Estimation of the critical constants of long-chain normal alkanes

Correlations between the critical constants of normal alkanes and the number of carbon atoms in a molecule have been considered. In an approximation of a self-consistent field for a polymeric fluid, an equation of state of the van der Waals type has been written, and the dependences of the critical constants of chain molecules on the number of mers have been obtained. It has been found that for an infinitely long alkyl chain, the limiting values of the critical temperature, the critical pressure, and the critical density are equal to, respectively, 1135 K, 0 MPa, and 0 kg . m(-3). A method of pulse heating of a wire probe immersed in the substance under investigation has been used to measure the dependence of the temperature of the attainable superheat T* of low-density polyethylene on the pressure p and the duration of heating pulse t*. Extrapolation has been used to obtain an estimation of the attainable-superheat temperature of polyethylene T*(p = 0, t* = 0) = 1175 K, which can be treated as the ''critical'' temperature of polyethylene.