Interfacial properties of novel surfactants based on maleic and succinic acid for potential application in personal care

Article abstract of DOI:10.1016/j.molliq.2021.117484

Hemiesters and Hemiamides of maleic and succinic acid viz. sodium lauryl succinate (C₁₂SE), sodium lauryl maleate (C₁₂ME), sodium lauryl succinamide (C₁₂SA), sodium lauryl maleamide (C₁₂MA), sodium hexadecyl succinate (C₁₆SE) and sodium hexadecyl maleate (C₁₆ME) were synthesized and investigated as surfactants in the pure water and aqueous hydrotrope [sodium p-toluene sulfonate (NaPTs)] solution. The chemical structures of the prepared surfactants were established by FTIR and ¹H NMR spectroscopy. The surface tension measurements depicted low CMC and a high adsorption efficiency (pC₂₀) which is highly beneficial for creating personal care formulations. The dynamic light scattering (DLS) technique indicated the formation of larger micelles which was important for skin care as larger micelles cannot penetrate the skin layer. Moreover, these surfactants depicted good foamability and stability attributed to faster monomer adsorption and small bubble size which was preferred for cleansing application. Additionally, low protein/lipid solubilization by these surfactants indicated its mild behaviour on skin as compared to other commonly used conventional anionic [sodium lauryl sulfate (SLS)], zwitterionic [cocamidopropyl betaine (CAPB)] and nonionic [decyl glucoside (DG)] surfactants. Viscosity measurements suggested decent thickening ability of surfactants in the presence of co-surfactant like lauramine oxide. Basis of all the properties discussed, these novel hemiesters and hemiamides seem promising as surfactants for improving various characteristics in potential personal care formulations.

Full text of DOI:10.1016/j.molliq.2021.117484

Journal of Molecular Liquids 342 (2021) 117484
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Interfacial properties of novel surfactants based on maleic and succinic
acid for potential application in personal care
Devi Sirisha Janni b, Gajendra Rajput a, Niki Pandya a, Gayathri Subramanyam b, Dharmesh Varade a,
⇑
a School of Engineering & Applied Science, Ahmedabad University, GICT Building, Central Campus, Ahmedabad 380 009, India
b Personal Care Division, ITC Limited, ITC Life Science & Technology Centre, Peenya Industrial Area, Phase 1, Bangalore 560 058, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2021
Revised 1 September 2021
Accepted 5 September 2021
Available online 08 September 2021
Hemiesters and Hemiamides of maleic and succinic acid viz. sodium lauryl succinate (C12SE), sodium lau-
ryl maleate (C12ME), sodium lauryl succinamide (C12SA), sodium lauryl maleamide (C12MA), sodium hex-
adecyl succinate (C16SE) and sodium hexadecyl maleate (C16ME) were synthesized and investigated as
surfactants in the pure water and aqueous hydrotrope [sodium p-toluene sulfonate (NaPTs)] solution.
The chemical structures of the prepared surfactants were established by FTIR and 1H NMR spectroscopy.
The surface tension measurements depicted low CMC and a high adsorption efficiency (pC20) which is
highly beneficial for creating personal care formulations. The dynamic light scattering (DLS) technique
indicated the formation of larger micelles which was important for skin care as larger micelles cannot
penetrate the skin layer. Moreover, these surfactants depicted good foamability and stability attributed
to faster monomer adsorption and small bubble size which was preferred for cleansing application.
Additionally, low protein/lipid solubilization by these surfactants indicated its mild behaviour on skin
as compared to other commonly used conventional anionic [sodium lauryl sulfate (SLS)], zwitterionic [co-
camidopropyl betaine (CAPB)] and nonionic [decyl glucoside (DG)] surfactants. Viscosity measurements
suggested decent thickening ability of surfactants in the presence of co-surfactant like lauramine oxide.
Basis of all the properties discussed, these novel hemiesters and hemiamides seem promising as surfac-
tants for improving various characteristics in potential personal care formulations.
Keywords:
Hemiesters
Hemiamides
Interfacial properties
Hydrotropes
Lipid solubilisation
Mild surfactants
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
of finding means to reduce the manufacturing cost for prevailing
surfactants, appeal for ‘greener’ products is the dominant driving
The self-assembly of amphiphilic molecules show an imperative
role in natural and industrial applications [1–5]. The driving force
for self-assembly stems from the interplay between hydrophobic
and hydrophilic interactions of the amphiphile. Depending on the
molecular architecture and solution conditions (presence of salt,
co-surfactant, temperature, etc.), surfactants form numerous
aggregates of well-defined shapes and sizes beyond the critical
micelle concentration [6–10]. Anionic surfactants have gained
much consideration from the scientific community in the past
few years and are expected to further increase owing to their
unique physicochemical properties [11–15]. This compels the
improvement of new anionic surfactants possessing a high surface
activity, excellent adsorptive ability, and is mild to skin by judi-
ciously tuning their molecular structure. Besides the continual task
force for surfactants development.
Numerous different classes of surfactants can be synthesized
from simple amino acid building blocks [16–18]. These surfactants
have been comprehensively explored with respect to dermatolog-
ical properties, surface activity and foaming properties and serves
as a vital asset in personal care formulations. Hemiesters and
hemiamides of maleic acid and succinic acid with different chain
lengths (C8-C16) of the hydrophobic alkyl group were synthesized
by Abele et al. and were used as surfactants in the emulsion poly-
merization of styrene and butyl acrylate [19,20]. However, they did
not report comprehensive interfacial properties of these surfac-
tants owing to their low solubility in pure water. The succinates
have an analogous structure to maleates, but they do not contain
any reactive double bonds. Hence it would be interesting to
explore their comprehensive surface properties.
In this context, we have synthesized hemiesters and hemi-
amides with lauryl chain length and hemiesters of maleic acid
and succinic acid with hexadecyl chain length by adopting the
method reported by Abele et al. [19] and finally neutralizing them
⇑
Corresponding author at: School of Engineering and Applied Science, Ahmed-
abad University, GICT Building, Central Campus, Navrangpura, Ahmedabad 380009,
Gujarat, India.
E-mail address: dharmesh.varade@ahduni.edu.in (D. Varade).
https://doi.org/10.1016/j.molliq.2021.117484
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.

Devi Sirisha Janni, G. Rajput, N. Pandya et al.
Journal of Molecular Liquids 342 (2021) 117484
with aqueous sodium bicarbonate solution. Physicochemical prop-
erties of these surfactants were explored for the first time which
could be highly beneficial for their application in personal care for-
mulations. These surfactants displayed intriguing interfacial prop-
erties but lower water solubility. However, we found that overall
solubility of these surfactant could be increased in the existence
of hydrotropes (sodium p-toluene sulfonate; NaPTs) which were
structurally similar to surfactants (but possess a weaker hydropho-
bic character) and have many practical applications [21,22]. On the
basis of various theoretical and experimental efforts, it is widely
accepted that there is a formation of a complex between hydro-
tropes and additives, which would then show a higher aqueous
solubility.
This paper will encompass fundamental information relating to
synthesis, basic characteristics, and the physicochemical proper-
ties of novel surfactants based on maleic and succinic acid like
sodium lauryl succinate (C12SE), sodium lauryl maleate (C12ME),
sodium lauryl succinamide (C12SA), sodium lauryl maleamide
(C12MA), sodium hexadecyl succinate (C16SE) and sodium hexade-
cyl maleate (C16ME) in pure water and aqueous NaPTs solution.
The chemical structures of the prepared surfactants were estab-
lished by FTIR and 1H NMR spectroscopy. Measurements of critical
micelle concentration (CMC) made by surface tension and size dis-
tribution of the micelles was derived using dynamic light scatter-
ing (DLS) techniques. In addition, the foaming behavior was
investigated and the protein/lipid solubilization by surfactants
was examined. Viscosity measurements were performed by using
Brookfield viscometer.
cylinder by vigorous shaking for 60 s. Sample taken in each cylin-
der was 10 mL; 0.3 wt%. The foamability was determined by noting
the foam height immediately after shaking and images were taken
at 27 1 °C. Foam stability was determined by observing the
change in foam height with respect to time. Foamability and foam
stability were also evaluated using Ross Miles method [23] with
these surfactants (250 mL; 10 mM). Initially column was rinsed
with double distilled water, then walls of column was rinsed with
surfactant solution till 50 mL surfactant solution was stored at bot-
tom and remaining approximately 200 mL of surfactant solution
was filled into a pipette. That pipette was immediately placed at
top of column and the stopcock of pipette was slowly released.
When all the solution had run out of the pipette, stopwatch was
started to take readings of the foam height at initial time (foama-
bility) and again reading of the foam height was recorded after
5 min which helps to determine the foam stability.
2.2.2. Protein solubilisation
Surfactants diminish the barrier properties of the skin proteins
leading to their swelling and denaturation. The tendency of surfac-
tants to interact with model proteins (e.g., Zein – one of the best
understood plant protein) has been concomitant with their harsh-
ness toward human skin. Thus, higher the tendency of a surfactant
to interact (higher the solubilization of Zein protein); higher is its
inclination to irritate human skin. Protein (Zein) solubilization by
as-synthesized surfactants was determined by gravimetric analy-
sis. Surfactant solutions were taken in a vial and approximately
15 wt% Zein powder was added to it (excess of Zein). Now, the mix-
ture of Surfactant-Zein was kept for continuous shaking for about
24 h. After that, the mixture of surfactant-Zein was filtered using
Whatman filter paper. In the end, solid collected was dried at
80 °C for 24 h. From the weight of insoluble Zein after 24 h of dry-
ing, the wt% of solubility of the protein in surfactant was calcu-
lated. This experiment can clearly give us the amount of Zein
protein solubilized by each surfactant at particular concentration.
From this data we can compare the mildness property (harshness)
of the surfactants for using them in personal care formulations.
2. Experimental section
2.1. Materials
Surfactants used in the study like sodium lauryl succinate,
sodium lauryl maleate, sodium lauryl succinamide, sodium lauryl
maleamide, sodium hexadecyl succinate and sodium hexadecyl
maleate were synthesized (were pure as confirmed by NMR mea-
surements in supporting information) in the laboratory. Sodium
p-toluene sulfonate (>98%, Sigma), Zein protein (>98%, Sigma)
and stearic acid (>98%, TCI) were used as received. All experiments
were carried out using Milli-Q water. Samples were allowed to
equilibrate for at least 24 h before measurement.
2.2.3. Lipid solubilisation
Lipid (stearic acid) solubilization was determined by gravimet-
ric analysis. Surfactant solutions were taken in a vial and approxi-
mately 1 wt% stearic acid powder was added to it (excess of stearic
acid). Now, the mixture of surfactant-stearic acid was kept for con-
tinuous shaking for about 24 h. After that, the mixture of
surfactant-stearic acid was filtered using Whatman filter paper.
In the end, solid collected was dried at 60 °C for 24 h. From the
weight of insoluble stearic acid after 24 h of drying, the wt% of sol-
ubility of the lipid in surfactant was calculated.
2.2. Methods
2.2.1. Surface tension
Surface tension of the various concentration of surfactant was
determined using a Du Nouy tensiometer (Type: K20, KRÜSS, Ger-
many) using a platinum plate method. Each experiment was con-
ducted by using 20 mL of the sample in a 50 mL beaker. In this
method, the liquid to be examined was taken in a vessel whose
position can be changed by moving it up and down with the help
of a screw. A platinum plate was positioned at the top of the vessel.
