Below you can see contact angle measurements of water on two surfaces (surface 1

Question

Below you can see contact angle measurements of water on two surfaces (surface 1 and surface 2). (a) Assuming that one of these materials is PTFE and one is PTFE with covalently-tethered PEG, which surface is which? Why? (b) Following is a plot of contact angle (theta_2) vs. Liquid-vapor surface tension (gamma_L V) for it interacts with several different liquids. Determine the critical surface tension (gamma_c) and calculate gamma LV for the system in which surface 2 interacts with water. Assume that you have tethered PEG on your PTFE surface to allow subsequent modification with a biological molecule. (c) PEG has been added to the surface to act as a "spacer arm" for the biological molecule. Why are spacer arms often needed when modifying surfaces with bioactive molecules?

Explanation / Answer

a) Surface 2 has covalently-tethered PEG, while surface 1 does not. Because surface 2 has a smaller contact angle, it is most likely more hydrophilic than surface 1 (indicating hydrophilic PEG surface modification of normally hydrophobic PTFE).

b) c = LV-value at =0°
c = 32 dynes/cm
  
= 2.2*LV – 70.4
40° = 2.2* LV – 70.4
LV = 50.2 dynes/cm

c) Spacer arms are often needed to provide physical space between the bioactive molecule and the substrate. They are inert molecules, but may allow for greater rotational freedom/maintenance of proper conformation, improving the activity of the biological molecule.

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