Design criteria for rod-like reverse micelles asviscosifiers
Masanobu Sagisakaa,*, Robert M. Enickb, Julian Eastoeca

Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki
University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, JAPANb
Department of Chemical and Petroleum Engineering, University of Pittsburgh, 940 Benedum Hall,
3700 O’Hara Street, Pittsburgh, Pennsylvania 15261, United StatescSchool of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
sagisaka@hirosaki-u.ac.jp

The formation and growth of rod-like micelles are known to affect the viscosity of surfactant
solutions. The rheological behavior of rod-like micelles plays an important role in fields, forexample, enhanced oil recovery, drug delivery, cements, cosmetics, pressure-loss reduction
agents. However, studies of rod-like reverse
micelles (RRMs) are scarce compared withdirect (non-reversed) rod-like micelle. Therefore,
the design of surfactant molecules and mixed
systems for the formation of RRMs is a less welldeveloped field. This review covers previous
reports of RRMs and discusses how surfactants
and mixed systems may be designed togenerate RRMs for practical applications.
Keywords: Reverse micelles, rod micelles, Supercritical CO2, Viscosity, Hydrotropes
Masanobu Sagisaka received his PhD degree from Tokyo University of Science in
2003. After working at National Institute of Advanced Industrial Science and Technology(AIST) under a JSPS research fellowship for young scientists, he joined Hirosaki
University as an Assistant Professor in 2004, and was promoted to Associate Professor
in 2014. He frequently spends time Prof Eastoe’s group at University of Bristol (UK) as
a visiting researcher funded by the Excellent Young Researcher Overseas Visit Programfrom JSPS.
Robert M. Enick received his PhD in Chemical Engineering from the University ofPittsburgh in 1985, where he is now the Bayer Professor of Chemical and Petroleum
Engineering. He is also an ORISE Faculty Fellow at the US DOE National Energy
Technology Laboratory (NETL). His research interests include CO2-soluble polymers,
surfactants, chelating agents, and thickeners, and solvents for pre-combustion andpost-combustion carbon capture. He also has studied thickeners for ethane, propane
and butane. He has recently initiated a study with NETL of novel enhanced oil
recovery methods for fractured shale formations.
Julian Eastoe is Professor of Chemistry at the University of Bristol UK. He has
published over 300 research articles with more than 150 co-authors. His researchinterests span colloids, surfactants and applications of neutron scattering. Julian is aco-Editor of the Journal of Colloid and Interface Science and Chair of the International
Colloids Conference. He is winner of the 2015 European Colloid and Interface SocietySolvay medal “for original scientific work of outstanding quality… in the previous five
years”.
Acc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

50Design criteria for rod-like reverse micelles asviscosifiers
Masanobu Sagisakaa,*, Robert M. Enickb, Julian Eastoeca

Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki
University, b Department of Chemical and Petroleum Engineering, University of Pittsburgh, c School ofChemistry, University of Bristol

1. Introduction

surfactants, oil, pH, temperature, pressure

Certain lipophilic surfactants in nonpolarand other additives like salts, hydrotropes,

organic solvents can form reverse micelles

cosurfactants and cosolvents.1-7(RMs) above a certain concentration (i.e. theTherecritical reverse micelle concentration), andmorphologies with shapes being not only

furthermore these micelles can solubilize

spherical but also ellipsoidal, rod-like (thread-water in their polar cores. RMs are surfactant

like or worm-like), ring-like and disk-like.7

molecular assemblies with hydrophilic head

Rod-like micelles can have significant effects

groups forming cores and shells of theon viscosity depending on the aspect ratiohydrophobic tails in contact with the external

and rod length.8 The rheological behavior

continuous solvent. RMs have attracted muchplays important roles in the fields of EOR,

attention as “universal solvents” for dry-drug delivery systems, cements, cosmetics,

cleaning, enhanced oil recovery (EOR),

pressure loss reduction agents, as typicaldyeing, extraction and enzymatic reactions.

examples.

Another promising application is in synthesis

reversed rod-like micelle (RRM) solutions hasofasbeen shown to be similar to that of aqueous

nanoreactors. These nanoreactors allow for

rod-like micellar solutions9-11. Although there

reactions between water-soluble components

are numerous studies dealing with aqueous

encapsulated in the closed polar nanospaces

rod-like micelles, RRMs are only occasionally

but also water-soluble and water-insoluble

reported. Therefore, the design of surfactant

reactants, which can interact with each othermolecules and mixed systems for theacross the water/nonpolar solvent interface.

formation of RRMs is a less well-developed

The size and shape of the nanoparticles

field.

produced depends on the dimensions of the

However, that said, RMs have been studied

RMs and the exchange rate between thein supercritical CO2 (scCO2), which can be

nanomaterials

using

the

RMs

1-4

are

variousThe

possible

micellarrheological behavior of

aqueous cores .

considered as an environmentally friendly,

For many surfactants water is considered an

nonpolar solvent. One particular application

essential component in stabilization of RMs5.

of scCO2 is in EOR (CO2-EOR) from porous

The sizes and shapes of RMs are generally

rocks12,13. The main disadvantage of using

affected by the water-to-surfactant molar ratio

CO2 in EOR is the very low viscosity, which

W0,

does not promote uniform flow of the CO2composition

andcombinations

Acc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

of51through oil-bearing reservoirs, but rather

1 have the ability to form reversed micelles

viscous fingering of the CO2 via a pathway of(spherical, rod-like).

4

least resistance through the porous media .

According to the HLB approach, a hydrophilic

There is therefore a need to develop systems

surfactant with a high HLB tends to form

that can enhance CO2 viscosity to a level

water-soluble normal micelles, whereas acomparable to that of the oil.

lipophilic surfactant, with a low HLB index,

Numerous(HC)-based

forms reversed oil-soluble micelles RMs. For

polymers, surfactants and cosolutes are

typical nonionic surfactants, HLB decreases

known to generate viscous phases in organic

with increasing temperature and increasing

mediahydrocarbon

14-16, but due to the limited solvency ofsalt concentration, as hydrogen bonding

CO2, they are unsuitable for use in thisstrength between polyoxyethylene (PEO)

medium. Based on this background, a CO2-

headgroups and water molecules decreases.

soluble thickener has been required for over

The CPP also depends on temperature andthe last three decades. An efficient CO2-EOR

the concentration of additives (salts, co-compatible CO2-thickener would prolong thesurfactants, cosolvents and hydrotropes) due

lifetime

to the headgroup area a0 being affected by

of

crude

oil

reserves

while

simultaneously sequestering CO2 and giving

hydrationscientists time to develop other clean andheadgroup-headgroup repulsions.

sustainable energy producing technologies.

The micellar surface curvature changes from

This review introduces previous reports ofpositive to zero (e.g. spherical micelles →RRMs

rod-like micelles → vesicles) and then toanddiscusses

howsurfactant

andelectrostaticandstericmolecules and mixed systems may be

negative (lamellar LCs

designed to generate RRMs for such practicalspherical RMs) with increasing CPP and/or

applications.

decreasing HLB. For example, spherical

→

RRMs

→

micelles in aqueous cetyltrimethylammonium
2. The main parameters to design RRMs

chloride (CTAB) solutions turn into rod-like

Importantaffectand worm-like micelles upon addition of

surfactant aggregate morphology are thesodium bromide or potassium bromide above

parameters

known

to
17

critical packing parameter (CPP)and thecertain salt concentrations18. HLB decreases

hydrophilic-lipophilic balance (HLB) . CPP is

and CPP increases due to a decrease ingiven by V/(a0 lc), where V is the volume andhydration caused by a salting out effect.

lc is the length of hydrophobic tail group, a0 is

For a

the interfacial area of the hydrophilic group.

system, the general CPP concept can also be

When CPP is <1/3, indicating a conical shape

applied. In terms of the HLB approach, theof the surfactant molecule, normal water-concept of “lipophilicity” can be replaced bysoluble spherical micelles are formed. CPP“CO2-philicity” giving the hydrophilic/CO2-

values of 1/2-1/3 and 1/2-1 are suitable forphilic balance HCB (instead of HLB)19. In

water-soluble rod-like micelles and vesicles

earlier studies of the phase behavior of(or lamellar liquid crystals at CPP=1),

emulsions in W/CO2 mixtures, HCB wasrespectively. Surfactants with CPP larger thandefined by equation (1):7

Acc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

water/supercritical

CO2(W/CO2)52

1/HCB = (ATC ‒ ATT ‒ ACC)/(AHW ‒ AHH ‒ AWW)

Compared with bile salts SC and SDC, a(1)large molecular size of taurine-based bilewhere Aij is the energy for various interactions

salts (STC and STDC) was indicated to be

between CO2 (C), surfactant tail (T), waterless effective at RRM formation.

