302
R. G. Weiss
7.2 A Short (Prejudiced) History of Gels
Although materials known to be gels have been mentioned for more than 3 millennia
[17], the time span during which gels have been studied scientifically is much
shorter. The author’s prejudiced view of some of the most important observations and
advances is summarized briefly below. In 1841, Lipowitz reported the first formal
scientific molecular hydrogel ‘sighting’–the gelation of aqueous solutions by lithium
urate [18]. In 1864, Thomas Graham, who became Master of the Mint in England,
began his studies of sol-gel chemistry; although these are not true gels as noted above,
mention of this work is seminal because the pathway leading to them can involve
‘true’ gels. In addition, he made some very important (and unusual!!) pronouncements, such as his support for the theory of ‘vitalism’: “While the rigidity of the
crystalline structure shuts out external expressions, the softness of the gelatinous
colloid partakes of fluidity, and enables the colloid to become a medium for liquid
diffusion, like water itself …. The colloid possesses energia. It may be looked upon
as the probable primary source of the force appearing in the phenomena of vitality.”
[19].
In 1871, Maddox used ‘dry’ gelatin plates with silver salts for photography. In
1888. Eastman made silver halide dispersions in gelatin on cellulose nitrate rolls of
film placed in a camera. In 1891, Meunier made gels with 1,3:2,4-di-O-benzylideneD-sorbitol as the gelator. In 1896, Liesegang reported reactions of molecules, as well
as their diffusion and crystallization, in gels. In 1899–1900, Hardy employed gel
electrophoresis and recorded microscopic images of gel networks. In 1907, Foster
and Jackson made organogels using camphoryl thioisemicarbazide as the gelator. In
1907, Cotton and Mouton performed studies of thixotropy in gels. In 1912, Zsigmondy and Bachmann reported gelation of aqueous and alcoholic liquids by fatty
acid salts. Notably, in the same year, Hardy synthesized thermally-reversible gels
with small organic molecules as the gelators and made serious attempts to formulate
a theoretical basis for how rod-shaped objects aggregate into colloids and, in some
cases, result in gels. Many discoveries have made in the field of gels, including molecular gels, in the intervening century [3, 20–25, 29]. They have been driven in part by
advances in instrumentation which allows processes and structures to be interrogated
at increasingly short time and length scales [26]. Despite these advances, some of the
frustrations noted by Dorothy Jordon Lloyd nearly a century ago [27]—“The colloid
condition, the gel, is one which is easier to recognize than to define”—persist today,
and the field of molecular gels remains an active area of inquiry. Unfortunately, part
of the reason for even greater progress is a lack of standardized, accepted methods
for preparing and analyzing gels [28].
R. G. Weiss
7.2 A Short (Prejudiced) History of Gels
Although materials known to be gels have been mentioned for more than 3 millennia
[17], the time span during which gels have been studied scientifically is much
shorter. The author’s prejudiced view of some of the most important observations and
advances is summarized briefly below. In 1841, Lipowitz reported the first formal
scientific molecular hydrogel ‘sighting’–the gelation of aqueous solutions by lithium
urate [18]. In 1864, Thomas Graham, who became Master of the Mint in England,
began his studies of sol-gel chemistry; although these are not true gels as noted above,
mention of this work is seminal because the pathway leading to them can involve
‘true’ gels. In addition, he made some very important (and unusual!!) pronouncements, such as his support for the theory of ‘vitalism’: “While the rigidity of the
crystalline structure shuts out external expressions, the softness of the gelatinous
colloid partakes of fluidity, and enables the colloid to become a medium for liquid
diffusion, like water itself …. The colloid possesses energia. It may be looked upon
as the probable primary source of the force appearing in the phenomena of vitality.”
[19].
In 1871, Maddox used ‘dry’ gelatin plates with silver salts for photography. In
1888. Eastman made silver halide dispersions in gelatin on cellulose nitrate rolls of
film placed in a camera. In 1891, Meunier made gels with 1,3:2,4-di-O-benzylideneD-sorbitol as the gelator. In 1896, Liesegang reported reactions of molecules, as well
as their diffusion and crystallization, in gels. In 1899–1900, Hardy employed gel
electrophoresis and recorded microscopic images of gel networks. In 1907, Foster
and Jackson made organogels using camphoryl thioisemicarbazide as the gelator. In
1907, Cotton and Mouton performed studies of thixotropy in gels. In 1912, Zsigmondy and Bachmann reported gelation of aqueous and alcoholic liquids by fatty
acid salts. Notably, in the same year, Hardy synthesized thermally-reversible gels
with small organic molecules as the gelators and made serious attempts to formulate
a theoretical basis for how rod-shaped objects aggregate into colloids and, in some
cases, result in gels. Many discoveries have made in the field of gels, including molecular gels, in the intervening century [3, 20–25, 29]. They have been driven in part by
advances in instrumentation which allows processes and structures to be interrogated
at increasingly short time and length scales [26]. Despite these advances, some of the
frustrations noted by Dorothy Jordon Lloyd nearly a century ago [27]—“The colloid
condition, the gel, is one which is easier to recognize than to define”—persist today,
and the field of molecular gels remains an active area of inquiry. Unfortunately, part
of the reason for even greater progress is a lack of standardized, accepted methods
for preparing and analyzing gels [28].