Reference: Sharh ‘Umdatil-Fiqh – Tape No.4, Question No.13
Question: Can soap be used as a substitute for soil [or sand] when purifying from the impurity of the dog[‘s saliva]?
Response: No, no it cannot be used as a substitute for soil [or sand]. That is because soil [and sand] has [specific] features which [is why] Allaah has rendered it as a substitute to water when removing an impurity; it is therefore not like soap. [Let us offer an example], if you were to do tayammum with soap [by itself], would that be correct? Would it remove the impurity? No, [because] it is not [have the specific features of] soil [or sand].