Fabrication and Simulation Properties of [SiO2 - TiO2] / Silan-Cellulose Acetate Nanocomposite for Self-Clean Surfaces Applications

INTRODUCTION
The non-wettability was observed in Nature such as in cicada’s wings and the lotus leaves [1, 2]. Lotus leaves showed fantastic properties of Wetness and self-cleaning surface “hydrophobicity”, typically determined by measuring the contact angle between the surface and the drop of water [3]. The super hydrophobic feature of any surface is defined as a situation where the drop of water is similar to the spherical shape, causing the drop of water to slide and roll on the surface even when the angle of deviation of the surface from the horizon “droplet angle” is very small. That is, when the contact angle is greater than 150 degrees and the droplet angle less than 10 degrees, superhydrophobic surfaces have numerous applications in industries. Super hydrophobicity can be obtained by controlling the roughness of the surface and the surface energy [4]. 
The surface acquires the status of hydrophobic when the topography of the surface contains nano and microstructure named hierarchical structure using materials having a low surface free energy. Self-clean surfaces 
can be manufactured in many physical and chemical preparation methods [2, 5]. Certain chemical compounds such as silan can be used to manufacture self-clean surfaces because they have low surface energy and can coat on different types of substrates [6].
  Thermal spray coating has a great interest when preparing self-clean films for its advantage of low cost, ease of work and the possibility of mixing several materials with each other, and substrate independence [7–8]. Li researcher developed coatings that have super hydrophobic coatings property using spraying technique lay-by-lay added materials [9]. Using the one-step method, Li et al. researchers enabled to manufacture of colorful superhydrophobic surfaces by spraying inorganic materials on metal substrates [10]. Wang et al. used micro and particles in addition to the polymer to obtain compositions with low surface energy to fabricate super hydrophobic surfaces deposited on the substrates of the magnesium alloy [11]. Hydrophobic coatings are adopted by spray technique is mostly used for the aggregation of hydrophobic and low surface energy particles [12]. Silicon dioxide is widely used to manufacture superhydrophobic surfaces [13, 14] because it’s low cost, low toxicity, environmentally friendly and chemically stable. One-step thermal spraying technique has been used to manufacture polymer nanocomposite film One-step thermal spraying technique used to manufacture polymer nanocomposite film (polydimethyldiclorosilane/SiO2- TiO2)   which characterized by high adhesion and through which large areas and various surfaces of materials can be painted.      
As it is known and scientifically proven, simulation is used for the purpose of obtaining results that increase the accuracy of the practical results, in addition to knowing the comparison between them to reach congruence.
 Recently, the Gaussian program has been used to implement simulations, especially in the field of materials science, where the Gaussian program depends on calculating the total energy of the system by applying the Schrödinger equation to the particles of the system using one of the theories of quantum mechanics according to a basis set and functions that describe the system. Gaussian program can perform simulations to prepare the molecules under study to calculate the structural, vibrational, optical, thermal and mechanical properties [15 – 17].

THEORETICAL CONSIDERATIONS
The contact angle can be defined as the angle formed between the surface and the tangent is drawn on the edge of the drop of water at the seam point between the surface and the drop (Fig. 1a). Based on the roughness and homogeneity of the topography of the surface, there are two models (Wenzel CA and Cassie CA) for interpreting the hydrophobic and hydrophilic phenomenon. When the water drop balanced on any homogeneous solid chemical surface, the contact angle called Young’s contact angle, as can be seen from the Fig. 1a. 
    Practically it is difficult to get a homogeneous surface ideal for sliding the drop of water because there must be a variation in the roughness and topography of the surface. The decrease on these surfaces can be in the case of stable balance, or the case of unstable balance, specifically the phenomenon of contact angle hysteria. Due to contact angle hysteria, when the surface is assigned at a certain angle and the drop can be stable in the form of a slope (1b), the contact angle becomes larger or smaller, i.e. the angle of progress or the shape of the receding angle (1b). If not for the contact angle, the drop of water will roll from the surface [18].
Fig. 1: Schematic illustration of (a) contact angle ª on the flat solid surface and (b) a stable droplet on the tilted surface owing to the contact angle hysteresis, advancing angle ªa and receding angle [18].
One of the theories of quantum mechanics is used in simulations implemented through the Gaussian program. One of the most significant methods to calculate the electronic structure of solids, molecules and atoms is Density functional theory (DFT). Having been on a large scale used for over 40 years by physicists working on the electronic properties of surfaces, defects solids, etc.,  DFT based on the distribution of electron density n(r), instead of many-electron wave function Ψ (r1, r2, r3...) [19].
The central quantity in DFT is the electron density ρ(r). It is defined as the integral over the spin coordinates of all electrons and over all but one of the spatial variables (x=r,s):
              

   Where ρ(r) represent probability of existence any electrons inside a volume element dr. Because ρ(r) represents the probability, this means that ρ(r) is a nonnegative function and vanishes at infinity and integrates to the total number of electrons N i.e [87] 23:

Density functional theory is very successful approach for the description of ground state properties of metals, semiconductors, and insulators. The success of density functional theory (DFT) not only involve standard bulk materials but also complex materials such as proteins and carbon nanotubes, While DFT in principle gives a good depiction of ground case characteristic, practical applications of DFT are based on approximations for the seeming exchange-correlation potential that gives superior accuracy to Hartree-Fock theory (Ab-initio and semiemperical approximations).
 The exchange-correlation potential describes the influence of the Pauli principle and the Coulomb potential beyond a pure electrostatic interaction of the electrons. Possessing the perfect exchange-correlation potential means that: the many-body problem solved exactly [20].

MATERIALS AND METHODS
The study procedures included two parts, the first part is practical and the second part is simulation. In this work, films were prepared from polymeric composites [SiO2 - TiO2 / silane- cellulose acetate] on glass substrates using (0, 10:1, 10:2, 10:3, 10:4) of (SiO2:TiO2) weight ratio. Substrates were cleaned using absolute ethanol for 10 min and washing again with deionized water. The substrates dried with before the spraying process. Silane and cellulose acetate were added to the alcohol. SiO2 and TiO2 nano particle has been added with different weight percentages for the solution. To obtain a homogeneous mixture, the solution was mixed using a magnetic stirrer for 10 minutes, and then the solution was placed in an ultrasonic homogenizer for 15 minutes.The process of preparing the films was carried out using a thermal spray system on the glass bases at a temperature of 70 ° C. 
Gauss view program has been designed to receive the input files from Gaussian program. Also, it is applied to display output files from Gaussian program with image dimensions. Gauss view does not use in calculations but it simplifies the work in Gaussian program and provides the users with three main benefits: The first, it makes the employer to draw the molecules; also, it enables to rotation, transformation and modification the size through the mouse. The second, it permits to investigate many Gaussian calculations, it deals with the complication in input setting for routine work by a progressive ways. The third, it allows to inspect of Gaussian calculations results by employ various geometrical techniques and this connect with equilibrium of molecular model, molecular orbital and electronic density surface [21].

RESULTS AND DISCUSSION
Hydrophobic nanocomposites were obtained through an overlapping mixture of nanomaterials (0, 10:1, 10:2, 10:3, 10:4) of (SiO2:TiO2) weight ratio with the matrix material (silan-cellulose acetate) using different weight ratios. Fig. 2 represents the stages of a drop of water falling on the surface of the nanocomposite. We note that the drop rolls off the surface and is not absorbed or adsorbed by the surface of the composite material, an indication of obtaining hydrophobic surface.
Microscopic images (FESEM) of the hydrophobic film coated on glass substrate surfaces are shown in Fig. 3 and for all the weight ratios added to the matrix material. Fig. 3 reveals the FESEM images of manufactured films nanostructures of (0, 10:1, 10:2, 10:3, 10:4) of (SiO2:TiO2) weight ratio. Images shows presence of rock breakers nanostructures with rough and agglomeration with diameter range 23.39 nm to 84.56 nm.
  Fig. 4 gives the relation shape between the effect of weight ratio and WCA of SiO2 to TiO2 nanostructures. The surface wettability measurements showed that the WCA increases from (126°) to (156 °). The optimum weight ratio is 10:3 with maximum CA equal 156.2 degree.
The simulation results included several properties. The first of these properties is to prove the optimization structure of each material that was used in the research. Fig. 5 shows the optimum stereo structure of silicon dioxide, titanium dioxide, silane and cellulose acetates. Fig. 5 shows the molecules structure and how the optimum structure was obtained, which benefits and helps to form an optimized composite material for the purpose of using it as a hydrophobic substance. The density functional theory was used in this research by employing (6-31 g (d)) as a basis set and with the help of the wave function (B3LYP) to obtain the structural results related to the lengths of bonds, triangles, and dihedral angles.
The Figs. 6 and 8 represent the electronic distribution in the valence band, which is called HOMO, and the electronic distribution in the conduction band, called LUMO of SiO2 and TiO2 respectively. The red color represents the ionization potential, while the green color represents the electronic affinity of of SiO2 and TiO2 respectively. 
Figs. 7 and 9 represents the total potential energy of SiO2 and TiO2, which is equal to zero. It can be noted that there is great stability represented by the green color, with a small presence of the ionization potential, which represents the red color.

CONCLUSION

The results showed that all films possess the property of self-clean surfaces. The addition of TiO2 improved this property of hydrophobicity. The contact angles that content TiO2 nanoparticles of the films were greater than that of the pure film. The optimal value of weight ratio was at the ratio 10: 3 with a contact angle equal to 156.2, so the films acquired the super hydrophobic surfaces. 
The simulation gave a clear understanding of the optimal spatial structure of the molecules that build up the composite under study. It can be concluded that the simulation was able to analyze the surface property of the composite and the reason for it having a hydrophobic property.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.