On the incorporation of the non-steroidal anti-inflammatory naproxen into cationic O/W microemulsions

Microemulsions (ME) containing hexadecyltrimethylammonium bromide (HTAB)/ethanol as surfactant, isopropylmyristate (IM) or butylstearate (BS) as oil phase and aqueous buffer were studied. Pseudo-ternary phase diagrams of the investigated systems were obtained at constant surfactant/cosurfactant molar ratio (1:5) by titration in order to characterize the proportions between the components to obtain clear systems. Oil in water microemulsions were prepared in a wide range of phase volume (phi). UV-vis absorption spectra of naproxen at pH 5.5 showed that the solubility of Np increases significantly in the presence of O/W ME in high phase volumes. For both, IM and BS microemulsions, the dynamic light scattering experiments showed that the size of the oil droplets remains constant in low values of phi, increasing abruptly in high phi values. Phase solubility study revealed that for both IM and BS microemulsions, the drug incorporation followed a straight-line profile in all range of phi. The data could be analyzed through the phase-separation model and the association constants (K) calculated varied from 27 to 90 M(-1), depending on the pH and on the microemulsion oil phase.     

Observation of two diffusive relaxation modes in microemulsions by dynamic light scattering

Water-in-oil microemulsions stabilized by AOT and dispersed in n-alkane oils with a constant molar water-to-surfactant ratio were studied by dynamic light scattering. A dilution series (in the range of volume fraction of water plus surfactant, phi approximately 0.02-0.52) was used, which allowed us to extract information about droplet sizes, diffusion coefficients, interactions, and polydispersity from experimental data. We report the observation of two diffusive relaxation modes in a concentrated microemulsion (0.20 < phi < 0.5) due to density (collective diffusion) and concentration or polydispersity (self-diffusion) fluctuations. Below this concentration it was difficult to resolve two exponentials unambiguously, and in this case one apparent relaxation mode was observed. It was found that for a given composition self-diffusion is more pronounced in apparent relaxation mode for a shorter chain length alkane. The concentration dependence of these diffusion coefficients reflects the effect of hard sphere and the supplementary attractive interactions. It was observed that the attractive part becomes more pronounced in the case of a large alkane chain oil at a given temperature. This explains the shift of the region of microemulsion stability to lower temperatures for higher chain length alkanes. Increase in hydrodynamic radius, Rh, obtained from the diffusion coefficient extrapolated to infinite dilution was observed with increase of alkane chain length. The polydispersity in microemulsion systems is dynamic in origin. Results indicate that the time scale for local polydispersity fluctuations is at least 3 orders of magnitude longer than the estimated time between droplet collisions.     

Properties of surfactant monolayers in relation to microemulsion phase behaviour

The relationship between the properties of surfactant monolayers at oil-water interfaces and the phase behaviour in bulk of mixtures of oil + water + surfactant is discussed. Such monolayer properties include the spontaneous curvature, c_o the interfacial tension, I γ, the elasticity K (or rigidity) associated with the mean curvature, and the elasticity K associated with the Gaussian curvature. The model system chosen for investigation is the anionic surfactant AOT + aqueous NaCl + n-alkane at 20°C. In such systems, inversion of microemulsion type from oil-in-water (o/w) to water-in-oil (w/o) is possible with increasing electrolyte concentration. The tension, γ, passes through an ultralow minimum value at conditions corresponding to the formation of three phases. Using small angle neutron scattering, we have determined the structure of surfactant-rich third phases (c_o ~ 0) formed with the different alkanes. Lamellar phases consisting of surfactant monolayers separated alternately by oil and water appear with short alkanes, whereas L₃ and bicontinuous phases form in systems containing longer alkanes. The bending elasticity K has been measured for planar monolayers at the oil-water interface by ellipsometry. K is independent of salt concentration but depends markedly on alkane chain length N, falling from ~ 1 k_BT for N < 11 to ~0.1 k_BT for N = 14. This is discussed in terms of the differing extents of oil penetration into the surfactant chains. Higher rigidities favouring lamellar phases and lower rigidities favouring bicontinuous microemulsions are in line with the theoretical predictions of de Gennes and Taupin. Estimates of the constant K have been obtained in droplet microemulsions (w/o) from a knowledge of their size, K and γ. The sign of the constant is in agreement with the geometry of the phases formed in three phase systems. Finally, the ideas and concepts developed in the oil-water systems described above are used to explain the wetting behaviour by alkanes of AOT monolayers at the air-water surface.     

Microstructure and structural transition in microemulsions stabilized by aldonamide-type surfactants

Significant efforts were undertaken to characterize the microstructure and structural properties of water-in-oil (w/o), oil-in-water (o/w), and bicontinuous (bc) microemulsions composed of N-alkyl-N-methylgluconamides (n-alkyl = n-C(12)H(25), n-C(14)H(29), n-C(16)H(33)) and n-alcohols (ethanol, n-propanol, n-butanol) or iso-alcohols (iso-propanol, iso-butanol) as cosurfactants, as well as iso-octane and water. The internal structure of so created four-component system was elucidated by means of an analysis of isotropic area magnitudes in phase diagrams and conductivity measurements. Dynamic light scattering (DLS) measurements provided the microemulsion size and polydispersity. Polarity and viscosity of microemulsion microenvironment were acquired by means of electron paramagnetic resonance (EPR), UV-vis absorption spectroscopy (in the case of w/o droplets), and steady-state fluorescence (SSF) (in the case of o/w droplets). The results show that both the surfactant and the cosurfactant types affect the shape and extent of microemulsions. The size of droplets depends strongly on the type of examined microemulsion and the type of cosurfactant (linear or brunched) but is almost independent of the length of the surfactant alkyl chain. The size of microemulsion droplets ranges from 8.1 to 22.6 nm and from 3.7 to 14.3 nm respectively, for o/w and o/w microemulsions, making them good candidates for both template-based reactions and household components solubilizing media.     

Effects of Mixed Anionic-Cationic Surfactants and Alcohol on Solubilization of Water-in-oil Microemulsions

The solubilization of water in w/o microemulsions formed with mixed-surfactants containing one anionic and one cationic surfactant and alcohol was studied as a function of alkyl chain length of oil (C6 to C16), mixed-surfactant (sodium dodecyl sulfate, SDS, and cetyltrimethylammonium bromide, CTAB, or cetylpyridinium bromide CPB), and alcohol (1-butanol, 1-pentanol, 1-hexanol). The results show that the solubilization of water in microemulsion systems increases significantly with the mixed-surfactants due to the synergistic effect resulting from the strong Coulombic interactions between cationic and anionic surfactants and the solubilizing efficiency increases as the chain length or concentration of alcohol increases. With increasing the oil chain length the solubilization for water increases, decreases, and has the chain length compatibility effect when the systems contain 1-hexanol, 1-butanol, 1-pentanol, respectively. The total solubilizing capacity increases as the surfactant concentration (keep the ratio of SDS to butanol constant) increases.     

RELATIVE WATER VAPOR PRESSURE OF WATER - IN - OIL MICROEMULSIONS BY HEADSPACE GAS CHROMATOGRAPHIC ANALYSIS

The headspace gas chromatography has been used for the determination of relative water vapor pressures of microemulsions consisting of Aerosol OT and various alkanes. The relative water vapor pressure of the micromulsions do not seem to vary significantly as a function of Chain function of chain length of n-alkane, i. e.from heptane to dodecane. This is in sharp contrast with its water solubilization capacity which increases markedly from the systems of heptane to nonane and it decreases sharply to that of dodecane. The results are explained in terms of the pseudophase model, and the effects of curvature and attraction on the microemulsion droplets.     

Phase behavior and microstructure of microemulsions with a room-temperature ionic liquid as the polar phase

Microemulsions of nonionic alkyl oligoethyleneoxide (CiEj) surfactants, alkanes, and ethylammonium nitrate (EAN), a room-temperature ionic liquid, have been prepared and characterized. Studies of phase behavior reveal that EAN microemulsions have many features in common with corresponding aqueous systems, the primary difference being that higher surfactant concentrations and longer surfactant tailgroups are required to offset the decreased solvophobicity the surfactant molecules in EAN compared with water. The response of the EAN microemulsions to variation in the length of the alkane, surfactant headgroup, and surfactant tailgroup has been found to parallel that observed in aqueous systems in most instances. EAN microemulsions exhibit a single broad small-angle X-ray scattering peak, like aqueous systems. These are well described by the Teubner-Strey model. A lamellar phase was also observed for surfactants with longer tails at lower temperatures. The scattering peaks of both microemulsion and lamellar phases move to lower wave vector on increasing temperature. This is ascribed to a decrease in the interfacial area of the surfactant layer. Phase behavior, small-angle X-ray scattering, and conductivity experiments have allowed the weakly to strongly structured transition to be identified for EAN systems.     

Solubilisation of triolein by microemulsions containing C12E4/hexadecane/water: equilibrium and dynamics

Increasing triolein content of oil-in-water microemulsions in the pure C(12)E(4)/water/n-hexadecane/triolein system while maintaining a fixed surfactant concentration and volume fraction of drops raises the temperature of the solubilisation boundary, where excess oil separates, but has only a slight effect on the (higher) cloud point temperature, where excess water appears. Thus, the temperature range of the single-phase microemulsion shrinks and ultimately disappears. When such microemulsions are in equilibrium with excess oil, the hexadecane/triolein ratio is greater in the microemulsion, probably because the larger triolein molecules are unable to penetrate the hydrocarbon chain region of the surfactant films of the microemulsion droplets. Indeed, monolayer studies and calculations based on microemulsion and excess oil compositions indicate that the films have minimal triolein and similar ratios of hexadecane to surfactant. Triolein drops brought into contact with hexadecane-in-water microemulsions first swell as they incorporate hexadecane, then shrink owing to solubilisation. Interfacial tension decreases during this process until it becomes almost constant near 0.01 mN m(-1), suggesting that the drops in the final stages of solubilisation have high hexadecane contents. A microemulsion containing 10 wt% C(12)E(4) and 15 wt% hexadecane was able to remove over 50% of triolein from polyester fabric at 25 degrees C, more than twice that removed by an oil-free solution with the same surfactant concentration in similar experiments.     

Ionic liquids in microemulsions: Formulation and characterization

In the last few decades ionic liquids (ILs) have been widely considered as a “green solvents” and they are used in various fields. ILs can be used in the formation of microemulsion as a dispersed medium, polar domain and recently as a surfactant. In this particular review our discussion is about the novel IL-based aqueous and non-aqueous microemulsions which are quite fascinating and interesting research field for scientists. Synthesis of double and triple chain containing surface active ionic liquid (SAILs) and formation of microemulsion as a surfactant with ILs as a polar core have been elaborated in this review. ILs with a certain surface activity having long alkyl chain substituents can self-aggregate and form ILs microemulsion with high-temperature stability and temperature insensitivity. Characterization of these ILs in oil microemulsion and different ultrafast processes which are performed inside these characterized systems are documented very well. We have highlighted the similarities and differences between the nonaqueous microemulsions and the aqueous microemulsions. Addition of water and effect of temperature are quite important in case of the ILs containing microemulsions.     

Recent advances in microemulsion electrokinetic chromatography

Microemulsion electrokinetic chromatography (MEEKC) is an electrodriven separation technique. Separations are typically achieved using oil-in-water microemulsions, which are composed of nanometre-sized droplets of oil suspended in aqueous buffer. The oil droplets are coated in surfactant molecules and the system is stabilised by the addition of a short-chain alcohol cosurfactant. The novel use of water-in-oil microemulsions for MEEKC separations has also been investigated recently. This report summarises the different microemulsion types and compositions used to-date and their applications with a focus on recent papers (2002-2004). The effects of key operating variables (pH, surfactant, cosurfactant, oil phase, buffer, additives, temperature, organic modifier) and methodology techniques are described.     

Surfactant Chain Length Effect on the Structural Parameters of Nonionic Microemulsions

In this study we estimated the structural parameters of (water+propylene glycol)/sucrose esters/(benzaldhyde+ethanol) systems. The weight ratios of water/propylene glycol and that of benzaldhyde/ethanol equal 2 and 1, respectively. The sucrose esters were sucrose laurates (L595, L1695, and SM1200), sucrose myristate (M1695), sucrose palmitate (P1670), sucrose oleate (O1570), and sucrose stearate (S1570). The pseudoternary phase behavior at 37°C was explored to determine the extension of the microemulsion phase regions. A one‐phase microemulsion region extending from the oil rich region to the water rich corner was observed in these systems. It was found that minor changes in the surfactant chain length, structure, and composition suffice to provoke a considerable change in the aggregation number, core radius and interfacial area per surfactant and cosurfactant molecules head groups in the formed microemulsions. The interfacial area per surfactant head group decreases with the increase in the surfactant chain length. For a sucrose ester with a given chain length the interfacial area per surfactant head group decreases with the increase in the surfactant monoester content. The interfacial area per surfactant head group increases with the increase in the surfactant concentration and the water core volume in the formulated microemulsions.     

Partition coefficients of nonionic surfactants in water/n-alkane systems

In water/oil systems, surfactants partition between the water phase and the oil phase according to their solubility in both phases. The ratio between the concentration of the surfactant in the oil phase and in the water phase at equilibrium is known as the partition or distribution coefficient (K(p)). The partition coefficient (K(p)) is an important fundamental parameter essential to understanding and controlling phenomena in water-oil-surfactant systems under both equilibrium and non-equilibrium conditions. In the present work we report on the partitioning of three different classes of nonionic surfactants in the pre-cmc regime, namely polyoxyethylene alkyl ethers (C(i)E(j)), alkyl dimethyl phosphine oxides (C(n)DMPO) and alkyl glycosides (β-C(n)G(m)) between water and different n-alkanes. We focus on the influence of the surfactant's molecular structure (alkyl chain length, head group size and type), and oil chain length on K(p) to derive systematic structure-property relationships. Moreover, we discuss the influence of the surfactant purity on partition coefficients of technical grade alkyl glycosides and polyoxyethylene alkyl ethers, respectively.     

Temperature-induced phase separation in pseudoternary mixtures of Triton X-100–butanol–kerosene–water

The transparent Winsor IV domain in the phase diagram of the mixtures of emulsifier (Triton X-100 and butanol), oil (kerosene), and water is found to be 34% of the total phase diagram in presence of emulsifier with surfactant:cosurfactant::1:1, and is water dominant. Increase in cosurfactant/surfactant ratio inverts the Winsor IV domain to become oil rich. The plot of conductance of the microemulsions prepared by substituting water by brine against water content depicts the existence of three distinct phases like oil-in-water, bicontinuous, and water-in-oil microemulsion in the phase diagram. The phase contrast micrographs of the mixtures of different compositions in these three different phases reveal the existence of microdroplets of oil dispersed in water and water dispersed in oil. Further, the dynamic light scattering studies of these solutions reveal an inhomogeneity in the size distribution of the droplets. A temperature-induced clouding in the microemulsion domain leading to phase separation has been observed. Additives like glucose, sucrose, and sodium chloride decrease the cloud point (CP), while addition of ammonium thiocyanate increases it. A quantitative relationship of the clouding temperature with the composition of the microemulsion has been established. With increase in oil and emulsifier, the cloud point of the microemulsion increases. The separated phases after the clouding have been used for preconcentration of water-soluble metal ions as well as oil-soluble dyes. The turbid systems on heating led to separation into three isotropic phases which are found to be stable at ambient temperature. The stability of these phases is ascribed to the formation of stable microemulsions by mass transfer from one phase to other.     

Alkyl polyglucoside microemulsion phase behavior

We review several key elements of alkyl polyglucoside (C_mG_n) microemulsion phase behavior. The low solubility of C_mG_n surfactants in oils such as alkanes makes producing C_mG_n microemulsions and subsequent study of their phase behavior difficult. Increasing the solubility of C_mG_n in oil is therefore helpful for the systematic study of C_mG_n-based microemulsion formulations. To this end, the role of cosurfactants in producing microemulsions with water, alkanes, and n-alkyl β-d-glucopyranosides is first discussed. Adding C₁₀βG₁ to mixtures of water–alkane–ethoxylated alcohol surfactants (C_iE_j) produces a region of the three-phase body (a ‘chimney’) that is independent of temperature; thus C_mβG₁ are not completely soluble in the co-oil formed of alkane and C_iE_j at higher temperatures. Then, through a novel approach using oxygenated ether oils (C_kOC₂OC_k), microemulsions are formed with water, C_kOC₂OC_k, and C_mβG₁ and the phase behavior studied as a function of temperature and composition. Increased C_mβG₁ solubility in the more hydrophilic ether oils produces patterns of phase behavior in water–C_kOC₂OC_k–C_mβG₁ mixtures that are identical to those observed in water–alkane–C_iE_j mixtures. Using the water–ether oil–C_mβG₁ mixtures as a base case, the role of C_mG_n surfactant structure in setting C_mG_n microemulsion phase behavior is explored. The solubility of the α-d anomer (n-alkyl α-d-glucopyranosides, C_mαG₁) in water is much less than that of the β-d surfactant, and these solubility boundaries extend to high surfactant and oil concentrations in water–C_kOC₂OC_k–C_mαG₁ mixtures. Adding C_mG₂ compounds to water–C_kOC₂OC_k–C_mβG₁ mixtures shifts the phase behavior to high temperatures, again demonstrating the extreme hydrophilic nature of the sugar headgroup. Finally, adding small amounts of ionic alkyl sulfate surfactants to water–C_kOC₂OC_k–C_mβG₁ mixtures dramatically reduces the total amount of surfactant needed to form a single-phase microemulsion.     

Viability and permeability across Caco-2 cells of CBZ solubilized in fully dilutable microemulsions

The purpose of this study was to evaluate the viability and permeability of carbamazepine (CBZ) solubilized in fully dilutable non-ionic microemulsions across Caco-2 cells used as a model for intestinal epithelium. Maximum solubilization capacity (SC) of CBZ was determined within water-in-oil (W/O), bicontinuous and oil-in-water (O/W) structures formed upon dilution. The effect of the nature of the oil phase, surfactant type, and the ratio between the oil phase and surfactant on the quantity of solubilized CBZ, droplets size, the viability of the cells and drug permeability was elucidated. We found that: (1) several fully dilutable microemulsions based on pharma-grade ingredients can be loaded with very significant amounts of CBZ, (2) W/O microemulsions (10wt% water) exhibit up to 3-fold higher solubilization capacity over the drug's solubility in oil (triacetin), (3) CBZ in the O/W microemulsions (80wt% water) exhibit up to 29-fold higher solubilization than in water, (4) the O/W droplets of the examined systems are 9-11nm in size, (5) the highest permeability was obtained in systems containing triacetin/alpha-tocopherol acetate/ethanol in 3/1/4wt% ratio as oil phase and Tween 60 as surfactant, (6) the replacement of alpha-tocopherol acetate by alpha-tocopherol inhibits CBZ release, (7) replacement of a saturated chain of Tween 60 by an unsaturated (Tween 80) or shorter chain (Tween 40) inhibited drug release, (8) the decrease in the oil phase to surfactant ratio leads to enhancement of drug release (dilution line 64>dilution line 73).     

微乳液脱除油泥砂中的原油

以煤油为油相, 十二烷基苯磺酸钠(SDBS)为主表面活性剂, 正戊醇为助表面活性剂, 通过改变NaCl浓度分别制备出上相、 中相和下相3种类型的微乳液, 研究了其对油泥砂中原油的脱除效率, 考察了微乳液类型、 油泥砂处理量、 时间和温度等的影响. 结果表明, 油相在下相微乳液中的脱油效率最高, 在中相中次之, 在上相中最低, 且油相脱油率与油泥砂处理量间具有良好的线性关系; 在所考察的时间(10~60 min)和温度(20~70℃)范围内, 脱油效率变化幅度不显著, 表明该体系可对油泥砂进行常温快速脱油处理.     

咪唑啉型表面活性剂组成微乳液的热力学性质

由咪唑啉型表面活性剂、醇、水和正十六烷组成微乳液，探讨了该微乳液中醇从油相转移到界面相时的自由能变化及温度对自由能变化的影响，算得了熵变和焓变。     

Interfacial composition, structural parameters and thermodynamic properties of water-in-oil microemulsions

The formation and structural characteristics of water-in-oil microemulsions comprising hexadecylpyridinium chloride (CPC), alkanols (C₄–C₆) and alkanes (C₅, C₈–C₁₀) have been investigated by the method of dilution. The compositions of the surfactant and the cosurfactant in the interfacial region (interphase) of the microemulsion droplets have been determined. The thermodynamics of transfer of the cosurfactants (alkanols) from the continuous oil (alkane) phase to the interface have been evaluated from dilution measurements at different temperatures. The structural parameters, radii of the droplet and the waterpool, aggregation numbers of CPC and the alkanols in the interphase of a droplet, and the nanoparticle density of solution have been estimated assuming monodispersity of the droplets. The thermodynamics and structural parameters have been examined in terms of the chain lengths of the alkanols and alkanes. Received: 12 September 2000 Accepted: 27 October 2000     

MICROEMULSION SYSTEMS WITH A NONIONIC COSURFACTANT

The conditions to obtain W/0 microemulsions using ionic surfactants and a nonionic cosurfactant, a polyoxyethylene alkyl ether, were investigated. The length of the polyoxyethylene chain was critical to obtain the typical water solubilization maximum

The variation of the W/0 microemulsion region with hydrocarbon content was different from that of the usual type of microemulsions having a medium chain length alcohol as cosurfactant. In the present systems the W/0 microemulsion region was not a direct continuation of the inverse micellar area at zero content of hydrocarbon. Addition of hydrocarbon was necessary for the formation of inverse micelles

The microemulsion regions were sensitive to the kind of hydrocarbon used; a sign of the importance of the nonionic surfactant for the stability of this kind of microemulsions.     

Phase behavior of monoglycerol fatty acid esters in nonpolar oils: reverse rodlike micelles at elevated temperatures

We have studied nonaqueous phase behavior and self-assemblies of monoglycerol fatty acid esters having different alkyl chain lengths in different nonpolar oils, namely, liquid paraffin (LP 70), squalane, and squalene. At lower temperatures, oil and solid surfactants do not mix at all compositions of mixing. Upon an increase in the temperature of the surfactant system, the solid melts to give isotropic single or two-liquid phases, depending on the nature of the oil and the surfactant. All monolaurin/oil systems form an isotropic single-phase liquid, but with a decreasing alkyl chain length of surfactant, they become less lipophilic and immiscible in oils. As a result, a two-phase domain is observed in the oil rich region of all monocaprylin/oil systems over a wide range of concentrations. Judging from the phase diagrams, the surfactants are the least miscible with squalane, and the order of miscibility tendency is squalene > LP 70 > squalane. With a further increase of temperature, the solubility of the surfactant in the oil increases, and the two-liquid phase transforms to an isotropic single phase. This phase transformation corresponds to the reverse of the cloud-point phenomenon observed in aqueous nonionic surfactant systems. Small-angle X-ray scattering (SAXS) measurements show the presence of reversed rodlike micelles in the isotropic single phase, and the length of the aggregates decreases with increasing temperature and increasing alkyl chain length of the surfactant. These results indicate a rod-sphere transformation with increasing lipophilicity of the surfactant and confirms the validity of Ninham's penetration model in the reversed system. An addition of a small amount of water dramatically enhances the elongation of the reverse micelles. Increasing the surfactant concentration or changing the oil from squalene to LP 70 also increases the length of the rodlike aggregates.