In contrast to SN2 reactions, the strength of the nucleophile generally does not affect the rate of SN1 reactions.
Why? Because the nucleophile is not involved in the rate-determining step, which is the formation of the carbocation intermediate. Once the carbocation is formed, any sufficiently reactive nucleophile can attack.
While the nucleophile concentration does not influence the rate of the reaction, it can affect the product distribution. A higher concentration of nucleophile can shift the equilibrium toward the substitution product, improving the yield.
In many SN1 reactions, the solvent itself acts as the nucleophile. This is known as solvolysis.
Solvolysis is common in SN1 chemistry, particularly in reactions such as: