Dissolution and ion release

The dissolution and ion release from dental silicate cement have been the most investigated characteristics with good reason, for they are central to its clinical performance. Erosion limits its life but release of fluoride has important clinical consequences. 

In neutral solution when fully hardened, dental silicate cements are resistant to aqueous attack. Before they have fully hardened, set cements contain soluble reaction intermediates - soluble sodium salts, acid phosphates and fluorides - which render them vulnerable to attack even by neutral solutions including saliva (Wilson, 1976). 

The composition of the leachates does not correspond to the composition of the cement at all (Wilson Batchelor, 1967a,b). The predominant species eluted are the soluble sodium salts of phosphate and fluorides, although sodium is only a minor constituent of the cement. For one example of cement examined, the leachate contained 0-28 % sodium and 0-20% phosphate (expressed as a percentage of the amount of the species contained in the cement). For the major constituents of the glass the figures were 0 07% fluoride, 0 02% Al Oj, 0 01 % SiOj and 0 003% CaO. 

The rate of elution declines sharply with time and the pattern of elution changes. The acid phosphate ions, H2PO4 and HPO4, are removed by further reaction or by elution, and the release of phosphate changes from predominant to minor. Thereafter, the rate of loss of phosphate is governed by the phosphate concentration of the solution indeed if the phosphate concentration of the solution is sufficiently high the process is reversed and the cement takes up phosphate. So, clearly, an ion exchange phenomenon is involved (Kuhn Wilson, 1985 Kuhn, Winter Tan, 1982). 

Elution of ions is accompanied by absorption of water and this can amount to as much as 20% by mass in five days (Kuhn et al., 1982). The extent of water uptake is affected by the ionic concentration of the solution. 

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