| 1. |
- Angelico, R, et al.
(författare)
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Biocompatible lecithin organogels: Structure and phase equilibria
- 2005
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 21:1, s. 140-148
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure of organogels formed upon the addition of tiny amounts of water to a solution of lecithin in fatty acid esters (viz. isopropylpalmitate and ethyloleate) was investigated by means of molecular self-diffusion measurements. In both systems lecithin and water form disconnected cylindrical reverse micelles. The ternary phase map for the lecithin/water/isopropylpalmitate has been investigated in detail. The organogel exists in a narrow region close to the lecithin-oil binary axis; for higher water content equilibrium between lamellae and reverse micelles is found. Lamellar phase occupies the lecithin-rich region, close to the lecithin corner (with the exception of a small island of hexagonal phase) and coexists with neat water close to the water-lecithin axis. The remaining part of the phase map shows the three-phase coexistence of water, oil, and lamellar phase.
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| 2. |
- Angelico, R, et al.
(författare)
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Deuterium NMR study of slow relaxation dynamics in a polymerlike micelles system after flow-induced orientation
- 2003
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:38, s. 10325-10328
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Tidskriftsartikel (refereegranskat)abstract
- We present a first time-resolved deuterium NMR study on wormlike micelles under shear. For the investigation, a micellar system made of soybean phosphatidylcholine (lecithin), D2O, and cyclohexane was studied in the concentrated regime, in the vicinity of the isotropic/nematic transition but positioned in the equilibrium isotropic phase. The D2O dispersed in the micellar core was used as a probe to study the relaxation of a shearing-induced nematic phase back to the isotropic liquid state, after cessation of shear. The whole process was followed through time-resolved deuterium spectra and was characterized by a continuous decrease of the measured quadrupolar splitting, proportional to the nematic order parameter.
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| 3. |
- Angelico, R., et al.
(författare)
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Flow-induced structures observed in a viscoelastic reverse wormlike micellar system by magnetic resonance imaging and NMR velocimetry
- 2016
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Ingår i: RSC Advances. - : Royal Society of Chemistry (RSC). - 2046-2069. ; 6:40, s. 33339-33347
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Tidskriftsartikel (refereegranskat)abstract
- The aim of the present work is to illustrate and discuss an application of rheo-NMR techniques in the investigation of the flow micro-morphology of a rheo-thinning fluid. The viscoelastic material is composed by weakly hydrated nonionic Wormlike Micelles (WM), stabilized by the biocompatible phospholipid in an organic solvent (lecithin organogel). By applying rheo-NMR techniques, such as micro-imaging and flow velocimetry in Couette flow, to lecithin organogels in the concentrated isotropic phase, a new phase nucleating inhomogeneously at the inner rotating cylinder showing periodic fluctuations in space in some cases, has been identified for applied shear rates within the isotropic-nematic stress plateau. On the other hand, evident slippage phenomena have been found in flow regimes consistent with a full shear-induced nematic state. Bulk rheometric investigations executed in oscillatory, steady state and transient mode have been finally carried out to bridge different flow micro-heterogeneities detected by rheo-NMR with a variety of mechanical responses manifested by lecithin WM.
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| 4. |
- Angelico, R., et al.
(författare)
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Ordering fluctuations in a shear-banding wormlike micellar system
- 2010
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 12:31, s. 8856-8862
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Tidskriftsartikel (refereegranskat)abstract
- We present a first investigation about the non-linear flow properties and transient orientational-order fluctuations observed in the shear-thinning lecithin-water-cyclohexane wormlike micellar system at a concentration near to the zero-shear isotropic-nematic phase transition. From rheological measurements the stress plateau was found shifted to very low values of the applied shear rate (gamma)over dot, compared to most of the concentrated living polymer systems reported in the literature. Rheo-small angle neutron scattering (Rheo-SANS) experiments performed in the flow-vorticity plane revealed periodical fluctuations of both the order parameter P-2 and the angular deviation phi from the vorticity axis as determined from the scattering peaks. The periods of the oscillations were not found to depend on imposed (gamma)over dot. A theoretical model was also developed to explain the oscillatory dynamics of the shear-induced nematic order parameter in terms of the presence of standing waves of the director orientation profile along the circumference of the Couette cell. The experimental results of the periodic order parameter fluctuations together with their theoretical modelling shed significant new insights on the shear banding phenomenon, particularly its microscopic mechanism.
