RESEARCH NOTE pubs.acs.org/IECR
Dissolution of Zein Using Protic Ionic Liquids: N-(2-Hydroxyethyl) Ammonium Formate and N-(2-Hydroxyethyl) Ammonium Acetate Hyung-Min Choi*,† and Inchan Kwon*,‡ †
Department of Organic Materials and Fiber Engineering, Soongsil University, Seoul, Republic of Korea 156-743. Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904, United States.
‡
bS Supporting Information ABSTRACT: Certain protic ionic liquids (PILs) were found to dissolve zein, which is one of the industrially important natural protein polymers. We prepared two PILs;N-(2-hydroxyethyl) ammonium formate (HEAF) and N-(2-hydroxyethyl) ammonium acetate (HEAA);and their ability to dissolve zein was investigated. Up to 70% zein solution may be prepared, but it was difficult to process, because of high viscosity. Zein solutions with 5%-20% concentration were easily prepared by heating at 95 °C or higher with aid of microwave treatment, even in the presence of small amounts of water. A two-step microwave-only heating process can also be useful in preparing solutions with up to 20% zein. No significant disparity was found between HEAF and HEAA, in terms of dissolving zein. Both HEAF and HEAA could be economically viable and environmentally benign solvents, not only over conventional organic solvent systems, but also over other imidazolinium-type ionic liquids, such as 1-butyl-3-methylimidazolinium, which had been suggested as a solvent for zein.
I
onic liquids (ILs) have received considerable attention over the past decade as an environmentally benign alternative class of solvent.1-7 This is mainly because of their negligible vapor pressures, wide liquid ranges, nonflammability, good thermal stabilities, solvating property for diverse substances, considerable electronic conductivities, and wide electrochemical windows, which are advantageous for a large number of applications.5-7 These characteristics of ILs make them comply with the rules of green chemistry. ILs can be generally classified into two broad categories: aprotic ionic liquids (AILs) and protic ionic liquids (PILs).6 To date, AILs have received much greater attention than PILs.1,2,6 AILs are mainly based on cations of heterogeneous cyclic amines, such as substituted imidazoliums and alkyl pyridiniums.1 These AILs are good solvents for various biopolymers, such as starch and cellulose,3 but they are generally difficult to prepare and expensive to use as an industrial solvent. Furthermore, there are some recent concerns over the high environmental toxicity and low biodegradability of some AILs.4 On the other hand, PILs are easily formed by the stoichiometric combination of a Bronsted acid with a Bronsted base.5,6 The PILs can form hydrogen bonds and mediate hydrocarbonsolvent interactions.5 One of the PILs, ethylammonium nitrate, was originally developed in 1914 as the first IL, but had been neglected since then.2 N-(2-Hydroxyethyl) ammonium formate (HEAF) was first synthesized in 2005.2 Unlike AILs, many of these PILs are mainly liquid at room temperature, because of their low melting point.5,6 For example, HEAF has a melting point of -82 °C.2 Therefore, the PILs such as HEAF can be considered as a “true” room-temperature ionic liquid. It is known that the addition of hydroxyl group substantially improves the biodegradability of quaternary ammonium groups.4 The methylated derivative of N-(2-hydroxyethyl) ammonium, (2-hydroxyethyl) trimethylammonium, is also known as the r 2010 American Chemical Society
choline cation, which has been approved as a food additive.2 Therefore, PILs, such as HEAF and N-(2-hydroxyethyl) ammonium acetate (HEAA), can be prepared by formic acid or glacial acetic acid with relatively low toxicity and commercially available low-cost chemicals.4 Zein is a natural protein polymer obtained during industrial corn processing.3,8 The recent growth of bioethanol industry has significantly increased the potential availability of zein. This biomolecule has been previously used in making fibers but now it is used as a coating for candy, nuts, fruit, pills, and other encapsulated foods and drugs.8 Zein has also been extensively studied as a film former for the controlled-release of active pharmaceutical ingredients in tablets and masking the taste of bitter orally administrated drugs.8,9 However, certain characteristics of zein, such as chemical inertness, globular structure, and lack of solubility, make its commercial applications limited.8 Zein is not soluble in water.3 The most common solvent for zein is an alcoholwater mixture (90:10 w/w).3 However, this solution cannot be employed for its chemical modification, since the alcohol/water mixture readily reacts with electrophilic reagents before zein does.3 Recent study has revealed that one of the most common ILs, 1-butyl-3-methylimidazolium chloride, dissolved zein and its benzoylation was successfully performed using benzoic anhydride and pyridine.3 Several different ILs were examined, but 1-butyl-3-methylimidazolium chloride was the only solvent to dissolve zein. Deep eutectic solvent mixtures such as choline chloride and citric acid or oxalic acid were also investigated with regard to dissolving zein, but they were not successful.3 However, Received: April 1, 2010 Accepted: December 19, 2010 Revised: December 15, 2010 Published: December 29, 2010 2452
dx.doi.org/10.1021/ie1007745 | Ind. Eng. Chem. Res. 2011, 50, 2452–2454
Industrial & Engineering Chemistry Research
Figure 1. Preparation of (a) N-(2-hydroxyethyl) ammonium formate (HEAF) and (b) N-(2-hydroxyethyl) ammonium acetate (HEAA).
Figure 2. 10% zein in (a) water (w/v), (b) HEAF (w/v), and (c) HEAA (w/w).
as far as we know, no study has been conducted to examine the dissolution properties of zein in PILs. PILs such as HEAF and HEAA are preferred solvents in regular dissolution or organic modification steps, because of their relatively low cost and toxicity over more common AILs. In addition, these PILs are known for their tendency to stabilize some proteins such as lysozyme.10 Therefore, in this study, we report dissolution characteristics of zein in two PILs: HEAF and HEAA. The success of this research will provide basic information for large-scale, environmentally benign industrial dissolution and, consequently, the modification of zein. The formation of the PIL reaction is a simple acid-base neutralization, creating a formate or acetate salt of N-(2-hydroxyethyl) amine11 (see Figure 1). The formation of PIL process can be easily carried out in any round-bottomed flask. (The experimental procedure is given as Supporting Information.) However, it is important to maintain stirring and an efficient cooling to below 10 °C, because of the highly exothermic nature of the acid-base reaction. The final compound obtained was a yellowish viscous clear solution for HEAF. Neither solid crystals nor precipitation was obtained for HEAF as described in the previous studies.2 On the other hand, the final compound of HEAA was a molten saltlike stage. However, a small increase of temperature readily melts HEAA completely to liquid. Since no further purification (i.e., distillation) was carried out, because of practical reasons, it is expected that the PIL should contain a trace amount of water. The previous study found that final amounts of water in these ionic liquids (ILs) were
