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