Comparison between lyotropic cholesteric phase behavior with partly fluorinated surfactants and their exact hydrogenated counterparts

Article abstract of DOI:10.1016/j.molliq.2018.03.031

In this study, we examined lyotropic intrinsic cholesteric phase properties of some amino acid-based chiral surfactants, with partly fluorinated and hydrogenated chains, to investigate the effect of the twist structure of fluorocarbon chain on the helical twisting power of chiral surfactants. We synthesized some chiral and achiral surfactants, and their fluorinated/hydrogenated counterpart surfactants, taking into account the rule that [1 CF₂ = 1.5 CH₂]. We prepared lyotropic mixtures exhibiting discotic cholesteric phases by dissolution of chiral surfactants L-alaninehydrochloride undecylester (L-AUnDE), L-serinehydrochloride undecylester (L-SUnDE) and their partly fluorinated counterparts (L-APFOE and L-SPFOE, respectively) into sodium chloride (NaCl)/water mixtures, separately. The pitch measurements were used to evaluate the helical twisting powers of each fluorinated/hydrogenated chiral surfactants. The results indicated that the twist structure of fluorocarbon chain provides higher helical twisting power with respect to the hydrocarbon chain.

Full text of DOI:10.1016/j.molliq.2018.03.031

Accepted Manuscript
Comparison between lyotropic cholesteric phase behavior with
partly fluorinated surfactants and their exact hydrogenated
counterparts
Erol Akpinar, Sinan Yurdakul, Antônio Martins Figueiredo Neto
PII:
DOI:
Reference:
S0167-7322(17)36219-0
doi:10.1016/j.molliq.2018.03.031
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12 February 2018
8 March 2018
Please cite this article as: Erol Akpinar, Sinan Yurdakul, Antônio Martins Figueiredo
Neto , Comparison between lyotropic cholesteric phase behavior with partly fluorinated
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ACCEPTED MANUSCRIPT
Comparison between lyotropic cholesteric phase behavior with
partly fluorinated surfactants and their exact hydrogenated
counterparts
Erol Akpinar1*, Sinan Yurdakul1 and Antônio Martins Figueiredo Neto2
1
Abant Izzet Baysal University, Arts and Sciences Faculty, Department of Chemistry, 14030, Golkoy, Bolu,
Turkey
2 Instituto de Física, Universidade de São Paulo, Rua do Matão, 1371, São Paulo - SP, 05508-090, Brazil
Abstract
In this study, we examined lyotropic intrinsic cholesteric phase properties of some amino acid
based chiral surfactants, with partly fluorinated and hydrogenated chains, to investigate the
effect of the twist structure of fluorocarbon chain on the helical twisting power of chiral
surfactants. We synthesized some chiral and achiral surfactants, and their
fluorinated/hydrogenated counterpart surfactants, taking into account the rule that [1 CF2=1.5
CH2]. We prepared lyotropic mixtures exhibiting discotic cholesteric phases by dissolution of
chiral surfactants L-alaninehydrochloride undecylester (L-AUnDE), L-serinehydrochloride
undecylester (L-SUnDE) and their partly fluorinated counterparts (L-APFOE and L-SPFOE,
respectively) into sodium chloride (NaCl)/water mixtures, separately. The pitch measurements
were used to evaluate the helical twisting powers of each fluorinated/hydrogenated chiral
surfactants. The results indicated that the twist structure of fluorocarbon chain provides higher
helical twisting power with respect to the hydrocarbon chain.
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Keywords: Lyotropic discotic cholesteric phase, chirality, chiral induction mechanism, chiral
fluorinated surfactants, chiral hydrogenated surfactants, pitch, helical twisting power,
isotropic micellar solutions, critical micelle concentrations, electrical conductivity, laser
diffraction.
Author for correspondence: Dr. Erol Akpinar; e-mail:akpinar_e@ibu.edu.tr
1. Introduction
Cholesteric phases are one of the most common structures of lyotropic liquid crystals.
They differ from their nematic counterparts since they show a helical structure with finite
pitch. While their thermotropic counterparts have been used in electro-optical devices such as
liquid crystal displays, optical imaging, thermometers etc. [1], lyotropics have potential
applications in biotechnology [2]. For instance, some studies showed that lyotropic cholesteric
phases were used for delivering drugs such as Cromolyn [3-5]. Models were proposed to
describe the chiral induction mechanism [6-8], however, it is not yet completely understood.
The lyotropic cholesteric mixtures may be classified in two families. If a chiral guest
molecule (e.g. brucine) is added to a lyotropic host mixture of an achiral surfactant, which
exhibits a nematic phase, the “induced or extrinsic” cholesteric mixture is obtained [9,10]. In
the case of “intrinsic” cholesteric mixtures, an enantiomeric surfactant (such as potassium N-
dodecanoyl-L-alaninate) is used to form the lyotropic cholesteric mixture [11,12]. These two
families (intrinsic and extrinsic) have been widely studied to understanding the chiral
induction mechanism. Comparing both families, the enantiomeric surfactants in the intrinsic
cholesteric mixtures, in general, transfer their chiralities to the whole structure with shorter
(higher) helical pitch length (helical twisting power) with respect to the chiral guest molecules
in the induced cholesteric mixtures [13].
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It is known that the racemic DL-forms of the enantiomeric surfactants originate
nematic phases [14]. So, keeping the molar compositions of the lyotropic mixtures D- or L-
enantiomeric surfactants and their DL-racemic forms the same, the differences observed
between DL-racemic nematic phase and D/L-cholesteric phase should come from the chirality
of the enantiomers. Though, the information obtained from the intrinsic cholesteric mixtures
are complementary to those from the induced ones.
For several years, researchers have been trying to understand the chiral-induction
mechanism studying the effects of some factors that affect the transfer of the chirality of the
chiral molecule to the mesophase structure [13,15-28]. Since the basic units in lyotropic liquid
crystals are micelles, these factors play important roles on the mesophase chirality via
eventual changes in the micellar structure. For instance, when an electrolyte concentration is
increased in the mixture, the helical-pitch length increases [29]. It is known that the addition
of electrolytes in isotropic dilute micellar solutions [30-35] and also in lyotropic liquid
crystals [36] causes micellar growth. Indeed, the addition of an electrolyte into a lyotropic
mixture presenting cholesteric phase decreases the repulsions between the surfactant head
groups at the micelle surface and hinders the solubilization of chiral guest molecule in the
micelles. This addition leads to lowering the helical twist [29].
The micellar growth, by the addition of electrolytes to the mixture, is due to
interactions between the ionic species present in the mixture, like ions of the electrolytes and
the polar or ionic head groups of the surfactants at the micelle surfaces. However, according
to our recent studies on lyotropic nematic phases, the modifications in the inner part of the
micelles also lead to the micellar growth [37,38]. If the surfactant alkyl-chain length is
increased, keeping the same surfactant head group (e.g., potassium undecanoate and
potassium dodecanoate) the maximum birefringence of the nematic phases increases. The
bigger the birefringences the higher the micellar shape anisotropy and may imply in micellar
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growth. Consequently, two factors, i.e. addition of the electrolyte in the lyotropic mixture and
the surfactant alkyl chain length, have to be taken into account when studying lyotropic
systems.
In the literature we find studies discussing the chiral induction mechanism in the
presence of chiral molecule in lyotropic mixtures [39]. In 2006, Acimis and Karaman [40]
investigated the helical twisting power of some cholesteric phases of L-enantiomeric esters
with fully hydrogenated aliphatic alkyl chain by comparing them with their partly fluorinated
counterparts. They chose L-alaninehydrochloride dodecylester (L-ADDE) and L-
serinehydrochloride dodecylester (L-SDDE) and their fluorinated counterparts L-
alaninehydrochloride
1H,1H,2H,2H-perflourooctylester
(L-APFOE)
and
L-
serinehydrochloride 1H,1H,2H,2H-perflourooctylester (L-SPFOE). They studied these
cholesteric mixtures to understand the effect of –CF2 groups on the chirality, assuming that L-
ADDE (L-SDDE) is a counterpart of L-APFOE (L-SPFOE). However, the condition for a
hydrogenated surfactant to be a counterpart of a fluorinated one is that both surfactants have
to exhibit approximately the same critical micelle concentrations (cmc), obtained from their
isotropic diluted binary surfactant/water solutions [41-43]. Due to the energy contribution of
each CH2 or CF2 group in the surfactant alkyl chain to the micellization (i.e., the micellization
Gibbs energy), one CF2 group corresponds to almost 1.5 CH2 for surfactants with the same
head group [44-46]. Thus, it is expected that the cmc of sodium dodecanoate (0.0261
mmol/kg) be similar to that of sodium perfluorooctanoate (0.0302 mmol/kg) [42]. However,
the cmcs of partly fluorinated surfactants are different from those of fully hydrogenated
surfactants [47-50]. So, partly fluorinated L-APFOE (L-SPFOE) could not be the counterpart
of fully hydrogenated L-ADDE (L-SDDE). Acimis and Karaman [40] showed that those
molecules are not counterparts one of another because the partly fluorinated L-APFOE and L-
SPFOE surfactants have approximately two times higher cmc values than L-ADDE and L-
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SDDE [51]. So, the study reported in [40] presents problems, since wrong surfactant
molecules were chosen.
In the present study, we aim to investigate the cholesteric phase behavior of partly
fluorinated surfactants by comparing them with their exact hydrogenated counterparts. To do
so, we synthesized some novel hydrogenated and partly fluorinated surfactants, in addition to
those given in Ref. [40]. The cmcs of all the surfactants synthesized were characterized by
electrical conductivity measurements on their diluted isotropic binary aqueous solutions.
Lyotropic liquid crystalline properties were investigated by using polarizing optical
microscopy (texture observation) and laser diffraction for the pitch measurements.
2. Experimental
All chemicals (sodium dodecylsulfate, undecanoic acid, dodecanoic acid, DL-alanine,
L-alanine, DL-serine, L-serine, 1-undecanol, 1-dodecanol, 1H,1H,2H,2H-perfluorooctanol,
chlorosulfonic acid, sodium carbonate, sodium chloride, sodium sulfate, potassium hydroxide,
sulfuric acid, chromium VI oxide, carbon tetrachloride, ethanol, butanol, acetone, ethyl
acetate and diethylether) were purchased from Sigma, Merck, Aldrich and Alfa-Aesar in high
purities (99%), except 1H,1H,2H,2H-perfluorooctanol of 97%. Ultrapure water was provided
by Millipore Direct-Q3 UV (18.2 MΩ.cm of resistivity at 25ºC) for the preparation of
isotropic micellar solutions and lyotropic liquid crystal samples.
Eighteen different surfactants were synthesized in this study, except sodium
dodecylsulfate. Amino acid based DL- and L-forms of ester surfactants (DL/L-
alaninehydrochloride undecylester, DL-AUnDE/L-AUnDE, and dodecylester, DL-ADDE/L-
ADDE; DL/L-serinehydrochloride undecylester, DL-SUnDE/L-SUnDE, and dodecylester,
DL-SDDE/L-SDDE; DL- and L-alaninehydrochloride 1H,1H,2H,2H-perfluorooctylester, DL-
APFOE/L-APFOE; DL- and L-serinehydrochloride 1H,1H,2H,2H-perfluorooctylester, DL-
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SPFOE/L-SPFOE) were synthesized from the reaction of the corresponding alcohols with the
amino acids alanine and serine, separately, according to the well-described procedures
[40,52,53]. The FT-IR spectra of all amino acid-based surfactants are in good agreement with
those reported in the Refs. [40,52-54].
Sodium undecylsulfate (NaC11S) and sodium 1H,1H,2H,2H-perfluorooctylsulfate
(NaPFOS) were prepared by the reaction of undecanol and 1H,1H,2H,2H-perfluorooctanol
with chlorosulfonic acid, respectively, from the well-known procedure for sodium
alkylsulfates [55]. The FT-IR spectra of NaC11S and NaPFOS are similar to that of sodium
dodecylsulfate [56], except that some extra peaks were observed between 1230-1190 cm-1 for
NaPFOS arising from –CF2 groups [57].
Potassium undecanoate (KC11) and potassium dodecanoate or laurate (KC12) were
obtained from the neutralization reactions of undecanoic and dodecanoic acid with potassium
hydroxide, respectively, according to Berejnov’s procedure [58]. For the preparation of
potassium 2H,2H-perfluorooctanoate (KFPO), first mid-product 2H,2H-perfluorooctanoic
acid (PFOA) was synthesized from oxidation reaction of 1H,1H,2H,2H-perfluorooctanol by
Jones’ reagent (solution of chromium VI oxide and sulfuric acid) [57,59] and characterized by
FT-IR. The compound PFOA exhibited the peaks at 3072 (3080) cm-1, 1724 (1720) cm-1 and
1249-1186 (1230-1190) cm-1 for –OH, −C=O and –CF2, respectively, where the numbers in
the parentheses are values from the literature [57]. Then, PFOA was neutralized with
potassium hydroxide, similar to KC11 or KC12, to give the final product KPFO. All
potassium alkanoate surfactants (KC11, KC12 and KPFO) were characterized by FT-IR. It is
known that [58,60-62] the disappearances of bands related to carboxyl group, −COOH (~1700
cm-1), and hydroxyl group, −OH (850-960 cm-1, 1240-1300 cm-1 and 2500-3400 cm-1) and the
appearance of potassium carboxylate, −COO−K+, band at ~1550 cm-1 confirmed that
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undecanoic acid, dodecanoic acid and PFOA were completely neutralized into KC11, KC12
and KPFO, respectively.
Isotropic micellar solutions and lyotropic liquid crystalline mixtures were prepared
into sealed glass test tubes and then well-homogenized by applying vortex and centrifuging
occasionally. Notice that, especially for lyotropic mixtures, sample preparation (careful
weighing and homogenization) is a very important key point to obtain reproducible and
reliable results.
Electrical conductivity measurements were performed (Mettler Toledo S470
SevenExcellence) at constant temperature. A conductivity cell was placed in a home-made
metallic (made from Al) sample holder and water circulated around it (Polyscience SD07R)
for providing stable temperature. The cell constant was determined as 1.03 cm-1 from the
dilute KCl solutions. The conductivities were measured as a function of the surfactant
concentration by the successive addition of stock solutions of surfactants with an Eppendorf
micropipette into the cell, including ultrapure water. For each surfactant/water binary
isotropic solutions, the conductivities of surfactant/water binary mixtures were measured at a
given total surfactant concentration until reaching the concentration of the surfactant to about
2-2.5 times of the critical micelle concentrations. For each surfactant/water binary system, the
electrical conductivity measurements were repeated at least three times.
Polarizing optical microscopy observations of textures of lyotropic discotic cholesteric
phases (ChD) were performed in a Nikon Eclipse E200POL microscope, connected with
Linkam heating/freezing stage (Linkam LTS120E T95-PE) temperature controller and water
circulating bath (Polyscience SD07R). A small amount of the sample was transferred into a
0.2 mm flat microslide. Then, to prevent water loss, both ends were closed with specific
photopolymer.
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The pitch of the lyotropic cholesteric phases can be precisely measured by laser
diffraction technique [63-66]. For the laser diffraction measurements, where HeNe laser was
used (=633 nm), the lyotropic samples were encapsulated between two circular optical
glasses (Hellma, 22.5 mm diameter and 1.5 mm thickness), were separated by 2.5 mm O-ring
(Hellma, 2.5 mm thickness). Pitches of the cholesteric phases were calculated from Bragg
diffraction [67-69]. The temperature of the sample was controlled by a Lakeshore 335
temperature controller (with Pt102 sensor and a precision of ±0.001°C) and homogeneous
heat distribution within the sample holder was provided by a water-circulating bath
(Polyscience SD07R with a precision of ±0.01°C). The errors in the pitch measurements are
less than ±5%.
