P.S. Goyal

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.

Molecular Modulation of Surfactant Aggregation in Water: Effect of the Incorporation of Multiple Headgroups on Micellar Properties

Surfactant molecules self-organize in water,[1] often producing nearly spherical aggregates called micelles in dilute solutions, and lyotropic mesophases at higher concentrations. The polar headgroups of these aggregates lie near the bulk aqueous phase, whereas the hydrocarbon chains extend inwardly to avoid unfavorable water contacts.

Dependence of the size of micelles on the salt effect in ionic micellar solutions

We report the effect of addition of salt KBr on the growth of different sizes of cationic micelles of alkyltrimethylammonium bromide surfactants in aqueous solution. The surfactants with different hydrophobic tail lengths have been used to vary the size of the micelles. It is found that the effect of salt on the growth of the micelles strongly decreases for the small micelles. We explain these results in terms of counterion condensation, which depends on the size of the micelles.

Role of counterion distribution on the structure of micelles in aqueous salt solutions: small-angle neutron scattering study

Small-angle neutron scattering studies have been carried out on aqueous micellar solutions of cationic surfactant cetyltrimethylammonium bromide (CTAB) at different temperatures in presence of varying concentration of salt KBr. It is found that the micelles grow strongly with the addition of salt at low temperatures and this effect is significantly reduced with the increase in the temperature. These results are explained in terms of excess of counterions around the micelles with the increase in the temperature.

Effect of the addition of n-alkylamines on the growth of sodium decyl sulfate micelles

The effect of the addition of n-alkylamines [CH3(CH2)mNH2; m= 5, 6, or 7] on the growth of sodium dodecyl sulfate (SDS) micelles has been studied by small-angle neutron scattering (SANS) and viscosity measurements at 30 °C. The viscosities of SDS (0.3 mol l–1 in D2O) solutions show a distinct rise at a certain amine concentration (XC) which depends on the chain length of the amine. This has been attributed to a change in micellar shape. To corroborate our viscosity results, SANS experiments were carried out on 0.3 mol l–1 SDS–amine micellar systems. SANS distribution showed a well defined peak for 0.3 mol l–1 SDS which shifts towards low Q-values on addition of different amines. The SANS spectra were analysed using the Hayter–Penfold model and shape and size parameters were computed. The results are interpreted in the light of micellar growth caused by the addition of amines. The micelles are shown to be ellipsoidal with the length of the semi-major axis depending on the chain length of the amine.

SANS study of salt induced micellization in PEO–PPO–PEO block copolymers

Abstract Small-angle neutron scattering (SANS) measurements have been carried out from the aqueous solutions of polyethylene oxide (PEO)–polypropylene oxide (PPO)–PEO block copolymers of pluronics F88 and P84 in presence of salt KCl. It is seen that the effect of addition of salt is similar to that of increasing temperature. The pluronics exist as unimers at low temperatures and their micellization takes place when the salt is added or the temperature is increased. The measurements for different salts of the lyotropic series on the above pluronic systems show that micellar structure is sensitive to the sizes of the hydrated ions of these salts.

Electron magnetic resonance of ferrofluids: Evidence for anisotropic resonance at 77 K in samples cooled in a magnetic field

Abstract Evidence is presented from studies of electron magnetic resonance for the formation of linear chains in kerosene based Mn 0.1 Fe 0.9 Fe 2 O 4 (MF1) ferrite ferrofluid cooled in a magnetic field. The resonance field at 77 K was found to depend on the field at which the sample was cooled. More interestingly, the samples cooled in a magnetic field exhibited anisotropy in a resonance field with 180° periodicity, giving evidence for frozen chains.

Micellar growth in the presence of quaternary ammonium salts A SANS study

The effect of the addition of quaternary ammonium bromides, R4NBr (R=H, CH3 or C4H9) on the growth of sodium dodecyl sulfate (SDS) micelles has been studied by small-angle neutron scattering (SANS) measurements at 30°C. SDS forms ellipsoidal micelles at 0.3 M concentration with an average aggregation number ns=110. The effect of adding 0.1 M R4NBr to 0.3 M SDS has been studied and compared with the effect of adding NaBr. The values of ns are in the order: no salt<NaBr≈(CH3)4NBr<NH4Br<(n-C4H9)4NBr which shows that the effectiveness of the cation (counterion) in increasing ns is in the order of decreasing hydrated radii. The unusual behaviour of (n-C4H9)4N+ is discussed in terms of its possible intercalation in the micelle. At higher [R4NBr], micellar growth is substantial in the case of (n-C4H9)4NBr which is explained in the light of an increase in the Mitchell–Ninham parameter of the surfactant monomer.

Combined SANS and SAXS studies on alkali metal dodecyl sulphate micelles

Combined small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS) studies have been performed on charged micelles of anionic surfactants of alkali metal dodecyl sulphate (MDS, M = Li, Na, Rb and Cs) in aqueous solution. The dimensions of the micelles have been obtained using SANS as the neutrons are mostly scattered by the hydrogenous core of the micelles. The contrast for the head groups and the counterions is very small, so they are not seen with the neutrons. On the other hand, x-rays are scattered by the electron density fluctuation, which is proportional to the atomic number, and thus x-rays have been used to see the high-Z components of the micelles directly. SAXS from LiDS gives the headgroup thickness, and the counterion condensation is measured directly by the scattering from the high-Z counterions of MDS (M = Na, Rb and Cs) micelles. Based on the results of these two techniques, the role of the different counterions in deciding the structure of micelles has been explained in terms of counterion condensation and the thickness over which they are condensed around the charged micelle. It is found that as the hydrated size of the counterion decreases, the counterion condensation increases and counterions are condensed over a smaller thickness, which results in larger charge neutralization and hence a larger size of the micelles.

Characterization of new gemini surfactant micelles with phosphate headgroups by SANS and fluorescence spectroscopy

Two series of bisphosphate surfactants having different hydrophobic tails and variable number of spacer methylene groups have been synthesized. Detailed small-angle neutron scattering studies were carried out with their aqueous solutions to study their micellar properties. For some of these new surfactants, critical micellar concentrations and micropolarities were also determined. It has been found that the micelles were ellipsoidal for surfactants with short spacers having C12H25 hydrophobic tails. However, the corresponding surfactants having C16H33 chains and similar m-values afforded micelles that were disk-like and rod-like