During the process, distance between the liquid surface and
immersed plate was kept approximately 3 mm to ensure the plate
was just below the liquid surface. The pre-programmed software in
the tensiometer measures the surface tension of the surfactant
solutions. The tensiometer was calibrated using deionized Milli-Q
water and surface tension values (71.8 0.1 mNmꢀ1) at 27 1 °C
agreed well with the literature. After each reading, the plate was
washed thoroughly by double distilled water and then heated in
burner flame. The readings were taken in triplicate to ensure the
reproducibility and accuracy of measurements.
2.2.4. Viscosity measurements
Viscosity measurements were performed with Brookfield vis-
cometer (DV 2T) with Spindle No. 15 for 1 min. Required amounts
of samples in vials were homogenized and kept at 27 1 °C for at
least 24 h to ensure equilibration before performing viscosity mea-
surements. Reproducibility (triplicate) was checked for the sam-
ples and no significant differences were observed.
2.2.5. Micellar size and zeta potential measurements
Micellar size and zeta potential was determined using Dynamic
Light Scattering (Zetasizer Nano-ZS, ZEN3600) Malvern instru-
ments Ltd. The surfactant solutions of different concentrations
(1 mM, 5 mM and 10 mM) were taken directly into the quartz cell.
The temperatures of the measurements were controlled around
27 1 °C.
Foaming Behavior: Foaming experiment was performed using a
simple handshake method in 100 mL self-standing measuring
2

Devi Sirisha Janni, G. Rajput, N. Pandya et al.
Journal of Molecular Liquids 342 (2021) 117484
2.2.6. Microstructure of foams
3.5. Synthesis of sodium lauryl succinamide: (C12SA)
At first, foam was generated by using the hand shaking method.
Then the microstructure of foam bubbles was observed with the
Olympus STM7CB Digital Microscope.
3. Results & discussion
3.1. Synthesis of novel surfactants
Synthesis of surfactants was started with maleic or succinic
anhydride as starting materials. Opening of these cyclic anhydrides
either with long chain alcohols or amines result in the formation of
hemiesters or hemiamides [19,20]. Later, the carboxylic acid group
was converted to respective sodium salts by treating with sodium
bicarbonate to get required surfactants of interest. Synthesized
surfactants were pure, as per the NMR recorded for the intermedi-
ates; hemiester carboxylic acids and hemiamide carboxylic acids.
They were converted into corresponding carboxylates by using
exactly one equivalent of sodium bicarbonate (NaHCO3) to get a
clean product without any unwanted side products or unreacted
reactants and reagents. Also the melting point determination for
intermediates; hemiester carboxylic acids and hemiamide car-
boxylic acids were in line with that reported by Abele et al [19].
The schematic representation of the synthesis of each surfactant
was presented below, while the experimental details pertaining
to synthetic procedures and analytical data of surfactants were
designated in the supporting information.
3.6. Synthesis of sodium hexadecyl maleate: (C16ME)
3.7. Synthesis of sodium hexadecyl succinate: (C16SE)
4. Adsorption at the air–liquid interface
Surface tension (c) measurements were executed to explore and
estimate the surface active properties of as-synthesized surfactants
in pure water. Longer alkyl chain surfactants were insoluble in
water because of their high hydrophobicity. The equilibrium sur-
face tension versus concentration plot obtained for sodium lauryl
succinate (C12SE) and sodium lauryl succinamide (C12SA) were
depicted in Fig. 1a, and for sodium lauryl maleate (C12ME) and
sodium lauryl maleamide (C12MA) was presented in Fig. 1b. The
measured surface tension isotherms for different concentrations
do show a characteristic decline in surface tension with increase
in surfactant concentrations and then reaches a plateau region,
beyond which a practically constant value for surface tension is
observed. The critical micelle concentration (CMC) values for each
of these surfactants were acquired by analyzing the intersection
point of the plots. Two essential factors, the efficacy of surface ten-
3.2. Synthesis of sodium lauryl maleate: (C12ME)
3.3. Synthesis of sodium lauryl succinate: (C12SE)
3.4. Synthesis of sodium lauryl maleamide: (C12MA)
sion decrease (pCMC) and the adsorption competence (pC20), can
also be inferred from the measured surface tension data using
the following Equations (1) and (2) respectively [24]. Typical struc-
tures, CMC values, pCMC and pC20 for all the individual surfactants
were listed in Table 1.
