1,721,151 research outputs found
Cellulose–solvent interactions from self-diffusion NMR
Full text linkMolecular self-diffusion coefficients were measured in solutions of microcrystalline cellulose (MCC) and dissolving pulp, in 40 wt% aqueous tetrabutylammonium hydroxide (TBAH), using pulsed field gradient stimulated echo NMR. From the cellulose diffusion coefficients, a weight averaged radius of hydration h>w = 6.1 nm for MCC and h>w = 15 nm for pulp were obtained. Water and TBA+ ions show a significantly different dependence on the cellulose concentration, revealing different molecular interactions with the polymer. Water-cellulose are essentially excluded volume. TBA+ ions, on the other hand, bind to cellulose with approximately 1.2 TBA+ ions per glucose unit
NMR relaxation in micelles formed by a long zwitterionic surfactant
No full textA frequency-dependent 14N and 13C NMR relaxation study of the micellar region formed by a very long zwitterionic surfactant, viz., 6-(dimethyleicosylammonio)hexanoate (C20AH), is presented. As a starting point the binary phase diagram of C20AH/D2O is determined. The phase diagram is similar to those found for shorter chain ionic surfactants with bulky head groups. The NMR relaxation data show a dependence upon the magnetic field strength and are discussed in terms of the two-step model of relaxation. The frequency dependence found at low frequencies in the 14N data is assumed to be caused by the rotational tumbling of the micelle, while it is argued that the frequency dependence found at high frequencies in the 13C relaxation data is caused by a local motion. The analysis of the data yields a radius of the micelle which is slightly shorter than an extended C20 chain, pointing to a situation where the methylene groups in the head-group dipole are embedded in the micelle. Moreover, the local motions in the head-group region are slow as compared to the corresponding motions in single-chain ionic surfactants
Phase Diagram And Phase Properties Of The System Lecithin-water-cyclohexane
No full textThe isothermal quasi-ternary-phase diagram of the lecithin - cyclohexane - water system was determined at 25 degrees C using a combination of polarizing microscopy, small-angle X-ray diffraction, and NMR techniques. The system contains four lyotropic liquid-crystalline phases and two isotropic liquid phases. Apart from the lamellar (L-alpha) phase, there are only reverse-type aggregates with a water interior, in addition to an essentially pure water phase, whose relative locations in the phase diagram follow the sequence (from the oil corner to the surfactant corner): reverse micellar solution (L-2), reverse anisotropic nematic (N-2), reverse micellar cubic (I-2), reverse hexagonal (H-2), and finally, the lamellar phase. The aggregates have a finite swelling with water, and coexistence with excess water is found at higher water contents. The area per lecithin molecule was determined in the H-2 and L-alpha phases,This area varies with the mole ratio [H2O]/[Lec] = W-0 at lower W-0 values, but saturates at an area of 90 Angstrom(2)/ molecule for W-0 greater than or equal to 15. The phase diagram is discussed in relation to the known formation of giant wormlike reverse micelles in the Liquid L-2 phase. Of particular interest here is the transition from liquid (L-2) to nematic (N-2) as the wormlike aggregate concentration is increased
Molecular Diffusion in a Living Network
No full textWe report on the diffusion of a surfactant confined in a branched cylindrical "micellar" network, formed by lecithin and small amounts of water in the solvent isooctane. By means of the pulsed field gradient H-1 NMR technique, the measured surfactant mean square displacement, , allows for a detailed investigation on the microstructure of the micellar network. Our results show that the structure depends weakly on the micellar volume fraction, Phi, and strongly on the water-to-lecithin molar ratio, W-0. We have studied the lecithin diffusion along two different oil dilution lines, corresponding to different water-to-lecithin molar ratios, 2 and 3. The time window in the diffusion experiments was varied in the range from 50 ms to 1 s. At W-0 = 3, a Gaussian diffusion, characterized by a mean square displacement varying linearly with time, was observed for all concentrations and all observation times investigated. Furthermore, the selfdiffusion coefficient was found to be independent of the concentration in the micellar volume fraction range studied from Phi = 0.1 to Phi = 0.38. The value of the diffusion coefficient is approximately 1/3 of the value of the lateral diffusion coefficient, D-c. At the second dilution line, W-0 = 2, the situation is markedly different. At lower concentrations (Phi scaling as t(1/2) consistent with curvilinear diffusion. For longer times, there was a crossover to a Gaussian diffusion with proportional to t. The observation time where there is a crossover from curvilinear to a Gaussian diffusion shifts to shorter times with increasing Phi. At higher concentrations, only a Gaussian diffusion was observed within the experimental time window. The diffusion coefficient evaluated from the Gaussian regime increases linearly with Phi, the value varying from D-c/100 to D-c/20. The high diffusion coefficients evaluated at W-0 = 3 clearly indicate that the structure is a branched micellar network where the curvilinear distance along the cylindrical micelles between two branch points is smaller than the persistence length. At W-0 = 2, the data can also be interpreted in terms of a branched network, however with a much smaller density of branch points. The branching density increases with increasing Phi. Finally, the measured water diffusion along the two oil dilution lines was found to be Gaussian with a time-independent, single diffusion coefficient. The dominating mechanism for the water diffusion was found to be the motion inside the giant wormlike reverse micelles mediated by an interaggregate exchange with a characteristic time of the order of microseconds
Optimum formulation conditions for cationic surfactants via rheo-titration in turbulent regime
No full textRecently a new, time saving, approach to the determination of the composition at which a microemulsion is balanced has been developed using the so called HLD-titration. The key of the method is the observation that in correspondence of the balanced state where the microemulsion coexist with excess oil and water (Winsor III phase equilibrium), the level reached by the three-phasic system under stirring has a consistent maximum allowing a fast and low-cost readout of the balanced state that permits the evaluation of the surfactants characteristic parameters (s). To better understand how the formation of a balanced microemulsion is related to rheology, here a titration experimental approach was adopted under turbulent flow meanwhile fluid friction. A mixture of equal volumes of brine and oil and didodecyldimethylammonium bromide (DDAB) is titrated with dodecyltrimethylammonium bromide (LTAB), in the Couette cell of a rheometer under continuous rotation, promoting the transition from a Winsor II (w/o microemulsion coexisting with brine) to a Winsor III phase equilibrium. The turbulent apparent viscosity was measured after each addition of the LTAB. We demonstrate that the turbulent apparent viscosity attains a minimum at the Winsor III phase equilibrium. Furthermore, we have investigated the microstructural evolution of the microemulsions found at the different DDAB/LTAB ratios that reproduces the HLD-titration by means of small-angle X-ray scattering (SAXS) and diffusion nuclear magnetic resonance (NMR) and rheo- small-angle light scattering (rheo-SALS). The anisotropy index, measured by rheo-SALS, increases upon increasing the shear rate suggesting the ability of oil and water domains, in the balanced state, to elongate along the streamlines
Surfactant curvilinear diffusion in giant wormlike micelles
No full textWe present the first experimental measurements of surfactant curvilinear diffusion in giant wormlike micelles. The surfactant diffusion was monitored by pulsed field gradient NMR for various observation times t ranging from 0.020 to 1.5 s. The surfactant mean-square displacement was found to scale as t(1/2). A model of lateral diffusion along wormlike micellar aggregates with Gaussian statistics is found to describe the echo attenuation well. This type of diffusion is analogous to polymer segment diffusion in the tube-reptation model
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