title: Shell structure of natural rubber particles: evidence of chemical stratification by electrokinetics and cryo-TEM.
abstract: The interfacial structure of natural rubber (NR) colloids is investigated by means of cryogenic transmission electron microscopy (cryo-TEM) and electrokinetics over a broad range of KNO3 electrolyte concentrations (4-300 mM) and pH values (1-8). The asymptotic plateau value reached by NR electrophoretic mobility (μ) in the thin double layer limit supports the presence of a soft (ion- and water-permeable) polyelectrolytic type of layer located at the periphery of the NR particles. This property is confirmed by the analysis of the electron density profile obtained from cryo-TEM that evidences a ∼2-4 nm thick corona surrounding the NR polyisoprene core. The dependence of μ on pH and salt concentration is further marked by a dramatic decrease of the point of zero electrophoretic mobility (PZM) from 3.6 to 0.8 with increasing electrolyte concentration in the range 4-300 mM. Using a recent theory for electrohydrodynamics of soft multilayered particles, this "anomalous" dependence of the PZM on electrolyte concentration is shown to be consistent with a radial organization of anionic and cationic groups across the peripheral NR structure. The NR electrokinetic response in the pH range 1-8 is indeed found to be equivalent to that of particles surrounded by a positively charged ∼3.5 nm thick layer (mean dissociation pK ∼ 4.2) supporting a thin and negatively charged outermost layer (0.6 nm in thickness, pK ∼ 0.7). Altogether, the strong dependence of the PZM on electrolyte concentration suggests that the electrostatic properties of the outer peripheral region of the NR shell are mediated by lipidic residues protruding from a shell containing a significant amount of protein-like charges. This proposed NR shell interfacial structure questions previously reported NR representations according to which the shell consists of either a fully mixed lipid-protein layer, or a layer of phospholipids residing exclusively beneath an outer proteic film.
authors.affiliation: Laboratoire Interdisciplinaire des Environnements Continentaux (LIEC), Université de Lorraine , UMR 7360, 15 avenue du Charmois, Vandœuvre-lès-Nancy, F-54501, France.
authors.forename: Christophe N
authors.initials: CN
authors.lastname: Rochette
authors.forename: Jérôme J
authors.initials: JJ
authors.lastname: Crassous
authors.forename: Markus
authors.initials: M
authors.lastname: Drechsler
authors.forename: Fabien
authors.initials: F
authors.lastname: Gaboriaud
authors.forename: Marie
authors.initials: M
authors.lastname: Eloy
authors.forename: Benoît
authors.initials: B
authors.lastname: de Gaudemaris
authors.forename: Jérôme F L
authors.initials: JF
authors.lastname: Duval
mesh_terms: D020285:Cryoelectron Microscopy; D055664:Electrochemical Techniques; D007700:Kinetics; D015394:Molecular Structure; D010316:Particle Size; D010743:Phospholipids / Q000737:chemistry; D011506:Proteins / Q000737:chemistry; D012408:Rubber / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D010743:Phospholipids; D011506:Proteins; D012408:Rubber