20(W), and surfactant head group (H) . As with
HLB, HCB strongly links to phase behavior
and dispersion stability and suggests theformation of W/CO2 dispersions at HCB < 1
and CO2/W dispersions at HCB > 1.
Formation of RRMs therefore requires CPP >1 and low HLB (or low HCB).
3. Earlier studies of W/O and W/CO2 RRM
systems
The formation of RRMs in W/O mixturesusing various surfactants (e.g. lecithin) andadditives has previously been studied.21-25
The chemical structures of these surfactants
and the additives are shown in Figure 1.
In these studies, low viscosity solutions of
cyclohexane, n-decane and n-hexadecane
containing spherical (globular) RMs weretransformed into viscoelastic organogels
containing long worm-like RRMs through theaddition bile acids and/or salts.21 The
efficiency of formation of these RRMs on
addition of bile salts followed the order(SodiumcholateSC

andSodiumdeoxycholate SDC) > taurine salts (Sodium
taurocholate

STC

and

Figure 1. Chemical structures of surfactants andadditives examined for W/O-type RRMs.

Sodiumtaurodeoxycholate STDC)> bile acids (Cholic

The formation of RMs in hexadecane throughacid CA and Deoxycholic acid DCA)21. This

the mixing two kinds of surfactants withsuggests that these additives alter thedifferent CPPs and HLBs was also examined.

surfactant CPP making the formation of

The triple-tail surfactant STO (Figure 1),

RRMs more favorable. From these results,

having high lipophilicity, formed globular RMs

the hydrogen bonding of the additives tobut these could be transformed into RRMs bylecithin and AOT was identified as a key tothe addition of the less lipophilic double-tailgenerating a larger headgroup area and,surfactant SDO21.

therefore reducing CPP, leading to the

In this RM system, the oil-insoluble SDO

formation of RRMs as shown in Figure 2.

molecules were solubilized in STO RMs

Acc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

53

Figure 2. Steric structures of lecithin with bile acids or bile salts hydrogen bonded
to the glycerol ester. The position of additive molecules to lecithin depends on
the arrangement of the hydrogen bonding groups on the additive structures.

resulting in mixed STO/SDO micelles withbenzenedimethylene (12-Ph-12), and anionichydrogen bonding between mutual sucrose

surfactant,sodiumlaurate

(SL)

in
23

groups. Therefore, the large CPP and low

cyclohexane was found to form long RRMs .

HLB of STO, usually favorable of globular RM

The maximum zero-shear viscosity of thisformation, was altered by mixing with thesystem was 1858 Pa s at a water to totallower CPP/higher HLB of SDO thus causing

surfactant molecule molar ratio, W0, of 14 andRRM formation.

0.2

As another example, the surfactant C12G2

concentration increased to 0.3 M, the(Figure1) formed longer RRMs at higher

viscosity increased to 2200 Pa s. The

surfactant concentration in cyclohexane anddynamic rheology showed the typical visco-22

n-octane .

ElevatingtemperatureM

12-Ph-12.

As

the

12-Ph-12andelastic response for thread-like or worm-likeincreasing the HC length of the surfactant

micelles at 0.2M 12-Ph-12 and W0 = 10, withcaused a transition from RRMs to globular

a relaxation time of 0.11 s.

micelles. In this case, the increase in HC

In other studies, dynamic control over thelength should hardly affect CPP, however,viscosity of RRM solutions by UV-vis light

HLB is lowered with increasing HC length andirradiation was investigated. One successful

therefore spherical RM morphology becomesexamplepreferred. For a nonionic surfactant, elevating

lecithin/SDC/cyclohexanewassolutions24

byincorporating

RRMs to be spherical as found for C12G219,22.

exposure to UV light, a structural transition

An equi-molar mixture of cationic Gemini

from SP to merocyanin (MC) (Figure 3) issurfactant,

C,C’-bis(sodium

induced and the long, viscosity enhancing,bromide)-p-

RRMs in the system transform into shorter

Acc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

(SP) .

intemperature also increases HLB and drives

dodecyldimethylammonium

spiropyranachieved

After

54

RRMs leading to a significant reduction in

irradiation, the transparent 0.3 M G14-azo

viscosity.

long-RRM-to-short-RRM/0.6M CTAB solution at W0 = 40 became

transition is reversible and the viscosity could

turbid and the viscosity decreased from 285

be recovered by irradiation with visible light or

Pa s to 0.3 Pa s, consistent with a transition

by standing for several minutes to reform thefrom worm-like RMs into spherical ones. The

worm-like RMs. The mechanism relies on

spherical micellar solution returned to its

different

original homogeneous state with a viscosityThiseffects

of

the

SP

and

MC

photoisomers on RMs (Figure 3). Theof ~290 Pa s under visible light irradiation.

hydrophobic SP molecules locate in theFormation of RRMs in scCO2 has beenhydrophobic

micellesexamined to develop an efficient and effective

composed of the solvent and the hydrophobic

CO2-EOR technology for the last threetails of lecithin. After UV irradiation, the SP

decades.26 Some double fluorocarbon (FC)-converts into the hydrophilic MC and strongly

tail surfactants and hybrid surfactants having

interacts with the lecithin headgroup leading

both fluorocarbon (FC) and HC tails wereto a concomitant decrease in the competing

often found to form not only globular RMs but

SDC-lecithin interaction. The headgroup area

also RRMs. Figure 4 shows chemical

is thus reduced resulting in a molecular

structures of the surfactants studied in thosegeometry closer to a cone and hence causing

papers.

regionsofthethe formation of shorter RRMs.

Figure 3. Mechanism of elongating/shortening oflecithin/SDC/SP-based RRMs.24

Another dynamic viscosity control study of
RRMs employed an equally-charged mixtureof an anionic Gemini surfactant, O,O ′ bis(sodium

2-tetradecylcarboxylate)-p-

azodiphendiol (G14-azo), and a cationic

Figure 4. Chemical structure of the CO2-philic

surfactant, cetyltrimethylammonium bromide

surfactants for W/CO2-type RRMs27(CTAB) in cyclohexane25. The low shear
viscosity of this system reached 4370 Pa s at

One example of RRMs in scCO2 was found

W0 = 13 and 0.2 M G14-azo. Under UV-lightusing the hybrid surfactant series M-F7H427

Acc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

55(where the counter ion, M = Li+, Na+, K+, or++

x = 2). The Na(di-HCF4) formed only

Rb ). RRMs appeared when M=Na , butspherical RMs and increased CO2 viscosity

ellipsoidal or spherical RMs where seen for

by

+

+

+

only 10%

even

at

the

surfactant

systems with Li , K or Rb . The length andconcentration of 10 wt%. In contrast withaspect ratio of Na-F7H4 RRMs reached 525

Na(di-HCF4),

Å and 10.5 respectively at a surfactant

HCF4)2 generated RRMs. The rod lengths

concentration of 4.4 wt% (75 mM), W0 = 12.5,

were 300 Å for Co(di-HCF4)2 and 700 Å for

400 bar and 40 ºC.

Ni(di-HCF4)2 at surfactant concentration of 50

Intrinsic viscosity [η] at infinite dilution is verymM, W0 = 10 at 25 ºC and 400 bar, and thesensitive to particle shape, for hard spheres

RMs increased CO2 viscosity by 20-90 % at

[η] = 2.5, whereas for one-dimensional,

6-10

anisotropic particles [η] is greater than thistransition metal ions as counterions for CO2-and can be approximated using equation

philic surfactants is a very effective approach

28,29

(2)

,

Co(di-HCF4)2

wt%31.

Thus,andemployingNi(di-

divalentfor RRM formation and increasing viscosity.
1.508[η] = 2.5 + 0.4075 (Xmicelle ‒ 1)

(2)

The counterion effect probably originates

where Xmicelle is the aspect ratio of the RM.

from a decrease in CPP due to an increase in

The value of [η] can be used to estimate thehydrophilic group volume containing a large

specific viscosity ηsp (ηsp= (η/η0) ‒ 1 where η

counter ion.

and η0 are viscosities of solution and pure

There are some studies investigating thesolvent, respectively). As such, equation (3)effect of hydrophobic chain lengths on RM

details an approximate formula which is valid

morphology. For the hybrid surfactant series

for the dilute regime of micelle volume fraction

FCm-HCn (where m and n are FC and HC

ϕp < 0.2, and the calculated viscosity has

lengths, m=4 or 6, n = 2, 4, 5, 6, or 8 as shown

been confirmed to be coincident with thein30experimental data .

Figure3)

RRM

morphology

wasinvestigated at different FC and HC chainηsp = [η] ϕp + KH [η]2 ϕp2(3)

where the KH is the Huggins coefficient for
29

lengths32,33. SANS studies revealed FC6-HCn
RMs to be ellipsoidal for n = 4, cylindrical forrods (in this case ~0.4) , calculated from then = 5 or 6, spherical for n = 8. The aqueous

estimated shear rate and rotational diffusion

core length and the aspect ratio of the FC6-

coefficient Drot; shear rate being obtained by

HC5 RRMs at 35 mM were estimated to beanalytical solution of the Navier–Stokes

about 880 Å and 36, respectively. The length

equation and Drot being calculated using thewas 5.3 times longer than that of FC6-HC6

SANS

neat

and identified to be the longest of the W/CO2

solution viscosity. The equations (2) and (3)systems studied. The intrinsic viscosity wasestimated a relative viscosity of ~2 for the Na-

calculated as [η] = 30.9 and specific viscosity

structuralparameters

and

27

F7H4 RRMs .

ηsp = 2.07 for the FC6-HC5 RRMs. When

The effect of counter ion on RM morphology

compared with the largest viscosity effect ofin scCO2 was also examined for a double FC-ηsp = 1.0 for the 75 mM Na-F7H4/W/CO2

tail surfactant M(di-HCF)x series (where M =systems with W0 = 12.5, the ηsp value of the+

Na with x = 1, Co

2+

2+

with x = 2, and Ni

Acc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

with

FC6-HC5/W/CO2 system achieves an even56

greater viscosity enhancement.

RRMs.
To tune the CPP and HCB of a CO2-philic
surfactant, additives with known effects in
W/O RMs were also examined in W/CO2 RMs.
34,35

One of the additives is a hydrotrope well-known to affect RM morphology in a W/Osystem.34

Figurestructures

of

6

shows

hydrotropes

chemicalsodiump-ethylbenzoate (C2benz) and sodium poctylbenzoate (C8benz) and the CO2-philic
surfactants tested in W/CO2 systems. Mixinghydrotropes C2benz and C8benz to a CO2philic surfactant DCF2 (the Na(di-HCF4)
analogue having double C2F5CH2CH2-tails)
induced ellipsoidal RMs. Change in HC
length of the hydrotrope did not show a clear
difference in RM morphology in scCO234. A
Figure 5. Change in CPP as a function of W0 for

mixture of the non-fluorinated co-surfactant

W/CO2

DIGSS

RMs

with

hybridanddouble-tail

and

the

low

F-content

hybridsurfactants or W/n-heptane RMs with AOT.

surfactant Hybrid CF2:SIS1 also generate

Experimental conditions were 45 ºC and 350 bar

ellipsoidal RMs at low CO2 pressures (lowfor 17mM FC6-HCn (n = 4-6) and 17mM

CO2 density)34. Here, the addition of DIGSSnFG(EO)2 (n = 4, 8), 23-40 ºC and 380-400 bar forprobably changed the CPP of CF2:SIS1 and78 mM Li-F7H4 and 73 mM K-F7H4, 75-79 mMthen curvature of the RMs.

Na-F7H4, 25 ºC and 350-400 bar for 25 mM Ni(di-Furthermore, the addition of DIGSS to theHCF4)2 in W/CO2 RMs, and 25 °C and 1 bar for

double fluorinated tail surfactant nFG(EO)2 (n32

50 mM AOT in W/n-heptane RMs.

= 6, 8) showed remarkable effects onelongation of RMs and stabilization of W/CO2

To find the optimum condition for RRM

microemulsions. The effects could be given

formation, a relationship between CPP andby the interaction between ethoxy groups of

W0 of RMs was investigated as shown in

DIGSS and the surfactant headgroups, andFigure 5.32 RRMs having rod lengths

short and branched HC tails of DIGSS

exceeding 250 Å could be obtained with thekeeping an appropriate HCB and CPP for RM

optimum conditions being W0 = ~10, at

system. As such hydrotropes can be applied

surfactant concentrations ≥ 25 mM, and CPP

to modify RM morphology of other fluorinatedof 1.4 ~1.7. In the case of FC6-HCn, thesurfactants. Unfortunately, the mixing effect of

longer HC length gave the larger headgrouphydrotropes on RM morphology in fluorine-

area and a smaller CPP. Therefore, FC6-HC5

free W/CO2 systems is not as pronounced as

is considered to have the optimum HC chain

in W/O systems35.

