A stable mixture comprising surface-modified particles which are obtained by reacting metal oxide particles, the metal oxide being a magnetic iron oxide particle, with at least one compound of the general formula R 1 n -Si(OR 2) 4-n in which R 1, R 2 and n have the following meanings: R 1 are each independently hydrogen, linear or branched, optionally functionalized C 1 -C 30 alkyl, linear or branched, optionally functionalized C 2 -C 30 alkenyl, linear or branched, optionally functionalized C 2 -C 30 alkynyl, optionally functionalized C 3 -C 20 cycloalkyl, optionally functionalized C 3 -C 20 cycloalkenyl, optionally functionalized C 1 -C 20 heteroalkyl, optionally functionalized C 5 -C 22 aryl, optionally functionalized C 6 -C 23 alkylaryl, optionally functionalized C 6 -C 23 arylalkyl, optionally functionalized C 5 -C 22 hetaryl, R 2 are each independently hydrogen, linear or branched, optionally functionalized C 1 -C 30 alkyl, linear or branched, optionally functionalized C 2 -C 30 alkenyl, linear or branched, optionally functionalized C 2 -C 30 alkynyl, optionally functionalized C 3 -C 20 cycloalkyl, optionally functionalized C 3 -C 20 cycloalkenyl, optionally functionalized C 1 -C 20 heteroalkyl, optionally functionalized C 5 -C 22 aryl, optionally functionalized C 6 -C 23 alkylaryl, optionally functionalized C 6 -C 23 arylalkyl, optionally functionalized C 5 -C 22 hetaryl, NR 1 4 +, where R 1 can each independently have the meanings given above, group of the general formula 1/(p-x ∗ y) M p+ X x- y, where M is a metal atom selected from the group consisting of metals of the main and transition groups of the Periodic Table of the Elements, X is an anion, p is the oxidation number of the metal atom M, x is an integer from 1, 2 or 3, and y is an integer from 0, 1 or 2, and/or group of the general formula (IIa) -SiR 1 m (OR 2) 3-m (IIa), where R 1 and R 2 each independently have the meanings given above and m can each independently be 0, 1, 2 or 3, n 1, 2 or 3, and comprising at least one solvent, at least one surface-active substance or a mixture thereof, wherein in the compound of the general formula or in the group of the general formula (IIa) at least one radical R 2 is NR 1 4 + or a group of the general formula 1/(p-x ∗ y) M p+ X x- y, with the meanings given above for R 1, p, x, y, M and X. R 1 are each independently hydrogen, linear or branched, optionally functionalized C 1 -C 30 alkyl, linear or branched, optionally functionalized C 2 -C 30 alkenyl, linear or branched, optionally functionalized C 2 -C 30 alkynyl, optionally functionalized C 3 -C 20 cycloalkyl, optionally functionalized C 3 -C 20 cycloalkenyl, optionally functionalized C 1 -C 20 heteroalkyl, optionally functionalized C 5 -C 22 aryl, optionally functionalized C 6 -C 23 alkylaryl, optionally functionalized C 6 -C 23 arylalkyl, optionally functionalized C 5 -C 22 hetaryl, R 2 are each independently hydrogen, linear or branched, optionally functionalized C 1 -C 30 alkyl, linear or branched, optionally functionalized C 2 -C 30 alkenyl, linear or branched, optionally functionalized C 2 -C 30 alkynyl, optionally functionalized C 3 -C 20 cycloalkyl, optionally functionalized C 3 -C 20 cycloalkenyl, optionally functionalized C 1 -C 20 heteroalkyl, optionally functionalized C 5 -C 22 aryl, optionally functionalized C 6 -C 23 alkylaryl, optionally functionalized C 6 -C 23 arylalkyl, optionally functionalized C 5 -C 22 hetaryl, NR 1 4 +, where R 1 can each independently have the meanings given above, group of the general formula 1/(p-x ∗ y) M p+ X x- y, where M is a metal atom selected from the group consisting of metals of the main and transition groups of the Periodic Table of the Elements, X is an anion, p is the oxidation number of the metal atom M, x is an integer from 1, 2 or 3, and y is an integer from 0, 1 or 2, and/or group of the general formula (IIa) -SiR 1 m (OR 2) 3-m (IIa), where R 1 and R 2 each independently have the meanings given above and m can each independently be 0, 1, 2 or 3, n 1, 2 or 3,