Hydration Contributions to Association in Polyelectrolyte Multilayers and Complexes: Visualizing Hydrophobicity

Abstract

Thin films of polyelectrolyte complex were assembled using the multilayering method with direct, in situ observation of all multilayer components using attenuated total internal reflectance FTIR (ATR-FTIR). Buildup and ion doping of two representative combinations of positive and negative polyelectrolytes are controlled by salt type. The internal hydration of multilayers, measured precisely by ATR-FTIR, depends on the chemical identities of the polymers and the salt ions. The efficiency of doping inversely tracks the degree of hydration: less hydrated ("hydrophobic") ions are more efficient dopants, and less hydrated polyelectrolye complexes are harder to dope. Given that polyelectrolyte complexation is essentially entropy-driven, driving forces for doping, or association (the inverse of doping), are rationalized by counting all species in the condensed polyelectrolyte phase, including water molecules. For any combination of uni-univalent salt ions and polyelectrolyte, the strength of polyelectrolyte association is described by a single universal parameter. The magnitudes of the interactions per repeat unit are not high--a few kT--and are proportional to the number of water molecules released from the polymers when they form ion pairs. Hydration within multilayers due to residual salt is extensive but may be removed by an external osmotic stressing agent.

Effect of ionic strength and type of ions on the structure of water swollen polyelectrolyte multilayers

“…Data set H 2 O vac and D 2 O vac are the same. Error bars are set in accordance to a level of 10% increase in w 2 49 also reported hydration contributions to association in polyelectrolyte multilayers. Our analysis based on neutron reflectivity indicates no structural change in the dry polyelectrolyte multilayers, prepared from light water H 2 O, dried and subsequently exposed to heavy water D 2 O for 8 hours and then dried.…”

Disassembly of redox responsive poly(ferrocenylsilane) multilayers: The effect of blocking layers, supporting electrolyte and polyion molar mass

“…The solubility of anions increases with decreasing hydration entropy DH h according to the Hofmeister series (for ClO À 4 DH h ffi À57 J K À1 mol À1 and for SO 2À 4 DH h ffi À2000 J K À1 mol À1 ) [39]. The hydration number of anions follows the order ClO À 4 < NO À 3 % Cl À < SO 2À 4 [40,41]. Since the hydration number differs for every anion, small, hard ions form large water balls and transport large amounts of water with itself due to the high level of solvation [42].…”

Charge Transport in Redox Polyelectrolyte Multilayer Films: The Dramatic Effects of Outmost Layer and Solution Ionic Strength

“…An important difference between chemically cross-linked hydrogels and PEMs is that the latter are ion-pair cross-linked films and therefore soluble salt plays an important role controlling the degree of crosslinking. [28][29][30] For instance, we have previously shown that the…”