Article pubs.acs.org/Langmuir
Adsorbed Mass of Polymers on Self-Assembled Monolayers: Effect of Surface Chemistry and Polymer Charge Plinio Maroni,† Francisco Javier Montes Ruiz-Cabello,†,‡ Catia Cardoso,† and Alberto Tiraferri*,†,§ †
Department of Inorganic and Analytical Chemistry, University of Geneva, Sciences II, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland ‡ Biocolloid and Fluid Physics Group, Applied Physics Department, Faculty of Sciences, University of Granada, Fuente Nueva s/n, 18071 Granada, Spain § Department of Land, Environment and Infrastructure Engineering (DIATI), Polytechnic University of Turin, C.so Duca degli Abruzzi 24, 10129 Torino, Italy S Supporting Information *
ABSTRACT: The adsorbed mass of polymers on surfaces with different chemistry is presented, and the related adsorption mechanism is discussed. Strong and weak polyelectrolytes of negative and positive charge are studied, as well as an uncharged polymer. Self-assembled monolayers of alkanethiols on gold are used in reflectometry and quartz crystal microbalance (QCM-D) experiments as adsorbing substrates bearing different terminal moieties, namely, methyl, hydroxyl, carboxyl, and amine groups. The various polymer-surface combinations allow the systematic investigation of the role of surface chemistry and polymer charge on adsorbed amount. Interactions of different nature and range drive polymer adsorption: the measured adsorbed amounts reveal information about their relative contribution. When electrostatic chain-surface attraction is present, the largest adsorbed masses are observed. However, significant mass is measured even when an electrostatic barrier to adsorption is present, suggesting the importance of forces of nonelectrostatic origin, which include both hydrophobic interactions and specific forces acting at short distances. This mechanism results in large adsorbed amounts for the adsorption of weak polyelectrolytes, and it is apparent especially in the adsorption behavior of a neutral polymer.
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maximum and finally decreases.6−8 While the mechanisms underlying layer formation and structure in oppositely charged systems have been studied extensively, limited information is available about adsorption in the absence of electrostatic attraction. When systems comprise PEs and surfaces of like charge, long-range electrostatic double-layer repulsions are expected to prevent or significantly thwart deposition. However, the presence of multivalent counterions can promote adsorption by condensation at the solid/water interface, thus reversing its electric potential.9,10 Evidence is in fact available that polymers adsorb even in the absence of electrostatic attraction between chains and the adsorbing substrate, suggesting that forces of nonelectrostatic origin are also important.11−13 These interactions include but are not limited to van der Waals forces, hydrogen bonding, and especially hydrophobic interactions. The relative importance of energetic and entropic contributions is also subject to debate, but entropic forces can be operative under certain conditions and especially for neutral polymers or PEs of low charge.11,14−16
INTRODUCTION When solid surfaces are immersed in an aqueous solution containing dissolved polymers, the polymers adsorb at the solid/water interface. The properties of the modified surfaces are then intimately related to the characteristics of the resulting polymeric films.1 Representative examples are the removal of suspended contaminant matter during the treatment of water and wastewater and the control of colloidal stability in the manufacturing of consumer products and in the food industry.2 Adsorption of organic matter in the environment governs the fate of both natural and anthropogenic nanomaterials.3 Understanding the mechanisms of formation and growth of polymeric films allows one to predict and to tailor the behavior of these systems. Within this effort, the mass of the adsorbed polymeric layer is the foremost parameter to be predicted. Adsorbed amounts of polymers on planar substrates are usually probed with optical reflectivity, ellipsometry, and quartz crystal microbalance.1,4−6 Adsorption of polyelectrolytes (PEs) on an oppositely charged surface is fast, irreversible, and quantitative until saturation, due to electrostatic attraction, which makes deposition favorable.1 In this case, the adsorbed mass normally increases with increasing solution ionic strength. However, this salt dependence is more complex for weakly charged PEs, whereby the adsorbed mass goes through a © XXXX American Chemical Society
Received: November 17, 2014 Revised: May 20, 2015
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DOI: 10.1021/acs.langmuir.5b01103 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
