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.     

Pulsed laser polymerization of styrene in compartmentalized systems

Pulsed laser polymerizations of styrene were performed in microemulsion droplets and in latex particles. From the molecular weight distribution of the resultant polymer either the propagation rate coefficient in the droplets or the monomer concentration in the particles could be determined. Furthermore for microemulsion droplets with a low initiator concentration the rate coefficient for transfer to monomer could be determined. Because in very small particles the termination rate can be very high, instantaneous termination may occur, leading to a shift of the low molecular weight inflection point as the best measure of the propagation rate coefficient to the maximum in the molecular weight distribution. forming pulsed laser polymerization experiments directly in emulsion systems gives specific information on the special features associated with doing polymerizations in heterogeneous and compartmentalized systems.     

Polymerization of oil (styrene and methylmethacrylate)-in-water microemulsions

 The polymerization of styrene-in-water and methylmeth-acrylate-in-water microemulsions stabilized by nonionic surfactants was investigated using different initiation techniques. Thermally induced initiation was carried out using potassium persulfate (water soluble) and azobisiso-butyronitrile (AIBN) (oil soluble) at 60° and 50°C, respectively. When the monomer concentration was kept below a certain limit, the particle size of the nanolatex was similar to the droplet size of the microemulsion precursor. At higher monomer concentrations, the latex produced was significantly larger than the microemulsion droplets, as a result of the possible coalescence of the microemulsion droplets during polymerization. By using chemically induced polymerization (hydrogen peroxide+ascorbic acid) at temperatures below the cloud point temperature of the microemulsion or by photochemically induced initiation at room temperature, it was possible to obtain nanolatex particles with similar size to the droplets up to 10% monomer content. In all cases, the particle size was determined using photon correlation spectroscopy (PCS). Electron micrographs of the microlatex particles were taken and these confirmed the measurements obtained by PCS. The molecular weight of the polymers produced was determined by gel permeation chromatography. The average number of polymer molecules per particle was calculated. It was shown in some cases that the nanolatex contained one polymer chain per particle. A mechanism was suggested for polymerization and particle growth. Received: 29 May 1997 Accepted: 28 May 1998     

Some Emulsion Features

Traditionally, emulsions have been defined as consisting of two liquids, of which one is dispersed in the other as macroscopic drops, stabilized by mono‐molecular layer of surfactant at the interface. However, a large majority of commercial emulsions are more complex than so and the added elements are essential for the properties of the emulsions including their stability.

With this in mind, this treatment of emulsions is divided into emulsions with mono‐layers and multilayers at the interface. In addition, additional elements in emulsions are described; such as lyotropic liquid crystals, vesicles, microemulsion droplets and solid particles, and their potential influence on the emulsion properties is indicated.     

Length scale of polyisopirene and dynamic light scattering and structure of sodium-2-diethylhexyl sulfosuccinate/water/decane

The mixture of polyisopirene (PI) and sodium-2-diethylhexyl sulfosuccinate /decane/water microemulsion (ME) at AOT to water molar ratio (X = 30) and droplet mass fraction (m_f,drop = 0.08) was studied with dynamic light scattering and small-angle X-ray scattering (SAXS). The light scattering was used to obtain the diffusion coefficient of Brownian motion of the nano-droplets at different polymer concentrations and molecular weights (1000 and 4700) in the ME. The dynamics of the nano-droplets decreased with the increase of molecular weight (from 1000 to 4700) and concentration (from 0.01 to 0.09) of PI. The study of the structure by SAXS showed that with increase of PI (MW = 1000) mass fraction from 0.01 to 0.09 at ME, the size of the droplets changes from 4.5 to 4.3 nm and with increase of PI (MW = 4700) concentration at ME, the size of droplets changes from 4.8 to 4.4 nm. The size ratio of droplets to polymer decreased with increase of concentration and molecular weight of polymer and also the interaction between the droplets increased with increase of polymer concentration.     

Drying of DNA droplets

The evaporation kinetics of droplets containing DNA was studied, as a function of DNA concentration. Drops containing very low DNA concentrations dried by maintaining a constant base, whereas those with high concentration dried with a constant contact angle. To understand this phenomenon, the distribution of the DNA inside the droplet was measured using confocal microscopy. The results indicated that the DNA was condensed mostly on the surface of the droplets. In the case of high concentration droplets, it formed a shell, whereas isolated islands were found for droplets of low DNA concentrations. Rheologic results indicate the formation of a hydro gel in the low concentration drops, whereas phase separation between the self-assembled DNA structures and the water phase occurred at higher concentration.     

Microemulsion polymerization of styrene in the presence of a cationic emulsifier

The principal subject discussed in the current paper is the radical polymerization of styrene in the three- and four component microemulsions stabilized by a cationic emulsifier. Polymerization in the o/w microemulsion is a new polymerization technique which allows to prepare the polymer latexes with the very high particle interface area and narrow particle size distribution. Polymers formed are very large with a very broad molecular weight distribution. In emulsion and microemulsion polymerizations, the reaction takes place in a large number of isolated loci dispersed in the continuous aqueous phase. However, in spite of the similarities between emulsion and microemulsion polymerization, there are large differences caused by the much larger amount of emulsifier in the latter process. In the emulsion polymerization there are three rate intervals. In the microemulsion polymerization only two reaction rate intervals are commonly detected: first, the polymerization rate increases rapidly with the reaction time and then decreases steadily. Essential features of microemulsion polymerization are as follows: (1) polymerization proceeds under non-stationary state conditions; (2) size and particle concentration increases throughout the course of polymerization; (3) chain-transfer to monomer/exit of transferred monomeric radical/radical re-entry events are operative; and (4) molecular weight is independent of conversion and distribution of resulting polymer is very broad. The number of microdroplets or monomer-starved micelles at higher conversion is high and they persist throughout the reaction. The high emulsifier/water ratio ensures that the emulsifier is undissociated and can penetrate into the microdroplets. The presence of a large amount of emulsifier strongly influences the reaction kinetics and the particle nucleation. The mixed mode particle nucleation is assumed to govern the polymerization process. At low emulsifier concentration the micellar nucleation is dominant while at a high emulsifier concentration the interaction-like homogeneous nucleation is operative. Furthermore, the paper is focused on the initiation and nucleation mechanisms, location of initiation locus, and growth and deactivation of latex particles. Furthermore, the relationship between kinetic and molecular weight parameters of the microemulsion polymerization process and colloidal (water/particle interface) parameters is discussed. In particular, we follow the effect of initiator and emulsifier type and concentration on the polymerization process. Besides, the effects of monomer concentration and additives are also evaluated.     

Phenomenological theories of globular microemulsions.

Simple phenomenological theory of noninteracting microemulsion water droplets dispersed in an oil phase (or vice versa) is presented. Several characteristics quantities such as radii, polydispersities, shape fluctuation amplitudes and interfacial tensions are evaluated. In particular, the different features of a one phase microemulsion and a two phase coexistence of the microemulsion with an excess water phase are emphasized. We discuss the relations to most existent theories of microemulsion droplets, micelles and nucleating droplets and briefly review recent experimental techniques to access the relevant parameters.     

羧甲基纤维素系列高分子表面活性剂形成微乳液的研究

采用紫外光谱、相图、动态激光光散射、同位素示踪、光学显微镜、电导率等研究了羧甲基纤维素系列高分子表面活性剂与甲苯-水-异丙醇体系微乳液的形成过程,发现微乳液粒子大小均一,形态一致,其尺寸比低分子表面活性剂所形成的微乳液粒子大得多.醇分子插入到油水界面,改变了两相界面结构,促使微乳液的生成.电导率测定表明所形成的微乳液区均为水包油结构,即使在富含甲苯区域,亦不会有油包水的反相胶束形成.     

