Presentation on theme: "A guide to Optimal Surfactants Professor Steven Abbott"— Presentation transcript:
1 A guide to Optimal Surfactants Professor Steven Abbott This is my non-expert personal view of the fieldIt has been used as the basis for a number of tutorial talksIt is provided “as is” to stimulate discussion/debatePlease send feedback toThe website for the HLD-NAC software package isHLD-NACA guide to Optimal SurfactantsProfessor Steven AbbottMy thanks to, in alphabetical order:Professors Acosta, Aubry, Salager and Sabatinifor their generous assistance in teaching me about HLD,and to Professor Acosta and his group for their continuing support and guidance in developing the software and database.Images from their papers are used with grateful acknowledgement
2 What I’m not talking about My interest is in microemulsions for cosmetics, oil-field recovery, nanoparticles etc.I have no particular interest in or knowledge of foamsI have no particular interest/knowledge in the dynamics of classical micelles and their CMC behaviourI can’t talk about polymeric surfactants because they, too, are outside my knowledge baseThis talk is pseudo-ternary-phase-diagram free
3 Just an ordinary scientist I’m a formulator, not a surfactant scientistI’ve read lots of surfactant theoriesNone of them provided a practical formulation toolI want a rational choice of surfactantsand I suspect that surfactant suppliers would love to reduce their inventories …I want to formulate efficiently, not trial-and-errorSurfactant formulation space is way too largeI want to rationally swap to “green” surfactants
4 Microemulsion terminology Winsor Type I, o/w, lower (denser) phaseWinsor Type II, w/o, upper (lighter) phaseWinsor Type III, bicontinuous middle phaseType IType IIIType IIIn reality are bicontinuousGenerally a uniform size
5 I want to get this first time EACN scan from Intelligent Formulation Green Surfactant project. Low EACN to left, high EACN to rightWater phase largerType I o/wOil phase largerType II w/oOptimal Type III
6 I want: To be in the right phase, first time Sometimes I want Type I, sometimes Type II, sometimes Type IIITo have a high solubility parameter (SP)ml water-in-oil/g surfactantml oil-in-water/g surfactantBecause surfactants are “bad” and “expensive”(To be free from liquid crystal phases)I’ve no solutions to this issue but high SP means low surfactant concentrations which will help
7 I want microemulsions because I can do clever cosmetic formulations with themBut note that most published microemulsion cosmetic/pharma papers use them only because “nano is good”These papers generally prove that if you shove a lot of surfactant onto skin you can get things to go through. Nothing to do with microemulsions
8 Not HLBHLB really aren’t much use because they really only apply well to medium-sized, standard EO non-ionicswhy should the % hydrophilic sorbitan be in any way equivalent to % hydrophilic EO?With a choice of 2,000 surfactants, maybe 200 have your HLB, yet their properties are very differentAnd they don’t predict much beyond basic o/w or w/o, and often not very wellDon’t predict effects of salinity, oil, temperature
9 Not CMC Critical Micelle Concentration Tells you a lot about its desire to escape from water and form micellesBut not a lot about making great w/o or o/w emulsionsGenerally lower is better because its not too water soluble, and probably has long tailsthe long tail is good for other reasonsCMC for extended surfactants are “meaningless”
11 Not CPPCPP=Vtail/(Ltail*Ahead)Critical Packing Parameter seemed to be a good idea for distinguishing between o/w and w/o surfactants based on whether head or tail is bulkierTheoretically bankrupt because microemulsion curvature is flat on the molecular scale …… and one can go seamlessly from o/w to w/o with surfactants of very different CPP valuesBut they’re great for those interested in complex surfactant phases (hexagonal, laminar…)And such phase behaviour in practice limits microemulsion practicality
12 CPP for phases in conc. surfactant Any complete surfactant story would include HLD-NAC and the tendency to form liquid crystalline phases ...
13 But … Knowing the key parameters used in CPP is very important: Head area, A(Extended) Tail length, LAnd there may be some routes from CPP to Cc (see later)But so far that’s speculative
14 HLD Hydrophilic Lipophilic Difference Apparently simple, but a lot of work to get it right from (in alphabetical order) Aubry, Sabatini and SalagerYou get the Optimal Surfactant when HLD=0lowest interfacial energyhighest SP (Solubility Parameter – not HSP)ml/g i.e. ml of oil dissolved in water per g of surfactantbest cleaning, oil recovery and microemulsions
15 Same(ish) equation for i and non-i EACNHexane = 6Decane = 10etc.ButBenzene = 0Limonene = 7IPM = 13Triolein = 16i.e. they behave like an alkane with those numbers of CIonicsHLD = ln(S)-K.EACN-αΔT+CcNon-ionicsHLD=b.S-K.EACN+cΔT+CcS = Salt concentration, g/100mlK= Constant ~0.17EACN = Effective Alkane Carbon Numberα=temperature coefficient ~0.01. Note the - signΔT=temperature difference from 25°Cb = Salt dependency for non-ionics ~ 0.13c=temperature coefficient ~0.06. Bigger effect, opposite signCc=Characteristic Curvature, unique to each surfactant
16 What does it mean? Ionics 0 HLD when ln(S)+ Cc = K.EACN + αΔTas S goes up, Water solubility goes downas Cc goes up, Water solubility goes downas EACN goes up, Oil solubility goes downas T goes up, Oil solubility goes downbecause ionic is more soluble in waterThe best surfactant for a hot cycle in a washing machine is often bad for a cold cycle – and vice versa
17 What does it mean? Non-Ionics 0 HLD when b.S+cΔT+Cc = K.EACNAs S goes up, Water solubility goes downAs T goes up, Water solubility goes downEO is a strange beast so water solubility decreases with TAs Cc goes up, Water solubility goes downAs EACN goes up, oil solubility goes down
18 Why does it matter? - 1If you’ve got an easy oil (e.g. heptane) and just want a modest amount of o/w or w/o microemulsion (e.g. for nanoparticle manufacture), then trial and error will get reasonable results without any theoryBut …
19 Why does it matter? -2 Difficult oils have large EACNs No problem – balance with Salt or a large +ve CcBut high Salt is often not practical + small Type IIIAnd large +ve Cc usually means v. long tail which generally means nasty liquid crystal phases and sticky messesAOT is an exception because its Cc=2.5 (SDS=-2.3) so can formulate with EACNs 25 higher than SDSThose two tails = lots of C but not too self-associativeThe branches help bulk out and reduce self-association
20 Why does it matter? -3To get large amounts of o/w, w/o or to get a whole, stable Type III 50:50 emulsion is very hard without theoryEspecially if you want a high SP – best emulsion with least surfactantFor cosmetics, surfactants are often “bad”, so less surfactant is desirable.
21 The parameterscΔTNo theoretical method to calculate c as EO solubilities are hard to understandBut c seems to be a good solid numberαΔTNo theoretical method to calculate αNo fundamental reason why different ionic headgroups should have the same αinsufficent data to know either way
22 The parameters (cont) EACN By definition OK for alkanes! But where do EACNs for other oils come from?My view is that HSP can predict these“Like dissolves like” so compatibility between surfactant and oil will decrease EACNBut so far only found one good dataset (Aubry) to be able to test
23 The parameters (cont) K Known to be smaller (~0.1) for Extended Surfactants than for “typical” surfactants (~0.17)Why is K the value it is?Can it be calculated?Is it truly a constant?Till there’s more data, assume that it is a constant
24 The parameters (cont) Cc Characteristic curvature A –ve value means that for “normal” conditions the emulsion is curved around the oil, so it’s o/w Type IA +ve value means curved around the water, so it’s w/o, Type IISounds a bit like CPP, but is simply an observed parameter, not something directly calculated (yet?) from shape/sizeSodium Di-octyl sulfosuccinate is +2.5Sodium Di-hexyl sulfosuccinate is -0.9why do 2 Me groups (+ branching) give such a big change?
25 Scans, not phase diagrams *Hope for SP of 10So 3% surfactant will dissolve 30g oil20:60:20 resultA proper phase diagram variesOil, water, surfactant, brine, temperatureNeed a 5D plot to visualise itLife’s too shortSo take 10 test tubes, 50:50 o/w, modest amount of surfactant*And look for 2 (o+o/w) to 3(o+o~w+w) to 2 (w/o+w) phases as salt goes lowhigh
27 More scansNote the relatively large oil phase to the right of S*, showing a relatively large amount of water brought into the oil. I’m not clear why you don’t get the same phase volume to the left, though LC phases are part of the problem
28 The simple scan gives: S* - optimal salinity where HLD=0 You know EACN, you know ΔT=0 so you get CcAnd you choose the oil so the data apply to youYou don’t care if things would be better with hexane!From the size of the middle phase you get SPIf you don’t get phases then put all tubes in a water bath and find T where you doEspecially good for non-ionicsThat’s the theory – it’s never worked for me
29 Scans are: Tedious Error prone Limited by practical stock solutions ThereforeHigh-throughput equipment is highly desirable!
