Hot water heater expansion tank

I have just installed a new gas hot water heater. I intend to install a thermal expansion tank. The easiest place to install it is directly above the cold water inlet on the heater. I am concerened that hot water will migrate up into the tank. Will this be a problem? Should I put a loop to force the hot water to be lower than the heater inlet? The inlet and outlet pipes come out of the wall directly to the top of the heater. Thanks for the help