Large scale aggregation in magnetic colloids induced by high frequency magnetic fields

Journal Pre-proofs Large scale aggregation in magnetic colloids induced by high frequency magnetic fields V. Socoliuc, R. Turcu PII: DOI: Reference:

S0304-8853(19)32428-X https://doi.org/10.1016/j.jmmm.2019.166348 MAGMA 166348

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Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

5 August 2019 11 December 2019 20 December 2019

Please cite this article as: V. Socoliuc, R. Turcu, Large scale aggregation in magnetic colloids induced by high frequency magnetic fields, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/ j.jmmm.2019.166348

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Large scale aggregation in magnetic colloids induced by high frequency magnetic fields V. Socoliuc1*, R. Turcu2 1Romanian

Academy – Timisoara Branch, Center for Fundamental and Advanced Technical Research, M. Viteazu Ave. 24, 300223 Timisoara, Romania 2National Institute for Research and Development of Isotopic and Molecular Technologies, Donat 67-103, 400293 Cluj-Napoca, Romania * corresponding author: [email protected], [email protected]

Abstract The paper presents evidence of large scale aggregation in an aqueous dispersion of magnetic microgels, induced by 100kHz AC magnetic fields with amplitude ranging from 40 Oe to 120 Oe. The microgels with 250 nm zaverage hydrodynamic diameter and 44 emu/g saturation magnetization are composed by 8 nm magnetite nanoparticles imbedded in (3acrylamidopropyl)-trimethylammonium chloride (APTAC) matrix. The magnetically induced aggregation was investigated by means of light extinction (LE) and small angle light scattering (SALS) at room temperature. The SALS experiments show the formation and growth of field direction elongated aggregates with thickness in the range of a few microns and length in the range of tens to hundreds microns. The extinction experiments show the increasing of aggregate total volume with increasing magnetic field amplitude. It was estimated that, at the saturation of the aggregation process in 120 Oe amplitude field, the aggregates consist of about 80% of the total amount of magnetic microgels within the sample. These findings have potential interest for magnetic composite applications in magnetic hyperthermia and heat transfer applications. Keywords: magnetic colloid; magnetic microgel; aggregation; high frequency magnetic field; magnetic hyperthermia; heat transfer;

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1. Introduction Magnetically induced large scale aggregation is one of the major cause for the loss of colloidal state in liquid magnetic colloids. Field oriented spindle-like aggregates with microns thick and hundred microns long form in equilibrium with the depleted colloid. In the case of ferrofluids, the aggregation is the consequence of magnetically induced phase separation and the large scale aggregates are actually the condensed phase [1-4]. In the case of magnetic composite dispersions, the aggregates form by magnetically induced composite chaining and subsequent chain zippering [5-7]. Due to drastic decrease of specific surface, the magnetically induced clustering has a significant detrimental effect on the efficiency of magnetic colloids’ bio-medical and engineering applications like magnetic hyperthermia, drug targeting , MRI and heat transfer [8-10]. The functionalization of magnetic nanoparticles with a polymeric shell is a promising approach for the design of well-defined composite materials. The main characteristics of these materials involves two aspects, namely adjusting the magnetic properties based on the interactions at the interface in polymer / magnetic nanostructures and the possibility to functionalize the polymer for specific applications. The hybrid nanostructures with magnetic core and polymeric shell have the advantage to be dispersible in good solvents for the polymeric shell without additional stabilizers. Magnetic microgels obtained by the encapsulation of magnetic nanoparticles from magnetic nanofluids into polymeric gels is a promising strategy resulting in homogenous dispersion, high loading of magnetic nanoparticles and superparamagnetic properties [1113]. In a previous work we investigated the DC magnetically induced large scale agglomeration in an aqueous dispersion of 60nm m-NIPA nanogels [7] by means of light scattering methods. We developed a method to determine the field dependence of the magnetically induced sample supersaturation, i.e. the fraction of magnetic microgels embedded in the aggregates, from light extinction and optical microscopy experiments (see Ref.[7] ESI). In this paper we develop a 250 nm (3-acrylamidopropyl)trimethylammonium chloride (APTAC) magnetic microgel water dispersion and investigate the magnetically induced large scale aggregation in AC high frequency magnetic fields. 2. Materials and methods 2.1 Sample preparation A 0.4w% water dispersion of magnetic microgels (MMG) was prepared for experiments. The magnetic microgels were prepared by free radical polymerization of (3-acrylamidopropyl)-trimethylammonium chloride (APTAC) in water solution containing the crosslinker N,N-methylene bis-acrylamide (BIS), ammonium persulfate (APS), tetramethylethylenediamine (TMEDA) and the oleic acid double layer coated iron oxide nanoparticles. The reaction is carried out under argon atmosphere at 70oC. The magnetic microgel particles were separated from the synthesis medium by centrifugation at 10000 rpm, then washed several times and redispersed in water. The hydrophilic iron oxide magnetic nanoparticles (MNP) were synthesized by means of chemical coprecipitation method and subsequent sterically stabilized with a double

