Influence of the Sophorolipid Molecular Geometry on their Self‐Assembled Structures

Abstract

The rare earth‐rich intermetallic cadmium compounds RE10TCd3 (RE = Y, Tb – Tm, Lu; T = Fe, Co, Ni, Ru, Rh, Pd) were synthesized by induction melting of the elements in sealed niobium ampoules. These cadmium intermetallics crystallize with an ordered variant of the hexagonal Co2Al5 type, space group P63/mmc. The three crystallographically independent rare earth atoms lie on the aluminum sites, while the transition metal and cadmium atoms are ordered on the two cobalt positions. The structures of Y10Co1.103Cd2.897, Y10NiCd3, Y10RuCd3, Y10PdCd3, Er9.92FeCd3.08, and Er9.82NiCd3.18 were refined from single‐crystal X‐ray diffraction data. Some of the crystals showed small degrees of RE/Cd, respectively T/Cd mixing. The striking structural building units are transition metal centered T@RE6 trigonal prisms (TP) that are condensed to empty RE6 octahedra (O), building infinite …TP–O–O… rows in c direction. The second motifs are slightly flattened RE3@Cd6RE16 icosahedra. The latter are condensed via common triangular faced along the c axis. Each …TP–O–O… row is enrolled by three rows of condensed RE3@Cd6RE16 icosahedra in the motif of a hexagonal rod packing.

Surface-induced assembly of sophorolipids

“…The former can be explained by the ability of sophorolipids to self-assemble into crystalline fibers, as described by previous authors 55,58 and recently explained by us; 63 the latter strongly recalls dewetting phenomena of surfactant solutions on surfaces, which depend on the autophobic effect, 31 probably driven by the local formation of a self-assembled lipid monolayer on the surface. 32,36 In the following paragraphs we tried to rationalize both possibilities.…”

“…60 In some cases, acidic sophorolipids form twisted ribbons under acidic pH conditions. 55,58 However, this was not the case for the compound used in the present study, which only formed a stable micellar solution. In a recent study, 66 we have given the conditions for a micellar vs. fibrillar stability of sophorolipids in water at acidic pH.…”

“…The formation of fibers, which may seem quite odd if one considers the stable micellar phase in solution, is actually not surprising as acidic sophorolipids have shown to form both micelles 56,62 and fibers in water under acidic pH conditions. 55,58 At the moment, the conditions that favor one phase over the other were only partially known 66 and a tentative explanation at the molecular level in terms of surface-sophorolipid interactions favouring fiber formation would be highly speculative. However, we must consider that similar effects have been described for amyloid-b peptides on mica and can undergo a micelle-to-fibril transition as a function of number density of the peptides on the surface, which depends on the deposition and diffusion rates of the peptides.…”

“…[55][56][57] It has also been established that the structure of the molecules greatly influenced the self-assembly. 58 Acidic monounsaturated sophorolipids form ellipsoidal micelles in water in a very broad pH range (from pH 11 to pH 3), 59 although at basic pH, formation of nanoplatelets is also observed. 60 We also reported the micellar structure and their surface charge induced by pH.…”

“…61,62 The same molecules were also reported to form giant ribbons. 55,58 We have recently shown that the micelle-ribbon duality depends on purity; a standard non-acetylated, monounsaturated, acidic sophorolipid batch forms micelles, unless it is contaminated with 15 wt% of the saturated congener, which induces the ribbon formation. 63 In terms of surface-adsorption properties of the sophorolipids, only alkyl ester derivatives have been explored, which nonetheless have shown adsorption on alumina being two orders of magnitude higher than on silica beyond a solution concentration of 10 À4 M. 64 The surface self-assembly and patterning ability of an aqueous micellar solution of non-acetylated, monounsaturated, acidic sophorolipids have been shown in the present investigation.…”