{HEAD ~ R16} and {TAIL ~ R17} are each, independently, hydrogen, alkyl of 1 12 carbon atoms, or n Ar 
{ HEAD ~ R16 } and { TAIL ~ R17 } are each , independently , hydrogen , alkyl of 1 12 carbon atoms , or n Ar
HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH