alkyl radicals, R2 and {HEAD ~ R3} are ##STR18## or a mixture of ##STR19## where {TAIL ~ R7} and R8 are selected from C.sub 
alkyl radicals , R2 and { HEAD ~ R3 } are # # STR18 # # or a mixture of # # STR19 # # where { TAIL ~ R7 } and R8 are selected from C.sub
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH