Unlike the aliphatic ester promoters above, the diffusion coefficients of the 4-n alkyl benzaldehydes are broadly similar, allowing for the estimated uncertainties. Here, increasing alkyl chain length, in the 4 position of the benzene ring, does not lead to a significant change in the total diffusion coefficient. In contrast, smaller and lighter benzaldehyde has a lower diffusion coefficient of all the promoters studied. In all cases, the diffusion coefficient is greater along the straight pores (b direction).