From a Landau theory the static scattering intensity distribution I(q) of microemulsions is obtained. As essential ingredient we have included a negative gradient term in the free energy expression. The form of I(q)∼(a2+c1q2+c2q4)−1 yields for a2>0, c1 0 a single broad scattering peak and a q−4 decay at large q, both properties experimentally observed for a variety of microemulsions containing comparable amounts of water and oil. The peak originates from the modulation in the corresponding space correlation function given by γ(r)=(d/2πr)⋅e−r/ξ⋅sin(2πr/d). It is shown that the scattering intensity relation describes experimental literature data remarkably well, using only three fit parameters.

Origin of the scattering peak in microemulsions

In this paper we highlight the advantageous features of both short- and long-chain surfactant systems in determining the most important properties of three-phase bodies and genuine microemulsions. To this end first the phase behavior of the simple system H2O–n-dodecane–C4E1 (n-butyl monoglycol ether) is discussed. From temperature–concentration sections through the phase prism at various water-to-oil ratios, a number of isothermal Gibbs triangles are constructed, which can quantitatively be compared with theories. Furthermore, the critical tie-lines of the three-phase body have been precisely determined. From these measurements it is evident that there is a non-negligible monomeric solubility of the surfactant in the excess phases. For long-chain surfactant systems, the more efficient the surfactant, the larger is the size of the microstructure of the microemulsion. We report a striking correlation between the efficiency of a surfactant and its monomeric solubility in the excess phases for 23 systems of the type H2O–n-alkane–CiEj. These findings suggest that the monomeric solubility of the surfactant in oil and water might be the key to describing the main features of these microemulsion systems.

On microemulsion phase behavior and the monomeric solubility of surfactant

http://www.sustainable-chemistry.de/06_GreenChem_glycerol%20monoethers_imprim.pdf

Short chain glycerol 1-monoethers—a new class of green solvo-surfactants

Short Chain Glycerol 1-Monoethers – a New Class of Green Solvo-Surfactants, Green Chemistry

The binary and ternary phase behavior of the trisiloxane E6 and E10 polyoxyethylene surfactants with water and three low molecular weight silicone oils has been determined. The silicone oils were the tetra- and pentacyclosiloxanes, D4 and D5, and the short linear tetrasiloxane oil, MD2M. Microstructures were investigated using small-angle X-ray scattering, polarized light microscopy, and cryogenic transmission electron microscopy. Our results illustrate the differences and similarities between the trisiloxane surfactants and conventional hydrocarbon surfactants. Both water/surfactant binary systems form the isotropic spongelike phase, L3, in a narrow temperature band above the lamellar phase region. The L3 phase is found at significantly higher temperatures for the E10 homolog. In the water/M(D‘E6)M system, no two-phase region was detected between the region labeled L3 and the isotropic surfactant-rich region, L2. The different birefringent appearance as one progresses between the L3, the L2, and the lame...

Phase Behavior and Microstructure of Water/Trisiloxane E6 and E10 Polyoxyethylene Surfactant/Silicone Oil Systems

Certain ternary mixtures of ethoxylated alcohols, n-alkanes, and water at 25/sup 0/C separate at equilibrium into three liquid phases. The development of an amphiphile-rich middle phase is sensitive to alcohol molecular weight, alkane carbon number (ACN), and temperature. A middle phase arises when all three variables are adjusted such that the alcohol has no preference to partition into either an oleic or aqueous phase. A change in one of these variables yields a sequence of ternary phase diagrams in which the range of the three phases is from one critical tie line to a second. There are two distinct patterns, however, in which the critical tie lines arise. With a low molecular weight ethoxylated alcohol, both critical tie lines lie in the miscibility gap between water-rich and alkane-rich phases. With larger ethoxylated alcohols, however, one of the critical tie lines arises from the fusion of that miscibility gap with the critical point on a second one between water-rich and alcohol-rich phases. Both patterns are modeled well by modifying the Flory-Huggins equation of state to account for the tendency of amphiphile to concentrate between water-rich and alkane-rich domains, thereby attenuating or screening the enthalpic repulsion between water and alkane. The screeningmore » is expressed by a factor which is exponential in amphiphile concentration. The type of three-phase equilibria which arises from fusion of two miscibility gaps requires a pair of screening factors. Other available thermodynamic models appear incapable of approximating this second pattern of three-phase equilibria.« less

C. A. Gorman

Papers

Patterns of phase behavior in ternary ethoxylated alcohol-n-alkane-water mixtures

Patterns of Phase Behavior in Ternary Ethoxylated Alcohol-n-Alkane-Water Mixtures.