Micellar formation of sodium dodecyl sulfate in sol-gel glasses probed by pyrene fluorescence

Matsui, Kazunori; Nakazawa, Takumi; Morisaki, Hitoshi

doi:10.1021/j100155a088

Cited by 30 publications

(24 citation statements)

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“… , An important consequence of this flexibility is the adaptability of the structure to the restrictive shape of the environment, that is, to the pore network of the silica sol−gel matrix . This picture is based also on other observations, such as the recent observation by Anderson et al that CTAB (like other ionic and nonionic surfactant systems) forms various micellar structures in the sol−gel starting reaction mixture, depending on the co-solvent (methanol) concentration.…”

Section: Discussionmentioning

confidence: 92%

Surfactant-Induced Modification of Dopants Reactivity in Sol−Gel Matrixes

et al. 1999

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Organically and bio-organically doped sol−gel materials have attracted much attention due to their ability to reproduce solution molecular activities within the ceramic environment. We take now this methodology one step forward and explore conditions under which the dopant properties can be modified by the matrix. Specifically we report that the co-entrapment of the surfactant cetyltrimethylammonium bromide (CTAB, the modifier) at low concentrations, with an extensive series of pH indicators representing several key molecular families (the primary dopant) within tetramethoxysilane (TMOS)-derived silica sol−gel matrixes, greatly modifies the indicating performance of the primary dopant. Thus, very large pK i shifts of up to 3−4 orders of magnitude obtained upon the co-entrapment cause methyl orange (MO) to become an indicator for higher acidities and phenolphthalein for higher basicities, compared to their solution behavior. In another example, the ΔpK i between the two indicating transitions of alizarin increased from ∼4.5 pH units in solution to ∼10.5 pH units (!) in the glass, transforming it into an indicator for both the high acidic and high basic pH ranges. In yet another example, the two indicating transitions of phenol red were shifted to a more acidic pH range, pushing the tail of the more acidic titration branch into the negative pH values range. These and other effects were found to be more pronounced by the co-entrapment than by the use of CTAB solutions or of sol−gel matrixes without CTAB, pointing to a synergetic effect between the surfactant and the silica cage. The indicators also proved highly sensitive in revealing the properties of the local environment created by the surfactant. Thus, the indicator molecules were shown to migrate and reorient within the hydrophobic and the hydrophilic regions of micellar environment, according to their acquired charge upon pH changes. The concentration-dependent and humidity-dependent surfactant aggregation processes within the silica cage were probed with MO, and the results were compared with the behavior of entrapped MO in sol−gel matrixes of varying hydrophobicities, obtained by the copolymerization of CH3Si(OCH3)3 with TMOS at various ratios, with and without CTAB. Of practical importance have been the observations that the dopant/surfactant co-entrapment greatly improves the leaching profiles: half-lives, ranging from several months to several years, were found for MO and methyl red; and using SAXS, surface area and porosity measurements, it was shown that CTAB can be used to stabilize the microscopic structure of the material upon heat-drying. This provides a potential solution to the problem of continuing structural changes, which take place with sol−gel materials long after the completion of their synthesis.

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Section: Discussionmentioning

confidence: 92%

Surfactant-Induced Modification of Dopants Reactivity in Sol−Gel Matrixes

et al. 1999

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“…Preparation of Dye-Doped Polysiloxane Films. The method of preparing sol−gel glasses by the acidic hydrolysis of TEOS in ethanol was adopted from the literature with minor modifications. MPTRS and TEOS was added to a solution of ethanol containing 10 -5 M pyrene.…”

Section: Methodsmentioning

confidence: 99%

Stabilization of Excited 2,3-Bis(2-methyl[b]benzothiophen-3-yl)maleic Anhydride in a Poly(ethylene glycol)-g-Polysiloxane

1998

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Fluorescence studies on 2,3-bis(2-methyl[b]benzothiophen-3-yl)maleic anhydride (BTMA)-doped polymer films were investigated. Poly(ethylene glycol) grafted polysiloxane (PEG-g-polysiloxane) films containing 5 wt % of BTMA were prepared by using a poly(ethylene glycol) modified sol−gel precursor and tetraethoxysilane (TEOS) in an acidic condition. The BTMA-doped film was transparent and showed a water contact angle of 40°. The fluorescence spectrum of BTMA in the PEG-g-polysiloxane film showed a large Stokes shift of 0.81 eV, compared to that of BTMA-doped polyethylene film at 0.65 eV. The Stokes shifts in the emission spectra of BTMA molecules in other polymeric hosts such as polystyrene, polycarbonate, and PMMA showed strong correlations with the solubility parameter and dielectric constant of the hosts and coincidentally the contact angle of the doped films. Thus it seems entirely plausible that there exist strong interactions between the excited BTMA molecules with a polar polymeric host. The more polar becomes the host, the stronger would be the interactions. The Stokes shift of 0.81 eV in PEG-g-polysiloxane is attributed to a stabilization of excited BTMA molecules in the polar PEG-g-polysiloxane system. From the plot of Stokes shift (νa − νf) against [(ε − 1)/(2ε + 1) − (n 2 − 1)/(2n 2 + 1)] for different polymeric media, it is proposed that the excited state of BTMA molecules have a larger dipole moment than those of the ground state.

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“…Surfactants can form various micellar structures in sol-gel starting reaction mixtures [33,34]. After gelation, these micellar structures can be retained inside the gels [35].…”

Section: Electrostatic Interactionsmentioning

confidence: 99%