VITAMIN E DISSOLUTION IN SULFOBETAINE ZW1TTERIONIC SURFACTANT SOLUTION AND ENHANCEMENT OF DIS SOLUTION BY ADDING SODIUM CHLORIDE

Vitamin E is widely used in pharmaceutical, food and cosmetic preparations.This paper discusses methods of preparing vitamin E emulsions by using sulfobetaine (SB) zwitterionic surfactants which have high electrolyte tolerance. The amount of vitamin E dissolved in water was analyzed by turbidity and UV absorption measurements. The emulsion droplet size was determined by laser light scattering. Microemulsions with small particle size and high resistance to oxidation in air can be obtained by solubilizing vitamin E in SB solutions at high concentrations (above %) The performance of a series of SB surfactants is compared in 1.5% solutions. A SB surfactant with a 12 carbon chain seems to be the most effective in dissolving vitamin E. The saturation value of vitamin E is increased about one order of magnitude by adding NaCl to the SB surfactant solutions. Emulsions formed in surfactant solutions containing NaCl (0 4-0.6 M) are stable at low surfactant concentrations such as 0.2-0.4 % The emulsion droplet sizes are below or around 200nm.     

DETERMINATION OF LONG-TERM STABILITY OF VITAMIN E EMULSION AND FORMATION OF MICROEMULSIONS BY USING LASER LIGHT SCATTERING AND UV ABSORPTION MEASUREMENTS

This paper presents methods of determining the long-term stability of vitamin E emulsion and formation of microemulsions. Several emulsion systems formed by using anionic, zwitterionic and cationic surfactants have been studied in the presence and absence of NaCI. Several conclusions can be drawn: (1) by using UV absorption and particle size measurements, one may be able to predict the long-term stability of an emulsion or the possibility of forming a microemulsion by measuring the initial properties of an emulsion, (2) in order to form a stable vitamin E emulsion or microemulsion, the initial properties of the emulsion should have the following features : (a) the particle size is ≤ 200 nm, (b) the surfactant system has a saturation value ≥ 1 and (c) the surfactant system can dissolve a substantial amount of vitamin E without causing an increase of the emulsion droplet size and (3) the saturation value and the stability of many vitamin E emulsion systems can be increased by adding an optimum amount of NaCI.     

Mixtures of n-octyl-beta-D-glucoside and triethylene glycol mono-n-octyl ether: phase behavior and micellar structure near the liquid-liquid phase boundary

The phase behavior and microstructure of aqueous mixtures of n-octyl-beta-D-glucoside (C8betaG1) and triethylene glycol mono-n-octyl ether (C8E3) is presented. C8betaG1 forms a one-phase micellar solution in water at surfactant concentrations up to 60 wt %, whereas mixtures with C8E3 show a liquid-liquid phase transition at low surfactant concentration. The position of this phase boundary for mixtures can be rationally shifted in the temperature-composition window by altering the ratio of the two surfactants. Small-angle neutron scattering is used to determine the size and shape of the mixed micelles and to characterize the nature of the fluctuations near the cloud point of the micellar solutions. The C8betaG1/C8E3 solutions are characterized by concentration fluctuations that become progressively stronger upon approach to the liquid-liquid phase boundary, whereas micellar growth is negligible. Such observations confirm previous views of the role of the surfactant phase boundary in tuning attractive micellar interactions, which can be used effectively to change the nature and strength of interparticle interactions in colloidal dispersions. Colloidal silica particles were then added to these surfactant mixtures and were found to aggregate at conditions near the cloud point. This finding is relevant to current strategies for protein crystallization.     

The Micellar Dissociation Concentration of Polyoxyethylene Mono N-Decylether Nonionic Surfactants in Water

This paper will demonstrate a method for determining the micellar dissociation concentration (MDC) of polyoxyethylene mono n-decylether nonionic surfactants in water. Turbidity and surface tension measurements were applied to commercial samples without further purification. Complicated curves containing turbidity maxima and surface tension minima were obtained for some impure samples. It is difficult to determine the critical micellar concentration (CMC) for impure surfactants. Explanations are given for all curves obtained for pure and impure samples and the method of identifying the MDC values is illustrated. It is more practical for surfactant users to determine the MDC values in their own laboratory than to search for the CMC values in the literature when the surfactants arc impure.     

The role of the surfactant head group in the emulsification process: binary (nonionic-ionic) surfactant mixtures

Dilute emulsions of dodecane in water were prepared under constant flow rate conditions with binary surfactant systems. The droplet size distribution was measured as a function of the mixed surfactant composition in solution. The systems studied were (a) the mixture of anionic sodium dodecyl sulfate (SDS) with nonionic hexa(ethyleneglycol) mono n-dodecylether (C12E6) and (b) the mixture of cationic dodecyl pyridinium chloride (DPC) with C12E6. At a constant concentration of SDS or DPC surfactant in solution (below the CMC) the mean emulsion droplet size decreases with the increase in the amount of C12E6 added to the solution. However, a sharp break of this droplet size occurs at a critical concentration and beyond this point the mean droplet size did not significantly change upon further increase of the C12E6. This point was found to corresponded to the CMC of the mixed surfactant systems (as previously determined from microcalorimetry measurements) and this result suggested the mixed adsorption layer on the emulsion droplet was similar to the surfactant composition on the mixed micelles. The emulsion droplet size as a function of composition at the interface was also studied. The mean emulsion droplet size in SDS-C12E6 solution was found to be lower than that in DPC-C12E6 system at the equivalent mole fraction of ionic surfactant at interface. This was explained by the stronger interactions between sulphate and polyoxyethylene head groups at the interface, which facilitate the droplet break-up. Counterion binding parameter (beta) was also determined from zeta-potential of dodecane droplets under the same conditions and it was found that (beta) was independent of the type of the head group and the mole fraction of ionic surfactant at interface.     

Characterization of mixed non-ionic surfactants n-octyl-β-D-thioglucoside and octaethylene-glycol monododecyl ether: micellization and microstructure

Mixed micelles of n-octyl-β-D-thioglucoside (OTG) and octaethylene-glycol monododecyl ether (C(12)E(8)), two non-ionic surfactants belonging to the alkyl glucosides and polyoxyethylene alkyl ether families, respectively, were investigated by using light scattering and fluorescence probe techniques. From the determination of the critical micelle concentration (cmc), by the well-established pyrene 1:3 ratio method, it was found that the mixed system behaves ideally, the micellization process being clearly controlled by the ethoxylated surfactant. The micellar hydrodynamic radius as a function of temperature, composition and concentration was obtained by dynamic light scattering measurements. It was observed that the micellar size increases with temperature, this growth being more pronounced as the relative proportion of the ethoxylated surfactant was increased. The behavior of the micellar size with the total surfactant concentration was also found to be dependent on temperature and composition. The clouding temperature, characteristic of the ethoxylated surfactants, was increased with the addition of the sugar surfactant. Lastly, possible structural changes in the micellar palisade layer were examined by steady-state fluorescence anisotropy in conjunction with time-resolved fluorescence studies with the hydrophobic probe coumarin 6 (C6). The obtained results indicate that the participation of the ethoxylated surfactant induces a slightly more polar palisade layer, whereas the probe carries out a faster rotational reorientation as a result of a less compact environment. All these observations were attributed to the different structure of the head groups of both surfactants and, as a consequence, to their different hydration.     

Phase behavior and nano-emulsion formation by the phase inversion temperature method

Formation of oil-in-water nano-emulsions has been studied in the water/C12E4/isohexadecane system by the phase inversion temperature emulsification method. Emulsification started at the corresponding hydrophilic-lipophilic balance temperature, and then the samples were quickly cooled to 25 degrees C. The influence of phase behavior on nano-emulsion droplet size and stability has been studied. Droplet size was determined by dynamic light scattering, and nano-emulsion stability was assessed, measuring the variation of droplet size as a function of time. The results obtained showed that the smallest droplet sizes were produced in samples where the emulsification started in a bicontinuous microemulsion (D) phase region or in a two-phase region consisting of a microemulsion (D) and a liquid crystalline phase (L(alpha)). Although the breakdown process of nano-emulsions could be attributed to the oil transference from the smaller to the bigger droplets, the increase in instability found with the increase in surfactant concentration may be related to the higher surfactant excess, favoring the oil micellar transport between the emulsion droplets.     

