We have compared calorimetric measurements with the nonlinear optical technique vibrational sum frequency scattering to investigate interface-solvent effects in colloidal gelation transitions. This allows us to explain the difference in gelation behaviour between dispersions of stearyl-coated silica particles in n-hexadecane and benzene or toluene. In n-hexadecane dispersions, an anomalous heat effect is observed, due to the formation of an ordered interface layer (that is not confined to the first monolayer and is composed of ∼1/3 surface crafted chains and ∼2/3 solvent molecules). For solvents that cannot interdigitate with the surface chains this transition does not occur and consequently no heat effect is observed. © 2005 IOP Publishing Ltd.