why use of 1uf capacitors in max232 ic, why does not use 10uf capacitor in max232 ic.?
Because if datasheet says 1uF for the charge pump then that is the best value it works with the given frequency of the charge pump. Higher capacitance charges more slowly so the charge pump would need slower frequency or it would take longer for the pumped voltage to settle. Lower capacitances would charge too fast and can not transfer enough charge per clock cycle, so the voltage would droop under load. There is a difference between MAX232 and MAX232A, the former needs 1uF capacitors while the latter only needs 100nF capacitors.