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Ion-acoustic envelope solitons in a collisionless unmagnetized electron-positron-ion plasma are studied. The Krylov-Bogoliubov-Mitropolsky perturbative technique is used to obtain the nonlinear Schrödinger equation. The critical wave number for the modulational instability depends upon the concentration of different species and the temperature ratios of electrons and positrons. In the limiting case of zero positron concentration we recover the previous results of electron-ion plasma. It is found that a small concentration of ions in the electron-positron plasmas can change the dynamics of the system significantly. The ions can introduce slow time and long spatial scales in the plasmas. Thus the electron-positron plasmas become richer in linear and nonlinear wave dynamics.

A criterion is presented to decide whether a produced plasma can be called a pure pair-ion plasma or not. The theory is discussed in the light of recent experiments which claim that a pure pair-ion fullerene (C60±) plasma has been produced. It is also shown that the ion acoustic wave is replaced by the pair ion convective cell (PPCC) mode as the electron density becomes vanishingly small in a magnetized plasma comprised of positive and negative ions. The nonlinear dynamics of pure pair plasmas is described by two coupled equations which have no analog in electron-ion plasmas. In a stationary frame, it becomes similar to the Hasegawa-Mima equation but does not contain drift waves and ion acoustic waves.

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