A compound of formula (I): wherein k is 1 or 2;Y is —CH2CH(CF3)2 or —(CH2)nCF;X is hydrogen, —(CH2)nRf, or —C(O)[CF(A)]p-B—Rf, wherein B is a divalent radical selected from the group consisting of a covalent bond, —O— and —(CH2)m—; A is —F or —CF3; and each Rf is independently C1 to C6 perfluorinated linear or branched alkyl, optionally interrupted by one or more oxygens;Ro is Ro1 or is a linear or branched alkyl having from about 10 to about 100 carbon atoms, interrupted or substituted by one or more hydrophilic groups selected from the group consisting of —O—, —OH, —NR—, —N(R)2, and —C(O)NR—, wherein a) the ratio of hydrophilic groups to carbon atoms is from about 1:2 to about 1:10; b) each carbon atom has at most one hydrophilic group bonded to it, and c) covalent bonding between hydrophilic groups is absent;R is hydrogen or a C1 to C4 linear or branched alkyl; andRo1 is a linear or branched aliphatic group of from about 10 to about 100 carbon atoms, interrupted by about from about 5 to about 50 ether oxygens, wherein a) the ratio of ether oxygen to carbon atoms is from about 1:2 to about 1:3, b) each carbon atom has at most one ether oxygen atom bonded to it, and c) covalent bonding between ether oxygen atoms is absent;provided that 1) when Y is —(CH2)nRf, Ro is Ro1; 2) when X is hydrogen, Y is —CH2CH(CF3)2; and 3) when X is —(CH2)nRf and Y is —(CH2)nRf, Ro is Ro1.