Acidity bond strength effects

FIGURE 3.3 A summary of periodic trends in relative acidity. Acidity increases from left to right across a given row (electronegativity effect) and from top to bottom in a given column (bond strength effect) of the periodic table. 

Bond Strength The effect of bond strength is easy to see by comparing the acidities of the hydrogen halides... 

The strength of an acid depends on the atom to which the proton is bonded The two mam factors are the strength of the H—X bond and the electronegativity of X Bond strength is more important for atoms m the same group of the periodic table electronegativity is more important for atoms m the same row Electronegative atoms elsewhere m the molecule can increase the acidity by inductive effects... 

Table 1 Hsts some of the physical properties of duoroboric acid. It is a strong acid in water, equal to most mineral acids in strength and has a p p o of —4.9 as compared to —4.3 for nitric acid (9). The duoroborate ion contains a neady tetrahedral boron atom with almost equidistant B—F bonds in the sohd state. Although lattice effects and hydrogen bonding distort the ion, the average B—F distance is 0.138 nm the F—B—F angles are neady the theoretical 109° (10,11). Raman spectra on molten, ie, Hquid NaBF agree with the symmetrical tetrahedral stmcture (12).

Surface treatment of the composite can have a significant effect on adhesion. Surface treatment enhances one or more of the mechanisms described previously. Wu et al. [15] studied the effects of surface treatment on adhesive bonding for AS-4/APC-2 laminates. They found that the greatest bond strength was achieved from acid etching and plasma etching the composite surface. Table 1 summarizes the various surface treatments that were evaluated. 

To explain features of acid strength, we need to look at the effects of bond strength, polarity, and charge. 

Peddy, M. (1981). The bond strength of polycarboxylic acid cements to dentine effect of surface modification and time after extraction. Australian Dental Journal, 26, 178-80. 

Prati, C., Nucci, C. Montanari, G. (1989). Effects of acid and cleansing agents on shear bond strength and marginal microleakage of glass-ionomer cements. Dental Materials, 5, 260-5. 

Other common examples are aluminium chloride, tin(rv) chloride, zinc chloride, etc. We shall, at this point, be concerned essentially with proton acids, and the effect of structure on the strength of a number of organic acids and bases will now be considered in turn. Compounds in which it is a C—H bond that is ionised will be considered subsequently (p. 270), however. 

Prior to the use of the adhesives, the dentin surface is etched with a conditioning solution which is usually an aqueous acidic solution of citric acid-ferric chloride, phosphoric acid, or even polyacrylic acid [192]. These solutions tend to demineralize the dentin and expose the collagen (organic fibers in the dentin) [193], and thereby result in higher bond strengths. The effects of various pre-conditioning treatments on the bond strength of 4-META to dentin have been discussed [194]. 

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