Controlling Infiltration Pressure of a Nanoporous Silica Gel via Surface Treatment

While it has been a well-known fact that a certain amount of energy would be dissipated as a liquid flows in a channel, not until recently with the development of nanoporous technology did this phenomenon receive wide attention for energy absorption applications. For instance, it was noticed that as water moved from compressive parts of a hydrophilic nanoporous silica to tensile parts, considerable energy dissipation took place. A more attractive way to dissipate energy, however, is based on the capillary effect, for which the nanoporous phase must be lyophobic. As a nonwetting liquid is forced to enter a nanoporous material, external work is transformed to the excess solid–liquid interfacial tension. Since in a number of systems this process is irreversible, it can be employed for developing advanced protection or damping structures, e.g., liquid armors and liquid bumpers. A key factor dominating the system performance is the specific surface area, which, for most of nanoporous materials, is in the range of 100–2000m/g, six to eight orders of magnitude larger than in bulk materials, resulting in a superior energy absorption efficiency. Another important design variable of the nanoporous energy absorption system (NEAS) is the degree of lyophobicity, or, if the liquid phase is water based, the degree of hydrophobicity, which is measured by the excess solid–liquid interfacial tension, 1⁄4 sl ð s þ lÞ, where sl is the effective solid–liquid interfacial tension at nanopore surface, and s and l are the effective surface tensions of the solid and the liquid, respectively. On the one hand, a higher degree of hydrophobicity would lead to a higher energy absorption efficiency. On the other hand, must be in an appropriate range so that the working pressure is harmless; otherwise the system would be too ‘‘rigid,’’ causing blast-lung type problems. That is, the degree of hydrophobicity must be controlled in a relatively broad range to meet various functional requirements. In a previous study, it was confirmed that using chemical admixtures can either increase or decrease the infiltration pressure. However, the addition of admixtures would cause heterogeneous liquid structures, making it difficult to develop reliable systems, especially when the system must work in a broad temperature range. Therefore, it is desirable to directly control the pore surface structure using surface treatment methods. Compared with ordinary surface treatment, the modification of properties of inner nanopore walls is relatively difficult owing to the poor surface accessibility. The molecular size of the functional agents must be much smaller than the nanopores, and the diffusion process might take a long time. On the other hand, one advantage of NEAS is that leaching is not a concern. Since many protection systems, such as car bumpers and body armors, work only under the first loading, even if the functional groups are deactivated as a high pressure is applied, it would not affect the energy absorption performance. In order to validate the above considerations, a Davisil236845 nanoporous silica gel was investigated experimentally. The average pore size was 16.2 nm; the specific pore volume was 1.07 cm/g; and the specific surface area was 305m/g. The as-received material was in powder form, with the particle size around 250mm. The surface treatment was performed in a 100-mL round bottom flask, by the technique discussed by Lim and Stein. Prior to the treatment, the silica gel was vacuum dried at 100 C for 24 h. Immediately after the flask was taken out of the oven, a glass topper was used to seal it. Then, 40mL of Sigma-244511 dry toluene was injected into the flask. While the solution was stirred gently, 1.0mL of Fluka-92360 chlorotrimethylsilane was injected. The light yellow mixture was stirred for 10min at ambient temperature and then refluxed in a hot mantle at 90 C. The treatment time was in the range of 1–48 h. A drying tube was attached to the reflux apparatus to minimize the influence of moisture. The silica gel was vacuum filtered, washed by dry toluene, and then vacuum dried at 100 C for 24 h to remove the residual chlorotrimethylsilane. Finally, the sample was washed by distilled water so as to hydrolyze remaining chloride groups and dried again in a vacuum oven. A stainless steel cylinder was sealed by a steel piston with reinforced gasket, which contained 0.5 g of surface-treated silica gel and 7 g of distilled water. The piston was compressed by a type 5583 Instron machine at a constant rate of 0.5mm/min into the cylinder, applying a quasi-hydrostatic pressure on the liquid phase. When the pressure exceeded 50MPa, the piston was moved out at the same speed. The loading–unloading cycles were repeated for a few times. The measured sorption isotherm