The coefficient of friction μ between plate glass and natural rubber subjected to boundary lubrication by various isotonic salt solutions was measured. All the monovalent chloride solutions tested (Na+, K+, Li+, NH4+, (CH3)3NH+and (CH3)4N+) reduced the value of μ below that obtained using distilled water as the lubricant. The divalent cations tested (Ca2+, Ba2+and Mg2+) and La3+increased μ. The effects off Fe3+and Al3+were anomalous. Little increase in friction was observed in the presence of these cations, but large increases in μ were obtained ;after they had been rinsed away with NaCl. Immersion in water increased the extensibility of the rubber relative to that obtained in air. The addition of monovalent cations had no measurable effect on the elastic properties of the rubber. The friction increased progressively when the pH of the NaCl solutions was reduced below 4 by adding HCl solution. Desorption experiments showed that the rubber adsorbed45Ca2+. The effect was competitively inhibited by the addition of 0.1 and 1 M non-isotopic CaCl2. The effects are interpreted as follows.(1)Monovalent cations reduce the friction by forming double-layer mutual repulsion forces.(2)Divalent cations increase the friction by bridging polar groups on 0 the rubbing surfaces.(3)The increase in μ when Fe3+and Al3+are rinsed with NaCl results0 from hydrolysis of adsorbed ions at the higher pH of the rinsing solution. Hydroxylated metal ions coagulate under these conditions and form a molecular sludge between the rubber and the glass. © 1997 Elsevier Science Ltd. All rights reserved.
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