A Simple, Long-Lasting Treatment for Concrete by Combining Hydrophobic Performance with a Photoinduced Superhydrophilic Surface for Easy Removal of Oil Pollutants.

Superhydrophobic surfaces present promising applications in the protection of building materials, such as the self-cleaning effect promoted by their high water-repellent properties. However, these surfaces easily lose their properties when exposed to oil contaminants. This is a critical weak point for their application in building facades, which are exposed to environmental pollutants such as hydrocarbons and vandalism (e.g., grafitti). A viable strategy to remove oils is to produce superhydrophilic surfaces, which present underwater superoleophobic behavior. In the case of buildings, the use of this strategy can be considered counterproductive because it promotes their interaction with water, the main vehicle of most decay agents. In this work, we have successfully combined the advantages of a superhydrophilic coating with a hydrophobic impregnation treatment, which prevents water ingress into the porous structure of the substrate. Specifically, a photoinduced superhydrophilic surface was produced on concrete by simple spraying of a starting sol containing TiO2NPs, which create a Cassie-Baxter state, a silica oligomer, producing a compatible matrix promoting good adhesion to the substrate and polydimethylsiloxane as a hydrophobic agent. After being exposed to sunlight, the treated surfaces switched from superhydrophobic (SCA 160°) to superhydrophilic (SCA < 10°). These surfaces presented underwater superoleophobicity (SCA 152° with CHCl3) and oil-contaminated dust was easily cleaned without employing detergents. The photoactivation does not alter the protection against water absorption (>85% reduction). The treatment showed suitable adhesion to the substrate and good resistance to rainfall and outdoor exposure due to the presence of the hydrophobic silica matrix in the concrete pore structure.

Ormosils loaded with SiO2 nanoparticles functionalized with Ag as multifunctional superhydrophobic/biocidal/consolidant treatments for buildings conservation.

The alarming increase of pollution has significantly increased buildings maintenance. Nowadays, the economic figures associated to repairing activities are even more relevant than those corresponding to new construction works, especially on heritage buildings. Since the degradation of building materials is the result of a complex combination of physical, chemical and biological agents, the development of multifunctional protective treatments remains a significant challenge. We report a simple strategy to produce a versatile biocidal/superhydrophobic/consolidant treatment by incorporating biocidal Ag nanoparticles (AgNPs) grafted to functionalized SiO2NPs into a silica sol, which can be applied by simple procedures such as spraying. The use of an Ag-SiO2 coupling agent increases biocidal effectiveness up to >90% values due to: (1) an increase of the AgNPs stability; (2) a hierarchical roughness due to the formation of Ag/SiO2NPs clusters; and (3) an enhanced contact with the cell walls. In addition, the synergistic effect allows for an easier removal of the dead cells, increasing the durability of the treatment.

Long-Term Effectiveness, under a Mountain Environment, of a Novel Conservation Nanomaterial Applied on Limestone from a Roman Archaeological Site.

A novel alkoxysilane-based product was applied on limestone samples from a Roman archaeological site. The study consisted of an initial phase to evaluate site environmental conditions in order to choose the most suitable product type to be applied. The decay that was produced in the site is mainly caused by natural action, with water being the main vehicle for the decay agents. Thus, the effectiveness of an innovative product with hydrophobic/consolidant properties and two commercial products (consolidant and hydrophobic agent) were evaluated on limestone from Acinipo site, under laboratory conditions. Next, the long-term effectiveness of the three products under study was evaluated by the exposure of limestone samples in the archaeological site for a period of three years. Since the recognized incompatibility between alkoxysilanes and pure carbonate stones, the interaction between the products and the limestones was widely investigated. The results that were obtained allow for it to be concluded that the innovative product presents adequate compatibility and adherence to the limestone under study, producing a long-term effective, homogeneous, and continuous coating with a depth of penetration of up to 10 mm. However, the commercial products produced discontinuous aggregates on the limestone surface, did not penetrate into its porous structure and it did not produce long-lasting effects.

TiO2-SiO2 Coatings with a Low Content of AuNPs for Producing Self-Cleaning Building Materials.

The high pollution levels in our cities are producing a significant increase of dust on buildings. An application of photoactive coatings on building materials can produce buildings with self-cleaning surfaces. In this study, we have developed a simple sol-gel route for producing Au-TiO2/SiO2 photocatalysts with application on buildings. The gold nanoparticles (AuNPs) improved the TiO2 photoactivity under solar radiation because they promoted absorption in the visible range. We varied the content of AuNPs in the sols under study, in order to investigate their effect on self-cleaning properties. The sols obtained were sprayed on a common building stone, producing coatings which adhere firmly to the stone and preserve their aesthetic qualities. We studied the decolourization efficiency of the photocatalysts under study against methylene blue and against soot (a real staining agent for buildings). Finally, we established that the coating with an intermediate Au content presented the best self-cleaning performance, due to the role played by its structure and texture on its photoactivity.

Producing lasting amphiphobic building surfaces with self-cleaning properties.

Nowadays, producing building surfaces that prevent water and oil uptake and which present self-cleaning activity is still a challenge. In this study, amphiphobic (superhydrophobic and oleophobic) building surfaces were successfully produced. A simple and low-cost process was developed, which is applicable to large-scale building surfaces, according the following procedure: (1) by spraying a SiO2 nanocomposite which produces a closely-packed nanoparticle uniform topography; (2) by functionalizing the previous coating with a fluorinated alkoxysilane, producing high hydrophobicity and oleophobicity. The formation of a Cassie-Baxter regime, in which air pockets could be trapped between the aggregates of particles, was confirmed by topographic study. The building surface demonstrated an excellent self-cleaning performance. Finally, the surface presented lasting superhydrophobicity with high stability against successive attachment/detachment force cycles. This high durability can be explained by the effective grafting of the silica nanocomposite coating skeleton with the substrate, and with the additional fluorinated coating produced by condensation reactions.

Producing superhydrophobic roof tiles.

Superhydrophobic materials can find promising applications in the field of building. However, their application has been very limited because the synthesis routes involve tedious processes, preventing large-scale application. A second drawback is related to their short-term life under outdoor conditions. A simple and low-cost synthesis route for producing superhydrophobic surfaces on building materials is developed and their effectiveness and their durability on clay roof tiles are evaluated. Specifically, an organic-inorganic hybrid gel containing silica nanoparticles is produced. The nanoparticles create a densely packed coating on the roof tile surface in which air is trapped. This roughness produces a Cassie-Baxter regime, promoting superhydrophobicity. A surfactant, n-octylamine, was also added to the starting sol to catalyze the sol-gel process and to coarsen the pore structure of the gel network, preventing cracking. The application of ultrasound obviates the need to use volatile organic compounds in the synthesis, thereby making a 'green' product. It was also demonstrated that a co-condensation process effective between the organic and inorganic species is crucial to obtain durable and effective coatings. After an aging test, high hydrophobicity was maintained and water absorption was completely prevented for the roof tile samples under study. However, a transition from a Cassie-Baxter to a Wenzel state regime was observed as a consequence of the increase in the distance between the roughness pitches produced by the aging of the coating.

Simple strategy for producing superhydrophobic nanocomposite coatings in situ on a building substrate.

Numerous superhydrophobic materials have been developed in recent years by using a combination of two strategies: reducing the surface free energy and roughening the surface. Most of these procedures have the serious drawback of involving tedious multistage processes, which prevent their large-scale application, such as on the external stone and similar material surfaces of buildings exposed to the weather. This paper describes an innovative synthesis route for producing superhydrophobic surface coatings. The coating can even be produced, outdoors, on the building by a low-cost process. We demonstrate that the addition of silica nanoparticles to a mixture of organic and inorganic silica oligomers in the presence of a surfactant produces a coating of closely packed particles. The effect of this is to trap air beneath the water droplets, thus significantly minimizing the contact area between droplet and surface. The organic component reduces the surface free energy of the material, resulting in a high static contact angle. This has the effect of repelling water because the water droplets that form simply roll rapidly down the coated surface. The surfactant plays a valuable role, acting as a sol-gel transition catalyst and, by coarsening the pore structure of the gel network, prevents the coating material from cracking.

2D and 3D characterization of a surfactant-synthesized TiO2-SiO2 mesoporous photocatalyst obtained at ambient temperature.

A mesoporous TiO(2)-SiO(2) nanocomposite photocatalyst has been prepared from TiO(2) nanoparticles and ethoxysilane oligomers in the presence of a non-ionic surfactant (n-octylamine). The 2D and 3D structure properties of the resulting nanomaterial are described. The use of 3D techniques, particularly HAADF-STEM electron tomography, together with 3D reconstructions and atomic force microscopy, provides insight into the fine structure of these materials. We find that n-octylamine creates a mesoporous silica structure in which titania nanoparticles are embedded, and that some of the titania is retained on the outer surface of the material. Rapid photodegradation of methylene blue dye is facilitated, due to the synergistic effect of: (1) its adsorption into the composite mesoporous structure, and (2) its photodegradation by the superficial TiO(2).

Producing surfactant-synthesized nanomaterials in situ on a building substrate, without volatile organic compounds.

This article describes a sol-gel route for nanomaterials production, without volatile organic compounds (VOCs). These materials are simply obtained by mixing a silica oligomer with a non-ionic surfactant under ultrasonic agitation. The surfactant acts as sol-gel transition catalyst and also as an agent that directs the pore structure of the material, reducing capillary pressure during drying. Thus, a crack-free monolithic material is produced. We also synthesize a novel product with hydrophobic properties by adding OH terminal-polydimethylsiloxane to the starting sol. Importantly, since our synthesis does not require calcination or other additional procedures, the sol can be applied directly onto substrates, particularly the external surface of buildings. Thus, an application of these nanomaterials is to restore and to protect building substrates. Our in-depth investigation of the structure of these materials, using several techniques (physisorption, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, nuclear resonance magnetic spectroscopy), reveals that they are composed of silica particles as a result of the role played by n-octylamine. In the hybrid materials, polydimethylsiloxane acts to form bridges linking the silica particles. Finally, we demonstrate the effectiveness of these products for consolidating one particular building stone and making it hydrophobic.

Surfactant-synthesized ormosils with application to stone restoration.

A challenging objective in monumental stone restoration is to synthesize crack-free silica materials for application as consolidants. Hydrophobicity is also a valuable property for such products; it is important to prevent the penetration of water because water is the main vehicle by which the agents of decay enter the pore structure of the stone. We report the development of a hydrophobic crack-free nanomaterial with application to stone restoration. Specifically, organically modified silicate (ormosil) has been synthesized by the co-condensation of tetraethoxysilane (TEOS) and hydroxyl-terminated polydimethylsiloxane (PDMS) in the presence of a nonionic surfactant (n-octylamine). The role played by the surfactant in the assembly of the organic-inorganic hybrid silica gel was investigated. We also prepared a crack-free material using the same synthesis but without adding PDMS to the starting sol. Finally, the effectiveness of the nanomaterials synthesized as a consolidant and hydrophobic protective treatment was evaluated on a particular widely used monumental stone. The high hydrophobicity of the organic-inorganic hybrid product synthesized in our laboratory is discussed as a function of the surface roughness of the material.

Ibuprofen loading in surfactant-templated silica: role of the solvent according to the polarizable continuum model.

Ibuprofen (an anti-inflammatory drug) has been loaded onto two different surfactant-templated silicas (SBA-15 and MCM-48). To evaluate the effect of the drug-solvent combination on the loading capacity of the silica, we have performed ibuprofen adsorption experiments using 10 different solvents; we have interpreted our experimental results assuming a chemical equilibrium between the ibuprofen adsorbed on the silica and that remaining in solution. To estimate the equilibrium constant for different solvents, we have calculated the free energy in solution for the ibuprofen molecule using the polarizable continuum model (PCM) to take the solvent into account. The results have been analyzed statistically to eliminate the effects of the dispersion of experimental data; results reveal a statistically significant (95-99%) linear relationship between the ibuprofen loading capacity and its free energy in solution calculated with the PCM solvation model. In addition, useful relationships between loading capacity and dielectric constant and molecular size of the solvents are established.

New nanomaterials for consolidating stone.

A novel sol-gel synthesis, in which a surfactant acts to make the pore size of the gel network more coarse and uniform, is shown to provide an effective alternative for the consolidation of stone. The new mesoporous silica avoids the main inconvenience of current commercial consolidants, which is their tendency to crack inside the pores of the stone. Since the cracking of xerogels is a well-known drawback of the sol-gel process, the synthesis presented here can be extended to other applications. Finally, preliminary studies of the effectiveness of the novel surfactant-templated sol in consolidating a typical biocalcareous stone are also discussed.

Structure of hybrid colloid-polymer xerogels.

Crack-free monolithic gels were prepared from different mixtures of colloidal silica with a sol solution containing tetraethoxysilane, under powerful ultrasonic agitation (sonosol). Recently, information on the structure of these gels, inferred from N2 adsorption and mercury intrusion porosimetry, was presented. In the present paper, these data were used to construct structural models of the gels using Monte Carlo calculations on the basis of random close packing (RPC) premises. In addition, the structure of gels under study was investigated by transmission and scanning electron microscopy. The material can be described as a composite in which the sonogel is the matrix and the colloid particles the reinforcing phase. For low colloid content, the colloid forms discrete clusters, and the main structural characteristic of sonogels, i.e., a network of uniformly sized particles of approximately 3-4-nm radius, remains unmodified. However, for high colloid silica content, a multimode distribution appears, the structure is discontinuous, and only colloid aggregates larger than 100 nm are observed. For medium colloid content, aggregates of approximately 50-100 nm can be seen, but the sonogel structure extends throughout the whole material. By the processing method and election of a suitable precursor concentration, it is possible to design the composite for specific purposes.