1. Introduction
The use of metal alloys in the manufacture of dental prostheses has been a common practice for decades. Several dental alloys have been used in the fabrication of these devices, such as titanium alloys, cobalt-chromium alloys, chromium-nickel alloys, etc. Although these alloys are biocompatible and relatively resistant to corrosion, when exposed to certain media (e.g., acid environments [
1,
2]), and also as a consequence of wear [
3] resulting from chewing, the release of metal ions in the oral cavity is a matter of concern. If these metal ions are constantly diffusing into body fluids, their concentration in the body will increase and, after a certain latency period, it can reach a toxic threshold value that can cause significant harmful effects on the body (by combining with biomolecules such as enzymes and proteins), leading to health problems [
4,
5].
The chromium released during the degradation of the Co-Cr alloys is essentially Cr(III), but it can be oxidized to Cr(VI) at the cellular level. Cr(VI) is mutagenic and carcinogenic, with its potential biological effects being controversial [
6]. In any case, metals such as chromium and cobalt, which may have potential or demonstrated oncogenic effects in humans, must be subject to strict regulations for the protection of humans [
7,
8].
The release of metal ions from orthodontic devices in the presence of mouthwashes is a matter of concern given the potential toxicity raise and function loss of devices [
9,
10,
11].
Among those devices, prosthetic restorations are subject to fretting corrosion and wear [
12,
13,
14], and the released wear debris may produce ionic species which have a potential toxicity depending on their concentration [
15]. Consequently, it is of utmost importance to analyze the behavior of those ions in the oral cavity. There are several studies that identify released elements or determine ion concentrations [
2,
15]. However, to the best of our knowledge, the use of mass transport by diffusion to assess the interaction between metal ions and mouthwashes is an innovative approach.
In the present manuscript, the interaction between metal ions, in particular divalent cobalt and trivalent chromium ions, and a pharmacological molecule: chlorhexidine digluconate (C
34H
54Cl
2N
10O
14,
Figure 1) (CHDG), which is present in many mouthwashes, is evaluated by measuring intermolecular diffusion coefficients using the Taylor dispersion method [
16,
17,
18]. We intend to conclude about the potential of CHDG to act as a carrier of those ions, facilitating their removal from the oral cavity and, thus, reducing the potential toxicity.
It is known that a considerable decrease in salivary pH occurs after drinking acidic beverages whose pH is around 2 [
19,
20], and several studies are related to the behavior of metal alloys under physiological conditions using a pH = 2.3 [
1,
21].
In the first part of this study, tracer diffusion coefficients, D, or apparent diffusion coefficients for aqueous solutions of cobalt (II) chloride and chromium (III) chloride at 0.001 mol dm−3, in artificial saliva at pH: 8.3, 7.0, and 2.3, and in artificial saliva with sodium fluoride at pH = 7.0, are reported. This study is complemented by the evaluation of the diffusion behavior of the chlorhexidine digluconate (as a pure compound or contained in commercial formulation) in aqueous solution in the absence or presence of metal ions’ salts. UV-visible spectroscopy was also used to assess the effect of chlorhexidine digluconate on Co(II) and Cr(III).
2. Results and Discussion
Table 1 shows the tracer diffusion coefficients,
D01, for aqueous solutions of cobalt (II) chloride and chromium (III) chloride in water and in CHDG-containing solutions. It can be seen that the
D01 value for CoCl
2 matches with that previously reported in [
22] and measured by using the open-ended capillary cell. Although the tracer diffusion coefficients are similar for both salts, and for the mixture as well, at pH 6.4, the predominant cationic species are Co
2+ [
23] and Cr
3(OH)
45+ [
24]. It can also be seen that, in general, the tracer diffusion coefficients decrease by decreasing the solution pH. In fact, the presence of GHDC either as pure or in a commercial formulation leads to a slight increase in the acidity of the media.
Table 2 shows the tracer diffusion coefficients of the same salts and mixture of salts in artificial saliva (AS). In order to evaluate the effect of ionic strength and pH on the tracer diffusion coefficients, we have used sodium fluoride and lactic acid, to control both parameters, respectively. The former has been chosen as it is an important component of several oral mouth rinses used for preventing dental caries [
25,
26], whilst the latter has been used to simulate the variation of pH occurring in the oral cavity [
27,
28].
Table 3 shows the average experimental diffusion coefficients of solutions of chlorhexidine digluconate (
DCHDG). These values are compared with those obtained by using a CHDG-based commercial formulation (
DCHDG-cf). It should be noted that information on apparent diffusion coefficients of CHDG in a commercial mixture is of significant importance for understanding the behavior of this compound in practical dental applications. The reproducibility of these diffusion coefficients is similar to that observed for other systems, i.e., within ±2% [
29,
30].
Tracer diffusion coefficients of CoCl
2 and CrCl
3 and the mixture of them increase significantly in artificial saliva and in artificial saliva with NaF pH = 7.0, when compared with those obtained in water. The deviations between the tracer diffusion coefficient values (
D01) in these media and the limiting diffusion coefficients of these salts in water (
D0) [
31,
32], at the same temperature, are positive (Δ
D0/
D0 = 44%, 62%, and 95%, for CoCl
2 and CrCl
3, and the mixture of them, respectively). The increase in these
D0 values when compared with the
D0 value in water indicates the presence of salting-out effects. These ions, as chlorides, will suffer less frictional resistance to motion through the fluid and, consequently, their diffusion coefficients in these media become higher, and can flow faster inside living tissues, which can cause severe disturbances associated with these heavy metal ions.
However, in acidic media (artificial saliva pH = 2.3 with and without NaF), tracer diffusion coefficient values are much smaller when compared with D0 values in water. This fact may be explained on the basis of an electrostatic mechanism. Considering that in acid solutions, H3O+ is one of the predominant species, due to its large mobility, a strong electric field will be generated by a concentration gradient in H+. Slowing down these H+ ions drives large counter-current fluxes of Co2+ and Cr3+ in aqueous solutions, and consequently, their values of D < 0 (salting-in). For Cr(III) solutions, an alteration in the structure of the solution due to modification of predominant species, as a function of pH, is also expected.
Table 3 shows the average experimental diffusion coefficients of chlorhexidine digluconate in water and in a commercial formulation (cf). It is observed that the diffusion coefficients of CHDG at cf are higher than in water. In the case of salts, it has been found that pH is slightly lower than water and that has an effect in the tracer diffusion coefficients. Another possible explanation comes from the occurrence of higher ionic strength in the cf, thus inducing less electrostatic interactions between the diffusing species [
33]. It can also be noticed that, in both cases, diffusion coefficients decrease by increasing the concentration; however, the effect of concentration in the diffusion of CHDG in the cf is more significant, when compared with water.
These results show that, although the interpretation of diffusion coefficients is not straightforward, they can provide relevant information on the effect of different conditions on the mobility of relevant ions, which can be of utmost importance for practical purposes. Keeping that in mind, ternary mutual diffusion coefficients for the systems {(CoCl
2) (1) + CHDG (2)} and {(CrCl
3) (1) + CHDG (2)} in water and in a commercial formulation (cf), were measured at tracer concentrations, and data are reported in
Table 4 and
Table 5, respectively.
Tracer diffusion coefficient values for chlorhexidine digluconate (at mole fraction
X2 = 0) are reported in
Table 4 and
Table 5. It should be noted that the main diffusion coefficients
D11 and
D22 were generally reproducible within ±0.02 × 10
−9 m
2 s
−1, whilst the cross-coefficients were reproducible within ±0.05 × 10
−9 m
2 s
−1.
The
D11 values are considerably larger than the
D22 values, and, in the case of CrCl
3, they are higher than the binary diffusion coefficient of this salt measured previously by the same technique [
31,
32].
It can also be seen that coupled diffusion is significant. Indeed, D21/D11 ratios indicate that a mole of diffusing salt can counter-transport up to 0.6 mol of chlorhexidine digluconate, whereas the values of the ratio D12/D22 show that a mole of diffusing chlorhexidine can counter-transport up to 1.2 mol of salt (CrCl3).
From the significant negative cross-diffusion coefficients for the system CrCl
3 and chlorhexidine, indicating counter-current coupled flows of this salt and chlorhexidine digluconate, we can infer that there is evidence of strong interactions between these two components. This mechanism also accounts for the large negative values of cross-diffusion as well as the increased diffusivity of the CrCl
3 and CoCl
2 components in aqueous chlorhexidine digluconate (
Table 4 and
Table 5). The presence of complex ions between Co(II) and Cr(III) ions and chlorhexidine digluconate may explain the obtaining of negative
D12 values. That is, in the region of the solution where the concentration of chlorhexidine digluconate is highest, there will be a more pronounced decrease of a large amount of the free Co
2+ and Cr
3+ ions resulting from the formation of those complex ions, hence the appearance of a gradient of these ions (Co(II) or Cr(III)) with a signal opposite to the gradient of cobalt chloride (or chromium chloride).
Support for this effect of chlorhexidine digluconate on chromium ions was further assessed by UV-visible spectroscopy.
Figure 2a shows the spectra of aqueous solutions of CrCl
3·6H
2O. The spectra show characteristics of maximum absorbances at 417 and 520 nm [
34]. These spectra are similar to those obtained for Cr(III) in the presence of CHDG (1 mM). However, an increase in the absorbance was found at both wavelengths: 417 and 590 nm (
Figure 2b), accompanied by a maximum 0.2 and 0.05 red-shift displacement. No alteration in the spectra of Co(II) in the absence and presence of CHDG was observed (not shown). This can be justified by changes in the conjugated system of ligand molecules. The Cr(III) water exchange rate, from the first hydration shell, is very low [
23], which confers some inert features for ligand substitution [
35]; consequently, it can be hypothesized that a strong interaction with some component of CHDG, acting as a ligand, takes place. Concomitantly, Cr(III) has high charge density when compared with Co(II) (the ionic radii of Co(II) and Cr(III) are 0.74 and 0.61 nm, respectively [
36]). The digluconate shows a high affinity towards metal ions (for example, calcium and aluminum) and, consequently, it could be the referred ligand [
37,
38].
4. Conclusions
Binary and ternary diffusion coefficients of cobalt chloride and chromium chloride, and chlorohexidine digluconate, alone and in mix solutions, respectively, were measured by the Taylor dispersion technique. In artificial saliva at pH = 2.3, containing lactic acid and sodium fluoride, a decrease of the diffusion coefficients of these salts from approximately 10% to 40% was observed when compared with those obtained in water. In the presence of saliva, a salting-in effect affecting the metal ion salts was observed.
However, in artificial saliva at pH = 7 and 8.0, the diffusion coefficients of these salts increased significantly (at most 60%), which indicates the presence of salting-out effects. These ions will suffer less frictional resistance to motion through the fluid and, consequently, their diffusion coefficients in these media become higher and can flow faster inside living tissues, causing severe disturbances associated with these heavy metal ions.
Interactions between the metal salts and CHDG were also observed, essentially for Cr(III), by UV-Vis spectroscopy.
The experimental results suggested that interactions between metal ions and CHDG might be justified by the occurrence of metal–digluconate interactions and are stronger for Cr(III), probably due to its high charge density.
We can conclude that the chlorhexidine digluconate may be used as a controlled heavy metal chromium and cobalt capture system, and therefore contribute to reduce the toxicity levels in the oral cavity.