脂肪酸十二烷基铵在四氯化碳中的聚集行为

通过碘光谱法和对水的增溶研究了乙酸十二烷基铵(DAA)、丙酸十二烷基铵(DAP)和丁酸十二烷基铵(DAB)在四氯化碳中的聚集作用; 碘光谱测得的(cmc)~I约比水增溶法测得的(cmc)~W低一个数量级。可能因为碘光谱法测得的是开始发生聚集时的cmc, 这时聚集体较小, 对水无增溶能力; 只有当聚集体随浓度升高而长大到一定程度时, 才能开始增溶水。实验表明, DAA, DAP和DAB的反胶束对水的饱和增溶能力, 分别相当于每一个表面活性分子增溶4.2, 9.4和13.5个水分子。根据球型反胶束模型, 计算了反胶束聚集数、捕集水团的半径和自由水团的半径。     

Molecular probe location in reverse micelles determined by NMR dipolar interactions

The location and interactions of solutes in microheterogeneous environments, such as reverse micelles, critically influence understanding of many phenomena that utilize probe molecules to characterize properties in chemical, biological, and physical systems. The information gained in such studies depends substantially on the location of the probe used. Often, intuition leads to the assumption that ionic probe molecules reside in the polar water pool of a system. In this work, the location of a charged polar transition metal coordination complex in a reverse micellar system is determined using NMR spectroscopy. Despite the expected Coulomb repulsion between the surfactant headgroups and the negatively charged complex, the complex spends significant time penetrating into the hydrophobic portion of the reverse micellar interface. These results challenge the assumption that ionic probe molecules reside solvated by water in microheterogeneous environments and suggest that probe molecule location be carefully considered before interpreting data from similar systems.     

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.     

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.     

Tuning the cooperativity of the helix-coil transition by aqueous reverse micelles

We show in this letter that the thermodynamic properties of helical peptides can be tuned by varying the degrees of backbone hydration. The latter was achieved by solubilizing peptides in the water pool of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles with different water contents or w0 values. Far-UV circular dichroism measurements on a series of alanine-rich peptides indicate that the helicity of shorter peptides is significantly increased in AOT reverse micelles at low w0 values, as compared to the corresponding helical content in buffer. This result therefore corroborates the previous simulation studies suggesting that desolvation of backbone CO and NH groups increases the stability of monomeric helices. In addition, it was found that the thermal unfolding transition of these peptides can either be very noncooperative or very cooperative, depending on w0 and peptide chain length. A simple model, which considers the heterogeneous distribution of the water molecules inside the polar core of AOT reverse micelles as well as the geometric confinement effect exerted on the peptide by the reverse micelles, was used to interpret these results.     

Ultrafast dynamics of water in cationic micelles

The effect of confinement on the dynamical properties of liquid water is investigated for water enclosed in cationic reverse micelles. The authors performed mid-infrared ultrafast pump-probe spectroscopy on the OH-stretch vibration of isotopically diluted HDO in D(2)O in cetyltrimethylammonium bromide (CTAB) reverse micelles of various sizes. The authors observe that the surfactant counterions are inhomogeneously distributed throughout the reverse micelle, and that regions of extreme salinity occur near the interfacial Stern layer. The authors find that the water molecules in the core of the micelles show similar orientational dynamics as bulk water, and that water molecules in the counterion-rich interfacial region are much less mobile. An explicit comparison is made with the dynamics of water confined in anionic sodium bis(2-ethythexyl) sulfosuccinate (AOT) reverse micelles. The authors find that interfacial water in cationic CTAB reverse micelles has a higher orientational mobility than water in anionic AOT reverse micelles.     

Confinement or the nature of the interface? Dynamics of nanoscopic water

The dynamics of water confined in two different types of reverse micelles are studied using ultrafast infrared pump-probe spectroscopy of the hydroxyl OD stretch of HOD in H2O. Reverse micelles of the surfactant Aerosol-OT (ionic head group) in isooctane and the surfactant Igepal CO 520 (nonionic head group) in 50/50 wt % cyclohexane/hexane are prepared to have the same diameter water nanopools. Measurements of the IR spectra and vibrational lifetimes show that the identity of the surfactant head groups affects the local environment experienced by the water molecules inside the reverse micelles. The orientational dynamics (time-dependent anisotropy), which is a measure of the hydrogen bond network rearrangement, are very similar for the confined water in the two types of reverse micelles. The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.     

Study of reverse micelles of di-isobutyl-phenoxy-ethoxy-ethyl-dimethyl-benzyl ammonium methacrylate in benzene by nuclear magnetic resonance spectroscopy

(1)H NMR spectroscopy was used to investigate the aggregation of the surfactant di-isobutyl-phenoxy-ethoxy-ethyl-dimethyl-benzyl ammonium methacrylate (Hyamine-M) in benzene. Adding water makes swollen reverse micelles (microemulsion droplets). The droplets also contain cadmium ions and the sodium salt of the methacrylic acid. The critical micelle concentration of Hyamine-M was determined by NMR to be 3.95 mM under the current conditions. Two-dimensional NMR NOESY spectra were used to study the conformation of the surfactant in the micelle and the spatial localization of water and counterions. We found that the surfactant molecules are folded with both phenyl fragments oriented toward the micelle exterior and the oxyethylene and NCH(3) groups in the micelle core. The water molecules and counterions are distributed around the surfactant polar groups in the micelle interior and penetrate up to both aromatic rings. The investigated system can be further utilized as a microemulsion matrix for the synthesis of cadmium-containing semiconductor nanocrystals, eventually capped with a polymer shell, or of polymer nanoparticles.     

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.     

An Experimental Study on the Relationship between the Physical Properties of CTAB/Hexanol/Water Reverse Micelles and ZrO2-Y2O3 Nanoparticles Prepared

Based on the studies of their physical properties such as aqueous solution uptake, electric conductivity, and microstructure, CTAB/hexanol/water reverse micelles (CTAB, cetyltrimethyl ammonium bromide) were used to prepare ZrO2-Y2O3 nanoparticles. The relationship between the micelle microstructure and size, morphology, and aggregate properties of particles prepared was also investigated. It has been found that with high CTAB concentration ([CTAB] > 0.8 mol/l), the reverse micelles can solubilize a sufficient amount of aqueous solution with high metallic ion concentration ( approximately 1.0 mol/L), while the microstructure of the reverse micelles keeps unchanged. The most important factor affecting the size and shape of reverse micelles was found to be the water content w0 (w0, molar ratio of water to surfactant used). When both the CTAB concentration and the w0 values are low, the diameters of reverse micelles are below 20 nm, and the ZrO2-Y2O3 particles prepared are also very small. However, the powders obtained were found to form a lot of aggregates after drying and calcination. High CTAB concentration, high w0 value, and high metallic ion concentration in the aqueous phase for high powder productivity were found to be the suitable compositions of reverse micelles for preparing high-quality ZrO2-Y2O3 nanoparticles. Under these conditions, the reverse micelles are still spherical in shape even the reverse micellar system is nearly saturated with aqueous solutions. These reverse micelles were found to have a diameter of between 60 and 150 nm and the ZrO2-Y2O3 particles prepared therefrom range from 30 to 70 nm with spherical shape and not easy to form aggregates. Copyright 1999 Academic Press.     

SPECTROSCOPIC AND DYNAMIC CHARACTERIZATION OF FMN IN REVERSED MICELLES ENTRAPPED WATER POOLS

Abstract— The encapsulation of FMN in surfactant entrapped water pools resulted into specific interactions of FMN with the polar head groups, the entrapped water molecules and the outer apolar solvent. Two positively charged surfactant/solvent systems were employed: dodecyl ammonium propionate (DAP) in toluene and hexadecyltrimethylammonium bromide (CTAB) in chloroform/ n -octane (6:5, vol/vol). Also a surfactant with a negatively charged polar head group, sodium bis (2-ethylhexyl) sulfosuccinate (AOT) in n -octane, was used. In CTAB and especially DAP reversed micellar systems the light absorption spectra revealed the localization of the flavin in a more apolar environment, while in AOT reversed micelles FMN appeared to reside mainly in the core of the water pool. The fluorescence spectra showed unresolved bands, which were blue-shifted in DAP and CTAB reversed micelles as compared to the spectra of aqueous FMN solutions. The fluorescence decay kinetics of FMN in enclosed water droplets is non-exponential. The heterogeneity can be explained assuming incomplete relaxation of partly immobilized water molecules during the lifetime of the excited singlet state. The relatively high anisotropy of the fluorescence of FMN in encapsulated water indicated a higher viscosity than in bulk water. This was confirmed by anisotropy decay measurements of FMN in DAP and AOT entrapped water, for which the rotational correlation times were much longer than for FMN in plain water.     

Organogels from a vitamin C-based surfactant

A new double chained surfactant, 2-octyl-dodecanoyl-6-O-ascorbic acid (8ASC10), with a L-ascorbic acid unit as the polar headgroup was synthesized for the first time. The behavior of the compound in the dry solid state has been characterized through DSC, XRD, and SAXS measurements. The surfactant forms stable viscous organogels in the presence of suitable organic solvents and also water-induced organogels upon addition of water to the organogel. These mixtures show shear-thinning properties and are birefringent. The behavior and properties of the organogels have been studied through rheology, DSC, and SAXS experiments. The organogels possess the same antioxidant properties of the original L-ascorbic acid ring and can be used to solubilize and protect valuable organic molecules.     

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.     

Reverse micellar aggregates: effect on ketone reduction. 2. Surfactant role

Kinetics of the reduction of 3-chloroacetophenone (CAF) with sodium borohydride (NaBH(4)) were followed by UV-vis spectroscopy at 27.0 degrees C in different reverse micellar media, toluene/BHDC/water and toluene/AOT/water, and compared with results in an isooctane/AOT/water reverse micellar system. AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate, and BHDC is benzyl-n-hexadecyl dimethylammonium chloride. The kinetic profiles were investigated as a function of variables such as surfactant and NaBH(4) concentration and the amount of water dispersed in the reverse micelles, W(0) = [H(2)O]/[surfactant]. In all cases, the first-order rate constant, k(obs), increases with the concentration of surfactant as a consequence of incorporating the substrate into the interface of the reverse micelles where the reaction takes place. The reaction is faster at the cationic interface than at the anionic one probably because the negative ion BH(4)(-) is part of the cationic interface. The effect of the external solvent on the reaction shows that reduction is favored in the isooctane/AOT/water reverse micellar system than that with an aromatic solvent. This is probably due to BH(4)(-) being more in the water pool of the toluene/AOT/water reverse micellar system. The kinetic profile upon water addition depends largely on the type of interface. In the BHDC system, k(obs) increases with W(0) in the whole range studied while in AOT the kinetic profile has a maximum at W(0) approximately 5, probably reflecting the fact that BH(4)(-) is part of the cationic interface while, in the anionic one, there is a strong interaction between water and the polar headgroup of AOT below W(0) = 5 and, above that, BH(4)(-) is repelled from the interface once the water pool has formed. Application of a kinetic model based on the pseudophase formalism, which considers the distribution of the ketone between the continuous medium and the interface and assumes that reaction takes place only at the interface, has enabled us to estimate rate constants at the interface of the reverse micellar systems. At W(0) < 10, it was considered that NaBH(4) is wholly at the interface and, at W(0) >/= 10, where there are free water molecules, also the partitioning between the interface and the water pool was taken into account. The results were used to evaluate CAF and NaBH(4) distribution constants between the different pseudophases as well as the second-order reaction rate constant of the reduction reaction in the micellar interface.     

When is water not water? Exploring water confined in large reverse micelles using a highly charged inorganic molecular probe

The interior water pool of aerosol OT (AOT) reverse micelles tends toward bulk water properties as the micelle size increases. Thus, deviations from bulk water behavior in large reverse micelles are less expected than in small reverse micelles. Probing the interior water pool of AOT reverse micelles with a highly charged decavanadate (V(10)) oligomer using (51)V NMR spectroscopy shows distinct changes in solute environment. For example, when an acidic stock solution of protonated V(10) is placed in a reverse micelle, the (51)V chemical shifts show that the V(10) is deprotonated consistent with a decreased proton concentration in the intramicellar water pool. Results indicate that a proton gradient exists inside the reverse micelles, leaving the interior neutral while the interfacial region is acidic.     

Quantitative model for prediction of hydrodynamic size of nonionic reverse micelles

The sizes of nonionic reverse micelles were investigated as a function of the molecular structure of the surfactant, the type of oil, the total concentration of surfactant [NP], the ratio of surfactant to total surfactant (r), the water to surfactant molar ratio (omega), temperature, salt concentration, and polar phase. The basis of our investigation was a mixture of nonylphenol polyethoxylates--NP4 and NP7, various polar phases, and several oils. Micelle sizes were determined using dynamic light scattering (DLS). A central composite experimental design was used to quantitatively model micelle size as a function of omega, surfactant concentration, and r. The model has demonstrated the capability of predicting the mean diameter of micelles from 4 to 13 with a precision of +/-2 nm as measured by DLS. This quantitative correlation between the size of reverse micelles and the synthetic variables provides the foundation for choosing experimental conditions to control reverse micelle size. In turn, this allows control of the size of nanoparticles synthesized within them.     

Adsorption kinetics of surfactant mixtures from micellar solutions as studied by maximum bubble pressure technique

The adsorption kinetics of micellar solutions of anionic/cationic SDS/DATB mixtures with mixing ratios of 10/1 and 10/2, respectively, are studied experimentally by means of the maximum bubble pressure method. For long adsorption times the adsorption of the highly surface-active anionic/cationic complex leads to a decrease of dynamic surface tension in comparison to the single SDS system. However, the situation is the reverse for short adsorption times where the dynamic surface tension is increased by addition of the cationic surfactant, although the overall concentration is increased. This unexpected behavior is explained by partial solubilization of free SDS molecules into micelles formed by SDS/DTAB complexes. With increasing overall concentration, when eventually the CMC of SDS is reached, the anionic/cationic complex itself is solubilized by SDS micelles. Finally, no complex micelles, which for their part can solubilize an excess of SDS molecules, are present. Hence, the dynamic properties of the solution are no longer influenced by the depletion of SDS molecules and the mixture tends to behave like a pure SDS solution.     

Nanoscale Control Over Interfacial Properties in Mixed Reverse Micelles Formulated by Using Sodium 1,4‐bis‐2‐ethylhexylsulfosuccinate and Tri‐n‐octyl Phosphine Oxide Surfactants

The interfacial properties of pure reverse micelles (RMs) are a consequence of the magnitude and nature of noncovalent interactions between confined water and the surfactant polar head. Addition of a second surfactant to form mixed RMs is expected to influence these interactions and thus affect these properties at the nanoscale level. Herein, pure and mixed RMs stabilized by sodium 1,4‐bis‐2‐ethylhexylsulfosuccinate and tri‐n‐octyl phosphine oxide (TOPO) surfactants in n‐heptane were formulated and studied by varying both the water content and the TOPO mole fraction. The microenvironment generated was sensed by following the solvatochromic behavior of the 1‐methyl‐8‐oxyquinolinium betaine probe and ³¹P NMR spectroscopy. The results reveal unique properties of mixed RMs and we give experimental evidence that free water can be detected in the polar core of the mixed RMs at very low water content. We anticipate that these findings will have an impact on the use of such media as nanoreactors for many types of chemical reactions, such as enzymatic reactions and nanoparticle synthesis.     

Effect of Amino‐Acid‐Based Polar Oils on the Krafft Temperature and Solubilization in Ionic and Nonionic Surfactant Solutions

Abstract

The Krafft temperature and solubilization power of ionic and nonionic surfactants in aqueous solutions are strongly affected by added polar oils such as amino‐acid‐based oils (e.g., N‐acylamino acid esters, AAE), because they tend to be solubilized in the surfactant palisade layer. The Krafft temperatures of 5 wt.% sodium dodecyl sulfate (SDS)‐water and octaoxyethylene octadecyl ether (C₁₈EO₈)‐water systems largely decreases upon addition of AAE and 1‐hexanol, whereas it decreases very slightly in isopropyl myristate (IPM) and n‐dodecane. The lowering of the Krafft temperature can be explained by the same mechanism as the melting‐temperature reduction of mixing two ordinary substances. Namely, the polar oils are solubilized in the surfactant palisade layer of micelles and reduce the melting temperature of hydrated solid‐surfactant (Krafft temperature). On the other hand, non‐polar oil such as dodecane is solubilized deep inside micelles and makes an oil pool. The solubilization of non‐polar oil is enhanced by mixing surfactant with AAE due to an increase in micellar size.     

混合量子-经典动力学方法研究反胶团水池中I2分子的振动频率位移

结合Monte Carlo模拟技术, 提出了一种反胶团溶液的快速数学建模新方法. 利用量子-经典动力学模拟方法, 考察了I2分子受限于两个不同尺寸的反胶团水池中振动频率的诱导位移及谱分布. 结果表明, 相比于体相水, 受限于反胶团水池中I2分子的诱导位移表现为蓝移, 且蓝移大小随水池尺寸变化不大. 通过对I2分子与周围环境相互作用的分解分析, 得到了水池水、表面活性剂以及有机溶剂分子对I2分子振动频率诱导位移的瞬态贡献, 揭示了I2分子振动弛豫的微观作用机制. 此外, 对于受限水池中水分子的诱导贡献及空间分布的研究表明, I2分子振动频率位移的诱导贡献主要来自于第一溶剂层, 它是由4个水分子蓝移贡献和2个水分子红移贡献组成.