138 research outputs found
Self-diffusion of nonfreezing water in porous carbohydrate polymer systems studied with nuclear magnetic resonance
Water is an integral part of the structure in biological porous materials such as wood and starch. A problem often encountered in the preparation of samples for, e.g., electron microscopy is that removal of water leads to a decreasing distance between supermolecular structural elements and a distortion of the structure. It is, therefore, of interest to find methods to investigate these materials in the native water-swollen state. We present a method to study water-swollen biological porous structures using NMR to determine the amount and self-diffusion of water within the porous objects. The contribution of bulk water to the NMR signal is eliminated by performing experiments below the bulk freezing temperature. Further decrease of the temperature leads to a gradual freezing of water within the porous objects. The contribution of he freezing water fraction to the migration of water through the porous network is, thus, estimated. The results are rationalized in terms of the ultrastructure of the samples studied, namely, wood pulp fibers and potato starch granules
The formation of hostâguest complexes between surfactants and cyclodextrins
Cyclodextrins are able to act as host molecules in supramolecular chemistry with applications ranging from pharmaceutics to detergency. Among guest molecules surfactants play an important role with both fundamental and practical applications. The formation of cyclodextrin/surfactant hostâguest compounds leads to an increase in the critical micelle concentration and in the solubility of surfactants. The possibility of changing the balance between several intermolecular forces, and thus allowing the study of, e.g., dehydration and steric hindrance effects upon association, makes surfactants ideal guest molecules for fundamental studies. Therefore, these systems allow for obtaining a deep insight into the hostâguest association mechanism. In this paper, we review the influence on the thermodynamic properties of CDâsurfactant association by highlighting the effect of different surfactant architectures (single tail, double-tailed, gemini and bolaform), with special emphasis on cationic surfactants. This is complemented with an assessment of the most common analytical techniques used to follow the association process. The applied methods for computation of the association stoichiometry and stability constants are also reviewed and discussed; this is an important point since there are significant discrepancies and scattered data for similar systems in the literature.
In general, the surfactantâcyclodextrin association is treated without reference to the kinetics of the process. However, there are several examples where the kinetics of the process can be investigated, in particular those where volumes of the CD cavity and surfactant (either the tail or in special cases the head group) are similar in magnitude. This will also be critically reviewed
Investigations of vesicle gels by pulsed and modulated gradient NMR diffusion techniques
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugĂ€nglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Vesicle gels are surfactant systems that form stiff gels with rather low amounts of surfactant. So far their structures have mostly been investigated using scattering techniques, which are generally appropriate for the study of structures on the nm-length-scale. Here we examine these gels using two complementary diffusion NMR techniques, which are both sensitive to structures on the ÎŒm-scale. The presented results imply structural features on the ÎŒm-scale, indicating a more complex structure than just that of densely packed small vesicles, as previously found for these systems. It is demonstrated that a combination of the diffusion NMR methods, described here, can provide useful insights, when morphological features extend over a wide range of length scales
Förvirring â en framgĂ„ngsrik metod att kontrollera inomhuslevande mottfjĂ€rilar
Förvirring â en framgĂ„ngsrik metod att kontrollera inomhuslevande mottfjĂ€rilar, Vad vi kan finna i maten:
Tre mycket vanliga mottfjÀrilar, kvarnmott (Ephestia kuehniella), mandelmott (Ephestia cautella) och indiskt mjölmott (Plodia interpunctella) angriper matprodukter över hela vÀrlden och orsakar stor skada i fabriker, lagerlokaler och i hem. Fram till vÀldigt nyligen bekÀmpades angrepp med kemiska bekÀmpningsmedel, t.ex. med metylbromid. En ökande insikt om mÀnsklig hÀlsovÄdlighet och insekters ökade resistens mot insekticider ledde till ett förbud mot anvÀndning av dessa kemikalier. Alternativa metoder krÀvs nu för att minska skadeinsekters angrepp. En alternativ metod Àr att anvÀnda feromoner.
