Sedimentation behaviour of disper-sions made from plain or decoratedparticles in dependence on pH-valueanalyzed.Results regarding the state of floc-culation compared with predictionsbased on Zeta potential data.Colloidal stability and degree ofparticle flocculation/agglomerationwell predicted by sedimentationbehaviour.In contrast, Zeta-potential was notable to cover all aspects of particleinteractions.The new approach can also be appliedfor soft particles and dispersionmedia regardless of their polarity.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 30 August 2012eceived in revised form 2 October 2012ccepted 3 October 2012vailable online 23 October 2012
eywords:locculation stateeta potentiallectrokinetic phenomenaoft particlesompressibilityediment density
a b s t r a c t
Dispersion based products have applications in every area of life. During formulation of new productsthe dispersion properties have to be adjusted to obtain the desired stability, textural and rheologicalproperties. Most often stable colloidal dispersions are required, sometime however, weak flocculation ispurposely induced to adjust structural properties. In other cases strong flocculation is helpful for disper-sion separation. From this it is evident that classification of the state of a dispersion regarding flocculation(net attractive particle interaction) and quantification of its degree are necessary and routine tasks inevery day formulation and optimization work. Zeta potential is commonly used to predict the stabilityof virtually all colloidal dispersions. This neglects that the Zeta potential concept is limited to classicalelectrostatically stabilized dispersions. It has to be emphasized, however, that nowadays new dispersionproducts are stabilized by different approaches (e.g. by steric or rheological stabilization).
Sedimentation analysis by multisample analytical centrifugation with photometric detection is a rathersimple but powerful and high throughput method to characterize the dispersed state/degree of particleinteraction. Visualization of in situ separation behaviour allows for the classification and differentiationbetween the various instability phenomena such as swarm sedimentation (stable dispersion) and zonesedimentation (flocculation, agglomeration). Even more, complex systems with subfractions of particlesexhibiting a different behaviour can also be analyzed.
ur of
Sedimentation behavio function of pH of the continuouculation and compared with prto measured Zeta potential theculation/agglomeration were acan also be applied for soft par
∗ Corresponding author.E-mail address: [email protected] (T. Sobisch).
different dispersions made from plain or decorated nanoparticles as a
s phase is presented and analyzed in terms of the degree and type of floc-edictions based on Zeta potential data. Results demonstrate that contrary
colloidal stability of the dispersed particles and the degree of particle floc-lways well predicted by the sedimentation behaviour. This new approachticles and dispersion media regardless of their polarity.
Dispersions are widely used in research, industry, agriculture,ood, pharmacy or home and health care. The state of dispersionoverns the behaviour of intermediate products during processing,s well as many functional aspects of the final product and its shelfife to a great extent. Crucial aspects in this regard are floccula-ion, agglutination or agglomeration, and the assembly of formerlyispersed particles into loosely coherent structures resulting in theestabilization of the dispersion. For most products these processesave to be prevented, yet for others such as for industrial scalehase separation, they are crucial. Flocs or agglomerates are heldogether by weak physical interactions and their formation is aeversible process. Therefore, flocs may be redispersed by moderateechanical forces. Particle–particle interaction at a given separa-
ion depends on the total interaction energy, which is obtained bydding up of all repulsive and attractive forces [1]. If attractionorces are larger than repulsive ones, flocculation and agglomer-tion are favoured.
Historically, stability of dispersions was often tailored bylectrostatic interaction. According to DLVO theory [1] the sur-ace potential governs the electrostatic interaction potential. Zetaotential obtained from the electrophoretic mobility [2] is tradi-ionally employed to characterize the power of the electrostaticnteraction. State-of-art is the assumption that dispersions tend toocculate rapidly, if the absolute value of Zeta potential is below0 mV. Below 30 mV a dispersion exhibits incipient instability. Forotentials higher than ±30 mV the dispersion is believed to betable (e.g. [3]). However, it has to be emphasized that the usualmoluchowski approach to calculate the Zeta potential from thexperimentally measured electrophoretic mobility is only valid foro called hard spheres [4]. These particles are characterized byhe following: (1) potential determining charges are exclusivelyocated at the particle surface (As/m2), (2) no surface conductiv-ty (Dukhin number « 1), (3) thin diffuse double layer comparedo particle size a [∗ » 1] and (4) no 3D-surface structure compara-le to Debey–Hückel length, e.g. Phenrat et al. [5]). Furthermore,he Smoluchowski approach implies that approaching the particleurface there is no change of the bulk dielectric constant and theiscosity of the continuous phase, respectively.
Advanced formulations, especially nanodispersions, focus morend more on steric and entropic stabilization as well as on sur-ace functionalization [6,7]. In these cases the particle core is oftenovered by a shell and/or an ion-permeable surface layer of poly-lectrolytes or amphoteric surface active molecules with extendedead groups similar to biological cells [8]. This type of particle isalled “soft” particle. The striking difference to “hard” particles ishe existence of an additional layer outside of the core particle sur-ace bearing a 3-D (spatial) distribution of the fixed electric chargesAs/m3 !) and gives place to hydrodynamic flow and electric con-uctivity. This ion-penetrable layer is called “particle corona” tomphasize the diffuse transition to the surrounding continuoushase [9]. The Smoluchowski approach cannot be applied to cal-ulate Zeta potential from experimental electrophoretic mobilityata of such soft particles. Since the first attempts for correspondingodels [10–12] in the eighties of the last century, enormous num-
ers of papers were published and theoretical models have beendvanced [13]. The published work clearly demonstrates that fromlectrophoretic mobility measurements reliable information abouthe electrostatic properties of soft particles can be only gained, ifhe structure/properties of the surface corona itself are known a pri-ri. Additionally, as electrophoresis is determined by the Donnan
otential in the surface layer, any calculated Zeta potential loses itshysical meaning for characterizing the surface potential of a parti-le [14]. Hence, the interaction potential between two soft particless not predictable and neither is dispersion stability.
sicochem. Eng. Aspects 440 (2014) 122– 130 123
In this paper we describe a new approach of the assessmentof particle–particle interaction. It is well known that phase sepa-ration behaviour under gravity or in centrifugal fields is stronglyinfluenced by the flocculation/agglomeration state of dispersion[15–18] and that separation behaviour may be in situ visualizedand quantified by STEP-Technology [19–25]. In this paper we aregoing to compare qualitative and quantitative experimental data ofphase separation kinetics obtained by analytical centrifugation fordifferent types of hard and soft particles and the correspondingmeasured Zeta potential (Smoluchowski-approach). The electricand steric surface particle structures were altered by changing thepH or the degree of polymer coating. It was demonstrated that forhard electrostatically stabilized particles both Zeta potential andanalytical centrifugation predict the suspension stability. In caseof soft particles only the new approach of in situ visualization ofseparation behaviour was able to classify the state of flocculationcorrectly.
2. Materials and methods
2.1. Materials
Ground AEROXIDE Alu C in aqueous dispersion (10%, m/m) waskindly provided by Technical University Braunschweig (Institutefor Particle Technology). Cumulative volume weighted size distri-bution was determined with the analytical centrifuge LUMiSizer(LUM, Germany). 80% of the particles fall into the range between100 and 250 nm (unpublished results). 1% (m/m) dispersions atvarying pH values were prepared by adding deionised water anddilute sodium hydroxide or hydrochloric acid solutions (no addi-tional background electrolyte). The pH of these dispersions wasdetermined after storage for one day before the sedimentationanalysis. According to the manufacturer [26] the Zeta potential isapprox. 40 mV (pH = 5), close to zero at pH = 9 and below −20 mVat pH = 11, respectively.
Titania – AEROXIDE P25 (Evonik Industries, Germany) and dis-persions therefrom were characterized by Paciejewska [19]. Bythis author size was determined to be in the range between 30and 100 nm. Sedimentation analysis was performed [19] for dis-persions with 5% (m/m) in 10 mM aqueous potassium bromidesolutions at varying pH with the analytical centrifuge LUMiFuge(LUM, Germany) one day after preparation.
Lysozyme coated silica nanoparticles (20 nm) were obtainedfrom Technical University Berlin, Institute for Chemistry–StranskiLaboratorium. For a detailed description and physicochemical char-acterization, see [20]. Dispersion characteristics were measuredas function of pH for 1% (m/m) silica dispersions with 1.1 mg/mllysozyme. The pH up to 7 was adjusted by 50 mM MES (2-(N-morpholino)-ethanesulfonic acid) buffer, for higher pH values witha 50 mM BICINE (N,N-bis(2-hydroxyethyl)glycine) buffer.
To optimize the performance of silica-iron oxide magneticnanoparticles (MNP) for gene delivery dispersion properties werecharacterized as function of surface decoration with 25-kDabranched polyethylene imine (PEI) [21]. A sterile-filtered (0.2 �m)PEI solution in water with a pH of 7.3 (adjusted with hydrochloricacid) was added to an equal volume of MNP suspension after vor-texing to obtain the required PEI-to-iron ratio. As described in [21]the core shell type particles are composed of an inner magnetitecore coated with a silicone dioxide layer with surface phosphonategroups. The sample without addition of the cationic polyelectrolytepolyethylene imine was labelled SO-Mag5. The hydrodynamicdiameter of these particles in aqueous suspension was determined
as 40 ± 14 nm [21]. Surface decoration was performed in the rangebetween 1 and 12% (m/m) PEI per iron (SO-Mag6-1 to SO-Mag6-12). Nanoparticle concentration expressed in mg Fe/ml suspensionwas approx. 10 throughout the measurements.
124 D. Lerche, T. Sobisch / Colloids and Surfaces A: Physicochem. Eng. Aspects 440 (2014) 122– 130
Fig. 1. (a) Sketch and transmission profiles for colloidal stable dispersion exhibit-ing polydisperse (swarm) sedimentation. Example 10% (m/m) aqueous dispersionof ground Alu C at pH 5, 2500 s centrifugation at 20 ◦C, centrifugal acceleration at thebottom of the cell 328 × g, interval between profiles 10 s. (b) Sketch and transmissionprofiles for strongly flocculated dispersion exhibiting zone sedimentation. Example10% (m/m) aqueous dispersion of ground Alu C at pH 9. 2500 s centrifugation at20 ◦C, centrifugal acceleration at the bottom of the cell 328 × g, interval betweenprofiles 10 s. (c) Sketch and transmission profiles for weakly flocculated dispersionexhibiting in the beginning swarm sedimentation followed by zone sedimentation.Example 10% (m/m) aqueous dispersion of ground Alu C at pH 11. 2500 s centrifu-gation at 20 ◦C, centrifugal acceleration at the bottom of the cell 328 × g, intervalbetween profiles 10 s.
Fig. 2. Zetapotential measured as function of pH for a 5% (m/m) dispersion ofAEROXIDE P25 in 10 mM aqueous potassium bromide solution (data provided byPaciejewska [19]).
2.2. Separation analysis
Sedimentation analysis was performed with analytical cen-trifuges (LUMiFuge, LUMiSizer; LUM, Germany). These mul-tisample analytical centrifuges display in situ changes ofparticle concentration instantaneously by NIR-light transmit-ted across the full length of the samples (STEP-Technology,http://www.lum-gmbh.com/technology.html, 2012-10-02). STEP-Technology stands as acronym for Space and Time ResolvedExtinction Profiles. Illuminating the dispersion across its entirety,and by having many thousand detectors measure the light sourceextinction profile instantaneously, even the smallest changes canbe detected. Changes in the extinction profile are representativefor the changes in particle concentration. The analytical centrifugesallow to measure the extinction profiles during centrifugation overthe whole sample length.
Throughout all measurements 2 mm PC-cells were used (sam-ple volume 0.4 ml). All experiments were run in duplicate. Themeasurement principle and the application of the instrument aredescribed in detail in [22].
3. Results and discussions
3.1. Example for the classical case of electrostatically stabilizedmetal oxide dispersion
In the case of one component hard sphere dispersions, a clearrelationship between Zeta potential and stability against floccula-tion has to be expected. Typical sedimentation behaviour patternof the alumina dispersions at pH 5, pH 9 and pH 11 are shown inFig. 1a–c.
At pH 5 and ZP = 40 mV the particles are well stabilized (col-loidal stable) as drawn on the left side in the upper part of Fig. 1a.They settle individually with different velocities according to thedifferences in particle size (swarm sedimentation, polydispersesedimentation). The corresponding consecutive transmission pro-files during centrifugation, i.e. the transmission as function of theposition within the sample column at a given sedimentation timeare shown in Fig. 1a. The position is given as distance from the cen-tre of rotation. The position of the filling height (meniscus) is closeto 106.5 mm. The latter is marked by a sharp insection. The position
of the cell bottom locates at 129.8 mm. Prior to separation, parti-cle concentration is equally distributed along the complete samplelength, i.e. transmission is constant at a low level (first transmis-sion profile with the lowest transmission). As particles settle out the
D. Lerche, T. Sobisch / Colloids and Surfaces A: Physicochem. Eng. Aspects 440 (2014) 122– 130 125
F 5 in 13 ofiles
smcitrycts
t(nvosaawtop
ig. 3. Comparison of transmission profiles for 5% (m/m) dispersions of AEROXIDE P25 ◦C, centrifugal acceleration at the bottom of the cell 287 × g, interval between pr
uspension their concentration gets lower in this region and trans-ission increases. Due to the polydisperse settling, the particle
oncentration gradually changes along the sample length, result-ng in the lowest particle concentration (highest transmission) nearhe filling height position. The last profile (highest transmission,esidual turbidity) indicates that the smallest particles have notet fully separated out from the supernatant within the appliedentrifugation time. Only a very small sediment is observed dueo a dense packing of particles characteristic for colloidal stableystems [6].
The separation pattern drastically changes at pH = 9, the isoelec-ric point (Fig. 1b) at which the dispersion is strongly flocculatedcp. sketch top left). As can be seen from the transmission profiles,o settling of individual particles or agglomerates occurs. Instead,ery steep transmission profiles, are moving towards the bottomf the cell indicating a sharp interface supernatant/dispersion. Thepacing between consecutive profiles narrows as the resistancegainst particle packing increases. This is typical for compression of
space filling flocculated network. During compression of the net-
ork the particle concentration increases leading to a decrease of
he baseline transmission. For the present case the sediment heightbtained is considerably larger as in Fig. 1a, confirming the looseacking characteristic for flocculated systems [6].
0 mM aqueous potassium bromide solutions as function of pH. 1200 s centrifugation,10 s.
If AluC-particles are dispersed at pH = 11, the Zeta potential of<−20 mV indicates charge reversal and the suspension restabilizes.As Fig. 1c reveals, the separation pattern for the first part of the sep-aration process (for about the first 10 min) again shows a gradualchange in concentration along the entire sample length, typical forswarm sedimentation. But the spacing between the consecutiveprofiles is larger than at pH 5, which reflects higher sedimenta-tion velocities due to partial flocculation (small individual flocs,cp. sketch on top left). As the separation process progresses, parti-cle concentration in the remaining sample volume increases, andpolydisperse sedimentation behaviour changes to zone sedimenta-tion. Corresponding transmission profiles steepen again as a sharpinterface forms (cp. sketch top middle). It should be mentioned thatwhether one observes polydisperse sedimentation of individualflocs or compression of a flocculated network, depends on whetherthe particle concentration is below or above the percolation thresh-old [27]. This is because the percolation threshold itself depends ontotal particle attraction energy.
In addition to the conclusions derived from Zeta potential values
regarding the degree of electrostatic particle stabilization, sedi-mentation analysis also provides more detailed information on theactual structure of the dispersions and the sediment formed, e.g.network formation, sediment packing density [27,28].
126 D. Lerche, T. Sobisch / Colloids and Surfaces A: Physicochem. Eng. Aspects 440 (2014) 122– 130
F mg/ma
3d
cbspcZp
Ptc5
sw(pipnhtstbfhdalc
ig. 4. Comparison of transmission profiles for 1% (m/m) silica dispersions with 1.1t the bottom of the cell 36 × g, interval between profiles 10 s.
.2. Example for electrostatically stabilized metal oxideispersions <100 nm
The smaller the particles, the more they deviate from the idealase of hard sphere particles, due to the Debye–Hückel-lengthecoming comparable to the particle size at even moderate ionictrength [29]. Generally, the thickness of adhering layers aroundarticles gets more comparable to the plain size of the neat parti-les. Therefore deviations from the expected relationship betweeneta potential calculated based on Smoluchowski approach andarticle attraction are likely.
The dependence of Zeta potential on pH as measured byaciejewska [19] is shown in Fig. 2. The corresponding sedimen-ation behaviour of titania dispersions at pH 2, 4, 5, 7, 9 and 10 isompared (Fig. 3). The Zeta potential at these pH values is approx.0, 47, 33 mV, around zero, −25 and −33 mV, respectively.
As expected, at pH 2 the particles are well stabilized (colloidaltable) due to their high charge density and settle individuallyith different velocities according to the differences in particle size
swarm sedimentation, Fig. 3, upper part left). While the polydis-erse nature of separation is still preserved at pH 4, sedimentation
s already much faster (Fig. 3, upper part right). Nearly all of thearticles have settled out and a clear interface between super-atant and sediment is detectable. The sedimentation velocity isigher because particles (aggregates) are markedly larger, even ifhere is only a small change in the already high Zeta potential. Theituation drastically changes if the Zeta potential is reduced fur-her towards 30 mV (pH 5, Fig. 3, middle part left). The dispersionecomes strongly flocculated and a close space filling network isormed. At the isoelectric point, large fast settling flocs are formed,owever, no large volume network was observed (pH 7, Fig. 3, mid-
le part right). After charge reversal at pH 9 and 10 the suspensionsgain exhibit a pronounced compression of a space filling floccu-ated network (Fig. 3, lower part). At pH 10 in addition to networkonsolidation, a gradual increase in transmission was observed in
l lysozyme as function of pH. 2850 s centrifugation, 20 ◦C, centrifugal acceleration
the supernatant. This is associated with the settling of fines notbound tightly to the network. The dispersion structure and sepa-ration behaviour is different in this respect from pH 5, despite theabsolute values of Zeta potential being nearly identical.
3.3. Enzyme covered silica nanoparticles
This case represents typical soft particles, as the size is in therange of 20 nm and the silica particles (IEP = 3) are covered by alayer of adsorped lysozyme [20]. Lysozyme itself has a radius ofgyration of 1.9 nm [30], a molar mass of 14.3 kD and an IEP of 10.7[20]. Zeta potential of coated particles as a function of pH is shownin Fig. 4 (left, upper part, data from Barti et al.). Above the isoelec-tric point of plain silica (IEP = 3) the Zeta potential increases dueto the interaction with the positively charged lysozyme and passesthrough a maximum (approx. at pH 6.5, >20 mV). The isoelectricpoint of lysozyme loaded silica nanoparticles is shifted to a pH valueof about 7.5. The sedimentation pattern behaviour of dispersions atpH 4.23, 5.45, 8.68 and 10.23 are presented in (Fig. 4, middle and leftpart). The Zeta potential at these pH values is approx. 5, 15, −30 and−40 mV, respectively. Based on the accepted rules, Zeta potentialwould predict about 4 domains of stability behaviour. Low stabilityat pH 4, an increase in stability towards pH 7, again low stabilitybetween pH 7 and 8.5, and high stability at pH above 9. Separationfingerprint patterns reveal a more complex and diverging picture,also demonstrated by photographs taken after the centrifugation(Fig. 4, left side, lower part).
At a pH value of 4.23 there are only very minor changes in trans-mission profiles observed during centrifugation. Over the entiresample height the transmission is very high (small attenuation) dueto the small size of primary nanoparticles (20 nm) and/or very small
aggregates. Furthermore, both transmission profiles and the photoafter centrifugation show no evidence for sediment formation. Thatmeans despite a very low absolute Zeta potential the dispersion isvery stable. Sedimentation fingerprint pattern show no detectable
Fig. 5. Zeta potential measured for a dispersion of silica-iron oxide magnetic
Fr
D. Lerche, T. Sobisch / Colloids and Surfaces A
igns of agglomerates or flocs. According to the measurement ofysozyme adsorption, lysozyme only covers a small fraction of theurface at this pH [20]. As observed by Kobayashi et al. [30], bareilica nanoparticles do not aggregate at pH 4. They attributed this to
‘hairy layer’ providing sterical stabilization. An important aspectf stability of silica nanoparticles prepared from tetraethoxysilaneas been revealed by Keizer et al. [31]. Charge inside the micropo-es is considerably larger than at the outer surface assessable byeta potential measurements.
In contrast, destabilization was found with increasing pHespite an increase in Zeta potential. Fig. 4 (upper part right) visu-lizes the separation behaviour at pH 5.45. Based on the lowerransmission (60%) for the first profile, we have to conclude thatrimary nanoparticles agglomerated and accordingly scatter more
ight. This conclusion is also supported by the distinct evolutionf the transmission profiles due to particle sedimentation. Settlingelocity is by orders of magnitude higher as at pH 4.23. Finger-rint pattern indicate swarm sedimentation at the beginning ofhe separation process. No subfraction is detectable. The state ofhe dispersion is therefore monomodal but polydisperse. It shoulde mentioned, that as the separation progresses the volume con-entration increases and a gradual transition from swarm to zoneeparation (network formation and consolidation) occurs. Increas-ng the pH further, flocculation is even more enhanced. The increasen sediment height from pH 5.45 to 7 (Fig. 4, see also photographs)mplies a decreasing sediment packing concentration, as the initial
olume concentration of the dispersion is the same. This finding isn additional evidence for the strongly flocculated state of the sus-ensions. Increasing the pH further, the flocculation progresses andinetics of separation behaviour is exemplarily shown for pH 8.68
ig. 6. Comparison of transmission profiles for a dispersion of silica-iron oxide magnetic natio). 2850 s centrifugation, 20 ◦C, centrifugal acceleration at the bottom of the cell 36 ×
nanoparticles (10 mg Fe/ml) as function of PEI loading (data provided by Mykhaylyket al. [21]).
(Fig. 4, lower part, middle). Zone sedimentation is obvious fromthe very beginning of centrifugation and separation results in avery thick sediment and consequently low sediment density. Bothare the sign of strong agglomeration/flocculation leading to forma-
tion of a closed space filling network. This is remarkable, as thecalculated Zeta potential is in the order of −30 mV, a much higherabsolute value as for the stable suspension at pH 4.23. Increasing thepH above 10 (Zeta-potential about −45 mV), the particle–particle
anoparticles (10 mg Fe/ml) as function of PEI loading (numbers give the PEI-to-irong, interval between profiles 10 s.
128 D. Lerche, T. Sobisch / Colloids and Surfaces A: Physicochem. Eng. Aspects 440 (2014) 122– 130
Fig. 7. Comparison of results for a dispersion of silica-iron oxide magnetic nanoparticles (10 mg Fe/ml) as function of PEI loading (5 corresponds to the bare particles, 6-1 to6 dimena n at th
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-12 to particles with increasing PEI-to iron ratio, from 1 to 12 respectively). (a) Seppearance after centrifugation. 2850 s centrifugation, 20 ◦C, centrifugal acceleratio
nteraction is markedly reduced, and recorded swarm orolydisperse sedimentation pattern indicate a stable dispersionFig. 4, lower part, right). Based on the initial and final transmis-ion (turbidity) it may be concluded that only small but compactggregates are present, and correspondingly only a small sedimentn the order of 1 mm can be detected.
.4. Polyelectrolyte coated magnetic nanoparticles for geneelivery
The core–shell type silica/iron oxide magnetic nanoparticlesecorated with polyelectrolyte have a very complex structure [21].round the magnetic core are subsequent layers of silicon dioxideith surface phosphonate groups and polyelectrolyte. This results
n a specific spatial fixed charge distribution very different fromhe classic two-dimensional surface charge distribution concept.ne should expect that the mechanism of polyelectrolyte floccu-
ation (bridging or patch mechanism or a mixture of both) willdd additional complexity. Indeed, we observed differences in sed-mentation behaviour, by only changing the order of addition fromolyelectrolyte into dispersions of magnetic particles to additionf magnetic particle suspension into the polyelectrolyte solution.hese differences were not elucidated by the calculated Zeta poten-ial, which remained virtually the same [21].
Decoration of negatively charged Silica-iron oxide magneticano particles with different amounts of positively charged PEIuantified in % m/m PEI per iron reverses the net charge of the parti-
les and traditional calculated Zeta potential alters as measured byykhaylyk et al. [21] and shown in Fig. 5. The Zeta potential changes
rom a negative value (−38 ± 2 mV) for the undecorated particlesSO-Mag5) to about +40 mV for SO-Mag6-5. Further increase in PEI
t height, (b) average transmission in the supernatant (118–120 mm) and (c) visuale bottom of the cell 36 × g.
addition did not change this value substantially. The Zeta potentialproved not to be a reliable indicator to optimize magnetotransduc-tion efficiency. Suitable dispersions were only obtained in the rangebetween 10 and 12% (m/m) PEI per iron [21].
Multisample analytical centrifugation delivered a very detailedpicture about the changes in the dispersion state as function of PEIdecoration (Fig. 6, selected samples).
For the undecorated sample (SO-Mag5), only minor changeswere observed (Fig. 6, 0, upper part left). A small degree of poly-disperse sedimentation can be traced just below the filling height(meniscus) due to the average particle size being very small. ForSO-Mag6-1 with reduced net negative charges (Fig. 6, 1, upperpart middle), the state of the suspension changes and the visu-alization of the separation pattern reveals that single particlestogether with individual aggregates (small flocs) compose the dis-perse phase. At the beginning of the separation process, particleshave enough space to separate individually and polydisperse sed-imentation occurs. Later, a sharp boundary between supernatantand sedimenting particles is formed. Inside the latter, a particlenetwork is built up, and therefore all particles are moving with thesame velocity (zone sedimentation). Above the interface, a gradualincrease in transmission is observed, which is related to slowly sett-ling individual fines that are not bound to or incorporated into thefaster settling particle network; i.e., the dispersion contains largerflocs and fines (primary particles and/or small aggregates).
As expected, the characteristics of the transmission profileschange with the PEI-to-iron ratios. By doubling the amount of PEI
(SO-Mag6-2, Fig. 6, 2, upper part right), the separation mechanismchanges completely to zone sedimentation. The flocculated particlenetwork is compressed rapidly. There is no residual turbidity, indi-cating that there is no sign of separate settling of a fine fraction. In
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D. Lerche, T. Sobisch / Colloids and Surfaces A
ther words, near the IEP, all of the particles are incorporated intohe flocs/network.
Further increasing the PEI concentration (SO-Mag6-2 → SO-ag6-4), the separation behaviour changes again. Very inter-
stingly, the separation pattern reveals that a distinct bimodaleparation behaviour takes place (Fig. 6, 4, lower part left). Theolydisperse sedimentation of fines in the region from 108 mm to12 mm (just below the meniscus/top of the sample) prove thathe suspension is again partially stabilized. The increase of trans-
ission up to about 75% in a tiny region just below the meniscus,ndicates that a subfraction of particles stay as single particles andmall agglomerates. Progression of clarification nevertheless indi-ates that for this fraction of the dispersed phase, stability is belowhat of sample SO-Mag5. The second fraction of the dispersed phasef sample SO-Mag6-4 exhibits distinct zone sedimentation, indicat-ng that these magnetic nanoparticles are interacting despite theeta potential is well above 30 mV. The reason for this bimodal-ty could be due to different particle PEI-coating, an insufficientrocessing reproducibility on the particle level or different prop-rties of plain particles. Increasing the PEI-loading further, theraction of fines increases, and the flocculated fraction decreasesradually despite Zeta potential staying unchanged and amountingbout 40 mV. At PEI-to-iron ratio of 12% (Si-Mag6-12), floccula-ion (zone sedimentation) was hardly detectable and the overallattern would have to be classified as polydisperse sedimenta-ion. The visual appearance of the samples after centrifugation isocumented in Fig. 7c.