pCMC
¼
c0
ꢀ
cCMC
ð1Þ
pC20 ¼ ꢀlogC20
ð2Þ
where c0 and cCMC was the surface tension of pure water and sur-
face tension of surfactant solution at CMC. While C20 was the con-
centration of surfactant required to reduce the surface tension of
water (i.e. 71.8 mN/m) by 20.0 mN/m.
It can be concluded from Table 1 as well as Fig. 1a and 1b that
CMC values of these surfactants were in accordance with the struc-
tural characteristics, i.e., C12SE and C12ME display slower adsorp-
tion rate (less surface active) at the surface and higher CMC
values compared to C12SA and C12MA with identical hydrocarbon
chain length respectively. The presence of amide group in C12SA
and C12MA attributes to lower CMC as it was prone to H-bonding
[25]. The solubility of surfactants in water was basically directed
3

Devi Sirisha Janni, G. Rajput, N. Pandya et al.
Journal of Molecular Liquids 342 (2021) 117484
almost insoluble in aqueous medium and hence the CMC values
were not reported here.
We have observed that solubility of C16SE and C16ME (which is
practically insoluble in pure water) could be enhanced in the pres-
ence of small amount (1 wt%) of hydrotropes such as sodium p-
toluene sulfonate (NaPTs). We had tried several hydrotropes like
NaS, Sodium salicylate; NaPTs, Sodium p-toluene sulphonate;
and NaXS, Sodium xylene sulphonate. However we found that
NaPTs worked well and also it is widely utilized in personal care
formulations. This allows to explore the interfacial behavior of
C16SE and C16ME in NaPTs solution at room temperature and their
CMC values are listed in Table 1. Fig. 1c and d show the surface
tension curves for C12SE and C12ME solutions in the absence and
presence of NaPTs. Since the hydrotropes are themselves weakly
surface active were capable of cooperatively forming aggregates
in the presence of other surfactants. The CMC values of C12ME
and C16ME agrees well with the reported data [19].
As indicated in Fig. 1c and d, presence of NaPTs considerably
lower the surface tension of C12SE and C12ME signifying increased
surface activity of the mixed system owing to favorable interac-
tions and allowing the micelle formation at much lower concentra-
tion (decrease in CMC) as compared to pure water. However, there
was no appreciable change in the final limiting value of the surface
tension. Furthermore smaller value of pC20 for C12SA and C12MA
indicates that it can more competently adsorbed at the interface
and decreases surface tension. The CMC values obtained for all
the surfactants in the presence and absence of NaPTs obtained
from the distinct break in the surface tension–log concentration
Fig. 1. Equilibrium surface tension versus concentration curve in pure water of (a)
C
12SE, C12SA (b) C12ME, C12MA and (c) C12SE (d) C12ME in pure water and 1 wt%
NaPTs solution.
by the balance amongst the hydrophobic and hydrophilic groups
and also on the ability of the molecules to pack into organized
structures. Sodium hexadecyl succinate (C16SE) and sodium hex-
adecyl maleate (C16ME) with longer hydrocarbon chains were
Table 1
Critical micelle concentration (CMC) of surfactants in pure water and 1 wt% NaPTs at 27 °C. * indicates CMC values from Ref. [19].
