# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Functional monomers
#### 6.1.3 Solvents

Table 1: Properties of solvents commonly used in dental adhesives [17]. stronger hydrogen bonds. Acetone has a higher dipole moment than water but forms weaker hydrogen bonds because it lacks the strong electronegative atom-hydrogen bonds that water has (O-H bonds). So, while both dipole moments and hydrogen bonds are related to molecular polarity, their strength and presence depend on the specific molecular structure and the types of atoms involved [17; 64]. The dielectric constant is the measure of a material's capacity to store electrical energy in an electric field. It is signified by the symbol \(\varepsilon\) (epsilon) and is a quantification of the degree to which a material can polarize in reaction to an external electric field. For solvents, the dielectric constant specifies the solvent's ability to reduce the strength of electrostatic forces between charged particles. Solvents with higher dielectric constants have a greater ability to stabilize charges and dissolve ionic or polar substances. In regard to the adhesive performance, solvents with higher dielectric constants can be beneficial because they are capable of effectively dissolving and removing water from the substrate, expediting the interaction between the adhesive and the substrate surface. Additionally, they aid in stabilizing the charged species within the adhesive, promoting better bonding and wetting [17]. The boiling point of a solvent is defined as the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure, leading to the transformation of a liquid into a gas. It is a crucial characteristic that determines the volatility and evaporation speed of the solvent [17; 48]. Acetone is an organic solvent, has a high vapor pressure of 200 mmHg, but lacks an H-bonding capacity, evaporates quickly (evaporation temperature, 56.2 \({}^{\circ}\)C), and requires the shortest drying time in the mouth [17; 26; 209; 214]. Acetone-based systems require and tolerate higher humidity levels without any loss of adhesion. Other systems, especially those based on ethanol, require a drier surface [26]. In acetone-based systems, the evaporation point of the water is lowered, while that of acetone is raised when acetone and water come into contact with dentin. This is known as the azeotropic phenomenon, which is defined as a mixture of two liquids that are distilled at a constant temperature and produce a set composition vapor at a certain pressure [215]. As a result, acetone and water evaporate, and the resin is able to pass through. For this reason, acetone-based adhesives should not be brushed during their application in order to avoid their rapid evaporation before the infiltration of the resin monomers into the demineralized dentin [48; 216]. It is important to note that achieving a moist surface does not imply tolerance to contaminants such as blood, gingival fluid, or saliva [17; 48]. Acetone allows a mixture of polar and nonpolar chemicals due to its dipole moment and dielectric constant, which facilitates the formulation of single-bottle adhesives that contain hydrophobic and hydrophilic monomers [17; 48; 217; 218]. However, due to its higher vapor pressure compared to ethanol, it is more volatile, necessitating higher solvent concentrations that lower monomer concentration and typically require the application of two or more bonding layers to achieve optimum BS [216]. In fact, reports have shown that acetone-containing adhesives have an inferior BS than ethanol-based solutions [219; 220; 215]. Additionally, acetone's higher volatilization compared to other solvents is likely to shorten its shelf-life and prevent certain manufacturers from using it [17]. Ethanol is a polar solvent, has a vapor pressure of 54.1 mmHg, average H-bonding capacity, evaporates more slowly (evaporation temperature, 78.5 \({}^{\circ}\)C) than acetone, and requires a moderate drying time [17; 48; 221; 222]. When combined with substances like water, ethanol forms hydrogen bonds [222].