3. Results and Discussions
3.1 Determination of Hydrogenated Counterparts of Partly Fluorinated Surfactants
Some studies in the literature stated that surfactants whose alkyl chains are fully
hydrogenated have almost similar critical micelle concentrations with respect to fully
fluorinated ones, where alkyl chains of the hydrogenated surfactants are 1.5 times longer than
those of the fluorinated surfactants [42,44-46]. These type of surfactants, with the same
surfactant head groups, are accepted as being counterparts, one with respect to another, and
exhibit similar hydrophobicity.
Surfactants are classified as kosmotrope or chaotrope, depending on their water-
matching affinities, i.e. Collins’s concept [70-72]. This classification is related to the
surfactant head groups [73,74] but not to the surfactant alkyl chains [75]. In the framework of
this study, the amino acid-based surfactants were chosen and there is no information whether
they are kosmotropes or chaotropes in the literature. Furthermore, to the best of our
knowledge, there is no systematic investigation to show whether the different functional head
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groups of the surfactants (e.g. carboxylate, sulfate, ester, amide, etc.) affect the partly
fluorinated surfactants to be the counterparts of the hydrogenated ones from being
kosmotrope or chaotrope point of view. To clarify this point, we synthesized two novel
surfactants potassium 2H,2H-perfluorooctanoate (KPFO) and sodium 1H,1H,2H,2H-
perfluorooctylsulfate (NaPFOS). The former (latter) has carboxylate (sulfate) head group,
which is known as kosmotrope (chaotrope) [73]. Their critical micelle concentrations were
determined from the electrical conductivity measurements as shown in Figure 1. The break
point observed in the specific conductivity versus total surfactant concentration corresponds
to the critical micelle concentration. We used the same technique to investigate the potassium
undecanoate, KC11, and potassium dodecanoate, KC12 (sodium undecylsulfate, NaC11S, and
sodium dodecylsulfate, NaC12S). By this way, we may decide which one, KC11 or KC12
(NaC11S or NaC12S), is a counterpart of KPFO (NaPFOS). The critical micelle
concentrations of those surfactants at 35°C are given in Table 1. This temperature was chosen
because some precipitates were observed for NaPFOS solutions below 35C.
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15
(a)
10
5
1
2
3
KPFO (1)
KC11 (2)
KC12 (3)
0
0.00
0.02
0.04
0.06
0.08
0.10
Csurf/mol.kg-1
2.5
2.0
1.5
1.0
0.5
0.0
(b)
1
2
3
NaPFOS (1)
NaC11S (2)
NaC12S (3)
0.00
0.01
0.02
0.03
0.04
Csurf/mol.kg-1
Figure 1 Specific conductivity (κ) versus total surfactant concentration (Csurf) for (a) KPFO,
KC11 and KC12 and (b) NaPFOS, NaC11S and NaC12S at 35°C.
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Table 1 The critical micelle concentrations of KPFO, KC11, KC12, NaPFOS, NaC11S and
NaC12S at 35°C.
Surfactant
KPFO
T/°C cmc/mmol.kg-1
Surfactant
NaPFOS
NaC11S
NaC12S
T/°C
35.0
35.0
35.0
cmc/mmol.kg-1
17.24±0.22
35.0
35.0
35.0
43.1±1.5
KC11
42.78±0.31
15.22±0.12 (16.0)b
8.41±0.06 (8.6c)
KC12
26.11±0.16 (27.2a)
a At 25C, from [76].
b At 21C, from [77]
c At 35C, from [78].
As seen from the critical micelle concentrations results (Table 1), the partly
fluorinated surfactants KPFO and NaPFOS have, within the experimental error, similar cmcs
when compared with KC11 and NaC11S, but not with KC12 and NaC12S, respectively. It
means that, taking into account the rule [1 CF2 = 1.5 CH2] and their molecular structures
(Figure 2), the counterpart surfactants of KPFO and NaPFOS have to be assumed as KC11
and NaC11S, respectively. In addition, we may say that the [1 CF2 = 1.5 CH2] rule is not
dependent on the kosmotrope or chaotrope characteristic of the surfactants.
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Figure 2 Comparison of molecular structures of the potassium alkanoates and sodium
alkylsulfates studied in terms of being counterparts one to another.
Electrical conductivity measurements were performed on the alanine and serine based
partly fluorinated and fully hydrogenated ester surfactants (Figure 3 and 4), and their critical
micelle concentrations are given in Table 2. While some of them were synthesized for the first
time in this study, others were previously studied in Refs. [40,52,54]. The cmc values of
fluorinated surfactants DL/L-APFOE and DL/L-SPFOE and hydrogenated ones with 12
carbon atoms in their alkyl chains (DL/L-ADDE and DL/L-SDDE) determined in this study
are, within the experimental error, very similar to those in the Ref. [51]. The data given not
only in this study but also in the Ref. [51] shows that the cmc values of both DL/L-APFOE
and DL/L-SPFOE are about two times higher than those of DL/L-ADDE and DL/L-SDDE,
respectively. In other words, DL/L-APFOE (DL/L-SPFOE) surfactants are not counterparts of
DL/L-ADDE (DL/L-SDDE). Alternatively, we synthesized and determined the critical
micelle concentrations of the DL/L-alaninehydrochloride undecylesters (DL/L-AUnDE) and
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DL/L-serinehydrochloride undecylesters (DL/L-SUnDE) at 25°C, Table 2. According to the
results, the cmc values of DL/L-AUnDE and DL/L-SUnDE are very similar to those of DL/L-
APFOE and DL/L-SPFOE, respectively, i.e. they are their counterparts. In other words, the
rule [1 CF2 = 1.5 CH2] is valid even if partly fluorinated surfactants are compared with their
hydrogenated ones, Figure 5.
