Based on the temperature behavior of the entropy increment induced by the probing electric field in the isotropic phase of mesogenic compounds belonging to the homologous series CnH2n+1PhCOOPhCN (n = 4-10), it is found that an ability to the dipolar aggregation of the molecules depends on the alkyl tail length, and, in particular, the ability is strongly reinforced when the number n changes from 7 to 8. The role of the molecular structure in the self-assembling process is discussed.