A structural description of liquid particle dispersions: Ultracentrifugation and small angle neutron scattering studies of microemulsionsWe have studied the phase behavior, wetting transitions, and small angle neutron scattering (SANS) of water, n-alkane, and n-alkyl polyglycol ether (C;E) 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 wel1-structured mixtures (exhibiting nonwetting middle phases and wel1-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 smal1er surfactant concentrations, and intersect there with the body of heterogeneous phases, depends on a number of factors that are discussed in some detail. phase microemulsions become able to wet the water/oil interface for thermodynamic reasons. 24 Widom has suggested

A small-angle neutron scattering study of nonionic surfactant molecules at the water-oil interface: Area per molecule, microemulsion domain size, and rigidity Small angle neutron scattering near Lifshitz lines: Transition from weakly structured mixtures to microemulsions Small-angle neutron scattering measurements were performed on symmetric microemulsions containing equal volume fractions of water/formamide, n-octane, and a sufficient amount of nalkyl polyglycol ether (CiE j ). By changing the surfactant chain length in the order C 8 E 3 , . C 6 E 2 , and C 4 E 1 in pure water, as well as for given CgE3 and C 6 E2 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 occurence of a n~nwetting ~ wetting transition in the vicinity of a Lifshitz line. Roughly speaking, mlcroemulslOns, 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. 8532

Spherical, cylindrical, and lamellar microstructures were examined by ͑SANS͒ in the binary system water ͑D 2 O͒/pentaethylene glycol mono-perdeutero-n-dodecyl ether ͑C 12 D 25 E 5 ͒ and in the ternary system ͑D 2 O͒/pentaethylene glycol mono-n-dodecyl ether ͑C 12 E 5 ͒/perdeuterated n-octane ͑C 8 D 18 ͒. 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.

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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→2̄ with rising temperature, with a second ternary mixture that shows a 2_→2̄ 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.

We present a procedure for the evaluation of small‐angle scattering data from microemulsions. A model independent picture of the structures in the various one‐phase samples emerges from the Indirect Fourier Transformation analysis of the measured small‐angle neutron scattering (SANS) spectra. The data analysis makes no a priori assumptions about the type of structure. Rather, the pair distance distribution functions are so unique that an unambiguous assignment of spherical, cylindrical or planar geometry is possible. In addition, the scattering length density profile normal to the interface is obtained by a deconvolution technique. We have investigated the microstructures in the binary system water (D2O)/pentaethylene glycol monoperdeutero‐n‐dodecyl ether (C12D25E5) and in the ternary system (D2O)/pentaethylene glycol mono‐n‐dodecyl ether (C12E5)/perdeuterated n‐octane (C8D18) by small‐angle neutron scattering. Spherical, cylindrical and planar morphologies were induced by changing the temperature of samples with fixed compositions. The same structural sequence exists in both the binary and the ternary mixtures. The spherical, cylindrical and planar structures are observed for water‐rich samples by increasing temperature. For oil‐rich samples the same structural sequence occurs as function of decreasing temperature. Both for water‐rich and oil‐rich samples, increasing the surfactant concentration at a constant ratio of surfactant to either oil or water, appropriately, causes the structures become more concentrated, but preserves the local structure and its dimensions.

We review and summarize the features of nonionic microemulsion phase behavior. Particular emphasis is given to the behavior of mixtures of oil, water or other polar materials with nonionic surfactants from the family of n‐alkyl polyglycol ethers, usually denoted as CiEj. The patterns of phase behavior are reviewed and the roles of various additives such as electrolytes and alcohols are discussed.

Microemulsions are, in general, prepared by mixing water, a nonpolar "oil", and an amphiphile, either nonionic or ionic. In this paper we study the effect of replacing water by another polar protic solvent, namely, formamide (FA). Because hydrocarbons are slightly more soluble in FA than in H20, the repulsive hydrophobic interaction between the hydrocarbon tails of the amphiphiles and FA is weaker than in H2O. As a consequence, both the mutual solubility and the cmc increase considerably upon replacing H2O by FA. This can be compensated by increasing the carbon number of the tails of the amphiphiles. With the nonionic CiEj one has to increase the carbon number i by about five, whereas with ionic amphiphiles one has to proceed from single-tailed to doubletailed amphiphiles as, e.g., (Cm)2DABr. If one does so, one may prepare microemulsions with FA having essentially the same properties as with H20 as polar solvent. Again one finds a reverse phase behavior for nonionic and ionic amphiphiles and comparable solubilization capacities, as well as the gradual evolution of a correlation peak in small-angle neutron scattering curves and a complete wetting -partial nonwetting transition as one proceeds from weakly to strongly structured mixtures.