As a matter of course, {HEAD ~ cation exchange membranes} permit selective permeation of {TAIL ~ cations} alone 
As a matter of course , { HEAD ~ cation exchange membranes } permit selective permeation of { TAIL ~ cations } alone
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH