Effect of compressed CO2 on the properties of lecithin reverse micelles

Lecithin is a very useful biosurfactant. In this work, the effects of compressed CO 2 on the critical micelle concentration (cmc) of lecithin in cyclohexane and solubilization of water, lysozyme, and PdCl 2 in the lecithin reverse micelles were studied. The micropolarity and pH value of the polar cores of the reverse micelles with and without CO 2 were also investigated. It was found that CO 2 could reduce the cmc of the micellar solution and enhance the capacity of the reverse micelles to solubilize water, the biomolecule, and the inorganic salt significantly. Moreover, the water pools could not be formed in the reverse micelles in the absence of CO 2 because of the limited amount of water solubilized. However, the water pools could be formed in the presence of CO 2 because large amounts of water could be solubilized. All of these provide more opportunity for effective utilization of this green surfactant. The possible mechanism for tuning the properties of the reverse micelles by CO 2 is discussed.     

Effect of compressed CO2 on the critical micelle concentration and aggregation number of AOT reverse micelles in isooctane

The effect of compressed CO2 on the critical micelle concentration (cmc) and aggregation number of sodium bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles in isooctane solution was studied by UV/Vis and fluorescence spectroscopy methods in the temperature range of 303.2-318.2 K and at different pressures or mole fractions of CO2 (X(CO2)). The capacity of the reverse micelles to solubilize water was also determined by direct observation. The standard Gibbs free energy (DeltaGo(m)), standard enthalpy (DeltaHo(m)), and standard entropy (DeltaSo(m)) for the formation of the reverse micelles were calculated by using the cmc data determined. It was discovered that the cmc versus X(CO2) curve and the DeltaGo(m) versus X(CO2) curve for a fixed temperature have a minimum, and the aggregation number and water-solubilization capacity of the reverse micelles reach a maximum at the X(CO2) value corresponding to that minimum. These results indicate that CO2 at a suitable concentration favors the formation of and can stabilize AOT reverse micelles. A detailed thermodynamic study showed that the driving force for the formation of the reverse micelles is entropy.     

An example of how to use AOT reverse micelle interfaces to control a photoinduced intramolecular charge-transfer process

6-Propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe due to its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer (ICT) states which can be particularly useful as sensors. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES), and time-resolved area normalized emission (TRANES) spectroscopies on PRODAN dissolved in nonaqueous reverse micelles. The reverse micelles are composed of polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane. Sequestered polar solvents included ethylene glycol (EG), propylene glycol (PG), glycerol (GY), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA). The experiments were performed with varying surfactant concentrations at a fixed molar ratio W(S) = [polar solvent]/[AOT]. In every reverse micelle studied, the results show that PRODAN undergoes a partition process between the external solvent and the reverse micelle interface. The partition constants, K(p), are quantified from the changes in the PRODAN emission and/or absorption spectra with the surfactant concentration. The K(p) values depend strongly on the encapsulated polar solvent and correlate quite well with the AOT reverse micelle interface's zones where PRODAN can exist and emits. Thus, the partition toward the reverse micelle interface is strongly favored in DMF and DMA containing micelles where the PRODAN emission comes only from an ICT state. For GY/AOT reverse micelles, the K(p) value is the lowest and only emission from the local excited (LE) state is observed. On the other hand, for EG/AOT, PG/AOT, and water/AOT reverse micelles, the K(p) values are practically the same and emission from both states (LE and ICT) is simultaneously detected. We show here that it is possible to control the PRODAN state emission by simply changing the properties of the AOT reverse micelle interfaces by choosing the appropriate polar solvent to make the reverse micelle media. Indeed, we present experimental evidence with the answer to the long time question about from which state does PRODAN emit, a process that can be controlled using the unique reverse micelle interfaces properties.     

Phase behaviors of AOT/longer chain n-alkanes reverse micelles in the presence of compressed ethylene

It was found that, in a suitable pressure range, ethylene could increase the amount of solubilized water in reverse micelles of sodium bis-2-ethylhexylsulfosuccinate (AOT) in longer chain n-alkanes considerably, where the phase separation was induced by a micelle-micelle interaction mechanism. The microenvironments in the ethylene-stabilized reverse micelles were investigated by the UV-vis adsorption spectra using methyl orange (MO) as a probe. The maximum absorption of MO decreased with the increase of ethylene pressure at constant W0 value. Conductivity measurements demonstrated that the percolation temperature of the reverse micellar system increased considerably after compressed ethylene was added. The results of this work confirm that some small-molecule gases have the function of cosurfactants to stabilize reverse micelles.     