Self-assembly of mixed anionic and nonionic surfactants in aqueous solution

We present the phase diagram and the microstructure of the binary surfactant mixture of AOT and C(12)E(4) in D(2)O as characterized by surface tension and small angle neutron scattering. The micellar region is considerably extended in composition and concentration compared to that observed for the pure surfactant systems, and two types of aggregates are formed. Spherical micelles are present for AOT-rich composition, whereas cylindrical micelles with a mean length between 80 and 300 ? are present in the nonionic-rich region. The size of the micelles depends on both concentration and molar ratio of the surfactant mixtures. At higher concentration, a swollen lamellar phase is formed, where electrostatic repulsions dominate over the Helfrich interaction in the mixed bilayers. At intermediate concentrations, a mixed micellar/lamellar phase exists.     

Emulsification through surfactant hydration: the PIC process revisited

We have performed sudden composition changes on a (surfactant + oil + water) system by adding water to a (surfactant + oil) solution. This composition change quenches the system into a metastable oil-in-water emulsion with a population in the 100 nm range. The conditions for a successful quench are as follows: the initial water content should be below a boundary called the "clearing boundary" (CB), the final water content should be sufficiently beyond CB, and the quench should be fast. We have used high purity components to avoid the complex phase separation patterns that occur with low purity ingredients: the surfactant is octaethylenehexadecyl ether (C(16)E(8)) and the oil is hexadecane (C(16)). Under these conditions, we show that the pathway for this type of quench proceeds through the swelling of the reverse micellar phase by the added water and the formation of a sponge phase. Then, further water addition causes the nucleation of oil droplets in this sponge phase, with a size that matches the spontaneous curvature of the sponge phase. Part of the surfactant remains adsorbed on these droplets, and the rest is expelled as micelles that coexist with the droplets. It is concluded that a PIC emulsification will always lead to a bimodal size distribution with surfactant "wasted" in small micelles. This is in contrast with the more efficient PIT emulsification.     

Vesicle growth and deformation in a surfactant solution below the Krafft temperature

We have studied vesicle growth and deformation in aqueous solutions of nonionic surfactant C(16)E(7) below the Krafft temperature by means of an optical microscope. It has been found that vesicles become larger by fusing together, and that the growth rate is slower than that of the unilamellar vesicle or emulsion systems due to the multilamellar structures of shells in a vesicle. The deformation of the vesicles depends on the temperature quench depth, and we found the transformation from spherical vesicles to string-like domains at a certain quench-temperature. From the small angle X-ray scattering and confocal microscope experiments, it can be deduced that the deformation of vesicles would be induced by osmotic pressure due to the micellar concentration difference between inside and outside of vesicles.     

Experimental and theoretical investigation of the micellar-assisted solubilization of ibuprofen in aqueous media

Surfactants can be used to increase the solubility of poorly soluble drugs in water and to increase drug bioavailability. In this article, the aqueous solubilization of the nonsteroidal, antiinflammatory drug ibuprofen is studied experimentally and theoretically in micellar solutions of anionic (sodium dodecyl sulfate, SDS), cationic (dodecyltrimethylammonium bromide, DTAB), and nonionic (dodecyl octa(ethylene oxide), C12E8) surfactants possessing the same hydrocarbon "tail" length but differing in their hydrophilic headgroups. We find that, for these three surfactants, the aqueous solubility of ibuprofen increases linearly with increasing surfactant concentration. In particular, we observed a 16-fold increase in the solubility of ibuprofen relative to that in the aqueous buffer upon the addition of 80 mM DTAB and 80 mM C12E8 but only a 5.5-fold solubility increase upon the addition of 80 mM SDS. The highest value of the molar solubilization capacity (chi) was obtained for DTAB (chi = 0.97), followed by C12E8 (chi = 0.72) and finally by SDS (chi = 0.23). A recently developed computer simulation/molecular-thermodynamic modeling approach was extended to predict theoretically the solubilization behavior of the three ibuprofen/surfactant mixtures considered. In this modeling approach, molecular-dynamics (MD) simulations were used to identify which portions of ibuprofen are exposed to water (hydrated) in a micellar environment by simulating a single ibuprofen molecule at an oil/water interface (modeling the micelle core/water interface). On the basis of this input, molecular-thermodynamic modeling was then implemented to predict (i) the micellar composition as a function of surfactant concentration, (ii) the aqueous solubility of ibuprofen as a function of surfactant concentration, and (iii) the molar solubilization capacity (chi). Our theoretical results on the solubility of ibuprofen in aqueous SDS and C12E8 surfactant solutions are in good agreement with the experimental data. The ibuprofen solubility in aqueous DTAB solutions was somewhat overpredicted because of challenges associated with accurately modeling the strong electrostatic interactions between the anionic ibuprofen and the cationic DTAB. Our results indicate that computer simulations of ibuprofen at a flat oil/water interface can be used to obtain accurate information about the hydrated and the unhydrated portions of ibuprofen in a micellar environment. This information can then be used as input to a molecular-thermodynamic model of self-assembly to successfully predict the aqueous solubilization behavior of ibuprofen in the three surfactant systems studied.     

Determining the Critical Micelle Concentration in O/W Emulsion Using the Rate Constant of Hydrolysis for Benzyl Acetate

An attempt to evaluate the kinetically effective critical micelle concentration CMC of sodium dodecyl sulfate (SDS) in micellar solutions and in O/W emulsions at 40°C and pH 9 utilizing the pseudo first order rate constant of benzyl acetate hydrolysis was implemented. The critical micelle concentration of SDS in micellar solutions was determined by both surface tension measurements utilizing Wilhelmy plate technique and by rate constant of hydrolysis. Hydrolysis reaction of benzyl acetate was monitored in surfactant solutions as well as in o/w emulsions as a function of time. Emulsion droplets were controlled using microfluidizer 110 T and oily droplets were separated from the emulsion by ultracentrifugation at (11,500 rpm or 9,800 g) prior to analysis by high performance liquid chromatography. The value of the critical micelle concentration (CMC) in micellar solutions in the presence of benzyl acetate as determined from the Wilhelmy plate technique was 7.8 × 10^?4 moles/L (CMC in micellar solution was 10 times lower than the value in literature due to use of buffer) while the CMC as determined from the kinetic study was 8.8 × 10^?4 moles/L. In emulsion systems, using 5% mineral oil, the CMC value was 8.6 × 10^?3 moles/L and at 10% oil, the value doubled to 1.73 × 10^?2 moles/L. The above results indicate that kinetics can be used to determine CMC in micellar solutions and in o/w emulsions.     

ATMOSPHERIC OXIDATION OF VITAMIN C AND E IN THE SURFACTANT SYSTEM

The association of ascorbic acid (H₂A, AA) with α-tocopherol (α-T) enhances the antioxidant capability of the two vitamins in a microemulsion formed by water, pentanol and sodium dodecyl sulphate. The comparison of vitamin C oxidation kinetics in the presence and absence of α-T in the system shows that there are no tangible differences in the time of the ascorbic acid decomposition. The main effect is the enhancement of the H₂A oxidation with increased pentanol concentration in the system. Ascorbic anion interacts with surfactant association structures, changing W/O microemulsion towards O/W system. The polar part of H₂A is exposed to aqueous solution and subjected to oxidation.

For the vitamin C to vitamin E weight ratio equal 0.13, ascorbic acid stimulates α-T decomposition as long, as it does in the non-oxidised form. When not all amounts of both vitamins are decomposed their activity is similar to each other in the anionic surfactant system. It means they are both good antioxidants in W/O microemulsions. After some time, when vitamin C is completely decomposed, all these micellar systems become more stable with regard to vitamin E. Vitamin E occurs to be a better antioxidant in O/W microemulsion.     

Highly Concentrated Emulsions: Physicochemical Principles of Formulation

This review deals with the preparation, stability, rheology and different applications of highly concentrated emulsions. These emulsions, which are known as high internal phase ratio emulsions (HIPRE), gel-emulsions, biliquid foams, etc., containing over 90% internal phase by volume, have a swollen micellar (L₁ or L₂) solution of nonionic or ionic surfactants as a continuous phase. These emulsions have the structure of biliquid foams and behave as gels since they present viscoelastic and plastic properties. The functional macroscopic properties of gel-emulsions are dependent on the structural parameters of the microemulsion continuous phase as well as of the interfacial properties (interfacial tension, bending modules, spontaneous curvature) of surfactant monolayers. The depletion interaction between emulsion droplets due to the non-compensated osmotic pressure of micelles is revealed as a main factor, along with surface forces, which determine the aggregative stability and the rheological properties of these emulsions. The effect of electrolyte and surfactant concentration, temperature, as well as other physicochemical parameters on the fiocculation threshold, stability, and yielding properties of highly concentrated emulsions is explained by the effect of these parameters on the critical micelle concentration (CMC) and the aggregation number of surfactants, and, consequently, on the depletion interaction. The thermodynamic theory of adhesion of fluid droplets in micellar solution and the suggested model of elasticity of gel-emulsions permit to explain the effect of mentioned physicochemical parameters on the elasticity modulus, the plastic strength and the linear deformation of these emulsions. A novel mechanism for the spontaneous formation of gel-emulsions by the phase inversion temperature (PIT) route is suggested, allows the selection of ternary systems able to yield these emulsions, and explains how the droplet size can be controlled during the PIT process. An original model for liquid film rupture is also suggested, and allows the prediction of the effect of structural parameters of micellar solutions and the interfacial properties of surfactant monolayers on the stability of gel-emulsions.     

A CO2-Switchable Polymer Surfactant Copolymerized with DMAEMA and AM as a Heavy Oil Emulsifier

A CO₂-switchable polymer surfactant was synthesized with acrylamide (AM) and 2-(dimethylamino)ethyl methacrylate (DMAEMA). The changes in conductivity, particle size, and ζ-potential were adopted to illustrate its switchability. The CMC of the surfactant was determined by the break point of the curve of surface tension versus concentration. An oil emulsion with 8 g/L surfactant almost reached the highest stability. The thermodynamic stability of the emulsion decreased sharply upon increase of the temperature. Adding an inorganic salt was hard to affect the emulsion stability because the surfactant is non-ionic. The emulsion could maintain its stability even if the concentration of NaCl was as high as 10 g/L. The emulsion could easily be broken by bubbling CO₂. Its dehydration rate was 155 times faster than that without the presence of CO₂, and the amount of residual oil in water was only 32.22 ppm, which displayed brilliant performance of de-emulsification.     

Self-aggregation and supramolecular structure investigations of Triton X-100 and SDP2S by NOESY and diffusion ordered NMR spectroscopy

The self-aggregation and supramolecular micellar structure of two surfactants in aqueous solution, the anionic surfactant SDP2S (sodium dodecyl dioxyethylene-2 sulfate) and the nonionic surfactant Triton X-100 (octylphenol-polyoxyethylene ether with 9.5 ethoxy groups), were investigated by NMR spectroscopy. The critical micellar concentration (CMC), the size, and shape of the aggregates were determined by diffusion ordered NMR spectroscopy (DOSY), while 2D NOESY NMR spectra were used to study the mutual spatial arrangement of surfactant molecules in the aggregated state. A nonlinear increase of the micellar hydrodynamic radius, indicating possible sphere-to-rod shape transition, was found for SDP2S at higher surfactant concentrations. Triton X-100 micelles were found to be almost spherical at low surfactant concentrations, but formation of ellipsoid shaped particles and/or micellar aggregation was observed at higher concentrations. The NOESY data show that at low concentration Triton X-100 forms a two-layer spherical structure in the micelles, with partially overlapping internal and external layers of Triton X-100 molecules and no distinct hydrophilic-hydrophobic boundary.     

