## 2 Conditioning Solutions
### Acidic Solutions  
Ethylenediaminetetraacetic acid (EDTA) can dissolve and chelate inorganic material by the removal of significant amounts of calcium (Ca\({}^{2+}\)) the root canal dentin [45], including hydroxyapatite, without having an effect on organic tissue [24; 25] and producing dentin demineralization to depths from 20 to 30 \(\upmu\)m [18]. Interestingly, studies using SEM have shown that 17% EDTA without activation produces a partial [16] or a complete [23] elimination of the SSL from dental tubules in the root canal, being an effective conditioning. The above is reflected in the POBS test, which shows a greater bond strength between resin adhesive and root canal, where the highest bond strength is found at the coronal level [10; 20; 22; 46] in comparison with the apical [10; 20; 22; 46; 47], meaning that EDTA might not have a good action deep inside the root canal. Moreover, there is evidence that short time canal irrigation by 17% EDTA for 20 s does not cause dentin damage [10]. Nonetheless, surprising one-minute procedures can lead to dentin erosion [11], indicating that the time factor is more relevant than concentration. Another pivotal factor is the solution form, since EDTA gel conduces to cohesive failure of a specimen cemented [46]. On the other hand, sometimes NaOCl is used as an irrigant solution with poor SSL removal [10; 11; 20; 22; 23; 27; 48; 49]; nevertheless, when it is synergistically applied with EDTA, their behavior increases from partial to complete removal of the smear layer, which was evaluated by SEM [9; 11; 20; 27; 36]. Interestingly, the results in the POBS test are lower in EDTA mixed with NaOCl at cervical, middle [6; 27; 28; 48], and apical levels [9; 28]. Orthophosphoric acid (OA) is a universal conditioner used in endodontic procedures to remove the SL, opening the dentinal tubules [40]. It can be found commercially in gel and liquid forms in a wide range of different concentrations from 5% to 37% [30; 50]. This conditioner evaluated by SEM showed a good rate for removal of layers, since it acts from partial removal with the gel form [30] to partial-complete removal with the liquid form [25; 30; 46], alone or combined with EDTA [16]. However, despite the optimal removal of the SSL by the OA in liquid form, better results have been shown in the post adhesion to the cervical, middle, and apical portions of the root canal using the POBS test when the gel form is used compared to the liquid form [30; 46]. Citric acid is a demineralizing solution that reacts with calcium ions in dentin [51]. In the endodontic therapy context, it exerts higher chelating effects than 17% EDTA when it is used at 1%, 5%, and 10% concentrations [42], even in the apical third [18]. Interestingly, when the citric acid is used at 10% during 90 s, after 60 s of 5.25% NaOCl treatment, there is a partial removal of the SSL from the root canal [9]. Also, this mixture showed similar bond strength values in the cervical, middle, and apical portions, however, these values are lower compared to bond strength values produced by the NaOCl/QMix, which showed a better adhesion to dentine; the NaOCl/citric acid mix was susceptible to present adhesive failure between the dentin and resin cement [9]. Maleic acid (MA) is a weak organic acid used as an irrigant in dentistry [18], since it produces demineralization at the intertubular dentin level resulting in a reduction of the substrate microhardness over a short period of time [18; 41]. Interestingly, it has been found by SEM imaging that treatment with 7% MA for 45 s followed by the application of 2.5%