Tribochemical Interactions of Acid-Base Chemistries

Tribochemical interactions of acid-base in tribosystem have been observed at two levels (i) acid-base reaction in oil formulation and (ii) acid-base reaction on metal surfaces. 

In mineral and synthetic engine oil formulations, hydrogen bond formation between anionic or molecular bases (A, B) and their conjugate acids HA or BH+, have a heavy impact on the activity of acids and their salts. For example, in mixtures of carboxylic acid with excess of sodium carboxylate in hydrocarbon and aprotic solvents, the activity of the acid is greatly decreased because of homoconjugation  

The formation constants K,(B11B ) for the hydrogen bonding of cationic acids, BH+ with free N-bases (B). 

Carboxylic acids and phenols R(CH2)nCOOH, ArOH, and cationic R(CH2) NH3+ conjugated acids exist in engine oil formulations together with their own or different bases, and will interact to form hydrogen-bonded complexes (Fox et al., 1991b). 

When this conjugation occurs, the level of active (corrosive) acid is substantially decreased. No simple quantitive correlation has been shown between the acidity (pKa) of acids in hydrocarbon formulation and low polar solvents (Coetzee, 1967). Acid-base interaction with and without proton transfer (PT) (BH+A B...(HA)m) has been related to acid and base enthalpies of reaction (Pawlak and Bates, 1982), the infrared carbonyl stretching band and gradual appearance of the asymmetric COO band (Lindeman and Zundel, 1972 Magonski and Pawlak, 1982), changes in pH (Kuna et al., 1982 Pawlak et al., 1982), NMR proton chemical shifts (Magonski and Pawlak, 1982), and dipole moments (Sobczyk and Pawelka, 1979). These parameters depend upon the acid-base strength of the partners, ApKa(PT) the difference between the pKa(acceptor) and pKa(donor) on the water scale (Sobczyk, 2001). 

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