Attheedgesofthekaoliniteparticles,therearebrokenchemicalbonds.Thesebondsmustbebalanced by adsorbed ionstopreserve electrical neutrality.Measurements arecommonlymade only of the number of cations required to balance the charge. The units used are thenumber of milliequivalents of cations per100grams of dry clay (Me/100 g).This number iscalled the cation exchange capacity.The word exchange is used because these cations areeasily replaced by other cations.Forkaolinite the cation exchange capacity is usually in theorderof3-10Me/100gForthesakeofcompletenessashortexplanationwillbegivenhere-afterabouttheexchangecapacity.Me = milliequivalents = 10-3 equivalents. An equivalent is the number of electrical charges inonemoleculeof solutionandequals6x10-3(Avogadro'snumber).An exchangecapacityof10Meper100gmeansthateach100gofsoilsolidscanexchange(10x10)(6x103)=6x 1021 electronic charges. If the exchangeable ion is univalent (e.g. Nat), 6 x 1021 sodium(Nat) ions can be replaced. If the exchangeable ions are divalent, such as Ca2+, 3 x 1021calciumionscanbereplaced.IlliteAn individual sheet of illite consists of an octahedral layer sandwiched between twotetrahedral layers.The symbolic representation is given in figure 15.GG(b)Figure15Symbolicstructureof illite[4].The thickness of one ilite sheet is 10 A. Again the bonds within each sheet are very strongcovalent bonds. The octahedral layer may contain Al, Mg,Fe or other cations. In thetetrahedral layer1 in 7Si4+ions are replaced byan A/3+iongiving the sheet a largedeficit ofpositive charge.Some deficiency also results from insufficient charges in the octahedral layer.Hence,theillitesheetmust notonlyadsorb cationstobalancebrokenbondsattheedges ofthe sheet, but must also adsorb cations to balance charge deficiencies inside each sheet.To balance the positive charge deficit in thetetrahedrons potassium ions,K+ ions enterbetweentwotetrahedrallayers.Theexteriortetrahedrallayersoftheillitesheetsarealmostin contact with each other since theadsorbed potassium ion fits very nicely in the triangularbase of the upper and lower tetrahedron.This is schematically shown below.2- ions at triangular base oftetrahedronK* ion fitting in base oftetrahedronThepotassium ions arefirmlybonded to bothsheetsand thereforeprovidea bridgeholdingthem together.22
22 At the edges of the kaolinite particles, there are broken chemical bonds. These bonds must be balanced by adsorbed ions to preserve electrical neutrality. Measurements are commonly made only of the number of cations required to balance the charge. The units used are the number of milliequivalents of cations per 100 grams of dry clay (Me/100 g). This number is called the cation exchange capacity. The word exchange is used because these cations are easily replaced by other cations. For kaolinite the cation exchange capacity is usually in the order of 3 – 10 Me/100 g. For the sake of completeness a short explanation will be given here-after about the exchange capacity. Me = milliequivalents = 10-3 equivalents. An equivalent is the number of electrical charges in one molecule of solution and equals 6 x 1023 (Avogadro’s number). An exchange capacity of 10 Me per 100 g means that each 100 g of soil solids can exchange (10 x 10-3 ) (6 x 1023) = 6 x 1021 electronic charges. If the exchangeable ion is univalent (e.g. Na+ ), 6 x 1021 sodium (Na+ ) ions can be replaced. If the exchangeable ions are divalent, such as Ca2+, 3 x 1021 calcium ions can be replaced. Illite An individual sheet of illite consists of an octahedral layer sandwiched between two tetrahedral layers. The symbolic representation is given in figure 15. Figure 15 Symbolic structure of illite [4]. The thickness of one illite sheet is 10 Å. Again the bonds within each sheet are very strong covalent bonds. The octahedral layer may contain Al, Mg, Fe or other cations. In the tetrahedral layer 1 in 7 Si4+ ions are replaced by an Al3+ ion giving the sheet a large deficit of positive charge. Some deficiency also results from insufficient charges in the octahedral layer. Hence, the illite sheet must not only adsorb cations to balance broken bonds at the edges of the sheet, but must also adsorb cations to balance charge deficiencies inside each sheet. To balance the positive charge deficit in the tetrahedrons potassium ions, K+ ions enter between two tetrahedral layers. The exterior tetrahedral layers of the illite sheets are almost in contact with each other since the adsorbed potassium ion fits very nicely in the triangular base of the upper and lower tetrahedron. This is schematically shown below. O 2- ions at triangular base of tetrahedron K + ion fitting in base of tetrahedron The potassium ions are firmly bonded to both sheets and therefore provide a bridge holding them together
Theexchangeablecations includeallthoseattheedgesplusthoseontheexteriorfacesplusafew from between the sheets. Cations inside the particle are unable to exchange.Thecation exchange capacity is usually in the range of 10 to 40 Me/100 g. Because of therelativelystrongbond, waterwill not easilypenetrateand swell and shrinkage isnot so muchofaproblemThe width of the platy like particle is 0.1 to 2 μm while the thickness is 1/10 of the width.MontmorilloniteStructurally,montmorilloniteis similar to illitein that each sheet consists of an octahedrallayer sandwiched between two tetrahedral layers.However,much of the charge deficiency inmontmorillonite comes from theoctahedral layer.This charge deficit must be balanced in thesame way as it is done with illite, i.e. by means of cations between the montmorillonitesheets. Since the distance between the charge deficit and the cation is larger with themontmorillonitethan with the illite,the bond between the montmorillonite sheets ismuchless than that of the illite sheets.Also the cations are less firmly held.This in turns meansthat the charge balancing cation is not necessarily Kt like in illite but can be any cation.Theadsorbed cations are unable to lock the sheets firmly together and as a result, particlesseparate into a single sheet ifmontmorillonite is dispersed in water.Allthisimpliesthatthesymbolicrepresentationof montmorilloniteisthesameasthatof illite(figure 13) but without the K ion connecting the sheets.Thelackofreinforcementbetweenthesheetsmakesitimpossibleforlargeparticlestoformand thus montmorllonite particles are very small. The width of the platy like particles is 0.1-1μmwhilethethicknessis1/100of thewidth.There are fewer adsorbed cations in the montmorillonite than in the illite but nearly all of themontmorillonite cations are available for exchange.This results in a cation exchange capacityof 80-130Me/100g.The properties of montmorillonite arevery dependent on thecation adsorbed.It will beshown lateronthatNaorientswaterloosely overacertaindistancewhileCaorientswatermuch more rigid over a much smaller distance. This makes Na montmorillonite much moreunstableand muchmoresensitivefor swellingand shrinkagethan Ca montmorillonite.Plasticity characteristics in relation to type of clay mineral and exchangeable ionThe influence of charge deficiency is discussed here-after. The distortion tends to restrictcrystal growth and thus limits the size of the crystals.Isomorphous substitution takes place in the following order.Lit <Nat <Ht < K* < NH+ << Mg2+<Ca2+ << A13+The order means that Al replaces Ca, Ca replaces Mg, etc.. Li is the easiest exchangeable ion.The effect of the exchangeable ion on the properties of the clay is quite significant as can beseen from table 6.MineralLiquidPlasticExchangeablePlasticityIonLimit [%]Limit [%]Index [%]61Na710656Montmorllonite510Ca429Fe29075215IliteNa120349656Ca100C1Fe1105332218 NaKaolinite3827Ca37Fe5922Table 6 Plasticity values in relation to clay mineral and exchangeable ion [4].23
23 The exchangeable cations include all those at the edges plus those on the exterior faces plus a few from between the sheets. Cations inside the particle are unable to exchange. The cation exchange capacity is usually in the range of 10 to 40 Me/100 g. Because of the relatively strong bond, water will not easily penetrate and swell and shrinkage is not so much of a problem. The width of the platy like particle is 0.1 to 2 m while the thickness is 1/10 of the width. Montmorillonite Structurally, montmorillonite is similar to illite in that each sheet consists of an octahedral layer sandwiched between two tetrahedral layers. However, much of the charge deficiency in montmorillonite comes from the octahedral layer. This charge deficit must be balanced in the same way as it is done with illite, i.e. by means of cations between the montmorillonite sheets. Since the distance between the charge deficit and the cation is larger with the montmorillonite than with the illite, the bond between the montmorillonite sheets is much less than that of the illite sheets. Also the cations are less firmly held. This in turns means that the charge balancing cation is not necessarily K+ like in illite but can be any cation. The adsorbed cations are unable to lock the sheets firmly together and as a result, particles separate into a single sheet if montmorillonite is dispersed in water. All this implies that the symbolic representation of montmorillonite is the same as that of illite (figure 13) but without the K+ ion connecting the sheets. The lack of reinforcement between the sheets makes it impossible for large particles to form and thus montmorillonite particles are very small. The width of the platy like particles is 0.1 – 1 m while the thickness is 1/100 of the width. There are fewer adsorbed cations in the montmorillonite than in the illite but nearly all of the montmorillonite cations are available for exchange. This results in a cation exchange capacity of 80 – 130 Me/100 g. The properties of montmorillonite are very dependent on the cation adsorbed. It will be shown later on that Na+ orients water loosely over a certain distance while Ca2+ orients water much more rigid over a much smaller distance. This makes Na montmorillonite much more unstable and much more sensitive for swelling and shrinkage than Ca montmorillonite. Plasticity characteristics in relation to type of clay mineral and exchangeable ion The influence of charge deficiency is discussed here-after. The distortion tends to restrict crystal growth and thus limits the size of the crystals. Isomorphous substitution takes place in the following order. Li+ Na+ H + K + NH+ Mg2+ Ca2+ Al3+ The order means that Al replaces Ca, Ca replaces Mg, etc. Li is the easiest exchangeable ion. The effect of the exchangeable ion on the properties of the clay is quite significant as can be seen from table 6. Mineral Exchangeable Liquid Plastic Plasticity Ion Limit [%] Limit [%] Index [%] Montmorillonite Na 710 54 656 Ca 510 81 429 Fe 290 75 215 Illite Na 120 53 67 Ca 100 45 55 Fe 110 49 61 Kaolinite Na 53 32 21 Ca 38 27 11 Fe 59 37 22 Table 6 Plasticity values in relation to clay mineral and exchangeable ion [4]
IsomorphoussubstitutionIt has already been mentioned that isomorphous substitution of cations may take place inclayminerals.Thismeansthat somecationsarereplacedbyothercations ofapproximatelythesamesize.Theconsequencesofisomorphoussubstitutionaretwofold:anet unitchargedeficiencymightoccurbecausethereplacingcationmightnothavethesamevalueasthereplaced cation,a slight distortion of the crystal lattice occurs sincethe ions are not precisely ofidentical size.5.3Theelectrical chargeonasoil particleand theinteractionwith waterThe previous sections have shown that soils with a high PI and a high LL are problem soils.Furthermore it isdiscussed howclayparticlesareformedand howtherebehaviourinrelationto moisture, indicated bytheplasticity values,depends on the cationsthatare attached totheclayparticle.Foragood understanding it is therefore necessarytodiscuss the effects oftheelectrical charge of a soil particle onitsbehaviour in greaterdetail.Thisdiscussion ispresented here-after. The text is taken from chapter 5 of reference [4].Every soil particle carries an electrical charge.This net electrical charge may arise from anyone or a combination of thefollowing factors:isomorphoussubstitutionsurface disassociation of hydroxyl ions,absenceof cations inthecrystal lattice,adsorptionofanions,presence of organic matter.Isomorphous substitution isthe most important of the causes.Althoughitis theoreticallypossiblethatthenetcharge ispositive,experimentshaveshownthatthenet chargeof clayparticles is negative. In addition to a net overall charge, a soil particle can carry a distributioncharge because the negative charge is located ina particular area which might also bethecaseofthepositivecharge.Sincethemagnitudeoftheelectricalchargeisdirectlyrelatedtothe particle surface area, the influence of this charge on the behaviour of the particle will bedirectly related to the surface areaper mass of the particles.The magnitude of the surfaceareapermass,thespecificsurface(sometimesthespecificsurfaceisdefinedassurfaceareapervolume)is thereforea good indication of therelativeinfluenceof electricalforces on thebehaviourof theparticle.In the remaining of this section the specific surface to volume ratio is used which is thesurfacearea of aparticledivided by itsarea.Fora cube whereL is thelength of theribs, thesurface area per volume is:6 x L2 / L3 = 6 / LFor a sphere,with diameterD, the surfacearea is6/DTable7showswhathappenswiththesurfaceareapervolumeifacubeof1minvolumeisfilledwithcubesofdifferentsizes.Length of cube sideNr. of cubesTotal surface areaSurfacearea/volume[m][m"][1 / m]611610-310960006000101810-660000006000000Table7surfacearea/volumeinrelationtothesizeof individual particles.Table7 clearly shows that the surface area per volume goes up if the particle size goesdown.24
24 Isomorphous substitution It has already been mentioned that isomorphous substitution of cations may take place in clay minerals. This means that some cations are replaced by other cations of approximately the same size. The consequences of isomorphous substitution are twofold: - a net unit charge deficiency might occur because the replacing cation might not have the same value as the replaced cation, - a slight distortion of the crystal lattice occurs since the ions are not precisely of identical size. 5.3 The electrical charge on a soil particle and the interaction with water The previous sections have shown that soils with a high PI and a high LL are problem soils. Furthermore it is discussed how clay particles are formed and how there behaviour in relation to moisture, indicated by the plasticity values, depends on the cations that are attached to the clay particle. For a good understanding it is therefore necessary to discuss the effects of the electrical charge of a soil particle on its behaviour in greater detail. This discussion is presented here-after. The text is taken from chapter 5 of reference [4]. Every soil particle carries an electrical charge. This net electrical charge may arise from any one or a combination of the following factors: - isomorphous substitution, - surface disassociation of hydroxyl ions, - absence of cations in the crystal lattice, - adsorption of anions, - presence of organic matter. Isomorphous substitution is the most important of the causes. Although it is theoretically possible that the net charge is positive, experiments have shown that the net charge of clay particles is negative. In addition to a net overall charge, a soil particle can carry a distribution charge because the negative charge is located in a particular area which might also be the case of the positive charge. Since the magnitude of the electrical charge is directly related to the particle surface area, the influence of this charge on the behaviour of the particle will be directly related to the surface area per mass of the particles. The magnitude of the surface area per mass, the specific surface (sometimes the specific surface is defined as surface area per volume) is therefore a good indication of the relative influence of electrical forces on the behaviour of the particle. In the remaining of this section the specific surface to volume ratio is used which is the surface area of a particle divided by its area. For a cube where L is the length of the ribs, the surface area per volume is: 6 x L2 / L3 = 6 / L For a sphere, with diameter D, the surface area is 6/D. Table 7 shows what happens with the surface area per volume if a cube of 1 m3 in volume is filled with cubes of different sizes. Length of cube side Nr. of cubes Total surface area Surface area / volume [m] [m2 ] [1 / m] 1 1 6 6 10-3 109 6000 6000 10-6 1018 6000000 6000000 Table 7 surface area / volume in relation to the size of individual particles. Table 7 clearly shows that the surface area per volume goes up if the particle size goes down
A soil particle in nature attracts ions to neutralize its net charge.Since these attracted ionsareusuallyweaklyheld ontheparticlesurfaceandcan bereadilyreplacedbyotherions,theyaretermed exchangeable ions.Thesoil particlewith its exchangeableion is neutral.Letusknowconsideramontmorilloniteandakaoliniteparticle.Figure16givesthesizeofamontmorilloniteparticlerelativetoakaoliniteparticle.(a)(b)Figure 16 (a) Montmorillonite particle 1000 A by 10 A thick. (b) Kaolinite particle 10000 A by1000 A thick [4].The two particles contain about 14000 exchangeable monovalent ions on the montmorilloniteand4oooooomonovalentionsperkaoliniteparticles(thesenumberscanbecalculatedbutthis is considered to be less relevantforthis course and this will therefore not be shownhere). Let us assume that the exchangeable ion is sodium (Na*). If both clay particles arenowdropped into water,both themineral surfaces andthe exchangeable ions pick up water,i.e.they hydrate. Upon hydration, the sodium ion grows about sevenfold as is illustrated infigure 17.-12As(a)(b)0+H20HydratedNat,R=7.8ALUnhydratedNa,R=0.98A(c)Figure17Soil surfacewith exchangeable ions.(a)Surfaceof drykaoliniteand sodiumions.(b)Surfaceof drymontmorillonitewith sodium ions.(c)Hydration of sodium ions [4].As the scaled drawings offigure17 indicate,the hydrated sodium ions aretoolargetofit intoamonoioniclayeronthemineralparticles.Actuallytheexchangeableionswiththeirshellsofwatermoveawayfromthemineralsurfacestopositionsofequilibrium.Theionsareattracted tothemineral surfaceto satisfythenegativecharge existingwithinthe surface;they also desire to move away from each otherbecause of their thermal energy;the actualpositionstheyoccupyarecompromisesbetweenthesetwotypesofforces.Thus,whenindividualparticlesaredropped intowatertheionsmoveawayfromthesurfacetoformwhatistermed thedoublelayer.25
25 A soil particle in nature attracts ions to neutralize its net charge. Since these attracted ions are usually weakly held on the particle surface and can be readily replaced by other ions, they are termed exchangeable ions. The soil particle with its exchangeable ion is neutral. Let us know consider a montmorillonite and a kaolinite particle. Figure 16 gives the size of a montmorillonite particle relative to a kaolinite particle. Figure 16 (a) Montmorillonite particle 1000 Å by 10 Å thick. (b) Kaolinite particle 10000 Å by 1000 Å thick [4]. The two particles contain about 14000 exchangeable monovalent ions on the montmorillonite and 4000000 monovalent ions per kaolinite particles (these numbers can be calculated but this is considered to be less relevant for this course and this will therefore not be shown here). Let us assume that the exchangeable ion is sodium (Na+ ). If both clay particles are now dropped into water, both the mineral surfaces and the exchangeable ions pick up water, i.e. they hydrate. Upon hydration, the sodium ion grows about sevenfold as is illustrated in figure 17. Figure 17 Soil surface with exchangeable ions. (a) Surface of dry kaolinite and sodium ions.(b) Surface of dry montmorillonite with sodium ions. (c) Hydration of sodium ions [4]. As the scaled drawings of figure 17 indicate, the hydrated sodium ions are too large to fit into a monoionic layer on the mineral particles. Actually the exchangeable ions with their shells of water move away from the mineral surfaces to positions of equilibrium. The ions are attracted to the mineral surface to satisfy the negative charge existing within the surface; they also desire to move away from each other because of their thermal energy; the actual positions they occupy are compromises between these two types of forces. Thus, when individual particles are dropped into water the ions move away from the surface to form what is termed the double layer
Figure18showsthesameparticlesasinfigure16butnowwiththeirfullydevelopeddoublelayer.Figure19isathreedimensionalpictureof figure17aftertheparticlesaredropped inwater.(a(b)Figure 18 Soil particles with water and ions. (a) Sodium montmorillonite. (b) Sodium kaolinite[4].000-0(a)R(6)Figure19Particlesurfaceswithwaterandions.(a)Sodiumkaolinite.(b)Sodiummontmorillonite [4].Figure2oshowstheconcentrationof theionsversusthedistancefromtheparticlesurface.Ata distanceofabout 4ooA,which isthethicknessof thedoublelayer,theconcentrationofsodium ions has become equal to that in the"pores" or"free"water. Also the electricalpotential versus distance from thethe surface is shown.Electrical potential is the workrequired to movea unit charge from infinity to thepoint in question.The double layer is thusthedistancefromthesurfacerequiredtoneutralizethenetchargeontheparticle.26
26 Figure 18 shows the same particles as in figure 16 but now with their fully developed double layer. Figure 19 is a three dimensional picture of figure 17 after the particles are dropped in water. Figure 18 Soil particles with water and ions. (a) Sodium montmorillonite. (b) Sodium kaolinite [4]. Figure 19 Particle surfaces with water and ions. (a) Sodium kaolinite. (b) Sodium montmorillonite [4]. Figure 20 shows the concentration of the ions versus the distance from the particle surface. At a distance of about 400 Å, which is the thickness of the double layer, the concentration of sodium ions has become equal to that in the “pores” or “free” water. Also the electrical potential versus distance from the the surface is shown. Electrical potential is the work required to move a unit charge from infinity to the point in question. The double layer is thus the distance from the surface required to neutralize the net charge on the particle