ozone cleaner for 20 min followed by a rinse with ethanol. The alkanethiol assemblies were spontaneously adsorbed on these surfaces by immersing them overnight (>10 h) into a solution containing the corresponding thiol dissolved in absolute ethanol (99.98%, VWR International, France). Solutions of 0.5 mM 11-mercapto-1-undecanol and 1-nonanethiol were used to form hydroxyl (OH) and methyl (CH3) terminated SAMs, respectively. These films were washed by rinsing both sides of the coated substrate with abundant ethanol. Carboxyl (COOH) and amine (NH2) terminated SAMs were produced following an improved method described elsewhere.20 An ethanolic solution of 0.5 mM 11-mercaptoundecanoic acid with 2% (v/v) CF3COOH was used to form COOH SAMs, whereas solutions of 0.5 mM 11-amino-1-undecanethiol hydrochloride with 3% (v/v) N(CH2CH3)3 were employed to fabricate NH2 SAMs. These films were rinsed sequentially with ethanol, an ethanolic solution of 10% (v/ v) NH4OH for carboxyl terminated films or 10% (v/v) CH3COOH for amine terminated films, and again ethanol. Following each polymer adsorption experiment, the gold surfaces were regenerated using a cleaning protocol that involved treatment in an oxygen-enriched UVozone cleaner for 20 min followed by immersion in 2% (w/w) sodium dodecyl sulfate for 30 min. The substrates were then rinsed with MilliQ water and finally dried in a flow of nitrogen before another 20 min cleaning cycle in the UV-ozone chamber. The sessile drop method was employed to measure the advancing contact angle of deionized water on the SAMs. The values of water contact angle evaluated for the four types of SAM are reported in Table 1 and are consistent with reports in the literature.20−22 No
Recently, gold−thiol self-assembled monolayers (SAMs) have been used in optical reflectometry experiments to investigate the adsorption kinetics and the adsorbed amount of strong PEs on surfaces with different chemistry.17 Layers comprising significant polymer amounts were shown to form also upon adsorption of PEs on surfaces with the same sign of electric potential. The formation of these layers was faster than what would be expected for systems involving an interaction barrier of electrostatic origins. Various combinations of SAMs and polymers bearing different moieties can be applied to elucidate the various contributions to the driving force for polymer adsorption, as well as more clearly underline the role of nonelectrostatic interactions. In this study, we fabricate and characterize SAMs of different hydrophobicities and signs of charge. These substrates are used to determine the adsorbed mass of strong PEs and weak PEs of both signs of charge, as well as one uncharged polymer. Reflectometry allows the accurate determination of the dry mass of adsorbed polymer, while QCM-D provides information on layer structure. The role of surface chemistry and polymer charge on the adsorbed amount at saturation is thus systematically studied. Adsorption is conducted from solutions of varying pH and concentration of monovalent electrolyte, allowing the identification of signature trends in the mass of the adsorbed layer as a function of solution chemistry, as well as the relative contribution of the interactions driving adsorption.
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Table 1. Contact Angle of Deionized Water (pH 5.6), Diffuse-Layer Potential, And Regulation Parameter for the Gold Surface and for the SAM Films Used for Adsorption Experiments
EXPERIMENTAL SECTION
Substrates and Polymers. The substrates used to produce SAMs consisted of an underlying layer of silicon, a middle adhesion layer of titanium, and an uppermost sputter-coated layer of gold. The thickness of the gold layer was 8.5 nm, while that of the adhesive titanium layer was 2.5 nm, as determined by ellipsometry (Multiskop, Optrel, Berlin, Germany).5 The surface roughness of the gold layer was analyzed using a Cypher AFM (Asylum Research, Santa Barbara, CA) in amplitude modulated mode, following the protocol described in the Supporting Information. The root-mean squared (RMS) roughness of the surfaces was ∼0.5 nm. Sodium poly(styrenesulfonate) (PSS, molar mass 666 kg/mol, number of monomers N ≈ 3000) and sodium poly(acrylate) (PAA, 1250 kg/mol, N ≈ 13 000) were used as a strong and a weak polyanion, respectively. Poly(diallyldimethylammonium chloride) (PDADMAC, 400−500 kg/mol, N ≈ 2500−3000) was employed as a strong polycation, while poly(allylamine hydrochloride) (PAH, 900 kg/mol, N ≈ 10 000) was chosen as a weak polycation. Strong polyelectrolytes (PEs) have a permanent charge and do not titrate with changes in solution pH, while the charges of weak PEs vary with pH and ionic strength. Acrylic acid has a pK of 4.5, and the ionization behavior of PAA was described in detail.18 The pK of allylamine is 9.5, and the degree of ionization of PAH is reported in the literature.18,19 Finally, poly(vinylpyrrolidone) (PVP, 360 kg/mol, N ≈ 3000) was used as an uncharged polymer. All polymers were purchased from Sigma-Aldrich, Switzerland, and dissolved in Milli-Q (Millipore) water to obtain stock solutions. The solutions for experiments were obtained by dilution of these stocks with Milli-Q water, with their pH adjusted with HCl or NaOH, and their ionic strength corrected using NaCl. The refractive index increment, dn/dc, of the polymers was determined using a refractometer (Abbemat, Anton Paar, Austria) at a wavelength of 532 nm. For the various polymers, the following values of dn/dc were measured, namely, 0.145 mL/g for PSS, 0.143 mL/g for PAA, 0.197 mL/g for PDADMAC, 0.205 mL/g for PAH, and 0.173 mL/g for PVP. Electrokinetic and size measurements of the polymers in 10 mM NaCl were performed by light scattering using a ZetaNano ZS (Malvern Instruments, Worcestershire, U.K.). More details on these tests can be found in the Supporting Information. Fabrication and Characterization of SAMs. Prior to fabrication of SAMs, gold surfaces were treated using an oxygen-enriched UV-
diffuse-layer potential, ψSa (mV)
regulation parameter, pS
surface
water contact angle (deg)
pH 4
pH 10
pH 4
pH 10
gold OH NH2 CH3 COOH
75 ± 7