2.5
(a)
2.0
1
1.5
2
1.0
DL-AUndE (1)
0.5
DL-APFOE (2)
3
DL-ADDE (3)
0.0
0.00
0.01
0.02
Csurf/mol.kg-1
0.03
2.5
2.0
1.5
1.0
0.5
0.0
(b)
2
3
1
L-AUndE (1)
L-APFOE (2)
L-ADDE (3)
0.00
0.01
0.02
Csurf/mol.kg-1
0.03
Figure 3 Specific conductivity versus total surfactant concentration graphs of (a) DL- and (b)
L-alaninehydrochloride hydrogenated and partly fluorinated alkylesters at 25°C. The arrows
show the break point observed on each curves, i.e. the critical micelle concentrations.
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2.5
(a)
2.0
1.5
1.0
0.5
0.0
2
1
3
DL-SUndE (1)
DL-SPFOE (2)
DL-SDDE (3)
0.00
0.01
0.02
Csurf/mol.kg-1
0.03
2.5
2.0
1.5
1.0
0.5
0.0
(b)
2
1
L-SUndE (1)
L-SPFOE (2)
L-SDDE (3)
3
0.00
0.01
0.02
Csurf/mol.kg-1
0.03
Figure 4 Specific conductivity versus total surfactant concentration graphs of (a) DL- and (b)
L-serinehydrochloride hydrogenated and partly fluorinated alkylesters at 25°C. The arrows
show the break point observed on each curves, i.e. the critical micelle concentrations.
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Table 2 Critical micelle concentrations of studied amino acid based ester surfactants
determined in this study (cmc) and in the Ref. [51] (cmc*).
Surfactant
DL-AUnDE
DL-APFOE
DL-ADDE
L-AUnDE
L-APFOE
L-ADDE
T/°C
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
25.0
cmc/mmol.kg-1
10.56±0.02
9.66±0.21
cmc*/mmol.kg-1
-----
9.32±0.04
5.24±0.04
-----
5.02±0.02
10.19±0.14 (10.89)a
9.51±0.03
9.38±0.03
5.21±0.01
-----
5.02±0.07 (5.04)a
12.16±0.13
12.34±0.17
6.00±0.13
DL-SUnDE
DL-SPFOE
DL-SDDE
L-SUnDE
L-SPFOE
L-SDDE
11.67±0.09
6.20±0.05
-----
12.37±0.10
12.64±0.04
6.43±0.13
11.72±0.12
6.29±0.02
a At 25.0C, from [54]. Cmc values were given in the unit of mM.
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Figure 5 Comparison of molecular structures of the partly fluorinated and hydrogenated
alanine and serine surfactants by omitting the enantiomerism, i.e. D-, L- or DL-forms of the
surfactant head groups. Notice that the D- or L-enantiomerism is omitted in the structure of
the head groups because DL- and L-forms exhibited similar results from the comparison of
partly fluorinated and hydrogenated surfactants point of view (Table 2).
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3.2 Lyotropic Intrinsic Cholesteric Phase Properties of Alanine and Serine Esters
It is clear from the electrical conductivity measurements that partly fluorinated
APFOE and SPFOE are counterparts of AUnDE and SUnDE, but not of ADDE and SDDE,
respectively. At first sight, one CH2 group difference between ADDE and AUnDE and
between SDDE and SUnDE might not be thought as important to discuss their lyotropic
cholesteric properties by comparing them with their partly fluorinated surfactants as did in the
Ref. [40]. However, according to our recent study related to the birefringences and small-
angle x-ray scattering (SAXS) measurements of lyotropic nematic phases [38], additional CH2
group in the surfactant alkyl chains changes the micelle shape anisotropy and the phase
structure efficiently. For instance, in that study, KC11/rubidium sulfate (Rb2SO4)/decanol
(DeOH)/water mixture presented an isotropic phase at a given mixture composition. If the
surfactant KC11 is replaced by KC12 (KC13, potassium tridecanoate), the resulting mixture
exhibited three nematic phases, discotic nematic, ND, calamitic nematic, NC, and biaxial
nematic, NB, (just the ND phase). In addition, the maximum birefringence of the ND phase in
the KC13 mixture is about twice that observed in the KC12 mixture. Higher birefringence
may be associated to higher micellar shape anisotropy and bigger micelles. This assumption is
supported by SAXS measurements (micelle volumes in the discotic nematic phase of KC12
and KC13 were calculated as about 32 nm3 and 74 nm3, respectively). So, additional CH2
group in the aliphatic chain causes the micellar growth. As expected, the same number of
chiral molecules present in a lyotropic mixture (induced cholesterics) may produce greater
twist in mixtures with smaller micelles with respect to those with bigger micelles. In other
words, the pitch decreases as the micelle size decreases, for the same amount of chiral dopant
added [79]. So, for the comparison of lyotropic liquid crystalline properties of the partly
fluorinated surfactants with those of hydrogenated ones, the exact counterparts have to be
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chosen. However, in the case of intrinsic cholesteric phase, when the micelles grow, larger
number of chiral surfactants are packed in the micelles, which may lead to shorter pitch.
Another problem of the conclusions drawn in Ref. [40] is that while the authors
prepared the mixtures of hydrogenated surfactants including the strong electrolyte NaCl, the
mixtures of fluorinated counterparts had only surfactant and water, i.e. no electrolyte was
added into the fluorinated mixtures. It is known that the presence of electrolyte in a lyotropic
mixture of induced cholesteric phase increases the pitch as the salt concentration is increased
[29]. It means that, in this case, the chiral molecule cannot effectively transfer its chirality to
the micelle and, of course, to the whole structure. Indeed, this is a result of the role of the
strong electrolyte in the mixture, where the interactions between the surfactant head groups
and the ions of the electrolytes at the micelle surfaces are present. When an electrolyte is
added to a lyotropic mixture it causes the micellar growth, similar to the effect of the
surfactant alkyl-chain length, as discussed before. Thus, the same amount of chiral molecule
will not chiralize the bigger micelles as they would do in the case of smaller ones.
Consequently, it is not correct to compare two different systems, one including electrolyte and
other not. In addition, we did not apply magnetic field for the sample alignment because, as
reported for lyotropic discotic nematic phases, the hydrogenated (fluorinated) straight alkyl
chains of the surfactants have negative (positive) diamagnetic susceptibility, <0 [2,80-82]
(>0) [2,83-86]. It means that if a magnetic field is applied to both lyotropic mixtures of
hydrogenated and fluorinated surfactants, which show nematic or cholesteric phases, they
exhibit opposite responses to the magnetic field. While the helix axes of lyotropic mixtures of
hydrogenated surfactants orient parallel to the magnetic field in the ChD phase [63,68,79] and
unwind in the ChC phase [87-89], those of fluorinated surfactants exhibit opposite behavior
[2]. We observed a decreasing of the twist as a result of the unwinding the helical structure in
discotic cholesteric phases of the L-APFOE and L-SPFOE. When we measured the pitch
18

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length in those phases, as a function of time, at constant temperature (Figure 6), we observed
that the pitch increased. This effect is due to the unwinding of the helical structure by the
surfaces of the sample holder and, as it is expected, the application of magnetic field to the
lyotropic cholesteric phase of the fluorinated surfactants causes the faster unwinding the
helical structure. Within the same time interval, we could not observe any significant change
in the pitch lengths for the cholesteric phases of the hydrogenated esters, L-AUnDE and L-
SUnDE. So, for the comparison of cholesteric phase properties of fluorinated surfactants with
their hydrogenated counterparts, the application of the magnetic field to the cholesteric phases
of L-APFOE and L-SPFOE is another serious mistake present in Ref. [40] from the
experimental design point of view.
5.0
(a)
4.5
4.0
P/m
3.5
3.0
2.5
0
1
2
3
4
5
6
7
8
9
10
t/h
50
40
30
20
10
0
(b)
P/m
0
1
2
3
4
5
6
7
8
9
10
t/h
Figure 6 Pitch measurements of (a) L-APFOE at 30.0C and (b) L-SPFOE at 22.0C with
time (in hour) in the absence of magnetic field. %XL-APFOE=0.788 given in Table 4 and %XL-
SPFOE=0.617 given in Table 6.
19

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In the case of lyotropic intrinsic cholesteric mixtures, it should be expected that the
micellar growth by the addition of strong electrolyte exhibits opposite situation with respect to
the lyotropic induced ones. In the latter mixtures, the ions of strong electrolytes screen the
repulsions between the surfactant head groups at the micelle surfaces and this may cause not
only the micellar growth as a result of denser packing of achiral surfactant molecules in the
micelles (like potassium laurate), but also the transfer of some parts of the chiral dopant from
the micelles to the intermicellar region [29]. Thus, the pitch (twist or helical twisting power)
of induced cholesteric phases increases by increasing the electrolyte concentration. When an
electrolyte is added to the intrinsic cholesteric lyotropic mixture, the micelles grow by
packing of larger amount of ‘chiral’ surfactants on them, leading to a shorter pitch. Thus, the
micelles of lyotropic intrinsic cholesteric mixtures are more efficiently organized in the chiral
structure when compared with those of lyotropic induced cholesterics.
For the comparison of lyotropic cholesteric liquid crystalline properties of partly
fluorinated surfactants with their hydrogenated ones, we prepared some lyotropic mixtures. It
was not possible to obtain the mixtures of partly fluorinated and hydrogenated surfactants in
exactly same concentrations because of phase separations, gel-like phase formation instead of
cholesteric phases. Thus, we prepared the lyotropic mixtures by keeping fixed the ratio of L-
enantiomeric surfactant/electrolyte in mole percent for partly fluorinated and hydrogenated
surfactants with different amount of water. By this way, it was provided that the effect of
electrolyte, NaCl, is similar in the mixtures of both partly fluorinated and hydrogenated
surfactants. The different water content in the mixtures does not include any risk for the
comparison of lyotropic cholesteric phase behaviors of partly fluorinated surfactants with
those of their hydrogenated counterparts. As it is well-known, the Coulombic interactions
between ions of electrolytes and ionic head groups of surfactants are strong in not only short
ranges but also long ranges, even if some amount of electrolyte ions stays in the intermicellar
20

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(water) region. The lyotropic mixture compositions of partly fluorinated and hydrogenated
chiral alanine and serine esters are given in Tables 3-6.
Table 3 Compositions of L-AUnDE/NaCl/water lyotropic mixtures in mole fractions (X) as
percent (X%). R is the ratio of XL-enantiomer to XNaCl. Errors in the pitch values were within
3%.
XL-AUnDE
6.141
6.175
6.210
6.244
6.279
6.313
6.347
XNaCl
1.689
1.688
1.688
1.687
1.687
1.686
1.685
Xwater
R
P/μm
19.84
20.53
21.01
22.08
22.62
23.81
25.77
P-1/μm-1
0.0504
0.0487
0.0476
0.0453
0.0442
0.0420
0.0388
92.170
92.136
92.102
92.069
92.035
92.001
91.967
3.64
3.66
3.68
3.70
3.72
3.74
3.77
Table 4 Compositions of L-APFOE/NaCl/water lyotropic mixtures in mole fractions (X) as
percent (X%). R is the ratio of XL-enantiomer to XNaCl. Errors in the pitch values were within
3%.
XL-APFOE
0.761
0.766
0.770
0.775
0.779
0.784
0.788
XNaCl
0.209
0.209
0.209
0.209
0.209
0.209
0.209
Xwater
99.030
99.025
99.021
99.016
99.012
99.007
99.003
R
P/μm
2.34
2.42
2.48
2.60
2.67
2.72
2.82
P-1/μm-1
0.4274
0.4139
0.4025
0.3845
0.3740
0.3678
0.3541
3.64
3.66
3.68
3.70
3.72
3.75
3.77
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Table 5 Compositions of L-SUnDE/NaCl/water lyotropic mixtures in mole fractions (X) as
percent (X%). R is the ratio of XL-enantiomer to XNaCl. Errors in the pitch values were within
4%.
XL-SUnDE
4.708
4.756
4.804
4.852
4.900
4.947
4.995
XNaCl
1.840
1.839
1.838
1.837
1.836
1.836
1.835
Xwater
R
P/μm
27.03
27.45
29.37
30.46
31.31
34.25
36.36
P-1/μm-1
0.0370
0.0364
0.0340
0.0328
0.0319
0.0292
0.0275
93.452
93.405
93.358
93.311
93.264
93.217
93.170
2.56
2.59
2.61
2.64
2.67
2.70
2.72
Table 6 Compositions of L-SPFOE/NaCl/water lyotropic mixtures in mole fractions (X) as
percent (X%). R is the ratio of XL-enantiomer to XNaCl. Errors in the pitch values were within
4%.
XL-SPFOE
0.580
0.586
0.592
0.598
0.604
0.611
0.617
XNaCl
0.226
0.226
0.227
0.227
0.227
0.226
0.226
Xwater
R
P/μm
8.11
8.44
8.48
8.88
9.04
9.42
9.74
P-1/μm-1
0.1233
0.1185
0.1180
0.1126
0.1106
0.1062
0.1027
99.194
99.188
99.181
99.175
99.169
99.163
99.157
2.56
2.59
2.61
2.64
2.67
2.70
2.72
The helical twisting powers (htps) of chiral ester surfactants were determined from the pitch
measurements via laser diffraction. The pitch (P) and twist (P-1) values obtained for each
lyotropic system are given in the Tables 3-6. The htps were evaluated from the slope of the
curve of P-1 as a function of XL-enantiomer, Figures 7a-d and Table 7.
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Figure 7 Twist as a function of the mole fraction of L-enantiomeric surfactant for (a) L-
AUnDE/NaCl/water at 30.0C, (b) L-APFOE/NaCl/water at 30.0C, (c) L-
SUnDE/NaCl/water at 22.0C and (d) L-SPFOE/NaCl/water at 22.0C.
Table 7 Helical twisting powers of alanine and ester surfactants evaluated in this study (htp)
and taken from the literature for the comparison (htpa).
Lyotropic mixture
htp/μm-1
-----
htpa/μm-1
−4.00.1
−157.86.7
-----
L-ADDE/NaCl/water
L-APFOE/water
-----
L-AUnDE/NaCl/water
L-APFOE/NaCl/water
L-SDDE/NaCl/water
L-SPFOE/water
−5.360.15
−270.25.3
-----
-----
−2.40.1
−13.70.7
-----
-----
L-SUnDE/NaCl/water
−3.400.12
−54.32.1
L-SPFOE/NaCl/water
-----
a From Ref. [40], at 22C.
23

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In ref. [40] the pitch measurements were carried out at 22C. However, in our case, the
addition of electrolyte to the mixtures of L-APFOE/water changed the phase-diagram
topology by shifting the lamellar-to-cholesteric phase transitions towards higher temperatures.
Then, on condition of obtaining homogeneous and stable mixtures, i.e. there is no phase
separation with time, we decreased the lamellar-to-cholesteric phase transitions to the lowest
temperature possible by increasing the water content on the L-APFOE/NaCl/water mixture.
Thus, the pitch measurements of L-APFOE/NaCl/water and L-AUnDE/NaCl/water were
performed at 30.0C. Since the main goal of this study is to compare the lyotropic cholesteric
phase properties of partly fluorinated surfactants with their hydrogenated counterparts,
choosing any constant temperature serves to this goal. So, temperature is not a key point in
the framework of this study. For L-SPFOE/NaCl/water and L-SUnDE/NaCl/water mixtures,
cholesteric phases were obtained for different mixture compositions at 22.0C and we had a
chance to compare the htps of these mixtures with those from the mixtures studied in the Ref.
[40].
As a result of the large electron affinity of fluorine atoms, they are partially negatively
charged [90]. As it is expected, these negatively charged fluorine atoms prefer to stay away
from one another, to reduce the electrostatic energy between them. Because of this, they tend
to produce a twist along fluorocarbon chain in a double helix structure around the carbon
atoms core, which causes them to have a rigid structure [90-96]. For instance, the molecular
structures of polytetrafluoroethylene and polyethylene [91,96] are good examples to show the
main difference between a fluorinated chain and a hydrogenated chain, Figure 8.
24

ACCEPTED MANUSCRIPT
Figure 8 Zigzag chain and side space-filling views of (a) polytetrafluoroethylene, PTFE, and
(b) polyethylene, PE. While there exists twisted zigzag chain for PTFE, PE has a molecular
zigzag chain. This figure is adopted (taken with small modifications) from a figure given in
Ref. [96].
If we take into account the structures of the fluorocarbon and hydrocarbon chains
given in the Figure 8, the comparison of the htps of the fluorinated surfactants with their
hydrogenated ones is more understandable. The ‘extra twist’ arising from the helical structure
of the fluorocarbon chains of L-APFOE and L-SPFOE leads to their lyotropic cholesteric
phases exhibiting higher htps with respect to the L-AUnDE and L-SUnDE, respectively. In
the Ref. [40], the result was explained by a ‘pivot’ effect of fluorocarbon chains of L-
enantiomeric esters from the rigid structure of the fluorinated chain point of view. However,
the higher htps of the L-APFOE and L-SPFOE may be attributed to the formation of the twist
structure of fluorocarbon chain, as it has been recently reported on the comparison of PTFE
and PE structures [96].
Some models were proposed to explain the chiral induction mechanism. Two
important models for lyotropic cholesteric phases were proposed by Radley and Saupe, which
are (a) the chiral sterical interaction model and (b) the chiral dispersion interaction model [6].
These models were mainly based on the intermicellar interactions. However, an alternative
25

ACCEPTED MANUSCRIPT
model, proposed by Hiltrop et.al., so-called ‘intramicellar chirality model’, states that the
macroscopic twist of lyotropic cholesteric phase arises from ‘intramicellar twist’ as a result of
the existing chiral guest molecule in the lyotropic mixture [7,97]. According to the htps of
chiral hydrogenated and fluorinated chiral counterpart surfactants, the more flexible
hydrogenated chains cannot allow the transfer of the chirality of the chiral head groups
towards the interior of the micelle with respect to less flexible (or more rigid) fluorinated
chains. From this respect, all results of L-enantiomeric surfactants obtained in this study have
strong evidences to support the intramicellar chirality model because we investigated the
modifications in the micelle core by comparing the fluorocarbon chain with the hydrogenated
counterparts, i.e. with the same hydrophobicity.
4. Conclusions
In this study, we mainly examined the isotropic micellar solutions and lyotropic
cholesteric liquid crystalline phase behaviors of some partly fluorinated and fully
hydrogenated DL-racemic and L-enantiomeric amino acid-based surfactants. Furthermore, we
synthesized some other non-amino acid based achiral surfactants. In the first part of the study,
we showed that [1CF2 = 1.5 CH2] rule, in terms of the hydrophobicity, is also valid for partly
fluorinated surfactants in addition to the fully fluorinated surfactants if they are compared
with their exact hydrogenated counterparts. According to the results, this rule is independent
on the surfactant head group type, i.e. carboxylate, sulfate or ester.
In the second part of the study, we prepared some lyotropic cholesteric phases of L-
amino acid based surfactants for partly fluorinated and fully hydrogenated surfactants. The
results indicated that fluorinated surfactants have higher helical twisting powers with respect
to their hydrogenated counterparts. The higher helical twisting powers of the former ones may
26

ACCEPTED MANUSCRIPT
be attributed to the twist structures of the fluorocarbon chains located in the micelles. From
this respect, this study supports the intramicellar chirality model which explains the chiral
induction mechanism for lyotropic cholesteric phases.
5. Acknowledgements
The authors thank Abant Izzet Baysal University Directorate of Research Projects
Commission (BAP) [grant number: 2017.03.03.1154] from Turkey for supporting this work.
CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) grant 2011/13616-
4, 465259/2014-6; São Paulo Research Foundation (FAPESP) 2008/57685-7, 2014/50983-3;
CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior); INCT-FCx
(Instituto Nacional de Ciência e Tecnologia de Fluidos Complexos); NAP-FCx (Núcleo de
Apoio à Pesquisa de Fluidos Complexos).
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