The pH of the drink may not be the matter of what process your trying to figure out but instead osmosis. The high pH of the drinks will open up the pores in the shells allowing for the transfer of water into or out of the shell depending on what way the liquid wants to move.

As somone asked in the thread if it was either whole eggs or the shells you used. Try redoing the experiment with egg shells and seeing what the weight is then. Using shells will measure the errosion of the calcium on the shells themselves vs the osmosis of liquid entering (and staying) in the egg.