First of all 100% ethanol is highly volatile and tends to evaporate before entering into the tissue significantly. Secondly when absolute ethanol enters the cell first it encounters the cell wall and immediately starts coagulating proteins present in the cell wall before actually considerably entering into the cell and therefore the protective coagulated layer stops further inflow of more ethanol into the cell. In this case the cell is protected and stays dormant and again functions normally when favourable conditions reappear. But if we use 70% ethanol it evaporates much slower than absolute ethanol undoubtedly and it also coagulates proteins and dissolves lipid but at a very slower rate; enough for it to completely get into the cell and then starts coagulating proteins and dissolving lipids and thus the cell dies.why 70% ethanol is standardized for surface sterilization? It may be because perhaps the dilution is enough for entering into the surface layers of the plant tissue where the pathogens may get into and does not go too deep into the plant tissue so that the plant cells beneath the surface tissues are not considerably harmed.
