For the first time, the thickness and refractive index of monolayers at the air/water interface have simultaneously been determined by null ellipsometry. Separation of refractive index from film thickness has been achieved by highly precise measurements of the two ellipsometric angles Ψ and Δ. In the solid state, film thicknesses of arachidic acid and valine gramicidin A obtained by ellipsometry are comparable with those obtained by the X-ray techniques. For arachidic acid in the condensed state, our results suggest that only the thickness of the hydrophobic moiety is measured. When highly hydrated, the thickness of the polar headgroup is not detected. This is presumably due to its refractive index being the same as that of the bulk water; hence, the calculated film thickness corresponds to the thickness of the hydrophobic part only. As molecular area is reduced, the polar headgroup gradually loses hydration water molecules causing its refractive index profile to become different from that of the bulk water. Our results suggest that the measurable thickness of the film-forming molecules increases as the degree of dehydration of the headgroup increases.

Measurement of oxidized mercury, Hg(II), in the atmosphere poses a significant analytical challenge as Hg(II) is present at ultra-trace concentrations (picograms per cubic meter air). Current technologies are sufficiently sensitive to measure the total Hg present as Hg(II) but cannot determine the chemical speciation of Hg(II). We detail here the development of a soft ionization mass spectrometric technique coupled with preconcentration onto nano- or microparticle-based traps prior to analysis for the measurement of mercury halides in air. The current methodology has comparable detection limits (4-11 pg m(-3)) to previously developed techniques for the measurement of total inorganic mercury in air while allowing for the identification of HgX2 in collected samples. Both mercury chloride and mercury bromide have been sporadically detected in Montreal urban and indoor air using atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). We discuss limitations and advantages of the current technique and discuss potential avenues for future research including quantitative trace measurements of a larger range of mercury compounds.

We have studied surface and spectroscopic properties of Photosystem II core complex (PS II CC) for the first time in monolayers at the nitrogen/water interface. A new instrument was thus specially built to perform absorption and fluorescence spectroscopic measurements directly at the nitrogen/water interface. The effect of initial surface density, incubation time, and compression speed have been studied. When PS II CC was spread at an initial surface pressure of 5.7 mN/m and immediately compressed at a speed of 40 nm 2 /molecule‚min, it retained its native spectroscopic characteristics. Even though a slower speed of compression (10 nm 2 /molecule‚min) produced more homogeneous films, the absorption maxima suffered a blue shift, indicating denaturation of PS II CC. Compression at a speed of 80 nm 2 /molecule‚min produced aggregates of intact PS II CC as indicated by ∆V-A isotherms, absorption spectra, and fluorescence micrographs. We also conclude that spreading of PS II CC at an initial surface pressure of 0.6 mN/m followed by a 30 min incubation time is inadequate to maintain PS II CC surface and spectral properties. Indeed, π-A and ∆V-A isotherms measured in that condition showed transitions which suggested that PS II CC underwent physical changes during compression. Moreover, absorption and fluorescence maxima were blue shifted, indicating that PS II CC is denatured under that condition.

We describe a photoacoustic cell which is simple to use, easy to construct, and which gives a great sensitivity, expressed as a signal-to-noise ratio of 3000. Calibration of the cell indicates that it can be used with confidence as a general purpose photoacoustic cell.

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Internal segregation and side chain ordering in hairy-rod polypeptide monolayers at the gas/water interface: An xray scattering study Structure of poly(γ-benzyl-L-glutamate) monolayers at the gas-water interface: A Brewster angle microscopy and x-ray scattering study J. Chem. Phys. 111, 9761 (1999); 10.1063/1.480312 Surfacepressureinduced reversible color change of a polydiacetylene monolayer at a gas-water interface Ellipsometry is a sensitive nondestructive optical technique which can be successfully used for determining the thickness of an optically transparent thin film deposited on a dielectric substrate. In this laboratory we have constructed a vertical null-type ellipsometer which has been used with a homemade aluminum Teflon~coated Langmuir trough, to study thin films at gas-water interface. Surface pressure, surface potential, and ellipsometric measurements have been performed simultaneously. Ellipsometric measurements have been done directly on the aqueous substrate contained in the trough, with and without the film, with reproducibility ( ± 0.02°) and good signal stability ( ± 0.1 mY) on the to-mY scale. The performance of the homemade instrument had been checked against the known ellipsometric properties of spread arachidic acid on acidic water solution. In this paper, details of construction and performance of the ellipsometer as wen as characteristics of the trough and ellipsometric study of .B-palmitoyl-a-oleoyl-L-a-phosphatidylcholine is presented. Stability of the photometric signa! as a function of the structure of the film-forming molecules is also discussed.

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