Christine E. DeWolf

Kinetic analysis of the ozone processing of an unsaturated organic monolayer as a model of an aerosol surface

It has been suggested that an organic aerosol containing unsaturated organic compounds at the surface would likely be processed by atmospheric ozone. The ozonolysis of oleic acid gives rise to the formation of products having shorter chain lengths than the starting molecule, and are consequently more water soluble than oleic acid. Hence, the exposure of a monolayer of oleic acid to ozone should lead to a decrease in surface activity at the air-water interface. A model system is used for real-time measurements of surface tension changes due to ozone exposure of a pendant drop that is coated by a fatty acid monolayer. The surface tension is measured based on an analysis of the shape profile of acquired images of the drop. A study of the kinetics of the gas-surface reaction is presented. Assuming that the uptake of ozone is dominated by the reaction at the surface, the measured reactive uptake coefficient of ozone gamma(meas) is estimated to be (2.6 +/- 0.1) x 10(-6).

Membrane selectivity and biophysical studies of the antimicrobial peptide GL13K.

GL13K is a short (13 amino acid) antimicrobial peptide derived from the parotid secretory protein. GL13K has been found to exhibit anti-inflammatory and antibacterial activities in physiological salt conditions. We investigated the mechanism of interaction of GL13K, with model membranes comprising 1, 2-dioleoylphosphatidylcholine (DOPC) and 1, 2-dioleoylphosphatidylglycerol (DOPG) using various biophysical and imaging techniques. Circular dichroism studies showed that GL13K adopts a β-sheet structure in the presence of negatively charged DOPG liposomes while it retains its random coil structure with zwitterionic DOPC liposomes. GL13K did not cause any fusion of these liposomes but was able to selectively disrupt the negatively charged membranes of DOPG leading to vesicular leakage. There was no or minimal evidence of GL13K interaction with DOPC liposomes, however an analysis of supported lipid bilayers (SLBs) using atomic force microscopic (AFM) imaging and dual polarization interferometry (DPI) suggested that GL13K can interact with the surface of a DOPC planar bilayer. In the case of DOPG bilayers, AFM and DPI clearly showed membrane thinned regions where a portion of lipid molecules has been removed. These results suggest that the mechanism of GL13K action on bacterial membranes involves localized removal of lipid from the membrane via peptide-induced micellization.

Real-time monitoring of the ozonolysis of unsaturated organic monolayers

The reaction of ozone with unsaturated organic molecules at the air-water interface of a pendant drop was followed by axisymmetric drop shape analysis (ADSA).

Space-Filling Trialkoxysilane: Synthesis and Self-Assembly into Low-Density Monolayers

A novel space-filling trialkoxysilane derivative was synthesized using a two-step strategy from commercially available starting materials to produce the precursor for the formation of low-density self-assembled monolayers. Self-assembled monolayers of the synthesized compound were prepared on three different substrates (Si/SiO(2), glass and ITO) and were characterized using contact angle, ellipsometry and sum-frequency generation spectroscopy. Removal of the space-filling protecting group, (2-chlorophenyl)diphenyl methanol, yields a carboxy-terminated surface. Correspondingly, the contact angle and film thickness decrease and the SFG spectra clearly indicate an increase in gauche defect concentration characteristic of a low-density disordered monolayer.

Model Lung Surfactant Films: Why Composition Matters

Lung surfactant replacement therapies, Survanta and Infasurf, and two lipid-only systems both containing saturated and unsaturated phospholipids and one containing additional palmitic acid were used to study the impact of buffered saline on the surface activity, morphology, rheology, and structure of Langmuir monolayer model membranes. Isotherms and Brewster angle microscopy show that buffered saline subphases induce a film expansion, except when the cationic protein, SP-B, is present in sufficient quantities to already screen electrostatic repulsion, thus limiting the effect of changing pH and adding counterions. Grazing incidence X-ray diffraction results indicate an expansion not only of the liquid expanded phase but also an expansion of the lattice of the condensed phase. The film expansion corresponded in all cases with a significant reduction in the viscosity and elasticity of the films. The viscoelastic parameters are dominated by liquid expanded phase properties and do not appear to be dependent on the structure of the condensed phase domains in a phase separated film. The results highlight that the choice of subphase and film composition is important for meaningful interpretations of measurements using model systems.

Surface Behavior of Boronic Acid-Terminated Silicones

Silicone polymers, with their high flexibility, lie in a monolayer at the air-water interface as they are compressed until a critical pressure is reached, at which point multilayers are formed. Surface pressure measurements demonstrate that, in contrast, silicones that are end-modified with polar groups take up lower surface areas under compression because the polar groups submerge into the water phase. Boronic acids have the ability to undergo coordination with Lewis bases. As part of a program to examine the surface properties of boronic acids, we have prepared boronic acid-modified silicones (SiBAs) and examined them at the air-water interface to better understand if they behave like other end-functional silicones. Monolayers of silicones, aminopropylsilicones, and SiBAs were characterized at the air-water interface as a function of end functionalization and silicone chain length. Brewster angle and atomic force microscopies confirm domain formation and similar film morphologies for both functionalized and non-functionalized silicone chains. There is a critical surface pressure (10 mN m(-1)) independent of chain length that corresponds to a first-order phase transition. Below this transition, the film appears to be a homogeneous monolayer, whose thickness is independent of the chain length. Ellipsometry at the air-water interface indicates that the boronic acid functionality leads to a significant increase of film thickness at low molecular areas that is not seen for non-functionalized silicone chains. What differentiates the boronic acids from simple silicones or other end-functionalized silicones, in particular, is the larger area occupied by the headgroup when under compression compared to other or non-end-functionalized silicones, which suggests an in-plane rather than submerged orientation that may be driven by boronic acid self-complexation.

Face- and edge-on orientations of octa-acid and -alcohol substituted tetraazaporphyrins in Langmuir and Langmuir–Blodgett monolayers

Polyaromatic dyes, such as phthalocyanines and tetraazaporphyrins (TAPs), arrange with an edge-on orientation of the macrocycle in almost all Langmuir and Langmuir–Blodgett (LB) monolayers, especially when condensed at higher surface pressures. Presented here is a new amphiphilic TAP that contains eight terminal carboxylic acid groups attached to the macrocycle via decyl spacers, which forms stable Langmuir and LB monolayers with a flat-on orientation of the macrocycle. A spider-like molecular conformation is proposed based on the measured surface area per molecule and film thickness determined by ellipsometry and AFM. This conformation allows all eight terminal carboxylic acid and carboxylate groups to attach to the polar interface while the aromatic macrocycle sits on top of the orthogonal decyl chains to generate a micro-segregated layer structure. Octa-acid TAPs with shorter alkyl spacers form edge-on structures in Langmuir monolayers upon compression and are not free of 3-dimensional aggregates based on observations by Brewster angle microscopy (BAM). All octa-acid TAPs are prone to spontaneous self-assembly that is attributed to strong interactions between carboxylic acid groups. Aggregation and spontaneous self-assembly can be minimized by deprotonation of the carboxylic acid groups but all octa-acid TAPs become water soluble when more than four carboxylic acid groups are deprotonated at a pH higher than 8. Cu(II) chelates of the octa-acid TAPs were also prepared but are unsuitable for Langmuir monolayer formation because of insufficient solubility in spreading solvents. Langmuir and LB monolayers of octa-hydroxy TAPs with propyl and undecyl spacers were also studied for comparison. An edge-on orientation of the macrocycle is obtained at higher surface pressures for both TAPs, although a flat-on orientation of the macrocycle in a spider-like confirmation may be present at low surface pressure for the derivative with undecyl spacers. No mesophases are formed by any of the octa-acid and -hydroxy TAPs as confirmed by polarized optical microscopy and thermal analysis measurements. In fact, of the octa-acid TAPs, only the derivative with the longest spacer (decyl) melts at a temperature below the onset of thermal decomposition.

Nanoparticle-induced structural changes in lung surfactant membranes: an X-ray scattering study

The effect of low concentration (0.001 wt%) of charged silica nanoparticles (NPs) on the molecular structure of lung surfactant monolayers was investigated at the air/water interface using grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR). The lipid systems investigated included 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), mixtures of DPPC and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), and Infasurf (a clinical lung surfactant formulation). In all cases, the anionic silica NPs interacted with the films but induced only small structural changes. By contrast, the cationic NPs induced a significant reduction of the alkyl chain tilt angle when anionic lipid was present. This appears to be associated with a condensation of the POPG lipids that would alter the ratio of liquid-expanded and condensed phases. Thus, this study reveals that low concentrations of cationic NPs can induce structural changes that could impact film mechanical properties that are important for pulmonary function.

Efficient Preparation of Novel Phenolic Surfactants for Self-Assembled Monolayers

Abstract Novel molecules have been synthesized that combine the phenolic nature of tannins and self-assembling properties of surfactants. These single-chain (C12) surfactants with potential biocompatibility have been synthesized with an ω-thiol or disulfide functionality, both commonly used anchors in self-assembly onto gold surfaces, using a modular route. Protecting groups for the phenol and thiol moieties played a key role for overcoming the challenges often associated with the purification of surfactants. The tasks of unmasking the thiol moiety and simultaneously deprotecting the acetyl protecting groups of the phenols were accomplished using sodium thiomethoxide. This modular route can be extended to synthesize other surfactants with the potential ability to form robust layers with biocompatible properties. [Supplementary materials are available for this article. Go to the publishers online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.] GRAPHICAL ABSTRACT

Effect of UV and UV–Ozone Treatment of Polyolefin Fibers on Toughness of Fiber Concrete Composite

Ultraviolet (UV) irradiation and a combination of UV irradiation and ozone treatment of polyolefin fibers were investigated for possible improvement of mechanical properties in concrete composites. In both treatment methods, various fiber exposure durations were used, and surface modification was monitored by measuring the water contact angle. Untreated and treated fibers were added to concrete at 0.32 % by volume, and the specimens were tested for flexural strength according to ASTM International specifications. Fiber surface treatment by UV alone was found to be an unattractive technique based on the results of flexural tests and contact angle measurements. Ten minutes of UV and ozone surface treatment of fibers was found to improve the composite peak stress by 11 % and the toughness by 4 %; longer periods of exposure led to lower flexural strength results, perhaps as a result of fiber deformation or degradation.