CHAPTER 11 Methodology for Chemical Analysis of Surfaces 11.1 Objectives of Analysis 11.2 Overview of Methodology 11.3 Microscopy 11.4 Diffraction 11.5 Contact Angle 11.6 Spectroscopy11.1 OBJECTIVES OF ANALYSIS 1) Determine how the surface chemistry (and, therefore, properties) differs from the bulk (relative to the function of the material in the device, effects on the body, and response to effects on the body). 2) Identify contaminants (viz., with respect to effects of the material on the body). 3) Identify chemical bonding possibilities for interactions with molecules in the biological milieu with respect to the effects of the material on the body (viz., bioadhesion) and the body on the material.11.2 OVERVIEW OF METHODOLOGY11.3 MICROSCOPY 11.3.1 Light Microscopy The resolution (lateral) of the light microscope is: D = 0.61l N sin a Where D = Smallest lateral dimension that can be resolved N = Refractive index of medium surrounding the specimen (i.e., air, 1.0, or oil, 1.5) a = Angular aperture = 1/2 angle of cone of light entering the objective lens from the specimen (depends on the width of the objective lens and distance from the specimen) -- increased by moving lens close to the specimen N sin a = Numerical aperture For specimens in air viewed by visible light: N = 1.0 l = 450 nm D = 292 (0.3 mm) For specimens in oil D = 200 nm (0.2 mm) For ultraviolet light l = 200 nm D is approximately 1/2 l Another important parameter is depth of focus Magnification Depth of Focus 10X 0.1 mm 100X 1 mm11.3.2 Comparison Of Light And Electron Microscopy Methods Incident Resolution Depth of Depth of Microscope Radiation l (nm) Penetration Focus Visible light Light 450 nm 200 - 1 mm @ 100X Ultraviolet light UV 200 nm 100 -Electron e -0.005 (at 50 kV) Scanning 2 1 mm 1 mm @ 100X Transmission 0.2 0.1 mm (thickness of section)11.4 DIFFRACTION METHODS Based on the principle that a monochromatic wave impinging on a regularly arrayed structure (e.g., a crystal) will be diffracted at specific angles only, related to the spacing between the features in the array (e.g., molecules). The wavelength of radiation needs to be on the order of (or less than) the spacing to be detected. Bragg's Law n l = 2d sin q n = integer indicating which multiple of the diffracted wave is being considered l = wavelength of radiation d = spacing between features in the structure (e.g., spacing between molecules in a crystal) q = the angle between incident and diffracted waves. d= n l 2 sin q Type of Diffraction Radiation l Optical Laser light 400 nm X- ray X-ray 0.154 nm (for copper) Electron e-0.005 nm at 50 kV Depth analyzed for x-ray diffraction (i.e., depth of penetration of the x-ray beam) is 1-10 mm.11.5 CONTACT ANGLE 11.5.1 Method q (included angle) = 0: complete wetting 0<q<90°: partial wetting q>90°: nonwetting At Equilibrium, ? surface tensions = 0 gSG-gLS-gGL cos q = 0 gSG-gSL = gLG cos q Young's equation Cannot solve for gSG because in equation gSG = gSL + gLG cos q There are 2 unknowns, gSL and gSG The experimental method employed to approximate gSG involves asking the question: what is the surface tension of a liquid that would completely wet the solid surface? This value is referred to as the critical surface tension of the solid. 11.5.2 Assumptions Equilibrium between the liquid droplet and solid surface has been reached (i.e., no absorption of liquid by the solid and no leaking of substances from the solid). If this assumption cannot be met then the "advancing angle" can be measured to determine the contact angle of the liquid with the dry surface and "receding angle" measured to determine the contact angle with the water absorbed surface. An alternative method is to measure the underwater (captive-air-bubble) contact angle that an air bubble makes with the immersed surface. This is particularly valuable for measuring surface that can switch from hydrophobic to hydrophilic depending on the environment. 11.5.3 Potential Problems 1) Contamination of the solid surface 2) Contamination of liquids11.6 SPECTROSCOPYMIT OpenCourseWarehttp://ocw.mit.edu 20.441J / 2.79J / 3.96J / HST.522J Biomaterials-Tissue Interactions Fall 2009 For information about citing these materials or our Terms of Use, visit: