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Chapter 3 Solution of Surfactants

Chapter 3 Solution of Surfactants. Micelle Formation & Applications. 2006.3.28. § 1 Solution Properties of Surfactants and CMC. Changes in some physical properties of an aqueous solution of surfactants in the neighborhood of CMC such as sodium dodecyl sulfate. 2. Micelle Formation and CMC

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Chapter 3 Solution of Surfactants

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  1. Chapter 3 Solutionof Surfactants Micelle Formation & Applications 2006.3.28.

  2. §1 Solution Properties of Surfactants and CMC • Changes in some physical properties of an aqueous solution of surfactants in the neighborhood of CMC such as sodium dodecyl sulfate

  3. 2. Micelle Formation and CMC • Definition – the concentration at which this phenomenon occurs is called the critical micelle concentration (CMC) • Formation mechanism Iceberg structure Sor G=H-TS Surface activity Adsorption at interface or surface Formation micelle in solution phase

  4. (3) Determination Electrical conductivity Molar (Equivalent) conductivity(当量电导) Surface tension Light scattering Refractive index Colligative property (osmotic pressure) Unit: mole/L or g/L Ionics 10-2 - 10-3 mole/L Nonionics 10-3 - 10-6 mole/L

  5. 3. Thermodynamics of micellization • Mass action model (a) Equilibrium constant K e.g. anionics: n anions, m counter ion micelle charge z = n-m nR- + mM+  Mm+Rn-(micelle) K = F[Mm+Rn-]/[R-]n [M+]m F = fMR/fRnfMm Dilute solution: K = [Mm+Rn-]/[R-]n [M+]m = [Mm+Rn-]/[R-]n [M+]n-z

  6. (b) Stantard Free Energy of micellization Gºm = -(1/n)RTlnK = -(1/n)RTln[Mm+Rn-]/[R-]n [M+]n-z =RTln [R-] [M+](n-z)/n [R-][M+]=CMC Gºm = RTlnCMC2-z/n = (2-z/n)RTlnCMC • If z=0 then n=m no surface charge Gºm= 2RTlnCMC • If z=n then no counter ion Gºm= RTlnCMC

  7. unit:CMC(mole/L) mole fraction G= RTlnXCMC= RTlnCMC/(1000/18) = RTlnCMC/55.5 (c) Nonionics nR  Rn k = F[Rn]/[R]n  [Rn]/[R]n Gºm = -(1/n)RTlnK = RTln[R] = RTlnCMC G= RTlnXCMC= RTlnCMC/(1000/18) = RTlnCMC/55.5

  8. (2) Phase separation model (pseudo phase拟相) m= 2 Pseudo phase: m= m0 (pure phase) Solution: 2 = 20 + RTlna2 Gºm = m- 20 = RTlna2 1-1type a2=a+a- = a±2 Gºm = 2RTlna± = 2RTln(CMC/55.5)

  9. 4. Micellar Structure and Shape • Structure of Micelle • Ionics • inner core - liquid phase hydrocarbon • Shell • diffuse electric double layer (b) Nonionics • inner core - liquid phase hydrocarbon • Shell Ionics Nonionics

  10. (2) Structure of Reversed Micelle(逆胶束) • In Polar Solvent – few & no micelle formation • In Nonpolar Solvent – a few small micelle formation: • Hydrophilic groups: Anionics>cationics>nonionics • Hydrophobic groups: R  reversed micellization

  11. (3) Micelle Shape – dependent of concentration (4) Micellar Aggregation numbers(n) • ( )TP C=CMC ~ 10CMC n is independent of concentration. c , numbers of micelle; • nionics = 50 – 60 ; nnonionics > 400

  12. (c) Factors affecting the n value • Hydrophobic groups: hydrophobicity(R, or SiR or FR), n  • Hydrophilic groups: Nonionics > Ionics (same R) • Electrolyte , Ionic Strength: I=(1/2)CiZi2 , ionics, hydrophilicity  , surface activity , the radius of ionic atmosphere , n

  13. Regulator of water structure(水结构调节剂) • Promoters – fructose，xylose; n • Breakers – urea,lower alcohol; n • Polar organic compound such as solubilization longchain polar organic compound n  • Temperature if T, then • Ionics: water-soluble, n; • Nonionics: water-soluble, n. • Difference between surfactants & solvents, n

  14. 5. Critical Micelle Concentration(CMC) • Mensuration of CMC • Surface tension - ~logc, error ~ 2-3% • Electric conductance (电导法) Molar conductance  = 0 +c1/2 Conductance(电导率)  ~ logc (c) Light scattling(光散射) (d) Dyeing (染色法) (e) Solubilization (增溶法)

