Head group specificity of novel functionalized surfactants: Synthesis, self-assembly and calcium tolerance

Article abstract of DOI:10.1016/j.tetlet.2014.08.111

The present work describes the synthesis, characterization and application of functionalized surfactants derived through simple organic reaction steps. These surfactants have been particularly tailor made to resist hardness due to calcium ions in water. It is unique of its kind because here the surfactants have an analogous hydrophobic chain but differ structurally in the composition of the head groups in terms of the position of attachment of the chain. The effect of this small variability in the head group on the surfactant property, adsorption, self assembly and calcium tolerance behaviour has been studied in detail. This kind of phenol-keto surfactants has not been reported before. It was also found that one of the surfactants was more tolerant towards Ca²⁺ion than the other. The individual packing behaviour of the surfactants at the air-water interface has been projected to cause this difference which is very interesting.

Full text of DOI:10.1016/j.tetlet.2014.08.111

Tetrahedron Letters 55 (2014) 5925–5931
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Head group specificity of novel functionalized surfactants: synthesis,
self-assembly and calcium tolerance
⇑
Deboleena Sarkar , Ravi Kant Shukla, Vijay Gadgil, Amitava Pramanik
Unilever R&D, 64 Main Road, Whitefield, Bangalore 560066, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The present work describes the synthesis, characterization and application of functionalized surfactants
derived through simple organic reaction steps. These surfactants have been particularly tailor made to
resist hardness due to calcium ions in water. It is unique of its kind because here the surfactants have
an analogous hydrophobic chain but differ structurally in the composition of the head groups in terms
of the position of attachment of the chain. The effect of this small variability in the head group on the
surfactant property, adsorption, self assembly and calcium tolerance behaviour has been studied in detail.
This kind of phenol–keto surfactants has not been reported before. It was also found that one of the sur-
factants was more tolerant towards Ca2+ ion than the other. The individual packing behaviour of the sur-
factants at the air–water interface has been projected to cause this difference which is very interesting.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 18 June 2014
Revised 26 August 2014
Accepted 29 August 2014
Available online 6 September 2014
Keywords:
Surfactant
NMR
CMC
Aggregation number
Surface tension
Ca2+ tolerance
There is currently much interest in the properties of bio surfac-
tants as replacements or partial replacements of conventional pet-
rochemical-derived surfactants in a wide range of detergent-based
products because of the attractions and potential benefits associ-
ated with bio sustainability and biodegradability.1–6 These serve
as an alternative to the common head group of the majority of non-
ionic surfactants, that is, polyoxyethylene chains. Also, anionic sur-
factants, which are major ingredients in many detergent-based
products, are notorious for their relative intolerance to water hard-
ness, and a number of strategies have been developed to minimize
the impact of hard water conditions.7–9
Biosurfactants that act as replacements or partial replacements
of conventional petrochemical derived surfactants are promising
due to their biodegradability, low toxicity and effectiveness in
enhancing biodegradation and solubilization of low solubility com-
pounds. However, greater difficulties in large-scale production and
purification have so far hampered their wider application, and as
such many applications or potential applications involve their
blending with conventional surfactants. Compared to conventional
surfactants, biosurfactants have, in general, lower toxicity, more
biodegradability, can be synthesized from sustainable sources,
and have higher tolerance to pH, temperature and hardness. The
higher tolerance of the biosurfactants to hard water conditions is
a potentially important feature. The problem of hardness occurs
due to the strong binding of multivalent counter ions and espe-
cially calcium, to form eventually insoluble precipitates. Hardness
tolerance can be defined as the minimum concentration of multi-
valent counter ions required to cause precipitation.5,6 Alargova
et al.10 and Petkov et al.11 have recently demonstrated and quanti-
fied how multivalent counterions (Ca2+, Al3+) prior to precipitation
promote micellar growth for anionic surfactants such as sodium
dodecyldiethylene glycol sulfate, sodium lauryl ether sulfate
(SLES). In other cases, such as the addition of Ca2+ to the anionic
surfactant, LAS (linear alkylbenzene sulfonate), micellar growth is
replaced by a transition from micellar to lamellar structures.12
These studies10,11 also highlight one of the frequently used strate-
gies to improve hardness tolerance: the incorporation of a nonionic
cosurfactant.8,9 It has now been demonstrated that, in addition to
the insensitivity of the ethyoxylate head group to electrolyte, the
larger head group of the nonionic cosurfactant provides a steric
hindrance which disrupts and ultimately prevents the formation
of the anionic surfactant–ion complexes which drive the micellar
growth, structural transitions and ultimately precipitation.10–12
Furthermore, it has been demonstrated that the ability to disrupt
that complex formation is directly associated with the size of the
ethylene oxide head group.
Surfactants and phospholipids bearing unsaturated fatty acid
chains are particularly useful in the reconstitution of membrane
proteins in vitro.12 In this respect, certain glycerophospholipids
and surfactants have been synthesized by Bhattacharya et al.13–16
Furthermore, another important application in the field of novel
⇑
Corresponding author. Tel.: +91 080 39831051.
E-mail address: Deboleena.Sarkar@unilever.com (D. Sarkar).
http://dx.doi.org/10.1016/j.tetlet.2014.08.111
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

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D. Sarkar et al. / Tetrahedron Letters 55 (2014) 5925–5931
surfactants with aromatic head groups lies in gene delivery.17–19
Researchers have also shown that multivalent cationic lipids with
an aromatic backbone hold a promise for superior gene transfec-
tion activities.20–24
in case of FS2 (Fig. 1). Increase in fluorescence intensity as well
as extent of blue shift was found to be significantly higher in case
of interaction of C153 with FS2 as compared to FS1 (Fig. 2). This
shows that the probe partitions better inside the micellar environ-
ment of FS2 and experiences a more non-polar environment. It can
therefore be assumed that FS2 is a more non-polar surfactant as
compared to FS1.
A plot of relative fluorescence intensity and concentration leads
us to accurately determine the CMC value of the two surfactants
from the emission studies as shown in Figure 3. The CMC is found
out to be 0.27 mM for FS1 and 0.45 mM for FS2. A slightly lower
value of CMC in case of FS1 compared to FS2 indicates that the
head group indeed plays a role in the micellization process, in spite
of having similar hydrophobic chain length.
This Letter describes the synthesis of two functionalized surfac-
tants. It is unique of its kind because here the surfactants have an
analogous hydrophobic chain but differ structurally in the compo-
sition of the head groups in terms of the position of attachment of
the chain. The structures of the two molecules are given in
Scheme 1. The molecules were chosen such that there will be the
possibility of a chelating complex between the OH group and the
Ca2+ ion at higher (detergent relevant) pH. Thus, these surfactants
have been tailored such that they become calcium tolerant in solu-
tion. Such phenol–keto surfactants which are environmentally
benign and calcium tolerant have not been reported before. The
effect of this variability in the head group on the surfactant prop-
erty, adsorption, self assembly and calcium tolerance behaviour
has been studied in detail. For convenience we have named the
first compound as FS1 and the second compound as FS2.
Micelles are formed at the critical micelle concentration (CMC),
which is detected as an inflection point when physicochemical/
microheterogeneous properties such as surface tension or fluores-
cence intensity ratio are plotted as a function of concentration. In
connection with a general investigation of the physicochemical
properties of FS1 and FS2 we have determined their CMC in water
by two independent methods: fluorescence measurements and
surface tension experiments.
The Gibb’s free energy of micellization was also found from the
following equation:
D
G0mic ¼ RT ln CMC
ð1Þ
This was found to be ꢁ20.36 kJ molꢁ1 for FS1 and ꢁ19.0
9 kJ molꢁ1 for FS2 indicating that the process of micellization is
highly favourable.
A greater lowering of surface tension was found in case of FS2 as
compared to FS1. This difference is expected considering the higher
CMC of the former species. The relationship between surface ten-
sion and the logarithm of the concentration of the surfactants is
shown in Figure 4. The breakpoint allows the ready evaluation of
the CMC for both the surfactants.
The CMC as calculated from surface tension measurement are
found to be 0.33 mM for FS1 and 0.50 mM for FS2. This is in good
agreement with those derived from the fluorescence studies. Also
it is evident that the micellization occurs at lower concentration
for FS1 as compared to FS2.
Fluorescence spectra were taken with stock solutions of both
FS1 and FS2 using a common fluorescent probe coumarin 153
(C153). C153 absorbs at ꢀ440 nm and emits at ꢀ545 nm in pure
aqueous solution. Addition of different concentrations of FS1 and
FS2 leads to a substantial increase in fluorescence intensity with
an appreciable blue shift of ꢀ30 nm in case of FS1 and ꢀ50 nm
Scheme 1. Structures of FS1 and FS2, respectively.
(a)
(b)
FS1
FS2
480
520
560
600
640
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Figure 1. Emission spectra of 5 lM C153 with increasing concentrations of FS1 and FS2. The concentration of both the surfactants varies from 0 mM to 2 mM.

D. Sarkar et al. / Tetrahedron Letters 55 (2014) 5925–5931
5927
550
540
530
520
510
500
490
1000
900
800
700
600
500
400
300
200
100
0
550
540
530
520
510
600
500
400
300
200
100
0
(a)
(b)
0.0
0.5
1.0
1.5
2.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
[FS1] (mM)
[FS2] (mM)
Figure 2. Variation of fluorescence intensity and wavelength of 5
lM C153 with increasing concentrations of FS1 and FS2.
16
(a)
(b)
10
14
CMC = 0.45 mM
12
10
8
8
6
4
2
0
CMC = 0.27 mM
6
4
2
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.5
1.0
1.5
2.0
[FS1] (mM)
[FS2] (mM)
Figure 3. Plot of relative fluorescence intensity vs. concentration of surfactants.
70
65
60
55
50
45
40
70
(a)
(b)
65
CMC = 0.50 mM
CMC = 0.33 mM
60
55
50
45
40
-2.0
-1.5
-1.0
-0.5
0.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
log [FS2]
log [FS1]
Figure 4. Plot of surface tension against log of the surfactants for FS1 and FS2, respectively.
17
The HLB values25–28 represent an empirical numericalcorrelation
of the emulsifying and solubilizing properties of different surface
active agents. Table 1 illustrates the various ranges of HLB values.
Davies29 found that HLB values for surface active agents can be
consistently calculated directly from the chemical formulae, using
group numbers. The group number for our surfactant system was
collected from ref
from which the HLB values were calculated
for FS1 and FS2, using the relation,
X
HLB ¼
ðhydrophilic group numbersÞ
ꢁ nðgroup number per CH2 groupÞ þ 7
ð2Þ

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D. Sarkar et al. / Tetrahedron Letters 55 (2014) 5925–5931
Table 1
with spherical bodies. The ratio of the radius of gyration to that
Classification of emulsifiers according to HLB values
of the hydrodynamic radius (RG/RH) has been evaluated to be close
pffiffiffiffiffiffiffiffiffiffiffiffi
to ð3=5Þ. This is the value for spherical micelles.32 This further
supports the presence of spherical micelles in the system. The plot
of hydrodynamic radius (at angle 90°) versus intensity for the two
surfactants is given in Figure 5.
Range of HLB values
Application
3.5–6
7–9
8–18
13–15
15–18
W/O emulsifier
Wetting agent
O/W emulsifier
Detergent
Furthermore, the second virial coefficient evaluated from the
Zimm plots for an aqueous solution of the surfactants in aqueous
solution was 1.25 ꢂ 103 mol cm3 gꢁ2 for FS1 and 2.12 ꢂ 103 -
mol cm3 gꢁ2 for FS2. The value is positive and close to that usually
found for spherical micelles.32 All these results suggest symmetry
in the micellar structure, pointing to a spherical shape of the
micelles. This is further evidenced by the polydispersity index of
the micellar solutions which has been calculated as 0.107 which
is very low indicating a fairly monodisperse solution.
The plots of surface tension as a function of concentration for
both FS1 and FS2 (Fig. 4a) show an effective reduction in the sur-
face tension. This is consistent with the efficient positive adsorp-
tion at the air/water interface. The lowering of surface tension
with increasing concentration of the surfactants can be explained
in terms of surface activity. It is well known that for surfactants,
the amphiphilic molecules orient at the air/water interface with
the hydrophobic part above the interface and the chains residing
in the aqueous phase. The relative ease with which a surfactant
molecule migrates to the interface depends on the solubility in
the bulk phase.
