Phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer and poly(oxyethylene) surfactant in water

The phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer (PI-PEO or C250EO70) and poly(oxyethylene) surfactant (C12EO3, C12EO5, C12EO6, C12EO7, and C12EO9) in water was investigated by phase study, small-angle X-ray scattering, and dynamic light scattering (DLS). The copolymer is not soluble in surfactant micellar cubic (I1), hexagonal (H1), and lamellar (Lalpha) liquid crystals, whereas an isotropic copolymer fluid phase coexists with these liquid crystals. Although the PI-PEO is relatively lipophilic, it increases the cloud temperatures of C12EO3-9 aqueous solutions at a relatively high PI-PEO content in the mixture. Most probably, in the copolymer-rich region, PI-PEO and C12EOn form a spherical composite micelle in which surfactant molecules are located at the interface and the PI chains form an oil pool inside. In the C12EO5/ and C12EO6/PI-PEO systems, one kind of micelles is produced in the wide range of mixing fraction, although macroscopic phase separation was observed within a few days after the sample preparation. On the other hand, small surfactant micelles coexist with copolymer giant micelles in C12EO7/ and C12EO9/PI-PEO aqueous solutions in the surfactant-rich region. The micellar shape and size are calculated using simple geometrical relations and compared with DLS data. Consequently, a large PI-PEO molecule is not soluble in surfactant bilayers (Lalpha phase), infinitely long rod micelles (H1 phase), and spherical micelles (I1 phase or hydrophilic spherical micelles) as a result of the packing constraint of the large PI chain. However, the copolymer is soluble in surfactant rod micelles (C12EO5 and C12EO6) because a rod-sphere transition of the surfactant micelles takes place and the long PI chains are incorporated inside the large spherical micelles.     

Effect of phenol on the aggregation characteristics of an ethylene oxide-propylene oxide triblock copolymer P65 in aqueous solution

The effects of phenol on the micellization, micellar growth, and phase separation of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic P65: EO19 PO30 EO19) in aqueous solution have been studied by cloud point, viscosity, dynamic light scattering (DLS), differential scanning calorimetry (DSC), fluorescence spectroscopy, and small-angle neutron scattering (SANS). Various concentrations of P65 have been chosen to estimate the effect of phenol on different concentration regions of P65. Phenol interacts quite differently at low concentrations (0-2%) than at high concentrations (2-10%) of P65, as per the observation that phenol is more predominant at smaller concentrations of P65. A marked decrease in the cloud points of the P65 solutions is observed in presence of phenol. The critical micelle temperature (CMT) of P65 shows a synergistic effect of phenol on P65 aggregates. Micellar transitions, phase separation, and aggregation behaviours like micellization and micellar growth in the presence of phenol have been observed by combining viscometry, DLS, DSC, and CP. DLS shows that the effect of phenol is predominant at high temperatures. SANS shows a high increase in axial ratio and aggregation numbers in the presence of phenol at fixed concentrations of P65. Fluorescence data illustrate that addition of phenol makes micelles polar but at the same time its favours aggregation. Water-soluble phenol (present in low concentrations) forms aggregates with P65, which can be separated by cloud point extraction, making this study interesting for separation of phenol from the phenol-water system.     

Tuning the structure of SDS micelles by substituted anilinium ions

The growth behavior of aggregates formed in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the cationic hydrophobic salts o-toluidine hydrochloride (OTHC) and m-toluidine hydrochloride (MTHC) has been studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS) techniques. DLS studies indicate a progressive growth of SDS micelles with addition of less than equimolar concentrations of hydrophobic salts. A prolate ellipsoidal model is used to analyze the DLS data, which is further supported by SANS measurements. We explain the propensity for the strong growth of micelles in the presence of OTHC and MTHC by the high charge neutralization provided by these salts as the aromatic counterions are adsorbed on the surface of the micelles. When the substitution is at the meta position, i.e., for MTHC, micellar growth is favored at lower salt concentrations than for OTHC. The variation in growth behavior is explained in terms of the difference in the chemical environments of the substituents at the ortho and meta positions. Micellar parameters obtained from SANS data at elevated temperature also support enhanced growth of micelles in the presence of MTHC as compared to OTHC.     

