Microstructure Determination of AOT + Phenol Organogels Utilizing Small-Angle X-ray Scattering and Atomic Force Microscopy

Abstract

Dry reverse micelles of the anionic twin-tailed surfactant bis(2-ethylhexyl) sulfosuccinate (AOT) dissolved in nonpolar solvents spontaneously form an organogel when p-chlorophenol is added in a 1:1 AOT:phenol molar ratio. The solvents used were benzene, toluene, m-xylene, 2,2,4-trimethylpentane (isooctane), decane, dodecane, tetradecane, hexadecane, and 2,6,10,14-tetramethylpentadecane (TMPD). The proposed microstructure of the gel is based on strands of stacked phenols linked to AOT through hydrogen bonding. Small-angle X-ray scattering (SAXS) spectra of the organogels suggest a characteristic length scale for these phenol-AOT strands that is independent of concentration but dependent on the chemical nature of the nonpolar solvent used. Correlation lengths determined from the SAXS spectra indicate that the strands self-assemble into fibers. Direct visualization of the gel in its native state is accomplished by using tapping mode atomic force microscopy (AFM). It is shown that these organogels consist of fiber bundle assemblies. The SAXS and AFM data reinforce the theory of a molecular architecture consisting of three length scales-AOT/phenolic strands (ca. 2 nm in diameter) that self-assemble into fibers (ca. 10 nm in diameter), which then aggregate into fiber bundles (ca. 20-100 nm in diameter) and form the organogel.

Lecithin organogels as a potential phospholipid-structured system for topical drug delivery: A review

“…24,25,28,30,65 The knowledge of molecular packing within the organogel network has been obtained using scanning and transmission electron microscopies (SEM and TEM), 20 dynamic and static light scattering (elastic or quasielastic light scattering techniques QLS), 21,22,[66][67][68] small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM). 21,22,[69][70][71][72] These techniques allow many features of organogels to be deciphered at 1 to 1000 nm scale. Recently, SAXS and AFM have become important tools in determining the molecular arrangement of long range structures such as LOs, along with the absolute quantities such as diameter, length, or topology in gels.…”

“…72,73 The scattering information (SAXS and SANS measurements) on organogels, which could be obtained even in undiluted samples (ie, without disturbing the originality of the system), combined with mathematical analysis provides such information as static correlation length "ξ,"mesh size of the network (or the number density of entanglements 'ν'), diffusion coefficients, and flexibility of the fibrous network, along with the structural features of the crosssections of LOs. 20,21,70,71,74,75 The direct visualization of the gel in its naïve state is possible using AFM, which allows observing the microstructures of the fibrous network throughout the gel mass. It also provides structural details on the larger length scales (ie, where micellar fibers or chains aggregate into large sized bundles).…”

Lecithin Organogels in Enhancing Skin Delivery of Drugs

“…The molecular packing of organogelator molecules inside the organogel network was studied by scanning and transmission electron microscopy (SEM and TEM, respectively) (Willimann and Luisi 1991 ), dynamic and static light scattering Addition of water (elastic or quasielastic light scattering (QLS) techniques) (Schurtenberger et al 1990(Schurtenberger et al , 1993Capitani et al 1996 ;Sato et al 1988 ;Aboofazeli et al 2000 ), small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM) (Zemb et al 1990 ;Terech and Weiss 1998 ;Simmons et al 2001 ;Gronwald et al 2002 ). These techniques allow many features of organogels to be studied at 1-1,000 nm scale.…”

“…The scattering information using SAXS and SANS measurements on organogels combined with mathematical analysis provides details about the static correlation length ξ, mesh size of the network (or the number density of entanglements "ν"), and diffusion coeffi cients. They also give information on the fl exibility of the fi brous network, along with the structural features of the cross sections of LOs (Terech and Weiss 1998 ;Simmons et al 2001 ;VanEsch and Feringa 2000 ;McAllister et al 2002 ). The direct visualization of the gel in its naïve state is possible using AFM, which allows observing the microstructures of the fi brous network throughout the gel mass.…”

Self‐Assembly of Two‐Component Gels: Stoichiometric Control and Component Selection

“…[4] A wide range of two-component systems based on different non-covalent interactions has been reported. [5][6][7][8] The two-component approach endows gel-phase materials with exquisite microstructural tunability, as either of the two components can be subtly modified. Furthermore, it is possible to tune the network structure by altering the molar ratio of the two components.…”