Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso Effects of enhanced clusterization of water at a surface of partially silylated nanosilica on adsorption of cations and anions from aqueous media I.F. Mironyuk a , V.M. Gun'ko b , H.V. Vasylyeva c , O.V. Goncharuk b , T.R. Tatarchuk a,d,∗ , V.I. Mandzyuk e , N.A. Bezruka f , T.V. Dmytrotsa f a Department of Chemistry, Vasyl Stefanyk Precarpathian National University, 57 Shevchenko Street, 76018, Ivano-Frankivsk, Ukraine b Chuiko Institute of Surface Chemistry, 17 General Naumov Street, 03164, Kyiv, Ukraine c Uzhhorod National University, 3 Narodna Square, 88000, Uzhhorod, Ukraine d Educational and Scientific Center of Material Science and Nanotechnology, Vasyl Stefanyk Precarpathian National University, Ivano-Frankivsk, 76018, Ukraine e Department of Computer Engineering and Electronics, Vasyl Stefanyk Precarpathian National University, 57 Shevchenko Street, 76018, Ivano-Frankivsk, Ukraine f Ivano-Frankivsk National Medical University, 2 Halytska Street, 76018, Ivano-Frankivsk, Ukraine ARTICLE INFO Keywords: Fumed silica Partially silylated nanosilica Cation adsorption Anion adsorption Zeta potential ABSTRACT Adsorption of metal species Ba(II), Sr(II), Zn(II), Ca(II), and Mg(II) and anions Cl − , Br − , and I − at a surface of unmodified and partially silylated nanosilica A-300 was studied in aqueous media upon changes in the degree of substitution of silanols to trimethylsilyl groups (Θ TMS ) at Θ TMS = 0.272–0.483. The cation adsorption increases in series Sr(II) < Mg(II) < Zn(II) < Ca(II) < Ba(II) with increasing value of Θ TMS . Nanosilica at Θ TMS = 0.483 adsorbs 1.8 mmol/g of Ba(II) from 0.01 M BaCl 2 solution that is three times higher than that for the unmodified silica. Among anions, the adsorption of Cl − is maximal (1.34 mmol/g of Cl − from 0.01 M CaCl 2 solution) onto silylated nanosilica at Θ TMS = 0.272 that is eight times greater than that for the unmodified nanosilica. An increased adsorption ability of partially silylated nanosilica in comparison to unmodified silica can be explained by nonuniformity of a modified silica surface resulting in enhanced clusterization of adsorbed water that leads to reduction of its activity as a solvent. Therefore, the desolvation energy decreases for adsorbed cations and anions having smaller solvated shells near the modified silica surface. 1. Introduction Fumed silica (nanosilica) is widely used in industry, medicine, and agriculture as an individual powder material or a component of com- posite materials such as filled polymers [1,2], powder or monolith so- lids [3,4], suspensions [5], etc. Flame (H 2 /O 2 /N 2 ) synthesis at 1000–1200 °C using SiCl 4 or SiCl 4-n R n (n =1–3, R is an organic func- tionality, e.g., OCH 3 , CH 3 , etc.) as precursors leads to certain general characteristics of fumed silicas at various specific surface area. Many of nanosilicas features are caused by the nano-particulate morphology and the absence of pores in primary nanoparticles [6–8]. Both totally and partially hydrophobized fumed silicas are of in- terest from a practical point of view because these materials are used as better fillers of nonpolar polymers or more appropriate materials for other practical applications than unmodified hydrophilic nanosilica. These aspects were described in detail in the literature [6–11]. There are various low-molecular weight modifiers, which can be used for hydrophobization of silica, such as chlorosilanes (Cl x SiR 4-x , where R = CH 3 or other organic functional groups) and organosilanes (((CH 3 O) x SiR 4-x ), hexamethyldisilazane, etc.). Clearly, an increase in the degree of silylation (Θ) of a silica surface leads to a decrease in the hydrophilicity and to an increase in the lyophilic properties of the materials. Despite a linear decrease in water adsorption with increasing value of Θ, the Gibbs free energy and enthalpy of immersion in water weakly depend on the Θ value up to Θ = 0.5, and at Θ ≈ 0.1 the in- teraction energy grows in comparison to unmodified silica due to strong nonuniformity of the surface [12]. Note that maximal nonuniformity of the interfacial layer at a silica surface modified by various modifiers is typically observed at Θ < 0.07 [11–13]. This nonuniformity affects the interactions of partially modified nanosilica with both polar and non- polar adsorbates both in gaseous and liquid media [11–15]. Equilibrium sorption of ions from aqueous media onto a solid sur- face depends on several factors. First, the Gibbs free energy of solvation (ΔG s ) in the bulk and partial desolvation (ΔG ds ) upon adsorption. https://doi.org/10.1016/j.micromeso.2018.10.016 Received 5 August 2018; Received in revised form 30 September 2018; Accepted 15 October 2018 ∗ Corresponding author. Educational and Scientific Center of Material Science and Nanotechnology, Vasyl Stefanyk Precarpathian National University, Ivano- Frankivsk, 76018, Ukraine. E-mail addresses: tatarchuk.tetyana@gmail.com, tetiana.tatarchuk@pu.if.ua (T.R. Tatarchuk), mandzyuk_vova@ukr.net (V.I. Mandzyuk). Microporous and Mesoporous Materials 277 (2019) 95–104 Available online 17 October 2018 1387-1811/ © 2018 Elsevier Inc. All rights reserved. T