ARTICLE pubs.acs.org/Langmuir
Compact Poly(ethylene oxide) Structures Adsorbed at the Ethylammonium Nitrate-Silica Interface Oliver Werzer,† Gregory G. Warr,‡ and Rob Atkin†,* † ‡
Centre for Organic Electronics, The University of Newcastle, Callaghan, NSW 2308 Australia School of Chemistry, The University of Sydney, NSW 2006 Australia
bS Supporting Information ABSTRACT: The adsorption of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) onto silica from ethylammonium nitrate (a protic ionic liquid) has been investigated using colloid probe AFM force curve measurements. Steric repulsive forces were measured for PEO, confirming that PEO can compete with the ethylammonium cation and adsorb onto silica. The range of the repulsion increases with polymer molecular weight (e.g., from 1.4 nm for 0.01 wt % 10 kDa PEO to 40 nm for 0.01 wt % 300 kDa PEO) and with concentration (e.g., from 16 nm at 0.001 wt % to 78 nm at 0.4 wt % for 300 kDa PEO). Fits to the force curve data could not be obtained using standard models for a polymer brush, but excellent fits were obtained using the mushroom model, suggesting the adsorbed polymer films are compressed and relatively poorly solvated. No evidence for adsorption of 3.5 kDa PPO could be detected up to its solubility limit.
’ INTRODUCTION Ionic liquids (ILs) consist entirely of ions. In contrast to high melting point inorganic salts, IL ions are typically bulky and sterically hindered, which results in melting points below 100 °C. ILs frequently have low volatility, high conductivity and are able to dissolve unusual combinations of solutes. ILs are attracting research interest in a diverse range of areas including particle stabilization,1 lubrication,2 synthesis,3 catalysis,4 extractions,5 in solar cells,6 as solvents for surfactant selfassembly,7-9 and polymers10 among many others. Much of this interest is a consequence of the ability to tune the physical properties of ILs through systematic variation in the molecular structure of the constituent ions. However, the array of interionic forces expressed in an IL (Coulombic, van der Waals, hydrogen bonding, and solvophopicity) can make prediction of physical properties difficult. Ethylammonium nitrate (EAN)11 was discovered a century ago and has a melting point of 12 °C. Its viscosity is about 30 times higher than water12 and its density is 1.2 g/mL. EAN is a protic IL, synthesized by proton transfer from nitric acid to ethylamine resulting in ethylammonium cations (EAþ) and nitrate anions. Each ion has three H-bonding sites, enabling the formation of an extensive H-bonding network.13 Recent neutron scattering experiments have shown that bulk EAN has pronounced (sponge-like) nanostructure.14,15 Electrostatic and hydrogen bonding attractions between the ethylammonium cation and the nitrate anion favor the formation of ionic domains which produces a solvophopic force16 that induces cation alkyl groups to cluster into nonpolar domains. In the vicinity of a macroscopic interface EAN structure becomes even more pronounced.17,18 r 2011 American Chemical Society
At the air-EAN interface vibrational sum frequency spectroscopy experiments have revealed that the ethylammonium cation is preferentially adsorbed with its alkyl group orientated toward the gas phase.19,20 X-ray reflectivity shows that a distinct, layered interfacial structure extends into the bulk to a depth of about 20 Å, corresponding to 5 ion pairs, before the sponge morphology is reached. Atomic force microscopy and surface force apparatus have shown that similar surface-induced structural transitions occur at solid-liquid interfaces.17,18 The depth to which interfacial structure extends depends on the surface species and charge,18 roughness, and temperature.21 The presence of polar and apolar domains in the liquid means surfactants have high solubility in EAN.22,23 For nonionic surfactants, the polyoxyethylene headgroup is solvated by polar H-bonding moieties and the tail by the nonpolar domains. Surfactant aggregates like micelles and lyotropic liquid crystals do form in EAN, but since the liquid structure can satisfy the polar and apolar surfactant moieties, critical micelle concentrations are orders of magnitude higher than in water.22,23 This makes surfactants inefficient in bulk EAN9 as much higher concentrations must be used to produce effects corresponding to aqueous systems. The situation is even more pronounced at a charged surface. We have been unable to detect any evidence of surfactant adsorption at concentrations up to 30 wt % on silica or mica, as adsorption is strongly hindered by competing ethylammonium cations. This means that surface properties cannot be modified by surfactant adsorption from EAN solution as they can in aqueous systems. Received: November 17, 2010 Revised: January 24, 2011 Published: February 25, 2011 3541
dx.doi.org/10.1021/la104577a | Langmuir 2011, 27, 3541–3549
Langmuir Recent work has shown that Pluronics (triblock copolymers consisting of polyethylene oxide (PEO) and polypropylene oxide (PPO)) can displace ethylammonium cations and adsorb at the EAN-silica interface,24 and small angle neutron scattering experiments have revealed that Pluronics form micelles and liquid crystalline phases in bulk EAN. However, these experiments could not elucidate whether surface adsorption is due to attractions between silica and the PEO block, the PPO block, or some combination of both. Even though the chemical structures of PEO and PPO are somewhat similar, their (temperaturedependent) solubilities in EAN are distinctly different and roughly in accordance with aqueous systems. PEO is highly soluble in EAN and water. PEO has a cloud point in water at 105 °C but remains soluble in EAN up to at least 150 °C. PPO is waterinsoluble and only soluble in EAN up to 1 wt %, with a cloud point at around 34 °C. Solubility differences between PEO and PPO may be attributed to PPO’s additional iso-CH3 group. The reduction in solubility with temperature for PEO in water, and PPO in EAN, are attributed to changes in the polymers’ polarity, and is determined by the sequence of trans and gauche arrangements about the C-C and C-O bonds.25 Less polar conformations are favored at elevated temperatures, reducing solubility. Atomic force microscope (AFM) force curve measurements are able to reveal the adsorbed conformation of polymers at liquid-solid interfaces. Previous experiments have elucidated the structure of PEO at the water-silica interface26 and Pluronics at the EAN-silica interface.24 Evaluation of force profiles is frequently made by comparison with theoretical descriptions which include the mushroom model,27 the Alexander-de Genes model (AdG),27 Milner-Witten-Cates model (MWC),28 and self-consistent field theory.29 These theories were originally derived for the approach of two flat surfaces, so application of these theories to an AFM experiment requires the use of the Derjaguin approximation, which in turn requires information on the interfacial interaction area through the radii of curvature of the interacting surfaces. As the interaction area between an AFM tip and the surface is poorly defined, a micrometer sized colloidal particle is attached to the end of the cantilever.30 This welldefined colloid acts as the probe and has a larger and much better defined area of interaction with the substrate compared to a conventional, sharp AFM tip. In this work, silica colloid probe AFM force measurements are performed at the silica-EAN interface. Force data is presented for the pure silica-EAN interface and in the presence of dissolved PEO and PPO. PPO is nonadsorbing up to its solubility limit. For PEO, the effect of molecular weight and concentration on the interfacial forces and structure is elucidated.
’ MATERIALS AND METHODS EAN was prepared by reacting equimolar amounts of ethylamine (Sigma Aldrich) and conc. nitric acid (BASF) in excess water. The solution temperature is maintained at 8-10 °C to prevent the formation of oxide impurities. Water is removed by rotary evaporation at 50 °C, then purging with nitrogen and heating at 105-110 °C for 12 h. This leads to a colorless liquid with water contents undetectable by Karl Fischer titration (
