Embodiments of the present
subject matter provide a biocidal composition comprising an aqueous mixture of (a) a
phosphonium compound and (b) a polymeric
ammonium compound, wherein the weight ratio of compound (a) to compound (b) is from 0.2:1 to 20:1, and whereinthe
phosphonium compound (a) has formula:wherein each R is independently a C1-C6
alkyl group which is unsubstituted or substituted by a cyano, hydroxyl, esterified hydroxyl or
aryl group;R1 represents a C8-C18
alkyl group which is substituted or unsubstituted; andX represents either
chlorine or
bromine; and whereinthe polymeric
ammonium compound (b) comprises the
repeat unit:wherein each R2 is independently a C1-C2
alkyl group which is substituted or unsubstituted;R3 represents a C2-C18 alkyl or alkenyl group which is substituted or unsubstituted; andR4 represents a C2-C18 alkyl or alkenyl group which is substituted or unsubstituted, a
diethyl ether group, an isopropanol group,a N,N-dipropylurea group, or a 2-
butene group.In some embodiments, the biocidal composition has a
synergy index (SI) of less than 1 in relation to the effect of the composition on the inhibition of growth of biological organisms.