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layer of oleic acid molecules [14]. The preparation method of the magnetic microgels is represented schematically in the Fig.1.

Fig.1 The preparation method of magnetic microgels. 2.2 Sample characterization The magnetic nanoparticles and microgels were investigated by transmission electron microscopy (TEM) on a JEOL 1010 equipment. The DC magnetization of the dried microgels was measured at room temperature by means of vibrating sample magnetometry (VSM) on an ADE Technologies VSM 880. The hydrodynamic diameter of the suspended microgels from the structure of the aqueous magnetic colloid was measured by means of dynamic light scattering (DLS) using a Malvern Zetasizer Nano ZS equipment. The DC magnetically induced aggregates in the magnetic colloid sample was investigated by means of optical microscopy using and OPTIKA N-400LD microscope. The AC magnetic field induced heat generation in the sample was measured under adiabatic conditions using an Optocon FOTEMP1-OEM optical fiber thermometer and a commercial 100kHz magnetic field generator with amplitude in 20 – 140 Oe range (see Fig.2). 2.3 Light extinction (LE) and Small Angle Light Scattering (SALS) experiments Light scattering is a useful tool for the investigation of large scale aggregation in magnetic colloids. Socoliuc and co workers developed a method for the investigation of magnetically induced large scale aggregation in magnetic colloids using light extinction (LE) and small angle light scattering (SALS) [4,7]. The experimental setup used in Ref.[7] was changed by replacing the electromagnet with a commercial AC magnetic field generator with fixed 100kHz frequency and 20 – 140 Oe field amplitude (Fig.2.b). The field

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amplitude was measured using an induction coil. Data acquisition was made using a National Instruments A/D converter and LabVIEW virtual instruments. The LE experiments consisted from the measurement of the forward scattered light within the 0.4 acceptance angle (Fig.2.a) using an iris (I), a light intensity accommodation filter (F1), a converging lens (L) and a Photomatrix IPL10530DAW photodetector (D1). The SALS 2D experiments were made by replacing the photodetection ensemble with a large sensor Thorlabs 8050M-GE CCD camera (Fig.2.a), that allows for more than 45 scattering angle measurement.

a) b) Fig.2 Experimental setup for LE and SALS in AC high frequency magnetic fields. 3. Results and discussion In Fig.3.a is presented a TEM picture of the MMGs flattened on the grid surface. The MNPs diameter was determined using ImageJ [15]: 7.71.6 nm. The MNPs are close packed in the polymer matrix, with 3 nm spacing. The DLS z-average hydrodynamic diameter of the MMGs is dMMG=250 nm with a polydispersity index PDI=0.273, which shows a wide diameter distribution as can be seen in Fig.3.b. The magnetization curve of dried MMGs was measured at room temperature. The MMGs are superparamagnetic with 42.9 emu/g saturation magnetization.

4

10

Intensity [%]

8 6 4 2 0

1

10

100

1000

10000

Dh [nm]

a)

b)

Fig.3 a) TEM picture of the flattened MMGs (scale bar is 200 nm), and b) DLS MMG size distribution by intensity. Based on the TEM, VSM and DLS findings, an estimation of the MMG interaction energy can be made in terms of both magnetic and van der Waals interactions. Because of the superparamagnetic nature of the MMGs, the magnetic dipole-dipole interaction energy is a function of the applied magnetic field H. The head-to-tail magnetic dipole-dipole attraction energy (uma) in kBT units is: 2 m H  1 0 uma  H,z     2  Eq.1 3 kBT 4  dMMG  z  where z is the distance between MMG’s surfaces, 0 is the vacuum magnetic permeability, kB is Boltzmann’s constant, T is the temperature, dMMG is the MMG diameter and m(H) is the MMG induced magnetic moment, parallel to the external magnetic field. Assuming the MMG superparamagnetism and identical MNPs with dMNP diameter, m(H) is: Eq.2 m  H   nMNP  MNP  L   H *   where H* is the local field inside the MMG, smaller then the applied field H due to demagnetization. nMNP is the MNP number in the structure of the MMG, MNP is the MNP permanent magnetic moment, L(x)=coth(x)-1/x is the Langevin function and (H*) is the dipole-field magnetic interaction parameter: H *     H   0 MNP , kBT Eq.3

MNP  Md VMNP  Md 



3  dMNP ,

6 where Md=480 kA/m is magnetite domain magnetization and VMNP is MNP’s volume. nMNP can be calculated assuming the MMG is composed of random packed MNPs, whose volume fractions is RPS=0.64, with a 3 nm polymer and surfactant layer spacing:

5

3

 d  nMNP   RPS   MMG  . Eq.4  dMNP    Derivation of m(H) from Eq.2 is not straightforward because the local field H* depends on m(H). For spherical MMGs: 1 H*  H  M  H  Eq.5 3 where m H  M H   Eq.6 VMMG is the MMG magnetization with: VMMG 



3 dMMG .

Eq.7 6 Considering the weak field amplitude, the explicit expression of m(H) can be derived from Eq.2 and Eq.5, using the L(x)x/3 approximation of the Langevin function. Having had obtained the expression of m(H), uma is a quadratic function of the AC field H. Because the field harmonic oscillating period is much lower than the Néel relaxation time, the time average is obtained by the multiplication with a ½ factor. Finally one gets: 2 12 03 RPS Md4 dMNP 3    uma  H0 ,z    6 46656  kBT 3  dMNP    

1



6 dMMG

 H02 .

Eq.8

6    dMMG  z  dMNP  0 RPS Md2  1    3   54 kBT   d   MNP   Using Eq.8, the H0 dependence of the magnetic dipole-dipole interaction parameter per particle can be calculated by setting z=0: 1 mdd  H0    uma  H0 ,z  0  , Eq.9 2 The AC field amplitude H0 dependence of <mdd> from Eq.9 is presented in Fig.4 for dMNP=8 nm, =3 nm and three values of the MMG diameter: 200, 250 and 300 nm. <mdd> increases quadratically with H0, reaching unity at H055Oe and 4.8 at H0=120 Oe for 250 nm MMG diameter. Thus, the magnetic dipole-dipole attraction among 250 nm MMGs is strong enough to allow thermally stable aggregation even in weak to moderate AC HF magnetic fields. For larger MMGs, the thermal stability is achieved at even lower field amplitudes. Following the DLS data, the MMG diameter is highly polydisperse, which is why, due to the big MMGs, large scale aggregation may occur in rather low intensity magnetic fields. This is similar to the case of polydisperse ferrofluids, where an amount of as low as 5% volume fraction of large particles may trigger magnetically induced phase separation [16]. 2

6

3

10 9

300 nm

8

<mdd> [-]

7 6 5

250 nm

4 3 2

200 nm

1 0

0

20

40

60

80

100

120

H0 [Oe]

Fig.4 AC HF magnetic field amplitude dependence of the time average magnetic dipole-dipole interaction parameter (<>). The shear size of the MMGs compels for considering the influence of the van der Waals attraction on the stability of the magnetically induced aggregates. The van der Waals interaction energy among identical MMGs is [17]: 2 2 dMMG dMMG 1 A  uvdW  z        2 kBT 12  2  dMMG  z 2 2  dMMG  z 2  dMMG  Eq.10 2 2   d  z   dMMG  ,  ln  MMG 2   d z   MMG    where the Hamaker constant A for magnetite in water is in the range 20-40 zJ [18]. For estimation purposes we shall use the lower value A=20 zJ, in order to account for the presence of the surfactant and polymer matrix in the structure of the MMGs. Fig.5 presents the MMG surface separation (z) dependence of the van der Waals energy and the magnetic dipole-dipole attraction energy for three values of the AC field amplitude. (Having no information on the existence of an external polymer shell, we did not considered the energy of hard sphere repulsion.) For intense fields, the magnetic interaction dominates the van der Waals down to very close proximity of the MMGs. In weaker fields however, the van der Waals attraction becomes dominant at surface separation as high as 20 nm. Thus, although short ranged, the van der Waals attraction is a major factor for aggregate stability once the long range magnetic field attraction brings the MMGs in close proximity. This is all the more important that, even in weak fields, small MMGs are attracted by magnetophoresis at the tips of the growing aggregates (see microscopy images below), very similar to the growing kinetics of the magnetically induced condensed phase spindle-like drops in ferrofluids [19]. As shown below, the MMG aggregates are micron thick and tens to hundreds microns long. The van der Waals attraction of this magnitude is also responsible for the lateral stability of the



7



aggregates, since the zippering magnetic repulsion energy is weak (eight times smaller than the head-to-tail attraction in absolute value).

2 0 -2 -4 u [-]

-6 -8

, H0= 40 Oe

-10

, H0= 80 Oe

-12

, H0=120 Oe

-14

uvdW , A=20 zJ

-16 0

10 20 30 40 50 60 70 80 90 100 110 z [nm]

Fig.5 The MMG surface separation dependence of the van der Waals energy and the magnetic dipole-dipole attraction energy for three values of the AC field amplitude. Optical microscopy of the MMG aggregation process was performed in DC magnetic field. A permanent magnet based field generator was adapted to the microscope table with 150 Oe magnetic field intensity, whose energy is equal to the average energy of an 211 Oe amplitude AC field. In Fig. 6 are presented the microscopy images of the MMG suspension before field application (Fig.6.a) and after 10 and 60 seconds of the DC field application (Fig.6.b and Fig.6.c respectively). The field oriented spindle-like magnetically induced MMG aggregates form and grow on the expense of the MMG depleted aqueous suspension. In the late stages of the process, the spindles reach ~2.5 micron average thickness and hundreds of microns length (Fig.6.c).

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Fig.6 Optical microscopy: time evolution of microgel clustering in an 150 Oe DC magnetic field. Because the aggregation process is strongly conditioned by temperature, the time dependence of the sample temperature was measured for 40, 80 and 120 Oe AC magnetic field amplitudes (Fig.7). The heating rates were found to be low enough to not affect the aggregation process during a couple of minutes of field exposure in the light scattering experiments.

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8 AC magnetic field heat generation

7

H0 = 40 Oe H0 = 80 Oe

6

H0 = 120 Oe

o

T [ C]

5 4 3 2 1 0 0

100

200

t [sec.]

300

400

H off

Fig.7 Time evolution of the sample temperature for three values of the AC magnetic field amplitude. The time evolution of the 2D scattered light pattern recorded during SALS experiment is presented in Fig.8. At t=0 sec an 120 Oe amplitude 100 kHz AC magnetic field was applied to the sample. The scattering pattern, isotropic before field application at t=0 sec, becomes increasingly concentrated in the scattering plane. This scattering pattern is typical for parallel prolate scatterers [20] oriented along the external magnetic field direction, which is perpendicular to the scattering plane. Thus, the SALS experiment reveals the evolving process of large scale prolate aggregate formation.

Fig.8 Time evolution of SALS 2D scattering pattern. The time dependence of the LE experiments is presented in Fig.9, where I(t) is the forward scattered light intensity measured by detector D1 in Fig.2. The lower I(t) the higher the extinction. After the field is turned on, the light extinction increases at a quasi-exponential rate. The rate increases with increasing field amplitude H0. When the field is turned off, the light extinction reverts to the initial value at a much higher rate. The decay of the forward scattered light is due to the formation of large scale spindle-like aggregates,

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oriented parallel to the external magnetic field. The extinction kinetics after the field is turned on is governed by the magnetically induced diffusion of MMGs in the process of the aggregate formation. The diffusion itself is governed by the magnetic dipole-dipole interaction among MMGs which in its turn, as shown above, is proportional to the square of the field amplitude H0. After the field is turned off, the extinction kinetics is governed by the MMG aggregates dissolution. A similar behavior in DC magnetic fields is presented in ref. [7]. 1.02 1.00

H on

H off

0.98

I(t) / I(0) [-]

0.96 0.94 0.92 0.90

Light extinction H0 = 40 Oe

0.88

H0 = 80 Oe

0.86

H0 = 120 Oe 0

20

40

60

80

460 480 500

t [sec.]

Fig.9 Time evolution of the forward scattered light intensity for three values of the AC magnetic field intensity. LE and optical microscopy data can be used to calculate the AC magnetic field amplitude H0 dependence of the sample magnetic supersaturation, defined as the percentage of the clustered MMGs. Socoliuc and coworkers [7] developed a method to calculate the field dependence of the magnetic supersaturation in colloidal dispersions of MMGs, using the asymptotic saturation values of the light extinction and the average thickness of the spindle-like aggregates(see Ref.[7] ESI). The asymptotic saturation values of the light extinction is obtained from the time dependence of the LE data. By fitting the time decay of the forward scattered light with the expression: Ext  t   P0  P1  e  P2 t Eq.11 where P0, P1 and P2 are fit parameters, the value of P0 gives the asymptotic saturation values of the light extinction for each of the three values of H0. The dashed curves in Fig.9 are the least squares fitted curves with Eq.11. As mentioned above, we were unable to conduct optical microscopy observations on the sample under the action of the AC HF magnetic field. The SALS 2D scattering patterns were also useless for the purpose of aggregate thickness determination due to its high polydispersity. Therefore, we based the supersaturation calculations on the estimated 2 microns average thickness value, obtained from optical microscopy images in DC field (Fig.6.c). This leads to an underestimation of the supersaturation for weaker field amplitudes where the aggregates’ thickness is likely to be smaller.

11

magnetic supersaturation [%]

100 80 60 40 20 0 40

60

80

100

120

H0 [Oe]

Fig.10 AC HF magnetic field amplitude dependence of the asymptotic magnetic supersaturation (the line is just a guide for the eye). Fig.10 presents the as calculated AC magnetic field amplitude dependence of the asymptotic magnetic supersaturation. The magnetic supersaturation increases with increasing AC magnetic field amplitude, reaching as high as 80% for H0=120 Oe. Extrapolating at higher field amplitudes, the magnetic supersaturation may approach 100%. 4. Conclusion AC high frequency magnetic fields with amplitudes as low as 40 Oe can induce large scale aggregation in colloidal dispersions of magnetic microgels or magnetic multicore-shell composites, provided that their magnetic nanoparticle content is large enough. An important factor in the stability of the magnetically induced aggregates is the van der Waals attraction among the microgels. The highly prolate spindle-like aggregates (~2.5 micron average thickness and hundreds of microns length) are oriented parallel to the external magnetic field. The aggregation reaches a considerable magnitude: up to 80% of the composites are entrapped in the spindle like aggregates. This finding has potential impact on magnetic hyperthermia and heat transfer applicability of magnetic composite colloids as well as on experimental data interpretation.

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Acknowledgements: The work of V. Socoliuc was supported by ARFT/CCTFA 2016-2020 research programme and Romanian Government STAR 176/2017 project. The work of R. Turcu was supported by the project RO-JINR 2019, theme No. 04-4-1121-2015/2020 “Structural investigations of ferrofluids in bulk and interfaces by neutron scattering methods”. We thank Dr Anca Petran for the sample preparation and Dr Lucian Barbu-Tudoran for TEM investigations. References: 1. J. C. Bacri and D. Salin, J. de Phys. 43 (1982) 21; 2. A. Cebers, J.Magn.Magn.Mater. 85 (1990) 20; 3. A. Yu. Zubarev, A.O. Ivanov, Phys.Rev.E 55 (1997) 7192; 4. V. Socoliuc and D. Bica, Prog. Colloid Polym. Sci. 117 (2002) 131; 5. G. Bossis, P. Lancon, A. Meunier, L. Iskakova, V. Kostenko, A. Zubarev, Physica A 392 (2013) 1567; 6. P. Liu, J. W. J. de Folter, A. V. Petukhov, A. P. Philipse, Soft Matter 11 (2015) 6201; 7. V. Socoliuc, L. Vekas, R. Turcu, Soft Matter 9 (2013) 3098; 8. E. Tombácz, R. Turcu, V. Socoliuc, L. Vékás, Biochemical and Biophysical Research Communications 468 (2015) 442; 9. S. Dutz, M. Kettering, I. Hilger, R. Muller, M. Zeisberger, Nanotechnology 22 (2011) 265102; 10. K. S. Hong, T.-K. Hong, Ho-S. Yang, Appl.Phys.Lett. 88 (2006) 031901; 11. E. Amstad, M. Textor and E. Reimhult, Nanoscale 3 (2011) 2819; 12. L. Borlido, A. M. Azevedo, A. C. A. Roque and M. R. Aires-Barros, Biotechnol. Adv. 31 (2013) 1374; 13. R. Turcu, V. Socoliuc, I. Craciunescu, A. Petran, A. Paulus, M. Franzreb, E. Vasile, L. Vekas, Soft Matter 11(5) (2015) 1008; 14. D. Bica, L.Vékás, M. V. Avdeev, O. Marinica, V. Socoliuc, M. Balasoiu, V. M. Garamus, J.Magn.Magn.Mater. 311 (2007) 17; 15. https://imagej.nih.gov/ij/ 16. A. O. Ivanov, J. Magn.Magn.Mat. 154 (1996) 66; 17. A. O. Ivanov, E. V. Novak, Colloid Journal 69 (3) (2007) 332; 18. A. O. Ivanov, O. B. Kuznetsova, Phys.Rev.E 64 (2001) 041405; 19. A. Yu. Zubarev, A. O. Ivanov, Phys.Rev.E 55(6) (1997) 7192; 20. C.F. Bohren and D.R. Huffman, Absorption and Scattering of Light by Small Particles, John Wiley & Sons, 1983.

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Highlights 

250 nm hydrodynamic diameter magnetic microgels were synthesized, with 8nm iron oxide nanoparticles embedded in APTAC matrix.



A 0.4w% aqueous dispersion of magnetic microgels was investigated.



Large scale aggregation of the magnetic microgels was discovered under the influence of weak 100kHz AC magnetic fields.



The aggregates are micron thick and hundred micron long, parallel to the external magnetic field.



The aggregates consist of up 80% of the total amount of magnetic microgels within the sample.

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