I am trying to take the cartridge out of the faucet of my kitchen sink. I removed the handle and that little cap. I cannot 'isolate' the cartridge. Here are two pictures. I tried to use a pipe wrench but always got the jaws slipped off the threads.
Don't use a pipe wrench on those threads, you'll ruin them. Take a pair of needle nosed pliers open them up and put them in against the barbs in the collar and turn it counter clockwise. that's what the barbs are for. There are also faucet spanner wrenches that will fit in there to loosen the collar. you can gt them at most home stores, see picture below.