Water distribution in dentin matrices: bound vs. unbound water

Abstract

Objective: This work measured the amount of bound versus unbound water in completely-demineralized dentin.

Methods: Dentin beams prepared from extracted human teeth were completely demineralized, rinsed and dried to constant mass. They were rehydrated in 41% relative humidity (RH), while gravimetrically measuring their mass increase until the first plateau was reached at 0.064 (vacuum) or 0.116 gH2O/g dry mass (Drierite). The specimens were then exposed to 60% RH until attaining the second plateau at 0.220 (vacuum) or 0.191 gH2O/g dry mass (Drierite), and subsequently exposed to 99% RH until attaining the third plateau at 0.493 (vacuum) or 0.401 gH2O/g dry mass (Drierite).

Results: Exposure of the first layer of bound water to 0% RH for 5 min produced a -0.3% loss of bound water; in the second layer of bound water it caused a -3.3% loss of bound water; in the third layer it caused a -6% loss of bound water. Immersion in 100% ethanol or acetone for 5 min produced a 2.8 and 1.9% loss of bound water from the first layer, respectively; it caused a -4 and -7% loss of bound water in the second layer, respectively; and a -17 and -23% loss of bound water in the third layer. Bound water represented 21-25% of total dentin water. Chemical dehydration of water-saturated dentin with ethanol/acetone for 1 min only removed between 25 and 35% of unbound water, respectively.

Significance: Attempts to remove bound water by evaporation were not very successful. Chemical dehydration with 100% acetone was more successful than 100% ethanol especially the third layer of bound water. Since unbound water represents between 75 and 79% of total matrix water, the more such water can be removed, the more resin can be infiltrated.

Keywords: Adhesive dentistry; Bound water; Bulk water; Collagen; Dentin; Hydrogen bonding.

Fig. 1

Adsorption of water vapor on dentin matrices that had been completely dried by high vacuum and elevated temperature. Water content is given in g H₂O/g dry mass. Symbols indicate the means of 10 separate specimens. The standard deviations were smaller than the size of the symbols. The water content of dry matrices was 0.001 g H₂O/g dry mass. Exposure of dry matrices to 41% RH produce absorption of bound water that plateaued at a water content of 0.064 g H₂O/g dry mass. This first layer of bound water is thought to be due to Ramachandran single water bridges with carbonyl oxygens and amide hydrogens. Exposure of the first layer of bound water to 0% RH for 5 min produced very small loss of mass (see oblong symbol). Exposure of these specimens to 60% RH water vapor induce further water binding that reached a second, higher plateau at 0.220 g H₂O/g dry mass. This layer of bound water is thought to be due to the formation of double water bridges. Exposure of the second layer of bound water to 0% RH for 5 min produced small loss of mass (see oblong symbol). When the specimens were exposed to 99% RH, they bound more water until they reached a third plateau at 0.493 g H₂O/g dry mass. This third increment in bound water is thought to be due to water binding to hydrophilic side chains in collagen peptides that extend away from the central axis. Exposure of the third layer to 0% RH for 5 min produced a larger loss of mass (see oblong symbol). After 5 min, the specimens were replaced in 99% RH and they regained their original mass.

Fig. 2

Change in water content (g H₂O/g dry mass) of absolutely dry demineralized dentin beams over time. The specimens that were dried for 2 hrs in the vacuum oven had a water content of 0.946 g H₂O/g dry mass (squares). This fell to 0.429 and then to 0.059 g H₂O/g dry mass in the vacuum oven at 37°C over the next 50 hrs. Then the oven temperature was increased 10°/hr until the water content fell to 0.001 g H₂O/g dry mass. Specimens dried in Drierite for 2 hrs had a water content of 1.474 g H₂O/g dry mass that fell to 0.001 g H₂O/g dry mass after 200 hrs in 0% RH Drierite. Then both groups (n = 10) were exposed to 41% RH and weighed until they reached their first plateau in mass gain, at a water content of 0.064 g H₂O/g dry mass (vacuum oven group, squares) or 1.116 g H₂O/g dry mass (Drierite group, diamonds). The beams were then exposed to 60% RH water vapor and reweighed until they reached their second plateau in mass at 0.220 g H₂O/g dry mass (squares) or 0.191 g H₂O/g dry mass. They were then placed in water vapor of 99% RH until they reached their third plateau in mass at 0.401 g H₂O/g dry mass for the Drierite group (diamonds) or 0.493 g H₂O/g dry mass for the vacuum oven group (squares). Both groups were then immersed in liquid water at 25°C for 1 hr to fully hydrate. The final water content was 1.71 g H₂O/g dry mass for the Drierite group (diamonds) or 1.71 g H₂O/g dry mass for the vacuum oven group (squares), showing that the vacuum group did not fully recover their initial hydration.