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  • 1
    Electronic Resource
    Electronic Resource
    Small angle neutron scattering near the wetting transition: Discrimination of microemulsions from weakly structured mixtures (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 3782-3787 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The wetting transitions of water, n-alkane, and n-alkyl polyglycol ether (CiEj) systems are examined in order to locate the transition between weakly structured mixtures and microemulsions. Using small angle neutron scattering (SANS) we determine the local structure and relate it to the phase behavior and wetting transitions observed by macroscopic measurements. We measure the SANS of the mixtures across the transition along two different experimental paths. One path begins with well-structured mixtures, and the effective chain length of the surfactant combination C6E2/C4E1 is decreased by increasing the C4E1 fraction. The other path starts with equal amounts of water and oil mixed by the strong amphiphile C8E3. The local structure of these "good'' microemulsions is weakened by increasing the temperature and concomitantly the oil/water volume ratio approaching the upper critical endpoint. As in previous studies analyzing the scattering experiments quantitatively permits determination of the amphiphilicity factor which is a measure of the strength of the surfactant. We confirm predictions that the amphiphilicity factor measured at the wetting transition becomes more negative as the temperature interval between the transition and the critical endpoint decreases. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471031
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  • 2
    Electronic Resource
    Electronic Resource
    Scaling of the interfacial tension of microemulsions: A phenomenological description (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 2875-2883 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent experiments revealed a striking scaling behavior of the low and ultralow interfacial tension of microemulsions. A description of this behavior based on the Helfrich elastic free energy, which is symmetric in the principal curvatures c1 and c2, appears to be inconsistent. We show that, within the phenomenological theory of membrane bending elasticity, symmetry breaking between the two principal curvatures seems to be required in order to explain the low, but nonzero, values of the interfacial tension and its temperature dependence. We propose two simple generalizations of the Helfrich free energy which describe the experimental results. The first considers a quadratic elastic free energy and anisotropy in the membrane which breaks the symmetry between the two principal curvatures. In the second, which is applicable to systems with positive saddle-splay rigidities, the symmetry between the two principal curvatures is spontaneously broken by inclusion of higher-order terms in the curvatures in order to stabilize the free energy of the system. This analysis provides a straightforward method to obtain estimates of the bending elastic constants from interfacial tension measurements. Experiments confirming the theoretical picture are presented and values for κ and κ¯, for a variety of systems, are obtained. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472149
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  • 3
    Electronic Resource
    Electronic Resource
    Ultralow interfacial tensions in water–n-alkane–surfactant systems (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 8606-8615 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interfacial tensions between water- and oil-rich phases in the presence of microemulsions have been measured for ternary systems of water, n-alkanes, and nonionic alkylpolyglycolether surfactants (CiEj). It is found that the minimum of the interfacial tension curve, which is observed for each system in conjunction with the well-known phenomenon of phase inversion, depends sensitively, but systematically, on the chemical nature of the oil and the surfactant. Specifically, the minimum value of the interfacial tension σ¯ab decreases by 1 order of magnitude on decreasing either the carbon number of the alkane k by 6, or the number of oxyethylene groups j by 3, or by increasing the number of carbon atoms in the surfactant tail i by 2. The numerical values of the interfacial tensions as a function of temperature are presented along with an empirical description previously suggested [R. Strey, Colloid and Polymer Sci. 272, 1005 (1994)]. From the analysis, in terms of bending energy one obtains estimates for the bending and saddle-splay constants. The similar shape of the interfacial tension curves permits a superposition of the data for all 19 systems in support of a scaling relation recently derived [S. Leitao, A. M. Somoza, M. M. Telo da Gama, T. Sottmann, and R. Strey, J. Chem. Phys. 105, 2875 (1996)]. Furthermore, we note a striking coincidence of the numerical values of critical amplitude ratio R=σ0ξ0〈sup ARRANGE="STAGGER"〉2=0.37 kT in near-critical systems and the product σ¯abξ¯2=0.44(±0.10) kT where ξ¯ is the maximum length scale in the bicontinuous microemulsions. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473916
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  • 4
    Electronic Resource
    Electronic Resource
    A small-angle neutron scattering study of nonionic surfactant molecules at the water–oil interface: Area per molecule, microemulsion domain size, and rigidity (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6483-6491 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The small-angle neutron scattering (SANS) of bicontinuous microemulsions of 19 different water-n-alkane-CiEj (n-alkylpolyglycolether) systems has been measured. All scattering curves exhibit a broad scattering peak which permits determining the characteristic length scale ξ for bicontinuous structures at symmetric water and oil volume fractions, i.e., φ=0.5. Various random models predict ξ=aδφ(1−φ)/φc. We find that ξ is indeed inversely proportional to the surfactant volume fraction φc. Approximating the effective surfactant chain length δ by δ=νc/ac, where ac and νc are the area and the volume of the surfactant molecule, the numerical value for a is determined to be a=7.16, which is close to, but significantly different from those used in theoretical models. The head group area ac at the water–oil interface is obtained from the large q part of the scattering curves. It is found to be independent of i and k, the carbon numbers of the alkyl chain of the surfactant and of the alkane, respectively. However, it depends strongly, and nearly linearly, on the head group size j of the surfactant. Within experimental error it is described by ac=29.3+6.20j (Å2). © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473638
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