Fracture surfaces of high-purity silica glasses, finely patterned with small amounts of either GeO2 or P2O5 substituted for SiO2, are investigated by atomic force microscopy under an electrolyte. The samples are cross sections of graded-index optical fibers of the type used for long-distance optical communications. While topography images are featureless, the frictional force is strikingly sensitive to the composition, down to similar to0.1% for the [P]/[Si] ratio. This unexpectedly large contrast depends on the pH of the electrolyte showing its relation to the acid-base properties of the oxides. This result holds considerable promise for the local exploration of acid-base properties of mixed oxides down to the nanometer scale.