Novel Conductivity-Enhancing Ionic Surfactants in Nonpolar Liquid

Submitted in partial fulfillment of the requirements for the degree of Master

of Science Chemical Engineering and

Colloids, Polymers, and Surfaces

Sixue Cheng

B.S., Applied Chemistry, South China University of Technology

M.S., Chemical Engineering, Carnegie Mellon University

Carnegie Mellon University Pittsburgh, PA

December 2014

i

Acknowledgement

I would like to use this opportunity to express my gratitude to everyone for the

positive impact on my Master study. I am thankful for their guidance, aspiring,

constructive criticism and friendly advice during my Master project work. For starters,

I would like to thank my advisor Professor Dennis Prieve and my co-advisor

Professor James Schneider. Throughout my whole project, you always give me so

much helpful suggestion and you are always being so inspiring. You two wonderful

advisors taught me a lot in this project, not just academic knowledge but also the

correct attitude toward research, which will inspire me in my future study and career.

Im sincerely appreciating the support from my colleagues and fellow graduate

students, Xiaoyu Liang, Yuan Fang, Ben Yezer and John Goldman. Ben Yezer and

John Goldman, thank you for your numerous technical supports during me experiment

and helping me place the orders. Xiaoyu Liang, thank you for your DLS data and

Mass Spectrometry result, and thank you for every constructive discussion we had.

Yuan Fang, thank you for your dye solubility data and your other supporting data.

I would also like to thank Professor Sides and Professor Khair, who provide a lot of

constructive suggestion on our weekly group meeting.

Finally, I would like to appreciate my parents who sponsor my Master education in

the United State.

ii

Abstract

Surfactants are not only widely used in aqueous systems as detergent, wetting agents

and dispersants, but also act as a conductivity- enhancing agent in nonpolar system.

Researches show that both ionic surfactant and nonionic surfactant could efficiently

increase the nonpolar liquids conductivity when added. This phenomenon is due to

reversed micelle structure formed in the nonpolar media, which the reversed micelle

acting as a charge carrier. This process has a very wide application, like in petroleum

handling, drug delivery, printer inks and electrophoretic display used in E-books.

Thus, the nature of surfactant charging process in nonpolar media has gained much

attention recently.

Previous works show that, the conductivity enhancement ability is related to varied

surfactant molecular properties: molecular structure and counter ion. In this research,

to better understand how these properties affect the charging process and the charging

mechanism, couple similar surfactants are synthesized: (1) Asp-(C8b)2-(pTs); (2)

Asp-(C12b)2-(pTs); (3) Asp-(C8b)2-(SO4); (4) Unbranched Variant. Those self-made

surfactants are compared with 2 similar surfactants: AOT; and a nonionic surfactant

sample from Dow company, the differences among them show on charging functional

group, length of tail and counter ion.

The conductivity and micellar size is determined for those surfactants solution in

dodecane. The result shows, there are a certain relationship among micellar size,

conductivity enhancement ability, functional group and tail length. And among those

surfactants, Asp-(C8b)2-(pTs), which is considered as a mixture of nonionic

surfactant and ionic surfactant, shows a greater exponent of 2 in conductivity-

concentration log-log function than other surfactant, which only around 1.2. Possible

explanations are provided for this behavior.

iii

Table of Contents

Acknowledgement-..............................................................................................................-i!Abstract-.................................................................................................................................-ii!Table-of-Contents-.............................................................................................................-iii!1.-Introduction-....................................................................................................................-1!2.-Materials-and-Method-..................................................................................................-5!2.1-Materials-Synthesis-..............................................................................................................-5!2.1.1!Asp((C8b)2((pTs)!...........................................................................................................................!6!2.1.2!Asp((C12b)2((pTs)!........................................................................................................................!7!2.1.3!Unbranched!Variants!....................................................................................................................!8!2.1.4!Dow!free!amine!Surfactant!and!Asp((C8b)2((SO4)!..........................................................!9!2.2-Purification-.........................................................................................................................-10!

3.-Experimental-Result-..................................................................................................-10!3.1-Purification-and-Composition Analysis-........................................................................-11!3.1.1!Purification:!CombiFlash!Solvent!.........................................................................................!11!3.1.2!Purification!Peak!Analysis!and!Mass!Spectrometry!Result!.......................................!13!3.2-Solubility-of-Surfactant-....................................................................................................-16!3.3-Conductivity-and-Micellar-Aggregation-.....................................................................-17!3.3.1!Conductivity!...................................................................................................................................!17!3.3.2!Micellar!Aggregation!Analysis!...............................................................................................!24!

4.-Discussion-.....................................................................................................................-25!5.-Conclusion-....................................................................................................................-28!6.-Reference-......................................................................................................................-29!

1

1. Introduction

Surfactants are a kind of compounds that lower the surface tension between two

liquids or between a liquid and a solid, acting as a surface-active agent. Surfactants

are usually organic compounds that are amphiphilic, containing a hydrophilic group

the head and a hydrophobic group the tail. This special structure allows it forming a

micelle structure in both aqueous and nonaqueous liquid (reversed micelle) when

surfactants molecule is concentrated enough in solution. In the case where an

aqueous solution of surfactant is mixed with a nonpolar liquid, surfactant will adsorb

at the interface: the hydrophobic tail will extend into the nonpolar phase and

aggregate together while the hydrophilic head group remains in the aqueous phase,

forming a micelle structure. Because of the micelle structure, the surfactant solution

above a certain concentration (critical micelle concentration), the solutions properties

may act very differently than without surfactant.

Since its so useful to manufacturing, surfactants in aqueous solution have been

widely studied and well understood in the past. Since about the 1950s, an interest has

risen in detergents in nonpolar solutions, more and more attention is focused on the

properties and applications of reversed micelles which are formed by surfactants

dissolved in nonpolar media. In aqueous environments, surface charge on colloidal

particles is very common and causes electrostatic stabilization of the colloidal

particles. Nonpolar solutions are considered to be charge free because of the large

energy needed to separate positive from negative charges. However, charging in

nonpolar media does occur and was first report in 1950s[1,2], the conductivity of

nonpolar media has even a much older history dating back to the 1890s[24]. Research

also show that the charge carrying species is the reversed micelle formed when

surfactants are dissolved in nonpolar media[3]. To understand this charging

phenomena and control this process is a very significant topic which could be applied

in area like petroleum industry, for stabilizing colloidal dispersions, preventing

explosions[4,5], drug delivery[6], printer inks and electrophoretic display used in E-

books[7].

2

How surfactant controls the charges in nonpolar solvent have been studied for years,

In 1993, Morrision[8] reviewed the electrical charge phenomena in nonpolar media

and display the research about ions stabilization in non polar liquid, reversed

micelles formation, conductivity enhancement and other related topics at that time.

This review is very helpful to understand the basic theory about charging process in

nonpolar media. Later, more research focused on controlling the charging process in

nonpolar liquid with surfactant, like ionic surfactants AOT is studied in nonpolar

media dodecane that by below CMC and above CMC by attached with particle

PMMA: when above CMC, there is little surface charge on particle while in

submicelle area, the solutions conductivity () shows ~ C1/2 and micelle regime of ~ C [8,9]. Nonionic surfactants like PIBS and sorbitan ester of the span family, which without ionic group have been also studied and identified as a powerful charge control

agents[10,11]. Some excellent reviews report recent studies about how ionic surfactant

and nonionic surfactant control charge stabilization [12, 13].

To better understand how charge process is performing in nonpolar media, we first

introduce a concept of the Bjerrum length. The Bjerrum length B is the distance

between two point charges (both equal to +/- e) at which the electrostatic potential

energy equals the thermal energy kT, B =e2

40rkBT, where 0 is vacuum

permittivity; r is relative permittivity; e is the elementary charge. If we treat an ion

pair as two point charges (having opposite sign), then the center-to-center distance

when they are associated might be compared to the Bjerrum length. If the distance is

larger than the Bjerrum length, then the ion pair might dissociate as a result of thermal

agitation to form charge carriers. If the distance is smaller than the Bjerrum length,

then the ion pair is unlikely to dissociate and no carriers will be formed. For polar

media like water at room temperature, r = 80 and B = 70nm .

Research shows that the charging process in nonpolar media is much more difficult

than aqueous solution due to the lower r[3] and consequently larger which means

that the charges in non-polar solvent need to be separated by a larger distance to carry

3

charge, such as they are in large enough sizes, or are carried by a large enough

structure like polymer or micellar structure. For example, in dodecane, because of the

relatively larger r, 28nm is needed to keep charge stable, while in water is only

0.79nm. In nonpolar media, its hard to keep ion stable, and micelle structure offer a

suitable environment to keep a large .So its reasonable that when above CMC and

surfactant molecules start forming micelle or pre-micelle structure, the solutions

overall conductivity will increasing because ion could stably exist in this environment.

For some kind of surfactant, instead of micelle or pre-micelle, they form oligomer

because of comparatively large r and head group radii d, which aggregation number

depends on surfactant concentration[14]. Previous research shows it may also increase

solutions conductivity [15].

From the surfactant forming reversed micelle structure in nonpolar solution, the

disproportionation mechanism[10] is a very well-accepted theory that how micelles

charging in nonpolar media: two neutral inverse micelles exchange charges, forming

two micelle with contrary charge, one negative and one positive. Karvars work report that[16], if we assume the aggregation number is constant, then the conductivity is proportional to the surfactant concentration .

So far, previous works show a theory about how charging process occurred in nonpolar media, however, the comprehensive phenomena remains elusive and there is

still some mystery need to be explored. This study focuses on how the surfactants

properties, like structure, counter ion and surfactants electrical polarity affect

charging stabilization. To conduct this, since commercial surfactant AOT is widely

studied in nonpolar solvent, we plan to synthesize a series AOT like surfactants,

varying the hydrophobic tails structure and the hydrophilic heads electrical polarity.

Then make a series of concentration solution of this surfactant and nonpolar solvent

dodecane, measuring the solutions conductivity. The conductivity measurement is a

common method to study the electrical charge carrying species. Research shows when

surfactant is added in nonpolar liquid, because of the charging process occurred on

micelles interface, it would efficiently enhance the solutions conductivity. Therefore,

by studying the conductivitys enhancing ability of certain surfactant, we a further

4

realized how the electrical stabilization is carried out by the surfactant. Furthermore,

DLS and dye solubility measurement is also applied to study the micelle size and

critical micelle concentration (CMC), which help us to understand surfactants

micellar properties

5

O

O

O

O

CH3

CH3

CH3

CH3

S

O

OO

-Na+

AOT's Structure

2. Materials and Method

2.1 Materials Synthesis

We synthesized a series of surfactants which share a similar structure with the

commercial surfactant Aerosol OT (AOT, Sodium di-2-ethylhexyl-sulfosuccinate ).

We decided to choose AOT as prototype because it has a good stabilizing and

charging properties when it is added into nonpolar media like dodecane. Furthermore,

the nonpolar solvent - surfactant (AOT) system is well studied recently. Thus, we

believe, by studying different kind of surfactants, which are similar to AOT could

help us understand how a surfactants structure could affect the charging property of

nonpolar media- surfactant system. Then, explore the mechanism of electrostatic

stabilization in Nonpolar Media.

The surfactant we synthesize to compared with AOT

are slightly different both in the tail structure and

counter ion. However, to keep the surfactants

solubility and charging properties similar to AOT, we

didnt change too much on the main structure. And the

AOT homologues can be divided into three types:

altering the tail; altering the counter ion; altering both the tails and the counter ion.

Even though the structure and synthesis schemes are similar, the product obtained

may have different properties which will be discussed in the next chapter. The

homologues structure and synthesis scheme is explained in this part.

To synthesize a series of homologues structure like AOT, we use the reaction

scheme similar to that for AOT which is cited from Asakuma et al[17] and Baczko et

al[18] and then use the Combiflash Chromatography to purify the product. All these

reactions go through a Fischer-Speoer esterification.

6

Scheme 1. FischerSpeier esterification[19]

2.1.1 Asp-(C8b)2-(pTs)

1,4-bis((2-ethylhexyl)oxy)-1,4-dioxobutan-2-aminium-methyl-benzenesulfonate

( Asp-(C8b)2-(pTs)) is synthesized in this part. The reactions are showed as Scheme 2.

Scheme 2

(1). Add Toluene as solvent and heated for about 5 to 6 hours. The two tails are attached to headgroup( aspartic acid). (2). Purify it with ComboFlash, the pTs counter ion is removed, leaving a free amine part (and some Asp-(Cb8)2-pTs)

7

Figure 1

30 ml (192mmole) 2-ethylhexanol (99%, Sigma-Aldrich Corporation),

12.77(96mmole) Aspartic acid (99% , Sigma-Aldrich Corporation), 20.06g (106

mmole) p-Toluenesulfonic acid (pTs, 99%, ACROS

ORGANIC Corporation) and 600ml toluene (99%, EMD

Millipore) as solvent are combined ( no reacted). All of

these are added into a 1000ml round flask and be placed

in a Dean-Stark apparatus (figure 1.) to monitor and

remove the water produced by the reaction. The

collection water is about 5.3 ml, which is predicted by

stoichiometry, after reaction time of 5 hours. After that,

the toluene will be evaporated by rotor evaporation, and

the residue purified using the Combiflash

Chromatography (CombiFlash Rf 200i, TELEVYME ISCO). Then a clear, viscous,

colorless product is obtained, which should be a mixture of the free amine and real

Asp-(C8b)2-pTs, but for convenience, we just denote as Asp-(C8b)2-(pTs) where

the parentheses around pTs indicating the synthesis method .

Asp-(C8b)2-(pTs) has the same tail as AOT, the only difference is the charge

carried by head group: For AOT, the head group carries a positive charge, it is a

cationic surfactant ; And Asp-(C8b)2-(pTs), before Combiflash, its head group carry

negative charge(Asp-(C8b)2-(pTs)); After purification, the pTs are removed from

some molecules head group, then the product is the mixture of free amine and

ionic, called Asp-(C8b)2-(pTs).

2.1.2 Asp-(C12b)2-(pTs)

1,4-bis((2-butyloctyl)oxy)- 1,4-dioxobutane-2-ammonium-methyl benzenesulfonate

(Asp-(C12b)2-(pTs)) is obtained through a same reactio. The same mole chemical

materials (from the same company as 2.1.1) are added in 1000 ml round flask and

heated until 5ml water is collected. Compared with Asp-(C8b)2-(pTs), this one need

longer time to reaction because of stronger steric hindrance. The total reaction time is

8

O

O

CH3

CH3

O

O

NH2

O

O

O

O

NH2 CH3

CH3

about 9 hours with 5ml water collection.

Similar with Asp-(C8b)2-(pTs), this product is also a mixture of free amine and

ionic surfactant. The difference from Asp-(C8b)2-(pTs) is the tail for Asp-(C12b)2-

(pTs) is longer by 2 carbon, both for main chain and branch.

Figure 2. Asp-(C12b)2-pTs with counter ion and without counter ion( free amine)

2.1.3 Unbranched Variants

In this part, we also try to synthesis a series of surfactants that dont have any

branched structure on their tail by the same scheme. The structures are shown in

Table 2 (the structure shown is the free amine):

Name

(tail name and source) Structure

Asp-(C6)2-Pts

(1-Hexanol

reagent grade, 98%, Sigma-Aldrich)

Asp-(C8)2-Pts

(1-Octanol

HPLC, 99%, Sigma-Aldrich)

O

O

O

O

NH3+

CH3

CH3

CH3

CH3

CH3

S

O

O O-

O

O

O

O

NH2

CH3

CH3

CH3

CH3

9

O

O

O

O

NH2

CH3

CH3

CH3

CH3DOW surfactant

Asp-(C14)2-Pts

(1-Decanol, 99%, Sigma-Aldrich)

O

O

O

O

NH2 CH3

CH3

Table1. Unbranched structures

Similarly, the reaction involved with longer tail need longer time to collect water,

and the reaction time vary from 4 to 10 hours. However, these straight-chain tail

surfactants are insoluble in dodecane. Thus, we didnt devote mush attention to them.

2.1.4 Dow free amine Surfactant and Asp-(C8b)2-(SO4)

We have chance to obtain some pre-purified sample surfactant directly from Dow

Chemical Company, the tag shows its pure free amine (Asp-(C8b)2). However, the

surfactant is yellow and not very viscous liquid, which is very

different from what we have from above synthesis. Thus, we tried

to synthesis some surfactant replacing pTs with sulfuric acid, with

the same mole proportion and scheme. The products appearance is

similar to Dow sample, with yellow color and low viscosity. In this

reaction, the time is similar to synthesis of Asp-(C8b)2-(pTs), but one should be very

careful about the heating process. Since concentrated sulfuric acid replaced pTs in this

reaction, its easily to be charred the materials and fail, so the heating should be well

controlled.

Similar with Asp-(C8b)2-(pTs), we believe the purification process will remove the

ionic part from the head group, form a mixture of free amine and ionic surfactant.

The differences between Asp-(C8b)2-(pTs) and Asp-(C8b)2-(SO4), besides the

remaining, ionic attached molecule, the proportion of free amine and ionic part are

different too: the result shows Asp-(C8b)2-(SO4) behaves more like free amine, while

Asp-(C8b)2-(pTs) still have some ionic surfactant properties.

