The present work is focused on better understanding of the interfacial interactions of SBA-15 mesoporous silica particles with flax fibers. In order to overcome the inherent complexity of flax fiber surface composition we have prepared model polysaccharide surfaces representing the main component of the flax fibers, e.g. cellulose, polygalacturonic acid (PGUA), and xyloglucan (XG) with thicknesses of about 200 nm, 100 nm, and 110 nm, respectively. The ξ-potential measurements of both silica and polysaccharides were performed in aqueous solutions as a function of pH and ionic strength. ξ-potential, AFM and SEM results supported the important role of electrostatic interactions in the silica adsorption on polysaccharide surfaces, since silica adsorption increased remarkably with ionic strength. The adsorption density of the SBA-15 onto the various polysaccharides was Cellulose > PGUA > XG, and the maximum was observed at pH = 4. Urea used as hydrogen bonds breaker reduced significantly the adsorption of SBA-15 on the polysaccharide surfaces, which highlighted the significant contribution of hydrogen bonding in the adsorption process. It was observed that most adsorbed SBA-15 particles were resistant to ultrasonic washing, which revealed their strong irreversible adsorption. Finally, direct adsorption experiments on both raw and treated real flax fibers yielded results consistent with those of model surfaces showing the important role of the surface fibers treatments on the improvement of the interfacial adhesion of the silica particles with flax fibers. The remarkable affinity of the SBA-15 particles with treated flax fibers is encouraging to design superinsulators composites with tuneable mechanical performances.
Developments of innovative efficient materials presenting high thermal insulation performances have recently attracted a great deal of interest because of their remarkable potential in building sector. In this context, a variety of inorganic materials have been used in designing of new materials with excellent physical and chemical struc- tures due to their very high porosity, nano-scale pore sizes and lightweight. Such promising kinds of materials are called aerogels well-known as superinsulators. Indeed these systems yield a significant decrease of solid and gaseous thermal conductivity due to air confinement effect [
Most of these inorganic superinsulating aerogels deal with silica materials that consist of a cross-linked in- ternal structure of silicon oxide chains with a large number of air-filled pores. However, the main disadvantage of these materials is nevertheless, their low mechanical strength resulting from their low mass density. The silica aerogels fragility restricts considerably their use in conventional industrial applications. In order to improve the mechanical resistance of these ultraporous structures, extensive efforts are being made to develop organic-based aerogels. Among the various proposed materials, one can cite resorcinol/formaldehyde aerogels [
Considering the increasing environmental awareness, the biggest challenge of thermal superinsulator mate- rials is not only their improving energetic behavior, but also their environmentally-friendly property which is of great interest from both economic and ecologic viewpoints [
In this context, we aimed at designing new porous material based on flax fibers coated with SBA-15 meso- porous silica particles. The interest of SBA-15 lies in its high specific surface area, ordered pore systems, and uniform and nano-scale pore diameters. In addition to the physico-chemical interest, the particles are synthe- sized via the well-known sol-gel method classified as soft chemistry process, which offers great potential in eco-friendly insulation applications [
Therefore, understanding the effects of fibers characteristics and their interfacial interactions with the silica particles is essential to the improvement and optimization of both the mechanical properties and the thermal insulating performances of the fiber-silica composite. However, the very complex composition and heterogeneity of the flax fibers, as reported above, make difficult the understanding of the adsorption mechanisms on real fiber. In order to make simplify the system we have represented the fiber by model surfaces of its main polysaccharides, as recently reported by our group in the study of the adhesion of PLA with flax fibers [
Microcristalline cellulose with an average size of 20 µm (Cellulose, Aldrich), polygalacturonic acid extracted from orange (PGUA, Sigma-Aldrich), it’s the major component of pectin, Xyloglucan from tamarind seed (XG, Megazyme), tetraethyl orthosilicate (TEOS, 98%, Aldrich), triblock copolymer P123 (PEO20PPO70PEO20), sodium chloride (NaCl, Sigma-Aldrich), calcium chloride (CaCl2, Sigma-Aldrich), urea (Sigma-Aldrich), ammonia (28%, Acros), absolute ethanol (Fisher Scientific), sodium hydroxide (NaOH, 99%, Sigma-Aldrich), hydrochloric acid (HCl, 37 wt%, Sigma-Aldrich) were used as received without further purification. The lithium chloride (LiCl, Aldrich) was dried at 200˚C for 1 day, and the N,N- dimethylacetamide (DMAc, Sigma) was freshly dried at 110˚C prior to use.
