Surfactant Weight Fraction Research Articles

Surfactant-Polymer Complexation and Competition on Drug Nanocrystal Surfaces Control Crystallinity.

Nanosizing drug crystals has emerged as a successful approach to enabling oral bioavailability, as increasing drug crystal surface area improves dissolution kinetics and effective solubility. Recently, bottom-up methods have been developed to directly assemble nanosized crystals by leveraging polymer and surfactant excipients during crystallization to control crystal size, morphology, and structure. However, while significant research has investigated how polymers and other single additives inhibit or promote crystallization in pharmaceutical systems, there is little work studying the mechanistic interactions of multiple excipients on drug crystal structure and the extent of crystallinity, which can influence formulation performance. This study explores how the structure and crystallinity of a model hydrophobic drug crystal, fenofibrate, change as a result of competitive interfacial chemisorption between common nonionic surfactants (polysorbate 80 and sorbitan monooleate) and a surface-active polymer excipient (methylcellulose). Classical molecular dynamics simulations highlight how key intermolecular interactions, including surfactant-polymer complexation and surfactant screening of the crystal surface, modify the resulting crystal structure. In parallel, experiments generating drug nanocrystals in hydrogel thin films validate that drug crystallinity increases with an increasing weight fraction of surfactant. Simulation results reveal a connection between accelerated dynamics in the bulk crystal and the experimentally measured extent of crystallinity. To our knowledge, these are the first simulations that directly characterize structural changes in a drug crystal as a result of excipient surface composition and relate the experimental extent of crystallinity to structural changes in the molecular crystal. Our approach provides a mechanistic understanding of crystallinity in nanocrystallization, which can expand the range of orally deliverable small molecule therapies.

Composition and temperature effects on the solution structure of SDS/octanol/brine by SANS, NMR and microscopy.

We investigate the solution structures of model sodium dodecyl sulfate/octanol/brine ternary mixtures across the lamellar (Lα), vesicle (L4) and micellar (L1) phases employing small angle neutron scattering (SANS), optical microscopy and nuclear magnetic resonance (NMR). Specifically, we examine the effect of co-surfactant octanol (0.2-9.48 w/v%) and temperature (25-65 °C) along dilution lines at fixed octanol : SDS ratios (0.08-1.21). A transition from Lα to sponge phase (L3) above 35 °C is found along the octanol : SDS = 1.21 isopleth, with phase coexistence above ϕ ≈ 0.14 weight fraction of surfactant and co-surfactant. The lamellar bilayers swell upon dilution, with an approximately linear increase of d-spacing, accompanied by a decrease of the Caillé parameter, indicative of greater membrane rigidity. At a lower octanol : SDS ratio of 0.62, coexistence of oblate micelles and vesicles is observed with preferential formation of vesicles at low concentrations. Dilution of the L1 phase, along octanol : SDS = 0.08, results in elongated micelles, as the NaCl : SDS ratio increases, while higher temperatures favour the formation of less elongated micelles. Our results provide a detailed map of the equilibrium structures found in the Lα vicinity of this extensively investigated flow-responsive surfactant system.

Effects of the Method of Preparation and Dispersion Media on the Optical Properties and Particle Sizes of Aqueous Dispersions of a Double-Chain Cationic Surfactant.

As inferred from visual observations and turbidity measurements, the average radius of the unilamellar vesicles formed in water from the cationic double-chain surfactant didodecyldimethylammonium bromide (DDAB) varies with the method of preparation, being ∼24 nm after sonication (SS method) and ∼74 nm after extrusion/ultrafiltration (SE method). The radii were larger when the vesicles were produced in 10 mM NaBr, ∼65 nm for the SS method and ∼280 nm for the SE method. The specific turbidity, or turbidity per unit path length divided by the surfactant weight fraction, w, of these vesicular dispersions increased with decreasing w until a constant value was reached at w*, which depends on the preparation method and the dispersion medium. The constant specific turbidities are indicative of single and independent scattering and were used to estimate vesicle radii by solving the specific turbidity equations derived for the Rayleigh-Debye-Gans (RDG) regime. Two turbidity equations were used, one accounting for absorbance errors due to some scattered light reaching the detector and another with no correction. Estimates of the average distances between the vesicles and their corresponding Debye lengths were obtained for evaluating the importance of intervesicle electrostatic interactions, which could lead to dependent scattering at higher weight fractions.

