Molecular Gels and their Fibrillar Networks - Soft Fibrillar Materials: Fabrication and Applications - page 2

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Molecular Gels and their Fibrillar Networks
Kevin L. Caran, Dong-Chan Lee, and Richard G. Weiss

1.1
Introduction

This chapter will review, in a non-comprehensive fashion, the formation and
properties of objects with very high aspect ratios [essentially one-dimensional (1D)
objects at the micron or larger distance scales] made from organic molecules
[topologically zero-dimensional (0D) objects at micron-range distance scales] which
are not linked covalently and aggregate upon separation from dilute organic
solutions or sols [1]. It will stress those 1D structures which undergo further
assembly into 3D networks [self-assembled fibrillar networks ( s)] that entrap

the liquid in which they form. It remains largely unknown how and why many
small organic molecules with very different shapes and functionalities [2] are able
to separate from dilute organic (NB, leading to organogels) or aqueous (NB, leading
to hydrogels) solutions or sols in the form of objects with very high aspect ratios [1].
The general name given to such materials is ''molecular gels'', and the molecules
that constitute them are referred to as low-molecular-mass organic gelators
(

SAFIN

s), although many of the materials may not meet the strict rheological
definition of a gel as required by their viscoelastic properties [3]. The smallest
known

LMOG

LMOG is N,N -dimethylurea, 88 DA [4], and the largest are limited arbi-

trarily at 2000 Da (although with some ''poetic license''). The range of small

molecules that can lead to gels via fiber and

<

formation is now in the
hundreds, if not more than one thousand [1]. Because the molecules are aggregated
but not linked covalently, the disassembly of the 1D objects (and their 3D networks)
can be accomplished by application of heat, dilution, shear, or other perturbations
which will be discussed.
The history of gels made from

SAFIN

s may go back as far as the fourteenth
century, although this example remains unsubstantiated and controversial [5].
The first formal description of a hydrogel of which we are aware, employing
lithium urate, was reported by Lipowitz in 1841 [6]. A description of gels with the
well-known and widely used

LMOG

), was
published in 1891 [7]. However, it was not until the middle of the twentieth century
that scientists began to confront the intricacies of SAFIN s and different forms of

LMOG , 1,3:2,4-di-O-benzylidene-d-sorbitol ( 1