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 iii 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 iv 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 v 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. vi 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 vii 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 viii 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 ix 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. x