Feromoner och möjligheten att anvĂ€nda dem i kontrollsyfte: Innan parning avger honan hos alla de tre nĂ€mnda arterna en specifik doft (feromon) som attraherar hanar. De tre arternas feromon bestĂ„r av tvĂ„ till fyra olika Ă€mnen som gör doften artspecifik (tillhör endast en art). Dessa nĂ€rbeslĂ€ktade mottfjĂ€rilar har emellertid samma huvudkomponent i sina feromoner ((Z,E)-9,12-tetradecadienylactetat) och denna ensam fungerar relativt vĂ€l för samtliga arter. âFörvirringâ Ă€r en metod som numera vinner mer och mer mark hos odlare över hela vĂ€rlden. Den övergripande hypotesen Ă€r att genom att sprida ut ett moln av det artspecifika doftĂ€mnet hindra hanen att hitta doftspĂ„ret till honan. DĂ€rigenom hindras parning och populationen minskar eller dör ut. Förvirringsmetoden anvĂ€nds oftare utomhus i odlingar men dess effekt inomhus har inte hittills blivit undersökt. Vi redovisar hĂ€r försök pĂ„ hur förvirring har minskat antalet mottfjĂ€rilar i olika typer av lokaler, i kvarnar (kvarnmott), i husdjursfoderlager (Indiskt mjölmott) och i en chokladfabrik (mandelmott) och som tyder pĂ„ att förvirring Ă€r en mycket lovande alternativ metod mot skadeangrepp av mottfjĂ€rilar.
Förvirring i kvarnar och chokladfabriker: Tre kvarnar har behandlats med förvirring dÀr populationsstorleken har mÀtts genom att rÀkna antalet fÄngade hanar i feromonfÀllor. FÀllfÄngster minskade drastiskt i alla lokaler och jÀmförelser mellan Är visar att metoden fungerar bÀttre under en lÀngre tid (cirka 20 mÄnader). Feromonkoncentrationen mÀttes med en bÀrbar elektroantennograf (EAG) som visade att feromonhalten ökade direkt efter uppsÀttandet av feromonavgivarna och att feromonet var jÀmnt fördelat över hela lokalerna. FÀrre mott kunde observeras i lokalerna och antalet kundreklamationer minskade under de Är försöken pÄgick. I chokladfabriken kunde vi Àven fÄnga mandelmott i vattenfÀllor som ett oberoende mÄtt pÄ populationsstorleken. FÄngster i dessa fÀllor visade en signifi kant nedgÄng av populationen av mandelmott. Av dessa förvirringsförsök kan vi sammanfatta att denna metod har stor möjlighet att, pÄ ett miljövÀnligt sÀtt, kunna minska populationer av dessa tre inomhuslevande mottfjÀrilar
The effect of polymers on the phase behavior of balanced microemulsions: diblock-copolymer and comb-polymers
Short range forces in surfactant systems. Specific ion-effects and ion competition
Electrostatic effects often dominate the behavior of charged surfactant systems. While descriptions on the level of the Poisson-Boltzmann equation are successful in describing many important features of such systems, they cannot predict those specific ion effects arising when changing counterions of the same valency and they often fail in predicting effects due to changing from a monovalent to a divalent counterion, everything else kept constant. We show in this communication two examples on how such specific ion effects are conveniently studied using equilibrium bulk surfactant systems. By systematically changing among alkali counterions in a lamellar liquid crystal while simultaneously measuring the NMR quadrupolar splitting of the ions, ion competition may be investigated. The evolution of the microstructure (as determined with NMR diffusometry) in a microemulsion when changing from Br- to SO42- as counterions to a cationic twin-tailed surfactant constitutes a convenient tool to investigate effects due to changing in the charge of the counterion. Finally, based on a suggestion originally due to Wennerstrom, we suggest an explanation on the effects observed when a monovalent counter ion is replaced with a divalent one in surfactant systems; an explanation which is based on the concept of image charges. (C) 2004 Elsevier Ltd. All rights reserved
Nuclear Magnetic Resonance Studies of Bicontinuous Liquid Crystalline Phases of Cubic Symmetry : Transport Properties from 2H Nuclear Magnetic Resonance Relaxation Rates
