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JANUARY 2012

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Surfactants

The value of sultaines

Regina Cosby and Frank Wagner of Rhodia take a new look at amphoteric surfactants*

Amphoteric surfactants are commonly used by

formulators in personal care applications, such as

body washes, shampoos and facial cleansers, and

in home care formulations like hard surface cleaners and

laundry and dish detergents. They typically allow

formulators to reduce the irritation potential of anionic

surfactants, develop formulation viscosity and enhance

foam quality in terms of texture, volume and stability.

Cocamidopropyl betaine (CAPB, Figure 1a) is a widely

used amphoteric surfactant in the home and personal

care industries, although it can limit formulators in

developing higher performance products at a lower

formulation cost. Sultaines, such as cocamidopropyl

hydroxysultaine (CAPHS, Figure 1b) are an amphoteric

option that offer improved foam quality, enhanced

mildness and cost-effectiveness and are environmentally

favourable.

The molecular design of sultaines enhances mildness

and foam performances and can solve key formulation

challenges of sulfate- and ethoxylate-free formulations in

personal care. Sultaines bring extra clarity and stability

when compared to betaines, due to their stronger

hydrotroping properties, and are extremely tolerant in

hard water environments. They have identical viscosity

development to betaines and can easily replace them in

home and personal care formulations.

Chemistry of sultaines

Sultaines are the inner sulfonic acid salt of a strong

inorganic acid and are commonly referred to as

‘sulfobetaines.’ They are similar to betaines, which are

inner carboxylic acid salts of a weak organic acid. 1

Both molecules are considered zwitterionic at neutral

pH where the nitrogen on the hydrophobic tail is

quaternary (cationic); the polar head groups are anionic

and add to the hydrophilic properties of the molecule.

Because the quaternary nitrogen is always positive, these

molecules cannot obtain anionic nature at any pH and are

not truly amphoteric, although they are commonly

referred to as such.

The alkyl chain length varies, depending on the

feedstock used. In personal care formulations, higher

hydrophobic alkyl chain lengths of C 12-14 are preferred for

optimum foaming, mildness and viscosity building

properties.

Foam volume (ml)

500

400

300

200

100

With disodium

Lauryl sulfosuccinate

CAPHS-FA

CAPB-FA

LAPHS-FA

LAPB-FA

CAPHS-T

CAPB-T

Figure 1 - CAPB & CAPMS

Figure 2 - Foam volume of

betaines & sultaines of

varying feedstocks with

common anionics

With sodium coco-sulfate

CAPH-FA

CAPB-FA

LAPHS-FA

LAPB-FA

CAPHS-T

CAPB-T

a - Cocamidopropyl

betaine

b - Cocamidopropyl

hydroxysultaine

O

CH 3

R N N + O -

H

Although sultaines and betaines are similar in structure,

the slight difference in the head group of the molecule is

what gives sultaines their different properties. A betaine’s

head group is a weakly anionic carboxylic acid that binds

easily to divalent cations, such as calcium and

magnesium. Sultaines consist of a strong anionic

sulfonate group that overpowers the cationic properties

of the quaternary nitrogen.

This small difference in the molecule is what gives

sultaines their enhanced properties over their betaine

counterparts, such as strong alkali stability, excellent lime

soap dispersion and enhanced coupling. When

manufactured, sultaines are naturally higher in terms of

active content than betaines without the use of solvents

and they are free of preservatives, processing aids and

chelants.

Improved foam properties

Foam tests of various anionic: amphoteric combinations

were evaluated at 0.1% actives at a ratio of 1.5:1 to

study further the properties of sultaines in personal care.

Since amphoterics display different properties at various

pH ranges, foam volume was measured at both pH 4.0

and 7.0. The testing parameters for all foam studies were:

no soil, an ambient temperature of 22°C and water with

a hardness of approximately 150 ppm calcium and

magnesium ions.