Scattering and Spectroscopic Study on the Hydration and Phase Behavior of Aqueous Alcohol Ethoxylate and Methyl Ester Ethoxylate: Effects of Terminal Groups in Hydrophilic Chains.

Using dielectric relaxation spectroscopy (DRS), small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and viscometry, we have investigated the hydration behavior, static structures, Brownian dynamics, and mechanical properties of aqueous solutions of alcohol ethoxylate (C12E15) and methyl ester ethoxylate (C12MEE), hereafter abbreviated as AE and MEE, respectively, in which we especially focus on the effects of the endcaps of these nonionic surfactants. We find that AE and MEE exhibit fairly different phase behaviors in water: AE produces liquid crystalline phases at w (surfactant weight fraction) > 0.35, whereas MEE retains a liquid phase in an extremely wide concentration range (w < 0.7) at ambient temperature. The structure factor deduced from SAXS intensities using a generalized indirect Fourier transformation technique and the effective hydration number evaluated from the negative excess bulk water relaxation amplitude revealed by DRS unambiguously demonstrate that hydration water molecules, exhibiting about 4-times-slower collective reorientational dynamics than that of bulk water, contribute to the excluded volume of the micelles. The blocked terminal hydrogen-bond donor/acceptor site of MEE leads to smaller hydration number of MEE than compared to that of AE, and consequently the lower excluded volume of the MEE micelles. The effective micellar volume fraction, ϕ(eff), should be defined by incorporating such different hydration effects. Importantly, voluminosity, defined as the micellar volume fraction per unit mass, is clearly a decreasing function of w, demonstrating progressive dehydration at a higher w. The collective diffusion constants determined by DLS for the AE and MEE micelles show a monotonous increase up to ϕ(eff) ≈ 0.5, as expected for the hard spheres. Low-shear-rate viscosities follow a Krieger-Dougherty model in the identical micellar packing fraction range. All static, dynamic, and mechanical properties of these micellar solutions can be explained in a consistent and quantitative manner only when the excluded volume of hydration water molecules is properly taken into account.

Effects of salts and sucrose on the phase behavior of ternary mixtures of water, decane, and mono-ethylene glycol butyl ether

Abstract 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 (C 4 E 1 ) 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 γ 0 (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 ˜ − γ 0 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 Na 2 SO 4 , 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.

Structure and Rheology of Charge-Free Reverse Micelles in Aromatic Liquid Phenyloctane

Reverse micelles formulation requires an inclusion of water or other polar molecules in the binary mixture of ionic surfactant and oil and generally exhibit spheroid geometry with a small aggregation number. Here, we report structure and rheology of charge-free (nonionic) reverse micelles in surfactant/oil systems. We have systematically investigated intrinsic parameters for the shape, size, and internal cross section structure control of such micelles using small-angle X-ray scattering (SAXS) and the rheometry. We found that diglycerol monomyristate (C14G2) when added into an aromatic organic liquid phenyloctane, spontaneously self-assembles into spheroid micelles with maximum diameter ca. 6.7 nm. Decrease in surfactant chain length favors globular-to-rod type transition and micellar aggregation number (N(agg)) increases significantly. On the other hand, increase in surfactant weight fraction induces one-dimensional (1-D) micellar growth; N(agg) increases in parallel to the surfactant concentration. Reverse micelles shrink with the rise of temperature, which is close to the rod-to-sphere type transition. However, water causes a significant micellar growth; N(agg) increases drastically, which shows that water not only increase reverse micellar size but also increases the number of surfactant molecules per micelle. All these microstructure transitions could be understood in terms of the modification of the critical packing parameter (cpp). The SAXS results are very well supported by the geometrical model fittings and rheometry.