The ternary system didodecyltrimethylammonium bromide, 1-decanol, and water forms an extended reversed continuous phase of cubic symmetry at 25 °C. The cubic phase belongs to the space group Im3m, as shown by small-angle X-ray experiments. We present extensive deuterium NMR relaxation data from this cubic phase for 1-decanol, deuterated at the carbon adjacent to the hydroxyl carbon position. 2H spin-lattice (R1) and spin-spin (R2) relaxation rates were measured over the existence region of the cubic phase, which extends from 0.2 to 0.6 in volume fraction of the dividing bilayer surface of the cubic phase. The data are interpreted with an existing theoretical framework for NMR spin relaxation in bicontinuous cubic phases, which takes its starting point in the description of bicontinuous phases using periodic minimal surfaces. Specifically, we obtain the self-diffusion coefficient over the minimal surface in one unit cell for 1-decanol. We also present pulsed field gradient NMR-derived self-diffusion data for didodecyltrimethylammonium bromide and compare the two sets of data. The diffusion data for both components show a mild, if any, dependence on the volume fraction of the bilayer surface. Furthermore, we present diffusion data for the water component in the cubic phase. Finally, we discuss the influences of the choice of the value of the product of the deuterium quadrupole constant and the order parameter S. Within the framework of the model used to analyze the relaxation data, a value for this parameter is required. As an initial value, we rely on measurements of deuterium quadrupolar splittings from deuterated decanol in an anisotropic phase
NMR Studies of Bicontinuous Liquid Crystalline Phases of Cubic Symmetry : Interpretation of Frequency-Dependent Relaxation Rates
Extensive deuterium NMR relaxation data are presented for two specifically deuterium labeled surfactants forming bicontinuous cubic phases with water. 2H spin-lattice (R1) and spin-spin (R2) relaxation rates were measured over an extended frequency range from 2 to 60 MHz. The data are interpreted with an existing theoretical framework for spin relaxation in bicontinuous cubic phases, which takes its starting point in the description of bicontinuous phases using periodic minimal surfaces. We show that the theory succeeds in accounting for the data and that the defining parameters of the theory, correlation times and order parameters, are in agreement with related data in other surfactant phase situations. Specifically, we obtain the surfactant self-diffusion coefficient over the minimal surface in one unit cell and show that it is in agreement with the corresponding macroscopic NMR diffusion data. By measuring two additional NMR relaxation parameters for each carbon on the surfactant hydrocarbon tail, we demonstrate how order parameter and correlation time profiles can be obtained. Finally, we analyze published molecular dynamics trajectories for a bicontinuous cubic phase. The analysis provides further support for the theoretical framework used to interpret relaxation data
Experimental determination of pore shape and size using q-space NMR microscopy in the long diffusion-time limit
The signal obtained with q-space NMR imaging applied to a confined liquid is directly related to the pore shape in the limit where all molecules have sampled the whole pore. We investigate the diffusion of water across a similar to50 mum thick film formed between planes of glass. The diffusion time t is changed almost three orders of magnitude. For short t, the root-mean-square displacement increases with a rate which is slightly less than for freely diffusing water. At t longer than 0.3 s, the displacement,is constant at 24 mum which implies that t he water is confined in the measuring direction defined by the applied gradient pulses. Perfectly smooth and aligned planes give rise to sharp diffraction-like features on the echo attenuation curve, i.e., NMR signal vs. the reciprocal space vector q. The experimental data with rather smooth local minima and maxima can be explained in terms of either surface roughness or a misalignment of the plan s. We discuss the averaging effect of diffusion along a laterally inhomogeneous film and propose two model-free methods to determine the pore shape from the echo attenuation curve obtained in the long-t limit. (C) 2003 Elsevier Science Inc. All rights reserved
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