Why shouldn't you mix baileys and cola? Some time ago I ordered a Becardi Cola, I however received a Baileys cola. The cola and Baileys seemed to have reacted, it turned into something spongy. How did this happen? What reaction took place?
Cream liqueurs curdle because of their high dairy content. Dairy products (such as milk or cream) curdle in the presence of an acidic liquid. Acids have a very low pH, which lowers the overall pH of the mixture. As the pH drops below 5.5, the casein proteins in the dairy ingredients begin to curdle. That’s what you called “spongelike texture”. To safeguard against this avoid mixing cream liqueurs with high-acid mixers such as citrus juices and soft drinks (most soft drinks do not taste sour, but they do contain high levels of citric and phosphoric acids and they have the same effect due to the pH levels).   
If you enjoy a mixed drink with cream liqueurs try coffee, hot chocolate, other cream liqueurs, or milkshake-style cocktails (in which the dairy enhances the smooth mouthfeel of the ice cream). Cream liqueurs are also delicious served alone on the rocks.