| 1. |
- Rosén, Olof, et al.
(författare)
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Anomalous surfactant diffusion in a gel of chemically cross-linked ethyl(hydroxyethyl) cellulose
- 2003
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:17, s. 4074-4079
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Tidskriftsartikel (refereegranskat)abstract
- The interactions of sodium dodecyl sulfate (SDS) with chemically cross-linked gels of ethyl(hydroxyethyl) cellulose (EHEC) were studied. Above the so-called critical association concentration (cac), binding of SDS gives rise to an increased swelling of the EHEC gels. The binding of SDS to the gels was measured with flame emission analysis of the sodium ion. The self-diffusion of the surfactant ion (DS) in the gels was studied by the NMR pulsed field gradient spin-echo technique. Both experiments were performed on gels swollen to equilibrium in SDS solutions of varying concentrations. Comparisons with the DS diffusion in a solution of non-crosslinked EHEC were also made. In the EHEC solutions the observed spin-echo decays for DS were always describable in terms of a single surfactant diffusion coefficient (Gaussian diffusion). In contrast, the DS diffusion in the gels above the cac (SDS) was clearly non-Gaussian, or anomalous. The echo decays in the gels could be fitte to a log-normal distribution of diffusion coefficients. When the time during which the diffusion was measured was increased, the width of the distribution increased, while the average diffusion coefficient remained constant. An increase in the width of distribution was also seen when the SDS concentration was increased. The anomalous diffusion is ascribed to inhomogeneities in the gel.
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| 2. |
- Groth, Cecilia, 1972, et al.
(författare)
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Diffusion of water in multilamellar vesicles of dialkyl and dialkyl ester ammonium surfactants
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 1873-4359 .- 0927-7757. ; 228:1-3, s. 64-73
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Tidskriftsartikel (refereegranskat)abstract
- The NMR self-diffusion technique is used for measuring diffusion of water in highly concentrated multilamellar vesicle solutions. The signal intensity of water, i.e. the water echo-decay, is monitored down to very small intensities thus providing an accurate measure of how water is diffusing in the solution. It is noted that a large curvature is dominating the functional form of the echo-decay indicating the presence of a large number of multilamellar vesicles. It is also concluded that in order to measure accurately the volume fraction of water inside and outside vesicles the experimental time scale can be changed. From a multiexponential fit to the echo-decay the fraction fast and the sum of all slow components can be extracted. When the apparent fraction "vesicle water", P-vw(app), is plotted versus the experimental time scale the graph produced is a good representation of the difference in how fast water diffuses over the different vesicle membranes. From an extrapolation to "zero time" the true fraction of trapped water can be extracted, i.e. a quantitative measure of the volume fraction of vesicles at a certain concentration of surfactant.
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| 5. |
- Uematsu, Takashi, 1968, et al.
(författare)
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Power Laws in Polymer Solution Dynamics
- 2003
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Ingår i: Physical Review E. - 1550-2376. ; 68:5, s. 051803-
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Tidskriftsartikel (refereegranskat)abstract
- The dynamical screening length xi(h) in semidilute to highly concentrated polymer solutions of poly(methyl methacrylate) in propylene carbonate has been examined using photon correlation spectroscopy and pulsed field gradient nuclear magnetic resonance. A crossover between different concentration dependent regimes, xi(h)similar tophi(-alpha), where alpha is found to be approximate to0.5, approximate to1, and approximate to2, is observed when the local viscosity is taken into account. Here phi is the volume fraction of polymer in the solution. Well-defined crossovers between alpha=0.5 and alpha=1 corresponding to a transition from a marginal solvent to a theta solvent behavior have been predicted to occur due to the reduction of excluded-volume effects between the spatially correlated polymer segments with increasing polymer volume fraction. However, a clear experimental validation of the crossover has never been presented before. The third regime (alphaapproximate to2) is observed in the highly concentrated region where the static screening length is comparable to the persistence length of the polymer. The observation indicates that the rigid rod model previously used to describe concentrated solutions is an oversimplification valid only in the very high concentration limit. The obtained results at high concentrations are discussed in the frame of a simple physical model where segments at the persistence length scale are treated as flexible rodlike segments.
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