Pratap Bahadur

Effect of additives on the micellization of PEO/PPO/PEO block copolymer F127 in aqueous solution

Abstract The micellization of an ethylene oxide-propylene oxide (PEO-PPO-PEO) symmetrical triblock copolymer (Pluronic®) F127 (EO 99 PO 65 EO 99 , mol. wt. PPO=1750, %PEO=70) in aqueous solution in the presence of various additives (i.e. sodium chloride, urea and sodium dodecyl sulfate (SDS)) is examined by cloud point, surface tension, dye spectral change, sound velocity, viscosity and dynamic light scattering measurements over the temperature range 25–50°C. The critical micelle concentration (CMC) of copolymer altered significantly in the presence of additives. While the addition of sodium chloride lowered the CMC, the addition of urea showed the reverse trend. The presence of added sodium chloride develops hydrophobicity in the PPO moiety and reduces hydrophilicity of PEO moieties, favoring micellization of the block copolymer at relatively lower concentrations than in water at ambient temperature. The increase in CMC of copolymer in presence of urea is interpreted in terms of enhanced solubility of semipolar PPO moiety and also PEO moiety. The critical micelle temperatures (CMTs) show a marked decrease in the presence of added sodium chloride. CMCs obtained by different methods are in good agreement. The addition of SDS to aqueous copolymer solutions leads to the formation of copolymer-SDS complex (or mixed micelle) showing polyelectrolyte nature. Surface tension/dye spectral change measurements reveal aggregation of SDS taking place at concentration much below its CMC, indicating clearly SDS-copolymer interaction. The addition of SDS suppresses the micellization of copolymer and beyond a particular SDS concentration only, SDS micelles with one or two copolymer molecules are present predominantly.

Effect of inorganic salts on the micellar behaviour of ethylene oxide-propylene oxide block copolymers in aqueous solution

The aggregation behaviour of two ethylene oxide-propylene oxide block copolymers (PEO-PPO-PEO) in aqueous solution has been investigated in the presence of added salts (KCNS, KI, KBr, KCl and KF) by viscosity, cloud point, light scattering, pulse gradient spin echo NMR, and solubilization measurements. The salts have a strong effect on the cloud points of the pluronics. Both P-85 and L-64 form micelles which increase in size and change into elongated shapes when the cloud point is approached. The changes of size and shape of the micelles, revealed by the intrinsic viscosity and rheological properties, seem to occur at the same temperature relative to the cloud point, independent of the nature of the salt. The onset of micelle formation is also shifted in the same direction as the cloud point by the salts, but by a much smaller amount.

Salt induced micellization and micelle structures of' PEO/PPO/PEO block copolymers in aqueous solution

Abstract Aqueous micellar solutions of two moderately hydrophilic polyethylene oxide/polypropylene oxide/polyethylene oxide (PEO/PPO/PEO) triblock copolymers, pluronics P84 and P104 are examined by small angle neutron scattering (SANS), viscosity and cloud point measurements. Micellar structures of pluronic P84 in aqueous solution are determined as a function of its concentrations (5 and 10 wt%) and added KCl concentrations (0–2 M). 5 wt% solutions of both the block copolymers contain spherical micelles with hydrophobic core of PPO and corona of highly hydrated ethylene oxide subchains. The presence of added neutral salt favors micellization at markedly lower concentration/temperature unlike conventional surfactants. The addition of the salt causes dehydration of ethylene oxide units from hydrated PEO shell from the side of PPO core, leading to an increase in core radius (RC) of spherical micelles. Micellar volume fraction (φ) increases with pluronic concentrations. It however increases at lower salt concentrations (up to 0.5 M) but is found to be independent at higher salt concentrations. Aggregation number (Nag) of P84 in aqueous salt solutions increases from 72 in water to 141 in 2 M KCl. No growth of spherical micelles is observed in unimer-to-micelle transition region and at temperatures below 20–22°C to its cloud point (CP) as hard sphere remains almost constant (Rhs∼70–74 A) with increasing salt concentration. Micelles remain spherical throughout the KCl concentration range studied.

Static and dynamic properties of a (PEOPPOPEO) block copolymer in aqueous solution

Abstract Aqueous solutions of a triblock copolymer of the type polyethyleneoxide-block-polypropyleneoxide-block-polyoxyleneoxide (Pluronic L-64, average composition 26 EO-units and 30 PO-units), were examined with static and dynamic light scattering (DLS), pulsed-gradient spin-echo (PGSE) NMR and fluorescence spectroscopy over a range of concentrations (0.2–25 wt%) and temperatures (15–60°C). Relaxation time distributions from DLS show L-64 to be molecularly dissolved at 21°C, and to form micelles at higher temperatures, which remain at high concentrations (25%) without formation of gel or liquid crystalline phases. The temperature where micelle formation starts is strongly concentration dependent, in contrast to the cloudpoint with remains fairly constant at 60°C. The hydrodynamic radii of the micelles, as obtained from DLS and PGSE NMR, are in reasonable agreement (60–80 A), and also agree with an aggregation number reported from fluorescence-quenching studies, whereas static light scattering, evaluated according to normal practice, indicates much smaller aggregates. This is due to the presence in the solution of a mixture of monomers, micelles, and at low temperatures, also some strongly scattering larger aggregates, possibly emanating from a small percentage of a diblock impurity in the preparation. The diblock impurity aggregates are dissolved by the proper micelles at higher temperatures. Their presence is indicated also by the anomalous excimer formation at low temperatures, caused by pyrene becoming concentrated in the premicellar aggregates.

Micelles from PEO–PPO–PEO block copolymers as nanocontainers for solubilization of a poorly water soluble drug hydrochlorothiazide

The effect of molecular characteristics of EO-PO triblock copolymers viz. Pluronic(®) P103 (EO(17)PO(60)PEO(17)), P123 (EO(19)PO(69)EO(19)), and F127 (EO(100)PO(65)EO(100)) on micellar behavior and solubilization of a diuretic drug, hydrochlorothiazide (HCT) was investigated. The critical micellization temperatures (CMTs) and size for empty as well as drug loaded micelles are reported. The CMTs and micelle size depended on the hydrophobicity and molecular weight of the copolymer; a decrease in CMT and increase in size was observed on solubilization. The solubilization of the drug hydrochlorothiazide (HCT) in the block copolymer nanoaggregates at different temperatures (28, 37, 45°C), pH (3.7, 5.0, 6.7) and in the presence of added salt (NaCl) was monitored by using UV-vis spectroscopy and solubility data were used to calculate the solubilization characteristics; micelle-water partition coefficient (P) and thermodynamic parameters of solubilization viz. Gibbs free energy (ΔG(s)°), enthalpy (ΔH(s)°) and entropy (ΔS(s)°). The solubility of the drug in copolymer increases with the trend: P103>P123>F127. The solubilized drug decreased the cloud point (CP) of copolymers. Results show that the drug solubility increases in the presence of salt but significantly enhances with the increase in the temperature and at a lower pH in which drug remains in the non-ionized form.

Viscoelastic micellar water/CTAB/NaNO3 solutions: Rheology, SANS and cryo-TEM analysis

Aqueous micellar solutions of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and sodium nitrate (NaNO(3)) were examined using steady and dynamic rheology, small-angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM). Upon addition of NaNO(3), the CTAB spherical micelles transform into long, flexible wormlike micelles, conveying viscoelastic properties to the solutions. The zero-shear viscosity (eta(0)) versus NaNO(3) concentration curve exhibits a well-defined maximum. Likewise, upon increase in temperature, the viscosity decreases. Dynamic rheological data of the entangled micellar solutions can be well described by the Maxwell model. Changes in the structural parameters of the micelles with addition of NaNO(3) were inferred from SANS measurements. The intensity of scattered neutrons at the low q region was found to increase with increasing NaNO(3) concentration. This suggests an increase in size of the micelles and/or decrease of intermicellar interactions with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between micelles indicates that addition of NaNO(3) leads to a decrease in the surface charge of the ellipsoidal micelles and consequently an increase in their length. The structural transition from spherical to entangled threadlike micelles, induced by the addition of NaNO(3) to CTAB micelles is further confirmed by cryo-TEM.

EFFECT OF SALT ON THE MICELLIZATION OF PLURONIC P65 IN AQUEOUS SOLUTION

Abstract Solutions of an ethylene oxide–propylene oxide triblock copolymer (PEO/PPO/PEO) pluronic® P65 (50–50 wt.% of each block with total molecular weight of 3400 g mol −1 ) in aqueous sodium chloride examined by different methods reveal salt-induced micellization. The critical micelle concentrations (CMCs) and critical micelle temperatures (CMTs) showed a marked decrease in the presence of added salt. Unimer-to-micelle transitions at low concentration (or temperature) and micellar growth at temperature close to cloud point are discussed.

Effect of Additives on the Clouding Behavior of an Ethylene Oxide-Propylene Oxide Block Copolymer in Aqueous Solution

Abstract Phase behavior of an ethylene oxide-propylene oxide block copolymeric surfactant Pluronic L-64 (MW = 2900, % PEO = 40) in the presence of various additives was studied from cloud point (CP) and phenol index measurements. Electrolytes with different sizes and polarizabilities of anions and cations showed an increase/decrease in cloud point; the effect of salts is discussed in terms of “salting in” and “salting out” and follows the Hofmeister series. The effect of various nonelectrolytes, viz., hydroxy compounds and amides, on the CP of L-64 was examined and discussed in terms of their influence on water structure. Ionic surfactant, viz., sodium dodecyl sulfate (SDS), increased the CP of L-64 which could be decreased drastically in the presence of small amounts of electrolytes. The clouding behavior of a mixed L-64 + SDS system in the absence/presence of salt is discussed primarily in terms of electrical charge on the micelle surface. The polyelectrolyte-like nature of the L- 64 + SDS complex is sh...

Micellar and solubilizing behaviour of Pluronic L-64 in water

Abstract The temperature effect in aqueous solutions of a polyoxyethylene—polyoxypropylene—poloxyethylene (POE—POP—POE) block copolymer, Pluronic L-64 in water, was examined by means of viscosity, sedimentation, scattering and sound velocity measurements. Micelle formation in L-64 was strongly dependent on concentration and temperature. An increase in temperature shifted the micelle formation markedly to lower concentration; the micelle grew large particularly at temperatures near the cloud point. The viscosity data were analyzed to estimate various parameters, including the hydrated micellar volume, hydration number, hydrodynamic radius, etc. An increase in diazepam solubility in L-64 micelles with increasing concentration and temperature was observed.

Interaction studies of styrene—ethylene oxide block copolymers with ionic surfactants in aqueous solution

Abstract The interaction of styrene—ethylene oxide block copolymers with four anionic surfactants (sodium dodecyl sulfate, sodium dodecanoate, sodium dodecylbenzenesulfonate, and sodium dodecanoyl sarcosinate), and two cationic surfactants (tetradecyl- and hexadecyl-trimethylammonium bromide), was studied and each surfactant showed a distinct interaction with the copolymer in aqueous solution. Usually two transitions, one below and one above the critical micelle concentration, CMC, of the surfactants, were observed from conductance, surface tension, and dye solubilization measurements. These transitions indicate the beginning and completion of polymer—surfactant interaction. The viscometric results showed the formation of a polyelectrolyte complex. The interaction between copolymer and sodium dodecyl sulfate was also examined by 1 H NMR. The influence of the molecular characteristics of the block copolymers, the nature and type of surfactants, temperature and added salt on the interaction is described. A possible mechanism for such an interaction is proposed.