Bacterial cellulose nanocrystals: impact of the sulfate content on the interaction with xyloglucan

In the present work, bacterial cellulose nanocrystals (BCN) were prepared with three distinct sulfate contents, namely 0, 0.42 and 0.65 % OSO3-, by using HCl (BCN-HCl), a mixture of HCl and H2SO4 (BCN-HCl/H2SO4) and H2SO4 (BCN-H2SO4) as hydrolytic media, respectively. BCN were characterized by means of elemental analysis, X-ray diffraction, atomic force microscopy (AFM) and zeta potential measurements. All BCN samples had crystallinity indices of about 82 %, about 10 % higher than the original BC. The zeta potential values determined for BCN-HCl, BCN-HCl/H2SO4 and BCN-H2SO4 amounted to -(5 +/- 1), -(40 +/- 1) and -(46 +/- 1) mV, respectively. The assembly between BCN-HCl, BCN-HCl/H2SO4 or BCN-H2SO4 and XG extracted from Tainarindus indica seeds was investigated by means of Freundlich isotherms, AFM, contact angle measurements, dynamic light scattering, ellipsometry and quartz crystal microbalance measurements. The adsorption experiments suggest that sulfate substitution on BCN surface as low as 0.65 % OSO3- impairs the original interaction with XG, thereby generating unstable layers with subsequent desorption. In contrast, the 0.42 % OSO3- samples presented favorable interactions with XG and exhibited highly improved colloidal stability compared to 0 % OSO3- samples. Films of XG adsorbed onto BCN with 0.42 % OSO3- were stable, homogeneous, and had low roughness and thickness. This is the first report showing that sulfate content on BCN surface can be tailored to promote high colloidal stability and sufficient interaction with XG.