What specific materials freeze solid beyond the 'ice line' enabling gas giant core accretion?

Water, methane, and ammonia

The established model for the formation of gas giants like Jupiter dictates that they must begin building their cores far from the star, in a region termed the 'ice line.' The significance of the ice line is that the temperature drops sufficiently low that volatile compounds, which are abundant in the protoplanetary disk, can freeze solid, turning into abundant icy material. These frozen compounds—specifically water, methane, and ammonia—provide the necessary solid mass for the planetary core to accrete quickly. Once this core reaches a critical mass, estimated to be around ten times the mass of Earth, its gravity becomes strong enough to rapidly sweep up the vast amounts of hydrogen and helium gas that remain in the surrounding environment, completing the formation of a Jovian-sized planet.