Determination of critical micellar concentration of homologous 2-alkoxyphenylcarbamoyloxyethyl-morpholinium chlorides

Article abstract of DOI:10.3390/molecules23051064

The critical micellar concentrations of selected alkyloxy homologues of local anesthetic 4-(2-[(2-alkoxyphenyl)carbamoyl]oxy ethyl)morpholin-4-ium chloride with n_c = 2, 4, 5, 6, 7, 8, and 9 carbons in alkyloxy tail were determined by absorption spectroscopy in the UV–vis spectral region with the use of a pyrene probe. Within the homologous series of the studied amphiphilic compounds, the ln(cmc) was observed to be dependent linearly on the number of carbon atoms n_c in the hydrophobic tail: ln(cmc) = 0.705–0.966 n_c. The Gibbs free energy, necessary for the transfer of the methylene group of the alkoxy chain from the water phase into the inner part of the micelle at the temperature of 25^?C and pH ≈ 4.5–5.0, was found to be ?2.39 kJ/mol. The experimentally determined cmc values showed good correlations with the predicted values of the bulkiness of the alkoxy tail expressed as the molar volume of substituent R, as well as with the surface tension of the compounds.

Full text of DOI:10.3390/molecules23051064

molecules
Article
Determination of Critical Micellar Concentration of
Homologous 2-Alkoxyphenylcarbamoyloxyethyl-
Morpholinium Chlorides
ˇ
Lenka Stopková, Jana Gališinová, Zuzana Šuchtová, Jozef Cižmárik and Fils Andriamainty *
Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Comenius University, Odbojárov 10,
83232 Bratislava, Slovakia; lenka.stopkova@gmail.com (L.S.); jana.galisinova@gmail.com (J.G.);
zuzanasuchtova@gmail.com (Z.S.); cizmarik@fpharm.uniba.sk (J.C.)
* Correspondence: andriamainty@fpharm.uniba.sk; Tel.: +421-50-117-226
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 16 March 2018; Accepted: 23 April 2018; Published: 2 May 2018
Abstract: The critical micellar concentrations of selected alkyloxy homologues of local anesthetic
4-(2-{[(2-alkoxyphenyl)carbamoyl]oxy}ethyl)morpholin-4-ium chloride with nc = 2, 4, 5, 6, 7, 8,
and 9 carbons in alkyloxy tail were determined by absorption spectroscopy in the UV–vis spectral
region with the use of a pyrene probe. Within the homologous series of the studied amphiphilic
compounds, the ln(cmc) was observed to be dependent linearly on the number of carbon atoms nc
in the hydrophobic tail: ln(cmc) = 0.705–0.966 nc. The Gibbs free energy, necessary for the transfer
of the methylene group of the alkoxy chain from the water phase into the inner part of the micelle
at the temperature of 25 ◦C and pH
determined cmc values showed good correlations with the predicted values of the bulkiness of the
alkoxy tail expressed as the molar volume of substituent R, as well as with the surface tension of
the compounds.
≈
4.5–5.0, was found to be
−
2.39 kJ/mol. The experimentally
Keywords: local anesthetic; morpholinium chloride; critical micellar concentration; pyrene
absorption; sigmoidal Boltzmann equation
1. Introduction
Amphipathics are synthetic or natural molecules that covalently bond hydrophilic (soluble in
water) and hydrophobic (insoluble in water) groups. In aqueous solution, such molecules self-assemble
into aggregates of diverse forms and sizes [
1]. These self-aggregated structures are habitually spherical
assemblies [ ], where the non-polar amphipathic tail groups are disposed within a hydrophilic shell
2–4
provided by the polar head groups of the amphipathics, and the created species is called the micelle.
Particularly spherical micelles can increase one-dimensionally into the cylindrical, or two-
dimensionally into discoidal ones, or bilayers. This process depends on a solution and its conditions, as
well as on the character of a surfactant. Surfactant’s polar groups ultimately control micelle extension.
It happens because both one-dimensional growths cause that the polar groups to get closer to each
other. This led to restriction of the available area of a surfactant molecule at the micelle surface and
influenced curvature of the surface as well [5,6].
The surfactant molecules in aqueous solutions of these micelle structures are directed with their
polar groups towards the aqueous phase, and with their hydrophobic fragments outside of this
phase. Regarding ionic micelles, an interphase area between aqueous and micelle phases includes
ionic polar groups, the Stern layer of the electrical double layer, which was merged with these
groups, approximately a half of a counter ion connected with both aqueous environment and micelle.
The leftover counter ions are surrounded within the Gouy–Chapman ratio of the double layer, which
Molecules 2018, 23, 1064; doi:10.3390/molecules23051064
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goes further into the aqueous phase. In the case that the electrolytes are present, double layer length is
a function of ionic power of the solution, and can be extremely compressed.
The Gibbs free increments per methylene group by different phase transitions ∆G*(CH2) can
be provided as measuring the strength of the interactions between species in aqueous solutions of
surfactant. For any homologous series of surfactants, the relationship between the critical micellar
concentration cmc and the number of carbon atoms nc in the hydrophobic group is defined as
ln(cmc) = A − B nc,
(1)
where A and B are constants reflecting the free energy changes caused in transferring the hydrophilic
group and a methylene component of the hydrophobic group, respectively, from an aqueous
surrounding to the micelle. The change in Gibbs free energy of transfer of any methylene group
between the phases (system of two water-immiscible or partially water-miscible phases, respectively,
for water (w) and micellar (m) phase at constant temperature (T), ∆G*(CH2), is given by [7–9]
∆G*(CH2) = −BRT,
(2)
where R is the universal gas constant (8.3144598 JK−1mol−1) and T is the absolute temperature.
There are various methods, like tensiometry, conductometry, fluorimetry, calorimetry, light
scattering, and nuclear magnetic resonance (NMR) spectroscopy for determination of the critical micellar
concentration (cmc) [10–15]. Spectral methods like absorption spectroscopy in ultraviolet/visible
(UV–vis) region of spectrum, and fluorescence spectroscopy using other compounds as probes, are also
used for the evaluation of cmc [16,17].
In this article, we applied the pyrene absorption method [16] for determining the cmc of the
solution containing the series of selected morpholine derivatives of 2-alkoxyphenylcarbamic acid.
2. Results
These studied compounds (selected morpholine derivatives of 2-alkoxyphenylcarbamic acid)
were evaluated on their local anesthetic potency [18]. It was found that their potency increases with
the number (nc) of carbon atoms in the alkoxy tail, up to hexyloxy and heptyloxy chain, respectively,
and then decreases. The loss/evident decrease of biological activity at bigger chain lengths is familiar
as a cut-off effect [19–21]. The effect of these compounds depends on membrane activity. The long
alkyl chain that gets through the membrane and disrupts its structure is essential for the biological
influence of this kind of compounds. It has been found that these characteristics can have an impact
on biological efficacy of similar kinds of compounds [22–27].
Generally, surfactants are known to be membrane perturbants, with hydrocarbon tail integrated
with the lipid bilayer of membranes [28,29]. This integration results in a disturbance of the membrane.
The size of the tail of the surfactants is supposed to add to the extension of membrane damage, as
longer chains can integrate into the lipid bilayers. The lowered activity of compounds with a longer
tail (cut-off effect) may be accompanied by their limited solubility. Because the tail lengthens, lipid
solubility rises at a quicker rate than that of partition coefficient change (lipid/aqueous). These longer
tails are with limited partitioning, thus, the concentration at the place of activity is not adequate to
have an important influence on the membrane of the cell wall [30]. However, the decrease of activity
with the tail reaching a particular length may also be explained like this: the final methyl group of
a compound can be found in the place of the final groups of the hydrocarbon chains of lipids that create
the opposite monolayer, where surfactants are not integrated [20]. The presence of such “sewing” of
both monolayers [14], and the terminal composition, can show a raised stability and lowered ability
of biological action. This feature is usually named interdigitation, and has been studied for some
amphiphilic compounds, alcohols, anesthetics, and other agents [31].
Surfactants are characterized by surface properties, therefore, molar volume (MV [cm3]) and
surface tension (ST [dyne/cm]) were predicted by means of ACD/Percepta (Advanced Chemistry

Molecules 2018, 23, 1064
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Development, Inc., Toronto, Canada). This software does not predict ionic compounds, therefore, both
descriptors were calculated for 2-(morpholin-4-yl)ethyl (2-alkoxyphenyl)carbamates. The values are
shown in Table 1.
In general, hydrophobic probes enter the micellar core and hydrophilic probes localize at
the micellar surface. Thus, micellar structure may be examined by using a probe designed to
localize in a particular micellar zone. This incorporation causes changes in intensities, respectively,
in ratio of intensities of the peaks, in the characteristic absorption and emission spectra of the probe.
The absorption spectrum of pyrene in water has showed eight peaks: strong (s) and weak (w) at 232w,
242s, 252w, 260w, 272s, 308w, 320s, and 336s nm, as depicted in Figure 1 [16,32,33]. The concentration
of used pyrene was 2
µM, which was within its solubility limit of 2–3
µM. Excimer formation was
expected to be absent at this concentration [16,34].
◦
Figure 1. Absorption spectrum of pyrene in water (2 µM) at 25 C.
In the absorption spectra of the studied compounds in water with pyrene concentration of 2 µM,
the characteristic pyrene peaks at wavelengths of 336 and 320 nm were visible. In addition, the peak at
308 nm was also visible in derivatives and . The other pyrene peaks were not observed due to the
6
7
strong absorption of these surfactants in the near UV region, and thus, they were masked.
Plots of the sum of absorbances of all major pyrene peaks (AT) against the surfactant concentration
(c) have been found to be increasingly sigmoidal in nature (see Figure 2). The absorbances of each peak
were also plotted separately against surfactant concentration in the inset.
◦
Figure 2. AT vs. concentration (c) profile for compound
4
in aqueous medium at 25 C (
ꢀ
); pH values
◦
vs. concentration (c) profile at 25 C (
#
). Inset: Plots of absorbances (A) of individual pyrene peaks (at
336 and 320 nm) vs. surfactant concentration.

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Table 1. Structures of investigated 4-(2-{[(2-alkoxyphenyl)carbamoyl]oxy}ethyl)morpholin-4-ium chloride
1
–
7
, parameters of fitting functions with determined critical
◦
micellar concentration (cmc) values at 25 C, and predicted values of molar volume (MV [cm3]) of substituent R and surface tension (ST [dyne/cm]).
Comp.
R
nc
z
x0
∆x
x0/∆x
r2
χ2
cmc1 [M]
cmc2 [M]
cmc [M]
MV * [cm3]
ST * [dyne/cm]
−2
−3
−2
−3
−3
−5
−5
−6
−6
−5
−5
−4
−6
−5
−2
−1
−2
1
2
3
4
5
6
7
C2H5
C4H9
2
4
5
6
7
8
9
28
42
69
42
28
29
21
0.0760
0.0476
0.0397
0.0172
0.0115
3.50
6.03
1.44
2.73
3.52
4.46
3.38
0.998
0.995
0.998
0.998
0.998
0.999
0.995
47.29
80.31
96.81
113.32
129.83
146.33
162.84
46.40
44.91
44.30
43.76
43.27
42.84
42.45
2.17 × 10
7.90 × 10
2.76 × 10
6.29 × 10
3.26 × 10
7.10 × 10
2.80 × 10
8.19 × 10
2.89 × 10
3.33 × 10
2.63 × 10
2.26 × 10
2.59 × 10
1.40 × 10
7.60 × 10
4.76 × 10
3.97 × 10
1.72 × 10
1.15 × 10
3.17 × 10
9.47 × 10
1.19 × 10
6.34 × 10
9.49 × 10
2.98 × 10
1.80 × 10
4.59 × 10
1.51 × 10
7.60 × 10
4.76 × 10
3.97 × 10
1.72 × 10
1.15 × 10
3.17 × 10
9.47 × 10
−2
−2
−2
−2
−4
−5
−2
−2
−2
−2
−4
−4
−2
−2
−2
−2
−4
−5
C5H11
C6H13
C7H15
C8H17
C9H19
−4
−5
3.17 × 10
9.47 × 10
* Predicted using of ACD/Percepta v. 2012 (Advanced Chemistry Development, Inc., Toronto, ON, Canada, 2012) not for final hydrochlorides but for 2-(morpholin-4-yl)ethyl
(2-alkoxyphenyl)carbamates.

