Robert G. Laughlin

Ion and Liquid Dependent Dielectric Failure in Electrowetting Systems

Electrowetting devices often utilize aqueous solutions with ionic surfactants and inorganic salts to modify the electrowetting response. It has been observed in low-voltage electrowetting devices (thin dielectric, <12 V) that a frequent onset of dielectric failure (electrolysis) occurs with use of ionic solutes such as potassium chloride (KCl) or sodium dodecyl sulfate. More detailed current-voltage investigations reveal less dielectric failure for the larger size ions. Specifically, improved resistance to failure is seen for surfactant ions carrying a long alkane chain. Therefore, a catanionic surfactant (in which both ions are amphiphilic) was custom synthesized, and elimination of dielectric failure was observed in both negative and positive voltage. Because water is a small molecule that easily penetrates dielectrics, further experiments were performed to show that dielectric failure can also be eliminated by use of larger size polar molecules such as propylene glycol. In addition to these results, important parameters such as conductivity and interfacial tensions are reported.

The role of swelling methods in surfactant phase science: past, present, and future

Abstract The foundation for surfactant phase science was established largely using isoplethal phase study methods, but some use has been made of qualitative isothermal penetration experiments. In 1987 a quantitative swelling method (the Diffusive Interfacial Transport method) was reported in which the analysis of phase compositions is based on refractive index data. Experience obtained to date during use of this DIT-NDX method is reviewed. These investigations have demonstrated the considerable value of swelling studies, but revealed serious flaws in the DIT-NDX method. Swelling studies are exceptionally efficient, reliable, and provide considerable information regarding the physical science of the system in addition to the phase diagram. Unfortunately, this method is incapable of accurately defining the compositions of many birefringent phases. A DIT-IR method, presently under development, should resolve this problem. Analysis of composition using the DIT-IR method will be based on infrared data obtained using a near-infrared microscope. In addition to providing better composition data, infrared studies are expected to provide information on conformational structure, crystal hydration, and hydration thermodynamics. The future of swelling methods is discussed.

Preparation and physical characterization of pure β-carotene

Pure all-trans beta-carotene has been prepared on the 10s of grams scale by isothermal Fractional Dissolution (FD) of commercial laboratory samples in tetrahydrofuran (THF). beta-Carotene purified in this way is black, with a faint brownish tinge. The electronic spectra of black samples extend into the near infrared, with end-absorption past 750 nm. Black samples react directly with dioxygen under mild conditions to yield the familiar orange or red powders. Pure beta-carotene rigorously obeys Beers Law in octane over the entire UV-Vis spectral range, while commercial laboratory samples and recrystallized samples do not. NMR self-diffusion coefficient data demonstrate that beta-carotene exists as simple molecular solutions in octane and toluene. The anomalously high crystallinity of beta-carotene can be attributed (from analysis using molecular mechanics) to the facts that: (1) the number of theoretically possible conformers of beta-carotene is extremely small, and (2) only a small fraction of these (ca. 12%, or 127) may actually exist in fluid phases.

Distearoylphosphatidyl choline/ammoniohexanoate surfactant interactions

Abstract The interactions which occur between a homologous series (C10–C20) of 6-alkyldimethylammoniohexanoates (AHs) and large unilamellar vesicles of DSPC have been investigated. The major findings are: (1) That when the temperature is below Tc of the DSPC and for short times (

Status of aqueous surfactant phase science

Recent phase studies of several surfactant-water systems, using new or refined methods, have revealed significant errors in earlier phase diagrams. These diagrams had been determined largely using methods based on the isoplethal phase studies principle. This principle has inherent limitations which do not exist in isothermal methods.Isothermal nuclear magnetic resonance and refined calorimetric methods have been extensively used in recent surfactant phase studies. Methods based on the new lyotrope gradient (swelling) principle show great promise as a means of improving the efficiency and quality of surfactant phase studies.

