Intermolecular Forces and Dissolving

Question from email------------------------------

I have a question in relation with our today's lecture, yogurt comes out from milk then why yogurt can not dissolve in water like milk, that completely mixes with water.

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The short answer is that yogurt contains networks of proteins that are solid enough to make yogurt thick, but not so solid that yogurt is just a big solid lump. To explain better, we need to think about why things dissolve. Let's start with a lump of sucrose (table sugar) and a glass of water. The sucrose is solid because the intermolecular forces between the sugar molecules are very strong. The water is liquid because the intermolecular forces are also quite strong, but the individual water molecules can slide past each other. When the lump of solid sugar is dropped into the glass of liquid water, the sugar dissolves. Energetically, we can think of this as a series of interactions being broken and formed, with the result being whatever state bring us to the lowest energy. If the sugar is going to dissolve in the water, we need to break sugar-sugar interaction (requires energy) and we need to break water-water interactions (requires energy). At the same time, we need to form water-sugar interactions, which liberates energy. If the energy we get back from forming water-sugar interactions is greater than the energy required to break the sugar-sugar and water-water interactions, then the sugar will dissolve in the water. This is the part of chemistry called thermodynamics, which looks at how changes in energy affect chemical reactions. Looking at the following figure:

Going from “A” to “B” represents breaking sugar-sugar and water-water interactions, both of which require energy to be added to the system, noted by the skinny red arrow. When water-sugar interactions are formed, energy is taken out of the system, represented by the skinny green arrows. If the amount of energy we get back from forming water-sugar interactions is relatively small (going from “B” to “C” in the figure), then the net change in energy for the whole process is positive, and the sugar will not dissolve. This overall change is represented by the fat red arrow. If, on the other hand, the amount of energy we get back from forming water-sugar interactions is relatively large (going from “B” to “D” in the figure), then the net change in energy for the whole process is negative (fat green arrow), and the sugar probably will dissolve.

Now back to the yogurt question. The intermolecular interactions we have to think about in yogurt are protein-protein, water-water, and protein-water. {Proteins make this a little trickier because proteins have portions that are more hydrophilic and portions that are more hydrophobic.} The protein-protein interactions in yogurt are pretty strong, so the proteins stick together to form nets, BUT there are also parts of the protein molecules that have fairly strong protein-water interactions. The proteins do not form a hard, compact, crystalline solid like a sucrose crystal because they can form a lot of protein-water interactions, but the parts of the protein molecules where the protein-protein interactions are strong prevent the whole molecule from dissolving in water.

The even deeper part of this question actually shows up in the words that were used. yogurt does not “dissolve” in water, but milk “completely mixes” with water. Remember, milk is an emulsion, so although it does mix with water, it's not really “dissolving”. In that sense, milk and yogurt are similar, the difference being that the parts of milk that do not dissolve are tiny little droplets and clusters that can freely float around in the aqueous part of the milk, while the part of yogurt that doesn't dissolve is a large, extended network of proteins that makes yogurt thick and clumpy. And delicious.