Initially the soy hulls (SH) were treated with a sodium hydroxide aqueous solution of 2% (w/w) for 4h at 100◦C under mechanical stirring and then washed several times with distilled water until the alkali was completely removed, and finally dried at 50 ◦ C for 12 h in an air-circulating oven. After this treatment, the fibers were bleached with a solution made up of equal parts (v:v) of acetate buffer (27 g NaOH and 75 mL glacial acetic acid, diluted to 1 L of distilled water) and aqueous sodium chlorite (1.7 wt% NaClO2 in water). This bleaching treatment was performed at 80 ◦ C for 4 h. The bleached fibers were washed repeatedly in distilled water until the pH of the fibers became neutral and subsequently dried at 50 ◦ C for 12 h in an air-circulating oven. The fiber content throughout these chemical treatments was about 4–6% (w/w) (de Rodriguez et al., 2006; Siqueira et al., 2010). The material which resulted after the purification was the treated soy hulls (TSH).
2.2. Extractionofcellulosenanocrystals
After the chemical treatment was completed, the TSH were milled with a blender and passed through a 35-mesh screen and then used for the extraction of nanocrystals by acid hydrolysis. The hydrolysis was performed at 40 ◦ C for 30 min or 40 min under vig- orous and constant stirring. For each gram of TSH we used 30 mL of H2SO4 64% (w/w). Immediately following the hydrolysis, the sus- pension was diluted 10-fold with cold water to stop the hydrolysis reaction, and centrifuged twice for 10 min at 7000 rpm to remove the excess acid. The precipitate was then dialyzed with tap water to remove non-reactive sulfate groups, salts and soluble sugars, until the neutral pH (5–7 days) be reached. Subsequently, the result- ing suspension of dialysis process was treated using a disperser type UltraTurrax for 5 min at 20,000 rpm and sonicated for 5 min. The colloidal suspension was stored in a refrigerator at 4 ◦ C with the addition of some drops of chloroform to avoid any bacterial growth. The cellulose nanocrystals or whiskers of soy hulls were labeled WSH30 or WSH40 depending on the time of extraction.
2.3. Gravimetricanalysis
The hydrolysis yield and the WSH concentration in the final sus- pension were calculated drying an aliquot of the suspension with a known volume at 105 ◦ C for 12 h in an air-circulating oven.