4
M. B. Shiflett et al.
Keywords Advanced materials · Commercialization · Ionic liquids · Ionic liquid
history · Ionic liquid processes and products
1.1 Introduction
Who discovered the first ionic liquid? This honor is now almost unanimously
attributed to the famous chemist Paul Walden (1863–1957), who was not only a
renowned professor but also a scientific celebrity during his eventful lifetime. In an
experimental study published in 1914 during his time as a professor at the Technical
University of Riga in the Russian Empire (now Latvia), he reported data on density,
capillarity, and electrical conductivity for 13 different alkyl- and aryl-substituted
water-free and low melting ammonium salts [1]. The ethylammonium nitrate was
found to have a melting point of about 13–14 °C, being a protic room-temperature
ionic liquid (RTIL) in present terms. Walden determined the degree of association
in the molten state using density and capillarity, a method commonly used in his time
[2]. Today, almost every work that includes a chapter on the historical development
of ionic liquids begins with this citation of a German paper in a Russian journal [1].
Nevertheless, Walden’s paper is probably one of the least read and—notably during
the beginning of modern-day research on ionic liquids—very often a wrongly cited
work of scientific literature.
According to the literature, there are several substances that can be retrospectively
classified as ionic liquids [3–5]. One candidate, first reported in 1877 and suggested
by several authors, is the “red oil” observed as a separate phase during Friedel–Crafts
reactions [4, 5]. This substance remained unspecified until 1976 when it was finally
identified as a chloroaluminate with an alkylated aromatic cyclic cation [6]. Pernak
et al. noted in their paper that Walden in his 1914 paper referred to an earlier work on
the electrical conductivity of homologous alkylchinolinium iodides by Schall (like
Walden a student of Wilhelm Ostwald at Riga) [7].
Other early ionic liquids found by Laus et al. were alkylpicolinium halides, first
described by Ramsay in 1876 [8], and quaternary anilinium salts made at the turn
of the twentieth century [3]. Even with their low melting points, these substances
did not draw particular attention. The paper by Pernak et al. gave an explanation,
namely that water was considered a universal solvent at that time, and salts with poor
water solubility fell out of interest. The old papers often report that different batches
of the same substance gave very divergent results. Purity and by-products could not
be characterized as they are today due to the analytical means available at the time.
These substances were also described as “intractable oils” and thus often ignored
[9].
In the literature, there is considerable information about Paul Walden that is oddly
if not wrongly interpreted. An autobiographical memoir was published in 1950 [10],
a Russian paper followed in 2003 [11], and Everts had a biographical sketch in
Chemical & Engineering News on the occasion of Latvia celebrating his 150th birthday [12]. The Latvian-born Walden started his academic career after graduation at
M. B. Shiflett et al.
Keywords Advanced materials · Commercialization · Ionic liquids · Ionic liquid
history · Ionic liquid processes and products
1.1 Introduction
Who discovered the first ionic liquid? This honor is now almost unanimously
attributed to the famous chemist Paul Walden (1863–1957), who was not only a
renowned professor but also a scientific celebrity during his eventful lifetime. In an
experimental study published in 1914 during his time as a professor at the Technical
University of Riga in the Russian Empire (now Latvia), he reported data on density,
capillarity, and electrical conductivity for 13 different alkyl- and aryl-substituted
water-free and low melting ammonium salts [1]. The ethylammonium nitrate was
found to have a melting point of about 13–14 °C, being a protic room-temperature
ionic liquid (RTIL) in present terms. Walden determined the degree of association
in the molten state using density and capillarity, a method commonly used in his time
[2]. Today, almost every work that includes a chapter on the historical development
of ionic liquids begins with this citation of a German paper in a Russian journal [1].
Nevertheless, Walden’s paper is probably one of the least read and—notably during
the beginning of modern-day research on ionic liquids—very often a wrongly cited
work of scientific literature.
According to the literature, there are several substances that can be retrospectively
classified as ionic liquids [3–5]. One candidate, first reported in 1877 and suggested
by several authors, is the “red oil” observed as a separate phase during Friedel–Crafts
reactions [4, 5]. This substance remained unspecified until 1976 when it was finally
identified as a chloroaluminate with an alkylated aromatic cyclic cation [6]. Pernak
et al. noted in their paper that Walden in his 1914 paper referred to an earlier work on
the electrical conductivity of homologous alkylchinolinium iodides by Schall (like
Walden a student of Wilhelm Ostwald at Riga) [7].
Other early ionic liquids found by Laus et al. were alkylpicolinium halides, first
described by Ramsay in 1876 [8], and quaternary anilinium salts made at the turn
of the twentieth century [3]. Even with their low melting points, these substances
did not draw particular attention. The paper by Pernak et al. gave an explanation,
namely that water was considered a universal solvent at that time, and salts with poor
water solubility fell out of interest. The old papers often report that different batches
of the same substance gave very divergent results. Purity and by-products could not
be characterized as they are today due to the analytical means available at the time.
These substances were also described as “intractable oils” and thus often ignored
[9].
In the literature, there is considerable information about Paul Walden that is oddly
if not wrongly interpreted. An autobiographical memoir was published in 1950 [10],
a Russian paper followed in 2003 [11], and Everts had a biographical sketch in
Chemical & Engineering News on the occasion of Latvia celebrating his 150th birthday [12]. The Latvian-born Walden started his academic career after graduation at