The process used an automatic cylinder shake foam

apparatus from Guam that consisted of placing 100 ml of

test solution in a 500 ml graduated cylinder and inverting

it ten consecutive times and at a frequency of 30 rpm. The

foam volume, expressed in ml, is measured immediately

following last inversion, t=0, and after five minutes, t=5.

The initial foam volume indicated flash foam properties

and the reading after five minutes indicated foam

stability. All samples were run in duplicate using a least

significant difference of 90%.

Figure 2 shows the foam volume of betaines and

sultaines of varying feedstock combined with two of the

anionic surfactants tested, disodium lauryl sulfosuccinate

and sodium coco-sulfate. The feedstock varieties included

betaine and sultaines derived from triglycerides (CAPB-T

and CAPHS-T), those derived from coconut fatty acid

(CAPB-FA and CAPHS-FA) and those derived from lauric

acid (LAPB-FA and LAPHS-FA).

26 Speciality Chemicals Magazine January 2012 www.specchemonline.com

R

O

N

H

CH 3

O

CH 3

N + S O

O O

CH 3 OH


Surfactants

Foam volume (ml)

500

450

400

350

300

250

SLES-2

SLES-3

Anionic neat

SLES-2

Combos

Triglyceride

Sultaines

Betaines

SLES-3

Combos

In all cases, at a 90% confidence level, sultaines

statistically outperformed their betaine counterpart of the

same feedstock in both pH environments. In terms of

foam stability, betaines and sultaines displayed similar

abilities to stabilise foam volume at five minutes. During

the study, it was observed that the sultaine combination

solutions were clear and the betaine combinations were

hazy when combined with each of the high Kraft point

anionics.

This mimics the hydrotroping properties of sultaines

that have been studied in hard surface cleaners. The

superior performance is most likely due to the greater

charge neutralisation of the molecule. Further studies are

planned to evaluate these coupling properties

quantitatively in a personal care formulation.

Sodium coco-sulfate has a dense, creamy foam quality

that is further stabilised with the addition of amphoterics.

Figure 2 shows that there was a significant improvement

in the foam quantity of sultaines over betaines with

sodium coco-sulfate in all feedstock cases.

Using the same conditions as this study, further foam

work focused on standard, triglyceride-based amphoterics

to explore the effect of the ethoxylation of the anionic on

the foam performance of sultaines. The triglyceride-based

sultaines outperformed the betaines and the foam volume

of the sodium laureth sulfate (SLES)/sultaine combinations

were significantly impacted by the degree of ethoxylation

on the SLES molecule (Figure 3).

SLES-3/sultaine foamed significantly higher than the

SLES-2/sultaine yet the ethoxylation differences did not

have an effect on the SLES/betaine combinations. All

anionic/amphoteric surfactant combinations have a

denser and more stable foam quality and do not have as

high a foam volume as the neat anionic surfactants.

Viscosity building

Viscosity comparisons used two base formulations that

only varied the test surfactant to evaluate viscosity

building properties with electrolytes. Viscosity readings

were measured when the samples reached 25°C in a

water bath by using a Brookfield LVT Viscometer, spindle

#3, after one minute. The speed on the viscometer was

adjusted accordingly from 3 to 12 rpm or when the

sample measured 50% torque.

Two common surfactant systems were analysed, based

on 12% active SLES-2/3% active test surfactant and 16%

active SLES-1/1.6% active cocamide MEA/1.5% active test

surfactant respectively. Similar to the foam studies, the

test surfactants consisted of sultaines and betaines

derived from various feedstocks including triglycerides

(whole coconut oil), coconut fatty acid or lauric fatty acid.

Figure 3 - Foam volume of

SLES-3/sultaine

combination

The objective was to determine if sultaines of various

feedstocks built viscosity as well as their corresponding

betaine counterparts. Each system was balanced to 100%

by weight with distilled water and adjusted to pH 7.0±0.2

using citric acid. Initial readings were measured, then

sodium chloride was added in 0.2-0.5% increments.

The results (Figure 4) demonstrate that a sultaine and

betaine derived from the same feedstock build viscosity in a

similar way. The addition of an alkanolamide made more of

an impact on viscosity response due to its strong hydrogen

bonding with electrolytes. For example, a sultaine and a

betaine, both based on coconut fatty acid, both built

viscosity in a similar way in the system that contained an

amide and in the one without it.

In the formulation without amide, both test

formulations with coconut fatty acid-based amphoterics

reached an ideal personal care formulation viscosity of

10,000-15,000 cps with 1.5% sodium chloride. When an

amide was present, the formulations reached this same

viscosity range at 0.5% sodium chloride, even though

half the amount of coconut fatty acid-based amphoterics

was used.

The same patterns were observed for lauric fatty acidor

triglyceride-based sultaines and their betaine

counterparts. Viscosity response showed the following

sequence between alkyls as contributors to the viscosity

build of amphoterics: in both systems tested, with or

without the addition of a non-ionic, lauric fatty acid >

coconut fatty acid > coconut oil.

Viscosity build is therefore a function of the alkyl chain

length and/or chain distribution of the amphoteric.

Betaines and sultaines build viscosity in a similar way.

Based on the two systems tested, sultaines can replace

betaines with minimal difference in viscosity building in

personal care formulations.

Reduced irritation

MatTek’s patented EpiOcular test was used to evaluate

and compare the effect that a sultaine and a betaine has

on reducing the irritation of anionic surfactants. Mildness

was evaluated in vitro by MatTek using its corneal model

that consists of cultured epidermal cells similar to those

found in the cornea. The model provides a predictive

means to assess ocular irritancy in vitro.

Table 1 - EpiOcular test results of SLES-2 or sodium coco-sulphate, sultaine & betaine

combinations

Sodium

laureth-2

sulfatebased

combinations

Sodium

cocosulfatebased

combinations

Sample description Draize Sample description Draize

score

score

15% active SLES-2 48.9 Johnson & Johnson NMT 9.1

15% active CAPHS 67.2 15% active CAPB 67.2

11.25% active SLES-2 11.25% active SLES-2

4.75% active CAPHS 40.7 4.75% active CAPB 37.1

7.5% active SLES-2 7.5% active CAPHS

7.5% SLES-2 14.4 7.5% active CAPB 18.0

4.75% active SLES-2 4.75% active SLES-2

11.25% active CAPHS 6.6 11.25% active CAPB 22.4

10% active coco-sulfate 102.5 Johnson & Johnson NMT 9.1

10% active CAPHS 97.0 10% active CAPB 65.4

7.5% active coco-sulfate 7.5% active coco-sulfate

2.5% active CAPHS 75.7 2.5% active CAPB 58.3

5.0% active coco-sulfate 5.0% coco-sulfate

5.0% active CAPHS 20.1 5.0% active CAPB 20.4

2.5% active coco-sulfate 2.5% active coco-sulfate

7.5% active CAPHS 13.8 7.5% active CAPB 17.3

www.specchemonline.com January 2012 Speciality Chemicals Magazine 27


Surfactants

Viscosity (cps)

120,000

100,000

80,000

60,000

40,000

20,000

0

0.0 0.5 1.0 1.5 2.0 2.5

NaCl (%)

Hydroxysultaines

SLES-1/CMEA

systems

CAPB-FA

LAPB-FA

LAPHS-FA

CAPHS-FA

CAPHS-T

CAPB-T

Betaines

SLES-2

systems

CAPB-FA

LAPB-FA

CAPHS-FA

LAPHS-FA

CAPHS-T

CAPB-T

CAPHS-FA LAPHS-FA CAPHS-T CAPB-FA LAPB-FA CAPB-T

The procedure uses a water-soluble, yellow tetrazolium

salt (MTT {3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium

bromide}), which is reduced by succinate

dehydrogenase in the mitochondria of viable cells to a

purple, insoluble formazan derivative. Substances which

damage this mitochondrial enzyme inhibit the reduction

of the tetrazolium salt. The amount of MTT reduced by a

culture is therefore proportional to the number of viable

cells.

As per MatTek’s protocol, the samples are diluted to a

20% solution and the appropriate tissue preparation is

made. 100 microlitres of the test article and distilled

water (negative control) were added to the micelles

containing the EpiOcular samples. The six-well plates

containing the dosed EpiOcular samples were then

incubated at 37°C, 5% CO 2 and >90% humidity. The

results were then converted from an ET-50 value into a

familiar estimated Draize score.

According to this test, the ingredient is considered

more or less irritating depending on the Draize Score: 0-

15 means non-irritating, minimal, 15.1-25 is mild, 25.1-

50 is moderate and 50.1-110 is considered severely

irritating or extreme. Traditionally, true amphoterics or

highly ethoxylated nonionics are used to reduce or

minimise eye irritation of alkyl ethoxylated sulfates and

alkyl sulfates. Previous studies show that betaines have

minimal impact on this property. 2

EpiOcular comparisons were made for two anionic

combinations, one with SLES-2 and one with sodium

coco-sulfate to determine if sultaines displayed similar

Figure 4 - Viscosity

responses of sultaines &

betaines

References

1. McIntyre Group, McIntyre

Chemistry Manual 1997, 46-60

2. T. Schoenberg, Formulating

with Betaine and Amphoteric

Surfactants 1997, 2.

3. F. Wagner, D. Colovic, J.

Kiplinger, G. Cosby & E. Leroy, A

Novel Look at Amphoteric

Surfactants, Poster, 2010.

Contact

Denis Bendejacq

Rhodia CTRA

Labo Home & Personal

Care

52, rue de la Haie Coq

F-93308 Aubervilliers

France

E-mail: denis.bendejacq@

eu.rhodia.com

Website: www.rhodia.com

behaviour to betaines. The results (Table 1) show that,

although all surfactants tested individually at 10-15%

active were severely irritating, the combination of an

anionic with either a betaine or sultaine starts to mitigate

the irritation potential.

Sultaines especially reduced the irritation when

incorporating more sultaine than anionic in the SLES-

2/amphoteric combinations. For example 11.25% CAPHS

and 4.75% SLES-2 had a score of 6.6, non-irritating and

the corresponding CAPB was 22.4, or mild. The score of

this SLES-2/CAPHS combination was even lower than the

benchmark baby shampoo that claims ‘no more tears’.

Similarly, the estimated Draize scores of sodium cocosulfate

were reduced when incorporating a betaine or

sultaine. The impact of the ethoxylation of the alkyl

sulfate in combination with a sultaine was more dramatic

than the sulfate/betaine combinations, as also indicated in

the foam studies.

Conclusion

Sultaines molecular design makes them an ideal

amphoteric choice for personal care formulations where

mildness and foam are important. They are free of

processing aids and preservatives and are able to reduce

the irritation of common anionics more efficiently than

betaines.

Sultaines are highly compatible with anionic surfactants

and enhance foaming properties of ethoxylate-free

surfactants such as sodium coco-sulfate and also sulfatefree

surfactants such as disodium lauryl sulfosuccinate. In

every combination of anionic/amphoteric, sultaines

delivered superior or equal foam when compared to

betaines at the 90% confidence level.

Chain length of the betaine or sultaine and anionic

selection is more important in boosting foam than pH

effect. The degree of ethoxylation on an alkyl sulfate in

combination with a triglyceride-based CAPHS has a

favourable impact on mildness and foam. Sultaines build

viscosity similarly to their betaine counterpart and can

easily substitute them into a personal care formulation

while simultaneously boosting foam and mildness.

* - The authors would like to thank Dusanka Colovic Vos of

Rhodia, University Park, for her assistance in running many of the

foam and viscosity tests, Jon Kiplinger and Tom Ruch of Rhodia

CRTA for their assistance with EpiOcular correspondence and

interpretation of results and Eric Leroy of CRTA for his guidance

on relating the results to molecular structure as originally

published in a poster format. 3 They would especially like to thank

Denis Bendejacq of CRTA for all of his assistance and guidance in

writing this article.

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