Intrinsic Parameters for the Structure Control of Nonionic Reverse Micelles in Styrene: SAXS and Rheometry Studies

Shape, size, and internal structure of nonionic reverse micelle in styrene depending on surfactant chain length, concentration, temperature, and water addition have been investigated using a small-angle X-ray scattering (SAXS) technique. The generalized indirect Fourier transformation (GIFT) method has been employed to deduce real-space structural information. The consistency of the GIFT method has been tested by the geometrical model fittings, and the micellar aggregation number (N(agg)) has been determined. It was found that diglycerol monocaprate (C(10)G(2)), diglycerol monolaurate (C(12)G(2)), and diglycerol monomyristate (C(14)G(2)), spontaneously self-assemble into reverse micelles in organic solvent styrene under ambient conditions. The micellar size and the N(agg) decrease with an increase in surfactant chain length, a scenario that could be understood from the modification of the critical packing parameter (cpp). A clear picture of one-dimensional (1-D) micellar growth was observed with an increase in surfactant weight fraction (W(s)) in the C(10)G(2) system, which eventually formed rodlike micelles at W(s) ≥ 15%. On the other hand, micelles shrunk favoring a rod-to-sphere type transition upon heating. Reverse micelles swelled with water, forming a water pool at the micellar core; the size of water-incorporated reverse micelles was much bigger than that of the empty micelles. Model fittings showed that water addition not only increase the micellar size but also increase the N(agg). Zero-shear viscosity was found to decrease with surfactant chain but increase with W(s), supporting the results derived from SAXS.

The production of high polymer to surfactant microlatexes

AbstractStarved‐feed microemulsion polymerization of styrene was investigated. The influence of the type (SDS or Dowfax 2A1) and concentration of anionic surfactant on the final particle size of latex made by the polymerization of microemulsions of styrene was studied. In addition, the influence of 1‐pentanol and acrylic acid as cosurfactants was examined. Latexes with 20% solids content and polymer to surfactant ratio of 22 were produced, with a particle diameter of 42 nm and very low polydispersity indexes. Smaller particles are produced using SDS than Dowfax 2A1 for the same weight fraction of surfactant; however, similar particle sizes were obtained with the same molar concentrations of SDS and Dowfax 2A1. Further shot additions of monomer increased solids level as high as 40% and polymer to surfactant ratios greater than 40, with particles remaining monodisperse with average diameter smaller than 60 nm. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 48–54, 2010

Cationic–anionic vesicle templating from fluorocarbon/fluorocarbon and hydrocarbon/fluorocarbon surfactants

Spontaneous catanionic vesicle formation is studied in systems comprising fluorinated surfactants, the cationic/anionic fluorinated surfactant system of 1,1,2,2-tetrahydroperfluorododecylpyridinium chloride (HFDPC)/sodium perfluorooctanoate (SPFO) and the analogous mixed hydrocarbon/fluorocarbon surfactant system of cetylpyridinium bromide (CPB)/SPFO. Aggregate formation is explored in the anionic-rich surfactant system (weight fraction of anionic surfactant, γ = 0.66–0.85) and a total surfactant concentration range of 0.1–2% wt/wt for the fluorinated system and 0.4–2.2% wt/wt for the mixed hydrocarbon/fluorocarbon system. Vesicle sizes range from approximately 40 to 200 nm for CPB/SPFO, as determined by negative staining transmission electron microscopy (TEM) and confirmed by dynamic light scattering. The primary vesicle diameter observed by TEM in the catanionic fluorinated/fluorinated surfactant system is smaller (20–50 nm). However, the relatively few larger vesicles (⩾100 nm) in the HFDPC/SPFO system dominate the dynamic light scattering measurements. Successful templating of silica hollow spheres is demonstrated in both HFDPC/SPFO and CPB/SPFO vesicle systems, using tetramethoxysilane (TMOS) as the silica precursor for the acid-catalyzed synthesis. The size of the resulting hollow silica particles is consistent with the templating of vesicles of the size range observed by TEM. Changes in zeta potential are used to monitor colloidal stability. At the conditions investigated (TMOS/surfactant weight ratios of 0.25–1.0, pH 3), the colloidal silica particles templated from fluorinated HFDPC/SPFO vesicles are more stable than the particles templated from the corresponding mixed fluorinated CPB/SPFO system. Further improvement of the stability of the colloidal particles is achieved in the CPB/SPFO systems by titrating the acid synthesis solution with base over the course of the particle synthesis.

Synthesis, characterization and kinetics of hydrophobically associating polyacrylamide

AbstractHAPAM was successfully prepared in inverse microemulsion. 1H-NMR, FT-IR, DSC and TG results show that the copolymerization of acrylamide (AM) with DM16 was successful. However, the ratio of AM to DM16 is not calculated exactly due to the low percentage of hydrophobic monomer in polymer. Therein, we only proved the success of the incorporation of DM16 onto polyacrylamide backbone. According to our experiment, the conversions of monomers and molecular weight have a strong dependence on initiator concentration and aqueous phase weight fraction. But it was not completely in accordance with literature. The difference may be ascribed to the major existence of locus of DM16 (at the interface of oil/water or in droplet). Compared with literature, the influence of initiator concentration on particle size was slight due to the high surfactant weight fraction, seeming to appear independent of initiator concentration. When the surfactant weight fraction reaches a certain degree, it did not affect the conversion. For the particle size and molecular weight, it was other way around, suggesting that there was an intense chain transfer reaction to the emulsifier. In this work, we could not calculate the average number of free radical per particle n, because the part of the surfactant in excess recombines to form small micelle and the DM16 may continuously transfer from micelle to another micelle, resulting in difficulty to estimate precisely the number of the particles. This was also reflected in the difference of kinetics between our system and literature. Finally, the occurrence of nucleation at high conversion was proved indirectly.

Variation of Characteristics of Pentaoxyethylene Decyl C10E5 and Hexaoxyethylene Tetradecyl C14E6 Ether Micelles with Uptake of n-Octanol

Wormlike micelles of the surfactant pentaoxyethylene decyl C10E5 and hexaoxyethylene tetradecyl C14E6 ethers were characterized by static (SLS) and dynamic light scattering (DLS) experiments to examine effects of uptake of n-octanol on the micellar characteristics. The SLS results have been successfully analyzed by the light scattering theory for micelle solutions to yield the molar mass Mw(c) as a function of concentration c along with the cross-sectional diameter d of the micelle. The apparent hydrodynamic radius RH,app(c) determined by DLS as a function of c has also been successfully analyzed by a fuzzy cylinder theory which takes into account the hydrodynamic and direct collision interactions among micelles, providing us with the values of the stiffness parameter lambda(-1). It has been found that the micellar length Lw increases with increasing surfactant weight fraction ws and increasing n-octanol content wo in the micelles or with raising temperature T. The values of d and lambda(-1) are found to increase with increasing wo, whereas the spacing s between hydrophilic tails of adjacent surfactant molecules on the micellar surface decreases with increasing wo. Comparison with our previous results for the C10E5 and C14E6 micelles containing n-dodecanol has revealed the salient features in change of the micellar characteristics with uptake of n-alcohols as follows: (i) The Lw values increase more significantly for the C14E6 micelles containing n-dodecanol than those containing n-octanol, whereas Lw of the C10E5 micelles increases by including n-dodecanol and n-octanol without a significant difference for the two alcohols. (ii) The values of d and lambda(-1) of the C10E5 and C14E6 micelles increase with uptake of n-octanol and n-dodecanol into the micelles. They are larger for the C10E5 micelles than for the C14E6 micelles, and their increase with alcohol content is less significant for the C14E6 micelles in comparison with the C10E5 micelles. (iii) The s values of the C10E5 and C14E6 micelles decrease with uptake of n-octanol and n-dodecanol into the micelles. They are somewhat larger in the latter micelles than in the former. (iv) The variation in d, s, and lambda(-1) with uptake of n-alcohol occurs with no difference in the effects for the two alcohols n-octanol and n-dodecanol.

