Supramolecular self-assembly of linear oligosilsesquioxanes on mica – AFM surface imaging and hydrophilicity studies

Abstract

Linear oligomeric [2-(carboxymethylthio)ethylsilsesquioxanes] (LPSQ-COOH) adsorb spontaneously on muscovite mica and form smooth, well-ordered lamellar structures at the liquid-solid interface. Side carboxylic groups, having donor-acceptor character with regard to hydrogen bonds, are engaged both in multipoint molecule-to-substrate interactions and intermolecular cross-linking. The unique arrangement of silsesquioxane macromolecules, with COOH groups situated at the interface with air, produces highly hydrophilic surfaces of good thermal and solvolytic stability. Supramolecular assemblies of LPSQ-COOH were studied using atomic force microscopy (AFM), angle-resolved X-ray photoelectron spectroscopy (ARXPS) and attenuated total reflectance (ATR) FTIR spectroscopy. Comparative height profile analysis combined with ATR-FTIR studies of the spectral regions characteristic of carboxylic groups and C1s core level envelope by XPS confirmed specific interactions between LPSQ-COOH and mica.

Nanostructured surfaces by supramolecular self-assembly of linear oligosilsesquioxanes with biocompatible side groups

“…The structure and reactivity of LPSQ-COOH/X makes the polymers suitable for the formation of planar PSAMs on various reactive surfaces. Side carboxylic groups in LPSQ-COOH/X allow for a very efficient polymer anchoring on the surface due to both multipoint ionic substrate–adsorbate interactions and adsorbate–adsorbate hydrogen bonding [ 37 ]. The formation of ordered SAMs and PSAMs at the liquid–solid interface can occur only if it is energetically allowed by entropy–enthalpy compensation [ 51 – 53 ].…”

“…1 ) of thickness that correlates well with the estimated macromolecule width. This was estimated to be about 1.6 nm, as calculated for the structure of a LPSQ-COOH oligomer constructed on HyperChem platform and modelled in vacuum using a molecular mechanics force field MM+ method (Polak–Ribiere/conjugate gradient optimization algorithm) and a semi-empirical PM3 method (single point energy calculations [ 37 ]. However, macromolecules of LPSQ-COOH/GSH, LPSQ-COOH/Cys-HCl and LPSQ-COOH/NAC do not easily extend parallel to the mica substrate.…”

“…The surface free energy (γ S ) of each studied PSAM sample was determined by measuring the contact angle of water and glycerol as reference liquids (sessile drop technique and Owens–Wendt geometric mean Equation S1 described in Supporting Information File 1 [ 55 ]). We have previously reported [ 37 – 38 ] that the very good wettability of mica coated with LPSQ-COOH is a result of the structure of adsorbed nanolayers and the fact that carboxylic groups attached to oligomers of LPSQ-COOH adopt a specific conformation at the interface with air. We have analysed the wettability of samples covered with other LPSQ-COOH/X schemes ( Fig.…”

“…We have recently found that linear oligosilsesquioxanes functionalized with 2-(carboxymethylthio)ethyl side groups (LPSQ-COOH) can adsorb from their solutions and spontaneously form well-ordered and stable, PSAM-type, 2D nanolayers at the surface of muscovite mica, which renders the surface exceptionally hydrophilic [ 37 – 38 ]. Muscovite mica, chosen as a substrate for the present study, is a layered aluminosilicate [KAl 2 (Si 3 AlO 10 )(OH) 2 ] that exhibits interesting surface properties and chemical specificity.…”

“…The character of the interactions between the oligomers and the substrate also defines the structure of the assemblies of LPSQ-COOH on mica [ 37 ]. It was thus of interest to study if the morphology of the surface layer of PSAMs and its physicochemical properties can be changed by alteration of the mechanism of adsorption on mica.…”