Energetics of formation and hydration of functionalized silica nanoparticles: An atomistic computational study
Graphical abstract
Introduction
Silica nanoparticles represent a versatile and interesting material with wide range of applications, such as optical [1], drug delivery vehicles [2], super hydrophobic surfaces [3], [5], [4], [6], [7], [8], [9] or as a surface tension modifier in fluid–fluid interfaces [10], [11]. Silica is a low expensive material and it is frequently used in applications that requires a large quantity of nanoparticles [12].
Additionally, silica based nanomaterials have received great attention for nanomedicine and biological applications [13] due to their biocompatible properties [14]. An important issue for in vivo applications of silica [2] is the retention of meso structures in the living organisms. In this case, nanoparticles can represent a proper choice due their smaller size [13]. Recent advances in this direction were reported by Liong et al. [15], where the authors produced SiO2 nanoparticles with a magnetic core suitable for applications in imaging diagnostic. Silica nanospheres also can be used as encapsulation materials for drug delivery [15], [16], [17]. To generate inorganic mesoporous structures for such applications, SiO2 nanospheres within CdS nanoparticles were functionalized with 2-(propyldisulfanyl)ethylamine to make the connections between nanoparticles [16].
Recently, considerable efforts have been also made for enhanced oil recovery (EOR) [18], [19], [20] and gas recovery [21] applications, where silica nanoparticles have been used to obtain smart surfaces with switchable superoleophilic and superoleophobicity properties in aqueous media for oil/water separation [22]. Suspended silica nanoparticles can be located at the water–oil interface [10], [11], [19], [20], [23], which can modify the oil–water surface tension, resulting in an improvement on EOR techniques [18], [19], [20]. However, the harsh reservoir conditions (high temperature, pressure and salt concentration) represent a special challenge for a nanoparticle suspension. In this direction, Garbin et al. have proposed a method to recover nanoparticles from oil/water interface [24], Eberle et al. employed octadecyl coated silica nanoparticles to develop a thermoreversible nanoparticle dispersion [25] and Athauda et al. [26] systematically studied the wettability of a glass surface coated by silica nanoparticles with diameter varying between 7 and 40 nm.
In most of these applications, nanoparticles are suspended in a fluid and the maintenance and stabilization of this suspension is crucial in order to avoid precipitation and consequent suspension collapse [27]. A common way to maintain the suspension consists to functionalize the nanoparticle [28] with either hydrophobic [29], [30], [31] and hydrophilic [32], [33], [34], [35], [36], [37], [38], [39] groups. The functional form, chain length [29], graft density and distribution of groups at surface can be tuned to kept the suspension and improve the nanoparticle functionality.
Atomistic simulations can be used as a systematic tool to evaluate and predict nanoparticle's suspension and precipitation. Lane et al. applied molecular dynamics (MD) calculations to study the approximation of two silica nanoparticles (5 nm diameter) functionalized with poly-ethylene oxide (PEO) groups in aqueous media [33]. They observed an intense repulsion between nanoparticles for distances shorter than 5 nm, concluding that the suspension is stable. In their work, a functional group graft density of 3.1 groups/nm2 was used, without further optimization. However, the authors commented on the importance of graft density optimization within modeling studies. In a recent contribution, Ewers et al. [34] considered SiO2 surfaces and nanoparticles functionalized by Alkylsilane groups by means of MD simulations, where it is observed that the group density and chain length are important parameters for the surface coverage and functional group structural properties. Considering both, experiments and simulations, Marschall et al. [35] studied a SiO2 pore passivated with functional groups that contain a sulfonic acid. The authors reported that distinct experimental methods can lead to different functional group coverage densities in SiO2 surfaces.
In this work, we have studied amorphous silica nanoparticles functionalized by different groups, namely, organosilane molecules with radical terminations with (i) polyethylene glycol (PEG) and (ii) sulfonic acid (SA) by means of first-principles methods, including van der Waals dispersion forces, and classical MD. Those functional groups are a common way to functionalize amorphous silica [36] and their head termination, namely, PEG and SA, represent hydrophilic and hydrophobic nanoparticle coverage, respectively. Those functional groups represent an interesting option to modify the silica nanoparticle surface and alter the dispersion properties, as showed by Metin et al. in a recent experimental study [37]. The aim of this work is the determination of the optimum graft density for silica nanoparticles, considering different functional groups in vacuum and aqueous media. This information represents a significant contribution on the optimization of functionalization processes for SiO2 nanoparticles and it can be extended to understand the other inorganic nanostructures.
Section snippets
Functionalized silica nanoparticle models
To generate amorphous silica nanoparticle structures, we adopt a Monte Carlo scheme [38] that can provide at the surface termination any percentage of Si(OH), Si(OH)2 and Si(OH)3. A total of three nanoparticle template models (NP) were constructed, containing 1482, 1438 and 1377 atoms, namely respectively as NP A, B and C. These models have 250, 232 and 199 hydrogen atoms on surface sites, where functional groups could possibly be attached. Based on experimental results [39], the surface of all
Methodology
First principles calculations based on density functional theory (DFT) [46], [47] was used. The correction for van der Waals (vdW) forces was included by means of the dispersion-corrected atom-centered potentials (DCACP) [48], [49]. For these first-principles calculations, an alpha-quartz (1 0 0) silica surface was adopted as model for functionalized nanoparticles since the fully atomistic models generated have a considerable computational cost.
SiO2 slabs with six and seven layers of Si and O
Results and discussion
The results are presented as follow: in Section 4.1 the first-principles results are showed, for both, hydrophilic and hydrophobic surface coverage. For each case, the silanol and geminal surface sites are evaluated considering standard DFT and vdW-DFT calculations. In Section 4.2, the results of the MD simulations for functionalized nanoparticles with hydrophobic and hydrophilic coverage are discussed in both, vacuum and aqueous solution.
Conclusions
We studied the functionalization of amorphous silica nanoparticles by hydrophilic and hydrophobic functional groups, using an atomistic computational approach including first principles and classical molecular dynamics (MD). Organosilane groups, containing head terminations of sulfonic acid (SA) or polyethylene-glycol (PEG) are used as representative of hydrophobic and hydrophilic groups, respectively. The (1 0 0) α-quartz silica surface was used as a model for first principles calculations, and
Acknowledgements
We acknowledge the financial support of the Brazilian agencies CAPES, FAPESP, CNPq, UFABC and the Advanced Energy Consortium (AEC). The calculations have been partially performed at CENAPAD-SP, CESUP-RS, and UFABC supercomputer facilities.
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