Colloid stability. Lyophobic sols. Stabilization of colloids. Levente Novák István Bányai Zoltán Nagy Department of Physical Chemistry

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1 Colloid stability. Lyophobic sols. Stabilization of colloids. Levente Novák István Bányai Zoltán Nagy Department of Physical Chemistry

2 Lyophilic and lyophobic sols Sols (lyosols) are dispersed colloidal size particles in a liquid medium (=solid/liquid dispersions) These sols can be Lyophilic : strong interactions exist between the particles and the solvent (the particles are weted) Thermodynamically stable Lyophobic: weak interactions between the particles and the solvent (partially weted or unweted particles) Thermodynamically always unstable ( aggregation) Can be kinetically stable or unstable

3 Solutions versus dispersions = Mixing is spontaneous. Mixing is reversible. The mixture is thermodynamically stable. Inhomogeneities are on molecular scale. Properties of the real solutions are independent on the way these solutions are prepared. Mixing is non-spontaneous (requires mechanical energy). Mixing is irreversible. Thermodynamically unstable (requires a stabilizing agent) and unmix spontaneously. Inhomogeneities on colloidal scale. Properties of the colloidal dispersions are strongly dependent on the way they are prepared for repeatability, empirical preparation procedures are followed

4 Thermodynamical stability Solutions are thermodynamically stable Gibbs energy of the components before mixing is higher than afer mixing (ΔG<0) Dispersions are thermodynamically unstable The Gibbs energy increases by mixing (ΔG>0) But if unmixing is slow enough kinetically stable dispersions

5 Kinetic stability In an unstable system the particles may adhere to one another and form aggregates of increasing size that may setle or cream out under the influence of gravity. An initially formed aggregate is called a floc or flocculate and the process is flocculation. The floc may or may not separate out. If the aggregate changes to a much denser form, it is said to undergo coagulation. An aggregate usually separates out either by sedimentation (if it is more dense than the medium) or by creaming (if it less dense than the medium). Usually coagulation is irreversible whereas flocculation can be reversed by the process of deflocculation.

6 Coagulation or flocculation Dense mass, irreversible aggregation cake or coagulate a) coagulated, b) flocculated particles Loosely adhering mass, it can be reversed by deflocculation floc or flocculate a)the suspended particles setle out and form a firm, dense mass (cake). The coagulate can not be redispersed by gentle agitation. b)the suspended particles form light, flufy agglomerates held together by strong van der Waals forces. The flocculated particles setle rapidly forming a loosely adhering mass with a large sediment height. Gentle agitation will easily resuspend the particles. Weak flocculation requires strong adhesion and a zeta potential of almost zero.

7 Strength of interparticle forces Encounters between particles occur as a result of Brownian motion and stability of a suspension is determined by the interaction between particles during these encounters. Stability depends on the balance of atractive and repulsive interactions. There is no repulsion Atraction comes from van der Waals forces between particles. Repulsion is a consequence of interaction between similarly charged electric double layers and/or particle-solvent afinity. Repulsion prevents particles to get close enough and atach. There is repulsion

8 Stability of lyophobic sols (summary) Lyophobic sols are thermodynamically unstable. However there are stabilizing factors (e.g. repulsion) kinetical stability can be atained. Whether aggregation does or does not occur depend of the balance of atractive and repulsive forces. F T =F A + F R For obtention of a stable lyosol, repulsive forces must dominate

9 Electrostatic and steric stabilization VR VS

10 Molecular origin of the van der Waals atraction There is atraction between atoms or molecules even in vacuum. l const V A (l )= 6 l VA: atraction potential (J). Dispersion atraction between atoms or molecules is additive so it also acts in case of macroscopic bodies too. The atraction depends on the geometry of the particles (composed of atoms) x r x A V A (x )= area 2 12 π x A is the Hamaker constant or atraction parameter (unit: J). V A (x )= Ar 6x

11 The Hamaker model Molecules in particle 1 2 A=π C ρ1 ρ2 C ρ1 ρ2 : : : Molecules in particle 2 x interaction energy constant (J m-6 for van der Waals interactions) number density of the surface 1 (m-3) number density of the surface 2 (m-3) The atraction of bodies arises from London (dispersion) atraction of molecules (all molecules act independently). The efect is additive: one molecule of the first colloid has a van der Waals atraction to each molecule in the second colloid, the total force is the sum of all forces. An atractive energy curve is used to indicate the variation in van der Waals force with distance (x) between the particles.