30 High ThroughputDo in one day what a skilled technician can do in a weekMuch more versatile because can weigh any amounts – no limits of stock solutionsPhoto record/measure of phase volumesA ChemSpeed Formax HT machine at VLCI
31 Measuring the parameters Simple phase scans to find “optimal salinity” S* followed by simple algebra give you:EACNs (find S* for same surfactant in different oils)Ccs (find S* for same oil, different surfactant)or extract more information from combined scansDOE principles greatly reduce number of scansChange proportion of unknown surfactant in known surfactant, and fit S* data to calculate Cc of unknown
32 Practical scans For experimental ease choose: An EACN scan to get Cc of surfactantFixed S, varying EACN e.g. TolHexadecane = 116 or Tol Squalane =124A surfactant scan using known Cc to get unknown CcFixed S, Fixed EACN, vary known/unknown surfactant ratioSalt scans are harder to doadding solid salt is hard without a robotsolid salt to EACN or surfactant scans can explore other ranges if original scan fails to find HLD=0
33 Scanning for EACN Fix S and unknown oil, scan two known surfactants A key step in building up the utility of HLD-NAC as too few real-world oils are knownNatural oils may show large variations due to differing levels of long chains and degrees of hydrolysis of glyceridesCustomers might require EACN values from suppliers so they can match with correct surfactant (blend)
34 Guesswork Impossible to scan full range with high accuracy So must guess Cc and plan scan around itIf guess is wrong then the scan helps narrow the range for future scansAs we build up more Cc values, there will be more prototypes for estimating Cc and more “right first time” scans
35 PlanningHLD-NAC software makes it easy to explore scan ranges and scan issuesA complex 2-surfactant situation optimal at 6% salt
36 QC via HLDThe “same” surfactant from different suppliers, and different batches from the same supplier might be different because of differing chain lengths, EO range, alcohol or acid levelsThe Cc will be an accurate QC tool to show functional equivalence of different batchsCan use small EACN steps around the Cc to get accurate values. Quick/simple test!
37 Calculating the parameters Cc can come from CxEOy correlationsSome evidence of link of Cc to CCPEACNs some rules of thumb, group contribution and, perhaps HSPBut till we have larger high quality databases, it’s hard to know if predictions are of great value
38 The Exxon model H/L = VH/VL = (VoH+VWH)/(VoL + VLo) Rather like CPP, but Vol_Head/Vol_TailVols made up of intrinsic VoH ,VoL (calculable) and associated VWH ,VLo (unknowable?)High H/L = Hydrophilic, Big Head = Head outside = o/wLow H/L = Lipophilic, Big Tail = Tail outside = w/oBut H/L is NOT a constant for a surfactantChanges with salinity, T, oilRobbins M.L. et al, J. Coll. Int. Sci. 124, , , 1988 & 126,
39 Exxon continued H/L changes with salinity “dehydration” of the associated waterCurvatures & Volumes calculableMany similarities with predictive capabilities of HLD-NACPhase volumes etc.But couldn’t compute τr/δrLittle referenced since
40 Volume Fractions & Phase Volumes A typical volume fraction scanSame thing but change cation!A lovely bit of work from Exxon
41 Exxon nukes CPPFig 3 of Paper II shows that small H/L ratio changes cause large S* shifts if L changes but small shifts if H changesBecause S* depends on (Head+Water)/(Tail+Oil)Fig 4 shows how the real sizes changeτr = τH/ τL δr =δH/δL
42 HLD-NACWe can do the HLD bit and get ourselves into the right formulation domainBut we can’t predict phase volumes, fish diagrams, viscosities …Exxon were close to a predictive model, but they didn’t have the simple HLD and they couldn’t quite get the right parametersHence we need HLD-NAC
43 What about NAC? First, the Net Curvature Net Curvature=1/Roil-1/Rwater If you’ve got w/o then Rwater is meaningful, what of Roil?Give it a pseudo value = 3*Voil/Asi.e. the ratio of volume to area of surfactantAs is calculated from number of molecules * surface area per moleculeSo what? See next slide
44 Total amount of water Net curvature also = -HLD/L Derived from scaling theory of the chemical potential (HLD) with the Kelvin equationNet curvature also = -HLD/LL= extended length of surfactant tailSo, 1/Rwater=HLD/L-1/RoilIf Roil is very large then 1/Rwater=HLD/L orRwater=L/HLDSo a long tail means large solubilityA small HLD also means large solubilityThat’s why long tails and small HLD are so good!Of course you can always increase total water by adding more surfactantor, for the same amount of water, more surfactant = smaller Rwater
45 Taming infinity At HLD=0 the solubility would be infinite That’s why we need NAC – Net Average CurvatureNAC=0.5*(1/Roil+1/Rwater)just the average of the two curvaturesNAC turns out to be equal to 1/ξξ (chi) is the De Gennes “coherence length”The longest length for which the surfactant “pallisade” can be considered to be a straight lineThe longer it is, the larger the curvatureSo Rwater is limited by the finite NAC
46 Everything now known The NAC limits the excesses of the NC 1/Roil-1/Rwater=HLD/L1/Roil+1/Rwater=2/ξCalculationsL from the structureHLD from S, T, EACN and CcVw/As requires known surfactant areaIf we knew ξ we’d know everythingBut we don’t, however intuition isn’t badFor large solubility:HLD = smallL = largeAs = largeξ = large
47 ChiImagine the microsphere is made up of straight sections of length ξThe bigger ξ, the bigger the sphere so the greater the solubilityξ =a exp(2πk/kBT) – so stiff chains are good for a large ξ …a=Tail+Boundary, k=stiffness, kBT - Boltzmann… but if the interface is too stiff then it loves to be ordered and we have liquid crystal phases
48 What happens at 50:50 Oil:Water? NC=0 (it’s neither o/w nor w/o)NAC ≠0 (it’s limited by ξ)So we have o/w curves and w/o curvesin other words we have a sinuous interweaving of the two phaseswith straight lines of length ξtypically 100ÅAnd there’s more...
49 More at 50:50If you have a small(ish) amount of surfactant you’ll have 3 phasesIf the SP of your surfactant is high (low HLD, large L, large As, large ξ) then the size of the middle, clear microemulsion phase is largewith essentially no surfactant in pure water or pure oilThis is what we look for in phase scans1 That we actually have three phases2 That the middle phase is large
50 Optimal insolubilityAny surfactant in the water or oil is doing nothingSo an optimal surfactant has zero solubility in the water and in the oilAnd is therefore 100% at the interfaceAn impossible ideal, but a simple, clear goal
51 Proof of optimal insolubility How much surfactant is needed to obtain 1 phase for 50:50 o:wSurfactant solubility in oil phase
52 Other calculations Rd and Ld – radius & cylinder length Vf - total dissolved volume fractionViscosityFish diagramVary % surfactant, calculate o/w and w/o volumes via ξ and therefore HLD and therefore T* for each transitionCan’t (yet) do the low % cutoff – needs surf. sol.
53 Calculating Shapes, Sizes, Viscosities Simultaneous equations in Hn (NAC) and Ha (AC) give us true shape* of “drops” – cylinders of radius Rd and Length LdCorrelates well with neutron scatteringCan calculate total Volume Fraction and Number Density from simple geometryViscosity can be predicted via “dilute rigid-rod theory”N = Number Density of dropletsL = Length, d=Diameter of rodscg = rigid rod concentration, ρ=µE density*As De Gennes points out, “whenever R> ξ, the shapes must be strongly non-spherical”
54 Formulation: failure is normal An average surfactant with an average “real world” oil will give no 3-phase at any sensible temperature or salinityCc is simply too far awayAnd high salinity tends to give narrow phases (Exxon)Even if there are 3 phases, the SP is smallL too small, As too small, ξ too small, HLD not zeroSo how do we choose the right surfactant(s)?
55 General Surfactant Design Rules Make the tail and head largeBut without making liquid crystal phases - hard!Use mixturesLong chains can be “balanced” by shorter chainsImpure is usually better than pureUse LinkersLong-chain alcohols as hydrophobic linkersSodium Mono/Dimethyl Naphthalene Sulphonate as hydrophlic linkerWorks pretty well to increase L and AsBut only pretty well...
56 Linkers Lipophilic linker – e.g. Octanol Di-block linker – e.g. PEP5-EO5No linkerLipophilic & Hydrophilic linkers
57 Extended surfactantsSo put a linker in between a conventional head and tailPO chains are neutral compared to EO heads and to HC tailsTypical example C14PO8EO2SO4NaC14 – a typical tailPO8 – a long “neutral” linker sectionEO2 – helps bulk up the headSO4Na – typical anionic head
58 Problems Predicting ξ Predicting Cc Predicting EACNs Predicting failure due to liquid-crystal phase formation instead of expected microemulsionUniting terminology and similar approaches (e.g. Exxon v HLD-NAC, %NaCl v g/100ml…)Cranking out public-domain datasets
59 HLD-NAC software It’s only as good as the theory and the data The theory seems better than anything else out thereAcosta has worked hard to supply dataSasol have published some on extendedsTwo ways forward?Groups take on measurement projectsSee the Intelligent Formulation Green Surfactant ProjectSurfactant suppliers supply data for everyone’s goodOne supplier gains competitive advantage …
60 AcknowledgementsIn alphabetical order, HLD-NAC is the fruit of work by Acosta, Aubry, Sabatini and SalagerEach has been generous in their discussions with meEdgar Acosta at U. Toronto is now the person taking forwad HLD-NAC and the current version of the software owes a lot to his helpMy colleagues at Syntopix, VLCI and Intelligent Formulation in the Green Surfactant project