The emergent properties that arise from self-assembly and molecular recognition phenomena are a direct consequence of non-covalent interactions. While gas-phase measurements and computational methods point to the dominance of dispersion forces in molecular association, solvent effects complicate the unambiguous quantification of these forces in solution. Here we have used synthetic molecular systems to measure interactions between apolar alkyl chains and aromatic rings. The dissected interaction energies are at least an order of magnitude smaller than estimates of dispersion forces derived from vaporisation enthalpies and dispersion-corrected calculations. Thus, the results provide the first quantitative experimental proof that dispersion forces between apolar hydrocarbons are almost completely cancelled by competitive interactions with the solvent. Instead, cohesive solvent-solvent interactions are shown to be the
major driving force behind apolar association in solution. These fundamental findings have implications for the development of supramolecular systems, catalysts and computational modelling.