from 0.1 to 10% by weight of a compound of formula : ##STR3## wherein the individual radicals R are identical are each a C1 C12 alkyl group or {TAIL ~ a polyoxyethylene radical} which contains 1 to 6 ethylene units and {HEAD ~ whose terminal OH group} is methylated; and 
from 0.1 to 10 % by weight of a compound of formula : # # STR3 # # wherein the individual radicals R are identical are each a C1 C12 alkyl group or { TAIL ~ a polyoxyethylene radical } which contains 1 to 6 ethylene units and { HEAD ~ whose terminal OH group } is methylated ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH