William R. Barger

The Alpha Substitution Effect on Phthalocyanine Aggregation

The dimerization and tendency to form higher aggregates of two isomeric metal-free phthalocyanine compounds with tetracumylphenoxy peripheral substitution at the alpha (1 or 4, 5 or 8, 9 or 12, 13 or 16) and beta (2 or 3, 6 or 7, 10 or 11, 14 or 15) positions has been analysed by spectroscopic measurements. The dimerization constants in chloroform solution are 1.3 × 101M−1 and 7.0 × 103M−1 respectively. At concentrations greater than 10−3M, the beta phthalocyanine isomer forms a spectroscopically resolvable higher aggregate. Q-band spectra of the dimer of each isomer and of the aggregate of the beta isomer in chloroform solution were generated. Comparison of these spectra with the corresponding thin film spectra of each isomer indicates that the stronger tendency toward aggregation of the beta isomer extends into the solid state. The difference in aggregating tendency of the two isomers was attributed to a steric constraint of the alpha isomer that restricts conformational freedom and, consequently, hinde...

Atomic Force Microscope Image Contrast Mechanisms on Supported Lipid Bilayers

This work presents a methodology to measure and quantitatively interpret force curves on supported lipid bilayers in water. We then use this method to correlate topographic imaging contrast in atomic force microscopy (AFM) images of phase-separated Langmuir-Blodgett bilayers with imaging load. Force curves collected on pure monolayers of both distearoylphosphatidylethanolamine (DSPE) and monogalactosylethanolamine (MGDG) and dioleoylethanolamine (DOPE) deposited at similar surface pressures onto a monolayer of DSPE show an abrupt breakthrough event at a repeatable, material-dependent force. The breakthrough force for DSPE and MGDG is sizable, whereas the breakthrough force for DOPE is too small to measure accurately. Contact-mode AFM images on 1:1 mixed monolayers of DSPE/DOPE and MGDG/DOPE have a high topographic contrast at loads between the breakthrough force of each phase, and a low topographic contrast at loads above the breakthrough force of both phases. Frictional contrast is inverted and magnified at loads above the breakthrough force of both phases. These results emphasize the important role that surface forces and mechanics can play in imaging multicomponent biomembranes with AFM.

Characterization of the physical properties of model biomembranes at the nanometer scale with the atomic force microscope

Interaction forces and topography of mixed phospholipid-glycolipid bilayers were investigated by atomic force microscopy (AFM) in aqueous conditions with probes functionalized with self-assembled monolayers terminating in hydroxy groups. Short-range repulsive forces were measured between the hydroxy-terminated probe and the surface of the two-dimensional (2-D) solid-like domains of distearoyl-phosphatidylethanolamine (DSPE) and digalactosyldiglyceride (DGDG). The form and range of the short-range repulsive force indicated that repulsive hydration/steric forces dominate the interaction at separation distances of 0.3-1.0 nm after which the probe makes mechanical contact with the bilayers. At loads < 5 nN the bilayer was elastically deformed by the probe, while at higher loads plastic deformation of the bilayer was observed. Surprisingly, a short-range repulsive force was not observed at the surface of the 2-D liquid-like dioleoylphosphatidylethanolamine (DOPE) film, despite the identical head groups of DOPE and DSPE. This provides direct evidence for the influence of the structure and mechanical properties of lipid bilayers on their interaction forces, an effect which may be a major importance in the control of biological processes such as cell adhesion and membrane fusion. The step height measured between lipid domains in the AFM topographic images was larger than could be accounted for by the thickness and mechanical properties of the molecules. A direct correlation was observed between the repulsive force range over the lipid domains and the topographic contrast, which provides direct insight into the fundamental mechanisms of AFM imaging in aqueous solutions. This study demonstrates that chemically modified AFM probes can be used in combination with patterned lipid bilayers as a novel and powerful approach to characterize the nanometer scale chemical and physical properties of heterogeneous biosurfaces such as cell membranes.

Derivatives of phthalocyanine prepared for deposition as thin films by the Langmuir-Blodgett technique☆

Abstract Film pressure versus area data for a series of organic derivatives of phthalocyanine are presented in detail. Classical monomolecular films were not observed. Compounds studied include tetraphenoxy, dicumylphenoxy and tetracumylphenoxy, tetraoctadecoxy and tetraneopentoxy phthalocyanine. Spread films of metal-substituted tetracumylphenoxy phthalocyanines that contained iron, cobalt, nickel, copper, zinc, palladium, platinum and lead were also examined along with 1:1 mole ratio mixed films of these compounds with octadecanol. Mixed films with varying mole ratios of nickel tetracumylphenoxy phthalocyanine and octadecanol are also described. On the basis of these data and considerations of recent studies of tetra- tert -butyl phthalocyanine by other investigators, a stacked phthalocyanine structure is indicated. The stack axis is not parallel to the plane of the film.

Trace Chemical Vapor Detection Using SAW Delay Line Oscillators

The resonant frequencies of SAW delay line oscillators are shown to be very sensitive to the presence of thin organic films that are deposited onto the delay line surface. Theory suggests that the sensi- tivity of the SAW device to mass loading depends on the square of the resonant frequency. This dependence has been studied experimentally using devices resonating at 31, 52, and 112 MHz that were coated with organic films of precisely controlled composition and thickness. The mass sensitivity of SAW oscillators can be exploited to make very sen- sitive chemical sensors if an appropriate sorptive coating is applied to the device. Results obtained from a delay line oscillator operating at 290 MHz (the highest frequency SAW chemical sensor reported to date) suggest that the rapid detection of organic vapors at concentra- tions substantially below 100 parts per billion (by volume) is readily achievable.

Surface chemical properties of banded sea slicks

Abstract Surface concentrations and film pressures of slick-forming organic constituents were measured in situ and on surface water samples from the Pacific Ocean, west of San Diego. The surface tension data, accurate to ±0·6 dyne/cm, indicated that film pressures ranged as low as a few dyne/cm in the banded slicks associated with internal waves. This observation is consistent with previously reported data which show that water-insoluble surface films damp capillary waves strongly at film pressures as low as 0·5–1·0 dyne/cm. Some surface-active material was present in all samples, whether from ‘clean’ rippled water or from visible slicks. Film pressures were measured as a function of the surface concentration of film-forming constituents. These data indicated that concentrations in many of the slicked zones were just sufficient to damp or resist the formation of ripples. This observation was also corroborated by the in situ surface tension data. It was estimated that a four-fold compression of the surface layer by horizontal convergent forces was required to create visible slicks at this location.

EFFECTS OF AN ARTIFICIAL SEA SLICK UPON THE ATMOSPHERE AND THE OCEAN

Abstract Vertical mean wind profiles, hot-film data, and wave height data measured during the passage of an artificial sea slick are compared with similar measurements without a sea slick. The effects of the slick are modifications of profile roughness length z0, and a possible increase in mean wind speed. Power spectral density plots of wave records obtained before, during and after the slick show wave energy modifications for wavelengths up to 10 m. Coherence values computed from a cross-spectral density of the wind field and simultaneously measured wave field both during and after the slick demonstrate the importance of small waves to air-sea interaction processes. Phase angles indicate the maximum horizontal velocity occurs over the wave trough for all spectral components, some of which are above and some below the wind-wave “matched layer.” During a stick, however, the horizontal velocity maximum occurs over the wave crest for those waves remaining coherent with the wind field.

Dissolved hydrogen in the Norwegian Sea : Mesoscale surface variability and deep-water distribution

Abstract The distribution of dissolved hydrogen (H 2 ) was studied during April 1979 in waters along the Iceland-Faroe Islands Ridge and in the Norwegian Sea. Surface water concentrations were well below air equilibrium throughout the region. Three areas (40 × 65 km) were sampled extensively by a surface pumping system allowing contour maps to be prepared with analysis spacings of 4 km. In general, H 2 concentrations were 6 to 7 nl l −1 on the ridge and decreased to 4 nl l −1 on the Voring Plateau. The results for the Norwegian Sea obtained before the spring plankton bloom suggest active consumption of H 2 , presumably by microorganisms, at the sea surface. Apparent net production of H 2 , which predominates in near-surface tropical North Atlantic waters, was not observed anywhere in the region. Vertical distributions showed that H 2 undersaturation persisted at all depths, averaging 4 nl l −1 below 300 m. Iceland-Scotland Overflow Water contained 4.5 nl l −1 . The apparent absence of net H 2 production at high latitudes is unexplained, but may reflect the presence and high activity of chemolithotrophic H 2 bacteria and the relative absence or low activity at high latitudes of H 2 -producing microorganisms such as nitrogen-fixers.

DESIGN AND CALIBRATION OF A SCANNING FORCE MICROSCOPE FOR FRICTION, ADHESION, AND CONTACT POTENTIAL STUDIES

We present the design and calibration of a scanning force microscope which can be used to study friction, adhesion, and contact potential differences between the cantilever tip and surface. The microscope uses a modular design where the laser, cantilever/sample holder, reflecting mirror, and detector are mounted directly on an optical table. The laser, reflecting mirror, and detector are mounted on translation and rotation stages. With this design the components can be rearranged to calibrate the Z piezo motion as a function of applied voltage. Using the detector micrometers, the detector response (voltage‐to‐distance relationship) can be determined after each series of measurements. The cantilever/sample holder is constructed such that the components are material matched and thermally compensated from a common reference point. This design feature minimizes thermal drift of the instrument. The instrument can be used in a contact scanning mode where both normal and lateral deflections of the cantilever are...

Morphology and properties of Langmuir films containing tetrakis(cumylphenoxy) phthalocyanines

Abstract Tetrakis(cumylphenoxy) phthalocyanine, H 2 Pc(Cp) 4 , and the family of related compounds containing transition metal ions in the center of the macrocyclic ring, MPc(Cp) 4 , will spread from organic solvents to give pressure vs. area isotherms typical of monolayers, but classical monomolecular films are not formed. Mixed films containing a classical component as a transfer promoter, such as stearamide, produce Langmuir-Blodgett (LB) multilayers of better quality than the MPc(Cp) 4 compounds alone. As a structural model, a simple mixed film containing stacked, ordered MPc(Cp) 4 molecules was originally postulated, but more recent observations by transmission electron microscopy and by IR reflectance-absorbance spectroscopy have revealed a more complex and less well-ordered morphology than previously suspected. In mixed films transferred by the LB technique, a two-dimensional colloidal dispersion of MPc(Cp) 4 aggregates is formed. Disk-like aggregates more than one monomolecular layer thick are observed in single-component films and are also seen dispersed in the second component of mixed films. Differential scanning calorimetry also suggests a significant degree of phase separation in the two-component LB films. Small melting point depressions relative to the melting points of single-component monolayers indicate that the melting points of two-component films containing MPc(Cp) 4 are determined by the transfer promoters.