10

O

O

O

O

NH3+

CH3

CH3

CH3

CH3

HSO4-

O

O

O

O

NH2

CH3

CH3

CH3

CH3

Figure 3. Asp-(C8b)2-(SO4 ) with counter ion and without counter ion( free amine)

2.2 Purification

First step to purification is carried out using Vacuum Pump Rotary

Evaporator(Rotavapor R-114, BUCHI) , for toluene, the water bath temperature

setting is about 60. Second, to obtain the sample with higher purity, CombiFlash

Chromatography(CombiFlash Rf 200i, TELEDYME ISOS) is applied to the mixture

of production after vaporizing the solvent Toluene. After evaporation, synthesis

products are clear and highly viscous liquid. Applied CombiFlash, the product after

rotated vaporizer is taken and then being dissolved in solvent, which is also the

mobile phase of CombiFlash, conducted the next step purification. The process is

carried out with parameter as below:

Separation Column properties: RedisSep Column with 120g Silica

Sample load each time: 12g mixture

Flow Rate: 85 ml/min

Solvent: 95% Chloroform + 5% Methanol

Detector Wavelength: 254nm(Red) & 280nm(Purple)

Evaporative Light Scattering (ELS) : on

Spray Temperature: 30

Drift Temperature: 60

Other parameter is set as default.

3. Experimental Result

Related apparatus and chemicals suppliers show as below:

Purification Chromatography: CombiFlash Rf 200i, TELEDYME ISOS

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Vacuum Rotavapor: Rotavapor R-114, BUCHI

Mass Spectrometry: LCQ ESI/APCI Ion Trap, Thermo-Fisher

Conductivity Meter: Non-aqueous Conductivity Probe DT700,

Dispersion Technology

Dynamic Light Scattering (DLS): Zeta Sizer Nano, Malvern

Dodecane (Solvent): n-Dodecane, for synthesis, EMD Millipore

3.1 Purification and Composition Analysis

To identify the product sample collected from purification, the Mass Spectrum is

applied to identify the molecular weight of the sample, making sure the sample

collected is objective product.

3.1.1 Purification: CombiFlash Solvent

CombiFlash mainly carries products purification. To choose a better solvent, two

different composition solvents are applied in the process:

i. 90% Ethyl acetate (HPLC, EMD Millipore) + 10% Chloroform (HPLC, EMD

Millipore)

ii. 95% Chloroform +5%Methanol (HPLC, EMD Millipore)

Figure4. CombiFlash Result with Solvent i.

12

Figure5. CombiFlash Result with solvent ii.

Figure 4 shows that separation result of Asp-(C8b)2-pTs with 90% Ethyl acetate +

10% Chloroform. At around 1 min, there is little red (UV, 254nm wavelength)

absorbance peak. From the CombiFlash operation manual, this peak should have a

benzene structure, which could be the remaining solvent toluene, since 254nm

wavelength is signature absorbance of a benzene ring. And at the time around 2min

and 3min, it shows two peaks on graph, but not completely separate, it might be a

mixture of product and unreacted pTs.

Figure 5 is the separation result with 95% Chloroform + 5% Methanol. Similar as

Figure 4, at around 1 min, there shows a absorbance peak in of 254nm wavelength,

however, different from figure1, there is only one peak at around 2min-3min,with a

high absorbance of evaporative light scattering detector (ELS, the green one,

evaporative light scattering) and comparatively lower absorbance of 254nm

wavelength, which is verified by mass spectroscopy to be objective product. The third

peak appears at a time of 9-10min, with absorbance of 254nm wavelength and ELS,

which is considered to be the un-reacted pTs.

Comparing the two different results coming from different solvents, the separation

is more complete with solvent ii, and it also give an obvious signal without overlap.

Thus, the conclusion could be made that solvent ii is more suitable for the separation.

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3.1.2 Purification Peak Analysis and Mass Spectrometry Result

Peak identification

To make sure we are collecting the right product, identification the component of

every peak on Figure1 is very significant. Since the first peak on Figure 2 only has

254nm absorbent, it should be very similar to benzene. For our mixture, the only

possibility is un-dried toluene.

For the second peak, there is both peak of ELS and UV (254nm) with the strongest

signal, which indicate that it might be the remains out after the solvent has been

evaporated, and it might have one or more functional group with UV absorbance.

Thus, we assume the second peak is the desired product.

The third peak is much weaker compared with peak 2, and it also shows ELS

absorbance and UV absorbance. We assume that the peak2 is objective product and

peak3 is remaining pTs. Here is the rationale: 1. Judging from intensity that peak2s

signal is higher than peak3. 2. The retention time in silica gel column of each

component is related to their polarity, components with higher polarity tend to have a

longer retention time in column. Thus, the eluted order should be Toluene > desired

product (Asp-(C8b)2 in this case)> pTs.

The Mass Spectrum result* proves the assumption. The results showing a very high

peak with molecular weight of 358, which is exactly the molecular weight of Asp-

(C8b)2. And to identify the component of peak 3, pure pTs solution is prepared and

run by CombiFlash alone, the result is show on figure3. Compared with the peak3 on

figure2, the shape is very similar: A wide absorbance both in ELS and UV, the ELS

absorbance is higher then UV; the retention time of peak on figure 2 is 10min and on

Figure 6 is 5min, since the later one is on pure pTs, the retention time might be

shorter, it is reasonable to believe that peak 3 is pTs, Therefore, for rest purification,

the sample of peaks 2 are collected.

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Figure 6. Pure pTs CombiFlash Result

Composition Analysis of Objective Product

Mass Spectrometry result of Asp-(C8b)2 shows under positive mode, which proves

that the objective product is obtained. From the negative mode result, we can see a

weak signal peak of 171, which is the molecular weight of pTs. This peak is also

show on other surfactants MS negative mode result with varied but weak intensity.

From the he negative mode result, we could assert that, there is no or not much pTs

or other counter ion attached at the head group because TLC (Thin-layer

chromatography) method might removed the pTs while separation. However, we still

can see a small peak on Mass Spectrometry graph, indicating there is still some pTs

attached on the molecule. Thus the product obtained after Combiflash might be a

mixture of Free Amine with no pTs attached and product with pTs, like what we

state before, which can also be supported by Figure 5.

On Figure 5, there is a UV absorbance peak for our product, indicating a functional

group like aromatic group or conjugated structure exists, however, our product

doesnt have these kinds of function group. Two possible reasons for this case:

1. The absorbance is cause by some Rayleigh Scattering. Its possible that some small

product particle in the solution that causes a Rayleigh scattering and gets

misunderstood by the detector as UV absorbance. It can be verified by doing a full-

length range of light absorption, if thats this case, the graph will be very different

15

from real UV absorbance.

2. Just like what we state before, the absorbance is caused by some remaining

attached pTs on molecule.

Thus, to try to get rid of remaining pTs and obtain pure free amine product, the

peak 2 sample coming from first CombiFlash run are collected and do the second run

with a new silica gel column.

Figure 7. Second run of purified sample

Only one peak shows on Figure 7, the plateau might be because loading sample

reaches to columns upper bound. The peak with strong signal should be the product,

however, the UV absorbance is still here, and at retention time of 6min, there is a very

weak ELS absorbance peak. The explanation is run CombiFlash multiple times might

be helpful to purified product, but it is not realistic. More studies need to be done to

obtain pure free amine product, like modify the solvent.

For other synthesized products, Asp-(C12b)2-(pTs), 470.3(product) on positive

mode, 171 on negative mode, but the intensity is lower than Asp-(C8b)2-(pTs),

indicating that pTs attached on Asp-(C12b)2-(pTs) might be easier to be removed by

CombiFlash; Dow Free Amine, 358(product) on positive mode with a high intense,

and nothing on negative mode; Asp-(C8b)2-(SO4), 358(product) on positive mode

with a high intense, and a very weak HSO4- peak(97) on negative mode.

The result indicate the main composition of product Asp-(C8b)2-(pTs) and Asp-

16

(C12b)2-(pTs) should be free amine, but still with some sample attached with pTs.

Particularly, the composition of attached pTs one should be fewer in C12 sample;

Dow surfactant should be high purity free amine; Most Asp-(C8b)2-(SO4) should be

amine with few Asp-(C8b)2 attached with HSO4-.

3.2 Solubility of Surfactant

Solubility of objective product is an important topic since the main goal is to study

the electrical property in nonpolar media (oil phase). In this case, dodecane is used as

nonpolar media because dodecane-AOT is well studied. After purify the product and

dried all solvent, the surfactants are in different physical form. And then we dissolve

them in dodecane (0.1gram sample in 10 gram solvent), however, the solubility in

dodecane of these surfactant are very different.

Surfactant Name Physical form Solubility in

dodecane

Asp-(C6)2-(pTs) white,fragile, crystalline No

Asp-(C8b)2-(pTs) clear,viscous,liquid,no color Yes

Asp-(C8)2-(pTs) white,fragile, crystalline No

Asp-(C8b)2-(SO4) clear,viscous,liquid, light yellow color Yes

Dow Sample

(Asp-(C8b)2) clear,viscous,liquid, light yellow color Yes

Asp-(C12b)2-(pTs) clear,viscous,liquid,no color Yes

Asp-(C14)2-(pTs) white,wax-like solid No

Table 2.

What obvious from the table is that surfactants solubility does not depend on

carbon chains length, but depend on branch on main chain. It could be found that all

soluble surfactants have branches in main carbon chain of the tails, while insoluble

one do not. The branches structure also affects their physical form, the straight chain

surfactants are all solid and branches surfactants are viscous liquid.

One interesting fact is that, from previous work done by Allison, surfactant Asp-

(C8b)2-pTs (with pTs attached) is insoluble in dodecane. In her work, the purification

17

was carried out by GPC (Gel permeation chromatography) which will not split pTs

from surfactant, and the MS result proved that pTs is attached on head group.

Therefore, an assumption can be draw that pure pTs surfactant is insoluble in

dodecane, while free amine surfactant is soluble. The solubility of free amine

could reach 10%( mass concentration) or even higher.

3.3 Conductivity and Micellar Aggregation

Conductivity and micellar size are two important properties of micelle solution, we

try to find our some kind of relationship between conductivity and micelle size, and

then compare with the commercial surfactant AOT. In this part, the source of AOTs

conductivity data is from published work by Dufresne[20].

3.3.1 Conductivity

The overall conductivity vs. concentration result shows as Figure 5, which shows

the tendency that how conductivity change with a concentration regime from 0.1% to

10%. Since only Asp-(C8b)2-(pTs), Asp-(C12b)2-(pTs), Asp-(C8b)2-(SO4), and Dow

Sample are soluble in dodecane, we only discuss these 4 surfactants solution. From

the figure, since the conductivity of pure dodecane is in a order of magnitudes of E-11,

its obvious that all surfactants help increase dodecanes conductivity, and the general

trend is as surfactants concentration increasing, solutions conductivity also

increasing. The solutions conductivity is proportional to surfactants concentration in

both two regimes (low concentration and high concentration), with different slope.

Particularly, Asp-(C8b)2-(pTs) conductivity is much higher than any other surfactants

solution. And the overall conductance order is:

Asp-(C8b)2-(pTs)> AOT>Asp-(C12b)2-(pTs)>Asp-(C8b)2-(SO4).

18

Figure 8. Overall Conductivity Tendency

To show the exponent correlation of these surfactant in dodecane, we plot the

conductivity vs. Mass concentration in log-log graph for surfactants AOT, Asp-

(C8b)2-(pTs), Asp-(C8b)2-(SO4) and Asp-(C12b)2-(pTs). Do power fitting at range

of 1%-10%, we obtain the exponent of the data. Surfactant Solution Exponent

Asp-(C8b)2-(pTs) in dodecane 2.18

Asp-(C8b)2-(SO4) in dodecane 1.26

Asp-(C12b)2-(pTs) in dodecane 1.17

AOT in dodecane 1.26

AOT in Heptane 0.52

Asp-(C8b)2-pTs in Toluene 1.17

Table 3.

19

From Figure 10, we can see, most surfactant solutions behavior is consistent with

power fitting in log-log graph. For AOT, Asp-(C8b)2-(SO4) and Asp-(C12b)2-(pTs),

the exponent( slope) are around 1.2, however, for Asp-(C8b)2-(pTs) shows an

exponent of 2.18. It means from mass concentration 1%-10%, the correlation could

also be described as second power exponent function (and linear-linear mentioned as

above), which is abnormal than other surfactants solution. We think its related to the

micelle formation.

Dow free Amine, Asp-(C8b)2-(SO4) and Asp-(C8b)2-(pTs)

Figure 11 compares conductivity of Asp-(C8b)2-(SO4) &Dow sample in a low

mass concentration range( 0.1% to 1%); Figure 12 shows the conductivity of Asp-

(C8b)2-(SO4) and Asp-(C8b)2-(pTs) in a concentration range of 0.1% to 10%. From

Figure 11, we can see that at low concentration, the conductivity behavior of Dow

free amine is very similar to the Asp-(C8b)2-(SO4), while Figure 12 shows, pTs

ones conductivity is higher than SO4 ones by about 20 times. This result indicates

1

10

100

1000

1 10

Conductivity vs. Concentration

Asp-C12b-pTsAsp-C8b-pTsAsp-C8b-SO4AOT

Con

duct

ivity

(nS

/m)

Mass Concentration%

0.1

1

10

100

1000

0.1 1 10

Conductivity vs Mass Concentration

Asp-C12b-pTsAsp-C8b-pTsAsp-C8b-SO4AOT

Con

duct

ivity

(nS

/m)

Mass Concentration%

Figure 9. Log-log Conductivity Figure 10. Power fitting of log-log conductivity

20

that this three surfactants, even though them have the same main chemical component,

Asp-(C8b)2 free amine, there must be something different among their components,

and that should be the reason why their conductivity behaviors are so different (It

would be good that we have Dow samples conductivity result in a same

concentration range, but we dont have enough Dow sample to conduct this

measurement).

From Figure 11, since the conductivity result in low concentration are very similar,

combine with MS result, it might be reasonable to conclude that the components of

Dow sample and Asp-(C8b)2-(SO4) are similar. However from figure 12, the

difference between Asp-(C8b)2-(SO4) and Asp-(C8b)2-(pTs) is very huge. We can

see that, for both linear regime, low concentration and high, Asp-(C8b)2-(pTs)s

conductivity is 20 times higher than Asp-(C8b)2-(SO4) .

One possible explanation is that, if Dow sample is real free amine, then Asp-

(C8b)2-(SO4) is more like to be free amine mix a small part of SO4 attached.

However, Asp-(C8b)2-(pTs) might still have some part of molecule attached with pTs

at their head group. Which means, its a mixture of free amine and cationic surfactant

as we mentioned in previous chapter. And that might be the reason why the

conductivity behavior is such a difference.

Figure 12.Conductivity of Asp-(C8b)2-(SO4) &Asp-(C8b)2-(pTs)

Figure 11.Conductivity of Asp-(C8b)2-(SO4) &Dow sample

21

Another information shows on Figure 12, for both surfactants, there is a turning

point in a certain mass concentration: both of them are around 6% or 5%. And both

range divided by this point shows a good linear fitting. For Asp-(C8b)2-(SO4), high

concentration slope is 3 times higher than low concentration regime; For Asp-(C8b)2-

(pTs), the multiple is over 10 times. This might related to the CMC: its possible that,

before this point, it is the dimer or other pre-micelle aggregation unit helps increase

the conductivity, and the point should be the critical micelle concentration (CMC) of

the surfactant. However, previous work shows the CMC (critical micelle

concentration) of AOT in dodecane is about 1mM in dodecane (about only 0.6% in

mass concentration), with an aggregation number of 30 [21]. Therefore, this problem

should be discuss later, in the summary part, after we show the conductivity graph of

AOT compared with Asp-(C8b)2-(pTs).

AOT and Asp-(C8b)2-(pTs)

Figure 13 and figure 14 shows the regular and log-log Conductivity vs. Mass

Concentration for surfactants solution Asp-(C8b)2-(pTs) and AOT in a mass

concentration range of 0.1% to 10%. Same as first part, Asp-(C8b)2-(pTs) has a much

higher conductivity than AOT, almost about 10 times. And theres also a turning point

for both surfactant solution, in both high concentration and low concentration regime;

The correlation of both regime are linear, with a very good fitting. For AOT, the high

concentration slope is also 3 times higher than lower regime, which is the same with

Asp-(C8b)2-(SO4) and Dow free amine.

One interesting thing is, if we assume Asp-(C8b)2-(pTs) is a mixture of cation

surfactant and nonionic surfactant, and that is the reason why it cause such a higher

concentration than Asp-(C8b)2-(SO4) and Dow sample. Then, AOT is anionic

surfactant, however its conductivity enhancing ability is lower then mixture Asp-

(C8b)2-(pTs). One possible explanation is the electropolarity due to this difference,

which still need to be discussed.

22

Asp-(C12b)2-(pTs) and Asp-(C8b)2-(pTs)

AOT and Asp-(C8b)2-(SO4)

Figure 15 shows the conductivity comparison of AOT and Asp-(C8b)2-(SO4),

which means it might be the comparison of anionic surfactant with non-ionic

surfactant. It might look similar from the overall figure (figure 8), but in the separated

Figure 14. Log-log Conductivity of Asp-(C8b)2-(pTs) and AOT

Figure 15. Conductivity of Asp-(C8b)2-(SO4) and AOT

Figure 16. Log-log Conductivity of Asp-(C8b)2-(SO4) and AOT

Figure 13. Conductivity of Asp-(C8b)2-(pTs) and AOT

23

plot we can find that the conductivity of AOT is 2 times higher than A-(C8b)2-SO4.

Besides, the turning point and the linear proportional correlation between mass

concentration and conductivity, are just like Asp-(C8b)2-(pTs).

Figure 17 is the conductance comparison between Asp-(C8b)2-(pTs) and Asp-

(C12b)2-(pTs). From part 2 we can see, these two products share the same synthesis

and purify process. Since it shows that Asp-(C8b)2-(pTs) might be the mixture of

cationic surfactant and nonionic surfactant, it make sense to say Asp-(C12b)2-(pTs) is

also a mixture. However, the graph shows a different result: the conductivity is very

different between them, still, Asp-(C8b)2-(pTs) is about 10 times higher than Asp-

(C12b)2-(pTs). For the structure, the only difference is that the Asp-(C12b)2-(pTs)

has a longer tail than Asp-(C8b)2-(pTs). It might be possible that the longer tail is the

reason why its conductivity is much lower. But if we compare its linear slope to

Asp-(C8b)-(SO4), we can find that the behavior is very similar, so, it might be

component is the reason cause the difference. Asp-(C12b)2-(pTs) has less ionic part

than Asp-(C8b)2-(pTs), more like free amine, and pTs is more likely be removed

from the surfactant which has a longer tail like Asp-(C8b)2-(pTs).

Figure 18. Log-log Conductivity of Asp-(C8b)2-(pTs) and Asp-(C12b)2-(pTs)

Figure 17. Conductivity of Asp-(C8b)2-(pTs) and Asp-(C12b)2-(pTs)

24

3.3.2 Micellar Aggregation Analysis

In this part, the DLS size measurement on surfactant solution of Asp-(C8b)2-(pTs)

and Asp-(C12b)2-(pTs) which is conducted by Xiaoyu Liang, is showed as below.

The AOTs micelle size data is from Bens work. Micelle Size diameter (nm)

mass concentration Asp-(C8b)2-(pTs) Asp-(C12b)2-(pTs) AOT

5% 2.578 2.331 NA

10% 3.587 2.845 3.3

Table 4.

From the result, we can see, at 10% concentration the micelle size:

Asp-(C8b)2-(pTs)> AOT> Asp-(C12b)2-(pTs)

This result is consistent with the conductance order we have above. It might indicate

fours points for reverse micelle in dodecane:

1. Larger micelle size induces higher conductivity.

2. Longer tails induce a larger micelle size.

3. For the same structure, non ionic and cationic mixture surfactant might form a

lager micelle at a same concentration.

4. Concentration could be a factor that affects the micelle size.

We do full range of concentration for Asp-(C8b)2-(pTs) and Asp-(C12b)-(pTs), but

the Zeta Sizer shows a very poor correlation function for those concentration lower

than 5%. Only above 5%, the correlation function shows reasonable, and for 10%, the

correlation function is very smooth. One possible reason is, under 5%, the molecule

aggregation size is too small to be detected. It might be that whats forming at the low

concentration is mixture of reverse micelle and pre-micelle structure. One abnormal

thing is, in aqueous solution, the aggregation number is independent on concentration,

but our result shows, at different concentrations, the micellar sizes are different. Think

about Mullers works[14], AOT is the type II ionic surfactant, which form reverse

micelle structure in nonpolar media, but there still some evidence shows it has some

type Is characters( forming oligomers). This suggest that there is no pure type I or

pure type II. And it might be a possible explanation to what we observed.

25

4. Discussion

From the result above, we can see Asp-(C8b)2-(pTs) is a mixture of pure free

amine and pTs attached surfactant, which means they a mixture of ionic

surfactant and non-ionic surfactant; Dow sample and Asp-(C8b)2-(SO4) might be

more like non-ionic surfactant, and Asp-(C12b)2-(pTs) is the also the mixture, but

different from Asp-(C8b)2-(pTs), we believe most of the component should be free

amine. This conclusion is supported by MS and Combiflash result, and also can be

verified by conductivity measuring. This result indicates that, the pTs ion is easier

separated if the head-group is attached with longer tail like C12b, because its more

non-polar.

When added to dodecane, this surfactant solution shows different properties. We

find out only the structure with branches that are soluble in dodecane. And the

surfactant solution shows a different conductivity behavior along concentration:

A-(C8b)2-(pTs)> AOT>A-(C12b)2-(pTs)>Asp-(C8b)2-(SO4)

This order is consistent with micellar size data, however, because the proportion of

ionic part and non-ionic part is hard to estimate, we know simply imply that is the

molecule structure that cause the differences.

By now, we can make an assumption that, DOW sample, Asp-(C8b)2-(SO4) and

Asp-(C12b)2-(pTs) should be more like non-ionic surfactant, maybe with some

cationic impurities; AOT is anionic surfactant; Asp-(C8b)2-(pTs) is a mixture of

cationic surfactant and nonionic surfactant. By seeing Figure 12, 13 and 17, we can

find that cation surfactants conductivity (Asp-(C8b)2-(pTs)) is much higher than

non-ionic surfactant solution and anion surfactant solution, by about 10 times for Asp-

(C8b)2-(SO4) and 5 times for AOT, while AOT solutions conductivity is 2 times

higher Asp-(C8b)2-(SO4). Thus, this result indicate that: for a surfactant with same

structure (head group and tails) the conductivity enhance ability should be: cation

surfactant> anion surfactant> non-ionic surfactant.

