Ripening Phenomena in Emulsions — A Calorimetry Investigation

Ripening phenomena occurring within different kinds of emulsions have been studied. The emulsions concerned are simple water-in-oil (W/O) or oil-in-water (O/W) emulsions, mixed emulsions obtained by the mixture of two simple emulsions, and multiple emulsions water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O) emulsions. Composition ripening due to a mass transfer and solid ripening due to the formation of solid particles from the undercooled droplets or due to the formation of solid hydrate around the droplets have been pointed out on using a suitable calorimetric technique. For that purpose a non-diluted emulsion sample is submitted to a cooling and heating cycle during which solidification and melting temperatures and energies of the different phases are analyzed. It has been shown that correlations between these quantities and the properties of the dispersed phase permit one to get information about the ripening phenomena under study. The solution-diffusion model used for mass transfer is in good agreement with the experimental results. From the shell model used for the hydrate formation, it has been possible to deduce the formation energy and the influence of salt upon the temperature of formation.     

NMR/MRI study of clathrate hydrate mechanisms

Clathrate hydrates are of great importance in many aspects. However, hydrate formation and dissociation mechanisms, essential to all hydrate applications, are still not well understood due to the limitations of experimental techniques capable of providing dynamic and structural information on a molecular level. NMR has been shown to be a powerful tool to noninvasively measure molecular level dynamic information. In this work, we measured nuclear magnetic resonance (NMR) spin lattice relaxation times (T1's) of tetrahydrofuran (THF) in liquid deuterium oxide (D2O) during THF hydrate formation and dissociation. At the same time, we also used magnetic resonance imaging (MRI) to monitor hydrate formation and dissociation patterns. The results showed that solid hydrate significantly influences coexisting fluid structure. Molecular evidence of residual structure was identified. Hydrate formation and dissociation mechanisms were proposed based on the NMR/MRI observations.     

纳米SiO2与阳离子表面活性剂的相互作用及其诱导的正辛烷-水乳状液的双重相转变

研究了3种不同结构的水溶性阳离子表面活性剂对纳米二氧化硅颗粒的原位表面活性化作用, 它们分别是单头单尾的十六烷基三甲基溴化铵(CTAB)、单头双尾的双十二烷基二甲基溴化铵(di-C12DMAB)和双头双尾的Gemini型阳离子三亚甲基-二(十四酰氧乙基溴化铵)(II-14-3), 并通过测定Zeta电位、吸附等温线及接触角等参数对相关机理进行了阐述. 结果表明, 阳离子表面活性剂吸附到颗粒/水界面形成以疏水基朝向水的单分子层, 从而增强了颗粒表面的疏水性是原位表面活性化的基础. 通过吸附CTAB和II-14-3, 颗粒的疏水性适当增强, 能吸附到正辛烷/水界面稳定O/W(1)型乳状液; 而通过吸附di-C12DMAB所形成的单分子层更加致密, 颗粒的疏水性进一步增强, 进而使乳状液从O/W(1)型转变为W/O型; 当表面活性剂浓度较高时, 由于链-链相互作用, 表面活性剂分子将在颗粒/水界面形成双层吸附, 使颗粒表面变得亲水而失去活性, 但此时体系中游离表面活性剂的浓度已增加到足以单独稳定O/W(2)型乳状液的程度. 因此当采用纳米二氧化硅和di-C12DMAB的混合物作乳化剂时, 通过增加di-C12DMAB的浓度即可诱导乳状液发生O/W(1)→W/O→O/W(2)双重相转变.     

Stabilization of O/W emulsion with hydrophilic/hydrophobic clay particles

In this study, a stabilizing behavior of clay in a 40/60 w/w oil-in-water (O/W) emulsion is investigated by macro- and microscopic morphological observations, rheology, and X-ray diffraction measurements. Hydrophilic and hydrophobic clays (Montmorillonites) are tested for stabilization of emulsion. When hydrophilic clay showing interfacial localization is added to the emulsion, emulsion is not stable to phase separation (creaming). With hydrophobic clay, the emulsion shows phase inversion to water-in-oil (W/O) emulsion due to the increase in oil viscosity which results in phase separation of sedimentation. On the other hand, with the mixture of hydrophilic and hydrophobic clays, the emulsion shows a synergistic macroscopic and microscopic stabilization due to the formation of composite structure at the interface by hydrophilic and hydrophobic clays.     

Maximum disjoining pressure in protein stabilized concentrated oil-in-water emulsions

A model for the prediction of the equilibrium profile of film thickness and continuous phase liquid holdup profile in a concentrated oil-in-water (O/W) emulsion is proposed. This model is employed to infer the maximum disjoining pressure in a concentrated corn oil-in-water emulsion stabilized by bovine serum albumin (BSA) from the experimental measurements of different proportions of oil, polyhedral O/W foam, and aqueous layers at different centrifugal accelerations. The inferred maximum disjoining pressures were found to be higher at higher concentrations of BSA, lower ionic strengths as well as at pH values farther away from pI. The predicted variations of disjoining pressure with film thickness for a concentrated O/W emulsion stabilized by BSA exhibited two maxima due to steric and electrostatic interactions, respectively. The experimental maximum disjoining pressures for toluene-in-water emulsion stabilized by BSA were found to be about two to three times the predicted maxima due to steric interactions but were two to three orders of magnitude higher than the maxima due to electrostatic interactions, thus indicating that steric interaction is the dominant stabilizing mechanism. The discrepancy between the experimental and predicted maximum disjoining pressures is believed to be mainly due to lack of information with regard to the thickness of the adsorbed protein layer at the oil—water interface.     

Intensification of methane and hydrogen storage in clathrate hydrate and future prospect

Gas hydrate is a new technology for energy gas (methane/hydrogen) storage due to its large capacity of gas storage and safe. But industrial application of hydrate storage process was hindered by some problems. For methane, the main problems are low formation rate and storage capacity, which can be solved by strengthening mass and heat transfer, such as adding additives, stirring, bubbling, etc. One kind of additives can change the equilibrium curve to reduce the formation pressure of methane hydrate, and the other kind of additives is surfactant, which can form micelle with water and increase the interface of water-gas. Dry water has the similar effects on the methane hydrate as surfactant. Additionally, stirring, bubbling, and spraying can increase formation rate and storage capacity due to mass transfer strengthened. Inserting internal or external heat exchange also can improve formation rate because of good heat transfer. For hydrogen, the main difficulties are very high pressure for hydrate formed. Tetrahydrofuran (THF), tetrabutylammonium bromide (TBAB) and tetrabutylammonium fluoride (TBAF) have been proved to be able to decrease the hydrogen hydrate formation pressure significantly.     

Accelerated nucleation of tetrahydrofuran(THF)hydrate in presence of ZIF-61

Clathrate hydrate can be used in energy gas storage and transportation,CO 2 capture and cool storage etc.However,these technologies are difficult to be used due to the low formation rate and long induction time of hydrate formation.In this paper,ZIF-61(zeolite imidazolate framework,ZIF) was first used in hydrate formation to stimulate hydrate nucleation.As an additive of clathrate hydrate,ZIF-61 promoted obviously the acceleration of tetrahydrofuran(THF) hydrate nucleation.It shortened the induction time of THF hydrate formation from 2-5 h to 0.3-1 h mainly due to the template function of ZIF-61 by which the nucleation of THF hydrate has been promoted.     

Intensification of methane and hydrogen storage inclathrate hydrate and future prospect

Gas hydrate is a new technology for energy gas(methane/hydrogen)storage due to its large capacity of gas storage and safe.But industrial application of hydrate storage process was hindered by someproblems.For methane,the main problems are low formation rateand storage capacity,which can be solved by strengthening mass andheat transfer,such as adding additives,stirring,bubbling,etc.Onekind of additives can change the equilibrium curve to reduce the formation pressure of methane hydrate,and the other kind of additivesis surfactant,which can form micelle with water and increase the interface of water-gas.Dry water has the similar effects on the methanehydrate as surfactant.Additionally,stirring,bubbling,and sprayingcan increase formation rate and storage capacity due to mass transferstrengthened.Inserting internal or external heat exchange also canimprove formation rate because of good heat transfer.For hydrogen,the main difficulties are very high pressure for hydrate formed.Tetrahydrofuran(THF),tetrabutylammonium bromide(TBAB) andtetrabutylammonium fluoride(TBAF) have been proved to be able todecrease the hydrogen hydrate formation pressure significantly.     

Fabrication of macroporous foam and microspheres of polystyrene by Pickering emulsion polymerization

Poly(styrene-co-methacrylic acid) (PS-co-MAA) particles were synthesized via surfactant-free emulsion polymerization and then used as particulate emulsifiers for preparation of Pickering emulsions. Our results showed that adjusting the solution pH can tune the wettability of PS-co-MAA particles to stabilize either water-in-oil (W/O) or oil-in-water (O/W) Pickering emulsions. Stable W/O emulsions were obtained with PS-co-MAA particles at low pH values due to their better affinity to the dispersed oil phase. In contrast, increasing the pH value significantly changed the stabilizing behavior of the PS-co-MAA particles, leading to the phase inversion and formation of stable O/W emulsions. We found that the oil/water ratio had a significant influence on pH value of the phase inversion. It decreased with decreasing the oil/water ratio, and no phase inversion occurred when the styrene volume fraction reduced to 10 %. Additionally, macroporous polystyrene (PS) foam and PS microspheres were obtained via polymerization of Pickering high internal phase emulsion (Pickering HIPE) and O/W Pickering emulsion, respectively.     

Effect of temperature fluctuation on hydrate-based CO_2 separation from fuel gas

A new method of temperature fluctuation is proposed to promote the process of hydrate-based CO2 separation from fuel gas in this work according to the dual nature of CO2 solubility in hydrate forming and non-hydrate forming regions [1].The temperature fluctuation operated in the process of hydrate formation improves the formation of gas hydrate observably.The amount of the gas consumed with temperature fluctuation is approximately 35% more than that without temperature fluctuation.It is found that only the temperature fluctuation operated in the period of forming hydrate leads to a good effect on CO2 separation.Meanwhile,with the proceeding of hydrate formation,the effect of temperature fluctuation on the gas hydrate gradually reduces,and little effect is left in the completion term.The CO2 separation efficiencies in the separation processes with the effective temperature fluctuations are improved remarkably.     

Gas-hydrate formation, agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

One of the main challenges in deep-water drilling is gas-hydrate plugs, which make the drilling unsafe. Some oil-based drilling fluids (OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation, agglomeration and inhibition by an experimental system under the temperature of 4 ℃ and pressure of 20 MPa, which would be similar to the case of 2000 m water depth. The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF. The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles. The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later. Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.     

Gas-hydrate formation,agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

One of the main challenges in deep-water drilling is gas-hydrate plugs,which make the drilling unsafe.Some oil-based drilling fluids(OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation,agglomeration and inhibition by an experimental system under the temperature of 4 ?C and pressure of 20 MPa,which would be similar to the case of 2000 m water depth.The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF.The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles.The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later.Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.     

Phase equilibria of mixed carbon dioxide and tetrahydrofuran hydrates in sodium chloride aqueous solutions

In this work, the competing effects of sodium chloride (NaCl) and tetrahydrofuran (THF) on carbon dioxide hydrate formation are investigated through phase equilibrium measurements. The phase behaviour in the hydrate forming region for the binary system carbon dioxide–water, the ternary systems carbon dioxide–tetrahydrofuran–water and ternary carbon dioxide–sodium chloride–water and, in addition, the quaternary system carbon dioxide–tetrahydrofuran–water–sodium chloride are determined experimentally, using a Cailletet apparatus. All measurements are made in a temperature and pressure region of 275–290 K and 0.5–7.0 MPa, respectively. In these ranges, three different hydrate equilibrium curves are measured namely: H-L_W-V, H-L_W-L_V-V and H-L_W-L_V. The formation of an organic-rich liquid phase in the systems due to a liquid–liquid two-phase split between water and tetrahydrofuran when pressurized with carbon dioxide causes the occurrence of an upper quadruple point (Q₂) to evolve into a four-phase H-L_W-L_V-V equilibrium line. The presence of sodium chloride in the quaternary system enhances the split between the two liquids due to the salting-out effect. It was found that the hydrate promoting effect of tetrahydrofuran is able to suppress the inhibiting effect of sodium chloride especially at lower concentration of sodium chloride.     

Phase relations and binary clathrate hydrate formation in the system H2-THF-H2O

Experimentally determined equilibrium phase relations are reported for the system H2-THF-H2O as a function of aqueous tetrahydrofuran (THF) concentration from 260 to 290 K at pressures up to 45 MPa. Data are consistent with the formation of cubic structure-II (CS-II) binary H2-THF clathrate hydrates with a stoichiometric THF-to-water ratio of 1:17, which can incorporate modest volumes of molecular hydrogen at elevated pressures. Direct compositional analyses of the clathrate phase, at both low (0.20 mol %) and stoichiometric (5.56 mol %) initial THF aqueous concentrations, are consistent with observed phase behavior, suggesting full occupancy of large hexakaidecahedral (51264) clathrate cavities by THF, coupled with largely complete (80-90%) filling of small dodecahedral (512) cages by single H2 molecules at pressures of >30 MPa, giving a clathrate formula of (H2) < or =2.THF.17H2O. Results should help to resolve the current controversy over binary H2-THF hydrate hydrogen contents; data confirm recent reports that suggest a maximum of approximately 1 mass % H2, this contradicting values of up to 4 mass % previously claimed for comparable conditions.     

Stabilization of Water‐Soluble Antioxidant in Water‐in‐Oil‐in‐Water Double Emulsions

Abstract

In this study, we are introducing a method that can effectively stabilize antioxidants in water‐in‐oil‐in‐water (W/O/W) double emulsions. Preliminarily, stable W/O/W double emulsions were produced by manipulating the characteristics of internal aqueous phase via two‐stage emulsification, resulting consequently in the formation of fine internal water droplets in the dispersed oil droplets. From conductivity measurements that can determine the elution amount of internal aqueous phase, it was confirmed that the double emulsion stability could be improved by treating the internal aqueous phase with a hydroxypropyl‐beta‐cyclodextrin. In this study, kojic acid, 5‐hydroxy‐2‐(hydroxymethyl)‐4‐pyrone was selected as a model antioxidant. The stabilization of kojic acid was attempted by locating it in the internal water droplets of the stable W/O/W double emulsions. The stability of kojic acid in the double emulsion system could be maintained at 90% for 10 weeks at high temperature. We believe that these stable W/O/W double emulsions could be used meaningfully as a carrier for many unstable antioxidants.     

Study of mass transfer in oil-water-oil multiple emulsions by differential scanning calorimetry

A multiple emulsion of the type O1/W/O2 is studied experimentally by means of differential scanning calorimetry (DSC). The aim of this work is to characterize and measure the time-dependent changes within the emulsion. In particular, interest is focused to quantify the concentration changes in the internal and external phases of the O1/W/O2 multiple emulsion. In order to accomplish the objective, the measurement and analysis carried out by DSC are based on the crystallization behavior of the emulsion. A volume of a few mm3 is periodically removed from the O1/W/O2 multiple emulsion. The sample is submitted to steady cooling and the crystallization thermogram is recorded. The experimental data provided by the crystallization thermogram makes it possible to quantify the crystallized mass for both phases, the internal and the external. In addition, the composition in each phase can also be deduced from the thermogram. To deduce the composition, a diagram of crystallization temperatures is elaborated, employing several mixtures of known composition. In addition to the main objective previously mentioned, the influence of formulation parameters such as surfactant concentration in the aqueous phase and the mass ratio of the internal and external phases are also analyzed. The experimental results made it possible to conclude that a mass transfer took place from the internal phase toward the external phase; this transfer is caused by the composition difference on both sides of the aqueous membrane. In this work we analyzed the mass transfer in the multiple emulsion carried out by a composition gradient through the aqueous membrane. The most likely mechanism of mass transfer through the aqueous membrane is a solution-diffusion of tetradecane enhanced by the micelles of the surfactant Tween 20. The model of mass transfer confirms that the osmotic pressure difference controls the kinetics of tetradecane transfer. It is also confirmed that an increment of surfactant concentration in the aqueous phase allows a faster kinetics of the tetradecane transfer.     

SiO2纳米颗粒与聚氧乙烯对Pickering乳液的协同稳定作用简

研究了聚氧乙烯(PEO)与SiO2纳米颗粒对水/二甲苯体系Pickering乳液的协同稳定作用. 实验发现,PEO的存在减小了乳液液滴的平均直径,抑制了乳液的相反转,有效阻止了乳液的熟化,使乳液具有更好的稳定性. 进一步对纳米颗粒膜的流变性质进行研究,结果表明,PEO高分子促进了纳米颗粒形成更大尺寸的聚集结构,提高了其在界面上的吸附性,增强了颗粒膜的力学性能,在较小颗粒用量条件下使得Gibbs稳定性判据得到满足.     

Water-in-oil-in-water double nanoemulsion induced by CO(2)

The cetyltrimethylammonium bromide (CTAB)/water/heptane emulsion system with different CO(2) pressure has been studied. The phase behavior investigation shows the nanoemulsion can be formed at suitable pressure range. The generalized indirect Fourier transformation (GIFT) analysis of the small-angle X-ray scattering (SAXS) data has drawn a clear picture of the structural information of the nanoemulsion, which reveals that the droplet of emulsion has a double structure with both the outer and inner droplet size in nanometre range. Furthermore, the investigation of the heptane/CTAB/water/CO(2) emulsion system by using electrical conductivity confirms the emulsion type transforms from O/W to W/O/W. In addition, the effect of different CTAB concentration on the nanoemulsion formation has been studied. It is found that enough CTAB concentration is necessary for the inclusion of continuous water into oil droplets. We also explored the application of the W/O/W double nanoemulsion in material synthesis. Interestingly, the hollow silica spheres with double shells were obtained in this CO(2)-induced double nanoemulsion.     

Applying the Z method to estimate temperatures of melting in structure II clathrate hydrates

Equilibrium melting temperatures for structure II THF hydrate and argon/xenon (Ar/Xe) binary hydrate have been calculated using molecular dynamics using two melting techniques, namely the Z method [Belonoshko et al., Phys. Rev. B, 2006, 73, 012201] (applied for the first time to complex molecular solids) and direct phase coexistence simulations. The two methods give results in moderate agreement: calculations with the Z method give T(fus) to be 250.7 K (0.77 katm) for THF and 244.3 K (1.86 katm) for Ar/Xe hydrate respectively; the corresponding direct phase coexistence calculations give T(fus) in the range 235-240 K (0.77 katm) for THF and 240-252.5 K (1.86 katm) for Ar/Xe hydrate. The Z method was found to define the key thermodynamic states with high precision, although required long simulation times with these multicomponent molecular systems to ensure the complete melting required by the method. In contrast, the direct phase coexistence method did bracket the equilibrium temperature with little difficulty, but small thermodynamic driving forces close to phase equilibrium generated long-lived fluctuations, that obscured the precise value of phase coexistence conditions within the bracketed range.     

A Three-Step Model for Submicron W/O Emulsion Formation in a Transitional-Phase Inversion Process

A three-step model of the transitional phase inversion (TPI) process for the formation of water-in-oil (W/O) emulsions is presented. Three types of emulsions exist in an emulsification process at different oil–water ratios and hydrophilic–lipophilic balance (HLB). A stable W/O emulsion was obtained using Sorbitan oleate (Span 80) and polyoxyethylenesorbitan monooleate (Tween 80) with a specified HLB and oil volume fraction. Oil was added into water, which contained the water-soluble surfactant, to dissolve the oil-soluble surfactant. This route allowed TPI to occur, and an interesting emulsification process was observed by varying the HLB, which corresponded to the change in the oil–water ratio. Two types of emulsions in the emulsification process were found: transition emulsion 1 (W/O/W high internal phase emulsion) and target emulsion 2 (W/O emulsion with low viscosity). This study describes the changes that occurred in the emulsification process.