sentence: This may be attributed to an elevated electrostatic interaction between strongly and positively charged PEI ( zetapotential : +35 – 40 Mv at pH 7 – 7.4 ) [ 40,41 ] and weakly and negatively charged cellulose surface ( zetapotential : − 18 to − 20 mV at pH 7 – 7.4 ) [ 42 ] .