, oil-continuous structure at lower temperatures to an oil-internal, water-continuous droplet structure at higher temperatures jumping across an intermediate region of a lamellar phase (Lα). However, at higher o-w weight fractions (α=80 and 90 %) the evolution goes through a stage of percolation of the water droplets first into extended water clusters, then the structural inversion takes place probably through a transition of these water clusters into an entangled tubular structure. At equal oil-to-water volume ration (α=40%), the structure can be described as bicontinuous at both low and high temperatures. In this case we are able to extract two lengths characterizing the structure from SANS data using different models for the scattering length density fluctuation correlation function of a bicontinuous microemulsion.

Correlation between the formation of a lamellar liquid crystalline phase and the hydrophile-lipophile balance (HLB) of surfactants

The hydrophile-lipophile balance of nonionic surfactants changes from hydrophilic to lipophilic with the rise in temperature and is considered to be just balanced in a temperature range at which a lamellar liquid crystalline phase forms in the surfactant-water system. The mid-temperature of that range is called the liquid crystal temperature, T LC. It was found that the following relation between the HLB numbers of nonionic surfactants and the LC temperature in an aqueous system is valid: T LC = k H 2 O (∑N HLB (i)W (i) − N H 2 O) where k H 2O = 17°C/HLB unit, N H 2O = 7.4 HLB unit, N HLB ( i) and W ( i) are the HLB number and the weight fraction of ith surfactant in total surfactants. This relationship is similar to that between the HLB number and the HLB temperature in a water/nonionic surfactant/oil system. Since the surfactant ratio to produce a lamellar liquid crystalline phase in a diluted region is almost independent of temperature in an ionicnonionic surfactant system, the condition to produce a lamellar liquid crystal was found at 30°C to be ∑N HLB (i)W (i) = 9.2 . This relation is equivalent to the above equation and is valid for an ionic-nonionic surfactant aqueous system.

Evaluation of the Hydrophile-Lipophile-Balance (HLB) of Long-chain Nonionic Surfactant

Correlation between hydrophile-lipophile-balance temperature, TcomHLB, and the HLB number of commercial nonionic surfactants (C12C18) was investigated. The empirical formula, Row (1/x-1) = A (T-TcomHLB) is valid for these surfactants. (Row is the weight fraction of oil in water+oil, x, the weight fraction of surfactant in a system, T, the temperature, and A, a constant.) TcomHLB (HLB temperature=Phase Inversion Temperature) is a characteristic temperature related to the NcomHLB (HLB number), i.e., TcomHLB=koil (NcomHLB-Noil), provided the HLB temperature is 1080°C or more. (koil, Noil are constants for a given oil.) Through the application of these equations, an HLB number of a surfactant may be evaluated from the HLB temperature and phase behavior or vice versa.

The swollen micelle—microemulsion transition

A rather abrupt transition takes place in the properties of a nonionic microemulsion at a certain value of hexadecane uptake, W o (the weight fraction of oil plus surfactant that is oil). When W o is greater than 0.06, the microemulsion behaves as spherical particles that can be diluted with water without change in their properties. The intrinsic viscosity, Huggins coefficient, and 4 A 2 M w have values close to those observed for the globular proteins. When W o is less than 0.06 the particles are anisotropic in shape, as shown by classical light scattering. Moreover, the anisotropic particles are undergoing open association so that dilution decreases their average size. When diluted, the rate of oil uptake by the microemulsion is orders of magnitude smaller than for the swollen micelles.

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In this study, liquid–liquid equilibriums are reported for different ternary systems of aqueous solutions with decane as oil and mono-ethylene glycol butyl ether (C4E1) as surfactant. The appearing Winsor phases and their evolution are shown by drawing temperature versus surfactant weight fraction for a given ratio of aqueous solution to oil. The obtained diagrams have the well-known fishlike shape. Four parameters are used to discuss the influence of the additives: the perpendicular and horizontal asymptotes to the realms-of-existence of the Winsor III phase. The two horizontal asymptotes, i.e., the LCST (lower critical solution temperature) and UCST (upper critical solution temperature) show no significant changes upon adding chloride salts, except for the triethylphenylammonium cation as counter-ion. By contrast, significant changes of the values of these two parameters are found when the nature of the anion of sodium salts changes. The two perpendicular parameters γ0 (minimum concentration of surfactant needed to form a Winsor III) and γ˜ (minimum surfactant concentration needed to reach the Winsor IV phase from a Winsor III one) are only little affected by the nature of the additive. Two other parameters in relation with Winsor III area are also studied: δ reflects the vertical extent of the realms-of-existence of Winsor III versus temperature (δ = UCST − LCST), while ξ=γ˜−γ0 reflects its horizontal extension. The value of δ is affected by the nature of the anion of sodium salts and strongly increases in presence of Na2SO4, but also with sucrose as additive. The effect of additives has in general a more pronounced influence on the values of the LCST and UCST than on ξ. The results are discussed in terms of Hofmeister effects, of salting-in and salting-out and of the resulting depletion and adsorption on the interfacial film of the Winsor III middle microemulsion.