Abstract:The phase behaviour and phase transition of the tetradecyldimethylaminoxide micellar system TDMAO/ D20 have been investigated by small-angle neutron scattering (SANS) at temperatures T between 287 and 325 K, pressures up to P--842 MPa and at TDMAO concentrations of 55 and 102mmoll -~. A pressureinduced structural (shape) phase transition was observed for the whole pressure versus temperature range, with a slope of the transition line of dTm/dP ~-0.31 ° MPa -lThe reversibility and reproducibility of this phase … Show more
“…Apart from that one observes a maximum in the SANS intensity distribution around Q max ≈ 1.6 nm -1 . Such a maximum was previously also found for other concentrations . This means that in these precipitates a characteristic length of l ‘ = 2π/ Q max ≈ 3.9 nm is present.…”
Section: Results Of the Sans Experiments And Their
Discussionsupporting
confidence: 81%
“…A spherocylinder consists of a cylinder of length L and radius R capped with two hemispheres of the same radius at the ends. The number of pressure experiments using neutron- or X-ray small-angle scattering techniques on surfactant/water systems is still limited. − We present a SANS study where in addition to temperature pressure was one of the external parameters.…”
By means of small-angle neutron scattering (SANS) experiments the dependence on pressure of the mean aggregation number 〈N〉 of rodlike tetradecyldimethylaminoxide (TDMAO) micelles in D2O was determined for pressures up to 2000 bar, at temperatures of 280.8, 294.9, and 308.1 K and at a TDMAO concentration c ) 118 mM L -1 . By an analysis via the so-called ladder model, the dependence of the difference of the chemical potentials on pressure for a monomer being located in the end cap and in the cylindrical part of the spherocylindrical micelles, respectively, was extracted. This difference depends on concentration c, pressure P, and strongly on temperature T. For the two lower temperatures, pressureinduced phase transition was observed.
“…Apart from that one observes a maximum in the SANS intensity distribution around Q max ≈ 1.6 nm -1 . Such a maximum was previously also found for other concentrations . This means that in these precipitates a characteristic length of l ‘ = 2π/ Q max ≈ 3.9 nm is present.…”
Section: Results Of the Sans Experiments And Their
Discussionsupporting
confidence: 81%
“…A spherocylinder consists of a cylinder of length L and radius R capped with two hemispheres of the same radius at the ends. The number of pressure experiments using neutron- or X-ray small-angle scattering techniques on surfactant/water systems is still limited. − We present a SANS study where in addition to temperature pressure was one of the external parameters.…”
By means of small-angle neutron scattering (SANS) experiments the dependence on pressure of the mean aggregation number 〈N〉 of rodlike tetradecyldimethylaminoxide (TDMAO) micelles in D2O was determined for pressures up to 2000 bar, at temperatures of 280.8, 294.9, and 308.1 K and at a TDMAO concentration c ) 118 mM L -1 . By an analysis via the so-called ladder model, the dependence of the difference of the chemical potentials on pressure for a monomer being located in the end cap and in the cylindrical part of the spherocylindrical micelles, respectively, was extracted. This difference depends on concentration c, pressure P, and strongly on temperature T. For the two lower temperatures, pressureinduced phase transition was observed.
“…This transition was also observed in the SANS spectra at lower temperatures and correspondingly lower pressures (data not shown). The identical transition has been reported by others in SANS studies of tetradecyldimethylaminoxide (TDMAO) micelles in D 2 O at pressures up to 3000 bar. , A determination of the microstructure in the new high-pressure phase was not attempted, although a lamellar structure was conjectured. 2 Measured SANS spectra at 20 °C for 1 wt % C 12 E 5 in D 2 O as a function of pressure and 49 wt % C 12 E 5 in D 2 O at ambient pressure.…”
We apply small-angle neutron scattering (SANS) to study the effect of pressure on micelle structure in a solution of 1% by weight pentaethylene glycol mono-n-dodecyl ether (C12E5) in D2O at 20 °C and pressures up to ∼3000 bar. At ambient pressure, the structure is a network of branched, semiflexible, cylindrical micelles with the branch points comprised of three-armed junctions. Our SANS results reveal that pressure induces a phase transition from this network of threadlike micelles to hexagonally ordered bundles of cylindrical micelles. Using geometric packing constraints for three-arm junctions and cylinders, we show that the formation of three-arm junctions becomes increasingly unfavorable with increasing pressure due to the compression of the micelle hydrophobic core, and as such, the network becomes unstable at pressures close to those observed in our SANS experiments. We also measured the temperature dependence of the transition pressure and find that it follows the pressure-temperature freezing curves for liquid n-alkanes of comparable hydrocarbon chain length. These observations lead us to propose that the phase transition is related to a loss of flexibility or conformational entropy of the C12E5 micelles upon the pressure-induced freezing of the micelle hydrophobic core to form an amorphous solid. The formation of hexagonally ordered bundles of cylindrical micelles follows as attractive van der Waals forces between the micelles are not offset by the loss of repulsive undulation forces arising from the fluidity of the hydrophobic core.
“…First high pressure setup at the small angle neutron scattering facility was realised using the Ti/Zr "null matrix" alloy [7]. The experiments with the surfactants C14MDAO and TDMAO provided new important information [7,8].…”
Section: High-pressure Setupmentioning
confidence: 99%
“…The temperature in the pressure cell is controlled by the LAUDA thermostat with an error ± 0,05°C. First P-V-T experiments with the waterdimyristoylphosphatidylcholine (DMPC) system have been done in [8]. Pressure dependence of isothermal compressibility of lipid membranes was studied.…”
Main features of the modernized small-angle neutron scattering spectrometer (YuMO) at IBR-2M pulsed reactor are described. New installations for sample environment of the spectrometer are highlighted. The modernized SANS instrument (YuMO) is equipped with a new type of position sensitive detector as well as two detector system which provide a unique dynamic range (Q max /Q min ratio is about 90).Sample environment is extended with a magnetic system (magnetic field about 2.5 Tesla), automated high pressure setup which allows simultaneous SANS and volumetric high pressure studies and light illumination system. In particular, these developments led to considerable improvements of resolution of the instrument (about 1%) and opened the possibility to study anisotropic materials and perform efficient high pressure studies.
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