Silica Nanocasts of Wood Fibers:  A Study of Cell-Wall Accessibility and Structure

Abstract

The porosity and the available surface area of a lignocellulosic fiber can influence the accessibility and reactivity in derivatization and modification reactions because the porous cell-wall network determines the upper size limit for molecules that can penetrate and react with the interior of the wall. To obtain information concerning the accessibility of the porous cell wall of wood fibers, surfactant-templated sol-gel mineralization has been examined. Wood and kraft pulp samples of Norway spruce were impregnated with a silica sol-gel and subsequently heated (calcined) and transformed into structured mesoporous silica. Microscopy studies (environmental scanning electron microscopy, transmission electron microsopy, TEM) on the silica casts showed that the three-dimensional architecture of the wood and pulp fiber cell wall was revealed down to the nanometer level. Image analysis of TEM micrographs of silica fragments from the never-dried pulp revealed complete infiltration of the cell-wall voids and microcavities (mean pore width 4.7 +/- 2 nm) by the sol-gel and the presence of cellulose fibrils with a width of 3.6 +/- 1 nm. Cellulose fibrils of the same width as that shown by image analysis were also identified by nitrogen adsorption measurements of the pore size distribution in the replicas.

The implication of chemical extraction treatments on the cell wall nanostructure of softwood

“…However, this study was primarily focussed on the structure of pulp fibres. On the other hand, difficulties in the observation of cellulose fibril aggregates in native, untreated cell walls have been reported (Daniel 2006) and other studies exist that report structures consisting of single fibrils in samples prepared for TEMobservations (Stoll and Fengel 1977;Deshpande et al 2006;Persson et al 2004).…”

Thermal modification of Southern pine combined with wax emulsion preimpregnation: effect on hydrophobicity and dimensional stability

“…In contrast, the wood samples impregnated with wax alone showed a rapid ASE decrement from 77.4% to 23.3% over the 90-min immersion irrespective of the higher initial ASE. The average particle size of the paraffin emulsion in this study is 220 nm, which is much larger than the average diameter of cavities in wood cell wall (2-12 nm) (Persson et al 2004), that is, the paraffin does not penetrate into the cell wall. Thus, the wax impregnation still allows unhindered diffusion of vapor and the formation bound water layers, which explains the gradual decrease in dimensional stability.…”

Silica replication of the hierarchical structure of wood with nanometer precision

“…Some of the authors of this study have recently been able to synthesize for the first time a ceria/zirconia ceramic replica with nanometer precision this way. 29 Other workers were able to produce silica replicates of wood, but they were either rather vague on the actual nature of the samples produced, suggesting that no monolithic samples were produced 30 or did claim, but not conclusively show the replication of the fibril structure of the templates used. 31 Nanometer-scale replication is, however, thought to be the key to novel functional properties based on hierarchical structuring, such as for instance actuators 10 or optical devices.…”