Solid surface free energy and liquid surface tension are the main factors that affect the wetting of a solid surface. The contact angle provides information about the wettability of a material and the surface energy of a solid surface. When the contact angle is > 90°, the liquid will flow away from the surface, while when the contact angle is < 90°, the liquid will wet the solid material, and when the contact angle is zero, the surface will be completely wet [17]. Therefore, knowing the contact angles of the solutions used in root canal irrigation, especially with respect to the root canal dentin, is a crucial criterion in determining their clinical effectiveness. Hence, in our current study, the contact angle measurements of the tested solutions were performed on the surfaces of root canal dentin. Although there are few studies in the literature evaluating the contact angle on the dentin surface, these studies used single-rooted human teeth [18,19,20]. In order to make comparison and meta-analysis more effective for past and future studies, human single-rooted teeth were preferred in our study. In addition, wide and straight canals in this type of single-rooted teeth enable irrigation solution application research and activation efficiency research to be carried out in laboratory environments in a more standardized manner [8, 21, 22].

The roughness of the solid surface is also an important factor that affects the rate of liquid absorption [17]. Polishing the surface alters roughness, thus affecting the resulting contact angle [23]. Studies have pointed out the significance of polishing the dentin surface before contact angle analyses [18]. For this reason, each sample surface was polished with #180-grit sandpaper under distilled water using a Tronic Ecopol 200 Polisher (Metcon, Bursa, Turkey) to ensure standardization before contact angle measurements and smooth dentin surfaces were obtained. However, the samples were not additionally tested for surface roughness before the measurements.

The behavior of irrigation solutions on smooth dentin surfaces can provide information regarding their ability to reach complex recesses in the root canals. Factors such as pressure applied during irrigation, temperature changes, root curvature, and tubule anatomy can affect the surface tension and contact angle of the irrigant [24], but the purpose of polishing dentin surfaces in this study was to standardize the calculation of the contact angle and assess the wettability of the fluids used in the experiment by reducing other factors. Thus, future studies involving staining or fluorescence techniques are needed to measure the extent to which irrigation solutions can reach the recesses within the root canal.

The most commonly used method for determining the contact angles of liquids is the sessile drop method, where a goniometer device is used for measurements [17]. Alongside measuring contact angles, surface tension can also be determined using a goniometer device [17]. Since surface tension measurements can be conducted using the same device and setup as contact angle measurements, in our study, liquid surface tension was measured using the pendant drop method. Since contact angle measurements are independent of the volume of the drop on the sample [17], the volume of the drops applied to the sample surfaces was not specifically mentioned in our study.

Based on the statistical data, the average surface tension values for the tested solutions in the current study can be ranked as follows: QMix = Biopure MTAD < CHX < distilled water = HOCl < ClO2 < NaOCl = BA < EDTA. Similarly, the average contact angle values of the same solutions on dentin surfaces can be ranked as Biopure MTAD = ClO2 = CHX < NaOCl = QMix = BA = HOCl < EDTA < distilled water. Considering these data, both null hypotheses in our study have been partially rejected.

Literature indicates various methods for surface tension measurement, including the DuNouy ring method [5],Wilhelmy plate method [25, 26], capillary method [4], or pendant drop techniques [16]. In our current study, the pendant drop technique was utilized for surface tension measurements. While these methods are fundamentally based on similar concepts, the surface tension levels measured using different methods for the same solutions can vary significantly [16, 25, 27, 28].

Göktürk et al. [27] conducted an in vitro study using the DuNouy ring method to examine various irrigation solutions, measuring the surface tension values of 17% EDTA and 5% NaOCl solutions as 59.26 and 70.36, respectively, at 22–23 °C. In their research, the surface tension of NaOCl was found to be statistically significantly higher than that of EDTA, contradicting the findings of our study. Giardino et al. [25], measured the surface tension values of Biopure MTAD, 17% EDTA, and 5% NaOCl solutions as 34.5, 46.8, and 49, respectively, at 22 °C using the Wilhemny plate method. Ulusoy et al. [28], using the sessile drop method, measured the surface tension of 17% EDTA and 5.25% NaOCl solutions at room temperature (20–22 °C) as 67.64 and 56.98, respectively, and found them to be statistically similar. Yılmaz et al. [16] used the pendant drop technique to measure the surface tension of 17% EDTA at different temperatures and pH levels. They determined this value to be 70 at 22 °C and pH 7.5. The same researchers also suggested that different preparation conditions, such as pH or temperature of EDTA, could lead to different surface tension levels, and the surface tension of the solution could also change depending on the shelf life of ready-made solutions [16]. In our study, the surface tension of 17% EDTA solution at pH 12.1 was determined to be 98.85 at 20 °C, which was statistically significantly higher than that of 5.25% NaOCl solution. In the study by Yılmaz et al. [16], it was reported that when prepared at room temperature, the EDTA solution showed similar surface tension levels at pH 5.5 and 10.5, while it exhibited significantly lower levels at pH 7.5. The same study also noted that increasing the temperature significantly reduced the surface tension of the EDTA solution at pH 5.5 and 7.5 but did not cause a significant change at pH 10.5. The higher surface tension measurement value in our study compared to other studies may be attributed to the measurements being conducted at 20 °C, differences in pH values of EDTA, and/or differences in the methodology.

Boric acid (BA) is a notable irrigation solution with potential diverse applications in endodontics, as it not only exhibits anti-inflammatory effects but also removes the smear layer, exposes collagen fibrils, and opens dentinal tubules due to these properties [29]. Sakhipgareev et al. [30] measured the surface tension of 5% BA using the duNouy ring method at 26 °C, obtaining a value of 71. They noted variations in surface tension, with values of 69 at 40 °C and 64 at 70 °C, indicating a dependency on concentration and temperature. In our current study, the surface tension of BA was measured as 82.06, potentially influenced by the temperature of 20 °C and the use of the pendant drop method in our experiment, explaining the difference.

Lee et al. [31] examined the contact angle of BA on solid surfaces, reporting a decrease in angle with increasing concentration. They noted that the contact angle of BA on solid surfaces ranged from 40° to 36.9° for concentrations between 0.2% and 0.6%. On a solid surface, the shape of the droplet and the contact angle of the solution are also dependent on the surface free energy and topography of the solid [17]. To achieve accurate contact angle measurements, it is essential for the solid surface to be both clean and smooth, as well as hard. However, root canal dentin surfaces do not possess the required level of smoothness even after the cleaning and shaping process. The concentration of BA used in our investigation was 5%, and we assessed the contact angle on polished dentin surfaces to be 53.81°. The discrepancy may be attributed to variations in the concentration that was used in prior investigations and the method of measuring contact angles on dentin surfaces. However, the lack of adequate evidence in the literature required further research to arrive at a conclusive assessment.

The present study found that EDTA and BA exhibited higher surface tension compared to distilled water; however, their contact angles were statistically significantly lower than those of distilled water. While there is literature available on contact angle measurements of various solutions, there are relatively few studies specifically evaluating contact angles on direct dentin surfaces [18,19,20]. There has not been a study specifically evaluating the contact angle of 17% EDTA and 5% BA on dentin surfaces. EDTA is a chelating agent that reacts with calcium ions in dentin to form soluble calcium chelates. [32]. It is also known that BA removes the smear layer [29]. It is also known that the contact angle is affected by chemical reactions occurring at the interface [33]. This reaction could explain why EDTA and BA exhibit a lower contact angle compared to distilled water in our study; however, further research is required to investigate this matter.

In the literature, the surface tension of NaOCl at various concentrations has been measured using different methods [34]. Leonardo et al. [34] measured the surface tension of NaOCl using the DuNouy ring method at 25 °C and found it to be 66.41. Spano et al. [35] measured the surface tension of 5% NaOCl using a tensiometer at 22 ± 2 °C and found it to be 77.3. Sevimay et al. [24] conducted surface tension measurements of NaOCl at various temperatures and concentrations using the du Nouy ring method. Their findings indicated that elevated temperatures correlated with a reduction in surface tension, whereas higher concentrations were associated with an increase in surface tension. Additionally, they determined the surface tension of 5.25% NaOCl at 21 °C to be 67.13. However, our study recorded this value as 81.77 at 20 °C.

BioPure MTAD is a final irrigation solution composed of doxycycline, citric acid, and Tween 80. With a pH value of 7.0, it is considered a biologically acceptable irrigant and is claimed to be usable as a single solution for final irrigation purposes [36]. Tween 80 (polyoxyethylene sorbitan monooleate) is a nonionic surfactant and detergent [36]. Çiçek et al. [37] measured the surface tensions of various irrigation solutions using the Traube stalagmometer with the drop method. They determined the surface tension values of BioPure MTAD, 2% CHX, and 5.25% NaOCl to be 48.53, 60.66, and 84.93, respectively, which is consistent with the findings of our study. Giardino et al. [26] measured the surface tension of Biopure MTAD and Qmix solutions using the Wilhelmy plate method, determining them to be 34.54 and 36.43, respectively. They found no statistically significant difference between these two solutions, which is consistent with the results of our study. Altundasar et al. [38] measured the surface tension of Biopure MTAD and 17% EDTA solutions at room temperature using the pendant drop method, determining them to be 37.45 and 79.90, respectively. Their findings align with the results of our study. While there are studies evaluating the surface tension of Biopure MTAD in the literature, there is no study measuring the contact angle on dentin surfaces. In this regard, our research contributes to the literature.

QMix, introduced to the market in 2011, serves as a final irrigation solution after NaOCl for the simultaneous removal of the smear layer and dentin disinfection. It contains EDTA, CHX, and a detergent (cetrimide), with a pH slightly above neutral [26]. In their in vitro study, Giardino et al. [26] examined the surface tension of QMix using the Wilhelmy plate method and measured it as 36.43. Our current research revealed a surface tension of 39.40 for QMix, and a contact angle of 49.98 on dentin surfaces. To our knowledge, there has been no prior investigation into the contact angle of QMix with root canal dentin, making our study the first to provide data in this regard. Consequently, further research is warranted to expand on this topic.

CHX is a broad-spectrum antimicrobial agent highly effective against gram-positive and gram-negative bacteria as well as yeasts. Because it is cationic, CHX can breakdown the outer layers of cells, letting things pass through them, and its electrostatic layers can stick to the outer walls of bacteria [39]. Sevimay et al. [24] measured the surface tension of 5.25% NaOCl and 2% CHX using the Du Nouy method at 21 °C, reporting that 2% CHX exhibited a statistically significant lower surface tension compared to 5.25% NaOCl. Additionally, according to the same study data, the researchers also reported a decrease in surface tension with increasing temperature. Abbaszadegan et al. [40] measured the surface tension of 5.25% NaOCl, 2% CHX, and 17% EDTA using the Wilhelmy plate method at 25 °C. They reported that 2% CHX exhibited a statistically significant lower surface tension compared to the other two solutions. These findings are consistent with our study results. Estrela et al. [41] calculated the surface tension value of 2% CHX using a tensiometer at 25 °C to be 55.5. However, our current study's results determined this value to be 61.72.

Basrani et al. [20] determined the contact angle of CHX on the dentin surface to be 28.4. Thirunarayanan and Hegde [19] calculated the contact angle of 2% CHX on the coronal root dentin surface to be 21.54. We determined this value to be 39.14 in our study. Natural variations in the root dentin surfaces used in our study, such as differences in the number and width of tubules depending on factors like the age of extracted teeth, could explain this discrepancy.

ClO2 has become a popular irrigation solution for root canal treatment in recent years. This is because it kills bacteria, dissolves organic tissue, and is less harmful to cells than NaOCl [8, 42]. Dang et al. [43] conducted an in vitro study, measuring the surface tension of 0.405% ClO2 at 26 °C using the pendant drop method, yielding a value of 59.28. We measured the surface tension value for 0.2% ClO2 in our current investigation to be 77.08. The disparity between the findings could stem from differences in temperature and concentration. No other study measuring both the surface tension of ClO2 and its contact angle on dentin surfaces was found in the literature; thus, our research contributes to the literature in this aspect.

HOCl is a potent oxidizing agent. In an aqueous solution, it dissociates into H + and OCl-, leading to the denaturation and aggregation of proteins [44]. HOCl also kills viruses by chlorinating them and creating chloramines and nitrogen-based radicals. These help break both single- and double-strand DNA, which makes viruses useless [45]. After reviewing the literature, several studies have investigated the use of HOCl as a surgical antiseptic and cavity disinfectant. Additionally, these studies investigated the impact of HOCl on enhancing the bonding of dentin bonding agents through the removal of proteins. Additionally, while there are articles exploring its role as a root canal antiseptic and pulp tissue solvent, as of now, there is no research evaluating its contact angle and surface tension properties [46,47,48]. Therefore, our study aims to contribute to the literature by investigating the contact angle and surface tension between HOCl and dentin surfaces.

In our study, although the surface tension values of NaOCl, ClO2, and HOCl were higher than those of distilled water, the contact angle with dentin surfaces was found to be lower than that with distilled water. It is observed that the high surface tension values of these solutions did not adversely affect the contact angle. Iglesias et al. [18] measured the contact angle of 2.5% NaOCl on root dentin, obtaining values of 44.7 in the cervical region and 44.63 apically. In our current study, the contact angle of 5.25% NaOCl was measured as 48.81. In an in vitro study conducted by Spano et al. [35], the surface tension and pH values of NaOCl before and after contact with pulp were examined, revealing a decrease in both values after contact with organic tissue. The researchers attributed this phenomenon to a variety of chemical reactions initiated by NaOCl upon contact with organic matter. Fatty acids react with sodium hydroxide to form soap and glycerol (saponification reaction); amino acids react with sodium hydroxide to form salt and water (neutralization reaction); and additionally, they react with HOCl to form chloramine and water. These reactions occur simultaneously and synergistically, leading to the liquefaction of organic tissue. It is also known that NaOCl deproteinizes hard tissues [49]. ClO2, a chlorine-based compound, is known for its organic tissue-dissolving effect [42]. In our study, the lower contact angles observed for NaOCl, ClO2, and HOCl compared to distilled water may be attributed to the decreased surface tensions resulting from their contact with the smear layer on dentin surfaces, as also suggested by Spano et al. [35], or to chemical reactions occurring at the interface [33].

Understanding both the surface tension and contact angles of irrigation solutions not only provides valuable data on the properties of these solutions but can also assist researchers in the development of advanced irrigation solutions for clinical applications. According to Giardino et al. (25), an irrigation solution with lower surface tension and/or dentin contact angle may remove microorganisms and debris in the root canal system more effectively. Therefore, the present study may guide the selection of clinically preferred solutions, and more successful treatment results may be achieved.

In our present study, although the surface tension values of EDTA, NaOCl, ClO2, BA, and HOCl were higher than those of distilled water, their contact angles with dentin surfaces were found to be lower than those of distilled water. Hence, it was observed that the high surface tension properties of the solutions did not adversely affect the contact angles of the root dentin. Therefore, it may be suggested that there might not be a direct linear relationship between surface tension and contact angles on different surfaces, and these values should be independently evaluated for each solution and surface type.