Scanning force microscopy was used in the contact mode to determine th
e adhesion force between a mica surface and a silicon nitride tip. The
measurements were performed in an aqueous solution of sodium and calc
ium chlorides. The adhesion force according to the Derjaguin-Landau-Ve
rwey-Overbeek theory depends on the competition between two kinds of f
orces: van der Waals and electrostatic ''double layer''. Two different
curves of adhesion force versus salt concentration were obtained from
the experiment with monovalent and divalent ions. The tip-surface adh
esion force was determined from a statistical analysis of data obtaine
d from the force vs. distance retracting curves.