Influence of structural and interfacial properties of microemulsion eluent on chromatographic separation of simvastatin and its impurities

In order to calculate the structural and compositional characteristics of microemulsions, used as eluents in the investigation of HPLC separation of simvastatin and its six impurities, predictive molecular thermodynamic approach is developed. For calculating fundamental interfacial properties of microemulsions, from pure component properties, the lattice fluid self-consistent field theory (SCF), in conjunction with new classical thermodynamic expressions, was applied. Calculation of predicted radii (PR), area per surfactant (ApS) and film thickness (FT), as well as is interfacial tension and bending moment enabled better understanding of separation of such a complex mixture. The microemulsion, which contained 1% (w/w) of diisopropyl ether, 2% (w/w) of sodium dodecyl sulphate (SDS), 6.6% (w/w) of co-surfactant such as n-butanol and 90.4% (w/w) of aqueous 25 mM disodium phosphate pH 7.0 enabled appropriate chromatographic separation between investigated compounds. It has been proved that this microemulsion had the smallest droplet radii and film thickness, which enabled optimal separation. Also the interfacial tension is the smallest, so the free energy change associated with dispersing the drops favoured a large number of small droplets. Hydrophobic interactions between solutes and stationary phase, as well as the microstructural characteristics of microemulsion eluents had a significant influence on chromatographic behavior of simvastatin and its six impurities.     

Covalently closed microemulsions in presence of triblock terpolymers

This paper is focused on the influence of polystyrene (PS)-poly(1,4-butadiene) (PB)-poly(ethylene oxide) (PEO) triblock terpolymers on the w/o microemulsion of the pseudo-ternary system water/sodium dodecylsulfate (SDS)/xylene-pentanol. Despite the insolubility of the copolymer in water as well as in the xylene-pentanol mixture, it can be incorporated into the w/o microemulsion and interactions between the triblock terpolymer molecules and the anionic surfactant headgroups can be detected by differential scanning calorimetry (DSC) measurements. Furthermore, dynamic light scattering measurements were used to determine the aggregate diameter of the modified microemulsions. For lower polymer concentrations large aggregates between 100 and 500 nm can be observed. Surprisingly, at a higher terpolymer concentration of 5 wt%, significant smaller aggregate diameters can be identified by dynamic light scattering and Cryo-SEM. One can conclude that the copolymers are incorporated in the inverse microemulsion droplets, where the PB blocks cover the water droplets. The thermally induced radical cross-linking of the butadiene units in the presence of azobisisobutyronitrile (AIBN) leads then to covalently closed nanocapsules with an average size of 10 nm.     

Influence of mass transfer at the monomer-water interface on polymerization emulsion

A quasi-spontaneous process of mass transfer takes place at the interface of a monomer and water in the presence of surface-active substances soluble in both phases as a result of their diffusion through the interfacial boundary. This process is accompanied by the formation of a microemulsion whose particle size depends on the emulsifier concentration and its molecular structure. While investigating various nonionic surface-active substances as emulsifiers in the emulsion polymerization of vinyl acetate, it was established that polymerization occurs in droplets of the monomer microemulsion in water, which are formed as a result of mass transfer at the interface.     

Investigation of the mobility of amphiphilic polymer--AOT reverse microemulsion systems using electron spin resonance

Amphiphilic polymers can be used as tools to manipulate the behavior of reverse microemulsions. EPR spectroscopy employing the spin probe 5-doxyl stearic acid was used to study the adsorption of a comb type polymer (polymaleic anhydride octyl vinyl ether) and a diblock polymer (polybutadiene-ethylene oxide) onto reverse microemulsion droplets formed from Aerosol-OT/heptane/water. The findings indicate that the comb type polymer was adsorbed by the reverse microemulsion drops at low polymer concentrations causing a structural change of the micelle.     

Interfacial tension between microemulsion and excess dispersed phases

Two-phase systems consisting of water-in-oil (W/O) microemulsions in equilibrium with excess water and oil-in-water (O/W) microemulsions in equilibrium with excess oil have been prepared using the surfactant sodium bis (2-ethylhexyl)sulphosuccinate (AOT) without cosurfactant. The interfacial tension of the planar interface separating the phases for the W/O case is only weakly dependent upon the volume fraction of droplets in the microemulsion phase whereas for the O/W case, the microemulsion droplet size increases and the tension drops as the dispersed volume fraction is increased.     

