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The size and composition of dust grains are critical in setting thedynamical, physical and chemical evolution of the media in which they arepresent. Thanks to facilities such as ALMA and in the future the SKA, theirthermal emission in the (sub)mm to cm has become a convenient way to tracegrain properties. Our aim is to understand the influence of the graincomposition and size distribution on the shape of their SED in dense ISMregions such as molecular clouds, prestellar cores, YSOs and protoplanetarydiscs. Starting from the optical constants defined in the THEMIS model foramorphous hydrogenated carbon and silicate grains in addition to water ice, wedefine 6 material mixtures representative of the expected dust composition indense ISM regions. The optical properties of 0.01 micron to 10 cm grains arethen calculated with effective medium and Mie theories. The corresponding SEDsare calculated for isolated clouds either externally heated by the ISRF aloneor in addition to an internal source. The 3 main outcomes of this study are asfollows. First, the dust mass absorption coefficient strongly depends on bothits composition and size distribution potentially leading to errors in dustmass estimates by factors up to 3 and 20, respectively. Second, it appearsalmost impossible to retrieve the grain composition from the (sub)mm to cm SEDshape alone as its spectral index for lambda > 3 mm does not depend oncomposition. Third, using the true dust opacity spectral index to estimategrain sizes may lead to erroneous findings as the observed spectral index canbe highly modified by the dust temperature distribution along theline-of-sight, which depends on the specific heating source and on the geometryof the studied region. Based on the interpretation of only the spectral shapeof (sub)mm to cm SEDs, the determination of the dust masses, compositions andsizes are highly uncertain.

From grains to pebbles: the influence of size distribution and chemical composition on dust emission properties