Article pubs.acs.org/Langmuir
Wettability of Azobenzene Self-Assembled Monolayers Silvio Pipolo*,† and Stefano Corni‡ †
Center S3, CNR Institute of Nanoscience and Department of Physics, University of Modena and Reggio Emilia, Via Campi 213/A, 41125 Modena, Italy ‡ Center S3, CNR Institute of Nanoscience, University of Modena and Reggio Emilia, Via Campi 213/A, 41121 Modena, Italy S Supporting Information *
ABSTRACT: The wettability properties of azobenzene selfassembled monolayers (SAMs), in the trans and cis forms, are investigated herein by classical Molecular Dynamics simulations of validated assembly structures described with a dedicated force field. The two different methodologies used for the calculation of the contact angle, one based on the Young’s equation and the other on geometrical models, have provided a consistent description of the SAMs wettability in line with available experimental results. Furthermore, we provide an atomistic description of the first layers of water molecules at the solvent−SAM interface, which rationalizes the wettability difference between the cis- and trans-SAMs.
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INTRODUCTION Azobenzenes are a family of organic compounds characterized by a reversible and efficient cis−trans photoisomerization reaction1 that was extensively investigated in gas phase, solution, and viscous media.2 Recently, this process was observed and monitored in self-assembled monolayers (SAMs),3−5 in particular, functionalized aryl-thiole SAMs displaying the azobenzene skeleton (4′-[(1,1′-biphenyl)-4yl]diazenyl-(1,1′-biphenyl)-4-thiol, thio-2DA in Figure 1) are
wettability were reported, many of which employing the photoisomerization reaction of the azobenzene molecules.11−13 A quantitative measurement of the wettability of a surface can be given either by specifying the contact angle “θ”,14 a macroscopic geometrical parameter connected with the shape of a solvent droplet put upon the surface, or by calculating the wetting coeff icient “k”,15 a thermodynamical parameter, that depends on the interface tension between the involved physical phases. When −1 ≤ k ≤ 1, these two indicators are connected through a simple equation:
k = cos(θ)
A computational evaluation of θ and k can be approached with atomistic simulations, as discussed in ref 16, where an overview of the methods employed so far for the evaluation of the wettability properties of a surface, using classical Molecular Dynamics (MD), is presented. In this regard, we underline that the determination of the wetting coefficient from the interface tensions of the involved phases, as described by the Young’s equation,15 can be done by a straightforward procedure.16 However, different approaches have been employed for the evaluation of the contact angle of a simulated droplet-like assembly of water molecules on the target surface. As an example, in the work of Hautman et al.,17 a spherical model is used to derive the contact angle of the simulated water droplet through the calculation of its center of mass from the simulation trajectory. Alternatively, Werder et al.18 used a circular fitting of a droplet bidimensional section generated through the evaluation of the water density profiles, and applied
Figure 1. 4′-[(1,1′-Biphenyl)-4-yl]diazenyl-(1,1′-biphenyl)-4-thiol molecule (thio-2DA).
found to self-assemble on gold surfaces and photoisomerize with high yields in compact assemblies.5 The possibility of controlling the azobenzene isomerization reaction within a SAM allows one to modulate its physical properties simply by irradiating the sample with light of the proper wavelength. Among the effects of the structural and electronic modification induced by the cis-trans interconversion reaction, the variation of the interaction with a solvent is particularly appealing because of the large variety of biochemical and environmental applications that exploit this feature: molecular recognition and binding processes in biology,6 hygroscopicity of aerosol particles in the atmosphere,7−9 and biosensors in a biological environment10 are some examples. Within those research fields, a variety of interesting results on photoresponsive surface © 2014 American Chemical Society
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Received: December 23, 2013 Revised: March 23, 2014 Published: March 27, 2014 4415
dx.doi.org/10.1021/la404922f | Langmuir 2014, 30, 4415−4421
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a correction to the microscopic contact angle through the modified Young’s equation, in order to compensate for the high linear tension in small droplets. A different strategy that does not require any size-dependent correction was proposed by Rafiee et al.,19 providing the simulation of a periodic canal of water molecules and evaluating the contact angle by fitting its section with a circular-shaped function as done by Werder et al.18 In this work, we present a classical Molecular Dynamics (MD) investigation of the wettability properties of a SelfAssembled Monolayer (SAM) of thio-2DA molecules on a gold (111) surface, through the evaluation of both the contact angle and the wetting coef f icient. The latter parameter is calculated employing the Young’s equation as described in ref 16, whereas a new approach is used for the determination of the contact angle. We simulate a periodic water canal as in the work of Rafiee et al.,19 but we analytically derive the equations that connect the contact angle both to the center of mass of the canal and to the mean squared position of the water molecules, employing a cylindric-shaped model. With this procedure, we avoid the linear-tension correction and any fitting procedure, and we are able to quantify the quality of the proposed geometrical model. Furthermore, this work provides a quantitative description of the wettability properties of the thio-2DA SAMs that enrich the knowledge of these promising photoresponsive systems and contributes to the validation of the dedicated force field and the SAMs atomistic models we developed in our previous works20,21 and that we use for this investigation. Finally, the atomistic simulations performed have provided a detailed picture of the solvent−SAM interface, allowing us to rationalize the wettability difference between the cis- and trans-SAMs.
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Figure 2. A sketch of the atomistic model (left) and the molecular arrangement (right)20 used in this work for the simulation the thio2DA SAMs, a = 2.93 Å.27 Red arrows refers to azobenzene-slab cell parameters, the SAM unit cell (including the gold slab) is pictured in black. Legend of atoms: yellow = gold, orange = sulfur, blue = nitrogen, light-blue = carbon, and white = hydrogen. k=−
γSL − γSV γLV
(2)
here γ identifies the interface tension and S, L, and V stand, respectively, for the solid, liquid, and vapor phase. The second one (“geometrical approach”) provides the explicit simulation of a water droplet and the evaluation of the average contact angle from the simulation trajectories by employing a geometrical model. In the following paragraphs, a detailed description of the two approaches is presented, we anticipate here that both of them treat water molecules with the three-center TIP3P model,28 because of its compatibility with the OPLS/AA force field that is used to simulate the SAMs. Thermodynamical Approach. Within this strategy, different simulations are needed to compute surface tensions, in fact they can be evaluated from the virial tensor (Π) as a proper combination of the diagonal components Πxx, Πyy, and Πzz:16
METHODS
Within this section, we provide a description of the computational schemes used to describe the investigated system at the classical level and to run classical dynamics simulations within the NVT ensemble. We further describe the methods we used to calculate the wettability properties, the simulation series, and the technical details. System Description. The thio-2DA molecule is described with a set of dedicated OPLS/AA22 force-field parameters we optimized in a previous work.21 The force field is of a Class I type23 with harmonic bond, harmonic angle, and periodic dihedral potential functions defined between series of bonded atoms. Restrained Electrostatic Potential (RESP) atomic charges24 are used for the electrostatic interactions and a Lennard−Jones (LJ) potential25 function is used for the description of the dispersion and repulsion forces. The structure of the azobenzene-based SAMs on gold surfaces was recently studied at the computational level by our group, as a result, we proposed an atomistic model for classical MD simulations.20 In view of the outcomes of the above-cited study, the atomistic approach used here employs the GolP model,26,27 for the modeling of the gold slab, an harmonic potential for the description of the gold− sulfur interaction, and a medium-density arrangement (4.0 molecules per nm2, compatible with experiments, see Figure 2), for the in-plane assembly of the thio-2DA molecules.20 In fact, this approach supplies a detailed description of the real system, and provides the best compromise between an ordered description of the monolayer, the stability of the cis arrangement and agreement with the experimental results.20 Computational Quantification of the Wettability. Two different computational strategies are used to quantify the wettability of the SAMs. The first one (dubbed here the “thermodynamical approach”) provides a description in terms of the wetting coefficient that is based on the calculation of the solid−liquid, liquid−vapor, and solid−vapor interface tensions (γ), as suggested by the Young’s equation:
Π γβδ =−
1 βδ βδ [2Πzz − (Πxx + Πβδ yy )] NintS
(3)
here Nint is the number of interfaces and Nint × S is the total contact surface area between the two phases (S is the cross section of the simulation box), in this case the SAM and the solvent. The symbols β and δ stand for L, S, or V. The virial tensor is computed from the forces Fij acting on the particles, at relative positions rij, as follows:
Πlm = −
1 2
Np
∑ r lijFijm (4)
i,j