Aqueous Phase Behavior of Diglycerol Fatty Acid Esters

We have studied the phase behavior of homologous series of diglycerol fatty acid esters (Qn‐D, for n=10, 12, 14, and 16, where n represents the carbon number in the alkyl chain length of amphiphile) in aqueous solution as a function of temperature and surfactant concentration. The different equilibrium phases present over a wide range of composition and temperature studied were characterized by means of visual observation under normal and polarized light, and x‐ray scattering techniques at small (SAXS) and wide angle (WAXS) regions. In diglycerol monocaprate (Q10‐D) and diglycerol monolaurate (Q12‐D)/H2O systems, lamellar liquid crystal (Lα) phase is present in the surfactant rich region and it swallows an appreciable amount of water. The amount of water swallowed by the Lα phase was determined by plotting the interlayer spacing, d, as a function of reciprocal of the surfactant weight fraction Ws . In the dilute regions, dispersion of Lα phase in water is observed over a wide range of temperature. At higher temperatures, the Lα phase melts to isotropic two‐liquid phases in water rich region whereas to isotropic reverse micellar solution (Om) in surfactant rich region. The Lα‐Om transition temperature is increased on increasing the hydrocarbon chain length of the surfactant from Q10‐D to Q12‐D. There is surfactant solid phase in equilibrium with water up to 25°C in diglycerol monomyristate (Q14‐D)/H2O system and the solid phase could solubilize 25 wt% water. The melting temperature of solid phase is practically constant in a wide range of compositions. Both the solid present region and the extent of water solubilization are increased in diglycerol monopalmitate (Q16‐D)/H2O system. At lower surfactant concentrations, excess water appears and dispersion of solid in water is formed. The structure of the solid is identified by WAXS measurement and it is confirmed to α‐solid. Normal vesicular aggregates are formed in Lα+W regions in the Q14‐D/H2O system at 25°C.

Wormlike Micelles of Hexaoxyethylene Decyl C10E6 and Tetradecyl C14E6 Ethers Containing n-Dodecanol

Wormlike micelles formed with the surfactant hexaoxyethylene decyl C10E6 and tetradecyl C14E6 ethers were studied by static (SLS) and dynamic light scattering (DLS) experiments to examine variation of the micellar characteristics with uptake of n-dodecanol. The SLS results have been analyzed by the light scattering theory for micelle solutions to yield the molar mass Mw(c) as a function of concentration c along with the cross-sectional diameter d of the micelle. The apparent hydrodynamic radius RH,app(c) determined by DLS as a function of c is also successfully analyzed by a fuzzy cylinder theory which takes into account the hydrodynamic and direct collision interactions among micelles, allowing us to evaluate the stiffness parameter λ−1. It has been found that the micellar length increases with increasing surfactant weight fraction ws or with raising temperature T irrespective of the composition of n-dodecanol content in the micelles. The length of the micelles at fixed ws and T steeply increases with increasing weight fraction wd of n-dodecanol. The length of the C14E6 micelles is extremely larger than that of the C10E6 micelles. The values of d and λ−1 are found to increase with increasing wd. It has been found that the increase in λ−1 is more significant for the micelles of the surfactant molecules with shorter hydrophobic chain length.

Phase Behavior of Pentaglycerol Monostearic and Monooleic Acid Esters in Water

The phase behavior in water of pentaglycerol monostearate (C18G5) and pentaglycerol monooleate (C18:1G5) surfactants has been studied as a function of temperature and surfactant weight fraction, W s . The equilibrium phases present at each composition and temperature studied were characterized by means of visual observation under normal and polarized light, differential scanning calorimetry (DSC), and X‐ray scattering, both at small (SAXS) and at wide angle (WAXS). In the temperature range 0–46°C, C18G5 presents a thermotropic α‐gel structure. However, at higher temperatures, the α‐gel phase melts and a lamellar liquid crystalline (Lα) phase is formed. The amount of water that can be solubilized by α‐gel and Lα was determined by plotting the interlayer distance, d, as a function of the reciprocal of W s . Water is soluble in the α‐gel phase up to 21 w/w% water concentration and in the Lα phase up to 30 w/w% water concentration. At higher water concentrations, excess water appears and a dispersion of α‐gel (α‐gel+W) and lamellar liquid crystal (Lα+W) in water is formed, respectively. In contrast, C18:1G5 is liquid in the whole range of temperatures studied (0–100°C). While at low temperatures, C18:1G5 presents a Lα structure, at about 63°C Lα melts and an isotropic liquid reverse micellar solution (Om) phase is formed. The amount of water that can be solubilized by both Om and Lα increases with temperature.

Theoretical and experimental studies of the aggregation number in the ammonium perfluorooctanoate/water binary mixture

The ammonium perfluorooctanoate/water (APFO/D 2O) system was studied by ‘ab initio’ quantum computational methods employing the 6-31 + G and 6-311 + G basis set at Hartree-Fock level. Infrared and Raman measurements were made in order to clearly understand the nature of the C–O bonds and also to verify the accuracy of the theoretical results. Small angle X-ray scattering experiments were performed for several samples at the desired surfactant weight fraction and from it the average micellar size and aggregation number were calculated using semi-empirical values for the monomer volume. These average aggregation numbers were compared with those obtained from the optimized geometry of the APFO, which showed that, for this fluorinated surfactant, there is some discrepancy between the results achieved by these two approaches. The results show that the average aggregation numbers obtained by semi-empirical methods are lower than those obtained by ab initio calculations.

Effect of temperature and salt on the phase behavior of nonionic and mixed nonionic–ionic microemulsions with fish-tail diagrams

The phase behavior of Brij-56/1-butanol/n-heptane/water is investigated at 30 °C with α [weight fraction of oil in (oil + water)] =0.5, wherein a 2̲→3→2¯ phase transition occurs with increasing W1 (weight fraction of 1-butanol in total amphiphile) at low X (weight fraction of both the amphiphiles in the mixture) and a 2̲→1→2¯ phase transition occurs at higher X. Addition of an ionic surfactant, sodium dodecylbenzene sulfonate, destroys the three-phase body and decreases the solubilization capacity of the system at different δ (weight fraction of ionic surfactant in total surfactant). A three-phase body appears at α=0.25, but not at α=0.75 for the single system. No three-phase body appears with the mixed system at either α value. Increased temperature increases the solubilization capacity of the Brij-56 system; on the other hand, a negligible effect of temperature on the Brij-56/SDBS mixed system has been observed. Addition of salt (NaCl) produces a three-phase body for both single and mixed systems and increases their solubilization capacities. The monomeric solubility of 1-butanol in oil (S1) and at the interface (S1s) has been calculated using the equation hydrophile–lipophile balance plane for both singles- and mixed-surfactant systems. These parameters have been utilized to explain the increase in solubilization capacity of these systems in the presence of NaCl.

Horseradish Peroxidase Activity in a Reverse Catanionic Microemulsion

In this paper we present the first results of enzymatic activities in a reverse microemulsion medium based on a mixture of an anionic and a cationic surfactant, called catanionic microemulsion. The studied system is composed of sodium dodecyl sulfate (SDS)/dodecyltrimethylammonium bromide (DTAB)/n-hexanol/citrate buffer/n-dodecane, with high SDS/(SDS + DTAB) weight fractions. It turns out that the results are similar to those obtained in classical reverse microemulsions, except that the presence of DTAB exerts an inhibiting effect on the enzyme. Nevertheless, enzymatic superactivities are found even at a DTAB to total surfactant ratio of 15%, corresponding to 3% weight fraction of cationic surfactant in the microemulsion. The influence of pH and hexanol content on the enzymatic activities is also studied.

