Mixed-emulsifier stabilised emulsions: Investigation of the effect of monoolein and hydrophilic silica particle mixtures on the stability against coalescence

An experimental circulating water loop has been constructed to study the deposition of hematite particles of average diameter 320 nm on polypropylene pipe walls in the ranges pH 4–11, Re 3300–17,700 at 25 °C. Real-time turbidimetric measurements were used to measure the deposited concentrations. Results showed that the deposition rate increased when pH decreased and when the flow rate increased. Adhesion was observed even under repulsive electrostatic conditions (pH > 7), where the surfaces of hematite and polypropylene were both negative, indicating that the kinetic energy of at least a part of the particles surpassed the electrostatic repulsive potentials. The experimental curves were fitted by a model assuming simultaneous adhesion and removal of particles, leading to adhesion and removal rate constants, whose values depend on pH and flow rate. Removal is negligible below pH 9.     

Theoretical studies of quantum amplified isomerizations for imaging systems involving hexamethyl Dewar benzene and related systems

The ring-opening reactions of the radical cations of hexamethyl Dewar benzene (1) and Dewar benzene have been studied using density functional theory (DFT) and complete active-space self-consistent field (CASSCF) calculations. Compound 1 is known to undergo photoinitiated ring opening by a radical cation chain mechanism, termed "quantum amplified isomerization" (QAI), which is due to the high quantum yield. Why QAI is efficient for 1 but not other reactions is explained computationally. Two radical cation minima of 1 and transition states located near avoided crossings are identified. The state crossings are characterized by conical intersections corresponding to degeneracy between doublet surfaces. Ring opening occurs by formation of the radical cation followed by a decrease in the flap dihedral angle. A rate-limiting Cs transition state leads to a second stable radical cation with an elongated transannular C-C bond and an increased flap dihedral. This structure proceeds through a conrotatory-like pathway of Cs symmetry to give the benzene radical cation. The role of electron transfer was investigated by evaluating oxidation of various systems using adiabatic ionization energies and electron affinities calculated from neutral and cation geometries. Electron-transfer theory was applied to 1 to investigate the limiting effects of back-electron transfer as it is related to the unusual stability of the two radical cations. Expected changes in optical properties between reactants and products of Dewar benzene compounds and other systems known to undergo QAI were characterized by computing frequency-dependent indices of refraction from isotropic polarizabilities. In particular, the reaction of 1 shows greater contrast in index of refraction than that of the Dewar benzene parent system.     

Lone-pair orbital interactions in polythiaadamantanes

The electronic structures of a series of polythiaadamantanes from thiaadamantane through 2,4,6,8,9,10-hexathiaadamantane (HTA) have been analyzed using density functional theory calculations in conjunction with Hückel and natural bond orbital analysis. The effects of multiple sulfur p-type lone-pair orbital interactions on ionization potentials, hole mobilities, and electronic coupling have been determined. An overall increase in the average energy of the lone-pair orbitals as the number of sulfur atoms increases is predicted, with the exact positioning of the HOMO depending on specific lone-pair interactions. Separation of through-bond (TB) and through-space (TS) interactions between intramolecular sulfur atoms has been performed using localized molecular orbitals and model systems based on interacting hydrogen sulfide molecules. TB interations were found to reduce orbital splitting, while TS interactions were found to increase orbital splitting. TS interactions were more or less constant from one polythiaadamantane to the next, and the contributions of TB effects to individual orbital energies vary depending on the relative orientation of sulfur atoms as determined by the sigma molecular framework. Electronic coupling between intermolecular sulfur lone-pair orbitals was determined by investigating unique dimer pairs observed in the crystal structure of HTA. Electronic coupling is not as strong as expected given the short intermolecular S-S distances observed in the crystal structure. In general, B3LYP/6-31G(d) and B3LYP/6-311+G(d,p) give very similar orbital energies and splittings.     

