Irreversible Adsorption from Dilute Polymer Solutions

TL;DR

This paper theoretically investigates irreversible polymer adsorption from dilute solutions, predicting universal features of non-equilibrium layers, and distinguishes between chemisorption and physisorption mechanisms with detailed kinetic and structural insights.

Contribution

It introduces a comprehensive theoretical framework for irreversible polymer adsorption, identifying universality classes and detailed layer formation dynamics for both chemisorption and physisorption.

Findings

Identifies three chemisorption universality classes: zipping, accelerated zipping, and homogeneous collapse.

Describes the transient loop distribution during accelerated zipping as omega(s) ~ s^{-7/5}.

Finds that the final layer's loop distribution and density profile match equilibrium layers, but chain properties depend on adsorption time.

Abstract

We study irreversible polymer adsorption from dilute solutions theoretically. Universal features of the resultant non-equilibrium layers are predicted. Two cases are considered, distinguished by the value of the local monomer-surface sticking rate Q: chemisorption (very small Q) and physisorption (large Q). Early stages of layer formation entail single chain adsorption. While single chain physisorption times tau_ads are typically microsecs, for chemisorbing chains of N units we find experimentally accessible times tau_ads = Q^{-1} N^{3/5}, ranging from secs to hrs. We establish 3 chemisorption universality classes, determined by a critical contact exponent: zipping, accelerated zipping and homogeneous collapse. For dilute solutions, the mechanism is accelerated zipping: zipping propagates outwards from the first attachment, accelerated by occasional formation of large loops which nucleate further zipping. This leads to a transient distribution omega(s) \sim s^{-7/5} of loop lengths s up to a size s_max \approx (Q t)^{5/3} after time t. By tau_ads the entire chain is adsorbed. The outcome of the single chain adsorption episode is a monolayer of fully collapsed chains. Having only a few vacant sites to adsorb onto, late arriving chains form a diffuse outer layer. In a simple picture we find for both chemisorption and physisorption a final loop distribution Omega(s) \sim s^{-11/5} and density profile c(z) \sim z^{-4/3} whose forms are the same as for equilibrium layers. In contrast to equilibrium layers, however, the statistical properties of a given chain depend on its adsorption time; the outer layer contains many classes of chain, each characterized by different fraction of adsorbed monomers f. Consistent with strong physisorption experiments, we find the f values follow a distribution P(f) \sim f^{-4/5}.