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Small-angle neutron scattering (SANS) and viscosity measurements are used to study the mixed micellar system comprising the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)-based triblock copolymer ([EO.sub.20])([PO.sub.70])([EO.sub.20]) and the anionic surfactant sodium dodecyl sulfate (SDS). Copolymers with high PO/EO ratios have showed an increase in the stability of the gel phase, whereas copolymers with low PO/EO ratios have showed a deterioration of the same in the presence of SDS.

LiCl-induced changes in the micellar hydration and gelation characteristics of aqueous solutions of the two triblock copolymers F127 and P123 are analyzed by small-angle neutron scattering (SANS) and viscometry. Viscosity studies on a poly(ethylene glycol) (PEG) homopolymer with a size equivalent to that of the PEO block in F127 has shown that the dehydrating effect of the [Cl.sup.-] ion on the PEG chain is compensated by its interaction with the hydrated [Li.sup.+] ions.

Dynamic light scattering, small-angle neutron scattering, and viscosity studies are carried out to examine the influence of NaCl and ethanol on the structure of triblock copolymer [(EO)(sub 20)(PO)(sub 70)-(EO)(sub 20) micelles in aqueous medium. The presence of small amount of ethanol induces a sphere to rod transition of micelles at high temperatures, while the pure triblock copolymer solutions do not show any significant growth of the micelles on approaching the cloud point.

The influence of methyl paraben (MP) and butyl paraben(BP) onthe aggregation characteristics of Pluronics in an aqueous mediumhas been investigated by DLS, SANS, viscometry, and fluorescence measurementtechniques. Parabens are extensively used as preservatives in cosmetic,pharmaceutical, and food products. In this paper, we show that theirinfluence on the restructuring and growth of Pluronics micelles varyquite significantly with their aqueous solubility and with the compositionof Pluronics. In the case of P105 and P104, MP reduces the sphere-to-rodtransition temperature down to room temperature, but BP with significantlyless aqueous solubility than MP suppresses such micellar transitionand leads to the formation of micellar clusters due to the onset ofintermicellar attractive interaction. In the case of more hydrophobicPluronic P103, on the other hand, both MP and BP are able to inducerapid room temperature sphere-to-rod micellar growth, which is notobserved in the presence of water structure making salts like NaCland Na3PO4. These observations have been attributedto modulation of growth and restructuring processes of the Pluronicmicelles arising due to different locations of parabens within themicellar corona as determined by their aqueous solubility and thehydrophobicity of the Pluronics. [ABSTRACT FROM AUTHOR]

Lipophile induced modulations of self-assembly characteristics in aqueous Pluronic systems merit attention because of wide-ranging uses of Pluronics as solubilizing agents of lipophilic substances. In this paper, we report unusual evolutions of structural and hydration properties in lavender essential oil (LO) solubilized Pluronic P85 aqueous micellar systems as a function of micellar volume fraction and temperature. Our DLS, SANS, and viscometry studies show that the spherical-to-wormlike micellar structural transition observed in 1% P85 solutions upon solubilization of LO quite unexpectedly gets suppressed with increased P85 concentration to ≥5%. Detailed SANS studies reveal that the core sizes of the oil solubilized micelles cannot attain the threshold value required for the onset of structural transition at higher copolymer concentrations due to their progressive shrinking with an increase in P85 concentration. Oil solubilized P85 solutions show two cloud points and very interestingly exhibit micellar growth upon cooling to their lower cloud points. Steady state fluorescence studies explain this based on increasing dehydration of micellar corona with a decrease in temperature, very much opposite to what is observed in pure aqueous Pluronic systems. The results give new insight into viscous flow properties and low temperature storage possibilities of oil solubilized aqueous Pluronic systems.

The influence of sodium hydroxide (NaOH) on the aggregation characteristics of the Tetronic 904 (T904) has been studied in the aqueous medium by dynamic light scattering (DLS), small angle neutron scattering (SANS), and viscometry methods. This polyethylene oxide (PEO)-polypropylene oxide (PPO) based X-shaped triblock copolymer shows pH sensitive aggregation characteristics due to the presence of a central amine group attached with the PPO block. The aqueous micellar solutions of this copolymer have been found to show a large increase in the room temperature (30 °C) relative viscosity with an increase in NaOH concentration, before they undergo phase separation at 1.25 M NaOH concentration. SANS and DLS studies ascribe this behavior to a sphere to rod growth of the copolymer micelles on approaching the cloud points of the copolymer solutions. DLS studies show that the observed micellar growth is accompanied by critical scattering due to the onset of an attractive intermicellar interaction. To understand the role of NaOH in inducing the observed micellar shape transition, we have also studied the effect of NaOH on the micellar solutions of Pluronic P84, which does not show pH sensitivity due to the absence of the central amine group but has the same weight fraction of the PEO block (40%) as that of T904. Quite interestingly, the P84 micelles too have been found to undergo a similar room temperature sphere to rod shape transition in the presence of NaOH. The observed growth and interaction of the Tetronic micelles have thus been attributed to the dual effects of NaOH in influencing the aggregation characteristics of the Tetronic molecules. On one hand, it stabilizes these micelles by the deprotonation of the copolymer molecules, and on the other hand, its dehydrating effect on the copolymer molecules promotes the onset of micellar growth and intermicellar attractive interaction at the room temperature. The observed results are the first of their kind in the aqueous Tetronic system.

Interaction of cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), with pluronics F88 (EO(103)-PO(39)-EO(103)) and P105 (EO(37)-PO(56)-EO(37)) micelles and its effect on modulating the location of an anionic solute in the mixed micelles have been investigated using time-resolved fluorescence and small angle neutron scattering (SANS) studies. SANS results indicate the formation of pluronic-CTAB supramolecular assemblies, in which the hydrophobic chains of CTAB occupy the hydrophobic core of the pluronic micelle while the positively charged head groups reside at the micellar core-corona interface. Rotational correlation time of the anionic probe in these supramolecular assemblies increases with an increase in the CTAB concentration, and the observed results are explained on the basis of the probe movement from the surface to the interior of the micelle due to the increased electrostatic attraction. Dynamic Stokes' shift measurements also support the movement of the probe due to the addition of the surfactant to the supramolecular assemblies. From the studies with different pluronics, it is indicated that the concentration of CTAB required to drag the probe molecule into the interior of the micelles is linearly correlated to the thickness of the corona region of the respective micelles.

Steady-state and time-resolved fluorescence anisotropy measurements using probes coumarin 153 (C153) and 4-heptadecylumbelliferon (HUF) have been carried out to understand the micelle to gel transition of an aqueous triblock copolymer P123 ((EO)(20)-(PO)(70)-(EO)(20)) (EO = ethylene oxide; PO = propylene oxide) solution. Anisotropy results with a normal fluorescent probe, C153, do not show a characteristic change due to the micelle to gel transition. However, the probe HUF having a long hydrocarbon chain that helps its strong association with the micelle shows an increase in anisotropy above the sol-gel transition point. This difference has been explained as invoking a substantial contribution from the micellar structural fluctuations to the depolarization of HUF as its hydrocarbon chain is embedded in the micellar structure, which is not sensed significantly by the normal probe C153. That the extent of change in anisotropy for HUF upon gelation is not that large is possibly caused by the collective motion of the physically interconnected nodes, as observed from the dynamic light scattering studies, which acts in favor of a relatively faster depolarization in the gel phase. Similar studies in other copolymers, such as P85 ((EO)(26)-(PO)(40)-(EO)(26)) and F127 ((EO)(100)-(PO)(65)-(EO)(100)), further demonstrate the potential of probes latched with hydrocarbon chains in displaying a characteristic change for the micelle to gel transition which otherwise remains obscured for normal fluorescent probes.

Dynamic Stokes' shift and fluorescence anisotropy measurements of coumarin 153 (C153) and coumarin 151 (C151) as fluorescence probes have been carried out to understand the influence of electrolytes (NaCl and LiCl) on the hydration behavior of aqueous (ethylene oxide)100-(propylene oxide)70-(ethylene oxide)100 (EO100-PO70-EO100, F127) block copolymer micelles. A small blue shift in the fluorescence spectra of C153 has been observed in presence of electrolytes due to the dehydration of the oxyethylene chains in the PEO-PPO region, although fluorescence spectra of C151 remain unaltered. The close vicinity of bulk water for C151 probably negates the effect of dehydration in the PEO region. Fluorescence anisotropy measurements indicate a gradual increase in microviscosity with electrolyte concentrations. The partial collapse of copolymer blocks in the presence of electrolytes has been suggested as a reason for the increase in microviscosity along with the strong hydration of ions in the corona region. The interplay between the ion hydration and the mechanically trapped water content, and specific interaction of ions, such as complexation of Li+ ions with the copolymer block, is found to control solvation dynamics in the corona region. In addition to that, it has been established that Na+ ions reside deep into the corona region whereas Li+ ions prefer to reside closer to the surface. Owing to its higher lyotropicity, LiCl influences the corona hydration to a greater extent than NaCl and sets in micelle-micelle interaction above the 2 M LiCl concentration, as reflected in the saturation of solvation time constants. The formation of larger clusters of F127 micelles above 2 M LiCl has been confirmed by dynamic light scattering measurements; however, such cluster formation is not evident with NaCl.