The mechanisms of nucleation and growth of carbon dust particles in circumstellarenvelopes of carbon-rich stars in the red giant and AGB phases of their evolution arepoorly understood. It has been proposed that the transition of gas phase species to solidparticles,is achieved by the formation of a critical nucleus composed of two PAHs heldtogether by van der Waals forces. Some insights into the validity of the nucleation of PAHmolecules in the envelope can be gained through the investigation of the thermodynamics ofdimers,representing the first stage towards condensation. We have performed experimentsto identify the temperature range over which small PAH clusters form in saturated uniformsupersonic flows. The kinetics of the formation has also been investigated. Theexperimental data have been combined with theoretical calculations. We unambiguouslydemonstrate that the association of small PAHs such as pyrene(C16H10) is slower than the destruction of the dimer in warm andhot environments such as IRC +10216. Our findings challenge a formation model based on thephysical stacking of small PAH units in circumstellar shells of carbon rich stars.