12 Efective Hamaker constant 1 x x 1 1 x x 1 2 V A (x )= Ar 6x The Hamaker constant (A) in vacuum depends on material properties: density, polarizability The efective Hamaker constant Aef also depends on the dispersion medium An atractive energy or atractive potential curve is used to indicate the variation in van der Waals force with distance between the particles. 2 Aef =( A12 A22 ) Aef r V A (x )= 6x Order of magnitude: Aef J VA(x) x

13 Repulsion: particles of the same charge κ(x x St ) Ψ =Ψ St e Most of the time the shear plane is close enough to the Stern plane, so we can consider ζ ΨSt St: outer Helmholtz plane = Stern plane

14 Repulsion between overlapping double layers V x: distance between the surfaces R The loosely held countercharges form electric double layers. The electrostatic repulsion results from the interpenetration of the difuse part of the double layer around each charged particle. 2 V R (x )= Ψ 0 e κ x An electrostatic repulsion curve is used to indicate the energy that must be overcome if the particles are to be forced together.

15 The Balance of Repulsion & Atraction (DLVOa theory) Notice the secondary minimum. The system flocculates, but the aggregates are weak this may imply reversible flocculation. x VT = VA + VR Ar V A (x )= 6x 2 2 V R (x )=r (kt ) γ z The point of maximum repulsive energy is called the energy barrier. Energy is required to overcome this repulsion. The height of the barrier indicates how stable the system is. The electrostatic stabilization is highly sensitive with respect to surface charge (ζ ~ Ψ ~ ph) and salt concentration (κ, z). γ= e e ze Ψ St 2 kt ze Ψ St 2 kt 2 e 1 +1 κ x

16 VT,VA, VR (J) the total, atractive and repulsive energy of two spherical particles at distance d (m). VT = VA + VR The height of the energy barrier depends upon ζ and 1/κ. sol gel Precipitate, or cake Primary minimum, irreversible coagulation In the secondary minimum there is a reversible flocculation or sol-gel transformation. Secondary minimum, weak flocculation (large sediment sedimentheight heightororgel) gel) VOan der Waals atraction will predominate at small and at large interparticle distances. At intermediate distances double layer repulsion may predominate, depending on the actual values of the forces. In order to agglomerate, two particles on a collision course must have suficient kinetic energy due to their velocity and mass, to jump over this barrier.

17 Electrostatic stability of dispersions Ionic strength: I1 < I2 < I3 < I4 < I5 Inverse Debye length: κ1 < κ2 < κ3 < κ4 < κ5 An increase in electrolyte concentration leads to a compression of the double layer (κ increase) the energy barrier to coagulation decreases or disappears. If the barrier is cleared, then the net interaction is all atractive the particles coagulate. This inner region is afer referred to as an energy trap since the colloids can be considered to be trapped together by van der Waals forces.

18 Critical coagulation concentration What concentration of salt (c =c.c.c.) eliminates the repulsive barrier completely? If the potential energy maximum is large compared to the thermal energy, kbt of the particles, the system should be stable; otherwise, the system should coagulate. Counterion lency c.c.c. mol/l) ~ z -6 va(in The c.c.c. is the concentration of salt that just eliminates the repulsive barrier.

19 Schulze Hardy Rule The Schulze Hardy rule states: the critical coagulation concentration (c.c.c.) inversely depends on the sixth power of the charge on the ions. c.c.c. (in mol/dm3) ~ z -6 cmonovalent : cdivalent : ctrivalent 1 : 2-6 : 3-6 = 1 : :

20 Rates of coagulation Rates of coagulation can be measured by the change in the number of particles, Smoluchowski equation: dn 2 2 =8 π D α N =k d N dt If there is an energy barrier, Vmax to coagulate then a fraction (α) of collisions is unsuccessful, so the rate of coagulation ks is slower. α e V max kbt kd is the rate of the difusion limited aggregation or rapid coagulation (no barrier, Vmax=0) The stability ratio: kd W ks The stability of dispersion is increased by: increase in particle radius, increase in electrokinetic potential ( ζ > 25 mvo), decrease in the Hamaker constant, decrease in the ionic strength, decrease in temperature. t : time (s), N: number of single particles per volume (dm-3), D : difusion coeficient (m2 s-1), ks: relative number of successful collisions (s-1), kd: number of total collisions (s-1), kb: Boltzman constant (J K-1), T: temperature (K), Vmax: maximal rate of aggregation (mol s-1)

21 Elementary steps of coagulation: N/N0 initial step N 1 = N 0 1+k N 0 t /2 N decreases with time, while the size of the resulting particles increases: V N =constant =V 0 N 0 V 1 N dn =k N =k t dt N N0 The decrease in the normalized number of total particles, singlets, doublets, and triplets according to the Smoluchowski theory as a function of time. If all flocculation rate constants are the same: Rate can be measured through the decrease of the total number (-dn/dt) or the increase of the average volume (dv/dt) for example by turbidity as a function of time: Turbidity ~ V 2 N = V (V N ) ~ V constant N 1 = N 0 1+k N 0 t /2 htp://apricot.polyu.edu.hk/~lam/dla/

22 Steric stabilization VR VS

23 Lyophilic macromolecules as stabilizers V S =V M +V VR VM Loose layers Protective action of adsorbed macromolecules (natural or synthetic) Entropic repulsion VVOR Polymer thickness Work is required to push the particles closer together than their polymer layers keep them apart. Dense layers Two efects

24 Steric stabilization VT = VS+VA Steric + atractive interaction Long tail Short tail ane factor of steric stabilization is the tail size Steric repulsion

25 Steric stabilization Steric stabilization by surface bound polymers: is not sensitive to surface charge and salt concentration works also in non-aqueous media works also in concentrated dispersions Disadvantage: more dificult to prepare. VT = VA + VS VR=0

26 How to avoid coagulation The stabilizing polymer must be in a good solvent environment V S =V M +V VR V T =V A +V S Efect of the temperature: Segments in the tail can move freely or not, the interaction between segments themselves is stronger or smaller than than the interaction between the segments and the solvent.

27 Configuration of adsorbed polymers Chemical adsorption (chemisorption) Sterically stabilized dispersions are stable when the stabilizing polymer is soluble in the solvent or at least it has one such part. The worse the solvent, the more unstable the colloidal dispersion. Cross-over from stabilization to flocculation: theta solvent at theta temperature (theta solvent: interactions between polymer segments are of the same strength than between a segment and the solvent).

28 Combined steric and electrostatic stabilization It can be achieved by polyelectrolytes (like proteins) or by the combination of charged surface and neutral polymers van der Waals Electrostatic atraction repulsion van der Waals Electrostatic repulsion atraction Steric repulsion VT = VA + VR VT = VA + VR + VS Plane of shear is pushed out farther Steric stabilization makes the potential minimum disappear (no net atraction)

29 Bridging flocculation Conditions: good adsorbent good solvent (very) low polymer density (very) long chain polymers Long polymers bind the colloids together in open flocs. Application: water purification (a few ppm of cationic polyelectrolyte is added to the dispersion flocculation, since most natural colloid surfaces are negative).

30 Stability of lyophilic colloids Lyophilic colloids Isostable: no precipitation at IEP Isolabile: precipitation at IEP

31 Stability of lyophilic colloids Stability of lyophilic sols comes from solvation and charge. If solvation interaction alone is strong enough the colloids stay stable at their isoelectric ph. If it is not, colloids coagulate at their isoelectric ph. Repulsion The isoelectric point of disappears casein is 4.6. at ph=4.6 Gelatin is stable at its isoelectric point so it is an isostable colloid, but it can be precipitated with much more salt or a dehydrating agent (acetone, alcohol). Casein is unstable at its isoelectric ph where it is uncharged, this is an isolabile protein. Casein precipitates at its IEP where there is no repulsion. The fermentation of milk sugar (lactose) produces lactic acid, which acts on milk protein casein coagulation and denaturation yoghurt (gel-like texture)