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Single Molecule Force Spectroscopy on Polyelectrolytes: Effect of Spacer on Adhesion Force and Linear Charge Density on Rigidity
Single Molecule Force Spectroscopy on Polyelectrolytes: Effect of Spacer on Adhesion Force and Linear Charge Density on Rigidity
In this paper, we have synthesized a new
kind of poly(2-acrylamido-2-methylpropanesulfonic acid)
with crown ether, and studied comparatively the single
molecule force spectroscopy of the polymers with and without crown ether, in
terms of desorption and elongation. The smooth desorption process enabled us to
calculate the loading rate of the stretching process. For the two polymers,
desorption forces were loading rate independent and ionic strength insensitive.
Interestingly, the desorption force of the two polymers were undistinguishable
in all conditions. These findings demonstrate: (1) the polymer chains adopt a
train-like (flat) conformation at the interface with a high
adsorption/desorption rate, (2) the spacer, which separates the charged group
from the hydrophobic backbone and combines the two properties together, should
account for the retained desorption force at high salt concentration, (3) the
20% less in linear charge density does not affect the desorption force
remarkably since hydrophobic interaction dominates the
adhesion force. In deionized water, PAMPS-co-Crown is less rigid than PAMPS since the uncharged
side groups separate the charged groups and thus the repulsion between adjacent
charged groups is reduced. As the salt concentration increased, the rigidity of
the two polymers both decreased, suggesting that the external salt would screen
the charges of the polyelectrolytes. The linear charge density and the ionic
strength affect only the rigidity of single polyelectrolyte chain but not the
adhesion force is another result of the “spacer effect”. This fundamental
finding, which reveals the non-electrostatic origin of the interfacial
interaction of polyelectrolytes, sheds new light on the understanding of
polyelectrolytes, especially for those containing spacers.
