Raymond A. Mackay

Chemical reaction in microemulsions

Abstract Microemulsions are colloidal dispersions consisting of monodisperse droplets of oil in water (o/w) or water in oil (w/o) ranging from about 8-80nm in diameter. These clear, stable fluids contain a high disperse phase volume fraction and a large amount of oil-water interfacial area. In recent years these media have been applied to the study of chemical reactions between oil and water soluble species. Although microemulsions with both oil and water continuous phases have been employed, studies in o/w systems will be emphasized. The chemical processes investigated include acid-base reactions, the formation of metalloporphyrins and other complex ions, and the hydrolysis of esters, as well as some photochemical and electrochemical reactions. Comparisons with similar processes in simple micellar systems are made, and both the similarities and unique differences between micelles and microemulsions are discussed.

Measurement of pH and pK in OW microemulsions

Abstract Glass electrode pH measurements have been performed in oil in water microemulsions stabilized by butanol or pentanol and cationic, anionic, and nonionic surfactant. In conjunction with the pH measurements, the p K a of chlorophenol red has been determined in these microemulsions using both cationic and anionic buffer systems. The results indicate that the interphase region of the microdroplet in which the indicator is located has an effective dielectric constant of about 20, and that the intrinsic p K a of a dye is the same in all microemulsions of this type provided both indicator species are in the same average environment. It is further suggested that the effective aqueous concentration of hydrogen ion is related to the geometric mean of the aqueous buffer and microemulsion concentrations, and that a pKa obtained using the average of the aqueous and microemulsion pH values is the most physically meaningful quantity.

Diffusion Measurements in Microemulsions.

ABSTRACT We have carried out diffusion coefficient measurements in both aqueous micelles and microemulsions using the techniques of palaeography and quasielastic light scattering (QLS) The former method involves the determination of the diffusion coefficient of an electroactive oil soluble probe at a polarizable microelectrode. For high water content microemulsions, both methods yield the same diffusion coefficients, which can be identified as the self diffusion coefficient For cetyltrimethylammonium bromide (CTAB) micelles, both methods yield the same result at the salt (NaBr) concentration at which the QLS measurements are independent of CTAB concentration. In more concentrated microemulsions, QLS data gave diffusion coefficients in agreement with polarography only for a sodium cetyl sulfate (SCS) system at 65-75 wt % water. For the SCS microemulsions at 60% water, and CTAB microemulsion at 60-75% water, the QLS data yielded rapid, nonexponential decays. However, consistent polarographic diffusion coeff...

Dielectric studies on nonionic microemulsions

Abstract The dielectric properties of a series of O/W microemulsions were measured at frequencies between 1 MHz and 15.4 GHz. The microemulsions were composed of hexadecane, an emulsifier consisting of Tween 60 (a nonionic surfactant) and n-pentanol as cosurfactant, and various quantities of water or 0.1 N NaCl. The dielectric data include (a) the conductivity of salt-containing microemulsions at radiofrequencies, (b) the permittivity of the microemulsions at radiofrequencies (0.5 to 100 MHz), and the permittivity and conductivity of the microemulsions at microwave frequencies from 0.73 to 15.4 GHz. The dielectric permittivity values do not vary significantly with frequency below ca. 1 GHz. At higher frequencies dispersion phenomena appear because of the dielectric relaxation of the water in the system. The data are analyzed using the Hanai and Maxwell mixture theories, to determine the dielectric properties of the water and suspended phases. The results suggest that in suspensions containing 40–75% water, volume of water amounting to ca. 40–70% of the volume of the suspended oil/emulsifier is associated with the droplets, and has a dielectric relaxation frequency and ionic conductivity both reduced by a factor of 4–10 from that exhibited by the bulk water. For a mean droplet diameter of 74 A, this is equivalent to a shell of ∼2 water monolayers, most of which is likely present as water of hydration of the ethylene oxide groups of the surfactant.

Self-diffusion of water in ionic and nonionic microemulsions

Abstract The self-diffusion coefficient of water in four microemulsion systems was measured using an NMR-pulsed field gradient technique. The systems studied were two containing ionic surfactants and two with nonionic surfactants. The dielectric relaxation properties of two of these systems had been previously shown to exhibit large hydration effects. Comparable self-diffusion properties were observed for the water in all of these systems that were interpreted as reflecting a combination of obstruction and hydration effects. An analysis using the Maxwell and Hanai mixture theories shows that the amount of motionally altered water corresponds to approximately two water molecules of hydration per ethylene oxide and hydroxyl group in the nonionic systems, which is in the range expected from chemical considerations. The present results show the large contribution of hydration effects to the bulk transport properties of these microemulsions.

