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
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
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
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
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
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
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
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
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”
Not CPP
CPP=Vtail/(Ltail*Ahead)
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
CPP for phases in conc. surfactant
Any complete surfactant story would
include HLD-NAC and the tendency to
form liquid crystalline phases ...
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
HLD
• Hydrophilic Lipophilic Difference
• 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
Same(ish) equation for i and non-i
• Ionics
– HLD = ln(S)-K.EACN-αΔT+Cc
• Non-ionics
– HLD=b.S-K.EACN+cΔT+Cc
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
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
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
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
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 …
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
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.
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
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
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
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-octyl sulfosuccinate is +2.5
• Sodium Di-hexyl sulfosuccinate is -0.9
• why do 2 Me groups (+ branching) give such a big change?
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
Scans not phase diagrams
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
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
Scans are:
• Tedious
• Error prone
• Limited by practical stock solutions
• Therefore
• High-throughput equipment is highly desirable!
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
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
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
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)
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
Planning
• HLD-NAC software makes it easy to explore scan ranges
and scan issues
• A complex 2-surfactant situation optimal at 6% salt
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!
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
The Exxon model
• H/L = VH/VL = (VoH+VWH)/(VoL + VLo)
– Rather like CPP, but Vol_Head/Vol_Tail
– Vols made up of intrinsic VoH ,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
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
Volume Fractions & Phase Volumes
A typical volume fraction scan
Same thing but change cation!
A lovely bit of work from Exxon
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/ τL δr =δH/δL
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
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
Total amount of water
• Net curvature also = -HLD/L
Derived from scaling theory of
the chemical potential (HLD)
with the Kelvin equation
– 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
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
Everything now known
•
•
•
•
The NAC limits the excesses of the NC
1/Roil-1/Rwater=HLD/L
For large solubility:
HLD = small
1/Roil+1/Rwater=2/ξ
L = large
A = large
Calculations
ξ = large
s
– 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
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
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
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
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
Proof of optimal insolubility
How much surfactant is needed
to obtain 1 phase for 50:50 o:w
Surfactant solubility in oil phase
Other calculations
•
•
•
•
Rd and Ld – radius & cylinder length
Vf - total dissolved volume fraction
Viscosity
Fish diagram
– Vary % surfactant, calculate o/w and w/o volumes
via ξ and therefore HLD and therefore T* for each
transition
– Can’t (yet) do the low % cutoff – needs surf. sol.
Calculating Shapes, Sizes, Viscosities
Simultaneous equations in Hn
(NAC) and Ha (AC) give us true
shape* of “drops” – cylinders
of radius Rd and Length Ld
Correlates well with neutron
scattering
Can calculate total Volume
Fraction and Number Density
from simple geometry
Viscosity can be predicted via
“dilute rigid-rod theory”
N = Number Density of droplets
L = Length, d=Diameter of rods
cg = rigid rod concentration, ρ=µE density
*As De Gennes points out,
“whenever R> ξ, the shapes must be
strongly non-spherical”
Formulation: failure is normal
• An average surfactant with an average “real
world” oil will give no 3-phase at any sensible
temperature or salinity
– Cc is simply too far away
– And high salinity tends to give narrow phases (Exxon)
• Even if there are 3 phases, the SP is small
– L too small, As too small, ξ too small, HLD not zero
• So how do we choose the right surfactant(s)?
General Surfactant Design Rules
• Make the tail and head large
– But without making liquid crystal phases - hard!
• Use mixtures
– Long chains can be “balanced” by shorter chains
– Impure is usually better than pure
• Use Linkers
– Long-chain alcohols as hydrophobic linkers
– Sodium Mono/Dimethyl Naphthalene Sulphonate as
hydrophlic linker
– Works pretty well to increase L and As
– But only pretty well...
Linkers
No linker
Lipophilic linker
– e.g. Octanol
Di-block linker –
e.g. PEP5-EO5
Lipophilic & Hydrophilic linkers
Extended surfactants
• So put a linker in between a conventional
head and tail
• PO chains are neutral compared to EO heads
and to HC tails
• Typical example C14PO8EO2SO4Na
– C14 – a typical tail
– PO8 – a long “neutral” linker section
– EO2 – helps bulk up the head
– SO4Na – typical anionic head
Problems
•
•
•
•
Predicting ξ
Predicting Cc
Predicting EACNs
Predicting failure due to liquid-crystal phase
formation instead of expected microemulsion
• Uniting terminology and similar approaches
(e.g. Exxon v HLD-NAC, %NaCl v g/100ml…)
• Cranking out public-domain datasets
HLD-NAC software
• It’s only as good as the theory and the data
• The theory seems better than anything else out
there
• Acosta has worked hard to supply data
• Sasol have published some on extendeds
• Two ways forward?
– Groups take on measurement projects
• See the Intelligent Formulation Green Surfactant Project
– Surfactant suppliers supply data for everyone’s good
– One supplier gains competitive advantage …
Acknowledgements
• In alphabetical order, HLD-NAC is the fruit of
work by Acosta, Aubry, Sabatini and Salager
• Each has been generous in their email
discussions with me
• Edgar Acosta at U. Toronto is now the person
taking forwad HLD-NAC and the current
version of the software owes a lot to his help
• My colleagues at Syntopix, VLCI and Intelligent
Formulation in the Green Surfactant project