Abstract: The adsorption isotherm of amphoteric phenolresin (APR) on clay and the ζ potential of clay/water interface are determined. Effects of the concentration of APR, adsorption amount, net negative charge numbers in the APR molecular
chain and the typeand concentration of inorganic salts on the ζ potential are studied. At the beginning the ζ potential increases with increasing the concentration of APR. When adsorption of APR reaches saturated state, the ζ potential reaches the maximum value. And if the concentration of APR increases continuously , the ζ potential decreases slowly. The more the saturated adsorption amount and the net negative charge numbers in the
macromolecular chain, the higher the absolute value of ζ potential. |ζ| decreases linearly with increasing the logarithmic value of NaCl concentration. The influences of CaCl₂ on ζ potential more obvious than NaCl, which conforms to the SchulzeHardy rule. The adsorption of APR on clay surfaces can allevi-
ate the effect of inorganic salts on the ζ potential greatly.

Key words: adsorption, electrical potential, interface, drilling fluid treatment, molecular structure