If we plot the conductivity data in regularly with linear axis, the correlation between

concentration and conductivity are linear in both low regime concentration and high

26

regime, and the high regimes slope is greater than low regime. The linear proportion

is consistent with strong electrolyte aqueous solution. However, when about 5%-

6%( 80mM-90mM) there is a turning point shows( thats how we divide high and low

concentration regime). For Behrenss work of the non-ionic surfactant span 85 in

hexane, there is a turning at around 25mM, above the CMC for 10mM[22]. For DLS

data of Asp-(C12b)2-(pTs) and Asp-(C8b)2-(pTs), we can see the micelle structure

signal around 8mM and 10mM. Thus, the turning points are actually above CMC,

rather than CMC. For CMC, not dramatic turning is observed, which might be

explained by Morrisons work[3].

Even though the slope change is not very dramatic, but its worth to pay attention

because the DLS also shows, when below that point, the micellar size is fluctuating

and the correlation function is very poor. Therefore, one possible reason is that there

should be some micelle structure change occurs at this point. Maybe the molecular

start form a more stable aggregation cluster with larger aggregation number.

Furthermore, if we plot it on log-log plot, we can find that by doing power fitting,

the exponent for Asp-(C8b)2-(pTs) reaches to 2.2, while others just around 1.2. Most

previous work shows, if the surfactant form a micelle structure in non-polar media,

the conductivity should be linear proportional to concentration[22, 9] (above CMC), or

conductivity ~c0.5 (concentration)[9](below CMC).

In Dufresnes work[9], a system of AOT in hexane is study, the below CMC part is

explained by weak dissociation that the law of mass action required

[Na+]=[AOT-] ~ [NaAOT]1/2. And for the above CMC part, one common explanation

is applied to explain[9, 23]: the charged micelle is the charge disproportion theory that

two originally neutral micelles exchange charge, forming two micelles with opposite

charge. In Behrens work[22], if the assumption of a monovalent micelle is made, a

expression of conductivity and concentration is generated:

//)( 02

112 CeCCe =+= +

)2/exp(2 aB =

Where C0 is the concentration of AOT and is equilibrium fraction of the charged

27

micelles, B is the solvents Bjerrum length, which is independent with micelle, and

2a is the Born diameter, which is identified with the size of the hydrophilic micelle

head group core where charge reside ; is the friction coefficient involved with the

micelles hydrodynamic radius which can describe by Stoke equation: HR 6= ,

where RH is the hydrodynamic radius of micelle and is viscosity of solution.

Obviously, above equation explained why the conductivity above CMC should have

linear correlation with concentration. However, for Asp-(C8b)2-(pTs) is consistence

with this equation. One possible reason might be the surfactant Asp-(C8b)2-(pTs)

form a mix micelle with both free amine molecule and pTs molecule. Recall the

previous work[15], the pure Asp-(C8b)2-pTs (cationic surfactant) is insoluble in

dodecane, even soluble in toluene, when doing the DLS size measuring, there is no

signal showing micelle formation (which is explained by Mullers theory[14] that the

pure pTs surfactant form some oligomers rather than micelle because of

comparatively large head-group radii). This fact is reasonable, since dodecane is more

non-polar than toluene, so the surfactant with a more polar ionic head group is

insoluble. However, Asp-(C8b)2-(pTs) can be dissolved in dodecane.

Thus, it indicates that, maybe that is the free amine (non-ionic surfactant) cause

the solubility in dodecane, which is more non-polar than toluene. And, in contrast, the

cationic component causes a higher conductivity. The possible reason might also be

this mixture surfactant form a mix reverse micelle. Previous works prove its possible

to from a mixed micelle both in aqueous media and non-polar media, but no enough

research report that how this structure, especially the ionic head group will affect the

micelle solutions electrical property.

Figure 18. Possible mixed reverse micelle structure of Asp-(C8b)2-(pTs)

28

5. Conclusion

Some AOT structure surfactants were synthesized: Asp-(C8b)2-(pTs), Asp-

(C12b)2-(pTs) and Asp-(C8b)2-(SO4), plus a surfactant sample from Dow(Asp-

(C8b)2). Combiflash instrument is applied to purify the surfactant, and mass

spectrometry, Solubility, Conductivity and DLS is applied to analyze the surfactant

solution in dodecane. The solubility result shows that, only branched-tail surfactants

are soluble in dodecane. And combine the result from MS and conductivity, we can

say the Combiflash will wipe off the ionic part from surfactant in different extent,

cause the product into free amine or free amine mix with ionic surfactant.

And all of these surfactants can enhance the dodecanes conductivity in different

level, which might be effect by the surfactants structure and component. For all

surfactant, there are turning point shows on regular conductivity vs. concentration plot,

the correlation is linear for both high concentration and low concentration, and the

slope in higher concentration regime is greater than lower concentration regime.

Combine with DLS measurement, we can see the micellar size is varied with

concentration, which it different with AOT, which might due to some oligomers

formed in this system. Besides, Among these surfactant, Asp-(C8b)2-(pTs) can

improve dodecanes conductivity much more efficiently by other surfactants.

By plotting the conductivity data in log-log graph, its obvious that Asp-(C8b)2-

(pTs) behave differently than other surfactant, showing a exponent of 2.2 while others

are about 1.2. To explain this abnormal behavior, and a possible explanation of mixed

reverse micelle is rising. The result shows its possible that conductivity and

concentration not obey a linear correlation even above CMC.

Even though we do have some progress, there still are some futures works need to be

done. Such as modifying the purification to obtain the pure ionic surfactant and non-

ionic surfactant then measuring the micelle size and conductivity, it would be helpful

to verify whether the mixture could actually enhance the solubility and conductivity.

29

Since most the AOTs conductance is greatly affected by water, we should also

devote attention to the waters concentration in the solution.

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