# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Functional monomers
#### 6.1.3 Solvents
Even though this solvent has a higher vapor pressure than water, it cannot be completely eliminated from the dental adhesive within a clinically feasible time frame [221]. A prior investigation revealed that ethanol remained in the adhesive for 60 s after volatilization [223]. Dental adhesives' sorption, solubility, and curing rates can all be hampered by excessive ethanol [17; 223]. According to [PERSON] et al. [224], ethanol concentrations of 20% or below are optimal for adhesive systems since they have a lower resin viscosity and enhance molecular mobility and polymer conversion. Higher concentrations may impair the adhesive'smechanical qualities and encourage the phase separation of hydrophobic and hydrophilic constituents [225]. The dental collagen matrix could expand and become more rigid when ethanol is added, which makes it easier for monomers to penetrate the demineralized dentin's collagen network. Nonetheless, as ethanol's hydroxyl group esterifies carboxylic groups, it may interfere with the capacity of the acidified chains to etch the tooth surface when paired with monomers that have carboxyl groups such as 10-MDP. Therefore, ethanol is inappropriate for monomers with carboxylic acid moieties [17; 48]. Furthermore, residual excess ethanol from air-drying may enhance the adhesive's water sorption and solubility, promoting hydrolytic breakdown and raising the risk of cytotoxicity for human dental pulp cells [225; 226]. The single-bottle adhesives' hydrophobic and hydrophilic monomers may not separate as a result of the presence of ethanol. Also, the aqueous ethanol solution creates an azeotrope, which facilitates vaporization of the remaining water within the demineralized dentin substrate [29; 227]. However, it is impossible to completely remove both bound and unbound water from the demineralized dentin [228], since ethanol vaporizes slower than acetone [29; 229]. Water is strongly polar, has a vapor pressure of 23.8 mmHg, and high H-bonding capacity [10], but because of the low vapor pressure and high boiling temperature (100 \({}^{\circ}\)C) [230], water evaporates very slowly and requires the longest drying time [48]. Agreeing to [PERSON] et al. [10], water has a strong capacity to break hydrogen bonds between collagen fibrils, allowing for further resin infiltration and re-expansion. This is essential for the creation of a HL in ER adhesive systems. As was previously mentioned, this solvent's capacity to ionize the acidic monomers found in SE adhesives--which are in charge of the chemical adhesion of these systems--is another crucial feature [231]. However, due to water's low vapor pressure, it is difficult to completely remove this solvent from the adhesive layer. As a result, to facilitate the evaporation of such a solvent, it is advised to combine it with additional solvents (ethanol and acetone), resulting in an azeotropic mixture [232]. This leads to superior vaporization, which will increase the adhesive's degree of conversion and create a higher-quality of HL that will strengthen the bond [17]. Achieving the optimal moist state can be challenging, particularly when dealing with massive tooth cavities and the intraradicular space. As a result, new adhesive compositions were created to decrease the sensitivity of modern adhesives to moisture. Certain materials enable proper dentin hybridization regardless of the substrate's moisture content. These adhesives often contain solvents other than water, ethanol, and acetone, such as tertiary butanol [233]. Tert-butanol, also known as 2-methyl-2-propanol, has a vapor pressure of nearly 26 mmHg that is close to the vapor pressure of water (46 mmHg) [17]. Compared to other alcohols, tert-butanol has a molecular chain with fewer hydrogen atoms and a superior molecular weight [234]. Contrary to ethanol-based systems, the molecular structure, which consists of four carbons and one alcohol group surrounded by three methyl groups, offers better stability and compatibility with both water and polymerizable resins [17; 235].