Anionic Surfactant Aerosol OT Research Articles

Adsorption kinetics of the partially dissociated anionic surfactant Aerosol-OT in 10 mM NaCl solution

The adsorption kinetics of anionic surfactant Aerosol-OT (AOT) in 10 mM NaCl solution was investigated by measuring and simulating the dynamic and equilibrium surface tension (ST) data. The relaxation of ST for AOT molecules adsorbing onto a freshly created bubble surface was measured using a video-enhanced pendant bubble tensiometer at eighteen AOT concentrations. The partially ionized Langmuir model was applied to best-fit the dynamic/equilibrium ST data. The subsequent findings revealed that the model with a dissociation degree α of 0.1 − 0.2 described the dynamic and equilibrium ST data very well. This implied that AOT behaved as a partially ionized surfactant in 10 mM NaCl solution. The adsorption for AOT onto a clean air–water interface was found to be either (i) diffusion-controlled with a diffusivity value that varied [between 2.5 and 4.6 (10−6 cm2/s)] depending on the bulk concentration or (ii) mixed-controlled with an adsorption rate constant of 2.4 ± 0.8 (107 cm3/mol.s).

The effect of surfactant micellization on the cytotoxicity of silver nanoparticles stabilized with aerosol-OT

The toxic action of surfactant used as a stabilizer of metal nanoparticles have been studied with the aim to determine separate contributions of surfactant monomers and micelles to cell viability decrease. Basing on (1) the well-known ability of surfactant molecules to form micelles in solution at a critical micelle concentration (CMC) and (2) the results reported in literature, showing that toxicity of various surfactants increases when their concentration exceeds CMC, we supposed that surfactant molecules and micelles may differ in their toxic effect on cells. This supposition was verified on the anionic surfactant aerosol-OT (AOT) used as a stabilizer of silver nanoparticles (AgNPs) in studies of their cytotoxicity on Jurkat cells by means of the MTT test. Two samples of AgNPs stabilized with AOT in concentrations higher (3 mM) and lower (1 mM) than its CMC in water were introduced to the cell medium as water solutions diluted to obtain nanoparticle concentrations in the range 1–7 μg/mL. Cell viability changes were registered after 24 h incubation. It was found that AgNPs of similar average size (about 16 nm), synthesized by the same procedure, introduced to the same concentrations in cell medium, produced a different effect on cell viability. Namely, decrease in cell viability was observed for AgNPs with 3 mM AOT, while no noticeable changes were registered for AgNPs with 1 mM AOT. A similar difference was detected for the corresponding dilutions of 3 mM and 1 mM AOT water solutions. We assumed that the toxicity dependence on AOT concentration originated from the difference in toxic action of the two different AOT forms – molecules (monomers) and micelles – present in the AgNPs and AOT solution. The approach was suggested for estimation of the separate contributions of monomers and micelles to the total AOT toxicity; changes of these contributions with AgNPs or AOT concentration were also determined. The results obtained may prove to be useful in studies of the biological activity of surfactants applied both as nanoparticle stabilizers and as agents working in medicine as suppressors of various infections.

Use of reflectance anisotropy spectroscopy for mapping the anisotropy of lyotropic liquid crystal dispersions in formulations

ABSTRACTLyotropic liquid crystal dispersions have commercial importance in the formulation of cleaning products and pharmaceuticals. The anisotropy of such dispersions is an important aspect of their properties and a simple method for measurement would be valuable to formulators. The relatively new optical technique of reflectance anisotropy spectroscopy (RAS) was considered to have potential in this respect. To test the idea, the anisotropy of a binary mixture of an anionic surfactant Aerosol OT (AOT) and glycerol (used as a model for water) was investigated using RAS. The variation in the measured anisotropy parallels the expected behaviour of a dispersion of lyotropic liquid crystals as a function of concentration and temperature. A response surface of anisotropy generated as a function of temperature and surfactant concentration demonstrates the use of RAS as a tool for mapping liquid anisotropy to facilitate the formulation of structured liquids.

Amide I IR probing of core and shell hydrogen-bond structures in reverse micelles

Abstract The properties of N-methylacetamide (NMA) molecules encapsulated in the reverse micelles (RMs) formed by anionic surfactant aerosol OT (AOT), are studied with vibrational spectroscopy and computation. Vibrational spectra of the amide I′ mode of the fully deuterated NMA-d 7 show gradual increase of peak frequencies and line broadening as the size of RMs decreases. Analyses of the spectral features reveal the presence of three states of NMA-d 7 that correspond to NMA located in the core of water phase (absorption frequency of 1606 cm–1) and two types of interfacial NMA near the surfactant layer (1620 and 1644 cm–1). In larger RMs with water content w 0 = [D2O]/[AOT] ≥ 10, only the first two states are observed, whereas in smaller RMs, the population of the third state grows up to 25 % at w 0 = 2. These results indicate the general validity of the two-state core/shell model for the confined aqueous solution of NMA, with small modifications due to the system-dependent solute-interface interaction. However, simulations of small RM systems with w 0 ≤ 15 show continuous variations of the population, frequency shifts, and the solute-solvent interaction strengths at solute-interface distance less than 4 Å. Thus, the distinction of solute core/shell states tends to be blurred in small RMs but is still effective in interpreting the average spectroscopic observables.

Effects of small ionic amphiphilic additives on reverse microemulsion morphology

HypothesisInitial studies (Hopkins Hatzopoulos et al. (2013)) have shown that ionic hydrotropic additives can drive a sphere-to-cylinder (ellipsoid) transition in water-in-oil (w/o) microemulsions stabilized by the anionic surfactant Aerosol-OT; however the origins of this behaviour remained unclear. Here systematic effects of chemical structure are explored with a new set of hydrotropes, in terms of an aromatic versus a saturated cyclic hydrophobic group, and linear chain length of alkyl carboxylates. It is proposed that hydrotrope-induced microemulsion sphere-to-cylinder (ellipsoid) transitions are linked to additive hydrophobicity, and so a correlation between the bulk aqueous phase critical aggregation concentration (cac) and perturbation of microemulsion structure is expected. ExperimentsWater-in-oil microemulsions were formulated as a function of water content w (= [water]/[AOT]) and concentration of different hydrotropes, being either cyclic (sodium benzoate or sodium cyclohexanoate), or linear chain systems (sodium hexanoate, sodium heptanoate and sodium octanoate). Phase behaviour studies were performed as a function of w, additive type and temperature at total surfactant concentration [ST]=0.10M and constant mole fraction x=0.10 (x=[hydrotrope]/[ST]). Microemulsion domain structures were investigated by small-angle neutron scattering (SANS), and these data were fitted by structural models to yield information on the shapes (spheres, ellipsoids or cylinders) and sizes of the nanodroplets. FindingsUnder the conditions of study hydrotrope chemical structure has a significant effect on microemulsion structure: sodium cyclohexanoate does not induce the formation of cylindrical/ellipsoidal nanodroplets, whereas the aromatic analogue sodium benzoate does. Furthermore, the short chain sodium hexanoate does not cause anisotropic microemulsions, but the more hydrophobic longer chain heptanoate and octanoate analogues do induce sphere-to-ellipsoid transitions. This study shows that underlying microemulsion structures can be tuned by hydrotropes, and that the strength of the effect can be identified with hydrotrope hydrophobicity in terms of the bulk aqueous phase cac.

In Situ X-ray Polymerization: From Swollen Lamellae to Polymer–Surfactant Complexes

The influence of the monomer diallyldimethylammonium chloride (D) on the lamellar liquid crystal formed by the anionic surfactant aerosol OT (AOT) and water is investigated, determining the lamellar spacings by SAXS and the quadrupolar splittings by deuterium NMR, as a function of the D or AOT concentrations. The cationic monomer D induces a destabilization of the AOT lamellar structure such that, at a critical concentration higher than 5 wt %, macroscopic phase separation takes place. When the monomer, which is dissolved in the AOT lamellae, is polymerized in situ by X-ray initiation, a new collapsed lamellar phase appears, corresponding to the complexation of the surfactant with the resulting polymer. A theoretical model is employed to analyze the variation of the interactions between the AOT bilayers and the stability of the lamellar structure.

Extraction and Purification of Lectin by Employing Anionic Surfactant Aerosol-OT Reversed Micellar System and Evaluation of its Antibacterial and Antioxidant Potentials

Extraction and Purification of Lectin by Employing Anionic Surfactant Aerosol-OT Reversed Micellar System and Evaluation of its Antibacterial and Antioxidant Potentials

Microstructure, drug binding and cytotoxicity of Pluronic P123–aerosol OT mixed micelles

The interaction between triblock copolymer Pluronic P123 and anionic surfactant Aerosol OT (AOT) has been studied using dynamic light scattering (DLS) and small angle neutron scattering (SANS). The copolymer/surfactant system has been investigated at a constant copolymer concentration of 0.5 wt% and with varying surfactant concentration up to ∼5 mM. DLS results reveal that addition of AOT suppresses the micellization of P123. With the addition of AOT, P123–AOT mixed micelles are formed via hydrophobic association of the surfactant with the block copolymer micelles leading to the development of negative surface charge on copolymer micelles. The solubilization of the hydrocarbon chains of the surfactant, AOT, into the polypropylene oxide core of the block-copolymer micelle leads to increased electrostatic repulsion between the P123–AOT mixed micelles. SANS studies show a steady decrease in the core size and aggregation number of the micelles with the increasing concentration of AOT, consistent with the DLS results. The binding ability of AOT to the anticancer drug doxorubicin hydrochloride (DOX) was confirmed by dynamic surface tension measurements. The in vitro cytotoxicity studies in various cancer cell lines revealed that anticancer activity of the drug is not affected by complexation with AOT and DOX-loaded micelles are more potent to MCF-7 human breast cancer cells compared to other cell lines. The present findings can form a basis to design block copolymer based micellar carriers for drug delivery.

Nonstoichiometric polymer-surfactant complexes obtained in a lamellar lyotropic medium

The addition of a polyelectrolyte to lamellar media formed by an oppositely charged surfactant often leads to the coexistence of several phases without macroscopic phase separation, which makes their characterization difficult. Here, the effect of the polydiallyldimethylammonium chloride (PD) on the lamellar liquid crystal formed by the anionic surfactant Aerosol OT (AOT) and water is investigated. Small-angle X-ray scattering results are discussed regarding the changes in the lamellar spacing as a function on the PD or AOT concentrations. In most of the samples, two lamellar phases, without macroscopic phase separation, are detected. One of them is a typical swollen phase, while the other is a collapsed phase, which corresponds to the polymer-surfactant complex. At concentrations of polymer up to 3 wt%, the two lamellar phases coexist; however, at a critical concentration higher than 3 wt%, the swollen phase becomes isotropic, and a macroscopic phase separation takes place. A simple model is proposed to calculate the composition of the phases when macroscopic phase separation does not occur. The results thus calculated show that generally the polymer-surfactant complexes are nonstoichiometric containing a lesser amount of polymer than ideally expected.

Impact of Multi-Interface Surfactant Adsorption on Wettability in Dense Nonaqueous Phase Liquid Systems

The wettability, and hence the distribution and transport, of dense nonaqueous phase liquid (DNAPL) wastes in the subsurface is strongly affected by surfactants that are present in the wastes. To understand their impact, this work examined the dependence of the contact angle of quartz/tetrachloroethylene (PCE)/water systems containing the anionic surfactant Aerosol OT (AOT) or/and the nonionic surfactant hexaoxyethylene glycol mono-n-dodecyl ether (C12E6) on the surfactants' adsorption at both the PCE/water and silica/water interfaces. Results showed that C12E6 enhanced oil wetness at low pH. However, the system reverted to water-wet upon the addition of AOT, which by itself, did not alter the wettability of the system. To mechanistically explain such behavior, it was proposed that, based on measurements of adsorption onto quartz and calculations of surface excess using an equation-of-state approach, the increased oil-wetness in systems containing C12E6 is due to the entropy-driven attraction between the oxyethylene groups of the adsorbed C12E6 at both the PCE/water and water/silica interfaces. Addition of AOT reduced the adsorbed C12E6 at both interfaces, lowering the attraction between PCE and silica. Thus, a consideration of surfactant adsorption at both interfaces, and the subsequent interaction between the molecules sorbed at both interfaces, is critical to explaining the wettability behavior of waste DNAPLs.

Polymer‐surfactant nanoparticles for sustained release of water‐soluble drugs

Poor drug encapsulation efficiency and rapid release of the encapsulated drug limit the use of nanoparticles in biomedical applications involving water-soluble drugs. We have developed a novel polymer-surfactant nanoparticle formulation, using the anionic surfactant Aerosol OT (AOT) and polysaccharide polymer alginate, for sustained release of water-soluble drugs. Particle size of nanoparticles, as determined by atomic force microscopy and transmission electron microscopy, was in the range of 40-70 nm. Weakly basic molecules like methylene blue, doxorubicin, rhodamine, verapamil, and clonidine could be encapsulated efficiently in AOT-alginate nanoparticles. In vitro release studies with basic drug molecules indicate that nanoparticles released 60-70% of the encapsulated drug over 4 weeks, with near zero-order release during the first 15 days. Studies with anionic drug molecules demonstrate poorer drug encapsulation efficiency and more rapid drug release than those observed with basic drugs. Further studies investigating the effect of sodium concentration in the release medium and the charge of the drug suggest that calcium-sodium exchange between nanoparticle matrix and release medium and electrostatic interaction between drug and nanoparticle matrix are important determinants of drug release. In conclusion, we have formulated a novel surfactant-polymer drug delivery carrier demonstrating sustained release of water-soluble drugs.

Dissipative Particle Dynamics Simulation of Phase Behavior of Aerosol OT/Water System

The phase behaviors of the binary mixture of an anionic surfactant aerosol OT (AOT) and water are investigated on a mesoscopic level using dissipative particle dynamics (DPD) computer simulations. With a simple surfactant model, various aggregation structures of AOT in water including the lamellar, viscous isotropic, and reverse hexagonal phases are obtained, which agree well with the experimental phase diagram. Special attention is given on the unusual lamellar regions. Water diffusivity shows much useful information to understand how the phase behaviors varied with concentration and temperature. It is proposed that the anomalous lamellar phenomena at intermediate AOT concentration (about 40%) are due to the formation of a defective structure, pseudoreversed hexagonal phase, which evidently decreases the water diffusivity. After increasing temperature above 328 K, the pseudoreversed hexagonal structure will be partly transformed to a normal lamellar phase structure and the system lamellar ordering is therefore enhanced.

Enzymatic hydrolysis of microcrystalline cellulose in reverse micelles

The activities of cellulases from Trichoderma reesei entrapped in three types of reverse micelles have been investigated using microcrystalline cellulose as the substrate. The reverse micellar systems are formed by nonionic surfactant Triton X-100, anionic surfactant Aerosol OT (AOT), and cationic surfactant cetyltrimethyl ammonium bromide (CTAB) in organic solvent mediae, respectively. The influences of the molar ratio of water to surfactant ω 0, one of characteristic parameters of reverse micelles, and other environmental conditions including pH and temperature, on the enzymatic activity have been studied in these reverse micellar systems. The results obtained indicate that these three reverse micelles are more effective than aqueous systems for microcrystalline cellulose hydrolysis, and cellulases show “superactivity” in these reverse micelles compared with that in aqueous systems under the same pH and temperature conditions. The enzymatic activity decreases with the increase of ω 0 in both AOT and Triton X-100 reverse micellar systems, but reaches a maximum at ω 0 of 16.7 for CTAB reverse micelles. Temperature and pH also influence the cellulose hydrolysis process. The structural changes of cellulases in AOT reverse micelles have been measured by intrinsic fluorescence method and a possible explanation for the activity changes of cellulases has been proposed.

Optical transition rates of a meso-substituted thiacarbocyanine in methanol-in-oil reverse micelles

We report the photophysical properties of 3,3′-diethyl-5,5′-dichloro-9-phenylthiacarbocyanine (DDPT) in methanol-in-oil (m/o) reverse micellar systems which form methanol droplets stabilized with anionic surfactant aerosol-OT (AOT) in n-heptane. The fluorescence quantum yield of DDPT is enhanced by a factor of 17 in the methanol droplet in comparison with bulk methanol. The fluorescence lifetimes of DDPT in m/o reverse micelles are prolonged up to 2.2 ns with increasing molar ratio of methanol to surfactant ( w 0=[MeOH]/[AOT]), whereas the fluorescence lifetime of DDPT in bulk methanol is 75 ps. The non-radiative rate constants of DDPT in the droplets are decreased by a factor of 40, resulting in a remarkable enhancement in quantum yields, indicating that internal motions of DDPT in the droplets are significantly reduced due to strong electrostatic interactions between the positively charged DDPT and the negatively charged sulfonate head-groups of AOT and the spatial confinement induced by the reverse micellar structure.

Polymerization of N,N-dimethylacrylamide in Aerosol OT–water mixtures: from lamellae to segregation

The polymerizable N,N-dimethylacrylamide (DMAA) is added to the lamellar liquid crystalline system formed by the anionic surfactant Aerosol OT and water. Small angle X-ray scattering, gives the same bilayer thickness and hydrophilic group cross-sectional area as without DMAA, and an environment sensitive fluorescent probe indicates that DMAA is likely to be in the water layers between the AOT bilayers. This shows that the structure of the AOT bilayer in the lamellar phase is not affected by DMAA. However, DMAA induces instability of the lamellar phase, and a second phase, an isotropic solution, appears for larger DMAA contents The effect of DMAA in stabilizing this phase with respect to lamellae is much larger than that expected from a simple dilution with water. Furthermore, there is a combined influence of temperature and DMAA concentration on the coexistence of the lamellar and the isotropic phases. When an initiator is added and the monomer polymerizes the tendency to phase separate increases strongly. The lamellar spacing after polymerization is below the value predicted by the global AOT/H2O ratio. This indicates that the polymer segregates from the AOT phase and that the lamellar phase is osmotically compressed by the polymer rich phase. (C) 2003 Elsevier Science B.V. All rights reserved.

Preparation and kinetic studies of surfactant–horseradish peroxidase ion paired complex in organic media

Horseradish peroxidase was solubilized in organic medium using the technique of ion pairing with the anionic surfactant Aerosol OT (AOT). System parameters like surfactant concentration, pH, ionic strength of the aqueous phase, cosolvent concentration etc. were found to influence the solubilization of HRP. Kinetic behaviour of the ion paired complex towards oxidation of o-phenylenediamine (OPD) in three different solvent systems—aqueous, aqueous-organic monophasic and aqueous-organic biphasic system was investigated. Steady state kinetics of ion paired HRP revealed that ion pairing of the enzyme with the surfactant resulted in an increase in the apparent K m and a decrease in V max values of the enzyme in aqueous medium. Enzyme–substrate affinity ( K m) showed an improvement for the ion paired enzyme in organic solvents and was found to decrease with increase in hydrophobicity of the solvent with the ion paired enzyme exhibiting the least K m value in the most hydrophobic solvent toluene. There was an eightfold decrease in K m value in toluene compared to that in aqueous medium. Among the two types of non-aqueous systems used for the study, V max was found to be higher in the microaqueous system (toluene). Ion paired HRP exhibited very high catalytic efficiency ( K cat/ K m) in toluene compared to that of native HRP.

Rheology at the Phase Transition Boundary: 1. Lamellar LαPhase of AOT Surfactant Solution

The phase behavior of the binary mixture of an anionic surfactant Aerosol OT (AOT) and glycerol (used as a model for water) is investigated using rheological, optical, and X-ray synchrotron techniques. In particular, the storage shear modulus G‘ response, as a function of temperature and strain amplitude, has been related to the structural transformation into the lamellar Lα phase surfactant bilayers. Results demonstrate a characteristic critical behavior at the phase transition boundary T = Tc. Stress−strain variation indicates the shear-induced alignment of the layer system with a universal scaling of stress−strain.

Equilibrium between Poly(N,N-dimethylacrylamide) and the Lamellar Phase of Aerosol OT/Water

A mixed amphiphilic system composed of the anionic surfactant Aerosol OT (AOT), in water forming a lamellar phase, to which is added a neutral noninteracting polymer, poly(N,N-dimethylacrylamide), is studied experimentally by SAXS, 2H NMR, and microscopy, in a range of surfactant and polymer compositions. Addition of the polymer produces a decrease in the lamellar spacing, the decrease by the polymer being almost twice that produced by an equal volume of AOT. Microscopy reveals heterogeneity, but no macroscopic phase separation occurs. 2H NMR detects that on increasing the polymer concentration some water is in an isotropic environment. It is inferred that the presence of the polymer induces a microscopic phase separation into a polymer-rich isotropic phase and a surfactant-rich lamellar phase, and this is tested theoretically by calculating the osmotic pressures in these two phases. In the lamellar phase, the effect of electrostatic, undulation, van der Waals, and hydration forces on the AOT bilayer is considered; in the isotropic phase, the osmotic contribution of the polymer is considered. These two pressures correlate well, supporting theoretically the hypothesis of the two phases in equilibrium.

Optimization of ion-paired lipase for non-aqueous media: acylation of doxorubicin based on surface models of fatty acid esterification

The lipase from Mucor javanicus was shown to catalyze the acylation of the primary hydroxyl (C14-OH) of doxorubicin (DOX), a potent anticancer compound. An ion-pairing method for solubilizing enzymes in organic solvents with the anionic surfactant Aerosol OT (AOT) was then adapted to enhance the non-aqueous activity of the lipase, representing the first demonstration of this solubilization and activation technique for a lipase. The pH and ionic strength of the aqueous phase during solubilization were identified as the factors having the greatest impact on the extraction efficiency and specific activity of the biocatalyst. A series of expanding experimental matrices yielded both solubility and specific activity surfaces as functions of [NaCl] and pH in the extraction of M. javanicus lipase. The activity response surfaces were generated with the esterification of octanoic acid with 1-nonanol in isooctane as a convenient model reaction, yet the results were shown to transfer to the acylation of DOX with 2-thiophene acetic acid vinyl ester, or vinyl butyrate, in toluene. A generalized approach to ion-paired lipase solubilization was thus developed, and a potentially high-value biotransformation was enhanced using a low-cost and easily-assayed reaction.

Effects of Fluorocarbon Surfactant Chain Structure on Stability of Water-in-Carbon Dioxide Microemulsions. Links between Aqueous Surface Tension and Microemulsion Stability

Formation and stability of water-in-carbon dioxide microemulsions are described with eight related fluorinated analogues of the anionic surfactant Aerosol-OT. The aim was to identify a structure−performance relationship in CO2 with surfactants of high surface chemical purity that can also be synthesized from readily available reagents. The most effective CO2-philes from this group were sodium bis(1H,1H-perfluoropentyl)-2-sulfosuccinate (di-CF4), sodium bis(1H,1H,2H,2H-perfluorohexyl)-2-sulfosuccinate (di-CF4H), and sodium bis(1H,1H-perfluoroheptyl)-2-sulfosuccinate (di-CF6). All three of these compounds stabilized microemulsions at CO2 bottle pressure (57 bar) at 15 °C, with a w value ([water]/[surf]) of 10. A close correlation is demonstrated between limiting aqueous phase surface tension of a given surfactant at its critical micelle concentration, γcmc, and its performance in water−CO2 microemulsions, as measured by the phase transition pressure Ptrans. This finding has important implications for the ra...