7 A Primer on Gels (with an Emphasis on Molecular Gels)
315
7.8.3 Some Tubule Assemblies of Steroidal Surfactants
Tubules with monodisperse diameters have been made by adding a simple bile acid,
lithocholic acid, to an aqueous sodium hydroxide solution [85]. Single-walled tubules
(52 and 49 nm outside and inside diameters) can be observed from the closure of
twisted ribbons shortly after mixing (Fig. 7.12a). They are easily aligned and become
multi-layered over longer times (Fig. 7.12b). They have been transformed into silica
and titanium oxide objects with very large aspect ratios via sol-gel processes and
calcination using tetraethyl orthosilicate (TEOS) and titanium(IV) isopropoxide
(Ti(OiPr) 4 ), as the precursors [86].
Fig. 7.12 Single-walled (a;
cryo-TEM) and multi-walled
tubules (b; freeze-fracture
replicas) of sodium
lithocholate that were formed
in water. Bars = 100 nm.
Reprinted with permission
from Langmuir 2002, 18,
7240. Copyright (2002)
American Chemical Society