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| 5. |
- Angelico, R, et al.
(författare)
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Phase behavior of the lecithin/water/isooctane and lecithin/water/decane systems
- 2004
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:3, s. 619-631
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Tidskriftsartikel (refereegranskat)abstract
- The isothermal pseudo-ternary-phase diagram was determined at 25 degreesC for systems composed of lecithin, water, and, as oil, either isooctane or decane. This was accomplished by a combination of polarizing microscopy, small-angle X-ray scattering, and NMR techniques. The lecithin-rich region of the phase diagram is dominated by a lamellar liquid-crystalline phase (L-alpha). For lecithin contents less than 60% and low hydration (mole ratio water/lecithin = W-0 < 5.5), the system forms a viscous gel of branched cylindrical reverse micelles. With increase in the water content, the system phase separates into two phases, which is either gel in equilibrium with essentially pure isooctane (for lecithin < 25%) or a gel in equilibrium with L-alpha, (for lecithin > 25%). These two-phase regions are very thin with respect to water dilution. For 8 < W-0 < 54 very stable water-in-oil emulsions form. It is only after ripening for more than I year that the large region occupied by the emulsion reveals a complex pattern of stable phases. Moving along water dilution lines, one finds (i) the coexistence of gel, isooctane and L-alpha,(ii) equilibrium between reverse micelles and spherulites, and, finally, (iii) disconnected reverse micelles that fail to solubilize water for W-0 > 54. This results in a Winsor II phase equilibrium at low lecithin content, while for lecithin > 20% the neat water is in equilibrium with a reverse hexagonal phase and an isotropic liquid-crystalline phase. The use of the decane as oil does not change the main features of the phase behavior.
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| 6. |
- Angelico, R, et al.
(författare)
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Relaxation of shear-aligned wormlike micelles
- 2002
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:10, s. 2426-2428
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We report on the first time-resolved small-angle neutron scattering (SANS) study of the structural relaxation from a shear-induced aligned state back to the isotropic state in a system of giant wormlike reverse micelles. The micelles are formed by lecithin and small amounts of water in cyclohexane. A liquid sample with the micellar volume fraction phi = 0.3 was exposed to a steady shear of (gamma) over dot = 10 s(-1) in a Couette shear cell. This was sufficient to transform the whole sample into an aligned state with the polymer-like micelles preferentially oriented parallel to the velocity direction. After cessation of shear, the sample relaxes back to the equilibrium liquid phase. By time-resolved SANS we could follow the relaxation process, which is characterized by a continuous decrease of the order parameter.
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| 7. |
- Olsson, Ulf, et al.
(författare)
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Slow dynamics of wormlike micelles
- 2010
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Ingår i: Soft Matter. - : Royal Society of Chemistry (RSC). - 1744-6848 .- 1744-683X. ; 6:8, s. 1769-1777
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Tidskriftsartikel (refereegranskat)abstract
- We present an extensive rheology study of the wormlike micelle system lecithin-water-cyclohexane. In this system the micelles are really wormlike, meaning that there are no signs of micellar branching, as it has previously been demonstrated by NMR self-diffusion experiments (R. Angelico, U. Olsson, G. Palazzo and A. Ceglie, Phys. Rev. Lett., 1998, 81, 2823). Wormlike micelles break and recombine, processes that are important for the stress relaxation. When branching is highly unfavorable, micelle recombination reactions only involve micelle ends, the concentration of which are very low when the micelles are very long. Hence, the break and recombination kinetics is very slow for true wormlike micelles. In the present system, the stress relaxation times are of the order of an hour. This is about three to four orders of magnitude longer than what commonly is observed in systems claimed to contain wormlike micelles. We conclude that systems with true wormlike micelles are very rare. An exponential stress relaxation is observed except at lower concentrations, where the micellar breaking time appears to exceed the reptation time. Because of the slow dynamics, the linear elastic modulus can be obtained from small rapid shear deformations, for which the system obeys Hooke's law. Larger deformations result in a fracture of the micellar network at a critical strain gamma* similar to phi(-1), where phi is the micelle volume fraction. For gamma < gamma* we may still obtain fracture, although with a lag time, tau*, that decreases with increasing gamma, and vanishes when gamma = gamma*. Extrapolating tau* to zero deformation we obtain the estimate approximate to 250 s at the highest concentration phi = 0.29. We interpret this time as the equilibrium micellar breaking time at rest, a quantity not previously measured.
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