Indian Journal of Chemical Technology
Vol. 11, November 2004, pp. 783-786
Turbidity studies on mixed surfactant systems in hard water: A new method for
estimation of water hardness
Naorem Homendra* & Ch. Indira Devi
Department of Chemistry, Manipur University, Canchipur, Imphal 795 003, India
Received 17 September 2003; revised received 10 May 2004; accepted 5 July 2004
Water hardness tolerance of anionic surfactants, viz. sodium dodecyl sulphate (SDS) and linear alkyl benzene sulphonate – sodium salt (LABS) has been investigated with the help of a digital Nephlo-turbidity meter. Effect of incorporation of
small amount of a nonionic surfactant, Triton X-100 (TX-100), on the water hardness tolerance i.e. the lime soap dispersing
ability (LSDA), of LABS has also been studied. It was observed that LABS generally exhibited superior LSDA than SDS
and that incorporation of a small amount of TX-100 led to significant improvement of the LSDA of the anionic surfactants
studied. In fact, incorporation of TX-100 beyond 10% concentration level, no turbidity was observed up to 300ppm hardness
of water. The small angle X-ray scattering profile of the systems revealed that, at the maximum turbidity point, the anionic
micelles were destroyed by the hardness causing ions. It was also observed that at certain range of the surfactant concentration in the pre-micellar region, the turbidity was found to vary linearly with the degree of hardness (ppm) of water. This linear relationship between the turbidity and the hardness can be effectively used for the estimation of the hardness causing ion
with good reproducibility.
IPC Code: C11D 1/02
Keywords: Turbidity, mixed surfactant systems, water hardness, lime soap
Aqueous solution of anionic surfactants generally imparts a turbid appearance in hard water containing
divalent cations such as calcium or magnesium ions
due to the formation of calcium or magnesium salt of
the anionic surfactant, known as lime soap1,2. The
phenomenon of precipitation of the anionic surfactant
as lime soap is of considerable interest from academic
as well as applied viewpoints since it is known to potentially limit the usefulness, especially, the efficacy
of the anionic surfactants in detergency applications
in hard water1-6. This phenomenon, however, has been
effectively employed in the recovery of surfactants
from surfactant based separation process by precipitating the surfactant in presence of multivalent cations7,8. Addition of electrolytes in surfactant solution
is known to cause a compression of the electric double layer, which leads to reduction in the mutual repulsion of similarly charged particles. This would, in
turn, result into increased adsorption and surface activity of anionic surfactant and consequently a decrease in the critical micelle concentration (cmc) of
the surfactant solution4,9,10. Presence of calcium or
magnesium ions in anionic surfactant solution would,
therefore, be expected to enhance the adsorption of
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*For correspondence (E-mail: [email protected] )
the surfactant. In view of the fact that calcium ions
compress the electrical double layer more effectively
than magnesium ions, negatively affecting the repulsion between the soil particles and the fibre, and that
the calcium salt of the surfactant is less soluble than
its magnesium salt, the presence of calcium ion not
only causes a more turbid solution but also causes
more deleterious effects in detergency as compared to
the magnesium ions5,9,11. The precipitated calcium or
magnesium salt of the anionic surfactant may be dispersed either at increased surfactant concentration or
by adding a good lime soap-dispersing agent or, simply, a nonionic surfactant 6,11. Detergency performance of pure anionic surfactants such as linear alkyl
benzene sulphonate-sodium salt (LABS), sodium dodecyl sulphate (SDS), etc. or mixed with a nonionic
surfactant such as Triton X-100 (TX-100) or nonyl
phenyl ethoxylate (NPE) in hard water has been investigated employing different techniques3,6,9,12,13.
Chang et al.14 reported that the cmc of (nonionic +
ionic) mixed surfactant systems containing an anionic,
a nonionic in pure water and also in presence of added
electrolytes, as determined from turbidity measurements, compared well with the results obtained from
other more elaborate experimental methods. Tulsani
et al.15 also reported a visually evaluable arsenazo test
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INDIAN J. CHEM. TECHNOL., NOVEMBER 2004
strip technique for qualitative and semi-quantitative
estimation of water hardness. A perusal of the literature, however, reveals that no attempt has been made
to study the precipitation of anionic surfactants as
lime soap in pure or mixed systems using a turbidity
meter. In view of this, it was considered worthwhile
to systematically investigate the turbidity behaviour
of pure and mixed surfactant systems in hard water. In
this paper, the results of the investigation on the turbidity behaviour of LABS or (LABS + TX-100)
mixed systems in hard waters of varying degrees of
hardness are reported. A new method for the estimation of hardness of water in presence of an anionic
surfactant based on turbidity measurement is also reported.
Experimental Procedure
Materials
The sample of LABS was obtained from Sigma.
Sodium dodecyl sulphate (SDS) with purity greater
than 99% (Loba Chemie, Bombay, India) was recrystalised. The nonionic surfactant Triton X-100,
procured from Sigma was used as received without
any further treatment. The solution of the anionic surfactants was standardized against standard solution of
benzethonium chloride (Sigma) using methylene blue
as indicator.
Methods
Turbidity of the solutions was measured using a
digital nephlo-turbidity meter (Systronics Model –
132). The principal operation of this instrument is
based on the Tyndall effect (more the amount of scattered light in the test solution, more is the turbidity),
which is measured in nephlo turbidity unit (NTU).
The instrument was calibrated with the help of standard formazin solution. Formazin was prepared in the
laboratory following standard procedures: 1.25g of
hydrazine sulphate and 12.5g of hexamethylene
tetramine were dissolved separately in 100 mL distilled water. The two solutions were then mixed and
the final volume made up to 250mL with distilled water. The solutions were shaken well and then allowed
to remain as such for 48 h at room temperature. This
gives a stock solution of 4000 NTU formazin solution. Standard solutions of 1000, 500, 100 and 50
NTU were prepared from the stock solution by dilution method. The turbidity of the solutions thus prepared was reproducible within ±1% and stable for 6-8
weeks. All the measurements were made at 25±1°C.
The sample of the hard water was
Fig. 1—Precipitation boundary for pure SDS and LABS at different water hardness levels as CaCO3 ppm (■- SDS, ●- LABS)
prepared by dissolving calculated amount of calcium
chloride and magnesium chloride in double distilled
water. The hardness of the water thus prepared was
estimated by standard EDTA titration16. Double distilled and de-ionized water was used throughout the
studies.
Results and Discussion
At room temperature all the anionic surfactants
gave clear solutions in pure water. However, at and
below 17°C within a range of concentration SDS solution in pure water gave a turbid appearance; with a
maximum turbidity at around 6.03 mM (cmc of SDS
at this temperature is ~ 6.5 mM). The turbidity
boundary or the phase diagram of LABS and SDS in
hard water is presented in Fig. 1. It is evident from
Fig. 1 that the precipitation boundary (the region
where the solution remained turbid) increased with
the water hardness, which corroborates with the fact
that the loss of the anionic surfactant as lime soap becomes significantly large at higher ppm of hard water3,6,9,12. It was also found, that, the hardness tolerance of LABS was generally superior to that of SDS.
The plot of turbidity against the logarithmic (log)
concentration of LABS in 20, 50, 100 and 150 ppm
hard water is presented in Fig. 2. It was observed that
turbidity when plotted against log concentration of the
surfactant gave a better break point. In view thereof,
turbidity is reported as a function of the log of the
concentration. It is evident from Fig. 2, that, the
breaks in the turbidity-log (concentration) curves are
rather broad at low ppm water but the broadness decreased significantly with increase in hardness and at
higher surfactant concentration indicating that the
HOMENDRA & INDIRA DEVI: TURBIDITY STUDIES ON MIXED SURFACTANT SYSTEMS IN HARD WATER
785
Fig. 2—Plot of turbidity against logarithmic concentration of pure
LABS at different water hardness level as CaCO3 ppm at 298 K
(■- 20 ppm, ●- 50 ppm, ▲- 100 ppm, ○- 150 ppm)
LABS is more effective as dispersing agent at higher
concentration. The turbidity of the solution was
greatly reduced by incorporation of small amount of
the nonionic surfactant, TX-100. The turbidity behaviour for the mixed systems containing 5 and 10% TX100 in hard water are shown in Fig. 3. A comparison
of Fig. 2 with Fig. 3 revealed, that, incorporation of
small amount TX-100 led to significant improvement
of the hardness tolerance of LABS3,12. It is also observed, that, the break point in the turbidity was much
sharper in the mixed systems as compared to the pure
LABS systems. This, perhaps, is due to the fact that
the lime soap dispersing ability of the mixed system is
much superior to that of pure LABS. No turbidity,
however, was observed even up to 300 ppm hardness
of water beyond 10% level of incorporation of TX100. All the turbidity versus concentration plots were
characterized by a point of maximum turbidity i.e. a
break point from where the solution became clear
with increased concentration of the surfactant. The
plot of the break points against ppm hardness for pure
and mixed systems exhibited a qualitative similarity
with those of the cmc versus ppm hardness curves17.
From the break points one can estimate the minimum
concentration level of a surfactant or a mixed surfactant system required for optimum lime soap dispersing properties. A profile of the Small Angle X-ray
Scattering (SAXS) of the pure and the mixed systems
in presence of hardness ions is shown in Figs 4 and 5.
The figures revealed the presence of micelles of size
of around 24-36Å in LABS in 0 ppm water but the
presence of micelles was not detected in hard water,
at least near the maximum turbidity point. This
would indicate that at the point
Fig. 3—Plot of turbidity against logarithmic concentration of
LABS in 5% and 10% TX-100 mixed systems at different water
hardness levels as CaCO3 ppm (■- 50 ppm, ●- 100 ppm, ▲- 150
ppm, ○- 200 ppm)
Fig. 4—SAXS profile of LABS in presence of calcium
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INDIAN J. CHEM. TECHNOL., NOVEMBER 2004
Fig. 5—SAXS profile of LABS without calcium ions
where there is maximum turbidity the calcium ions
are destroying the micelles resulting into formation of
less soluble lime soap.
Turbidity of the solution was also measured as a
function of water hardness at a fixed surfactant
concentration. A plot of turbidity against water
hardness for 1:0, 4:1, 2:1 and 1:1 calcium to
magnesium ratio (Fig. 6) showed a linear variation of
turbidity with hardness ppm at a fixed surfactant
concentration i.e. 2.67 mM. The linearity, however,
was not observed if the surfactant concentration was
either above 6.0 or much below 2.0 mM. At higher
surfactant concentration micelles would start
dispersing the lime soap, which would render the
solution less turbid or clear thereby interfering with
the linear relationship between turbidity and hardness
ppm. At low surfactant concentration, because of low
turbidity in the system the turbidity measurements
become less precise. These plots have been
effectively employed for the estimation of calcium or
magnesium from unknown samples within an
accuracy of ±0.5%. The presence of magnesium ions
resulted in considerable decrease in the turbidity of
the surfactant solution. By choosing normal tape
water or a sample of natural water as standard, it was
also found that the turbidity method may be used to
estimate the degree of hardness of water from other
natural sources within an accuracy of ±0.5%. The
present method gives more accurate results than the
arsenazo strip test15, since it is based on the turbidity
measurement rather than visual comparison of colored
strips. It must be pointed out that the method is rather
sensitive to calcium and magnesium ratio present in
the water sample. This method, however, can be
conveniently used for the estimation of calcium or
magnesium ions from an unknown sample with
reasonable degree of accuracy.
Fig. 6—Effect of calcium and magnesium hardness as CaCO3
ppm on the turbidity at fixed concentration of SDS (2.67 mM) at
298 K (■- 1:0, ●- 4:1, ▲-2:1, ○-1:1 Ca/Mg)
Acknowledgements
The financial assistance from the CSIR, New
Delhi, in the form of a research project is gratefully
acknowledged. The authors are thankful to Dr. Puyam
S. Singh of the Australian Open University for the
SAXS profiles.
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