There are a lot of misconceptions as to why gel packs are a better option for keeping goods cold during shipping than simply using frozen water. While it is true that a gel pack is mostly water, it also has an additive that increases its viscosity. Probably the most confusing statement about gel packs is that they “last longer than water,” which does not seem to make a lot of scientific sense at first glance.
The water in the gel pack is the substance that is going to absorb and release energy, so if the gel has the additive detracting from its ability to do that, why would it last longer? The key here is to understand that the gel pack is being compared to ice in cubes, also known as wet ice in the cold chain industry. The term “wet ice” is even a bit of a misnomer, as ice made of water will naturally make wet whatever it touches. The usage here is more a reference to the ice not being in a water-tight container like a gel pack. Therefore when the ice melts, everything in the package gets wet.
Understanding Melting: Frozen Water VS Gel Packs
Understanding all of that is helpful in really getting down to the root of the question. When something is melting, a factor in its melting rate is the amount of exposed surface area there is available for heat to transfer through. Ice cubes in an unorganized jumble will have many small exterior faces that will together add up to much more total area than the gel pack will, since that will be frozen as a single rectangular solid. So the gel pack does not have more pure absorption capacity as water—it is actually less pound for pound since the gelling additives subtract from the amount of water. Therefore, it will take longer to melt due to the lower overall surface area. Additionally, the product that the gel pack is keeping cold will not get wet like it would with wet ice, as the refrigerant gel is sealed into a plastic pouch.
Being a single item also gives gel packs the advantages of ease of use and storage, with the added benefit of being reusable if so desired. For pharmaceutical items, just the fact that gel packs will not contaminate the product makes them a must. For food items, the gels can give the customer a feeling of quality that wet ice cannot provide.