Promiscuous stabilisation behaviour of silicic acid by cationic macromolecules: the case of phosphonium-grafted dicationic ethylene oxide bolaamphiphiles{ Konstantinos D. Demadis,* a Anna Tsistraki, a Adriana Popa, b Gheorghe Ilia b and Aurelia Visa b Received 13th July 2011, Accepted 29th September 2011 DOI: 10.1039/c1ra00448d Phosphonium-based bolaamphiphiles have been found to stabilise silicic acid beyond its solubility limit (y150 ppm). Three bolaamphiphiles have been tested having a quaternary phosphonium group on each end, linked by a number of ethylene oxide (EO) units (5, 21, and 91, resulting in PEGP + -200, PEGP + -1000, and PEGP + -4000 dicationic bolaamphiphiles, respectively). Specifically, the ability of PEGP + -200, PEGP + -1000, and PEGP + -4000 to retard silicic acid condensation at circumneutral pH in aqueous supersaturated solutions was explored. The goal was to investigate the effect of P-based cationic molecules, EO chain length (and by inference the P-to-P spatial separation) on silicic acid stabilisation performance. PEGP + -200 showed no stabilisation ability in ‘‘long term’’ tests (i.e. 24, 48, 72 h). For PEGP + -1000, and PEGP + -4000, it was discovered that in ‘‘short-term’’ (0–8 h) and ‘‘long term’’ (. 24 h) studies the inhibitory activity is additive dosage-dependent, demonstrating that there is a clear increase in stabilisation ability upon phosphonium PEG dosage increase. Specifically, soluble silicic acid levels reach 420 ppm and 400 ppm after 24 h in the presence of 150 ppm PEGP + - 1000, or PEGP + -4000, respectively. PEG additives (PEG-200, PEG-1000, and PEG-4000) containing no phosphonium cations were also tested. Although PEG-200 and PEG-1000 showed no silicic acid stabilisation effects, PEG-4000, surprisingly, was a strong stabiliser. In fact, the inhibitory efficiencies of PEGP + -4000 and PEG-4000 were virtually identical. These results present strong proof that the polyethylene chain beyond a certain length strongly contributes to silicic acid stabilisation. Lastly, the effects of these boloamphiphiles on silica particle morphology were investigated by SEM. Spherical particles and their aggregates, irregularly shaped particles and porous structures, are obtained depending on the additive. Introduction Biosilicification is the directed formation of amorphous hydrated silica (biosilica) in living organisms, such as marine/freshwater diatoms, sponges and terrestrial plants. 1 It has been estimated that marine biological systems process the amount of about 6.7 gigatonnes of silicon. 2 This translates into gross production of y240 ¡ 40 terramoles of ‘‘silicon’’ per annum in surface waters. Biosilicification presents itself as a special kind of biomineraliza- tion in that biosilica is different from the plethora of various biogenic, metal-containing minerals (e.g. calcite, aragonite, vaterite, octacalcium phosphate, hydroxyapatite, iron sulfides, strontium/barium sulfates etc.). Metal carbonate, phosphate or sulfate solids are crystalline ionic materials whose formation is governed by cation–anion association and solubility equilibria, biosilica is an oxide of amorphous nature formed by a complicated inorganic condensation process, controlled by biomacromolecules. Thus, the ‘‘exotic’’ silica-containing elabo- rate morphologies at the micron scale. 3 The diatom is an ideal biosystem for investigation of the mechanism of silicon transport, which is an integral part of the biosilicification process. 4 As the environmental concentrations of ‘‘dissolved silicon’’ are rather low (y70 mM), diatoms must have an efficient transport system. Silicon (as orthosilicic acid or silicate) must not only be transported into the cell, but also transported intracellularly into the Silica Deposition Vesicle (SDV) where silica morphogenesis occurs. The cells maintain pools of dissolved silicon (in whichever chemical form) in relatively high silicon concentrations. It should be noted that although the intracellular silica pool can be as high as 450 to 700 nM/cell, 5 the actual level seems to range from less than 1 mM to about 20 mM (equivalent to a solution of y1% w/v SiO 2 ) as recalculated from the silica content and the biovolume a Crystal Engineering, Growth and Design Laboratory, Department of Chemistry, University of Crete, Heraklion, Crete, GR-71003, Greece. E-mail: demadis@chemistry.uoc.gr b Institute of Chemistry Timisoara of Romanian Academy, 24 Mihai Viteazul Blv., RO-300223, Timisoara, Romania { Electronic Supplementary Information (ESI) available: Mass, FT-IR, UV-vis and NMR spectra of the three PEGP + phosphonium compounds, and EDS of silica precipitates. See DOI: 10.1039/c1ra00448d/ RSC Advances Dynamic Article Links Cite this: RSC Advances, 2012, 2, 631–641 www.rsc.org/advances PAPER This journal is ß The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 631–641 | 631