Andy Looney, a guy who designs games and maintains a website full of, among other things, random weekly thoughts or observations, noted this week:
I don’t know if he really will test it out, but in the meantime, I thought it would be fun to throw it out to the SDMB crowd. Does anyone know offhand if he’s right? If so, why? If not, why does it seem that way?
I have no idea 
but as far as I know, the big difference between the two drinks is all that sugar. My crappy understanding of freshman chemistry would lead me to believe all that dissolved sugar messes with the properties of the water - I guess in the same way adding salt to water messes with the boiling point - and so it melts ice faster.
Just my WAG.
Yeah, but there are a lot things dissolved in Diet Coke too. And freezing point depression doesn’t rely on what is in the water, just the concentration/molality (if I learned that right.)
If this is true, I’d think it’d be due to the greater carbonation of DC, because it feels… crisper or something on my tongue.
Wouldn’t this go in the opposite direction? Just as salt melts ice, the sugar in non-diet Coke would make it melt ice faster, right?
Welp, back when I worked in a convienance (sp?) store, Diet Coke used to freeze in the coolers. That’s right, in the coolers, above freezing temps. That has to factor in here somehow, heh. =D
Cheers!
BA
Complete WAG: the sugar (actually corn syrup) makes regular Coke more viscous, which means less heat transfer by convection.
Until someone like Chronos shows up to do the heavy lifting, you might want to read This discussion. While nothing definitive comes from it, IMHO, it might provoke ideas.
That reminds me of something that bothered me about diet coke a few years ago. I used to have one of those slushy cups that you keep in the freezer and then you’d put some pop in it and stir it and it would freeze into a slushy. It would work fine with Coke but would not work at all with Diet coke.
The molality of regular Coke is much higher. There’s far more sugar than anything else dissolved in Coke, and artificial sweeteners are so sweet that they work in very low concentrations by comparison - hence the viscous consistency of regular soda. Coke has a much higher concentration of dissolved substances, which would make me assume that it would cause ice to melt faster. But there’s probably something else at work here.
Duh! I had forgotten that artificial sweeteners are sweeter than sugar.
But hold on. If DC freezes at a higher temperature, then that would mean less freezing point depression, right? But, if it melts faster than coke, that would mean greater FP depression… Hmm. I shall stand by the carbonation comment… lol, until you come along and tell me that Coke is more carbonated. 
I think you guys are approaching this problem backward… it isn’t a colligative properties problem, it’s a thermodynamics problem.
The ability to melt 2 identical masses of liquid ice should depend entirely on the ability of the liquid to transmit its heat to the ice. This would be directly via heat conduction throught the liquid, and convection of the ice as it melts.
I am pretty sure that it would impede convection of the liquid. The flow of melted ice water normally would sink to the bottom, but in a more dense sugar solution this flow might be retarded somewhat. As for conduction, I don’t know how the higher molality of the sugar solution would affect its ability to directly conduct heat. I think that would be the biggest factor, but google isn’t showing me anything at the moment.