HLD-NAC

1. This is my non-expert personal view of the field It has been used as the basis for a number of tutorial talks It is provided “as is” to stimulate discussion/debate Please send feedback to steven@stevenabbott.co.uk The website for the HLD-NAC software package is www.stevenabbott.co.uk/HLD-NAC.html HLD-NAC A guide to Optimal Surfactants Professor Steven Abbott My thanks to, in alphabetical order: Professors Acosta, Aubry, Salager and Sabatini for 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 foams • I have no particular interest/knowledge in the dynamics of classical micelles and their CMC behaviour • I can’t talk about polymeric surfactants because they, too, are outside my knowledge base • This talk is pseudo-ternary-phase-diagram free

3. Just an ordinary scientist • I’m a formulator, not a surfactant scientist • I’ve read lots of surfactant theories • None of them provided a practical formulation tool • I want a rational choice of surfactants • and I suspect that surfactant suppliers would love to reduce their inventories … • I want to formulate efficiently, not trial-and-error • Surfactant formulation space is way too large • I want to rationally swap to “green” surfactants

4. Microemulsion terminology • Winsor Type I, o/w, lower (denser) phase • Winsor Type II, w/o, upper (lighter) phase • Winsor Type III, bicontinuous middle phase Type I Type III Type II In reality are bicontinuous Generally 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 right Water phase larger Type I o/w Oil phase larger Type II w/o Optimal Type III

6. I want: • To be in the right phase, first time • Sometimes I want Type I, sometimes Type II, sometimes Type III • To have a high solubility parameter (SP) • ml water-in-oil/g surfactant • ml oil-in-water/g surfactant • Because 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 them • But 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 HLB • HLB really aren’t much use because they really only apply well to medium-sized, standard EO non-ionics • why 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 different • And they don’t predict much beyond basic o/w or w/o, and often not very well • Don’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 micelles • But not a lot about making great w/o or o/w emulsions • Generally lower is better because its not too water soluble, and probably has long tails • the long tail is good for other reasons • CMC for extended surfactants are “meaningless”

10. Not CPP CPP=Vtail/(Ltail*Ahead)

11. Not CPP CPP=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 bulkier • Theoretically 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 values • But 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, L • And there may be some routes from CPP to Cc (see later) • But so far that’s speculative

14. HLD • Hydrophilic LipophilicDifference • Apparently simple, but a lot of work to get it right from (in alphabetical order) Aubry, Sabatini and Salager • You get the Optimal Surfactant when HLD=0 • lowest interfacial energy • highest SP (Solubility Parameter – not HSP) • ml/g i.e. ml of oil dissolved in water per g of surfactant • best cleaning, oil recovery and microemulsions

15. Same(ish) equation for i and non-i EACN Hexane = 6 Decane = 10 etc. But Benzene = 0 Limonene = 7 IPM = 13 Triolein = 16 i.e. they behave like an alkane with those numbers of C • Ionics • HLD = ln(S)-K.EACN-αΔT+Cc • Non-ionics • HLD=b.S-K.EACN+cΔT+Cc S = Salt concentration, g/100ml K= Constant ~0.17 EACN = Effective Alkane Carbon Number α=temperature coefficient ~0.01. Note the - sign ΔT=temperature difference from 25°C b = Salt dependency for non-ionics ~ 0.13 c=temperature coefficient ~0.06. Bigger effect, opposite sign Cc=Characteristic Curvature, unique to each surfactant

16. What does it mean? Ionics • 0 HLD when ln(S)+ Cc = K.EACN + αΔT • as S goes up, Water solubility goes down • as Cc goes up, Water solubility goes down • as EACN goes up, Oil solubility goes down • as T goes up, Oil solubility goes down • because ionic is more soluble in water • The 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.EACN • As S goes up, Water solubility goes down • As T goes up, Water solubility goes down • EO is a strange beast so water solubility decreases with T • As Cc goes up, Water solubility goes down • As EACN goes up, oil solubility goes down

18. Why does it matter? - 1 • If 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 theory • But …

19. Why does it matter? -2 • Difficult oils have large EACNs • No problem – balance with Salt or a large +ve Cc • But high Salt is often not practical + small Type III • And large +ve Cc usually means v. long tail which generally means nasty liquid crystal phases and sticky messes • AOT is an exception because its Cc=2.5 (SDS=-2.3) so can formulate with EACNs 25 higher than SDS • Those two tails = lots of C but not too self-associative • The branches help bulk out and reduce self-association

20. Why does it matter? -3 • To get large amounts of o/w, w/o or to get a whole, stable Type III 50:50 emulsion is very hard without theory • Especially if you want a high SP – best emulsion with least surfactant • For cosmetics, surfactants are often “bad”, so less surfactant is desirable.

21. The parameters • cΔT • No theoretical method to calculate c as EO solubilities are hard to understand • But c seems to be a good solid number • αΔT • No theoretical method to calculate α • No fundamental reason why different ionic headgroups should have the same α • insufficent 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 EACN • But 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 I • A +ve value means curved around the water, so it’s w/o, Type II • Sounds a bit like CPP, but is simply an observed parameter, not something directly calculated (yet?) from shape/size • Sodium Di-octylsulfosuccinate is +2.5 • Sodium Di-hexylsulfosuccinate is -0.9 • why do 2 Me groups (+ branching) give such a big change?

