Surface nanostructure of Hevea brasiliensis natural rubber latex particles

Natural rubber (NR) from the Hevea brasiliensis tree is the main commercial feedstock for rubber and latex dipping industries. NR latex particles in a matured commercial latex concentrate are stabilized by charged groups derived from proteins, long-chain fatty acid soaps and polypeptides adsorbed on the particle surface. The colloidal stability of the latex is extremely sensitive to pH as well as to the ionic environment of the dispersing medium. Previous work on particle microelectrophoresis has shown that the charge on the particle surface is derived mainly from carboxylic groups of long chain fatty acids (ca. 86%). Freshly collected field latex from the tree, on the other hand, is believed to be stabilized by proteins and phospholipids before hydrolysis sets in upon leaving the tree. The exact arrangement of the adsorbed proteins and phospholipids and the thickness of this adsorbed layer have yet to be successfully determined. The present study has been focused on unraveling the arrangement of these proteins and phospholipids on the particle surface of a freshly tapped NR latex. In order to visualize these molecules, they were imaged using phase contrast atomic force microscopy (AFM). Moreover, the molecules in the particle were labeled with fluorescent Rhodamine B (RB) and uncharged fluorescein-5(6)-isothiocyanate (FITC), and monitored by confocal laser scanning microscopy (CLSM) thereby permitting an in situ observation of their locations on and inside the particles. The selective labeling of the proteins and phospholipids with RB and FITC in protein-free as well as in protein-and-lipid-free latex particles permitted an unequivocal determination of their presence both on the surface and in the interior of the particles. An indentation study of the surface layer of the original untreated NR particle was also carried out to confirm the hypothesis of a core–shell structure with a mixed layer of proteins and phospholipids surrounding the hydrophobic core of polyisoprene particles. According to the obtained results, the surface of the particles in freshly tapped latex is distinctly different from that of a matured latex concentrate.