CTMAB-C5H11OH-H2O体系微乳液、液晶及其增溶特性

微乳液、液晶及其增溶作用具有重要的理论意义及应用价值,已引起重视与关注。本文研究了CTMAB-C_5H_(11)OH-H_2O体系微乳液、层状液晶及其对C_7H_(16)的增溶特性。1 实验方法十六烷基三甲基溴化铵(分析纯,简称CTMAB)、正戊醇、正庚烷,皆为分析纯,H_2O为二次蒸馏水。1.2 各向同性区域的测定先以滴定法粗略确定单相区域的相界线,然后在相界线两侧均匀选点配样,于25.0±     

Preparation of metal nanoparticles in water-in-oil (w/o) microemulsions

The use of an inorganic phase in water-in-oil microemulsions has received considerable attention for preparing metal particles. This is a new technique, which allows preparation of ultrafine metal particles within the size range 5 nm相似文献   

微乳液中苯乙烯聚合反应的研究

测定了十二烷基磺酸钠(As)/正丁醇/20%苯乙烯/水体系相平衡。用油溶性偶氮二异丁腈(AIBN)和水溶性过二硫酸钾(K~2S~2O~8)为引发剂,研究了油包水(W/O)、双连续(BC)和水包油(O/W)型微乳液介质中苯乙烯的聚合反应。得到了苯乙烯转化率和聚苯乙烯分子量与体系水含量之间的关系,讨论了微乳液结构对聚合作用的影响。并通过电镜观察了聚苯乙烯的形貌,求得了聚苯乙烯的粒径,同时用^1HNMR研究了苯乙烯在微乳液液滴中的增溶位置,分析了聚合作用的实验结果。     

Polyelectrolyte-modified inverse microemulsions and their use as templates for the formation of magnetite nanoparticles

This paper is focused on the characterization of polyelectrolyte-modified inverse microemulsions and their use as templates for the synthesis of magnetite nanoparticles. It is shown that the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) of low molar mass can be incorporated into the individual inverse microemulsion droplets (L2 phase) consisting of heptanol, water, and an amphoteric surfactant with a sulfobetaine head group. Up to a polymer concentration of 20% by weight in the aqueous phase and for different molecular weights of the polymer, an isotropic phase still exists. At a PDADMAC concentration of 10% the area of the isotropic L2 phase is shifted in direction to the water corner. In the percolated area of the L2 phase, i.e., at higher water content, a temperature-dependent change in the conductivity can by observed, and bulk water can be detected by means of differential scanning calorimetry measurements. The unusual temperature-dependent behavior of the polymer-modified system, i.e., the conductivity decrease with increasing temperature, can be explained by temperature-sensitive polyelectrolyte-surfactant interactions, influencing the droplet-droplet interactions. These PDADMAC-modified microemulsions can be successfully used as a template for the formation of ultrafine magnetite particles, in contrast to the nonmodified microemulsion, where the process is misdirected due to the "disturbing" effect of the surfactants. However, in the presence of PDADMAC the surfactant head groups were masked, and therefore magnetite can be synthesized. During the process of magnetite formation the PDADMAC controls the particle growing and stabilizes spherical magnetite particles with a diameter of 17 nm, which can be redispersed without a change in size.     

Localized electric field induced transition and miniaturization of two‐phase flow patterns inside microchannels

Strategic application of external electrostatic field on a pressure‐driven two‐phase flow inside a microchannel can transform the stratified or slug flow patterns into droplets. The localized electrohydrodynamic stress at the interface of the immiscible liquids can engender a liquid‐dielectrophoretic deformation, which disrupts the balance of the viscous, capillary, and inertial forces of a pressure‐driven flow to engender such flow morphologies. Interestingly, the size, shape, and frequency of the droplets can be tuned by varying the field intensity, location of the electric field, surface properties of the channel or fluids, viscosity ratio of the fluids, and the flow ratio of the phases. Higher field intensity with lower interfacial tension is found to facilitate the oil droplet formation with a higher throughput inside the hydrophilic microchannels. The method is successful in breaking down the regular pressure‐driven flow patterns even when the fluid inlets are exchanged in the microchannel. The simulations identify the conditions to develop interesting flow morphologies, such as (i) an array of miniaturized spherical or hemispherical or elongated oil drops in continuous water phase, (ii) “oil‐in‐water” microemulsion with varying size and shape of oil droplets. The results reported can be of significance in improving the efficiency of multiphase microreactors where the flow patterns composed of droplets are preferred because of the availability of higher interfacial area for reactions or heat and mass exchange.