  15. (2)Factors affecting CMC • Structure of surfactants • Hydrophobic groups • Fluorocarbons < Silicones < Hydrocarbon < Branched Hydrocarbon • RnlogCMC = A – B n (ionics:n=8-18; nonionics:n12) Parameter A: ionics:A=1.4-1.6; nonionics:A=2-2.4. Parameter B: ionics:B=0.3; nonionics:B=0.5

  16. Branching degree , to loosely packed, CMC  • Position of polar groups: endmiddle, CMC  • Unsaturation & polar groups: hydrophilicity , CMC  • Hydrophilic groups • ionics » nonionics 1-2 orders of magnitude • polarity, CMC RCOO > RSO3 > RSO4

  17. The Counterion in Ionics • the CMC in aq. reflects the degree of binding of the counterion to the micelle. • Increased binding of the counterion, in aq. cases a decrease in the CMC of surfactants. • The extent of binding of the counterion increases with increases in its polarizability and valence(化合价), and decreases with increases in its hydrated radius(水化半径)

  18. e.g. anionic lauryl sulfates: Li+>Na+>K+>Cs+>N(CH3)4+>N(C2H5)4+>Ca2+, Mg2+, in particular RNH3, R  CMC Cationic quaternary ammonium salt F->Cl->Br->I- ,CMC • The degree of binding of the counterion(DBC) to the micelle depends on the surface charge density of the micelle. charge or surface area, DBC, CMC  R(CH3)3Br, R , tighter packing, DBC, CMC

  19. (b) Temperature • Ionics T , CMC  • Nonionics T , CMC (c) Additives • Ionic Strength: I=(1/2)CiZi , hydrophilicity  , surface activity , CMC  • Electrolyte: Anionics ~ Cationics > Zwitterionics > Nonionics • A&C: logCMC = -alogCi + b (Ci-total counterion concentration)

  20. Anionics: PO43->B4O72->OH->CO3-=>HCO3-> SO42->NO3->Cl- Z&N: logCMC = -KCs+ constant Cs- concentration of electrolyte (mole/L) • Regulator of water structure(水结构调节剂) • Promoters – fructose，xylose； CMC  • Breakers – urea,lower alcohol; CMC • Polar organic compounds • Short Chain: lower affect • Long Chain: synergism (协同作用）

  21. (3) Factors affecting the CMC/C20 • Ionics (C10-C16) R, CMC/C20 • Branching degree  , CMC/C20 • Polar hydrophilic head  , CMC/C20 • Ionic strength , CMC/C20 • T(10-40ºC) , CMC/C20 • Fluorocarbons > Silicones >Hydrocarbon • Saturated aliphatic hydrocarbon-water interface CMC/C20 then G-Water (h) unsaturated aliphatic hydrocarbon or short chain aromatic hydrocarbon -water interface CMC/C20 then G-Water (i) PEO nonionics > zwitterionics > ionics

  22. §2. Surfaceactivity in Mixtures of Surfactants • Two important works in researches and exploitation of surfactants • Relation between structure of surfactants and properties • Molecular design – synthesization of new surfactants • Application – exploitation according to structure of surfactants (2) Synergism in mixtures of surfactants • Composite

  23. 2. Adsorption of surfactant mixtures (N surfactants) • Surface excess concentration iof i component mixtures: -(d/RT) =  i dlnCi if other j components Cj = constant then dCj = dlnCj = 0 -(d/RT) = i (dlnCi)T,P,nj i = -(1/RT)[d/dlnCi]T,P,nj • i is measurable. [d/dlnCi]T,P,nj mean that the rate of change between surface tension of solution and dlnCi when the concentration of other j components are constant. Ionics: i = -(1/xRT)[d/dlnCi]T,P,nj

  24. (2) Adsorption isotherm of surfactant mixtures e.q. two surfactants: component 1 & 2 Rate of adsorption:dna1/dt = ka1( - 1 - 2 )C1 Rate of desorption :dnd1/dt = kd11 Adsorption equilibrium Rate of adsorption = Rate of desorption ka1( - 1 - 2 )C1 = kd11 If k1= ka1/kd1，then 1 (1+k1C1) = [k1C1( - 2 )] ① Same 2 (1+k2C2) = [k2C2( - 1 )] ②

  25. ①+② : 1(1+k1C1) +2(1+k2C2) = [k1C1( - 2 )] + [k2C2( - 1 )] =  (k1C1 + k2C2) - 2k1C1+1k2C2 (1 +2)(1+k1C1+k2C2) =  (k1C1 + k2C2)  = (1 +2) =  (k1C1 + k2C2)/(1+k1C1+k2C2) =  kiCi/(1+  kiCi) i =  kiCi/(1+  kiCi) If Xi the molar fraction of I component, Ci and C the concentration i and total component Then Ci = XiC, i =  CkiXi/(1+ CkiXi) i =  CkiXi/(1+ C kiXi)

  26. If K =  kiXi, then i =  CkiXi/(1+ C kiXi) =  CkiXi/(1+ CK)  = i =  CkiXi/(1+ CK) =  CK/(1+ CK) 3. Micelle formation of surfactant mixtures (N surfactants) • Ideal mixed micelle formation of surfactant mixtures – surfactant homologous mixtures • The surfactant solution is ideal • The mixed micelle is ideal solution • The interaction of i component in pure or mixed micelle

  27. Phase separation model (pseudo phase拟相) chemical potential of i component (micelle) mi= i (solution) Pseudo phase: mi= mi0 + RTlnXmi Solution: i = i0 + RTlnCTXi RTlnCTXi = (mi- i0)+RTlnXmi lnCTXi = A’-K0ln(CT+CS)+RTlnXmi ①K0 – Constant about electric work Cs – Concentration of salt In pure i component micelle solution lnCi = A’-K0ln(Ci+CS) ②

  28. ② - ①ln(CiXmi)/(CTXi) = ln[(CT+CS)/(Ci+CS)]K0 Xmi/Xi = [CT(CT+CS)/Ci(Ci+CS)]K0 Xmi = Xi [CT(CT+CS)/Ci(Ci+CS)]K0 ③∵Xmi= 1, from③ [CT(CT+Cs)k0/Ci(Ci+Cs)k0] Xi = 1  Xi /Ci(Ci+Cs)k0 = 1/CT(CT+Cs)k0 ④ ∵Xi= 1, from ③  XmiCi(Ci+Cs)k0 = CT(CT+Cs)k0 ⑤ CT = CMC – CMC of mixed micelle Ci = CMCiº - CMC of i pure component

  29. ④1/CMC(CMC+Cs)k0= Xi /CMCºi(CMCºi+Cs)k0 ⑤CMC(CMC+Cs)k0 =  XmiCMCºi(CMCºi+Cs)k0 1/CMC(CMC+Cs)k0= Xi /CMCºi(CMCºi+Cs)k0 CMC(CMC+Cs)k0 =  XmiCMCºi(CMCºi+Cs)k0 Discussion: • Nonionics , no electric interaction, K0=0 1/CMC= Xi /CMCºi ,, 1/CMC = X1 /CMCº1 + X2 /CMCº2 CMC=  XmiCMCºi CMC= Xm1CMCº1 +Xm2CMCº2

  30. Ionics K0≠0, if CS= 0 1/CMC1+k0= Xi /CMCºI 1+k0 CMC1+k0 =  XmiCMCºi1+k0 If CS=  , CS» CT & CMC & CMCºI 1/CMC(CMC+Cs)k0= Xi /CMCºi(CMCºi+Cs)k0 1/CMC (Cs )k0= Xi /CMCºi(Cs)k0 1/CMC = Xi /CMCºi CMC=  XmiCMCºI Same with nonionics

  31. (2) non ideal mixed micelle formation e.g. nonionics 1/CT=Xi/fiCi CT=XmifiCi fi – the activity coefficient of i component in solution 3. Synergism(协同作用) in mixtures of surfactants • Molecular Interaction Parameters (MIP) < 0  - Molecular Interaction Parameters for mixed monolayer formations  - Molecular Interaction Parameters for mixed micelle formations

  32. (2) Effect of chemcal structure and molecular environment on MIP • Effect of various types on || anionic-cationic > anionic-zwitterionic > ion (anionic, cationic)-POE nonionic > betaine – cationic > betaine - POE nonionic > POE nonionic – POE nonionic (b) R, || : || appears to be maximum when the lengths R of the two surfactants; |M| becomes more positive with increase in the total number of carbon atoms of two surfactants.

  33. (c) Electrolyte || (d) Temperature , || (10 - 40°C) Values of Molecular Interaction Parameters

  34. Values of Molecular Interaction Parameters

  35. Values of Molecular Interaction Parameters

  36. Values of Molecular Interaction Parameters

  37. Values of Molecular Interaction Parameters

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