Deposition of Latex Particles:
Theoretical and ExperimentalAspects
P. Somasundaran,SudhirShrotriand K.P.Ananthapadmanabhan1
LangmuirCenterfor Colloidsand Interfaces,ColumbiaUniversity,New York,NY 10027
145River Road, UnileverResearchUS Inc., Edge Water,NJ, 07020
USA
ABSTRACT
Deposition of colloidal particles on surfaces is important in the processing of various produ~ts as
minerals, cosmetics, detergents, and semiconductors.Several factors govern deposition making the
phenomenarather complex. The main objective of this work is to elucidatethe roles of the nature of the
depositing particles and environmental conditions on deposition and to identify critical factors that
govern deposition.
In the current study, deposition of anionic, cationic and zwitterionic latex particles on glass surface
was done as a function of deposition time, pH, and particle concentration.The charge groups on latex
particles are sulfate, carboxylic and amidine groups which are attachedto dIe surface by hydrocarbon
chains ( referred to as "hairy groups" later). The data was subjected to dteoretical treatment by
consideringforces involved in the depositionof different latex particles.
While amidine latex particles ( IEP aroundpH II) showedgood deposition,sulfate l~tex particles did
Dot show appreciabledeposition. Sincethe glasssurface is negatively chargedin the pH range (3-12) of
the deposition tests, the deposition of the opposite chargedparticles was as expectedand conforms to
the theoretical calculations. In contrastto this, deposition in the presenceof energy barrier was difficult.
However the exceptionwere zwitterionic latex particles, which showedconsiderabledeposition on glass
even when they were negatively charged. The deposition of zwitterionic latex particles with a negative
zeta potential similar to dtat of sulfate latex was contrary to dIe theoretical predictions. We proposedtat
the hairy groups on zwitterionic particles rearrange as they approach the surface to enable their
attachment.
INTRODUCTION
Deposition plays a vital role in many technological and natural processes,such as, involving drugs,l,2
cosmetics, detergents,3paper making,4 bacterial adhesion,Scarrier flotation, colloidal contaminants
transport,6filtration,7 and semiconductors.8The study of particle deposition is also of intrinsic interest
to the field of colloid sciencefrom a fundamentalpoint of view.
There have been numerous studies encompassingdifferent aspectsof deposition, employing a wide
array of colloidal particles and substrates.Researchershave studied deposition of latex particles,9.10,11
Surface Chemistry ofThiobacil/us
Ferrooxidans with Respect to Interaction with Sulphide Minerals
125
Sulphur- grown cells and liquid ferrous iron grown cells exhibit different IEP valuesindicating differences
in their surface chemistry. A proteinaceouscell surface appendagewas fonned on mineral grown cells,
facilitating their adhesion.Adhesionof the bacterialcells on sulphidemineral surfacespromotebioleaching
through the direct attack mechanism.Bacterial interaction witM' sphalerite and galena influenced their
floatability. Through bacterial pretreatment, selective separa1r10n
of sphalerite and galena could be
achieved.
~
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D.H.
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14. A.J. Vogel: Text Book of QuantitativeInorganicAnaly
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ux and H.J. Busscher: J. Bacteriol, 1988,vol. 170;pp.
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127
Deposition of Latex Particles: Theoretical and Experimental Aspects
mineral particles,12,13carbon black,i2 bacteria and other biological colloids on various substratessuch
as glass,\2 polymers\2,14and minerals.I 1,14The initial deposition processis generally describedby set
of transport equations, also known as Fokker Plank25 equations which take into account particle substratesurface interactions. During this stage a constant rate of particle deposition rate is generally
observed.15,16In contrast later stagesof deposition are thought to be governed by desorption, surface
hetrogenity,17blocking of deposition sites and excluded area effects.IS Colloidal forces dominating
deposition processl9-22and kinetic process23,24
involved have beenwidely reported and reviewed.25
However there are some important issuessuch as, the importance of the discrete charges,and their
rearrangementwhich have not been yet resolved. The presentstudy was undertakento elucidate these
factors. The objective was to understandthe influence of surface chemical features of particles and
substrates;in this context the model systemwas chosenspecifically to incorporate the effects such as
discrete as well as rearrangablecharges.Deposition of three different type of latex particles on glass
substratewas conductedand the results are discussedin terms of relevant colloidal forces presentin the
system.
MA.TERIALS AND METHODS
Materials
The latex particles were obtained from Interfacial Dynamics Corporation spheres(IDC) were used as
received with out any further cleaning. Three different types of polymer lattices particles were
employed, namely amidine, sulfate and zwitterionic lattices. These lattices are stabilized by the charge
groupspresenton the surface(seeTable I).
Table I: Latex Particle Specifications
Latex Type
Diameter( 1Jm)
Charge Group
Charge Behavior
Arnidine Latex
1.84
Amidine
Cationic
Sulfate latex
2.04
Sulfate
Anionic
Zwitterionic Latex
0.86
Amidine and Carboxyl
Mixed
A schematicof the latex is shownin Fig. I. Microscopic,frostedglassslidespurchasedfrom Fisher
Scientificcompanywereusedassubstrates
for thedepositionof theparticles.
Methods
Sample Preparation
Latex suspensionpreparation: In order to comparethe extent of deposition, latex ~uspensionof desired
initial particle concentration was preparedby diluting known weights of latex suspensiondrops with
triply distilled water with/without salt. HCI or NaOH, was added as required to adjust the pH and the
weight was adjustedto 3.0 g. The suspensionwas then mixed for 4 minutes using a magnetic stirrer and
desiredquantity transferredonto the substrateusing a micro pipette for the deposition test.
128
Mineral Processing: Recent Advances and Fuhue Trends
Figure A schematicof latex particleswith chargegroupspresenton the surface(figure not to scale).
Substratepreparation: As received slides were immersedin I: I Nitric acid solution for two minutes and
washedwith triple distilled water and air dried in an inverted slanted position to reduce deposition of
dust particles.
Deposition
Deposition studieswere carried out as a function of pH. A simple and reproducible deposition technique
basedon free settling adopted for the presentstudy involved placing of , 50 1.11
of the latex suspension
with the help of a micro pipette, on a precleanedglassslide. The slide was coveredwith a watch glassto
prevent any dust contamination and left for free settling for desired interval. The slide was then washed
with triple distilled water, air dried, and observedunder-amicroscope.
Zeta Potential Measurements
Electrokinetic characterization of glass slides was done by measuring streaming potential using a
Brookhaven AP Paar Electrokineticanalyzer.The samplesto be measuredwere taken in the
rectangularcell holder. Latex particles were characterizedusing Pen Kern Lazer Zee meter, model 500,
that measureselectrophoretic mobility. All experimentswere performed at room temperatureand ionic
strengthwas maintained at 10-4Musing NaCI solution.
-
RESULTSAND DISCUSSIONS
The zeta potential behavior of the slide was measuredusing the streaming potential technique and is
illustrated in Fig. 2 as a function of pH. The substrateis negatively chargedthroughout the pH range of
3-12.
The zeta potential of latex particles shown in Figs. 3 and 4 as a function of pH show the sulfate latex
to be negatively charged under all pH conditions studied. and the zwitterionic latex particles and the
amidine latex to possessan isoelectric points aroundpH 4 and II respectively.
[)r;pO.Yitionof Latex Particles: 11IeoreIicaJand Experimental Aspecl.r
129
G
-5
-10
>:
.
--IS
J
-10
s-u
...
",-~...~..,...,...,
2
J
4
5
,
,...,
7
.
,
,
"""=,,
10
II
.
12
...
Figure 2: Zeta potential, converted from streaming potential values, as a function of suspension pH for the glass slides,
ionic strength I0-4M
iii
Figure 3: Zeta potential of amidine latex particlesasa function of pH. ionic strength10-4 M.
Deposition of Latex Suspension
Deposition study was done as a function of the pH and the micrographs for representativecases,are
shown in Figs. 5 and 6. Glass surface is negatively chargedunder all test conditions and. the extent of
deposition in the caseof amidine latex, is governedby it's zeta-potential.Around the IEP as expected
the amidine latex particles had a tendency to aggregateand deposit on the slide as clumps. Above
isoelectricpoint, the deposition is found to decreasedrastically.
Depositionon glasssurfaceas a function of pH was also done with sulfate latex and zwitterionic latex.
In the pH range where the zwitterionic is positively charged.as expectedgood deposition was found.
Interestingly,above IEP under conditionswhen both the zwitterionic and sulfate latex particleshad same
130
Mi".ra/
Processing: Recent Advances and Future Trends
Figure4: Zeta potential measurementof sulfate (e) and zwitterionic (8) latex as a function of suspensionpH, ionic
strength lo-4M.
zeta potential the fonner deposited well while the latter did not (see Fig. 6). It is to be noted that
deposition of zwitterionic latex particle took place even though it is similarly charged to the glass
substrate.
THEORY
The deposition was done here under free settling conditions. Assuming that there is no convective
transfer and quiescentconditions prevail throughout the deposition process.Forcesassociatedwith the
attachmentof latex particles on to glass arise from the overlap of the electrical double layers of the
particle and substrate(FeU, van der Waals forces (Fvdw) and the gravitational force (Fg): Hence, the
total interaction force can be expressedas a sum of:
F~ Fel + Fvdw + Fg
(1)
The hydrodynamic force between the particle and substrate is negligible under these conditions.
Expressionsdevelopedto estimatethe magnitudeof theseinteractionsare discussedbelow.
Electrical Double Layer Interactions
Various expressionshave been developed for electrical double layer overlaps in the past and differ
essentially in the choice of boundary conditions, namely, constant charge or cons~antpotential of the
interacting surfacesduring their mutual approach.26-29The constant potential condition is suitable for
caseswhere electrochemical equilibrium of the potential determining ions and adsorbed species is
maintainedduring the interaction. Under constantchargeconditions, the adsorbedor chemically bound
speciesthat are responsiblefor the chargedevelopmentat the interface are not regulatedrapidly enough
during the approach of surfaces.These conditions are however the twoextremesand in real systems,
Deposition of Latex Particles: 77leoretical and Experimental Aspects
131
both charge and potential can undergo regulation. For the purpose of this study, nonetheless,const~t
chargeexpressionsare utilized, as the chargesare chemically bound to the surface.
(a)
(b)
(c)
Figure S: Representativedeposition micrographsof arnidine latex particles on glass when they are (a) positively
charged(b) nearisoelctric point (c) negativelycharged.
Mineral Processing:RecentAdva~s and Future Trends
132
(a)
(b)
Figure 6: Representativedeposition micrographsof (a) sulfate latex particles and (b) zwitterionic latex particles on
glasswhen both particleswere similarily chargedto the glass.
For the case when the radius of particle, rp, is much smaller than the radius of curvature of the
substrate,rs, Fel , basedon the model by Hogg et al.29canbe expressedas:30
F~
-
4". & r, K
oKrp H)
" ,. { 1+exp(exp(oK
r,H}}
(2)
Deposition of Latex Particles: 11IeoreticaJ and aperinlenlal
133
Aspects
where l;p is the elecuokinetic potential of particle, c;. is the electrokinetic potential of subsb"ate,ICis the
debye parameter,£ is the permittivity (= 78.54 x 8.8542 X 10.12at 25°C) F/m and H is the approach
distance.
van der Waa"s Interactions
For the case of a flat plate (1) and a sphere with radius rp (2) that interact across a medium (3) the
following equation is a satisfactoryapproximation:30
Fvdw
-
-
AI32
(3)
6 rtMf2
and 2) in a medium (3) and can be
where A132 is the effective Hamaker constantfor two materials (
expressedas:31
A132. ( JAII- JA33)
( .fin - /""in
(4)
where All' An. AJJ are the Hamaker"constants for the materials (I and 2) and the medium (3)
respectively.
Gravitational Force
The gravity force acting on the particle, neglectingthe resistive force by the medium is:32
Fg
-
-~ K r; (p - P
3
P
I
(5)
)g
WhereP p is the particle density, P I is the density of medium and g is the gravitational force
THEORETICAL CALCUU TIONS
The calculated forces for the amidine, zwitterionic and sulfate latex are shown Figs. 7, 8 and 9. Typical
valuesusedfor the calculation are also shown in Table II.
'Fable II: Typical Parameters usedfor the Calculation of Interaction Forces for the Latex Particles
1Ype01Latu
Amidine
Sulfate
Zwitterionic
-
Hamaker COlUlanI. A 131
Zeta
(in J)
potentialParticles
(in mY)
uta
Deb}'e length
potential Substrate innm
(in mV)
4.5 x 10-21
4.5 x 10-21
4.5 x 10 -21
+40.0
-20.0
-40.0
-20.0
30
-40.0
-20.0
30
134
MiMral Processing: Recent Advances and FIItIIrC Tnllds
1
10
APPCQIChDiSIaIQ
100
( nm )
Figure 8: Interactionforce betweensulfatelatex particlesand glasssurfKe as a function of approachdistance.
As expected for the Amidine latex deposition, an attractive force prevails at all distances of
approacheswhen the latex is positively charged.Gravitational force, though negligible comparedto the
double layer and van der Waals forces, also assiststhe deposition. The sulfate latex particles and the
substrateare similarly charged and thus an energy barrier is encountereddue to electrostatic repulsion
when they approachthe substrate.The energy barrier is also substantiallymore than kT, preventing any
thennal or diffusional escapeof the particles. Under the present deposition schemethe only external
force is the gravitational one and thus is negligible comparedto the barrier and hence deposition is not
feasible.
For zwitterionclatex, the conditionsare very muchthe sameas that for the sulfatelatex and once
againan energybarrier is encountered.
This barrieris also substantiallymorethan kT and againtht"
gravitationalpull is negligiblecomparedto thebarrier.
135
LHposition of Lola Particles: Theoretical and Experimental Aspects
IC
~
,
z
""2
0
J .,
-10
-.,
1
10
APP'f*;h D'-
100
<IUD>
Figure 9: Interactionfon:e betweenzwittcrionic latex paniclesand glasssurfaceasa function of approachdistance.
Based on these calculations, it is expected that zwitterionic particles should behave similar to the
sulfate latex and deposition should not be feasible under the present deposition scheme. Quite contrary
to this, a good amount of deposition was observed in this study.
UNCONVENTIONAL DEPOSITION OF ZWITTER/ONIC LA TEX PARTICLES'
As seenfrom both the experimental and the theoretical calculations,zwitterionic particles deposit in an
unexpectedfashion. Deposition of colloids on similarly chargedsubstratesis not new particularly in the
biological field. There are several examples where negatively charged bacteria would deposit on a
negatively charged surface. Similar behavior by the mixed charge latex is interesting since it can
indicate possible mechanismsfor such natural !>rocesses.We propose that the mixed charge groups
presenton the latex surfacerearrangein such a manner such that the positive charge sites are extended
and the negative ones retracted. Thus even though the over all average zeta potential is negative, the
hairy chargesare proposedto rearrangewhen the two surfacescan feel eachother; the schematicof this
is shown in Fig. 10.
~
(;.t)
(::J.
I
)
~i-c:e-
~(.7)
0
1!'111111
~
(-t.)
Figure 10: Schematicof proposedrearrangementof mixed chargeson a zwitterionic latex particle as it approachesa
similarly chargedsurface.
136
Mineral Processing: Recent Advances and FIIhIn TrendY
SUMMAR Y
Conventional deposition was found for amidine and sulfate latex particles. The electrostaticforces play
a dominant role in the deposition in thesesystems.The presenceof an attractive electrostaticforce leads
to a favorable deposition of amidine latex particles whereas that of a repulsive electrostatic force,
deposition was difficult for sulfate latex.
Zwitterionic particles behaved in an unconventional fashion and in spite of the presenceof an
apparentelectrostatic repulsion, good deposition was observed.Modification and rearrangementof the
mixed hairy charge groups are proposedto bring about this effect.
ACKNOWLEDGMENTS
Financial support from Unilever ResearchUS Inc. and National Science Foundation for this study is
acknowledged.
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Deposition of Latex Particles: Theoretical and Experimental Aspects
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