A method for hair treatment, in which the hair comprises two or more free thiol groups, comprising: (a) applying to the hair a formulation comprising a bonding agent of Formula I ** Formula ** in which A, B, C and D are reactive moieties capable of reacting with the free thiol groups each containing one or more charges, wherein each of the reactive moieties A, B, C and D independently contains a moiety selected from the group consisting of vinyl sulfone, an acrylate group, a methacrylate group, a styrene group, an acrylamide group, a methacrylamide group, a maleate group, a fumarate group and an itaconate group； (R) n is a linker containing two or more charges, where the charges are opposite to the charges of the reactive moieties, where R is a monomer, n>； = 1-10, and where (R) n no it is a polymer, and the sum of the charges is zero, and where the reactive moieties are ionically bound to the linker； where each occurrence of p, q, rys is, independently, an integer from 0 to 25, where the sum of p + q + r + s is equal to or greater than 2, in an effective amount to join of covalently free thiol groups； with the proviso that the binding agent of formula I is not a trimaleate salt of H2 N- (CH2) 3-N1 (G) - (CH2) 4-NH2 wherein G is a substituent that is linked to the group of secondary spermidine amine, selected from linear or branched, saturated or unsaturated alkyl groups of 1 to 6 carbon atoms, wherein one or more carbon atoms are optionally substituted by fluorine, namely, methyl, ethyl, trifluoromethyl, trifluoroethyl, propyl, isopropyl, butyl, isobutyl, pentyl, hexyl, ethylene, vinyl, propylene, butylene； aryl or aryl-alkyl groups, such as phenyl, naphthyl, benzyl, tolyl, wherein one or more carbon atoms are optionally substituted with fluorine, and wherein said aryl-alkyl groups include linear or branched, saturated or unsaturated alkyl groups formed by 1 to 6 carbon atoms, in which one or more carbon atoms are optionally substituted with fluorine, namely meth