25. Scans, not phase diagrams *Hope for SP of 10 So 3% surfactant will dissolve 30g oil 20:60:20 result • A proper phase diagram varies • Oil, water, surfactant, brine, temperature • Need a 5D plot to visualise it • Life’s too short • So 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 lowhigh

26. Scans not phase diagrams

27. More scans Note 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 Cc • And you choose the oil so the data apply to you • You don’t care if things would be better with hexane! • From the size of the middle phase you get SP • If you don’t get 2-3-2 phases then put all tubes in a water bath and find T where you do • Especially good for non-ionics • That’s the theory – it’s never worked for me

29. Scans are: • Tedious • Error prone • Limited by practical stock solutions • Therefore • High-throughput equipment is highly desirable!

30. High Throughput • Do in one day what a skilled technician can do in a week • Much more versatile because can weigh any amounts – no limits of stock solutions • Photo record/measure of phase volumes A 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 scans DOE principles greatly reduce number of scans Change 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 surfactant • Fixed S, varying EACN e.g. TolHexadecane = 116 or Tol Squalane =124 • A surfactant scan using known Cc to get unknown Cc • Fixed S, Fixed EACN, vary known/unknown surfactant ratio • Salt scans are harder to do • adding solid salt is hard without a robot • solid 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 known • Natural oils may show large variations due to differing levels of long chains and degrees of hydrolysis of glycerides • Customers 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 it • If guess is wrong then the scan helps narrow the range for future scans • As we build up more Cc values, there will be more prototypes for estimating Cc and more “right first time” scans

35. Planning • HLD-NAC software makes it easy to explore scan ranges and scan issues • A complex 2-surfactant situation optimal at 6% salt

36. QC via HLD • The “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 levels • The Cc will be an accurate QC tool to show functional equivalence of different batchs • Can use small EACN steps around the Cc to get accurate values. Quick/simple test!

37. Calculating the parameters • Cc can come from CxEOy correlations • Some evidence of link of Cc to CCP • EACNs some rules of thumb, group contribution and, perhaps HSP • But 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_Tail • Vols made up of intrinsicVoH ,VoL (calculable) and associated VWH ,VLo (unknowable?) • High H/L = Hydrophilic, Big Head = Head outside = o/w • Low H/L = Lipophilic, Big Tail = Tail outside = w/o • But H/L is NOT a constant for a surfactant • Changes with salinity, T, oil Robbins M.L. et al, J. Coll. Int. Sci. 124, 462-485, 486-503, 1988 & 126, 114-132

39. Exxon continued • H/L changes with salinity • “dehydration” of the associated water • Curvatures & Volumes calculable • Many similarities with predictive capabilities of HLD-NAC • Phase volumes etc. • But couldn’t compute τr/δr • Little referenced since

40. Volume Fractions & Phase Volumes A typical volume fraction scan Same thing but change cation! A lovely bit of work from Exxon

41. Exxon nukes CPP • Fig 3 of Paper II shows that small H/L ratio changes cause large S* shifts if L changes but small shifts if H changes • Because S* depends on (Head+Water)/(Tail+Oil) • Fig 4 shows how the real sizes change τr = τH/ τLδr =δH/δL

42. HLD-NAC • We can do the HLD bit and get ourselves into the right formulation domain • But 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 parameters • Hence 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/As • i.e. the ratio of volume to area of surfactant • As is calculated from number of molecules * surface area per molecule • So what? See next slide

44. Total amount of water Derived from scaling theory of the chemical potential (HLD) with the Kelvin equation • Net curvature also = -HLD/L • L= extended length of surfactant tail • So, 1/Rwater=HLD/L-1/Roil • If Roil is very large then 1/Rwater=HLD/L or • Rwater=L/HLD • So a long tail means large solubility • A small HLD also means large solubility • That’s why long tails and small HLD are so good! Of course you can always increase total water by adding more surfactant or, 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 Curvature • NAC=0.5*(1/Roil+1/Rwater) • just the average of the two curvatures • NAC turns out to be equal to 1/ξ • ξ (chi) is the De Gennes “coherence length” • The longest length for which the surfactant “pallisade” can be considered to be a straight line • The longer it is, the larger the curvature • So Rwater is limited by the finite NAC

46. Everything now known • The NAC limits the excesses of the NC • 1/Roil-1/Rwater=HLD/L • 1/Roil+1/Rwater=2/ξ • Calculations • L from the structure • HLD from S, T, EACN and Cc • Vw/As requires known surfactant area • If we knew ξ we’d know everything • But we don’t, however intuition isn’t bad For large solubility: HLD = small L = large As = large ξ = large

47. Chi • Imagine the microsphere is made up of straight sections of length ξ • The bigger ξ, the bigger the sphere so the greater the solubility • ξ =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 curves • in other words we have a sinuous interweaving of the two phases • with straight lines of length ξ • typically 100Å • And there’s more... http://met.iisc.ernet.in/~lord/webfiles/s.jpg

49. More at 50:50 • If you have a small(ish) amount of surfactant you’ll have 3 phases • If 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 large • with essentially no surfactant in pure water or pure oil • This is what we look for in phase scans • 1 That we actually have three phases • 2 That the middle phase is large

50. Optimal insolubility • Any surfactant in the water or oil is doing nothing • So an optimal surfactant has zero solubility in the water and in the oil • And is therefore 100% at the interface • An impossible ideal, but a simple, clear goal