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Fitting the AT vs. concentration (c) profiles to the sigmoidal Boltzmann equation (SBE) was herein
employed for cmc calculation [16,17,34–36]. Hence,
ꢀ
ꢁ
ai − af
AT =
+ af ,
(3)
1 + e(x−x )/∆x
0
where x is the overall concentration of surfactant, ai and af are the upper and lower limits of the
sigmoid, respectively, x0 is the center of the sigmoid, and x is directly related to the independent
variable range where the abrupt change of the dependent variable occurs. The sigmoidal plot can
produce two singular values of cmc, one at x0 and the other at (x0 + 2 x), respectively. Further, the ratio
∆
∆
x0/
x0/
∆
∆
x facilitates the decision which one is the right cmc [16
x < 10 produce cmc1 = x0, and those which yield x0/
,
∆
17]. The surfactant systems that provide
x > 10 by the SBE process produce cmc2
= (x0 + 2∆x). For aqueous solutions of all studied derivatives, the ratio x0/∆x had the value of less
than 10, indicating that the cmc is considered to be cmc1, and takes the value x0 according to Aguiar’s
theory [17]. The parameters of each sigmoidal function (the number of fitting points z, the number
of carbon atoms in a hydrophobic tail nc, x0,
chi-square χ2, value cmc1, cmc2, and cmc) are listed in the summary Table 1.
During the study of substances , and , the significantly tarnished solutions occurred
∆x, the ratio x0/∆
x, the coefficient of determination r2,
5,
6
7
throughout the successive dilution process. At that moment, the absorbances significantly increased,
and the solutions were not measurable until reaching the particular concentration at which a clear, and
again, measurable solution was obtained; see Figures 3–5. Only in this low concentration range was
there located the inflection point which determined the cmc (Figures 4 and 5).
Figure 3. AT of main pyrene peaks at 336 and 320 nm vs. concentration (c) profile for compound
aqueous medium at 25 C in whole measured concentration range.
5
in
◦
One hypothesis for significant opalescence changes in solutions of compounds
change in the pH value. Therefore, the further research has been moving in this direction. Dependence
of the pH value on the concentration (c) of compounds and in aqueous solutions (Figures 4 and 5)
5, 6, and 7 was the
6
7
showed that the pH was unlikely to have an effect on opalescence change, because (a) the pH value
was almost constant before and during the tarnishing process, and (b) the same pH vs. concentration
(c) profile was also seen for compound 4 (see Figure 2), where the opalescence change was not seen
at all.
Another hypothesis for this process was that the phase transition and the change in micelle shape
from a lower to higher level of complexity occurred at a certain concentration of surfactant in the
solution. This hypothesis might be the basis for further examination to confirm this apparent change,
for example, using the small angle neutron scattering (SANS) that could describe the structure of
molecules and aggregates in solution [37,38].

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In this study, the ln(cmc) values were calculated, and the plot of ln(cmc) vs. the number of carbon
atoms, nc, in the hydrophobic chain is illustrated in Figure 6. The critical micellar concentration values
decreases with increasing number of carbon atoms.
Figure 4. AT of main pyrene peaks at 336 and 320 nm vs. concentration (c) profile for compound
6
in
◦
aqueous medium at 25 C in whole measured concentration range (
ꢀ
); circle indicates concentration
◦
range where cmc was determined; pH values vs. concentration (c) profile at 2 C (#).
Figure 5. AT of main pyrene peaks at 336 and 320 nm vs. concentration (c) profile for compound
7
in
◦
aqueous medium at 25 C in whole measured concentration range (
ꢀ
); circle indicates concentration
◦
range where cmc was determined; pH values vs. concentration (c) profile at 25 C (#).
Figure 6. Dependence of ln(cmc) values on nc in hydrophobic chain.

Molecules 2018, 23, 1064
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A
study of micellization of the homologues of 2-alkoxyphenylcarbamoyloxyethyl-
pyrrolidinium [39] and 2-alkoxyphenylcarbamoyloxyethylpiperidinium chloride [40] compared with
2-alkoxyphenylcarbamoyloxyethylmorpholinium chloride homologues in the distilled water medium
◦
and at temperature 25 C showed that the cmc values increase in the order pyrrolidino- < morpholino-
< piperidinoethyl derivatives. The most probable reason is the presence of the oxygen atom in the
morpholine cycle of side chain, which remarkably changes the lipophilicity of the entire molecule in
favor of its hydrophilic properties, which lead to increased solubility in water. Increased solubility in
water can create a better environment for the formation of quantitative and qualitative parameters
of micelles.
The ln(cmc) values of compounds with the number of carbon atoms nc = 2, 4, 5, 6, 7, 8, and 9
are linearly dependent on nc: ln(cmc) = 0.705
−
0.966 nc with the coefficient of determination r2 =
0.741. The cmc values decrease with increasing number of carbon atoms. However, it seems that the
ln(cmc) dependence on nc deviates from linearity for substances nc = 8 and 9, consequently, there is
a low value for the coefficient of determination (r2 = 0.741). In this case, the curve could probably
have a quasi-parabolic dependence. This deviation is presumably due to longer alkoxy chain and
the decreased solubility of these substances. They form micelles at low concentrations ranging from
10−5–10−4 M, as compared to homologues having a lower number of carbon atoms in the alkoxy chain,
which form micelles at concentrations over 10−2 M. A large difference between these concentration
values could probably cause the deviation from the linearity. The obtained experimental results
support a model, according to which the quasi-parabolic dependencies of biological potencies of
amphiphilic compounds on the length of their hydrophobic substituent (“cut-off” effect) could be
caused by structural defects in lipid bilayer of the membrane induced by amphiphilic compounds or
by micellization [19
group of the hydrophobic chain from an aqueous phase into the micelle interior at the temperature
of 25 ◦C and pH
4.5–5.0, has, according to Equation (2), the value of 2.39 kJ/mol. For alkyloxy
homologues of local anesthetic heptacainium chloride determined by the ion-selective electrode,
the value of G*(CH2) in water and in the 0.1 M NaCl solution is 1.21 kJ/mol and 1.54 kJ/mol,
,40]. The Gibbs free energy ∆G*(CH2), necessary for the transfer of the methylene
≈
−
∆
−
−
respectively [41]. It is assumed that the differences between these two results might be caused by the
different species of the polar groups of two anesthetics, and by the composition of the system.
Using the Equations (1) and (2), and the experimental value of B = 0.966 given in Figure 6, the free
energy change ∆G*(CH2), involved in the transfer of the methylene group of the hydrophobic chain
from an aqueous phase into the micelle internal, is negative, accordingly favoring micellization. This
accounts also for the fact that the cmc values decrease with increasing length of the hydrophobic group.
Using Equation (1) and the value of A = 0.705 depicted in Figure 6, the free energy change, involved
in the transport of the hydrophilic group from an aqueous surroundings to the micelle external, is
positive, and then goes up against micellization.
As mentioned above, the discussed compounds can be considered as surfactants, i.e., their
biological effect is connected with the increasing surface activity (i.e., a longer tail as R substituent
is preferable). This parameter is reflected in the bulkiness/molar volume (MV [cm3]) of the alkoxy
chain (substituent R). The surface tension (ST [dyne/cm]) of the compound is also connected with
the length of the alkoxy tail. The calculated values of surface tension increase (i.e., surface activity
decreases) with decreasing bulkiness, expressed as molar volume (reflecting a decrease in the length of
the alkoxy tail). The dependence of the cmc values of the discussed compounds on the bulkiness/molar
volume (MV [cm3]) of the alkoxy chain is illustrated in Figure 7. It can be stated that the cmc values of
the compounds linearly decrease (nc = 7, r2 = 0.960) with increasing bulkiness (the length of the tail).
On the other hand, but logically, the cmc values linearly increase (nc = 7, r2 = 0.979) with increasing
surface tension (i.e., with a decrease of surface activity), see Figure 8, where the dependence of the
cmc values on the surface tension of the compounds is illustrated. These linear dependences showed
in Figures 7 and 8 underline the accuracy of the determined cmc values of the discussed compounds
and their usefulness for investigation of compounds of a similar type. Our results showed a close

Molecules 2018, 23, 1064
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relationship between the experimentally determined cmc and surface activity, respectively, biological
activity of the compounds.
Figure 7. Dependence of cmc values on bulkiness alkoxy tail expressed as molar volume (MV [cm3]) of
substituent R.
Figure 8. Dependence of cmc values on surface tension (ST [dyne/cm]) of whole compound.
3. Materials and Methods
3.1. Chemistry
The investigated compounds were prepared according to Scheme 1 by Cizmarik et al. [18,42].
Briefly, the starting material 2-aminophenol with acetanhydride yielded 2-acetaminophenol that, by
condensation with appropriate alkylbromide, gave a series of seven alkoxyacetanilides, the cleavage
of which by an aqueous solution of hydrochloric acid provided the corresponding alkoxyanilines.
By reaction with phosgene in anhydrous toluene, the latter yielded alkoxyphenylisocyanates, that
gave 2-(morpholin-4-yl)ethyl (2-alkoxyphenyl)carbamates by reaction with 2-morpholinoethan-1-ol.
Hydrochloride salts 1–7 (see Scheme 1 and Table 1) of these carbamates were prepared using
an equimolar amount of ethereal hydrogen chloride.

