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Examines binary and ternary mixtures of C[sub 12] E[sub 5], n-octane, and D[sub 2] O by neutron scattering experiments. Use of generalized indirect Fourier transformation to determine particle shape and structure in real space; Evaluation of microemulsion data.

In a recent publication we compared phase behavior and scattering data obtained from SANS in water, n-alkane, n-alkyl polyglycol ether (CiEj) mixtures. By analyzing the scattering spectra using the Teubner–Strey formula, it is possible to determine a measure for the amphiphilic strength of each system called the amphiphilicity factor, fa. It was demonstrated that the amphiphilicity factor is constant on a variety of surfaces within the three-dimensional space of composition and temperature. For instance, the Lifshitz surface, where fa=0, depends on composition and temperature and may be a precursor for the Lα phase. Here we show that the same sequence exists in the binary system water/C4E1, where the surfaces become lines in the two-dimensional composition-temperature planes. Variations in amphiphile concentration and solution temperature allow one to reach almost the entire accessible amphiphilicity scale (∞>=fa>=-1) within this simple binary mixture. The occurrence of the Lifshitz surface at high C4E1 concentrations and its disappearance as a function of temperature give more evidence that the Lifshitz region can be treated as a precursor of the lamellar liquid crystalline phase. While earlier investigations of water/C4E1 solutions in the dilute regime support the existence of micellar aggregates, the current work provides the first definitive evidence of the presence of supramolecular aggregates and short range order in the water/C4E1 system in the amphiphile-rich region. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied the phase behavior, wetting transitions, and small angle neutron scattering (SANS) of water, n-alkane, and n-alkyl polyglycol ether (CiEj) systems in order to locate the transition between weakly structured mixtures and microemulsions, and to provide a measure for the transition. We first determined the wetting transition by macroscopic measurements and then measured the location of the Lifshitz lines by SANS. Starting with well-structured mixtures (exhibiting nonwetting middle phases and well-expressed scattering peaks, features that qualify them as microemulsions) the wetting transition was induced by increasing the chain length of the alkane or by changing the oil/water volume ratio, and then the Lifshitz line was crossed. Further, starting with systems past the disorder line (weakly structured mixtures that display wetting middle phases and no scattering peaks), local structure was induced by either increasing the surfactant concentration or decreasing the oil/water volume ratio or the temperature. In each case a Lifshitz line was crossed. Analyzing the scattering experiments quantitatively, allows determination of the amphiphilicity factor, which is a measure of the strength of the surfactant. The results suggest there is a sequence of roughly parallel surfaces within the three-dimensional composition-temperature space. As the amphiphilicity factor increases, first a disorder surface is encountered, then a Lifshitz surface, and finally a wetting transition surface. How and to what extent these surfaces move in the one-phase region toward smaller surfactant concentrations, and intersect there with the body of heterogeneous phases, depends on a number of factors that are discussed in some detail. [ABSTRACT FROM AUTHOR]

Small-angle neutron scattering measurements were performed on symmetric microemulsions containing equal volume fractions of water/formamide, n-octane, and a sufficient amount of n-alkyl polyglycol ether (CiEj). By changing the surfactant chain length in the order C8E3, C6E2, and C4E1 in pure water, as well as for given C8E3 and C6E2 by increasing the relative amount of formamide in the water/formamide mixture, the amphiphilic strength (the amphiphilicity) is reduced. We observe that the characteristic scattering peak becomes weaker and its position moves into q=0 showing that the microstructure becomes disordered. However, a q-2 and a q-4 behavior for large q in film and bulk contrast, respectively, is still observed indicating the persistence of internal interfaces. As the microstructure is further weakened, a disorder line is passed at which the behavior of the real space correlation function changes from a damped oscillatory behavior to a monotonically decreasing one. We determine that this line has been passed from fits to the bulk scattering intensity. Simultaneously, the film scattering intensities show a transition from correlated to uncorrelated films as the disorder line is passed. Closely related to the weakening of the microstructure is the occurrence of a nonwetting→wetting transition in the vicinity of a Lifshitz line. Roughly speaking, microemulsions, that do show a scattering peak do not wet the water–oil interface. As predicted by Landau–Ginzburg theories, the wetting transition occurs on the microemulsion side of the disorder line. As the amphiphilicity is even further reduced, a tricritical point is reached with the interesting observation that substantial local structure still persists. [ABSTRACT FROM AUTHOR]

Quaternary mixtures of water (A), an oil (B), a nonionic amphiphile (C), and an appropriately chosen fourth component offer an opportunity for searching for tricritical points (tcp) at atmospheric pressure. It is shown that for reaching a tcp, one has to couple an A–B–C mixture that shows the phase sequence 2_→3→2 with rising temperature, with a second ternary mixture that shows a 2_→2 transition, the bar denoting in which of the two phases the amphiphile is mainly dissolved. With weakly structured solutions, that is, with short-chain amphiphiles as (C) this can be done by either adding an oil with a lower carbon number, or by adding a nonaqueous polar protic solvent such as formamide. With strongly structured solutions, that is, with long-chain amphiphiles, one has to add a short-chain amphiphile for destroying the structure as a prerequisite for reaching a tcp. Insofar, our earlier presumption that with long-chain amphiphiles, a tcp may also be reached, either by increasing their amphiphilicity or by lowering the carbon number of the oil, does not seem to apply. Experience shows that in A–B–C’ mixtures with sufficiently short-chain amphiphiles as C’ that separate into three phases: the amphiphile-rich middle phase always wets the A/B interface. If a short-chain amphiphile is added to an A–B–C mixture with a nonwetting middle phase one will, therefore, inevitably find a nonwetting→wetting transition as one approaches a tcp. [ABSTRACT FROM AUTHOR]

Spherical, cylindrical, and lamellar microstructures were examined by (SANS) in the binary system water (D2O)/pentaethylene glycol mono-perdeutero-n-dodecyl ether (C12D25E5) and in the ternary system (D2O)/pentaethylene glycol mono-n-dodecyl ether (C12E5)/perdeuterated n-octane (C8D18). A model-independent picture of the structures emerges from Fourier transformation of the measured SANS spectra. The data analysis makes no a priori assumptions about the type of structure. Rather, the pair distribution functions obtained are so unique that an unambiguous assignment of the local geometry is possible. The different structures for a given mixture are obtained by changing temperature. For water-rich samples the sequence of spherical, cylindrical, and planar structures is obtained with increasing temperature. For oil-rich samples the same sequence occurs as a function of decreasing temperature. As an important additional result, the scattering length density profile normal to the interface is obtained. The interfacial profile is found to be rather diffuse, apparently because of solvent penetration. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]