Investigation of the Adsorption Processes of Fluorocarbon and Hydrocarbon Anions at the Solid-Solution Interface of Macromolecular Imprinted Polymer Materials Abdalla H. Karoyo and Lee D. Wilson* Department of Chemistry, University of Saskatchewan, Saskatoon S7N 5C9, Canada ABSTRACT: This study details the isotherm adsorption properties of cross-linked polymers containing β-cyclodextrin (β-CD), hereafter referred to as macromolecular imprinted materials (MIMs). The isotherm sorption parameters of the MIMs are reported with perfluorooctanoic acid (PFOA), octanoic acid (OA), and perfluorooctanesulfonate (PFOS) anions. The Sips and BET models describe variable solid-- solution interfacial interactions for the MIMs with perfluor- ocarbon (PFC) and hydrocarbon (HC) anions, respectively. Typical monolayer adsorption occurs for the MIMs/OA system via the hydrocarbon alkyl tail of OA. Atypical adsorption is observed for PFOA and is driven via interactions between the MIMs surface with the carboxylate headgroup, as evidenced by the formation of multilayer adsorption and aggregation of the PFC alkyl chain at elevated levels. Molecular level support for the unique interfacial adsorption phenomenon is provided by several complementary methods: contact angle, infrared (IR) spectral shifts and intensity variations, differential scanning calorimetry (DSC), and isothermal titration calorimetry (ITC). Isotherm studies reveal the formation of monolayers of a hydrocarbon surfactant anion (OA) vs self-assembled multilayer aggregates for PFC anion species (PFOA and PFOS) at the MIMs solid-solution interface. The surface interactions are interpreted as a balance between headgroup electrostatic interactions and hydrophobic effects of the amphiphile chain at the adsorbent-solution interface. 1. INTRODUCTION The uptake of perfluorocarbon (PFC) and hydrocarbon (HC) surface active agents from aqueous solution onto synthetically modified polymer surfaces has been widely reported using solid-solution adsorption isotherms. 1-15 The adsorption process of such anionic surfactants onto polymer surfaces was reported to result in the formation of unique types of molecular aggregates such as micelles/hemimicelles and layered struc- tures. 15-17 The driving force of such interactions at the solid- solution interface depends on the mutual compatibility of interactions between the polymer adsorbent surface, adsorbate species, and solvent medium. 18-20 The surface activity of PFC and HC surfactants varies markedly as evidenced by the systematically lower critical micelle concentration (CMC) of PFCs, indicating that self-assembly via micelle formation occurs more readily for PFC surfactants. The formation of host-guest complexes between cyclo- dextrins (CDs) and surfactants results in a reduction of the surface activity of such guests, especially for PFC surfactants over their HC counterparts. 21 CDs are a group of structurally related macrocyclic host molecules comprised of six (α-), seven (β-), and eight (γ-CD) glucopyranose units with α-(1 → 4)- linkages that are formed during bacterial digestion of starch. 22,23 α-, β-, and γ-CDs have been reported to form stable host-guest complexes with surfactants at concentrations (C s ) below 23-32 and above 34 the CMC of the surfactant. An understanding of the host-guest chemistry of a CD with a surfactant micellar phase (C s > CMC) in solution is further complicated due to the occurrence of multiple equilibria, surfactant aggregation vs host-guest complex formation of a dispersed surfactant. Competitive equilibria of aggregates and host-guest complex formation depend on the relative magnitude of the equilibrium constant for such processes. For instance, the propensity of self- assembly of sodium perfluorononanoate (SPFN) was observed to occur along with the formation of 1:2 CD/SPFN inclusion complexes, according to an apparent molar volume and NMR studies. 30,31 Thus, multiple equilibria may result in the competitive formation of CD inclusion complexes or self- assembly since such processes lower the Gibbs energy of amphiphilic systems in solution. Competitive equilibria in solution may extend to polymer-based CD systems because the structural dynamics and accessibility of the CD inclusion site are influenced by the topology and the cross-linker density. The interaction between surfactants with CD polymers can occur within the cavity (inclusion sites) or onto the surface domains of the polymer framework (interstitial sites). The formation of micelles/hemimicelles may occur at the interstitial sites of the Received: December 14, 2015 Revised: March 2, 2016 Published: March 3, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 6553 DOI: 10.1021/acs.jpcc.5b12246 J. Phys. Chem. C 2016, 120, 6553-6568