The transmission level obtained in the region where the floc-ulated fraction of the particles had settled out, allows for auantitative measure of the flocculated subfraction. Fig. 7a com-ares the averaged residual transmission in the middle part ofhe sample cell (118–120 mm) of the supernatant after 43 minf phase separation, and supports the flocculation classification.ransmission is highest for SO-Mag6-3 particles (highest degree ofocculation, no separate fines remaining), and gradually decreases
or higher PEI-loading. Without going into detail, the flocculationtate may be also be quantified by the height of the sediment25,28]. Fig. 7 comprises these data. The sediment height is largestor SO-Mag6-2 particles and the sediment height decreases ashe loading becomes more intense. Sediment height and residualransmission analysis clearly underlines quantitatively the aboveualitatively discussed flocculation state, based on visualizationf sedimentation behaviour, and general fingerprint pattern fortudied soft particles. It should be mentioned that optimization ofilica/iron magnetic particles based on the above approach was, inontrast to Zeta potential, successful to improve magnetotransduc-ion efficiency in practice [21].
. Conclusion
In situ visualization of separation behaviour under gravity orn centrifugal field delivers detailed direct information on particleurface alterations, particle agglomeration/flocculation and col-oidal stability. Analysis of fingerprint patterns and quantificationf residual turbidity or of sediment height allows for quantifyinghe state of dispersion and to optimize formulations. It is veryuitable for dispersions of hard and soft micro- and nanoparti-les, sterical, electrosterical, and rheological stabilization, mixturesf different particle types (e.g. metal oxides), at any pH-valuend ionic strength, as well as for organic solvents and ionic liq-ids.
Zeta potential of soft particles calculated by conventionalnalysis does not reflect the state of agglomeration/flocculation.ispersions of soft particles may be flocculated despite high Zetaotentials.
[
sicochem. Eng. Aspects 440 (2014) 122– 130 129
Acknowledgements
We thank the Bundesministerium für Wirtschaft und Technolo-gie of Germany (grant INNO-WATT IW082075) and Senatsverwal-tung für Wirtschaft (grant 10141296) for financial support. Theauthors thank K. M. Paciejewska (TU Dresden), B. Bharti (TU Berlin)and O. Mykhaylyk (TU Munich) for providing samples and resultsof Zeta potential measurements.
[2] ISO. DIS 13099-1. Colloidal systems – methods of Zeta potential determination– Part 1: electroacoustic and electrokinetic phenomena.
[3] S. Vallar, D. Houivet, J. El Fallah, D. Kervadec, J.M. Haussonne, Oxide slurries sta-bility and powders dispersion: optimization with zeta potential and rheologicalmeasurements, J. Eur. Ceram. Soc. 19 (1999) 1017–1021.
[4] A.V. Delgado, F. González-Caballero, R.J. Hunter, L.K. Koopal, J. Lyklema, Mea-surement and interpretation of electrokinetic phenomena, Pure Appl. Chem.77 (2005) 1753–1805.
[5] J. Phenrat, N. Saleh, K. Sirk, H.J. Kim, R.D. Tilton, G.V. Lowry, Stabilization ofaqueous nanoscale zerovalent iron dispersions by anionic polyelectrolytes:adsorbed anionic polyelectrolyte layer properties and their effect on aggre-gation and sedimentation, J. Nanopart. Res. 10 (2008) 795–814.
[6] J.A. Lewis, Colloidal processing of ceramics, J. Am. Ceram. Soc. 83 (2000)2341–2359.
[7] T.F. Tadros, Applied Surfactants Principles and applications, first edition, Wiley-VCH, Weinheim, 2005.
[8] S. Tsuneda, J. Jung, H. Hayashi, H. Aikawa, A. Hirata, H. Sasaki, Influence of extra-cellular polymers on electrokinetic properties of heterotrophic bacterial cellsexamined by softparticle electrophoresis theory, Colloids Surf. B: Biointerfaces29 (2003) 181–188.
[9] V. Hakley, Plenary Lecture at Particulate System Analysis, Edinburgh UK, 5-7-September 2011.
10] H. Oshima, T. Kondo, Electrophoretic mobility and Donnan potential of a largecolloidal particle with a surface charge layer, J. Colloid Interface Sci. 116 (1987)305–311.
11] E. Donath, D. Lerche, Electrostatic and structural properties of the surface ofhuman erythrocytes. I – Cell-electrophoretic studies following neuraminidasetreatment, Bioelectrochem. Bioenergetics 7 (1980) 41–53.
12] D. Lerche, A biophysical model for interaction of cells with a surface coat (gly-cocalyc). I. Electrostatic interaction profile, J. Theor. Biol. 104 (1983) 231–248.
13] H. Oshima, Theory of Colloid and Interfacial Electric Phenomena, AcademicPress, London, 2006, pp 473.
14] H. Oshima, Electrical phenomena in a suspension of soft particles, Soft Mater.8 (2012) 3511–3514.
15] C. Allain, M. Cloitre, M. Wafra, Aggregation and sedimentation in colloidal sus-pensions, Phys. Rev. Lett. 74 (1995) 1478–1481.
16] P. Mpofu, J. Addai-Mensah, J. Ralston, Temperature influence of nonionicpolyethylene oxide and anionic polyacrylamide on flocculation and dewateringbehaviour of kaolinite dispersions, J. Colloid Interface Sci. 271 (2004) 145–156.
17] L. Besra, D.K. Sengupta, S.K. Roy, P. Ay, Influence of polymer adsorption andconformation on flocculation and dewatering of kaolin suspension, Sep. Purif.Technol. 37 (2004) 231–246.
18] M. Beiser, G. Bickert, P. Scharfer, Comparison of sedimentation behavior andstructure analysis with regard to destabilization processes in suspensions,Chem. Eng. Technol. 27 (2004) 1084–1088.
19] K.M. Paciejewska, Untersuchung des Stabilitätsverhaltens von binärenkolloidalen Suspensionen, Karina Paciejewska Selbstverlag, Dresden,2011, http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-65050 (accessed07.06.12).
20] B. Bharti, J. Meissner, G.H. Findenegg, Aggregation of silica nanoparticlesdirected by adsorption of lysozyme, Langmuir 27 (2011) 9823–9833.
21] O. Mykhaylyk, T. Sobisch, I. Almstätter, Y. Sanchez-Antequera, S. Brandt, M.Anton, M. Döblinger, D. Eberbeck, M. Settles, R. Braren, D. Lerche, C. Plank,Silica-iron oxide magnetic nanoparticles modified for gene delivery: a searchfor optimum and quantitative criteria, Pharm. Res. 29 (2012) 1344–1365.
22] D. Lerche, Dispersion stability and particle characterization by sedimentationkinetics in a centrifugal field, J. Disp. Sci. Technol. 23 (2002) 699–799.
23] G.G. Badolato, F. Aguilar, H.P. Schuchmann, T. Sobisch, D. Lerche, Evaluation oflong term stability of model emulsions by multisample analytical centrifuga-tion, Progr. Colloid Polym. Sci. 134 (2008) 66–73.
24] T. Sobisch, D. Lerche, Thickener performance traced by multisample analyt-ical centrifugation, Colloids Surf. A: Physicochem. Eng. Aspects 331 (2008)114–118.
25] T. Sobisch, D. Lerche, T. Detloff, M. Beiser, A. Erk, Tracing the centrifugal sepa-ration of fine-particle slurries by analytical centrifugation, Filtration 6 (2006)
313–321.
26] Evonik Industries, Industry information 2242, Metal oxides and silicabased materials, 2012, http://www.sipernat.com/sites/dc/Downloadcenter/Evonik/Product/SIPERNAT/11-01-344-vnd-ii-2242-katalysatoren-us.pdf[accessed 07.06.12].
30 D. Lerche, T. Sobisch / Colloids and Surfaces A
27] R. Buscal, L.R. White, The consolidation of concentrated suspensions, J. Chem.Soc. Faraday Trans. 83 (1987) 873–891.
28] D. Lerche, T. Sobisch, Consolidation of concentrated dispersions of nano- andmicroparticles determined by analytical centrifugation, Powder Technol. 174(2007) 46–49.
29] A.S. Parmar, M. Muschol, Hydration and hydrodynamic interactions of
lysozyme: effects of chaotropic versus kosmotropic ions, Biophys. J. 97 (2009)590–598.
30] M. Kobayashi, F. Juillerat, P. Galletto, P. Bowen, M. Borkovec, Aggregation andcharging of colloidal silica particles: effect of particle size, Langmuir 21 (2005)5761–5769.
sicochem. Eng. Aspects 440 (2014) 122– 130
31] A. de Keizer, E.M. van der Ent, L.K. Koopal, Surface and volume charge densitiesof monodisperse porous silicas, Colloids Surf. A: Physicochem. Eng. Aspects 142(1998) 303–313.