Covalently functionalized single-walled carbon nanotubes at reverse micellar interface: a strategy to improve lipase activity

The present work reports covalent functionalization of single-walled carbon nanotubes (f-SWNTs) to introduce hydrophilicity to the otherwise amphiphobic nanotubes. The charge and spacer length of the functional moiety were varied by using quaternized ethylene diamine, 6-aminocaproate, quaternized (ethylenedioxy)bis(ethylamine), and a poly(ethylene glycol) (PEG) unit (f-SWNT-1 to f-SWNT-4, respectively). These f-SWNTs with varying degrees of hydrophilicity were incorporated within cetyltrimethyl ammonium bromide (CTAB) reverse micelles to develop stable self-assembled nanohybrids. An optimum hydrophilicity on the SWNT surface led to interfacial localization of f-SWNTs resulting in the augmentation of space at the interface. A surface-active enzyme, lipase, localized at this enhanced interface of f-SWNT-containing CTAB reverse micelles exhibited significant activation (2.5-fold) compared to that in the absence of the nanoconstructs. This improvement in lipase activity was mainly due to the smooth occupancy of lipase and also presumably because of the increase in the concentrations of both substrate and the enzyme at the augmented interface. Interestingly, the f-SWNTs that activate lipase in reverse micelles deactivate the same enzyme in water. The dispersion of f-SWNTs in water and its matching integration at the interface of reverse micelles were confirmed through transmission electron microscopic (TEM) investigations. The interfacial localization of these nanoconstructs was also established from the distinct fluorescence behavior of a hydrophobic fluorescent probe, fluorescein isothiocyanate (FITC), adsorbed onto the f-SWNT surface. In concurrence with the observed lipase activity, the corresponding changes in the enzyme conformation within f-SWNTs integrated reverse micelle as well as in aqueous medium were studied by circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopy.     

Ion exchange in reverse micelles

The distribution and dynamics of alkali cations inside Na-AOT reverse micelles have been investigated using Monte Carlo and molecular dynamics simulations. Water is modeled using the extended simple point charge (SPC/E) model. Simulations were carried out for alkali salts of Li+, Na+, K+, and Cs+ placed into the aqueous core of the reverse micelle, for situations corresponding to one and three molecules of added salt. In all cases, we observe that the larger K+ and Cs+ ions exchange with the Na+ counterion; however, the smaller Li+ ion prefers to remains solvated within the core of the reverse micelle. Our study reveals that the oil-water interface of the Na-AOT reverse micelle has the greatest selectivity toward Cs+ followed by K+ and Li+. A model based on enthalpic contributions illustrates that the solvation energies of the different cations in water control the ion-exchange process. The hydration number of the first water shell for Li+ situated in the aqueous core of the reverse micelle with radius R = 14.1 A was similar to that observed at infinite dilution in bulk water.     

Effect of structure of PEO-PPO-PEO copolymers on reverse micelle formation induced by compressed CO2

The micellization of PEO-PPO-PEO block copolymers in p-xylene has been studied in the presence of CO2. With the application of CO2, some copolymers with suitable molecular weights and EO ratios can form reverse micelles with critical micellization pressure up to 5.8 MPa. For the copolymers with the same length of PO block, higher EO ratios facilitate reverse micelle formation. For the copolymers with the same composition, higher molecular weight is favorable to form reverse micelles. With the suitable composition and molecular weight, the critical micelle pressure (CMP) of copolymers decreases with the increase in the lengths of PEO and PPO blocks due to the hydrophilic and folding effects, respectively. Both the EO ratios and the molecular weights are important for the formation of reverse micelle. The reverse micelle solution can solubilize water with W0 (molar ratio of water to EO segment) up to 3.3.     

Exciplex-type behavior and partition of 3-substituted indole derivatives in reverse micelles made with benzylhexadecyldimethylammonium chloride, water and benzene

The fluorescence properties of 3-methylindole (MI), 3-indoleacetic acid (IAA), 3-indoleethyltrimethylammonium bromide (IETA), L-tryptophan (Trp) and tryptamine hydrochloride (TA) were studied in reverse micelles solutions made with the cationic surfactant benzylhexadecyldimethylammonium chloride (BHDC) in benzene as a function of the molar ratio water/surfactant R (= [H2O]/[BHDC]). The fluorescence quenching of the model compound MI by benzene in cyclohexane solutions and by BHDC in benzene solutions were also studied in detail. The fluorescence of MI in benzene is characteristic of a charge-transfer exciplex. The exciplex is quenched by the presence of BHDC, due to the interactions of the surfactant ion pairs with the polar exciplex. In reverse micelle solutions at low R values, all the indoles show exciplex-type fluorescence. As R increases, the fluorescence behavior strongly depends on the nature of the indole derivative. The anionic IAA remains anchored to the cationic interface and its fluorescence is quenched upon water addition due to the increases of interface's micropolarity. For IETA, TA and Trp an initial fluorescence quenching is observed at increasing R, but a fluorescence recovery is observed at R > 5, indicating a probe partition between the micellar interface and the water pool. For the neutral MI, the fluorescence changes with R indicate the partition of the probe between the micellar interface and the bulk benzene pseudophase. A simple two-site model is proposed for the calculation of the partition constants K as a function of R. In all cases, the calculation showed that even at the highest R value, about 90% of the indole molecules remain associated at the micellar interface.     

