Experimental studies on irreversibility of electrostatic adsorption of silica nanoparticles at solid–liquid interface Xue Li, Olivia Niitsoo, Alexander Couzis ⇑ Department of Chemical Engineering, The City College of New York, NY 10031, United States article info Article history: Received 5 October 2013 Accepted 12 December 2013 Available online 11 January 2014 Keywords: Electrostatic adsorption Irreversible Silica Ag/SiO 2 core/shell Nanoparticles Solid–liquid interface Amino-teminated substrate abstract Adsorption of colloidal nanoparticles (NPs) at solid–liquid interface is a scientifically interesting and tech- nologically important phenomenon due to its fundamental importance in many industrial, environmen- tal, and biological processes, such as wastewater treatment, printing, coating of surfaces, chromatography, papermaking, or biocompatibility. The process is well understood theoretically by the random sequential adsorption (RSA) model, based on the assumption of irreversible adsorption. Irrevers- ible adsorption is defined as a process in which, once adsorbed, a particle can neither desorb, nor to move laterally on the surface. However, published experimental data that verifies the irreversibility of particle adsorption are very limited. In this study, we demonstrate the irreversibility of electrostatically driven nanoparticle adsorption utilizing a carefully selected set of experiments. A simple method was employed by uniquely introducing Ag@SiO 2 core/shell NPs to perform exchange adsorptions experiments. Stöber SiO 2 NPs with a diameter of 50–80 nm were initially electrostatically adsorbed onto amino-functional- ized silicon wafer substrates followed by the subsequent adsorption of Ag@SiO 2 NPs. The Ag@SiO 2 NPs have the same surface chemistry as the neat SiO 2 NPs. For the second step the adsorption time was varied from 1 min to 1 week so as to get a thorough understanding of the process irreversibility. Surface cover- age quantification has shown that the surface coverage of the initially adsorbed SiO 2 NPs stays the same independent of the duration of the second step adsorption using the Ag@SiO 2 core/shell NPs. This obser- vation directly confirms the irreversibility of electrostatic adsorption of NPs. Published by Elsevier Inc. 1. Introduction Adsorption of colloidal nanoparticles (NPs) at solid–liquid inter- face plays an important role due to its wide range of practical applications in emerging areas of nanoscience and technology. Especially important are colloidal nanoparticles, in the form of monolayers and films deposited on solid surfaces, which have been attracting ever-increasing attention due to the fundamental impor- tance in many industrial, environmental, and biological processes, such as water and wastewater filtration [1], antireflection coatings [2], antibacterial coatings [3], catalytic materials [4], papermaking [5], cosmetics [6] and drug delivery [7,8]. Thus, there is a high de- mand on understanding the adsorption behavior of nanoparticles at interface in general. Many studies on the adsorption of nanopar- ticles at gas–liquid [8,9] and liquid–liquid [10–13] interface have been reported in the past few years. The nanoparticle adsorption could be controlled by manipulating the particle’s and the surface’s properties [14,15]. Tuning of the electrostatic interaction between particles and surface turned out to be useful for controlling the adsorption process [16–18]. However, there is little data accessible concerning the adsorption at solid–liquid interfaces which provide detailed information about the structural arrangement of NPs and stability of the adsorbed particles, arising from the irreversibility of the particle adsorption. The adsorption process at solid–liquid interfaces has been widely studied and is well understood theoretically within the framework of the random sequential adsorption (RSA) model [19–21]. In the RSA model, particles arriving at the substrate sur- face are adsorbed sequentially. Adsorbed particles are anchored on random sites irreversibly and the newly arrived particles are not allowed to occupy sites that lead to overlapping of previously anchored particles. Using the model, adsorption kinetics, structure of particle monolayers and jamming coverage can be determined [22]. It has been well accepted that the RSA model is based on the assumption of irreversible adsorption. Irreversible adsorption is defined as a process in which a particle, once adsorbed, can neither desorb, nor move laterally on the surface. A great number of exper- imental data has been reported indicating the accurate prediction of adsorption kinetics by the RSA model [23,24]. Particularly, the significant influence of the electrostatic interactions on the maximum coverage has been substantiated by using the direct microscope observation methods, such as optical, AFM, and SEM 0021-9797/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.jcis.2013.12.030 ⇑ Corresponding author. Fax: +1 (212) 650 6660. E-mail address: couzis@ccny.cuny.edu (A. Couzis). Journal of Colloid and Interface Science 420 (2014) 50–56 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis