Samim Ali

241 Pages·2015·21.14 MB·English

EXPERIMENTAL INVESTIGATIONS OF
DISPERSIBILITY, STABILITY AND RHEOLOGICAL
BEHAVIOR OF AQUEOUS SUSPENSIONS OF
COLLOIDAL CLAYS
By
amim li
S A
AthesissubmittedtotheJawaharlalNehruUniversity
forthedegreeofDoctorofPhilosophy
DepartmentofSoftCondensedMatter
RamanResearchInstitute
Bangalore560080
October2015
©SamimAli,2015.
TypesetinLATEX2ε.
ii
Certificate:
This is to certify that the thesis entitled “Experimental investigations of dispersibility,
stabilityandrheologicalbehaviorofaqueoussuspensionsofcolloidalclays”submitted
by Samim Ali for the award of the degree of Doctor of Philosophy of Jawaharlal Nehru
University is his original work. This has not been published or submitted to any other
UniversityforanyotherDegreeorDiploma.
Prof. RaviSubrahmanyan Dr. RanjiniBandyopadhyay
(Director) (ThesisSupervisor)
RamanResearchInstitute
Bangalore560080
India
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Declaration:
I hereby declare that the work reported in this thesis is entirely original. This thesis is
composed independently by me at Raman Research Institute under the supervision of Dr.
Ranjini Bandyopadhyay. I further declare that the subject matter presented in this thesis
has not previously formed the basis for the award of any degree, diploma, membership,
associateship, fellowship or any other similar title of any university or institution. I also
declarethatIhaverunitthroughTurnitinplagiarismdetectionsoftware.
Dr. RanjiniBandyopadhyay SamimAli
SoftCondensedMatterGroup
RamanResearchInstitute
Bangalore560080
India
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Acknowledgements
I would like to express my sincere gratitude to my supervisor and mentor, Ranjini
Bandyopadhyay, for giving the opportunity to work with her. The completion of this
thesis would not have been possible without her help, guidance, and support. I would
also like to thank Dr. Pramod Pullarkat and Dr. Sadiq Rangwala for their positive
disposition to be members of the advisory committee and their valuable advice and
comments on this research work. I am grateful to Prof. Yogesh M. Joshi, IIT Kanpur,
forilluminatingdiscussionsandactivecollaboration.
I would like to convey my sincere gratitude to Rajib Basak and Debasish Saha for
helping me familiarize myself to various lab instruments during my initial months in
thelabandgettingmestartedwithresearchatRRI.Ithasbeenagreatpleasuretohave
regulardiscussionswithSanjayandVenketesh. Ithankthemfortheirhelpwithsample
preparation and measurements. My hearty thanks are also due to Jyothi for her help
withtheeditingofthereferencelistandthecartoonspresentedinthisthesis.
I thank Dr. Sanjib Sabhapandit and Dr. Abhishek Dhar for introducing me to
the nonequilibrium statistical mechanics during my course work. I would also like to
express my gratitude to Dr. V. Lakshminarayanan, Dr. V. A. Raghunathan, Dr. Arun
Roy, Dr. Yashodhan Hatwalne for their encouragements, advice and for illuminating
discussionsonseveraloccasions.
My sincere thanks also go to Mr. Ram, Mr. M. Mani, Mr. A. Dhason, Mr. Durai
Chelvan for sharing their expertise during development of experimental setups and
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0. Acknowledgements
for help with AFM and cryo-SEM imaging. I thank Mr. K. Radhakrishna for all his
assistancewithadministrativeworks. IwouldalsoliketoexpressmygratitudetoRaja
andMuraliforextendingtheirhelpinghandsoncountlessoccasions.
I would also like to take this opportunity to thank the director and all the people
in various departments- administration, library, computer and networking, canteen,
engineering-workshop,horticultureandcleanersinRRIforprovidingthebasicsupport
andfriendlyenvironmentthatwererequiredformyresearchwork.
IamluckytohavefoundmanygoodfriendsinRRI.Theyhavemademystayhere
really enjoyable through their love and affection. I thank all of them for being friends,
forregularconversationsonvarioustopicsandfortheirconstructivecriticism. Iwould
alwayscherishthememoryoftimesIspentwiththem.
Lastly, I express my deepest gratitude and love to my mother, my sister and my
teacher,Mr. GobordhanSarkar,fortheirconstantsupportandencouragement,without
whichIwouldnothavecomeoutofmyvillageforhigherstudies.
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Synopsis
Colloidalparticlesareabundantinnatureandthesuspensionsofthesesubmicronpar-
ticles have become an important part of soft condensed matter research. The collec-
tive behavior of these systems is determined by the interactions that exist between
the colloidal particles in their suspensions. These colloidal interactions can easily be
manipulatedbyexternalmeanstoachievedesiredbulkproperties.
This thesis work reports experimental studies of the dispersibility, stability and
rheological behavior of aqueous suspensions of synthetic Laponite and natural Na-
montmorillonite clays. The primary clay particle, known as platelet, has negative
chargesonitsflatsurfacesandpHdependentpositivechargesonitsedge. Indryform,
these platelets form stacks, known as tactoids, with positive counterions in the inter-
calated spaces. In aqueous suspensions, these tactoids exfoliate into single platelets
with electric double layers on their surfaces. Due to screened repulsive interactions
between the anisotropic electric double layers, the effective excluded volume is much
higher than the geometrical volume of individual platelets. As a result, clay suspen-
sions exhibit spontaneous phase transitions from freely flowing liquids to kinetically
arrested, disordered glassy solids at particle volume fractions that lie far below the
jamming transition concentration of hard spherical particles. The interaction force be-
tweentwoplateletsisafunctionoftheinterparticleseparation,mutualorientationsand
thicknessoftheelectricdoublelayers. Therefore,itispossibletotunetheinteractions
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Synopsis
between the clay particles externally by controlling the particle and ionic concentra-
tions in the dispersing medium and solution pH. Addition of appropriate amounts of
salt leads to attractions between the screened electric double layers, which can result
intheformationofvolume-spanningnetworkstructuresknownasgels. Kineticallyar-
rested phases, such as glasses and gels, are also characterized by their aging behavior,
duetowhichthedynamicsofclayparticlesslowdownwithtime,leadingtochangesin
theirbulkproperties. Besidestheirobviousimportanceinfundamentalresearch,aque-
ous suspensions of these clay colloids find wide applications in the pharmaceutical,
cosmetics,petroleum,coating,andfoodindustries.
Chapter 1 of this thesis contains introductory material on colloids and colloidal
interactions in suspensions. The formations of glass and gel phases in these systems
are next described briefly. This is followed by a detailed description of the structures,
swelling and exfoliation processes of Laponite and Na-montmorillonite in aqueous
suspensions. Various phases that these clays exhibit at different particle and ionic
concentrationsarealsopresentedindetail. Ageneraldescriptionofseveralrheological
models, including soft glassy rheology, is presented. The chapter concludes with a
shortdiscussionontherheologicalbehaviorofclaysuspensions.
Chapter 2 describes the various experimental methods used in this thesis work.
The experimental techniques used here include rheological measurements, ultrasound
attenuation spectroscopy, electroacoustics, atomic force microscopy and cryogenic
scanning electron microscopy. Finally, a light-based, table-top setup for monitoring
thestabilityofclaysuspensionsisdescribed.
Chapter 3 describes the exfoliation kinetics of Laponite tactoids in an aqueous
medium using ultrasound attenuation spectroscopy. The attenuation spectra measured
in the frequency range 3 − 100 MHz are used to estimate bimodal size distributions
of tactoids for different ages of clay suspensions in a concentration range where these
suspensions show glassy behaviour. A theory of equivalent spherical diameter of col-
loidal discs under Stokes drag is used to estimate the average thicknesses of these
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tactoids. This analysis shows that a substantial fraction of clay aggregates in suspen-
sion are tactoids that are composed of more than one platelet. This is understood in
termsoftheenhancementofintertactoidCoulombrepulsionsduringtheexfoliationof
clayaggregateswithageofthesuspension.
Chapter 4 describes the exfoliation behavior of Na-montmorillonite in aqueous
suspensions below and above the jamming concentrations using ultrasound attenua-
tion spectroscopy. It is seen, using a similar formalism developed in Chapter 3, that
complete exfoliation is not achieved even below the glass transition concentration.
Conductivity measurements in these suspensions are performed after varying the clay
mineral concentration. The measurements indicate that the repulsive interactions may
not be the sole factor for incomplete exfoliation of this clay. As the bulk concen-
tration of the hydrated Na+ counterions dissociated from the intra-tactoid spaces in-
creasesduringexfoliation,theosmoticpressuredifferencebetweenthebulkphaseand
the intra-tactoid space decreases with age. It is seen that this has a reverse effect on
the exfoliation kinetics and leads to a reduction in the exfoliation rate at higher ages
and clay concentration. This is verified by performing experiments after dispersing
Na-montmorillonite in brine water. It is found that the presence of excess hydrated
counterions in the bulk medium hinders the exfoliation process. The exfoliation of
clay aggregates is also studied in a condition where counterions are added after dis-
persing the clay minerals in deionized water. It is found that the size of tactoids in
this suspension increases before eventual saturation. This phenomenon is interpreted
intermsofaprocessoftactoidreformationdrivenbythereverseosmoticpressurethat
is generated by the addition of counterions after dispersing clay. The influence of tac-
toid exfoliation on the suspension viscoelasticity and the stability of this natural clay
isassessedusingrheometryandelectroacoustics.
Chapter5describesthesalt-inducedmorphologicalchangesinNa-montmorillonite
gels and their macroscopic mechanical responses in oscillatory strain sweep experi-
ments. When the salt concentration in the dispersing medium is increased systemati-
cally, viscoelasticity and yield stress of the suspensions exhibit a monotonic increase
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Synopsis
upto a critical salt concentration before decreasing with the further addition of salt.
Thissignifiesamorphologicalchangeinthedispersionmicrostructuresacrossthecrit-
icalsaltconcentration. Wedirectlyvisualizethemicroscopicstructuresofthesekinet-
ically arrested phases using cryogenic scanning electron microscopy. We observe the
existence of honeycomb-like network morphologies for a wide range of salt concen-
trations. The transition of the gel morphology, dominated by overlapping coin (OC)
and house of cards (HoC) associations of clay particles at low salt concentrations to
a new network structure dominated by face-face coagulation of platelets, is observed
across the critical salt concentration. Using electroacoustics, it is seen that these gels
are highly stable under gravity over a wide salt concentration range even above the
criticalvalueofsaltconcentration.
Chapter 6 discusses the aggregation and stability behaviors of highly polydisperse
andcharge-anisotropicclayplateletsofNa-montmorilloniteinaqueoussuspensionsat
a dilute concentration well below that required to achieve the glass transition. Using
light transmission method and electroacoustics, it is seen that these platelets are not
stable in salt-free aqueous suspensions in the dilute concentration regime and sedi-
mentundergravitywithaparticleconcentrationgradientalongthesampleheight. The
additionofsalttotheclaysuspensionresultsintheformationofweakgels. Bydirectly
trackingthegelheightwithsampleageusingalighttransmissionmethod,itisfurther
shown that the addition of salt after dispersing the clay mineral results in more stable
gels even in very dilute suspensions. These weak gels are seen to exhibit transient
collapseafterafinitedelaytime,aphenomenonobservedpreviouslyindepletiongels.
The velocity of the collapse oscillates with age of the sample. However, the average
velocity of collapse increases with sample age upto a peak value before decreasing at
higher ages. With increasing salt concentration, the delay time for transient collapse
decreases, while the peak value of the collapsing velocity increases. Using ultrasound
attenuation spectroscopy, rheology and cryogenic scanning electron microscopy, we
confirm that morphological changes of the gel network assembly, facilitated by ther-
malfluctuations,leadtotheobservedcollapsephenomenon.
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