Abstract
The structural properties of halloysite/biopolymer aqueous mixtures were firstly investigated by means of combining different techniques, including small-angle neutron scattering (SANS), electric birefringence (EBR) and fluorescence correlation spectroscopy (FCS). Among the biopolymers, non-ionic hydroxypropylcellulose and polyelectrolytes (anionic alginate and cationic chitosan) were selected. On this basis, the specific supramolecular interactions were correlated to the structural behavior of the halloysite/biopolymer mixtures. SANS data were analyzed in order to investigate the influence of the biopolymer adsorption on the halloysite gyration radius. In addition, a morphological description of the biopolymer-coated halloysite nanotubes (HNTs) was obtained by the simulation of SANS curves. EBR experiments evidenced that the orientation dynamics of the nanotubes in the electric field is influenced by the specific interactions with the polymers. Namely, both variations of the polymer charge and/or wrapping mechanisms strongly affected the HNT alignment process and, consequently, the rotational mobility of the nanotubes. FCS measurements with fluorescently labeled biopolymers allowed us to study the aqueous dynamic behavior of ionic biopolymers after their adsorption onto the HNT surfaces. The combination of EBR and FCS results revealed that the adsorption process reduces the mobility in water of both components. These effects are strongly enhanced by HNT/polyelectrolyte electrostatic interactions and wrapping processes occurring in the halloysite/chitosan mixture. The attained findings can be useful for designing halloysite/polymer hybrids with controlled structural properties.
Highlights
Paper supramolecular functionalization of halloysite with oppositely charged PNIPAAm polymers, such as amine-terminated PNIPAAM and PNIPAAM/methacrylic acid copolymer.[23]
We investigated the structural behavior of aqueous mixtures composed of halloysite nanotubes (HNTs) and differently charged biopolymers, such as cationic chitosan, anionic alginate and non-ionic hydroxypropylcellulose
The simulation of small-angle neutron scattering (SANS) curves by a hollow cylinder model evidenced that the biopolymer coated nanotubes possesses similar geometrical features as those previously observed for pure HNT
Summary
Paper supramolecular functionalization of halloysite with oppositely charged PNIPAAm polymers, such as amine-terminated PNIPAAM and PNIPAAM/methacrylic acid copolymer.[23]. As a consequence of the different acid–base equilibria of alumina and silica groups, the HNT inner and external surfaces are positively and negatively charged, respectively, within an extended pH interval and, in addition, the charge conditions can be tuned by pH.[28] Rheological measurements evidenced that HNT aqueous suspensions can form a lyotropic liquid crystalline phase depending on the pH conditions.[29] The HNT dispersions exhibited stronger shear-thinning behavior by the addition of microcrystalline cellulose.[30] The HNT surfaces can be selectively modified by ionic molecules through electrostatic interactions.[20] The adsorption of cationic alkyltrimethylbromides onto the halloysite outer surface allows fabrication of inorganic reverse micelles,[31] which were used to synthesize alginate-based nanohydrogels within the HNT lumen.[32] The attractions between anionic surfactants and the positively charged internal surface generated functionalized nanotubes with a hydrophobic cavity.[33,34] As evidenced by SANS studies,[33] the structural organization of the adsorbed surfactants affects the hydrophobization degree of the modified halloysite lumen. The immobilization of several enzymes within the HNT lumen was controlled by pH conditions, which influence the electrostatic forces occurring between proteins and halloysite surfaces.[35]
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