A combined SEMSANS/SANS study of surfactant-templated mesoporous silica film growth at the air–water interface (Schmitt et al., 2020) established the bulk-solution pathway — silicate incorporation into surfactant micelles, micelle elongation, then precipitation of micrometre-sized mesostructured particles — but explicitly left open where silicate sits inside the evolving micelles, which ordered mesophase the subphase particles adopt, and what fraction of particles migrates to the interface versus remaining in bulk. The problem — completing the quantitative film-formation mechanism for dilute acidic CTAB/C16EO8 + TMOS solutions — remains open.

Background

Mesoporous silica is a class of inorganic materials with ordered, surfactant-templated pores a few nanometres across, made by co-assembly of an amphiphilic surfactant with a polymerizing silica source. Since 1996 it has been known that, in dilute acidic solution, such materials can form as thin mesostructured films that grow spontaneously at the air–water interface of the reaction solution — an evaporation-free route of interest for membranes, sensors, catalysis supports and drug-delivery hosts. In the system studied here the surfactant is either CTAB (cetyltrimethylammonium bromide, cationic) or C16EO8 (octaethylene glycol monohexadecyl ether, nonionic), and the silica source is TMOS (tetramethyl orthosilicate), which hydrolyses and condenses in acid. The formation mechanism has been debated for two decades. In the silica-coated-micelle picture, hydrolysed silicate species adsorb into the corona (palisade) region of surfactant micelles, elongating them; the film then forms either by migration of silica-enriched micelles to the interface or by their aggregation into mesostructured particles in the subphase (the bulk solution below the surface) that subsequently migrate. In the competing silica-droplet (phase-separation) picture, silica–surfactant-rich droplets or flocs appear first and nucleate the ordering.

Small-angle neutron scattering (SANS) measures elastic scattering at small angles to resolve structures of roughly 1–100 nm as a function of the magnitude of the scattering vector qq (the momentum transfer, which is inversely related to the length scale probed); its contrast is the difference in neutron scattering length density (SLD) between components, and by varying the D2O/H2O ratio of the solvent (contrast variation) selected components can be made nearly invisible, so their spatial location can be determined. Spin-echo modulated SANS (SEMSANS, closely related to SESANS) adds neutron spin-echo encoding (neutron spin precession used to encode scattering angles very sensitively) to a SANS instrument, extending sensitivity to much larger structures, up to the spin-echo length δSE\delta_{SE} (micrometres here), measured simultaneously with conventional SANS.

Prior work with several solvent contrasts established that the CTAB micelles in these film-forming solutions are core-shell objects (Brennan et al. 2002, DOI 10.1021/cm020399l), that the formation mechanism switches with concentration and silica:surfactant ratio between particle accumulation and gentler interfacial growth (Brennan et al. 2002, DOI 10.1021/la026192o), and that the final CTAB-templated films are well ordered (Edler et al. 2003, DOI 10.1016/s1387-1811(03)00402-5); time-resolved scattering on block-copolymer-templated systems had traced particle nucleation (Manet et al. 2011, DOI 10.1021/jp200213k; Schmitt et al. 2016, DOI 10.1021/acs.langmuir.6b00572). Schmitt et al. (2020, DOI 10.1021/acsami.0c03287) then combined SEMSANS and SANS on the Larmor instrument at the ISIS Neutron and Muon Source (UK) to follow the whole subphase evolution of CTAB- and C16EO8-templated film-forming solutions for the first time: micelles elongate as silicate incorporates (the fitted silica fraction xsilicax_{silica} in the micelles rising to about 30% for CTAB at silica:surfactant molar ratio rmol=3.6r_{mol}=3.6), then micrometre-sized mesostructured particles appear concomitantly with the first Bragg peak (a diffraction peak from an ordered mesophase, presumed 2D-hexagonal, i.e. a hexagonal array of parallel cylindrical micelles), and free micelles are progressively consumed into the particles. This supports the particle-formation-and-migration picture at low and moderate rmolr_{mol}, while at rmol=10.81r_{mol}=10.81 no mesostructured particles are seen, consistent with film growth there by adsorption of individual elongated micelles at the interface. The authors state precisely what this single-contrast, limited-qq, limited-δSE\delta_{SE} study could not determine — and those acknowledged gaps define the open problem below.

Problem Statement

For dilute acidic film-forming solutions of CTAB or C16EO8 with TMOS, determine the pieces of the air–water-interface film-formation mechanism that Schmitt et al. (2020) explicitly could not resolve, namely: (i) the location of hydrolysed silicate species within the evolving surfactant micelles — core, corona, or solvent inclusion — and the true silica fraction xsilicax_{silica} in the micelles (the single-contrast fit, which assumes the micelles contain only surfactant and silica and neglects D2O inclusion, may overestimate it); (ii) the mesophase symmetry of the ordered phase inside the subphase particles — the single accessible Bragg peak cannot distinguish 2D-hexagonal from cubic order and cannot decorrelate the micelle number density n′n' within domains from the effective silica-wall scattering length density ρ0′\rho'_0; (iii) the fate of the mesostructured particles: what fraction migrates to and assembles at the air–water interface versus remaining in bulk, including the high-silica regime (rmol=10.81r_{mol}=10.81) where the earliest kinetic stages remain unmeasured; and (iv) the size distribution of aggregates larger than the δSE≤2.5\delta_{SE}\leq 2.5 µm spin-echo range or below the low-qq limit, whose neglect may bias particle and domain-size estimates. Jointly, (i)–(iv) serve one objective: a quantitative, experimentally grounded mechanism of surfactant-templated mesoporous silica film formation at the air–water interface, from molecular incorporation in the micelles to interfacial assembly.

References

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