# A Literature Review of Adhesive Systems in Density: Key Components and Their Clinical Applications
## 6 Resin Adhesive Systems
### Functional monomers
#### 6.1.3 Solvents  
The wetting ability of adhesives is amplified by the addition of different solvents and hydrophilic monomers [26; 171]. The former eliminates the water, which is present between the collagen fibers, and dissolves the amphiphilic resin which increases the surface tension. The main solvents used in adhesive systems are acetone, ethanol, and water, each with a different evaporation time [17]. It is important to recall that the air-drying method is more intricate and reliant on pressure, position, angle, distance, and time [209]. Collagen fibrils collapse as a result of this technique, which also reduces monomer penetration and produces weaker BSs. It is advisable to investigate a different, more regulated approach. One of these methods is the blot-drying method, which quickly eliminates extra water by using a foam pellet (tissue or a sponge) to ensure that adhesives penetrate well [22]. The adhesive should ideally be allowed to air-dry on the dental surface in order to remove any leftover solvent, as this could disrupt the polymerization of the resin monomers and leave unwanted voids in the adhesive interface that could serve as defect initiator sites and weaken the BS [210]. Solvent evaporation is a critical factor in determining the bonding effectiveness of modern adhesives [211]. Various factors need to be considered to effectively carry out this clinical step. In this regard, the clinician must take into account that the evaporation capacity varies throughout solvents, which is caused by the vapor pressure (mmHg) [17; 212]. This pressure defines the point at which a liquid transforms into a gas [213]. The higher the vapor pressure, the more volatile the solvent is; that is to say, it evaporates more easily [49]. The capacity of solvents to attach to hydrogen is another crucial feature; this property permits collagen to expand again upon dehydration, hence enhancing resin dispersion and self-bonding [17] (Table 1). Polar solvents play a crucial role in dental adhesive systems. They have the ability to form hydrogen bonds with their solutes, facilitating effective bonding. Dipole moment is a measure of the separation of positive and negative electrical charges within a molecule. The Debye (D) is a unit used to express dipole moments. It is an indication of the degree of polarity of a molecule, with higher dipole moments demonstrating greater polarity. In summary, it measures how much a molecule's electrons are shared unequally between atoms, resulting in a partial positive charge on one end and a partial negative charge on the other. In the context of solvents, those with high dipole moments have strong polar properties, which make them effective at dissolving polar substances and enhancing interactions with other polar molecules, such as water [64]. Additionally, solvents with high dipole moments and excellent evaporation capacities exhibit good water-removing capabilities, which are essential for optimal adhesive performance. The hydrogen-bonding capacity of a solvent is particularly significant in re-expanding the shrunken demineralized collagen network after dehydration. Solvents with a higher affinity for forming hydrogen bonds can disrupt the stabilizing hydrogen bonds and other forces that maintain collagen in a shrunken state, thereby facilitating adhesive interactions [17; 48]. However, dipole moments and hydrogen bonding are not directly proportional. Just because a molecule has a higher dipole moment (like acetone) doesn't mean it forms \begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Solvent**} & **Dipole Moment** & **Dielectric** & **Boiling Point (\({}^{\circ}\)C)** & **Vapor Pressure** & **Ability to Form** \\  & **(D)** & **Constant** & & **(mmHg 25 \({}^{\circ}\)C)** & **Hydrogen Bonds** \\ \hline Water & 1.85 & 80 & 100 & 23.8 & High \\ Ethanol & 1.69 & 24.3 & 78.5 & 54.1 & Medium \\ Acetone & 2.88 & 20.7 & 56.2 & 200 & Low \\ Tert-Butanol & 1.7 & 12.5 & 82.4 & 46 & Medium \\ \hline \hline \end{tabular}
\end{table}