1. Introduction
Polysaccharide, one of the biomaterials, attracts attention as renewable resources. Polysaccharide is abundant in nature and is known as environmentally friendly biomaterials. In particular, chitin and chitosan are the second most abundant biopolymers in the world. Chitin is found in the exoskeletons (shells) of crustaceans such as shrimp and crabs, and insects, while chitosan is found in some fungi, diatoms, sponges, worms, and mollusks and is generally produced industrially by chemically deacetylating the chitin. Chitin has a hydroxyl group (-OH) and N-acetaldehyde (NHCOCH
3), and chitosan has a hydroxyl group (-OH) and an amino group (-NH
2), which are highly polar functional groups. Furthermore, these reactive groups can be easily functionally converted by chemical modification [
1,
2]. Their chemical structures are very similar, and they have excellent characteristics such as antibacterial, moisture retention, biocompatibility, safety, chelating, selective ion permeability, and low cost due to their natural origin [
3,
4]. Despite the fact that chitin and chitosan have a high versatility, several tons of them are still discarded each year. They can be easily formed into different shapes and are utilized in industries in various forms, such as fibers, hydrogels, beads, sponges, membranes, etc. Recently, chitin and chitosan have been used in cancer treatment as a vehicle for delivering cancer drugs to a specific site and in tissue/organ regeneration and repair [
5,
6]. Furthermore, it has been reported that the hydrated chitin and chitosan films in humidified conditions become proton conductors similar to other biomaterials [
7,
8,
9,
10,
11,
12,
13,
14,
15], and that can be used as the fuel cell electrolyte where the chemical energy by hydrogen and oxygen is directly extracted as electric energy. Thus, chitin and chitosan are versatile and have the potential to play an important role in a wide range of industries, and are the most attractive biomaterials. The fuel cell is a key device for achieving a carbon-neutral society without CO
2 emission [
16,
17]. Some problems for the currently-used fuel cell, such that electrolyte and platinum electrodes are expensive and cause significant environmental impact in production and disposal, can be solved by using biomaterials with low-cost and environmentally friendly properties.
Figure 1 shows the temperature dependence of ionic conductivities (
σAC) of hydrated chitin membranes with H
2O (Milli-Q) or D
2O (heavy water) described as “Chitin-H
2O” and “Chitin-D
2O” and chitosan membranes hydrated with H
2O and D
2O described as “Chitosan-H
2O” and “Chitosan-D
2O”. The temperature dependence of the ionic conductivity of Chitin-H
2O and Chitin-D
2O was measured using a precision LCR meter (Agilent E4980A) in the temperature range 230–280 K with the Chitin-H
2O and Chitin-D
2O membranes of 0.08 μm thickness, according to the same technique that determined that of Chitosan-H
2O and Chitosan-D
2O [
18]. The ionic conductivities of Chitin-H
2O and Chitosan-H
2O increase as the temperature increase, and these values are higher than that of Chitin-D
2O and Chitosan-D
2O. Furthermore, the ionic conductivity of Chitin-H
2O is ~30 times higher than that of Chitosan-H
2O. Considering that deacetylation of chitin modifies the N-acetaldehyde to the amino group, it is expected that the acetyl group plays an important role in the increase of proton conductivity [
14]. Regarding the electrolyte of a fuel cell, it is very important to have high proton conductivity, and clarifying the proton conduction mechanism from a microscopic viewpoint is necessary for the future development of fuel cells.
Recently, we have measured the hydrated chitosan from the microscopic viewpoint to reveal water dynamics via quasi-elastic neutron scattering (QENS) [
18]. QENS provides quantitative observation of mono-particle dynamics information such as a self-diffusion and site-to-site jump distance by analyzing the momentum transfers (
Q) and energy transfers (Δ
E = Δ
ħω) between an incident neutron and a neutron scattered from a nucleus [
19]. The neutron energy and momentum transfer provide information on the time and space distributions of the particle. From the extremely large incoherent scattering cross section of hydrogen, the scattering spectra obtained from H
2O-containing materials focus on self-diffusion dynamics of hydrogen atoms derived from the materials and water molecules interacting with the material, while the scattering spectra obtained from D
2O-containing materials only focus on hydrogen atoms derived from the materials. Hydrated water dynamics has been evaluated by subtraction of the spectrum for the material hydrated with D
2O from that hydrated with H
2O, assuming no functional difference in the dynamics of the material between the D
2O and H
2O systems. The results of water dynamics in chitosan by QENS measurements showed that mobile hydration water exhibits jump diffusion motion and a diffusion constant increased from 1.33 × 10
−6 cm
2/s at 238 K to 1.34 × 10
−5 cm
2/s at 283 K [
18]. Moreover, a part of the hydrogen atoms (i.e., mobile hydrogen atoms) attached to chitosan dissociates as the protons and undergoes jump diffuses. In addition, the amount of the mobile hydrogen well corresponds with the proportion of the mobile hydration water surrounding the dissociable hydrogen atoms in chitosan, which is observed at each temperature. From these results, it was found that the transition of protons between the hydration water molecule and hydrogen atoms of chitosan is significantly important to realize proton conduction in the hydrated chitosan.
In the present study, we have investigated water dynamics in the hydrated chitin by QENS measurement and revealed differences in proton conductivity between the hydrated chitin and chitosan from the microscopic viewpoint. The results in the present work will lead not only to the mechanism of proton conductivity in chitin and chitosan but to the development of the application of new environmentally friendly bio-electric devices such as fuel cells and hydrogen sensors.
5. Discussion
The main objective of this paper is to understand the proton conduction mechanism of hydrated chitin by investigating the relationship between the hydration water in chitin and the hydrogen atoms from the microscopic viewpoint and to determine the differences in proton conduction between the hydrated chitin and chitosan. The analysis results of
Satom (
Q,
ω) showed that a part of the hydrogen atoms in chitin was mobile even at 238 K and dissociated from chitin as protons. In addition, the analysis results of
Swater (
Q,
ω) also showed a part of the hydration water in chitin was mobile even at 238 K, where bulk water is frozen, and had jump-diffusive obeying the jump-diffusion model [
19]. The fraction of the slow mobile hydration water and the mobile hydrogen atoms in chitin increased as the temperature increased. These results are similar to trends of the results reported in the QENS experiments for the hydrated chitosan [
18].
However, a quantitatively significant difference between the hydrogen atoms dynamics of chitin and chitosan was shown. More specifically, D of the mobile hydrogen atoms in chitin was two times larger, and τ was two times faster than those in chitosan. These results suggest that the difference in ionic conductivity between chitin and chitosan is due to D and τ.
Regarding the mobile hydrogen atom and slow mobile hydration water dynamics in chitin,
D of the mobile hydrogen atom was 1.5 times larger,
τ was two times faster, and
l was 0.4 Å shorter than those of the slow mobile hydration. However, the activation energy estimated from these
D and
τ in the mobile hydrogen atoms and the slow mobile hydration water dynamics were consistent with 0.28 eV (cf.
Figure 7 and
Figure 12), suggesting that these dynamics are closely related.
Two conduction mechanisms of proton transport are generally well known, namely the Grotthuss and the Vehicle mechanism [
27]. The Grotthuss mechanism is the proton hopping transfer within the hydrogen bonding network of water molecules. It is assumed that protons transfer around water molecules to form hydronium ions (H
3O
+) in the cluster and simultaneously break hydrogen bonds and subsequent rearrangement between the neighborhood water molecules. On the other hand, the vehicle mechanism is carried out by self-diffusion of proton-producing species. In a hydrated polymer electrolyte, the QENS results support that the water dynamics mechanism in Nafion is a diffusion mechanism in which H
3O
+ moves rather than a concerted jump motion in which H
3O
+ continuously builds up and disappears [
28,
29]. In addition, it is considered that the hydrated polymer electrolyte undergoes formation of H
3O
+ via a slow jump mode with a characteristic time from 500 ps to 150 ps and long-range diffusion of water molecules within the confinement region via a fast jump mode with a characteristic time from 8.0 ps to 2.5 ps [
28,
29]. The residence times of mobile hydrogen atoms in chitin ranged from 0.26 ps to 3.1 ps is much faster than these motions observed in the hydrated polymer electrolyte. Furthermore, these conduction mechanisms can be distinguished by their activation energies, and the proton conduction process by the Grotthuss mechanism generally requires activation energy ≤ 0.4 eV, while the vehicle mechanism requires a larger energy ≥ 0.4 eV because it transports a mass larger than the proton [
27,
30,
31]. Since the activation energies for the hydrogen atoms and hydration water dynamics in chitin were below 0.4 eV, it is clear that proton conduction processes in the hydrated chitin are the Grotthuss mechanism.
In the proton conduction process of the hydrated chitosan, the hydrogen atoms of the hydroxyl and amino groups in chitosan dissociate as protons and diffuse into the neighboring hydration water (i.e., fast water or slow mobile water) to form H
3O
+. Subsequently, the mobile hydration water assists in the proton transfer between H
3O
+ and additional hydration water molecules, thereby realizing proton conduction. However, Chitosan-D
2O does not exhibit a significant increase in ionic conductivity as temperature increases. (cf.
Figure 1). Therefore, it is speculated that hydrogen atoms are unable to transfer to D
2O hydration continuously. This means that the hydrogen atom of H
3O
+ would be returned to the chitosan when a proton acceptor is D
2O. However, the ionic conductivity in Chitin-D
2O increased as the temperature increased and exhibited high ionic conductivity (cf.
Figure 1). Therefore, it is speculated that Chitin
D and Chitosan
D are different in the proton conduction process. For this reason, in the proton conduction process in chitin based on the above consideration, it is possible that the hydrogen atoms derived from chitin dissociate and jump to the neighboring hydration water molecules to form H
3O
+, and then the hydrogen atom of H
3O
+ directly transfers to the neighboring chitin. In order to determine a suitable model for the process leading to proton conduction in the hydrated chitin, the model for the relationship between the mobile hydration water and mobile hydrogen atoms was formed based on the following assumptions. (1) Among the thirteen hydrogen atoms per chitin monomer, there are enough mobile hydration waters around three dissociable hydrogen atoms (i.e., the hydroxyl and the amino groups of N-acetaldehyde groups) at 283 K, and these hydrogen atoms preferentially transfer to the slow mobile hydration water; (2) The dissociable hydrogen atoms equivalently transfer to the slow mobile hydration water to form H
3O
+; (3) The hydrogen atom of H
3O
+ can transfer directly to the proton acceptor of neighboring chitin. Thus the following equation was formulated,
where
α is the number of proton acceptors of neighboring chitin.
Figure 15 shows the results of the theoretical equation when 3 is substituted for
α and the fraction of the mobile hydrogen atoms in chitin (cf.
Figure 14). The theoretical formula substituting
α = 3 and the experimental data are equivalent, which indicates that the hydrogen atoms in chitin transfer to the proton acceptors of neighboring chitin via H
3O
+. These results indicate that the proton conduction process in the hydrated chitin, in addition to the proton transfer process between H
3O
+ and another hydration water observed in the hydrated chitosan, involves the preferential transfer of dissociable hydrogen atoms in chitin as protons to the slow mobile hydrogen water to form H
3O
+, and then the hydrogen atoms of H
3O
+ transfer to three proton acceptors of neighboring chitin.
These results indicate that the hydrogen atoms and hydration water are involved in the proton conduction of the hydrated chitin in the water dynamics observed in QENS, as schematically shown in
Figure 16. In addition, it is known that the difference in proton conduction between the hydrated chitin and chitosan is attributed to the hydrogen atom dynamics. More in detail, the proton conduction process in the hydrated chitin, the dissociable hydrogen atoms (i.e., the hydroxyl and the amino groups of N-acetaldehyde groups) in chitin preferentially transfer as protons to the slow mobile hydration water located 2.1 Å in the distance to form H
3O
+, and then the hydrogen atom of H
3O
+ transfers to another mobile hydration water or the proton acceptors of neighboring chitin, thereby realizing proton conduction. Therefore, it is revealed that the difference in proton conductivity between the hydrated chitin and chitosan is determined by diffusion and residence time of hydrogen atom dynamics and the presence of proton acceptors in the distance that the hydrogen atoms of H
3O
+ can transfer and the number of proton acceptors for the hydrogen atoms of H
3O
+. We expect that the N-acetaldehyde groups in chitin play a role in controlling the location of the hydration water in order to ensure that dissociative hydrogen atoms can transfer directly to the proton acceptors of neighboring chitin.
6. Conclusions
We reported the results of neutron quasi-elastic scattering on ChitinH and ChitinD samples to reveal the proton conduction process in the hydrated chitin and differences in proton conductivity between the hydrated chitin and the hydrated chitosan from the microscopic viewpoint.
QENS results in chitin showed that a part of the hydration water and hydrogen atoms in chitin could be mobile even at 238 K, and the amount of mobility and diffusion increased as the temperature increased, which was qualitatively similar to the QENS results for chitosan. However, D of the mobile hydrogen atoms in chitin was ~two times larger, and τ was ~two times faster than those in chitosan, indicating a quantitative difference between chitin and chitosan. In the proton conduction process of the hydrated chitosan, the easily dissociable hydrogen atom (i.e., the hydroxyl and amino groups) dissociate as protons and transfer to the neighboring hydration water (i.e., the fast or slow mobile hydration water) to form H3O+, and then hydrogen atom of H3O+ transfers to an additional hydration water molecule to realize proton conduction. However, the ionic conductivity of ChitosanD did not significantly increase as ChitinD. From these results, it is clear that the proton conduction process between chitin and chitosan is different. From the results of QENS measurements in chitin, the experimentally determined fraction of mobile hydrogen atoms was well correlated with the proportion that the dissociable hydrogen atoms (i.e., the hydroxyl and the amino groups of N-acetaldehyde groups) in chitin preferentially transfer to the slow mobile hydrogen atoms and then to the three proton acceptors of neighboring chitin. These results indicate that the proton conduction process of the hydrated chitin is not only the proton transfer process observed in the hydrated chitosan but also the dissociable hydrogen atoms in the chitin preferentially transfer as protons to the slow mobile hydrogen water to form H3O+, and then the hydrogen atoms of H3O+ transfer to 3 proton acceptors of neighboring chitin. Therefore, it is revealed that the difference in proton conductivity between the hydrated chitin and chitosan is yielded by the differences in D and τ determined by hydrogen-atom dynamics and the location and number of proton acceptors.
Lastly, we concluded that the QENS measurements in chitin provide insight into the proton conduction process in the hydrated chitin by distinguishing between hydration water and hydrogen atom dynamics and understanding the ionic conductivity differences between chitin and chitosan from the microscopic viewpoint. These results are useful for research on the application of biopolymer chitin and chitosan, an environmentally friendly material, as a fuel cell electrolyte and for the potential functionality of chitin and chitosan for the creation of new products.