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Browsing by Author "Clark, Spencer C."

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    ATR-FTIR Measurements of Cationic Surfactant Exchange Rates at the Solid-Liquid Interface
    Clark, Spencer C. (Virginia Tech, 2003-05-01)
    In many experiments, surfactant adsorption and desorption at solid-liquid interfaces is found to be quite slow, considering that surfactants are small molecules. Attenuated total reflectance Fourier transform infrared spectroscopy was used to study the adsorption, desorption, and exchange of tetradecyltrimethylammonium bromide (C14TABr) at the silicon oxide surface. The exchange of surfactant was monitored using protonated and perdeuterated C14TABr. The data show that exchange of C14TABr between the surface and the bulk solution is very fast, complete exchange occurs in less than 10 seconds. A simple exchange model suggests that the disassociation rate constant of a single monomer is no less than 1 s-1, which is ~ 104 times slower than monomer exchange in bulk solutions. The actual exchange rate may be greater than observed in the present work due to transport phenomena. The rates of exchange are similar at concentrations above and below the critical micellar concentration. Adsorption is similarly rapid, but under some circumstances there is a small residue of surfactant that is slow to desorb. Desorption experiments utilizing KBr solutions of high and low ionic strength show that two thirds of each adsorbed micelle is held by hydrophobic association, and the other third is electrostatically bound. Adsorption, desorption, and exchange experiments at temperatures of 11°C above and 8°C below the Krafft temperature (14.4°C) show similar kinetics.
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    Development of the Evanescent Wave Atomic Force Microscope
    Clark, Spencer C. (Virginia Tech, 2005-07-26)
    The conventional atomic force microscope (AFM) is equipped with a single optical detection system. Probe-sample separation is determined in an independent deflection with respect to AFM z-translation experiment. This method of determining probe-surface separation is relative, susceptible to drift and does not provide real time separation information. The evanescent wave atomic force microscope (EW-AFM) utilizes a second, independent detection system to determine absolute probe-surface separation in real time. The EW-AFM can simultaneously acquire real-time force and probe-sample separation information using the optical lever and evanescent scattering detection systems, respectively. The EW-AFM may be configured with feedback on the optical-lever system for constant force applications or with feedback on evanescent wave scattering intensity for constant height applications. Scattering of the evanescent wave exponential decay profile is used to determine probe-surface separation. Sub-micron sized dielectric and metallic probes show exponential scattering profiles, micron sized polystyrene and borosilicate microspheres show non-exponential profiles when they are affixed beneath the cantilever tip. By affixing the microspheres to the end of the AFM cantilever exponential and non-exponential profiles were observed. The EW-AFM can be used to conduct force-distance and imaging experiments. The EW-AFM was used to measure the thickness of surfactant bilayers formed at the silica-solution interface using silicon nitride AFM tips. The presence of a refractive index difference between the surfactant bilayer and the solution does not influence the accuracy of the surfactant bilayer thickness measurement. The EW-AFM was used to scan a 2 x 2 micron area in constant height mode. The probe was brought to within 6 nanometers of a planar dielectric surface using the evanescent wave intensity as a height reference with accuracy of ± 1 nm. This capability may be utilized to observe charge heterogeneity at the solid-liquid interface with nanometer lateral resolution or to map chemical functional group heterogeneity based on perturbations to the electrical double layer. The EW-AFM evanescent scattering system has an absolute separation resolution of 0.3 nm compared to 1.0 nm relative separation resolution for the optical lever system. In constant scattering (constant height) mode the real time separation precision is about 2 nm.
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    Method and apparatus for evanescent filed measuring of particle-solid separation
    (United States Patent and Trademark Office, 2007-06-26)
    Evanescent wave scattering by a scanning probe in a scanning probe microscope is utilized to determine and monitor separation between a scanning probe and a sample. A laser light is totally internally reflected at the interface between a more optically dense (incident) medium and less optically dense (transmitting) medium, exciting a decaying evanescent field in the less optically dense medium. A scanning probe, such as a colloidal probe, is dipped into the evanescent field, which scatters off the scanning probe. The portion of the scattered field propagates back into the incident medium and is then detected by a detector. A dependency between the intensity of the scattered evanescent field and the separation between the probe and the incident medium was measured and used in determining the separation. This dependency of intensity is used to prepare images or maps of interfaces. A particular application of determining the separation between the probe and the sample in an atomic force microscope is disclosed.