The determination of polar lipid solubilization boundaries in dilute solutions by light scattering

An apparatus and procedure have been developed which facilitate the acquisition of particle free water by recirculation through a fine membrane filter in a closed system. The system also permits one to vary, independently, composition of two solutes over a few hundred mg/l range, with ± 1 mg/l accuracy, while monitoring scattering intensities. Using these techniques it has been possible to define the phase boundary governing the solubilization of distearoylphosphatidyl choline (DSPC), above its chain melting temperature, by three homologous (C16, C18, C20) ammoniohexanoate (AH) surfactants. The data suggest that negligible (< 1 mg/l) solubilization occurs up to an onset concentration of surfactant, Con, which for C16AH is 0.77 of the cmc. Beyond Con finite solubilization occurs with an appreciable efficiency, which peaks at C18AH at 1.35 g/g.

Methodologies for phase studies of amphiphilic compounds : recent developments

Abstract Isoplethal phase study methods, during which temperature is varied at constant composition, have seen little change during the last few years. However, isothermal methods (in which composition is varied at constant temperature) have undergone significant development. The deuterium-NMR variant of the classic isothermal analytic method has grown in importance. Also, more and better use has been made of qualitative isothermal swelling methods (‘penetration experiments’) to supplement other data. The quantitative Diffusive Interfacial Transport (DIT) method, first reported in 1987, has been much improved by replacing interferometry, as the basis for analysis, with the determination of water by NIR microspectroscopy. The resulting DIT-NIR method appears to be very general, and should see broad application in the future.

Surfactant phase science

Variations in both the lipophilic and the hydrophilic structural elements of synthetic surfactant molecules and phase studies of the compounds continue apace. Lyotropic nematic liquid crystals may exist in selected non-ionics (C 16 E 6 ), and ‘intermediate’ liquid crystal phases continue to be found. As with ionic surfactants, attaching two lipophilic chains to a polyether non-ionic group has been shown to promote formation of the lamellar liquid crystal phase (if the chains are sufficiently long). Considerable attention is being given to catanionic surfactants (ionic surfactants in which both ions are amphophilic), but as these compounds are often formed in situ, interpretation of the present data is often uncertain. Ternary systems of singly and doubly charged cationic surfactants and water have been studied, as have quaternary systems consisting of a semipolar amine oxide, an anionic (or a cationic) surfactant, oil and water. The profound influence that amphiphilic oils have on aqueous surfactant behavior (found earlier in ionic surfactant-oil-water systems) has now been documented in a non-ionic system. The phase behavior of perfume oils in combination with several surfactants has been studied.

An improved Senarmont retardation analysis method

The phase difference between two plane polarized light rays whose electric field vectors (planes of polarization) are at right angles is often determined by Senarmont analysis. In this analysis, a quarter‐wave plate whose vibration axes lie at an azimuth of ±45° to the planes of polarization of the two rays is inserted. The rotation of the analyser that leads to extinction is then determined by manual adjustment. This classical method has been significantly improved by, instead, measuring light intensities at thirty‐two evenly spaced analyser angles which span 180° of analyser rotation. An unbiased estimate of the phase difference may then be extracted with high accuracy by analysis as a designed experiment.

Phosphine oxide surfactants revisited.

This review summarizes everything we currently know about the nonionic surfactants alkyl dimethyl (C(n)DMPO) and alkyl diethyl (C(n)DEPO) phosphine oxide (PO surfactants). The review starts with the synthesis and the general properties (Section 2) of these compounds and continues with their interfacial properties (Section 3) such as surface tension, surface rheology, interfacial tension and adsorption at solid surfaces. We discuss studies on thin liquid films and foams stabilized by PO surfactants (Section 4) as well as studies on their self-assembly into lyotropic liquid crystals and microemulsions, respectively (Section 5). We aim at encouraging colleagues from both academia and industry to take on board PO surfactants whenever possible and feasible because of their broad variety of excellent properties.