Molecules 2018, 23, 1064
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Finally, studied compounds
1–
7
were recrystallized before investigations. All studied compounds
are white crystalline substances. The structural identity and purity of each of compounds were
verified using high-resolution mass spectrometry, and were measured using a high-performance liquid
chromatograph Dionex UltiMate® 3000 (Thermo Scientific, Waltham, MA, USA) coupled with a LTQ
Orbitrap XLTMHybrid Ion Trap-Orbitrap Fourier Transform Mass Spectrometer (Thermo Scientific,
Waltham, MA, USA) with injection into HESI II in the positive or negative mode. Pyrene ( 99%,
≥
Sigma-Aldrich, Buchs, Switzerland). Experimental assays were performed at laboratory temperature
25 ◦C. The critical micellar concentration (cmc) values were established by methods [16,17].
Scheme 1. Synthesis of 4-(2-{[(2-alkoxyphenyl)carbamoyl]oxy}ethyl)morpholin-4-ium chloride
Reagents and conditions: ( ) (CH3CO)2O, room temperature, 1 h; ( ) Na, EtOH, alkylbromides, reflux,
3 h; ( ) 18% HCl, reflux, 2 h; ( ) COCl2, toluene, reflux, 3 h; ( ) toluene, reflux, 3 h; ( ) HCl, Et2O [10 11].
1–7.
a
b
c
d
e
f
,
3.2. Preparation of Pyrene Stock Solution
All investigated compounds were easily soluble in the water. A store solution of pyrene was
prepared by adding a known weight of the compound in 20 wt % ethanol in water. The mixture was
sonicated in order to prepare a clear solution. Its dilution provided the experimental 2 M solution of
1–7
µ
pyrene, in which a concentration of ethanol was 0.25%. Such a minimal concentration of ethanol was
considered unable to influence the self-aggregation processes and spectral performance of amphiphiles
as well.
3.3. Absorbance Measurements
An absorbance study was performed by UV–vis spectrophotometer Spekol 1300 (Analytic Jena
AG, Jena, Germany) using 10 mm path length quartz cuvette. The spectra were recorded in the 200–400
nm wavelength scope.
3.4. pH Measurements
pH values of studied local anesthetics were measured by inoLab Cond 7110 pH meter (Xylem
Analytics Germany Sales GmbH & Co., KG, WTW, Weilheim, Germany).
4. Conclusions
Critical micellar concentrations within a homologous series of 4-(2-{[(2-alkoxyphenyl)carbamoyl]
oxy}ethyl)morpholin-4-ium chlorides were determined. These compounds were prepared as local
anesthetics, i.e., surfactant-type compounds; therefore, their surface activity characterization is
important for their biological potential. UV absorption of pyrene in surfactant solution has been
an easy method to establish the cmc of surfactant. The values of cmc with the number of carbon atoms
nc = 2, 4, 5, 6, 7, 8 ,and 9 linearly dependent on nc: ln(cmc) = 0.705
change G*(CH2), necessary for the transfer of the methylene group of the hydrophobic chain from
an aqueous phase into the micelle interior, has a value of
2.39 kJ/mol at 25 ◦C. It can be provided
−
0.966 nc. The Gibbs free energy
∆
−

Molecules 2018, 23, 1064
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as a measure of the intensity of the interactions between species in aqueous solutions of surfactant.
The experimentally determined cmc values showed good correlations with the predicted values of the
bulkiness of the alkoxy tail, expressed as the molar volume (MV [cm3]) of substituent R, as well as
with the surface tension (ST [dyne/cm]) of the compounds.
Author Contributions: Lenka Stopková, Jana Gališinová, Zuzana Šuchtová and Fils Andriamainty conceived and
designed the experiments; Lenka Stopková, Jana Gališinová and Zuzana Šuchtová performed the experiments
using the UV–vis spectroscopy method; Lenka Stopková, Jana Gališinová and Fils Andriamainty analysed
ˇ
all measured and calculated data; Jozef Cižmárik contributed studied substances; Lenka Stopková and Fils
Andriamainty wrote the paper; all authors equally contributed to final paper revising.
Acknowledgments: This study was supported by SANOFI-AVENTIS Pharma Slovakia.
Conflicts of Interest: The authors declare no conflict of interest.
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ˇ
Sample Availability: Samples of compounds 1–7 are available from author F. Andriamainty and J. Cižmárik.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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