Ongoing tensions in cosmology have motivated the study of extensions to Lambda-CDM. A promising route is to consider interactions in the dark sector, with a transfer of energy-momentum between dark matter and dark energy. In this talk, I will discuss the cosmological effects of new classes of interacting dark sector models, which couple the intrinsic entropy of dark matter fluids with scalar field dark energy. I show that these models give rise to a pure-momentum transfer, modifying only the Euler equation up to linear order, while leaving the background expansion history unchanged. The effects of these interactions can be seen in the suppression of the matter power spectrum and structure growth at late times. Meanwhile, key features of the primary CMB anisotropies are unchanged, with only small ISW effects occurring on large scales. I conclude by outlining ongoing work on potential observational signatures associated with intrinsic entropy perturbations.