Effects of Ionic Strength and pH on the Polyelectrolyte Star Conformation

7 Effects of Ionic Strength and pH on the Polyelectrolyte Star Conformation 

Erom the summary of the theoretical results presented above, it follows that the intramolecular volume of a star-branched PE, with a sufficiently large number of arms, is essentially electroneutral. That is, the bare charge of a star polymer is neutralized by mobile counterions. These counterions are predominantly retained inside the macroion volume, even if the star is immersed in a dilute salt-free solution. Moreover, if the intrinsic Debye length associated with the intramolecular concentration of entrapped counterions, = l paNis used as an upper estimate for the intramolecular electrostatic screening length, one finds that in the osmotic PE star, p a), the electrostatic interactions are screened at 

Therefore, a detailed analysis of the conformations of many-armed star-like PEs may take place within the so-called local electroneutrality approximation (LEA). The latter also allows for an equilibrium of the ionization of the macroion, hence also covering the pH-responsive (annealing) star-like PEs. 

As long as monovalent salt ions are added to the solution, the total ion concentration  

The effect of salt on the conformations of a many-arm (osmotic) PE star becomes important when the salt-controlled bulk Debye screening length, given by (33), becomes smaller than the intrinsic screening length, in a salt-free osmotic PE star. This is also true, equivalently, when the concentration of added salt exceeds the intramolecular concentration of counterions in the osmotic star. Clearly, the local electroneutrality in this case is ensured, and the LEA is applicable for analysis of the PE star conformations on a length scale larger than td. 

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