Effect of added poly(oxyethylene)dodecyl ether on the phase and rheological behavior of wormlike micelles in aqueous SDS solutions

Upon the addition of a short EO chain nonionic surfactant, poly(oxyethylene) dodecyl ether (C12EOn), to dilute micellar solution of sodium dodecyl sulfate (SDS) above a particular concentration, a sharp increase in viscosity occurs and a highly viscoelastic micellar solution is formed. The oscillatory-shear rheological behavior of the viscoselastic solutions can be described by the Maxwell model at low shear frequency and combined Maxwell-Rouse model at high shear frequency. This property is typical of wormlike micelles entangled to form a transient network. It is found that when C12EO4 in the mixed system is replaced by C12EO3 the micellar growth occurs more effectively. However, with the further decrease in EO chain length, phase separation occurs before a viscoelastic solution is formed. As a result, the maximum zero-shear viscosity is observed at an appropriate mixing fraction of surfactant in the SDS-C12EO3 system. We also investigated the micellar growth in the mixed surfactant systems by means of small-angle X-ray scattering (SAXS). It was found from the SAXS data that the one-dimensional growth of micelles was obtained in all the SDS-C12EOn (n=0-4) aqueous solutions. In a short EO chain C12EOn system, the micelles grow faster at a low mixing fraction of nonionic surfactant.     

Pressure-induced reentrant micellization of amphiphilic block copolymers in dilute aqueous solutions

The pressure-induced structural changes of a block copolymer, poly(2-ethoxyethoxyethyl vinyl ether)-block-poly(2-hydroxyethyl vinyl ether) (pEOEOVE-b-pHOVE) in aqueous solutions, were studied by means of small-angle neutron scattering (SANS) and dynamic light scattering (DLS) from atmospheric pressure up to 400 MPa. pEOEOVE-b-pHOVE formed a spherical micellar structure above 40 degrees C due to poor solubility of pEOEOVE. Micellization phase diagram was determined by DLS, and a covex-upward pressure-temperature (P-T) phase diagram was obtained having a peak around (P,T)=(150 MPa,48 degrees C). The SANS curves at 50 degrees C were analyzed as a function of P. The micellar core size decreased by pressurizing at low P's (P相似文献   

Reverse-micelle formation in the partitioning of trivalent F-element cations by biphasic systems containing a tetraalkyldiglycolamide

The conditions for reverse-micelle formation were studied for solutions of tetra-n-octyldiglycolamide (TODGA) in alkane diluents equilibrated with aqueous solutions of nitric or hydrochloric acids in the presence and absence of Nd3+. Small-angle neutron scattering, vapor-pressure osmometry, and tensiometry are all consistent with the partial formation of TODGA dimers at the lowest acidities, transitioning to a polydisperse mixture containing TODGA monomers, dimers, and small reverse-micelles of TODGA tetramers at aqueous nitric acid acidities of 0.7 M or higher in the absence of Nd. Application of the Baxter model to the samples containing 0.005-0.015 M Nd reveals the persistence of tetrameric TODGA reverse-micelles with significant interparticle attraction between the polar cores of the micelles that increases with increasing organic phase concentrations of acid or Nd. Our experimental findings suggest that the peculiar behavior of TODGA with respect to the extraction of trivalent lanthanide and actinide cations arises from the affinity of these metal cations for the preformed TODGA reverse-micelle tetramers.     

Influence of a hydrophobic diol on the micellar transitions of Pluronic P85 in aqueous solution

Micellization behavior of an amphiphilic ethylene oxide-propylene oxide-ethylene oxide tri-block copolymer Pluronic P85 [(EO)(26)(PO)(39)-(EO)(26)] in aqueous solution and in the presence of a hydrophobic C(14)diol (also known as Surfynol104) was examined by physico-chemical methods such as viscometry, cloud point (CP) and scattering techniques viz. dynamic light scattering (DLS) and small angle neutron scattering (SANS). The addition of diol decreases the cloud point and gelation temperature of aqueous Pluronic P85 copolymer solution. DLS and SANS measurements of the polymer in aqueous solution indicated micellar growth and sphere to rod transition in the presence of diol. Surfynol 104 is a sparingly water soluble diol surfactant with a solubility of approximately 0.1 wt%. However, up on addition to Pluronic solution, diol gets incorporated in the block copolymer micelles and leads to structural transition of the micelles. An increase in the temperature and the presence of added sodium chloride in the solution further enhances this effect. The addition of hydrophobic C(14)diol increases the hydrodynamic size and aggregation numbers of the micellar system. The micellar parameters for the copolymer in the presence of C(14)diol are reported at different temperatures and added sodium chloride concentrations.     

The microstructure of di-alkyl chain cationic/nonionic surfactant mixtures: observation of coexisting lamellar and micellar phases and depletion induced phase separation

The evolution of the microstructure and composition occurring in the aqueous solutions of di-alkyl chain cationic/nonionic surfactant mixtures has been studied in detail using small angle neutron scattering, SANS. For all the systems studied we observe an evolution from a predominantly lamellar phase, for solutions rich in di-alkyl chain cationic surfactant, to mixed cationic/nonionic micelles, for solutions rich in the nonionic surfactant. At intermediate solution compositions there is a region of coexistence of lamellar and micellar phases, where the relative amounts change with solution composition. A number of different di-alkyl chain cationic surfactants, DHDAB, 2HT, DHTAC, DHTA methyl sulfate, and DISDA methyl sulfate, and nonionic surfactants, C12E12 and C12E23, are investigated. For these systems the differences in phase behavior is discussed, and for the mixture DHDAB/C12E12 a direct comparison with theoretical predictions of phase behavior is made. It is shown that the phase separation that can occur in these mixed systems is induced by a depletion force arising from the micellar component, and that the size and volume fraction of the micelles are critical factors.     

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.     

Effect of SDS on the self-assembly behavior of the PEO-PPO-PEO triblock copolymer (EO)20(PO)70(EO)20

The mixed micellar system comprising the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)-based triblock copolymer (EO)(20)(PO)(70)(EO)(20) (P123) and the anionic surfactant sodium dodecyl sulfate (SDS) has been investigated in aqueous media by small-angle neutron scattering (SANS) and viscosity measurements. The aggregation number of the copolymer in the micelles decreases upon addition of SDS, but a simultaneous enhancement in the degree of micellar hydration leads to a significant increase in the micellar volume fraction at a fixed copolymer concentration. This enhancement in the micellar hydration leads to a marked increase in the stability of the micellar gel phase until it is destroyed at very high SDS concentration. Mixed micellar systems with low and intermediate SDS concentrations form the micellar gel phase in much wider temperature and copolymer concentration ranges than the pure copolymer micellar solution. A comparison of the observed results with those for the copolymers (EO)(26)(PO)(40)(EO)(26) (P85) and (EO)(99)(PO)(70)(EO)(99) (F127) suggests that the composition of the copolymers plays a significant role in determining the influence of SDS on the gelation characteristics of the aqueous copolymer solutions. Copolymers with high PO/EO ratios show an enhancement in the stability of the gel phase, whereas copolymers with low PO/EO ratios show a deterioration of the same in the presence of SDS.     

Room temperature sphere-to-rod growth and gelation of PEO-PPO-PEO triblock copolymers in aqueous salt solutions

The effects of NaCl and KF on the sphere-to-rod micellar growth behavior of triblock copolymers having two different compositions, (EO)20(PO)70(EO)20 (P123) and (EO)26(PO)40(EO)26 (P85), have been studied by dynamic light scattering (DLS), small angle neutron scattering (SANS) and dilute solution viscometry. NaCl can effectively tune the sphere-to-rod growth temperature of the micelles of both these copolymers and induce micellar growth down to the room temperature and below. The growth behavior is found to be dependent on the composition of the copolymer as P123 being more hydrophobic shows the room temperature growth in the presence of ethanol at significantly lesser NaCl concentration than the less hydrophobic copolymer P85. DLS studies depict for the first time the growth driven transition of the copolymer solutions from dilute to semi-dilute regime as a function of copolymer and salt concentrations. KF can also induce room temperature growth of the P123 micelles at lesser salt concentration than NaCl but it fails to induce any such growth of the P85 micelles. A pseudo-binary temperature-concentration phase diagram on 15% copolymer solutions shows the variation of the sphere-to-rod transition temperature and the cloud point of the copolymer solutions as a function of salt concentration.     

Degradation patterns of tetracycline antibiotics in reverse micelles and water

The objective of this study was to determine the chemical stability of tetracycline and oxytetracycline hydro-chlorides in reverse micelles. Their reverse micellar solutions were prepared using cetyltrimethylammonium bromide, water and ethyl formate. The aqueous solutions of the tetracycline antibiotics were also prepared for comparison. The reverse micellar and aqueous solutions were stored at 37 degrees C. Samples were analyzed by high performance liquid chromatography. When evaluation was performed on an aqueous tetracycline HCl solution, its half-life was estimated to be 329 h. Its chemical stability was not improved after being dissolved in the reverse micelles, and a similar half-life of 330 h was observed. However, there were noticeable differences between the two systems in terms of degradation kinetics and degradation byproducts. On the other hand, oxytetracycline HCl was unstable in water so that its half-life was only 34 h. Very interestingly, pronounced improvement in stability was attained with the reverse micellar system: upon dissolving in the reverse micelles, its half-life was increased to 2402 h. There were also marked differences in degradation patterns and mechanisms of oxytetracycline HCl in water and the reverse micelles. Our study indicates that the reverse micellar system has potential applications in solubilizing and stabilizing oxytetracycline HCl, thereby contributing to the development of its dosage forms.     

Binding constants and partial molar volumes of primary alcohols in sodium dodecylsulfate micelles

The densities of methanol, ethanol, 1-propanol, 1-butanol and 1-hexanol were measured in aqueous solutions of sodium dodecylsulfate at 25°C. The partial molar volumes of the alcohols at infinite dilution in the aqueous surfactants solutions were calculated and discussed using a mass-action model for the alcohol distribution between the aqueous and the micellar phase. The partial molar volumes of the alcohols in the aqueous and in the micellar phases, and the ratios between the binding constant and the aggregation number, were calculated. The partial molar volume for all the alcohols in micellar phase is 10 cm³-mol^–1 smaller than that in octane. This can be related to the strong hydrophilic interaction between the head groups of the alcohol and the micellized surfactant. From the extrapolated values of the distribution constant and the partial molar volumes in the aqueous and micellar phases, the standard partial molar volume of heptanol in micellar solutions was found to decrease with increasing surfactant concentration. The standard free energy of transfer of alcohols from water to micelles was rationalized in terms of hydrophilic and hydrophobic contributions. A model is proposed in which the empty space around each solute is assumed to be the same in the gas and liquid phases, and is used to explain the behavior of micelles in the presence of amphiphilic solutes.     

Morphological characterization of self-assembled peptide nucleic acid amphiphiles

Peptide nucleic acid amphiphiles (PNAA) are a promising set of materials for sequence-specific separation of nucleic acids from complex mixtures. To implement PNAA in micellar separations, the morphology and size of PNAA micelles in the presence and absence of a sodium dodecyl sulfate (SDS) cosurfactant have been studied by small-angle X-ray scattering and dynamic light scattering. We find that a 6-mer PNAA with a 12-carbon n-alkane tail forms ellipsoidal micelles (a = 5.15 nm; b = 3.20 nm) above its critical micelle concentration (CMC) of 110.9 microM. On addition of a stoichiometric amount of complementary DNA, PNAA hybridizes to DNA, suppressing the formation of PNAA micelles. At a ratio of 19:1 SDS/PNAA (total concentration = 20 mM), spherical micelles are formed with outer radius Rs = 2.67 nm, slightly larger than spherical micelles of pure SDS. Capillary electrophoresis studies show that PNAA/DNA duplexes do not comicellize with SDS micelles. No such effects are observed using noncomplementary DNA. The shape and size of the PNAA micelles is also verified by dynamic light scattering (DLS) studies. These results provide an interesting case study with competing electrostatic, hydrophobic, and hydrogen-bonding interactions in micellar systems and make possible the use of PNAA in micellar separations of DNA oligomers.     

Dynamic light scattering study of the dynamics of a gelled polymeric micellar system

The dynamics of the E(92)B(18)/water system are studied by dynamic light scattering (DLS) in the liquid, soft gel, and hard gel phases. Both the liquid and the soft gel phases are micellar phases, although the structural order is higher in the soft gel phase than in the liquid phase. The hard gel phase corresponds to a face-centered cubic arrangement of micelles. DLS results show that the dilute liquid phase is characterized by a single characteristic time tau(1) associated with the diffusion of the micelles. In addition, a second characteristic time tau(2) associated with the presence of micellar clusters in the system is identified in the concentrated liquid and in the soft gel phases. According to these results, DLS suggests that the structure of the soft gel phase comprises micellar clusters coexisting with micellar fluid, in good agreement with hypotheses from our previous work. The dynamics of the system slows down as the hard gel phase is approached and a plateau is observed in the DLS correlation function. The structure of the hard gel is "softened" upon increasing temperature and/or decreasing concentration.     

Flocculation of anionic surfactant micelles in the presence of hydrocarbons

Adsorptive micellar flocculation (AMF) is a surfactant-based separation process based on the flocculation of micelles of suitable anionic surfactants by Al³⁺. The micelles form large amorphous flocs which can be removed by filtration, which is attractive because soluble pollutants are captured by the floc, thus providing a simple separation method. As the primary aim of AMF is the removal of anionic pollutants from aqueous streams, it is important to investigate the influence of the various substances which may affect it. This article discusses the influence of the presence of insoluble hydrocarbons on the flocculation of micelles of the anionic surfactant dodecyl sulfate by Al³⁺. The ratio between surfactant remaining in solution and total surfactant and the ratio between Al³⁺ and surfactant in the flocculate are determined in systems composed of an aqueous solution containing a constant dodecyl sulfate concentration of 0.05 M and a variable Al³⁺ concentration and an organic phase (decane) with phase volume ratios of decane/water ranging from 0 (no decane) to 0.15. The flocculation is only slightly affected by the presence of decane for decane/solution ratios below 0.05, while the effect (lower flocculated fraction) is more marked above this ratio. Received: 25 October 1999/Accepted: 7 February 2000     

Fluorescence relaxation dynamics of acridine orange in nanosized micellar systems and DNA

In this paper, we report a detailed study of the fluorescence relaxation dynamics of a well-known fluorescent DNA intercalator, acridine orange (AO), in reverse micelles (RM), micelles, and DNA using picosecond resolved fluorescence spectroscopy. Solvation studies of AO in AOT reverse micelles (RM) containing water indicate the locations of AO close to the interface and those in RM containing NaOH; there are two types of AO--one in the nonpolar oil phase and the other at the interface. The bound water at the reverse micellar interface is found to be much more rigid than that at the micellar interface of sodium dodecyl sulfate (SDS) micelles. Dynamic light scattering (DLS) studies allow for the determination of the hydrodynamic radius and the overall tumbling motion of the macromolecules. Wobbling-in-cone data analysis of the temporal fluorescence anisotropy decay allows for determination of restriction on the motion of fluorophores attached to the macromolecules. This model further applied to AO-intercalated genomic DNA and synthetic oligonucleotides within their structural integrity (as confirmed through circular dichroism (CD) studies) shows that AO experiences less restriction in genomic salmon sperm DNA compared with that in synthetic oligonucleotides, and among the oligonucleotides, the ones with AT base pairs are much more rigid. This study would invoke further research on the dynamical nature of AO in restricted environments.     

Application of linear solvation energy relationships to polymeric pseudostationary phases in micellar electrokinetic chromatography

Polymerized sodium 11-acrylamidoundecanoate (poly(Na 11-AAU)) was used as a pseudostationary phase (PSP) for micellar electrokinetic chromatography to separate uncharged compounds. The polymer PSP showed signifcantly different solute migration behaviors from conventional micelles including sodium dodecyl sulfate and poly (sodium 10-undecylenate), giving high separation efficiencies (>200000 theoretical plates/m). Linear solvation energy relationships were used to evaluate and characterize the chemical interactions that influence the retention behavior in the poly (Na 11-AAU) micellar system. It was found that the solute volume and solute hydrogen bond basicity mainly influenced the retention. The characteristic feature of the poly (Na 11-AAU) micellar system is that the micelle has a significantly higher capacity for dipole-dipole and dipole-induced dipole interactions as well as a slightly higher capacity for electron pair interactions than the aqueous phase. Due to its unique selectivity, the poly(Na 11-AAU) micellar system would become an attractive new option for selectivity optimization on methods development.     

Formation of rodlike micelles of dimethyloleylamine oxide in aqueous solutions: effects of addition of HCl and NaCl on the micelle size and the intermicellar interaction

Angular dependence of light scattering has been measured for aqueous solutions of dimethyloleylamine oxide in the presence of HCl and NaCl. In micellar solutions more concentrated than 0.1×10^–2 g cm^–3, rodlike micelles are dominantly formed, and their properties are strongly reflected in the characteristics of the solutions. The aggregation number, radius of gyration and even flexibility of the rodlike micelles increase with the addition of HCl as well as NaCl. The increase of HCl concentration up to 10^–3 N makes the aggregation number of rodlike micelles as large as 58,000, when 0.01 M NaCl is present. The large micelle size would be stabilized by the dehydration of the amine oxide group and the hydrogen bonding between nonprotonated and protonated molecules in a micelle.In aqueous solutions without HCl and NaCl or in their presence at very low concentrations, the light scattering is subject to the effects of both external and internal interferences. The effect of external interference can be separated from the effect of internal interference by applying the Zernike-Prins equation to the observed angular dependence of light scattering. Then the second virial coefficient and the pair interaction potential of rodlike micelles can be derived by means of certain approximate methods. The addition of HCl to 10^–3 N makes both repulsive and attractive forces stronger and the resulting potential well deeper, but the addition of NaCl depresses such an effect of HCl considerably.