Smoothing capacitor for half wave rectification
I don't know what you mean here with isolation, but the opamp does not isolate from mains, don't be lured into false sense of safety.

You have an ideal simulator, the op-amp will buffer whatever voltage there is at the input, and it will do that no matter how big capacitor you put at the op-amp output, it will get charged and discharged by the op-amp.