length giving the most appropriate CPP forAcc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

57

though the HLB or HCB. Therefore, this often
dominatesanddestabilizes

the

RMs,

showing the delicate balance in moleculararchitecture needed to generate RRMs. A few
successful examples of W/CO2 RMs havebeen demonstrated with the hybrid surfactant
series FCm-HCn 24 and the double-FC tailsurfactant series M(di-HCFn)x 23. In the caseof FCm-HCn, RMs did not form with FC
length m = 4 and/or HC length n=2, but did
with m = 6 and n = 4-8, depending on CO2
affinity of the FCs or HCB of the surfactants.
When HC length increased with the constant
FC length of n = 6, RM morphology changedin the sequence of spherical (n = 4), rod-like(n = 5 and 6), returning to spherical at n = 8.
Figure 6. Chemical structure of hydrotropes andAs an another example of M(di-HCFn)x,

CO2-philic surfactants for W/CO2-type RRMs.

longer RRMs tend to form with the longer FC
length of ≥ 4. These are successful examples

4. Surfactant and mixed system designs

showing how it is necessary to balance up

for RRMs

HCB and CPP by carefully designing the

As described in previous sections, the mosthydrophobic group to achieve RRM formation.

important parameters for forming RRMs are

Reasons why thethe CPP, and the affinity of the surfactant for

especially the hybrid surfactant, are effective

nonpolar solvents (namely the HLB or HCB).

at forming the W/CO2-type RRMs, could be:

CPP values of 1/3-1/2 are known to form rod-FC surfactants can achieve a stable RM

like micelles in water and appropriate CPP

system with high solubility; and hybrid

values for RRMs in nonpolar solvents are

surfactants can develop micro-segregation

expected to be in a range of the inverse of

between the HC and FC chains, helping withthese numbers (i.e. 2-3). However, longthe necessary uniaxial micellar growth.

RRMs were found to appear at CPPs of 1.4-

Another advantage

1.7 as shown in Figure 5. Obtaining thesurfactant is the hydrophilic group area a0 cansmaller CPP values needed for RRMs can be

be adjusted by varying the HC-chain lengthachieved by decreasing V at the same chain

and enlarging the molecular distance (Figure

length lc (i.e. increasing the number of tail4). This is identified as a good way to tunegroups triple-tail → double-tail → single-

CPPtail) or increasing lc at constant V (branchedorientation of HC chains along the W/CO2

tail → less-branched tail, like iso-stearyl →interface is likely to occur due to then-stearyl). Unfortunately, this affects not onlyhydrophobicity,

CPP but also the affinity toward solvents,

phobicity. Tuning hydrophilic group area a0 to

Acc. Mater. Surf. Res. 2019, Vol.4 (No.2), 50-60.

toinduceFC surfactant, and

of

RRM

using

a

formation.

CO2-phobicity

andhybrid

The

FC-

58

obtain RRMs can also be achieved by

15KK0221,

Grant-in-Aidchanging spacer length in Gemini surfactants.

Research (Exploratory), No.17K19002], andAs introduced in the previous section, adding

Leading Research Organizations (RCUKhydrophilic additives (bile salts, hydrotropes,[through EPSRC EP/I018301/1], ANR [13-and transition metal ions) to the typical

G8ME-0003])

spherical RMs is another approach to control

Councils Initiative for Multilateral Researcha0 and favour RRMs.

Funding –G8-2012. We also acknowledge

under

the

for

Challenging

G8

Research

STFC for the allocation of neutron scattering
5. Conclusion

beam time.

Authors thank Mr TakahiroThis review details previous studies involving

Kawasaki (Hirosaki Univ), Prof Atsushi

various approaches for RRM formation in oil

Yoshizawa (Hirosaki Univ), Azmi Mohamed

as well as in supercritical CO2 solvents.

(Univ Pendidikan Sultan Idris), Prof Frédéric

Design criteria for surfactant molecules andGuittard (Univ Cote d'Azur), Dr Craig James

mixed surfactant/additive systems for RRMs

(Cardiff Univ) for supporting experiments andand thickening ability were discussed in detail.

help with preparation of this review.

As shown in this review, long RRMs can bereadily generated to control rheological

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