Use of Liquid Crystals as Vapor Detectors

Abstract The visual detection between crossed polarizers of organic vapors on various liquid crystal films is described. The liquid crystal is utilized as a smear on glass or as an impregnant in microporous polypropylene. The organic vapor induces a change of the liquid crystal from a birefringent mesophase to an isotropic liquid. The effect is generally reversible, i.e., removal of the film from the contaminated atmosphere restores the original state of the film. Non-selective detection is achieved easily in the ppm range. Of the liquid crystals examined, the most sensitive detector was found to be N-(p-methoxybenzilidine)-p-n-butylaniline (MBBA). Optical reflection from cholesteric materials, has been utilized previously for detection purposes. However, dependence on temperature and the need for instrumental monitoring to achieve high sensitivity do not currently allow the achievement of simple detector devices. In contrast, temperature effects and the need for instrumental monitoring are not as importa...

Light scattering measurements in nonionic microemulsions

Abstract Light scattering measurements have been performed on two nonionic microemulsion systems of the type hexadecane/water/alcohol/surfactant. The alcohol/surfactant combinations employed were butanol/Brij 96 and pentanol/Tween 60. By suitable treatment of the light scattering data, droplet diameters were obtained as a function of composition. These diameters are observed to increase exponentially with increasing oil content. In the Tween 60 microemulsion, a correlation of the droplet diameters with conductivity data indicates that the number concentration of droplets varies in a continuous fashion with oil content, as opposed to a previously postulated abrupt change. Coupled with model calculations, the results suggest that the fraction of alcohol cosurfactant in the interphase region of these nonionic systems decreases with increasing oil content.

Conductivity measurements in nonionic microemulsions

The conductivity has been measured of a number of nonionic oil-in-water (O/W) microemulsions containing added electrolyte. The systems examined are of the type O/W/n-pentanol/surfactant employing hexadecane or Nujol as the oil and various Tweens as the nonionic surfactant. The equivalent conductance (..lambda..) can be fit to an equation of the form ..lambda.. = ..lambda../sup 0/(1 - a phi/sub comp/)/sup n/, where phi/sub comp/ is the phase volume as determined by composition. The a value is relatively constant in all cases, consistent with the interpretation that the correct value of phi = a phi/sub comp/ reflecting the binding of a constant amount of water by the micro-droplet. The exponent n has a limiting value of 1.5 at low oil content, and decreases with increasing oil content toward the value of 0.5 expected for coarse O/W emulsions. Using the above equation for ..lambda.. in conjunction with Waldens Rule leads to an expression for the effective relative viscosity experienced by a solute in the continuous phase of the form eta/sub rel/ = (1 - phi)/sup (n+1)/. For n = 1.5 this coincides with an equation for the bulk relative viscosity of a suspension of random spheres. (12 refs.)

Microwave dielectric properties of ionic and nonionic microemulsions

Abstract The dielectric properties of several series of O W microemulsions were measured at frequencies between 1 and 15.4 GHz. The microemulsions were composed of an oil (hexadecane or mineral oil), a cosurfactant (pentanol or butanol), and an ionic or nonionic surfactant (Tween 60, Brij 96, SCS, or CTAB). For comparison, dielectric properties were also measured for a suspension of silica particles of 17–25 nm diameter. The dielectric properties exhibit strong dispersions at microwave frequencies, which arise primarily from dipolar relaxation of the suspending water. The data are analyzed using the Maxwell and Hanai mixture theories, to determine the dielectric properties of the water. The results suggest that the amount of hydration water in the microemulsions corresponds to about 2 water molecules per ethylene oxide plus hydroxyl groups in the nonionic systems, and approximately 10 and 20 water molecules per CTAB or SCS molecules, respectively, in the ionic microemulsions. The apparent dielectric relaxation frequency of this hydration water is approximately 3–4 GHz in all of these systems. In view of the large total fraction of hydration water in the microemulsions with high phase volume, hydration phenomena significantly affect the dielectric relaxation and presumably other transport properties of these systems.

Electron spin resonance studies with copper/silver/chromium impregnated charcoals

It has been found that cyanogen chloride reacts with humidified ASC whetlerite (an activated charcoal impregnated with copper/silver/chromium) to give a strong electron spin resonance (esr) signal. The microwave absorption consists of a symmetric line with a g value of 2.12 and a width of 190 gauss, and a weaker, broader line under the high field tail of the sharper line. Neither hydrogen chloride, tert-butylamine nor carbon monoxide were found to give a signal. However, exposure of the humidified whetlerite to hydrogen cyanide gave a broad, intense, esr line (width 475 gauss, g value 2.00). The results of these studies are discussed in terms of the mechanism of reaction, the nature of the active species, and the quantitative relationship between esr signal strength and extent of reaction.