Enhanced stabilization of reverse micelles by compressed CO2

The effect of compressed CO2 on the solubilization capacity of water in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in longer chain n-alkanes was studied at different temperatures and pressures. It was found that the amount of solubilized water is increased considerably by CO2 in a suitable pressure range. The suitable CO2 pressure range in which the solubilization capacity of water could be enhanced decreased with increasing W0 (water-to-AOT molar ratio). The microenvironments in the CO2-stabilized reverse micelles were investigated by UV/Vis adsorption spectroscopy with methyl orange (MO) as probe. The mechanism by which the reverse micelles are stabilized by CO2 is discussed in detail. The main reason is likely to be that CO2 has a much smaller molecular volume than the n-alkane solvents studied in this work. Therefore, it can penetrate the interfacial film of the reverse micelles and stabilize them by increasing the rigidity of the micellar interface and thus reducing the attractive interaction between the droplets. However, if the CO2 pressure is too high, the solvent strength of the solvents is reduced markedly, and this induces phase separation in the micellar solution.     

Higher order structure of proteins solubilized in AOT reverse micelles

The higher order structure of proteins solubilized in an bis(2-ethylhexyl) sulfosuccinate sodium (AOT) reverse micellar system was investigated. From circular dichroic (CD) measurement, CD spectra of cytochrome c, which is solubilized at the interface of reverse micelles, markedly changed on going from buffer solution to the reverse micellar solution, and the ellipticity values in the far- and near-UV regions decreased with decreasing the water content (W₀: molar ratio of water to AOT), indicating that the secondary and tertiary structures of cytochrome c changed with the water content. The ellipticity of ribonuclease A, which is solubilized in the center of micellar water pool, in the near-UV region was dependent on W₀ and became minimum when W₀ of ca. 8 while the ellipticity in the far-UV region was almost constant, indicating that the tertiary structure of ribonuclease A was affected by the water content, but the secondary structure was conserved. The degree of curvature of the micellar interface appears to influence the protein structure because the reverse micelle size is linearly proportional to the W₀ value. As evidence of this, when the micelle size was comparable to the protein's dimensions, the structures were more affected by the water content. Judging from the dependence of the factor influencing the protein structure on the protein species, the location of solubilized protein in reverse micelles is significantly related to whether the protein structure in the system is affected by the micellar interface. In the cases of cytochrome c and lysozyme, the ellipticity against W₀ was dependent on the AOT concentration. In contrast, ribonuclease A gave very similar ellipticity values whatever the AOT concentration. In the n-hexane micellar system, cytochrome c exhibited lower ellipticity values and ribonuclease A in the lower W₀ range (W₀ < ca. 8) higher ellipticity values. These results indicated that the interaction between the protein and the micellar interface is a dominant factor influencing the protein structure in reverse micelles, and that it is governed by the location of solubilized proteins and the state of the micellar interface.     

Solvent effects on AOT reverse micelles in liquid and compressed alkanes investigated by neutron spin-echo spectroscopy

Neutron Spin-Echo (NSE) spectroscopy has been employed to study the interfacial properties of reverse micelles formed with the common surfactant sodium bis-2-ethylhexyl-sulfosuccinate (AOT) in liquid alkane solvents and compressed propane. NSE spectroscopy provides a means to measure small energy transfers for incident neutrons that correspond to thermal fluctuations on the nanosecond time scale and has been applied to the study of colloidal systems. NSE offers the unique ability to perform dynamic measurements of thermally induced shape fluctuation in the AOT surfactant monolayer. This study investigates the effects of the bulk solvent properties, water content, and the addition of octanol cosurfactant on the bending elasticity of AOT reverse micelles and the reverse micelle dynamics. By altering these solvent properties, specific trends in the bending elasticity constant, k, are observed where increasing k corresponds to an increase in micelle rigidity and a decrease in intermicellar exchange rate, k(ex). The observed corresponding trends in k and k(ex) are significant in relating the dynamics of microemulsions and their application as a reaction media. Compressed propane was also examined for the first time with a high-pressure, compressible bulk solvent where variations in temperature and pressure are used to tune the properties of the bulk phase. A decrease in the bending elasticity is observed for the d-propane/AOT/W = 8 reverse micelle system by simultaneously increasing the temperature and pressure, maintaining constant density. With isopycnic conditions, a constant translational diffusion of the reverse micelles through the bulk phase is observed, conforming to the Stokes-Einstein relationship.     

Compressed CO2-enhanced solubilization of 1-butyl-3-methylimidazolium tetrafluoroborate in reverse micelles of Triton X-100

We carried out the first study about the effect of a compressed gas on the properties of reverse micellar solutions with ionic liquid (IL) polar cores. And the properties of compressed CO2/cyclohexane/1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4])/Triton X-100 (TX-100) system were investigated at 288.2, 293.2, 298.2, 308.2 K and different pressures by using phase behavior measurement, small-angle x-ray scattering, and UV-Vis techniques. The concentration of the surfactant in the solution was 0.3 mol/l (M). It was found that compressed CO2 could enhance solubilization of the IL in the reverse micelles considerably at suitable pressures, and formation of the reverse micelles could be controlled easily by pressure. Increase of CO2 pressure resulted in decrease of the micellar sizes at fixed [bmim][BF4]-to-surfactant molar ratios (w), and the size of the reverse micelles increased with the increase of w values. The polarity of the IL cores increased continuously with increasing w value.     

Endogenous growth of the population of reverse micelles

The coupling reaction between cetylbromide (CB) and trimethylamine (TMA) to yield the surfactant cetyltrimethylammonium bromide (CTAB) is studied in the system chloroform/isooctane (2/1,v/v)/water in which CTAB forms reverse micelles. This system affords an endogenous micelle population growth, i.e., an increase of the concentration of the micelles due to appearance of the surfactant in situ. The reaction is studied in the presence of preexisting CTAB reverse micelles. The rate of CTAB formation is measured by NMR spectroscopy, and the endogenous micelle population growth is directly monitored by time-resolved fluorescence quenching analysis. Under our experimental conditions, a 100% yield of the chemical reaction brings about a fourfold increase in the population of the reverse micelles. Since the water concentration is constant during chemical reaction, the newly formed water pools are formed at the expense of the initial ones, which brings about a decrease of the average water pool radius during micellar growth. The implication of the endogenous micelle population growth as a model for biological systems is briefly discussed.     

庚烷中C12-EOx-C12•2Br反胶团的形成

在助表面活性剂正己醇存在下, 季铵盐Gemini表面活性剂C12-EOx-C12•2Br(x=1, 2, 3)在正庚烷中形成了反胶团. 以碘光谱法测定了临界反胶团浓度(cmch), 该值小于它们在水中形成正胶团时的临界浓度(cmcaq), 但两者随x的变化规律一致, 均呈单调增长. 反胶团饱和增溶水量随x增加或温度升高而增大.     

Intramolecular charge transfer at reverse micelle-water pool interface: p-N,N-dimethylaminobenzoic acid in AOT/cyclohexane/water reverse micelle

Photoinduced intramolecular charge transfer (ICT) of p-N,N-dimethylaminobenzoic acid (DMABOA) in AOT/cyclohexane/H2O reverse micelle was investigated and compared with that in CTAB/1-heptanol/H2O reverse micelle. It is proposed that the DMABOA molecule exists at the AOT reverse micelle water pool interface with its carboxylic group heading toward the water pool while the dimethylaminophenyl moiety buried in the micellar phase. Dual fluorescence of DMABOA that is indicative of the ICT reaction in the excited state was observed over the investigated water pool size, W of 3-17, in the AOT reverse micelle. The ICT emission of DMABOA in the AOT reverse micelle-water pool interface was found to be much weaker than that in the CTAB reverse micelle-water pool interface, and was attributed to the parallel direction of the electric field at the AOT reverse micelle-water pool interface to the charge transfer.     

Enhanced solubilization of bovine serum albumin in reverse micelles by compressed CO2

The effect of compressed CO2 on the solubilization of bovine serum albumin (BSA) in water/sodium bis-(2-ethylhexyl) sulfosuccinate (AOT)/isooctane reverse micelles was studied by observing phase behavior and recording UV-visible spectra under different conditions. The pH values within the water cores of reverse micelles at different CO2 pressures were also determined. The solubilization capacity of the reverse micelles for the protein increased considerably as CO2 pressure increased within the low-pressure range, but decreased at higher CO2 pressures, so that the micelles eventually lost their ability to solubilize the protein. The effect of CO2 on the stability of the reverse micelles played an important role in the relationship between pressure and protein solubility. A "multicomplex" model was proposed to explain these effects. The different solublization capacities within different pressure ranges demonstrates the unique advantage of using compressed CO2 in the extraction of proteins with reverse micelles.     

混合反胶束的电导与微结构研究

反胶束是两亲分子在非极性溶剂中形成的一种有序组合体，在医药、化工、采油、胶束催化及酶催化等领域中有重要应用．与胶束溶液相比，人们对反胶束的形成与结构的了解至今仍不充分．特别是对于由混合表面活性剂形成的反胶束的研究几乎无人涉及．本文采用动态光散射、电导及荧光光谱等手段对阴离子表面活性剂AOT与非离子表面活性剂形成的混合反胶束进行了研究，旨在探讨利用表面活性剂的复配来调节和控制反胶束的结构和性能．亚实验部分二异辛基磺化琉璃酸钠（AOT，Sigma公司）；Brij30为含4个氧乙烯基（EO基）的十二碳醇（AcrosOrgani…     

Activity and Conformation of Yeast Alcohol Dehydrogenase (YADH) Entrapped in Reverse Micelles

Yeast alcohol dehydrogenase (YADH) solubilized in reverse micelles of aerosol OT (i.e., AOT or sodium bis (2-ethyl hexyl) sulfosuccinate) in isooctane has been shown to be catalytically more active than that in aqueous buffer under optimum conditions of pH, temperature, and water content in reverse micelles. Studies of the secondary structure conformational changes of the enzyme in reverse micelles have been made from circular dichroism spectroscopy. It has been seen that the conformation of YADH in reverse micelles is extremely sensitive to pH, temperature, and water content. A comparison has been made between the catalytic activity of the enzyme and the α-helix content in the conformation and it has been observed that the enzyme is most active at the maximum α-helix content. While the β-sheet content in the conformation of the entrapped enzyme was found to be dependent on the enzyme–micelle interface interaction, the α-helix and random coil conformations are governed by the degree of entrapment and the extent of rigidity provided by the micelle core to the enzyme structure.     

Enhancement of the solubilization capacity of water in Triton X-100/cyclohexane/water system by compressed gases

The effect of compressed CO2 and ethylene on the properties of Triton X-100/cyclohexane/water systems was studied at different temperatures and pressures. Surprisingly, it was discovered that the compressed gases had the functions of co-surfactants. At suitable pressures, the water-to-surfactant molar ratio (W0) was enhanced significantly by the dissolution of the gas in the solution. The microenvironment in the reverse micelles was investigated by UV-visible spectroscopy by using methyl orange (MO) as a probe. The influence of n-hexane, Na2CO3, NaHCO3, H2C2O4, and CaCl2 at various concentrations on the solubilization of water in the absence of compressed gases was also investigated in order to obtain some information about the mechanism of the interesting phenomenon. This new route to stabilize reverse micelles may have potential applications to other similar systems. Moreover, the results of this work provide some useful information to get insight into the mechanism of co-surfactants, because a conventional co-surfactant usually contains both polar group and hydrocarbon chain, and it is very difficult to clarify the functions these two groups, while the gases used in this work are small nonpolar molecules, which solely have the function of the hydrocarbon chain in a co-surfactant.     

Do probe molecules influence water in confinement?

The water inside reverse micelles can differ dramatically from bulk water. Some changes in properties can be attributed to the interaction of water molecules with the micellar interface, forming a layer of shell water inside the reverse micelle. The work reported here monitors changes in intramicellar water through chemical shifts and signal line widths in 51V NMR spectra of a large polyoxometalate probe, decavanadate, and from infrared spectroscopy of isotopically labeled water, to obtain information on the water in the water pool in AOT reverse micelles formed in isooctane. The studies reveal several things about the reverse micellar water pool. First, in agreement with our previous measurements, the proton equilibrium of the decavanadate solubilized within the reverse micelles differs from that in bulk aqueous solution, indicating a more basic environment compared to the starting stock solutions from which the reverse micelles were formed. Below a certain size, reverse micelles do not form when the polyoxometalate is present; this indicates that the polyanionic probe requires a layer of water to solvate it in addition to the water that solvates the surfactant headgroups. Finally, the polyoxometalate probe appears to perturb the water hydrogen-bonding network in a fashion similar to that in the interior surface of the reverse micelles. These measurements demonstrate the dramatic differences possible for water environments in confined spaces.