Nanoparticle surface modification by amphiphilic polymers in aqueous media: Role of polar organic solvents Biswajit Sarkar, Vinithra Venugopal, Andrew M. Bodratti, Marina Tsianou ⇑ , Paschalis Alexandridis ⇑ Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York (SUNY), Buffalo, NY 14260-4200, USA article info Article history: Received 27 July 2012 Accepted 12 January 2013 Available online 29 January 2013 Keywords: Silica nanoparticles Block copolymers Poly(ethylene glycol) Adsorption Surface micellization Displacer, cosolvent abstract We investigate the role of three polar organic solvents (dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), and glycerol) on the interfacial behavior of Pluronic P105 poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers on protonated silica nanoparticles in an aqueous dispersion. The polymer adsorption and self-assembly have been assessed from critical surface micelle concentration (csmc, measured by pyrene fluorescence spectroscopy) and adsorbed layer thick- ness (measured by capillary viscometry) data. Above its csmc, PEO–PPO–PEO block copolymers form hydrophobic domains on the nanoparticle surface. Below a critical concentration in water (known as crit- ical displacer concentration, cdc), organic solvents act as displacers (molecules that can displace adsorbed polymer from a solid surface). The critical displacer concentration is obtained from the csmc and the polymer adsorbed layer thickness data. The cdc is found to be dependent on both the amount of nano- particles present in the system as well as the nature of the displacer. Below the cdc, the csmc increases and the adsorbed polymer layer thickness decreases with increasing organic solvent concentration. Inter- facial free energy calculations suggest that DMF, DMSO, and glycerol can adsorb onto the silica particles by displacing adsorbed PEO. These calculations are consistent with the experimental results in that, as a displacer, glycerol is the most effective and DMF is the least effective. Above the cdc, the influence of glyc- erol or DMSO on csmc is opposite to that of DMF which is attributed to the cosolvent effect. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Various additives, especially organic solvents, are frequently used in formulations containing nanoparticles and amphiphiles such as paints, inks, and emulsions [1–3]. The presence of an addi- tive may influence the particle–polymer interface, the polymer organization in the bulk as well as on the particle surface, and con- sequently the properties of the formulation. For example, additives having stronger attractive interactions with nanoparticles than an adsorbed polymer may cause polymer desorption [4–6]. Such addi- tives are known as displacers [4,7–11]. This desorption of polymers from a solid surface can judiciously be used to (i) control the colloi- dal stability of nanoparticles [12], (ii) control the rheological prop- erties of polymer-assisted nanoparticle dispersions [3], (iii) assess the strength of polymer–particle interfacial interactions [4,7,13,14], and (iv) regenerate adsorbate and adsorbent [4]. Relatively few studies are available on the role of organic sol- vents in affecting adsorption of polymers on solid surfaces [4,5,7,8,13–20]. Dimethyl formamide (DMF) and dimethyl sulfox- ide (DMSO) in aqueous solution were found to disfavor the adsorp- tion of PEO homopolymer on the protonated silica surface [8]. The ability of organic solvents in controlling particle–polymer interac- tions not only depends on the relative affinity of polymer and or- ganic solvents to particle surface but also on other binary interactions such as polymer–water, polymer–organic solvent, or- ganic solvent–water. Subsequently, a complex relation exists be- tween adsorbed polymers, structure of polymer, solid surface, solvent, and displacers (additives). While the additives may cause desorption of polymers from the particle surface, some of the additives can also modify the bulk properties of the solvent by acting as a cosolvent [21] and influence the organization of non-adsorbed polymers. A topic that remains unexplored is the effect of the addition to water of organic solvents which can act simultaneously as displacer and as cosolvent and control the polymer organization. The present investigation is di- rected to establish the role of organic solvents as displacers and as well as cosolvents on nanoparticle–polymer interactions and self-assembly. Recently, we investigated the effect of silica nanoparticle sur- face chemistry, size, and concentration on the organization of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymers [22,23] at the particle–water interface [24]. Above a certain critical surface micelle concentra- tion (csmc), Pluronic P105 (EO 37 PO 56 EO 37 ) formed an adsorbed 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.01.034 ⇑ Corresponding authors. E-mail addresses: mtsianou@buffalo.edu (M. Tsianou), palexand@buffalo.edu (P. Alexandridis). Journal of Colloid and Interface Science 397 (2013) 1–8 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis