The Stability of L3Sponge Phase in Acidic SolutionsShilpa H. Bhansali, Abds-Sami Malik, Jessica M. Jarvis, Ilke Akartuna, Daniel M. Dabbs,Jeffrey D. Carbeck, and Ilhan A. Aksay*Department of Chemical Engineering, Princeton UniVersity, Princeton, New Jersey 08544-5263ReceiVed August 17, 2005In the synthesis of the disordered lyotropic liquid crystalline L3sponge phase prepared with the cosurfactantscetylpyridinium chloride and hexanol, aqueous NaCl solution is used as the solvent. When this sponge phase is usedasthetemplateforL3silica-phaseprocessing,wereplaceNaClwithHCltofacilitatetheacidcatalysisoftetramethoxysilanein forming a templated silica gel, assuming that changing the solvent from NaCl(aq) to HCl(aq) of equivalent ionicstrength does not affect the stability range of the L3phase. In this work, we confirm that changing the pH of the solventfrom neutral to acidic (with HCl) has negligible effect on the L3phase region. Equivalent ionic strength is providedby either NaCl(aq) or HCl(aq) solvent; therefore, a similar phase behavior is observed regardless of which aqueoussolvent is used.IntroductionAmphiphilic surfactant molecules assemble into variousspatiallyorganizedstructures includingone-dimensionalsphericalmicelles, two-dimensional sheetlike smectic mesophases, andthree-dimensionalcubic andhexagonal matrixes.1,2Thestabilityof the phases depends on the surfactant concentration, solutioncomposition, solution ionic strength, and temperature.1-5Sur-factantmesophases havebeen usedas templatesfor synthesizingmesostructured organic-inorganic composites.6-8These tem-platedmaterialshave found applicationsinbiologicalseparations,9as catalysts and catalyst supports,10and as membranes.9Aparticularly interesting structure is the L3or “sponge” phase thatexists in the quasi-ternary system composed of cetylpyridiniumchloride, hexanol, and 0.2 M (aq) NaCl or HCl solvent (Figure1).11The L3phase is a disordered isotropic structure that fills thevolumeof the liquid-crystallinesolution(Figure 2).1Thedistortedbilayer forms two interpenetrating networks of channels idealfor the transport of materials throughout the structure. Charac-teristic to the liquid-crystalline L3phase is the universal scalinglaw.1,11For a system that follows the universal scaling law, thetopological form of the system remains unchanged when dilutedwith solvent and the characteristic distance increases linearly asa function of the reciprocal of the membrane volume fraction.11Suchsystems areknownto beself-similaras they appearidenticalat all length scales.We have previously used the L3phase of the cetylpyridiniumchloride monohydrate (CpCl‚H2O) surfactant/hexanol cosur-factant/0.2 M HCl(aq) system to synthesize templated silicagels.12,13For this purpose, we replaced the brine solution (0.2M NaCl) used by McGrath11with a 0.2 M HCl solvent in order* To whom correspondence should be addressed. E-mail: [email protected].(1) Porte, G. J. Phys. Condens. Matter 1992, 4, 8649-8670.(2) Roux, D.; Coulon, C. J. Phys. Chem. 1992, 96, 4174-4187.(3) Anderson, D.; Wennerstrom, H.; Olsson, U. J. Phys. Chem. 1989, 93,4243-4253.(4) Gomati, R.; Daoud, M.; Gharbi, A. Physica B 1997, 239, 405-412.(5) Gomati, R.; Appell, J.; Bassereau, P.; Marignan, J.; Porte, G. J. Phys.Chem. 1987, 91, 6203-6210.(6) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.;Schmitt,C.;Chu,T.-W.; Olson,K. H.;Sheppard, E.W.; McCullen,S. B.;Higgins,J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843.(7) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S.Nature 1992, 359, 710-712.(8) Kim, S. S.; Zhang, W.; Pinnavaia, T. J. Science 1998, 282, 1302-1305.(9) Davis, M. E. Nature 1993, 364, 391-393.(10) Maschmeyer, T. Curr. Opin. Solid State Mater. Sci. 1998,3,71-78.(11) McGrath, K. M. Langmuir 1997, 13, 1987-1995.(12) McGrath, K. M.; Dabbs, D. M.; Yao, N.; Aksay, I. A.; Gruner, S. M.Science 1997, 277, 552-556.Figure 1. Phase diagram as determined by McGrath (ref 11) forthe CpCl‚H2O, NaCl(aq), and hexanol system. The quasi-ternaryhas been reduced to a pseudo-binary phase diagram by using theratio of the co-surfactants to define the vertical axis. The L3phaseregion is highlighted in yellow.Figure 2. Schematic of L3crystalline phase (modified from ref 1).10.1021/la0522449 CCC: $33.50 © xxxx American Chemical SocietyPAGE EST: 4.4Published on Web 03/31/2006to facilitate the hydrolysis of the silica precursor. The stabilityof the surfactant mesophase is critical in retaining the desiredstructure during templation, ensuring that the structure of thesurfactant phase is reproduced in the inorganic phase.8Whilesynthesizing L3-templated silica, we noted that it was difficultto consistently synthesize silica gels in the concentrated regimeof the phase diagram (55 wt% < solvent fraction < 65 wt%).We attributed these inconsistencies in the synthesis of theL3-templated silica to be due to (1) the L3template used in thepreparationof thesegels wasnot prepared inthe L3phasedomainand/or (2) addition of TMOS to synthesize the silica gel waspossibly pushing the system out of the L3phase field. Ourexperimental observations supported the former since we notedthat some solutions appeared turbid and showed optical bire-fringence under cross-polarizer optics, contrary to the lowviscosity,optically clearsolutionexpected ofanL3phase,despiteusing a hexanol/CpCl‚H2O ratio (h/c) of 1.15 consistent with thepreviously published phase diagram.11,14Further, samples thatwere characterized to have the L3structure went on to generateL3-templated silica gels, as determined by small-angle X-rayscattering (SAXS).15,16When replacing a 0.2 M NaCl solvent with a 0.2 M HClsolvent,wealter thepH ofthe lyotropicliquid crystal.The Helfrichmodel for bilayer phases indicates that altering the electrostaticsof a surfactant system by changing the pH of the solvent canresult in a phase transformation.17,18The bending energy of abilayer is dependent on the mean curvature modulus, K, and theGaussian curvature modulus, Kh.18Strongly negative values ofKh favorthe formationof disconnectedmicellarstructures,whereaspositive values favor the formation of a multiconnected, largeaggregatestructure.1Thechargedensity ona surfactantmembranesurface can affect both the bending rigidity (KM) as well as theGaussian curvature of the bilayer (KhM).17In the Poisson-Boltzmannregime,the electricalcontribution canlowertheDebyelength,resulting inagreater effecton KhMthanon