The used flax fibers (Marylin variety) are grown in Neubourg (France) in 2009. 18 MΩ Milli-Q (Millipore) water was used in the preparation of all the aqueous solutions.
The amount of silica adsorbed amount is determined using ImageJ software (NIH USA).
Spherical silica nanoparticles were prepared using Stöber sol-gel procedure. A mixture of 14 mL of TEOS and 56 mL of ethanol was added to a solution of 144 mL of ethanol, 18 mL of water and 7 mL of ammonia, and the mixture was stirred at room temperature under nitrogen atmosphere for 2 h. The resulting nanoparticles were thoroughly washed with ethanol via three repeated cycles of centrifugation/dispersion, and then dried in oven at 110˚C overnight.
SBA-15 hexagonal mesoporous silica particles were prepared using non ionic surfactant, Pluronic P123 as structured agent; typically 4 g of P123 was dissolved in a mixture of 19.5 ml HCl 12 M and 127 ml of distilled water. After 3 h of vigorous stirring at 40˚C, 8.62 g of TEOS were added under stirring, and after 5 min of TEOS addition the stirring was stopped and the solution was maintained at 40˚C for 2 h. Then the product was filtered and thoroughly washed with distilled water and then dried in oven at 70˚C for 48 h. In order to liberate the porosity, by removing the template, the as prepared particles were calcinated in a muffle furnace at 500˚C for 4 h.
The model surfaces preparation is a two-step process, the dissolution of the polysaccharides and then deposition of these solutions on freshly cleaved mica surfaces cut to 1 cm squares, as reported previously by Raj et al. [
The cellulose, XG, and PGUA model surfaces were incubated separately on 10 mL of 3 g·L−1 silica suspension with agitation using a magnetic stirrer during 2 h at room temperature. The suspensions were adjusted to the de- sired pH ranging from 2 to 8, by adding aliquots of HNO3 (1 M) or NaOH (1 M) at the beginning of the experi- ment. Afterward surfaces were washed with Milli-Q water under sonication, and dried under air. In order to study the effect of ionic strength on silica particles adsorption, experiments were conducted in presence of CaCl2 at four different concentrations varying from 5 × 10−3 to 5 × 10−2 M.
The flax fibers have been treated with different solvents, as soxhlet, water and sodium hydroxide. All these treatments procedure were described in detail in literature [
Morphology, roughness and thickness of polysaccharide model surfaces, and their surface adsorption of Stöber silica nanoparticles were accomplished by tapping mode AFM under ambient conditions (23˚C and 56% RH) using a Nanoscope IIIa multimode scanning probe microscope from Veeco, USA.
Adsorption and dispersion of SBA-15 nanoparticles on the different polysaccharide surfaces were investigated using a Jeol JSM 6460LV scanning electron microscope. All the samples were sputter-coated with gold before analysis.
The room temperature static contact angle of water on the different polysaccharide films was determined with a Digidrop GBX. The contact angles were measured after 20 ms water droplet deposition.
The zeta potentials measurements of cellulose, PGUA, XG, and silica nanoparticles at pH 2 - 8 rang were de- termined by dynamic light scattering (DLS) at 25˚C using a Malvern Zetasizer Nano ZS instrument. The solu- tions were prepared dissolving or dispersing the polysaccharides and the silica particles in Milli-Q water at a concentration of 1 g/L, using ultrasonic bath during 2 h, then magnetic stirrer during 15 h. Each data value is an average of three measurements.
Investigation of the relations between electric charges is essential for understanding the mechanism of silica par- ticles interaction with flax fiber biopolymers. The surface electric charge can be assessed by zeta potential, known as a measurable parameter related to the charge and electric double layer of surfaces in aqueous solutions. The zeta potential can also be described as the electrical potential at a hypothetical “slip plane” adjacent to a charged surface [
where ξ is the zeta potential (mV), ε, µE and η are the dielectric constant of the solution, the electrophoretic mo- bility and viscosity, respectively.
It is noteworthy that the zeta potential is not only used to determine the surface charge of colloidal systems especially silica particles, but can be extended to the macromolecules materials [
The surface electric charge on aqueous solutions of the studied polysaccharides (
obtained at high acidic pH. It can also be observed that the surface charge became strongly negative with in- creasing pH, except for XG for which, the zeta potential was only slightly modified versus pH. The values ob- tained for cellulose and PGUA indicate that the surface of the materials present a sufficient number of anionic charges (
AFM surface morphology and height profiles images (
The properties of polysaccharide surfaces and silica particles are strongly pH-dependent due to the dissociation of OH-groups of hydroxyl, carboxylic acid, and silanol functions of cellulose and XG, PGUA, and SiO2, respec- tively. Several works reported the pH of isoelectric point at 25˚C of silica that ranges from 1.7 to 3.5 and that is close to 3 for most of the polysaccharides [
The effect of pH on the adsorption of silica nanoparticles onto the polysaccharide thin films was evaluated at pH ranging from 2 to about 8 (lower and higher pH values were not studied because of the possibility of dissolu- tion of silica particles) via AFM and SEM characterizations (
the obtained results are given as adsorbed quantity per 5 µm2 of polysaccharide surface versus pH as illustrated in
It clearly appears that pH plays an important role in immobilization of SBA-15, due to the variation of its surface chemistry with pH. As can be observed from both SEM and AFM results, cellulose films exhibited higher ability to bind and to disperse homogeneously on the surface the silica particles than PGUA and XG films, respectively, at all pH values (
An interesting result that can be observed from the AFM and SEM images is that at pH = 2 and 4, the par- ticles are densely packed and distributed on the film surfaces. This means that the attraction between silica par- ticles and their interactions with polysaccharides surfaces is enhanced close to the pH of isoelectric point. Indeed
To explore the role of electrostatic interactions on adsorption of silica particles at the surface of the different polysaccharides, tests were carried out at pH 4 in the presence of CaCl2 (0.005 M to 0.05 M), using an initial SBA-15 concentration of 1 g/L.
Adsorption capacities of SBA-15 calculated from SEM images at different salt concentrations are shown in
To check the contribution of the hydrogen bonding in the silica particles adsorption behaviour on the polysac- charide films, we have studied this adsorption efficiency in the presence of urea by immersing the polysaccha- ride surface-coated SBA-15 in a 1 M urea solution. It is known that urea can be used as a strong hydrogen bond breaker [
The obtained result emphasizes the important role played by hydrogen bonding in the adhesion of the SBA-15 particles on the polysaccharide films.
Cellulose PGUA XG
To evaluate the different flax fibers surface treatments on the adsorption of SBA-15 particles, experiments were performed using untreated and three various treated (soxhlet, water, and NaOH) bundles of fibers at pH 4. SEM images shown in
The PGUA, XG and cellulose thin films investigated in this work nicely model these three treatments. SBA- 15 adsorption on the flax fibers is in excellent agreement with those obtained on polysaccharide model surfaces.
Differences on the surface chemical composition of the fibers due to the different treatments are of crucial role in the interfacial adhesion of the SBA-15 with the flax fibers. Indeed, the highest affinity of the two materials is obtained in the case of NaOH-treated fibers known for their cellulose rich surface in good agreement with the results of the adsorption on the model surfaces. The adsorption of SBA 15 ranked from PGUA < XG < Cellu- lose.
This study aimed at identifying and understanding the nature of interactions between SBA-15 mesoporous silica particles and flax fibers. Because of the complexity of the flax fibers system, we have studied these interactions independently on the main constituents of the flax fibers representing them by model surfaces, by following the adsorption of the silica particles onto the polysaccharide model surfaces. PGUA and XG surfaces exhibited smooth topography with RMS roughness of 0.7 nm, while cellulose film was rougher with an RMS of 4 nm. All the model surfaces show a hydrophilic character, owing to the strong interaction between the hydroxyl groups of polysaccharides and water molecules. The magnitude of ξ-potential values of both silica and polysaccharides in- creased with increasing pH, whereas they decreased with increasing ionic strength. AFM and SEM results demonstrated that silica adsorption density varied significantly in pH with the highest at pH 4, while it increased with increasing of the Ca2+ amount. Silica was adsorbed preferentially onto cellulose, followed by PGUA, then XG surfaces. These results support the important role of electrostatic interactions in the adsorption of SBA-15 onto polysaccharide surfaces and the high contribution of hydrogen bonding. Finally, results of adsorption of SBA-15 directly on both raw and treated real flax fibers are consistent with those obtained with model surfaces. SEM images have shown that the NaOH-treatment of the surface fibers improve strongly their adhesion with SBA-15 mesoporous silica particles.