Effect of homologous series of alkyl sulfate surfactants on the emulsion polymerization of styrene. I. Equal total concentrations

There is evidence in the literature that the rates of emulsion polymerization increase by a large factor as the alkyl chain length increases for a homologous series of surfactants. However, the area occupied by a surfactant molecule in a saturated monolayer at the polystryene/water interface is independent of chain length for alkyl sulfates so that, on the basis of Gardon's theory, equal rates of polymerization would be expected when equal concentrations of surfactants are used. There is a large increase in the number of polymer latex particles formed and in the rate of emulsion polymerization as the surfactant concentration is increased through the critical micelle concentration; this accounts for the large increases reported, because the lower members of the homologous series are below their critical micelle concentrations in most of the published studies. When a common concentration is chosen that is above the critical micelle concentration even for the lowest member of the series, only a relatively small increase in latex particle number and rate of emulsion polymerization with alkyl chain length of the surfactant is observed. This is attributable to an increase in the concentration of surfactant micelles. Good agreement with Gardon's theory is obtained when the concentration of micellar surfactant is used instead of the total surfactant concentration.     

Role of the nonionic surfactant Triton X-405 in emulsion polymerization. I. Homopolymerization of styrene

The emulsion polymerization of styrene was studied using the nonionic surfactant Triton X-405 (octylphenoxy polyethoxy ethanol). Two separate nucleation periods were noted in these polymerizations resulting in bimodal final latex particle size distributions. The partitioning of the surfactant between the phases was found to play the major role in determining the nucleation mechanism(s) in these polymerizations. Although the total concentration of the emulsifier was always added at a level above its critical micelle concentration (CMC) based on the water phase in the recipe, it was found that the portion of the surfactant initially present in the aqueous phase was below its CMC due to the partitioning. This CMC was also found to increase with increasing total surfactant because the distribution of the surfactant (varying ethylene oxide chain length) depended on the partitioning between the phases. Under these conditions, the first of the two nucleation periods was attributed to homogeneous nucleation, while the second was attributed to micellar nucleation. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3813–3825, 1997     

Volumes and compressibilities of pentanol in aqueous dodecyltrimethylammonium bromide solutions at 15, 25 and 35°C

Ultrasonic velocities and densities of the water-dodecyltrimethylammonium bromide (DTAB)-pentanol (PentOH) ternary system were measured at 15, 25 and 35°C as a function of the surfactant and alcohol concentrations. The apparent molar volumes and isentropic compressibilities of PentOH were calculated. The standard partial molar volumes increase with surfactant concentration continuously whereas the standard partial molar isentropic compressibilities show sharp changes in slope at about 0.25 mol-kg^–1 DTAB, which can be ascribed to a micellar structural transition. The volume data for alcohol in micellar solutions were treated by a model reported for the distribution of polar additives between aqueous and micellar phases. In the application of the model to compressibility, the contributions due to the pressure effect on the shift of both the micellization equilibrium and the alcohol distribution constant cannot be neglected. This is in contrast to what is found in the case of heat capacity. The distribution constant and the partial molar volumes and compressibilities of PentOH in the micellar phase have been derived by linear regression. Also, the apparent molar volumes and isentropic compressibilities of DTAB in water-pentanol mixed solvents at fixed composition have been calculated. These properties as a function of the surfactant concentration show maxima depending on the temperature and the mixed solvent composition. The decrease beyond the maximum can be attributed to the extraction of PentOH from the aqueous into the micellar phase, where its concentration tends to zero with the progressive increase of the surfactant concentration. As a consequence, by increasing the surfactant concentration, the apparent molar properties of the surfactant in the mixed solvent shifts towards the value in water.