Lyotropic Mesophases Next to Highly Efficient Microemulsions: A 2H NMR Study

Phase diagram investigations of symmetric microemulsions ofwater-n-decane-C 1 0 E 4 -PEP5-PEO5 in the semidilute regime suggested the existence of anisotropic mesophases (C 1 0 E 4 , n-decyl tetraoxyethylene glycol ether; PEP5-PEO5, poly(ethylenepropylene)-co-poly(ethylene oxide), M P E P = M P E O = 5 kg mol - 1 ). Performing a temperature scan on a sample at fixed composition, we observed an alternating sequence of eight two-phase regions and six one-phase regions. The composition of the sample was chosen to contain equal volumes of water and oil and an overall weight fraction of surfactant and polymer of 0.20. We examined the nature of the lyotropic mesophases and the coexisting phases by measuring the line splitting of 2 H NMR spectra. The experiments unequivocally reveal the existence not only of a lamellar (L α ) but also of a normal hexagonal (H 1 ) and a reverse hexagonal (H 2 ) phase.

Solubilization of Micellar Cubic Phases and Their Structural Relationships in Anionic−Cationic Surfactant−Dodecane−Water Systems

The morphology and solubilization of micellar cubic phases are investigated as a function of the cationic weight fraction in the mixture of two surfactants, the surfactant-to-water ratio, and the oil weight fraction in the water−dodecane−sodium dodecyl sulfate (SDS)−dodecyltrimethylammonium bromide (DTAB) or didodecyl-dimethylammonium bromide (DDAB) systems at 25 °C. One phase transition H1 → I1 appears at low cationic weight fraction, Y, and the other Lα → I1 takes place at high Y upon addition of oil. Solubilization for dodecane of the micellar solution and of the micellar cubic phase increases with an increase in the cationic surfactant weight fraction Y from 0 to 0.30. SAXS measurements index the structure of the cubic phase as the discontinuous Pm3n cubic phase for single surfactant systems and mixed surfactant systems at different levels of oil. The cross-sectional area of the hydrophilic head of the surfactant keeps constant with increase in the oil weight fraction at a fixed cationic surfactant weight fraction as the interlayer spacing of the Pm3n cubic phase greatly increases. However, it decreases with an increase in the cationic surfactant weight fraction at fixed oil weight fraction as the interlayer spacing of the Pm3n cubic phase slowly increases. So the solubilization for oil can be increased by adjusting the mean curvature of the surfactant interface layer with the addition of mixed surfactants and long-chain oil.

Formation of Cubic-Phase Microemulsions with Anionic and Cationic Surfactants at Equal Amounts of Oil and Water

The formation and microstructure of cubic phases were investigated in anionic and cationic surfactant-containing systems at 25°C. In the system sodium dodecyl sulfate(SDS)–dodecyltrimethylammonium bromide(DTAB)–water, mixing of two surfactants shows the phase transition hexagonal phase (H1)→surfactant precipitate, accompanied by an obvious decrease in the cross-sectional area per surfactant in the rod micelles of the hexagonal liquid crystal. In the mixed systems brine(A)–dodecane(B)–SDS(C)–DTAB(D)–hexanol(E), the isotropic discontinuous cubic phase is formed from the H1 phase at a low cationic surfactant weight fraction, Y=D/(C+D), and from the lamellar phase at high Y upon dilution with equal amounts of oil and brine, respectively. The minimum surfactant concentration to form the cubic phase decreases with increases both in cationic surfactant weight fraction Y from 0 to 0.30 and in hexanol weight fraction, W1=E/(C+D+E), accordingly. The maximum solubilization for oil of the cubic phase reaches 43 wt% at 14 wt% of mixed surfactants and alcohol.