Surfactant
Structure
CMC, mM
Water
0.50
cCMC
pC20
pCMC
NaPTs
0.40
NaPTs
27.3
Sodium lauryl succinate (C12SE)
0.022
44.7
O
ONa
O
C
12H25
O
O
Sodium lauryl maleate (C12ME)
0.40
0.15
0.27 (0.32)*
28.1
38
0.018
0.014
43.9
34
ONa
O
C12H25
O
O
Sodium lauryl succinamide (C12SA)
0.14
ONa
NH
C
12H25
O
O
Sodium lauryl maleamide (C12MA)
Sodium hexadecyl succinate (C16SE)
0.37
0.27
0.14
29
0.016
0.024
43
ONa
NH
C12H25
O
O
Insoluble
32.2
39.8
ONa
O
C16H33
O
O
Sodium hexadecyl maleate (C16ME)
Insoluble
0.22 (0.23)*
30.9
0.014
41.1
ONa
O
C
16H33
O
4

Devi Sirisha Janni, G. Rajput, N. Pandya et al.
Journal of Molecular Liquids 342 (2021) 117484
plots were also recorded in Table 1. Additionally, we have also
observed that use of potassium or organic counterions such as Tri-
ethanolamine (TEA) can also increase the overall solubility of syn-
thesized surfactants to some extent (data not shown).
6. Foaming behavior
Foam creating power of the liquid or foamability was referred to
the capability of the surfactants to accomplish low surface tension
in a short time when a fresh interface was created. The foam stabil-
ity of a surfactant solution was defined as the alteration in foam
volume which was primarily controlled by the drain speed and
intensity of the interfacial film. Foam construction and perma-
nency were imperative in numerous applications [26–28]. The sur-
factant molecules adsorb to the air/water frontier, allowing the
formation of stable foams. While low surface tension was more
advantageous for foam creation, several factors play decisive role
in the stability of aqueous foams once it is created, for e.g., interfa-
cial tension, bubble size, film rigidity and drainage.
Fig. 2(a) depicts representative plot for foaming behavior
observed by Ross Miles method for each surfactant (10 mM;
250 mL) in the presence of 1 wt% NaPTs. Foaming behavior of all
the surfactants shown in Fig. 2 was studied in the presence of
NaPTs since C16SE and C16ME were practically insoluble in pure
water it and would not be possible to study their foaming behavior
in water. All the studied surfactants exhibited good foaming capac-
ity within the tested concentration range. However, relatively
speaking, the foamability of lauryl chain surfactant C12SE, C12SA,
5. Micellar size and charge
Micellar size and micellar charge was evaluated for each surfac-
tant based on dynamic light scattering (DLS) and zeta potential
measurements at CMC in pure water and NaPTs solution (1 wt%).
The obtained results were reported in Table 2. We have also
included widely utilized conventional surfactants in personal care
like anionic sodium lauryl sulfate (SLS), zwitterionic cocamido-
propyl betaine (CAPB) and nonionic decyl glucoside (DG) for
comparison.
The results from DLS clearly indicates that micellar size of syn-
thesized surfactants was much larger both in pure water and in the
presence of aqueous NaPTs which could be attributed to the forma-
tion of some kind of aggregated structure. However, micellar sizes
of SLS, CAPB and DG was much smaller than synthesized surfac-
tants measured at CMC. This clearly indicates different aggregation
behavior of synthesized surfactants compared to conventional sur-
factants. Furthermore, C12SE and C12ME displays bigger micellar
size and micellar charge at similar concentrations compared to
same hydrocarbon chain length C12SA and C12MA respectively. Big-
ger micelles were preferred for skin care formulations as they can-
not penetrate the skin easily.
C
C
12ME and C12MA was slightly better than that of hexadecyl chain
16SE and C16ME. The average foaming volumes (foam height) and
half-lives of each surfactant exceed 150 mL and 30 min, respec-
tively, at a defined concentration. It has also been inferred from
surface tension measurements that each of these surfactants have
the ability to lower surface tension of water to as low as 30 mN/m
Table 2
Micellar size and zeta potential for surfactants determined at CMC in pure water and NaPTs solution at 27 °C.
Surfactant
Water
NaPTs
Size (nm)
Zeta potential (mv)
Size (nm)
Zeta potential (mv)
C
C
C
C
C
C
12SE
105
127.5
67.6
32.6
–
–
9.6
15.8
19.4
ꢀ45.6
ꢀ60.4
ꢀ13.2
ꢀ36.1
–
109.3
130.1
72.1
37.2
75.6
98.2
11.6
17.8
22.1
ꢀ50.3
ꢀ63.6
ꢀ20.1
ꢀ40.3
ꢀ60.2
ꢀ66.4
ꢀ61.4
ꢀ3.4
12ME
12SA
12MA
16SE
16ME
–
SLS
CAPB
DG
ꢀ57.6
ꢀ1.4
ꢀ19.5
ꢀ21.8
Fig. 2. (a) shows the plot of foam height of each surfactant (10 mM) and (b) shows the representative photograph of foaming behaviour C12SE in the presence of 1 wt% NaPTs
at 27 °C.
5

Devi Sirisha Janni, G. Rajput, N. Pandya et al.
Journal of Molecular Liquids 342 (2021) 117484
solutions, which in many cases are non-Newtonian. Viscoelastic
properties have been reported in anionic and mixed-surfactant
systems comprising of cationic–anionic and anionic–zwitterionic
surfactants [29,30]. Because of the electrostatic interactions
between anionic surfactant headgroups, the growth of micelles in
the solution is inadequate. But with inclusion of salts, cosurfac-
tants or other additives to the solution, electrostatic repulsive
forces in an ionic micelle are reduced, thus promoting micelle
growth. We have studied the viscosity behavior for C12SE and
C12ME in the presence of lauramine oxide. Also, we have used
SLS and DG for comparison under similar conditions.
Fig. 4 illustrates the plot of viscosity of surfactants like C12SE,
C12ME, SLS, and DG in the presence of varying concentrations (as
active %) of lauramine oxide (LAO) which is an amine oxide based
nonionic surfactant, with a C12 (dodecyl) alkyl tail. Lauramine
oxide is a common co-surfactant used often as foam booster in
consumer products. Pure lauramine oxide showed negligible
change in viscosity with concentration as indicated by dash line.
We have found that C12SE or C12ME solution (5 wt%) in the pres-
ence of lauramine oxide at pH 5.5 could result in formation of long
flexible micelles imparting high viscosity to the sample. This can be
explained by assuming that the repulsion between the charged
anionics C12SE or C12ME is shielded by the presence of the lau-
ramine oxide in the mixed micelle. We adjusted the pH to 5.5
(by using citric acid) to achieve the desired viscosity for that partic-
ular binary surfactant composition as lauramine oxide being zwit-
terionic exhibits variation with change in pH. Moreover, Fig. 4 also
reflects that SLS shows slightly higher viscosity than as-
synthesized surfactants, while DG shows almost negligible viscos-
ity change in the presence of lauramine oxide. Viscosity results for
as-synthesized surfactants clearly specify their good prospective
with decent thickening ability in the presence of the co-
surfactant like lauramine oxide. It can be seen from Figure that
Fig. 3. Optical microscopy images of foam generated from 10 mM surfactants
solution (a) C12SE, (b) SLS, (c) CAPB and (d) DG in the presence of 1 wt% NaPTs
solution at 27 °C. Inset shows the photo of the foam for the corresponding
surfactants.
which in turn can be good at stabilizing the initial foam created
during the lathering phase. The foam once created also stays intact
for few hours depicting the reasonable foam stability required for
many applications. The foamability and foam stability measure-
ments from the Ross Miles methods were also supported by hand
shaking methods shown in photo in Fig. 2(b) for C12SE.
We had also generated foams using solution (10 mM) of surfac-
tant C12SE, SLS, CAPB and DG in 1 wt% NaPTs and observed under
the microscope to understand the bubble size and foam structures
as depicted in Fig. 3. Foam generated from SLS and CAPB contained
higher liquid fraction and large bubble sizes. While DG and C12SE
produces foam with low liquid fraction and much smaller bubble
sizes. This observations indicate that synthesized surfactants could
serve as a good foaming agents for several applications.
C12SE and C12ME exhibit moderate viscosity build-up with LAO
performing better than non-ionics though not as good as SLS.
In C12SE-lauramine oxide or C12ME-lauramine oxide mixtures at
lower total surfactant concentration (below 8 wt%) the viscosity
remains low. While, sphere-to-rod micellar transition was
observed in both C12SE and C12ME at higher surfactant concentra-
tion in the presence of lauramine oxide as evidenced by the inset
photo in Fig. 4 which depicts the air bubbles trapped in the viscous
solution. This brings about a sharp increase in the solution viscos-
ity (>500 cP) attributed to the ability of lauramine oxide to reduce
the average cross sectional area of the C12SE or C12ME and SLS
therefore induce one-dimensional micellar growth leading to the
formation of the elongated micelles. The addition of lauramine
oxide to C12SE or C12ME also causes an escalation in the bulk vis-
cosity by enhancing the packing at the air–liquid interface. At rest,
the micelles were arranged randomly, which leads to a very high
viscosity and clear solution as shown in inset photo. With increas-
ing shear, the micelles get oriented parallel – and hence the viscos-
ity decreases. This process was reversible: with decreasing shear,
7. Viscosity measurements
Measurement of the rheological properties is one of the domi-
nant tools to characterize the properties of different surfactant
the micelles were arranged randomly again
– the viscosity
increases. The entire viscosity spectrum represented in figure gives
us very clear understanding of the variation in viscosity with sur-
factants content. This helps us to choose the content of mixed sur-
factants with anticipated viscosity range for designing the
formulations.
8. Skin mildness of surfactants
Fig. 4. Viscosity measurements performed for various surfactants (5 wt%) in the
presence of lauramine oxide (as active %) at 27 °C and pH 5.5. Dash line was only for
pure lauramine oxide without any other surfactant. Inset photo showed the
micellar transition from sphere-rod as indicated by trapping of the air bubbles for
Stratum corneum (SC), the uppermost cover of the epidermis,
affords a vital barrier function in intact skin. Surfactants reduce
the barrier properties of the SC proteins leading to their swelling
and denaturation, though the molecular mechanisms involved
C
12SE.
6

Devi Sirisha Janni, G. Rajput, N. Pandya et al.
Journal of Molecular Liquids 342 (2021) 117484
‘mildness’ with respect to skin, a property desired for use in topical
preparations. Taken together, these surfactants had many of the
characteristics that were desirable for industrial cleansing applica-
tions and hence was probable that this class of surfactants may in
future become a part of formulators’ tool-kit.
CRediT authorship contribution statement
Devi Sirisha Janni: Conceptualization, Methodology. Gajendra
Rajput: Visualization, Investigation. Niki Pandya: Data curation.
Gayathri Subramanyam: Supervision, Project administration.
Dharmesh Varade: Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Fig. 5. Plot displaying protein and lipid dissolution by various surfactants (0.3 wt%)
at 27 °C.
Acknowledgements
have not yet been entirely revealed [31]. The propensity of surfac-
tants to interact with model proteins has also been associated with
their severity toward human skin. Thus, higher the tendency of a
surfactant to interact; higher was its inclination to irritate human
skin. We have performed the dissolution study of Zein protein and
stearic acid as representative for protein and lipid by as-
synthesized surfactants (0.3 wt%) and the results are presented in
Fig. 5. We have also made comparisons with standard sodium lau-
ryl sulphate (SLS) which is known to interact strongly with Zein
protein and was considered as very harsh to the skin [32]. As can
be seen, the tendency of as-synthesized surfactants to dissolve Zein
protein and stearic acid was very low (0.15–0.18 wt%) as compared
to SLS (0.7 wt%) indicating ‘‘mildness” of these surfactants w.r.t
topical/ skin cleansing applications.
This work was supported by Seed Grant from Ahmedabad
University and UGC-DAE (CRS-M-304).
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molliq.2021.117484.
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