Synthesis and characterization on a novel series of protic pyrrolidinium surfactants

Article abstract of DOI:10.1016/j.cclet.2009.12.031

A novel series of protic tertiary pyrrolidinium surfactants were prepared and characterized by different techniques. These compounds show good conductivity and efficient ability to reduce surface tension. Thermogravimetric analysis proves their high thermal stability at decomposition temperatures over 250 °C. Their lyotropic and thermotropic liquid crystalline properties are also discussed.

Full text of DOI:10.1016/j.cclet.2009.12.031

Available online at www.sciencedirect.com
Chinese Chemical Letters 21 (2010) 385–387
www.elsevier.com/locate/cclet
Synthesis and characterization on a novel series of protic
pyrrolidinium surfactants
*
Fu Min Ma, Xiao Chen , Xu Dong Wang, Yu Rong Zhao, Qiu Hong Li,
Xiu Yue, Chao Lv
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China
Received 17 August 2009
Abstract
A novel series of protic tertiary pyrrolidinium surfactants were prepared and characterized by different techniques. These
compounds show good conductivity and efficient ability to reduce surface tension. Thermogravimetric analysis proves their high
thermal stability at decomposition temperatures over 250 8C. Their lyotropic and thermotropic liquid crystalline properties are also
discussed.
# 2009 Xiao Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Protic pyrrolidinium surfactants; Liquid crystal; Conductivity; Surface tension
Pyrrolidinium salts are generally composed of short or long tailorable alkyl chains, which could lead them to have
specific physicochemical properties through structural modifications. With short alkyl chains, such salts are usually
adopted as ionic liquids and plastic crystal materials [1–3], which are becoming more and more popular to substitute
the traditional volatile organic solvents for applications in the area of organic synthesis, catalysis, electrochemistry,
liquid/liquid extraction, and preparation of novel materials. Meanwhile, their high ionic conductivity in both solid and
liquid states has been paid much attention for uses as electrolytes in lithium batteries [4], solvent-free dye-sensitized
solar cells [5], and so on. With long alkyl chains, however, pyrrolidinium salts behave like surfactive 1-alkyl-3-
methylimidazolium salts ([Cnmim]X) to show amphiphilicity [6,7]. They are attracting increased attention now for
their fascinating ionic liquid crystalline properties except for good conductivity [8,9].
Most pyrrolidinium ionic liquids or surfactants contain an N-methyl-N-alkylpyrrolidinium organic cation and an
anion of different types like bromide, iodide, nitrate, bis-(trifluoromethane sulfonyl)imide, tetrafluoroborate or
hexafluorophosphate. On that basis, several novel protic pyrrolidinium-cations ionic liquids with short alkyl chains
have been developed recently [10], which can be used as proton conducting electrolyte in fuel cells, double layer
capacitors, dye-sensitized solar cell, or actuators. To our best knowledge, however, there are not reports on protic
surfactive pyrrolidinium salts and their liquid crystalline behaviour.
* Corresponding author.
E-mail address: xchen@sdu.edu.cn (X. Chen).
1001-8417/$ – see front matter # 2009 Xiao Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2009.12.031

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F.M. Ma et al. / Chinese Chemical Letters 21 (2010) 385–387
Scheme 1. Synthesis route of the protic pyrrolidinium cationic surfactants (1) and their side reactions (2 and 3). n = 9, 11, 13 for compounds 1–3.
Motivated by this, we report here a range of novel protic pyrrolidinium surfactants with different chain lengths
substituted on pyrrolidine ring. They were prepared by heating pyrrolidine with CnH2n+1Br (n = 12, 14, 16) as outlined
in Scheme 1(1). Considering that pyrrolidine is a cyclic secondary amine in nature, over-alkylation will occur in the
Hofmann alkylation process and give rise to a mixture of secondary or tertiary amines, and tertiary, quaternary
ammonium salts. Therefore, some side reactions will exist during the reaction of pyrrolidine with long-chain alkyl
bromides as listed in Scheme 1(2 and 3). Such a process is not like a quaternisation (Menschutkin) reaction, which is
usually used for the preparation of quaternary ammonium salts and no side reactions occur. To get pure protic tertiary
ammonium salts, some facile and effective steps are needed. First, to prevent the Menschutkin reaction, a slight excess
of pyrrolidine was employed. Then, to remove by-products, 1-alkylpyrrolidine, pyrrolidine and long-chain alkyl
bromides, the produced precipitate was filtered off and washed carefully with toluene and ethyl acetate after rotary
evaporation. Finally, the precipitate was recrystallized from tetrahydrofuran. Due to the well dissolution in
tetrahydrofuran for reactants and by-products, like pyrrolidine hydrobromide, 1-alkylpyrrolidine, pyrrolidine, and
long-chain alkyl bromides, only the product was crystallized and a white solid was obtained. After being dried in
vacuo, the pure products are characterized and the NMR data of compounds 1–3 were listed in Ref. [12].
The obtained compounds show good conductivity, efficient ability to reduce surface tension and colorful lyotropic
liquid crystalline textures in their aqueous solutions. Take N-dodecylpyrrolidinium bromide as an example, the
conductivity of its 0.02 mol/L solution at 25 8C is 1108 ms/cm, which is similar to that of ionic liquid [C12mim][Br]
[11], and the surface tension can be decreased to 31.5 mN/m, which is more efficient than [C12mim][Br] (39.4 mN/m),
and sodium dodecyl sulfate (SDS) (38 mN/m). With increasing concentration, colorful fanlike lyotropic liquid
crystalline texture was observed (Fig. 1a) corresponding to a hexagonal phase. The thermal behaviours of these protic
pyrrolidinium surfactants are also examined by thermogravimetric analysis (TGA), differential scanning calorimetry
(DSC) (Fig. 2) and polarizing optical microscopy (POM) (Fig. 1). The decomposition temperatures of all these
Fig. 1. Polarized optical micrographs of the representative lyotropic and thermotropic liquid crystals. (a) Fanlike texture of lyotropic liquid
crystalline phase of compound 1/H2O systems with the composition of 50/50 wt%, 25 8C; (b) lancet-like texture of the T phase of compound 3 at
152 8C.

F.M. Ma et al. / Chinese Chemical Letters 21 (2010) 385–387
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Fig. 2. DSC traces and thermograms of the pyrrolidinium salts. Abbreviations: Cr, crystalline phase; T, T phase; I, isotropic liquid; a–c for
compounds 1–3.
compounds are over 250 8C, showing high thermal stability. DSC data demonstrate that several phases existed before
the melting points. The existence of thermotropic liquid crystal phases is confirmed by POM. For example, Fig. 1b
shows a typical texture of thermotropic liquid crystal of N-hexadecylpyrrolidinium bromide corresponding to a T
phase. Compared to 1-alkylimidazolium salts with the same chain length, T phase is only formed for pyrrolidinium
surfactants but not for 1-alkylimidazolium salts, which mainly exhibit the SmA phase. Other protic pyrrolidinium
surfactants show similar properties and the systematic studies are underway.
In summary, a series of protic N-alkylpyrrolidinium halide salts have been synthesized and investigated. These
compounds show efficient ability to reduce surface tension as potentially useful and tunable detergents. Their good
conductivity and high thermal stability allow their application as proton conducting electrolyte. They also show
fascinating lyotropic and thermotropic liquid crystalline properties, which will make them adopted widely in material
fields.
Acknowledgment
Supports from the National Natural Science Foundation of China (Nos. 20573066 and 20773080) are gratefully
acknowledged.
References
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[12] Compound 1 (purity >98%): 1H NMR (400 MHz, CDCl3, 25 8C, TMS): d 0.86–0.89 (t, 3H; CH3), 1.25–1.32 (m, 18H; CH2), 1.87–1.95 (m, 2H;
NCH2CH2), 2.05–2.12 and 2.22–2.32 (m, 4H; pyrr. H-3 and H-4), 2.73–2.82 and 2.99–3.05 (m, 4H; pyrr. H-2 and H-5); 3.79–3.83 (m, 2H;
NCH2), 11.41 (s, 1H; pyrr. NH); ESI-MS m/z: 240.3 ([MÀBrÀ]+). Compound 2 (purity >98%): 1H NMR (400 MHz, CDCl3, 25 8C, TMS): d
0.86–0.89 (t, 3H; CH3), 1.25–1.32 (m, 22H; CH2), 1.87–1.94 (m, 2H; NCH2CH2), 2.05–2.12 and 2.22–2.27 (m, 4H; pyrr. H-3 and H-4), 2.74–
2.83 and 2.99–3.05 (m, 4H; pyrr. H-2 and H-5); 3.80–3.84 (m, 2H; NCH2), 11.37 (s, 1H; pyrr. NH); ESI-MS m/z: 268.3 ([MÀBrÀ]+).
Compound 3 (purity >98%): 1H NMR (400 MHz, CDCl3, 25 8C, TMS): d 0.86–0.90 (t, 3H; CH3), 1.25–1.32 (m, 26H; CH2), 1.87–1.94 (m, 2H;
NCH2CH2), 2.06–2.13 and 2.17–2.28 (m, 4H; pyrr. H-3 and H-4), 2.71–2.82 and 2.98–3.05 (m, 4H; pyrr. H-2 and H-5); 3.78–3.87 (m, 2H;
NCH2), 11.58 (s, 1H; pyrr. NH); ESI-MS m/z: 296.3 ([MÀBrÀ]+).