A modified particle comprising an aggregate, said aggregate having attached a group having the formula: ##STR20## wherein Ar represents an aromatic group; R1 represents a bond, an arylene group, an alkylene group, ##STR21## wherein R4 is an alkyl or alkylene group or an aryl or arylene group; R4 ' is an alkyl group or an aryl group; R2 and R3, which can be the same or different, represent hydrogen