Donald B. Siano

Layered sedimentation in suspensions of monodisperse spherical colloidal particles

Abstract Stratification of the supernatant of suspensions of polydisperse sedimenting colloidal-sized particles, hitherto ascribed to polydispersity, aggregation, convection, or sedimentation potential effects, is also shown to occur in dispersions of highly monodisperse, spherical polystyrene particles in the size range from 2.02 to 0.109 μm. The layers appeared in suspensions with concentrations as low as 0.01 wt% up to at least 0.4 wt%. The qualitative behavior of the layers with their associated interfaces was seen to be highly dependent upon concentration gradients initially imposed on the suspension but to be relatively insensitive to salt (as long as its concentration was below the fluctuation value). Examination of samples from the layers showed that the composition of each layer was the same—only the concentration varied. Many features of the data suggest that the process is in fact due to the mechanism of spinodal decomposition during the separation of two phases (colloid-rich sediment and colloid-poor supernatant) as the system moves toward its final equilibrium state.

The swollen micelle—microemulsion transition

Abstract A rather abrupt transition takes place in the properties of a nonionic microemulsion at a certain value of hexadecane uptake, Wo (the weight fraction of oil plus surfactant that is oil). When Wo is greater than 0.06, the microemulsion behaves as spherical particles that can be diluted with water without change in their properties. The intrinsic viscosity, Huggins coefficient, and 4A2Mw have values close to those observed for the globular proteins. When Wo is less than 0.06 the particles are anisotropic in shape, as shown by classical light scattering. Moreover, the anisotropic particles are undergoing open association so that dilution decreases their average size. When diluted, the rate of oil uptake by the microemulsion is orders of magnitude smaller than for the swollen micelles.

A polymer—microemulsion interaction: The coacervation model

A model which successfully predicts many of the qualitative features of the phase separation can be based upon the assumption that the microemulsion particle retains its integrity when diluted with external phase or when mixed with a polymer (which does not complex with the surfactants). In this case, the microemulsion particle behaves thermodynamically in a manner similar to a macromolecule so that one can predict that the polymer—microemulsion phase diagram will have qualitative similarities to that exhibited by the polymer—polymer2-solvent case. Most of the experiments were conducted by following the phase boundaries by measurements of the cloud points of the polymermicroemulsion mixtures. The cloud points of the mixtures were found to be (usually) linear functions of the weight fraction of the added polymer and decreased with increasing molecular weight. Plots of the logarithm of the slopes of these curves against the logarithm of the polymer molecular weight were linear, with a slope designated as β. The values for β for twenty-five systems studied fell within the narrow range of 0.55 ± 0.15. These values were seen to be the same as the Mark—Houwink exponents in the relationship between the intrinsic viscosity and the polymer molecular weight. The reason for this correspondence appears to be that the coacervation occurs at a polymer concentration about equal to the threshold overlap concentration. Ternary phase diagrams with microemulsion particles, brine, and polymer selected as pseudocomponents also exhibit many of the features predicted by the model. The phase boundary asymmetry is in the direction expected and asymptotes with the expected independence upon microemulsion and polymer molecular weights are also demonstrated. In agreement with the model, it is shown that the controlling variable is polymer weight average, rather than z or number average molecular weight.

Orientational analysis—a supplement to dimensional analysis—I

Abstract A method for extending the methods of dimensional analysis is based upon assigning “orientational” symbols to physical quantities such as area, force and angle which are spatially oriented. These symbols are shown to form a noncyclic group with four members, and they can be used to derive additional information that resolve problems incompletely solved by conventional dimensional analysis.

Perturbation of a nonionic microemulsion by the introduction of a charged surfactant. I: Light scattering

Abstract A nonionic microemulsion with a small fraction of its nonionic surfactant, Brij 96, replaced by sodium oleate was used to vary the charge on the presumed microemulsion droplets in a controllable fashion. Cloud points and other evidence indicate that the mixed microemulsion was formed. The molecular weight and second virial coefficient of the droplets were measured as a function of added salt and showed that the molecular weight of the ionized microemulsion was only slightly changed from the nonionic case, while the changes in second virial coefficient could be described by a hard sphere plus double-layer potential. The particles were shown to undergo ordering at very low salt concentrations.

Orientational analysis, tensor analysis and the group properties of the SI supplementary units—II

Abstract An extension of the methods of dimensional analysis to include the orientations of physical quantities shows that a useful approach is to assign orientations to the supplementary units, radian and steradian, and to require that physical equations be orientationally as well as dimensionally homogeneous. Orientational symbols representing the intuitive orientational character are shown to form a noncyclic Abelian group with four members. The methods of dimensional analysis derives from the fact that the laws of physics must be independent of the units; orientational analysis derives from the fact that they must also be independent of the coordinate system, i.e. they are expressible as tensor equations.

Perturbation of a nonionic microemulsion by the introduction of charged surfactant: II. Potentiometric titrations

Abstract A nonionic microemulsion which was perturbed by the addition of a small fraction of ionic surfactant, oleic acid, was studied by means of potentiometric titrations. The data were analyzed by a simple Debye-Hu¨ckle model for the electrolyte. The titrations gave a value of 6.6 for the intrinsicp K which implies that at the start of the titrations the car☐yl was in a region of low dielectric strength. Plots of the effectivep K vs the degree of ionization as a function of added electrolyte were analyzed to give an apparent radius of about 69A˚compared to the 50A˚found by classical light scattering.

Volumetric properties of a nonionic microemulsion

Abstract A model nonionic microemulsion system was studied by dilatometric methods as a function of temperature (15–30°C) and volume fraction of dispersed phase, W 2 (0.2–0.01). From these measurements the partial specific volume, v 2 , of the dispersed phase was derived, and the results were extrapolated to infinite dilution to give v 2 0 . It was found that v 2 was linear with W 2 ; no breaks were evident, indicating that the microemulsion is “dilutable.” A plot of v 2 0 against temperature was slightly concave down, indicating that the microemulsion surface was a water structure breaker, as expected for a polyethylene oxide covering. The excess volume V E and partial excess volume ∂ v E /∂W 2 were also determined. Since V E was significantly different from zero, ideal mixing did not hold. The partial excess volume extrapolated to infinite dilution was found to be about −0.042 ml g −1 , which is also comparable to that found for polyethylene oxide, about −0.048 g −1 . The volume decrease on mixing for the microemulsion with the (nominal) value of 0.10 was found to be about 40 × 10 −4 ml g −1 , indicating that the microemulsion composition is closer to a lower critical solution temperature.

Microemulsion order of mixing effects

Abstract A kinetic order of mixing effect, which does not appear to be due to sharp qualitative change in overall molecular organization, is demonstrated in a nonionic microemulsion. Above a certain concentration (in water) of surfactant plus cosurfactant, added oil is incorporated into the microemulsion very quickly whereas at a lower concentrain the added oil is taken up much more slowly. The surfactant concentration at which the break in solubilization rate occurs is termed the rapid solubilization concentration (rsc). The difference in rate of oil solubilization above and below the rsc is reflected by interfacial tension and turbidity data. NMR experiments indicate no change in the restriction of the carbons at the presumed hydrophobic-hydrophylic interface. Light scattering experiments indicate, in addition, that the microemulsions formed by dilution, sonication, or heating are thermodynamically stable.