Optogalvanic spectroscopy of metastable states in Yb+

The metastable ²F_7/2 and ²D_3/2 states of Yb⁺ are of interest for applications in metrology and quantum information and also act as dark states in laser cooling. These metastable states are commonly repumped to the ground state via the 638.6 nm ²F_7/2–¹D[5/2]_5/2 and 935.2?nm ²D_3/2–³D[3/2]_1/2 transitions. We have performed optogalvanic spectroscopy of these transitions in Yb⁺ ions generated in a discharge. We measure the pressure broadening coefficient for the 638.6 nm transition to be 70±10?MHz?mbar^?1. We place an upper bound of 375 MHz/nucleon on the 638.6 nm isotope splitting and show that our observations are consistent with theory for the hyperfine splitting. Our measurements of the 935.2 nm transition extend those made by Sugiyama et al., showing well-resolved isotope and hyperfine splitting (Sugiyama and Yoda in IEEE Trans. Instrum. Meas. 44: 140, 1995). We obtain high signal-to-noise, sufficient for laser stabilisation applications (Streed et al. in Appl. Phys. Lett. 93: 071103, 2008).     

My Life in Nuclear Physics, Photography, and Opera

I sketch my life as an experimental nuclear physicist, beginning as a graduate student at Harvard University from 1948 to 1951, then as a postdoctoral fellow at the Cavendish Laboratory from 1951 to 1952, and finally as a faculty member at the University of Minnesota from 1952 until my retirement in 1991. I also carried out research at the Niels Bohr Institute in Copenhagen, Indiana University, and Los Alamos National Laboratory, and I participated in a number of summer schools and international conferences on nuclear physics. I also have worked in photography and opera. Over the years, I met and collaborated with many people in many walks of life who became friends for life.     

Competitive adsorption of surfactants and hydrophilic silica particles at the oil-water interface: interfacial tension and contact angle studies

The effect of surfactants' type and concentration on the interfacial tension and contact angle in the presence of hydrophilic silica particles was investigated. Silica particles have been shown to have an antagonistic effect on interfacial tension and contact angle in the presence of both W/O and O/W surfactants. Silica particles, combined with W/O surfactant, have no effect on interfacial tension, which is only dictated by the surfactant concentration, while they strongly affect interfacial tension when combined with O/W surfactants. At low O/W surfactant, both particles and surfactant are adsorbed at the interface, modifying the interface structure. At higher concentration, interfacial tension is only dictated by the surfactant. By increasing the surfactant concentration, the contact angle that a drop of aqueous phase assumes on a glass substrate placed in oil media decreases or increases depending on whether the surfactant is of W/O or O/W type, respectively. This is due to the modification of the wettability of the glass by the oil or water induced by the surfactants. Regardless of the surfactant's type, the contact angle profile was dictated by both particles and surfactant at low surfactant concentration, whereas it is dictated by the surfactant only at high concentration.     

Plasma treatment of the Mg:Ag/tris-(8-hydroxyquinoline) aluminum interface in OLEDs: effects on adhesion and performance

The adhesion of a Mg:Ag cathode to the tris-(8-hydroxyquinoline) aluminum (Alq₃) in organic light emitting devices (OLEDs) can be greatly enhanced by a remote plasma treatment of the Alq₃ layer using either air or N₂ prior to metal deposition. The altered surface properties which lead to increased sticking coefficients of Mg and Ag, as well as enhanced adhesion, are attributed to the introduction of new functional groups into the organic layer, as observed by X-ray photoelectron spectroscopy (XPS). The storage life of the plasma treated devices in air without any capping treatment, as judged by a visible deterioration of the cathode, was increased by approximately five to six times compared to untreated OLEDs. Current–voltage characteristics and EL efficiency, however, were shown to deteriorate for devices incorporating either an air or an N₂ plasma treated Alq₃ layer. For OLEDs subjected to short treatment times with an N₂ plasma, only a very slight increase in the turn-on voltage, of about 0.2 V, was observed. An investigation of black spot formation revealed that an air plasma treatment resulted in a five-fold decrease in the time required for 50% of the device to become non-emissive. N₂ treated devices on the other hand, developed black spots at a comparable rate to the non-treated devices. Thus, a short N₂ plasma treatment of the Alq₃ layer prior to metal deposition improves the adhesion at the interface, thereby reducing the oxidation and degradation of the device through exposure to ambient conditions, particularly in storage.