Solubilization
Here, n is again the number of –CH2– groups in the molecule of
emulsifying agent. It must be emphasized that the HLB system is
not concerned with the stability of the emulsion once formed: it
is only a correlation of function, not of efficacy.27,28 The HLB value
was found out to be ꢀ4 for FS1 and ꢀ3 for FS2. It shows that the
surfactants are essentially lipid soluble surfactants and can act as
water-oil emulsifiers. These surfactants should also have a prop-
erty of acting as anti-foaming agents.29
Static light scattering measurements were employed for the
determination of the aggregation number of the surfactants. Typi-
cal Zimm plots30 of FS1 and FS2 aqueous solution at different con-
centrations above the CMC were used for the estimation of the
molecular weight of the aggregated states in the micellar solutions
as well as the radius of gyration and the second virial coefficient.
The aggregation number was calculated by simply dividing the rel-
ative molar mass of the aggregated states by that of the monomeric
state. It is interesting to see that the aggregation number of FS2 is
higher than that of FS1, though the area/molecule is lesser in the
former. This implies a closer packing of the surfactant molecules
in the air–water interface. The evaluated values are tabulated in
Table 2.
The Gibbs adsorption isotherm was used for the estimation of
the limiting area occupied by one molecule of FS1 and FS2 at the
air/water interface. The simplified Gibbs adsorption equation in
terms of concentration can be expressed as:
In the concentration range studied, angular dissymmetry,
Z60 = R60/R120, for a micellar solution of the surfactants was found
Cex ¼ ꢁð1=RTÞðd
c
=d ln cÞ
ð3Þ
to range ꢀ1.04 for FS1 and ꢀ0.91 for FS2. For spherical micelles
where Cex is the excess concentration of surfactant molecules at the
pffiffiffi
the value is close to unity and for rigid rods the value is 3, that
is, 1.73. It can therefore be concluded that the micelles formed
are typically spherical in nature.31 The calculated radius of gyration
was 33 2 nm for FS1 and 67 2 nm for FS2 which is compatible
interface, that is, the amount of surfactant adsorbed per unit area
(mol/cm2),
c is the surface tension of the solution, c is the concen-
tration of the surfactant in the bulk of the solution, T is the temper-
ature and R is the gas constant.
The slope of the surface tension versus log concentration plot
(Fig. 3b and c) (below the CMC) was used to estimate Cex. The lim-
iting area per molecule (Amin) was then calculated from the
equation
Table 2
Surfactant relevant properties of FS1 and FS2 in aqueous solutions
Surfactant
CMC (mM)
Area (Å2/molecule)
Mw
N
Amin ¼ 1=ðNCex
Þ
ð4Þ
FS1
FS2
ꢀ0.3
ꢀ0.5
117.2
85.7
2.85 ꢂ 104
ꢀ85
ꢀ135
2
5
4.25 ꢂ 104
where N is Avogadro number.
100
100
(a)
(b)
80
60
40
20
0
80
60
40
20
0
0
100
200
300
400
500
0
100
200
300
400
500
Hydrodynamic radius (nm)
Hydrodynamic radius (nm)
Figure 5. Plot of hydrodynamic radius vs. intensity for (a) FS1 and (b) FS2. [FS1] = 0.5 mM and [FS2] = 1 mM.

D. Sarkar et al. / Tetrahedron Letters 55 (2014) 5925–5931
5929
50
45
40
35
30
25
20
15
10
5
The limiting areas calculated for FS1 and FS2 were 117.2 and
85.7 Å2/molecule, respectively. The values match well with the
commercially available spherical surfactants. It is interesting that
the area per molecule at the air–water interface is smaller for
FS2 than for FS1. It seems that more favourable intra- or intermo-
lecular hydrogen bonding leads to better packing of the FS2.
The oil–water interfacial tension was measured using KRUSS
spinning drop tensiometer. The surfactant concentrations were
chosen well above CMC, 1.0 mM for both FS1 and FS2. The oil taken
was n-heptane with a density of 0.684 gm/cm3. The IFT was found
to be 0.5167 mN/m for FS1 and 0.8460 mN/m for FS2. Detergency
of oily soil is a complex kinetic process that includes contributions
from the wash system physical properties, time and temperature
of wash, and the hydrodynamic forces exerted during the wash
process. Generally, surfactant mixtures that exhibit a low oil–
water interfacial tension are considered to provide superior oily
soil detergency. A quantitative understanding of detergency will
be imperative in robust formulation design. There have been
attempts to correlate oily soil removal with easily measurable
physical property of the wash system. In particular, the oil–water
interfacial tension is one property that has been used to correlate
oily soil detergency.33–37 A preliminary look at correlating a com-
plex kinetic process like detergency with a thermodynamic param-
eter like interfacial tension appears to be flawed. The works of
Verma and Kumar38 indicate that a general correlation of oil–water
interfacial tension with oily soil removal does not exist. Detergency
of oily soils is a complex kinetic phenomenon, which can be
described by the mass transfer process, which includes surfactant
adsorption at various interfaces, detachment of oil from the surface
and suspension of these molecules in surfactant micelles, or as
emulsified drops. It is also obvious that the soil removal/suspend-
ing capacity of the wash liquor is defined and intimately related to
various thermodynamic quantities such as the oil–water interfacial
tension. Also, the spinning drop tensiometer is only useful for mea-
surement of the oil–water interface on the hydrophilic side of the
phase inversion temperature for nonionic surfactants. Above this
temperature, the surfactant will be relatively more oleophilic and
will try to form a water-in-oil emulsion in an agitated state. This
implies that the curvature will be concave towards the oil phase.
In the spinning drop tensiometer, this will just cause the oil droplet
to spread along the length giving misleading results for interfacial
tension. Therefore, instead of attempting to predict the detergency
of these surfactants, we can just say that the values are comparable
to the commercially available EO group of surfactants.38 Actual
study with formulations and soil will be required to understand
the detergency of these surfactants which is underway.
FS 1
FS2
0
0
20
40
60
80
100
[Ca2+] (mM)
Figure 6. Turbidimetric plots for interaction of FS1 and FS2 with Ca2+ ions.
[FS1] = 0.5 mM and [FS2] = 1 mM.
the surfactants have high level of Ca(II) tolerance. Up to ꢀ40 mM
for FS1 the turbidity does not change significantly indicating resis-
tance of the surfactants to Ca2+ precipitation. At higher concentra-
tion however, Ca2+ start precipitating leading to higher turbidity of
the solutions as shown in Figure 6. However, it is interesting to see
that FS2 seems to be even more Ca2+ tolerant compared to FS1. The
turbidity of the solutions hardly changes within the experimental
limits of our study. This shows that the effectivity of FS2 as a Ca(II)
tolerant surfactant is highly potential. This is probably happening
because the packing of the FS2 molecules are much better than
FS1 as we found out from our light scattering studies. In other
words, the steric crowding at the carbonyl oxygen which helps in
forming stable complexes with Ca(II) ions, is lesser in case of FS2
as compared to FS1. Perhaps the presence of a long hydrophobic
tail attached with the carbonyl oxygen in case of FS1 leads to effec-
tive binding with Ca2+ ions and better packing of surfactants. This
ultimately results in reduction in the precipitation of Ca2+ ions and
therefore increases the Ca2+ ion tolerance of FS2 compared to FS1.
Surface tensiometry
CaCl2 was added to an aqueous solution of the surfactants at a
concentration much higher than the CMC and the surface tension
was monitored. The results are shown in Figure 7. The surface ten-
sion remained approximately constant up to a CaCl2 concentration
of ꢀ40 mM for FS1, after which it rapidly increased. The strong
increase is most likely due to precipitation of the surfactant as
the calcium salt. Similarly the surface tension of FS2 with increas-
ing Ca(II) concentration was found to remain almost constant
within our experimental range. The results comply with those
obtained from the turbidimetric studies and confirm high toler-
ance of these synthesized surfactants towards Ca(II), especially
for that of FS2.
The effect of calcium ion on the behaviour of the two surfac-
tants was studied to understand the Ca2+ resistivity of the surfac-
tants. This method has been previously exploited by Bordes et al.
and is a very well known method for determining calcium resistiv-
ity of a particular surfactant.39 CaCl2 solutions of increasing con-
centrations were progressively added to
a stock surfactant
solution of FS1 and FS2 well above their CMC concentrations. The
effect of precipitation (of absence of it thereof) was monitored
through studying their physicochemical properties to estimate
the Ca2+ resistivity of the surfactants. This was studied using two
methods: (i) turbidimetry and (ii) surface tensiometry. The results
have been discussed in the following section:
NMR studies
NMR was used to determine quantitatively the amount of sur-
factant remaining in solution as the CaCl2 concentration was
increased. The results, shown in Figure 8, confirm that the FS2 sur-
factant is more calcium tolerant than the two other surfactants.
A tentative explanation for the difference is the following. These
surfactants should be capable of forming intramolecular chelates
with the divalent calcium ion, leading to stable membered rings.
However, the steric crowding for forming this in case of FS1 > FS2,
Turbidimetry
The effect of addition of Ca(II) ions on the precipitation behav-
iour of FS1 and FS2 was studied by monitoring the turbidity of the
solutions with progressively increasing concentration of Ca(II)
ions. The results are given in Figure 6. The results depict that both

5930
D. Sarkar et al. / Tetrahedron Letters 55 (2014) 5925–5931
80
75
70
65
60
55
50
45
40
which is perhaps why this is less favourable for FS1. The intramolec-
ular binding is likely to be less effective in terms of surfactant pack-
ing at the interface than the intermolecular binding where the
calciumionstrigger closer packingof theanionic surfactants. Precip-
itation seems to be favoured by the intramolecular binding 350 of
calcium ions, which is not surprising since calcium soaps of divalent
acids are known to precipitate readily. The intermolecular binding is
likely to be less effective in terms of surfactant packing at the inter-
face than the intramolecular binding where the calcium ions trigger
closer packing of the surfactants. The intramolecular chelating of
calcium ions is assisted by interaction between the carbonyl oxygen
and the cation (Scheme 2). Such interactions have been observed
before for similar chelating agents and divalent cations.27–30
Between FS1 and FS2, we find that FS2 is more effective in terms of
both surfactant packing at the air-water interface as well as its
Ca2+ ion resistant property. This has been explained by the higher
degree of steric crowding at the carbonyl oxygen in case of FS1,
which helps in forming stable complexes with Ca(II) ions, due to
the presence of a long hydrophobic tail attached with the carbonyl
oxygen. This perhaps leads to effective binding with Ca2+ ions and
better packing of surfactants that lead to reduction in the precipita-
tion of Ca2+ ions and therefore increase the Ca2+ ion tolerance of FS2
compared to FS1. Such interactions have been observed before for
similar chelating agents and divalent cations.40–42 Close packing of
all the surfactants may also be favoured by formation of hydrogen
bonds aligned at the interface.38
We also synthesized and extracted two derivatives of FS1 and
FS2, with –OCH3 groups substituted as shown in Scheme 3, naming
it as FS3 and FS4. The surface active properties of both the com-
pounds as well as their Ca2+ tolerant behaviour were characterized.
It was found that FS1 and FS3 had analogous properties while FS2
and FS4 had analogous properties. This conclusively shows that the
difference of property between FS1 and FS2 is mainly because of
the packing behaviour of the head groups at the air–water inter-
face which is governed by the steric crowding at the carbonyl
oxygen.
This Letter describes the synthesis of two unique functionalized
surfactants, FS1 and FS2 having an analogous hydrophobic chain
but differing structurally in the composition of the head groups
in terms of the position of attachment of the chain. The molecules
were chosen such that there will be possibility of a chelating
FS1
FS2
0
20
40
60
80
100
[Ca2+] (mM)
Figure 7. Surface tension plots for interaction of FS1 and FS2 with Ca2+ ions.
[FS1] = 0.5 mM and [FS2] = 1 mM.
1.1
FS1
FS2
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1
10
100
Ratio of CaCl2 : Surfactant
Figure 8. Relative amount of dissolved surfactant as a function of molar ratio of
added CaCl2 to surfactant. [FS1] = 0.5 mM and [FS2] = 1 mM.
Scheme 2. Intramolecular complex between Ca(II) and FS1 and FS2.
Scheme 3. Structures of FS3 and FS4, respectively,

D. Sarkar et al. / Tetrahedron Letters 55 (2014) 5925–5931
5931
complex between the OH group and the Ca2+ ion. These molecules
are therefore tailor-made to suit our purpose and this kind of
phenol–keto surfactants has not been reported here before. The
effect of this variability in the head group on the surfactant prop-
erty, adsorption, self assembly and calcium tolerance behaviour
has been studied in detail. The Ca2+ ion resistivity of both the sur-
factants was monitored and preliminary results have been pre-
sented, which seem to be promising. Comparing the surfactants
it was found that FS2 was more Ca2+ ion tolerant than FS1. The
individual packing behaviour of the surfactants at the air-water
interface has been projected to cause this difference which is inter-
esting. The higher tolerance of these surfactants to hard water con-
ditions poses a potentially important feature and opens a new vista
towards development of further such surfactants.
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