Relative Assessment of Four Synthetic Surfactants

ORIGINAL ARTICLE

Relative assessment of density and stability of foamproduced with four synthetic surfactants

Indu Siva Ranjani • K. Ramamurthy

Received: 12 March 2009 / Accepted: 5 January 2010 / Published online: 13 January 2010

� RILEM 2010

Abstract Selection of the surfactant has an impact

on many of the foam properties as it affects the

surface tension and gas–liquid interfacial properties.

The objective is to produce stable aqueous foam of

required density. These two characteristics are influ-

enced by the type of surfactant, its concentration and

foam generation pressure. This study compares the

density and stability of foam produced using four

synthetic surfactants namely sodium lauryl sulfate,

sodium lauryl ether sulfate, sulfanol and cocodie-

thanolamide through a systematic experiment design

based on response surface methodology. The relative

performance has also been assessed in terms of their

suitability for use in foamed concrete production

based on ASTM test method. The effect of surfactant

concentration has relatively lesser effect on foam

density for sodium lauryl sulfate and sulfanol

irrespective of foam generation pressure adopted.

The drainage is proportional to the initial foam

density for all the surfactant concentration for ionic

surfactants at different foam generation pressures.

For all the four surfactants under the optimum foam

generation pressure, a stable foam with drainage less

than 12% in 300 s (by considering economy as a

factor) is achieved. From the foam stability test based

on ASTM C 796-97, it is observed that all the four

surfactants are suitable for use in foamed concrete

production when optimized foam production param-

eters are adopted.

Keywords Density � Stability � Foam �Sodium lauryl sulfate � Sodium lauryl ether sulfate �Sulfanol � Cocodiethanolamide

1 Introduction

Foaming agents are surfactants which when present in

small amounts in solution facilitate the formation of

foam and ensures stability by preventing collapse.

These surfactants can be either natural or synthetic

based (origin), and ionic or non-ionic [1, 2]. Selection

of surfactant has an impact on the properties of foam as

it affects the surface tension and gas–liquid interfacial

properties. The nature of the surfactant also modifies

the properties of the thin liquid film which separates

the bubbles [3]. Stable aqueous foams are required in

many of the industrial applications. Several techniques

have been used in earlier studies to evaluate the

properties of foam produced with surfactants. Foam

density is an important property which determines its

volume to be added for achieving a desired density of

foam concrete. For this purpose the initial foam

density is presently being used as the basis. But the

I. S. Ranjani � K. Ramamurthy (&)

Building Technology and Construction Management

Division, Department of Civil Engineering, Indian

Institute of Technology Madras, Chennai 600036, India

e-mail: [email protected]

Materials and Structures (2010) 43:1317–1325

DOI 10.1617/s11527-010-9582-z

stability of the foam may be affected depending on the

surfactant, its concentration and foam generation

pressure. The fundamental physical mechanisms caus-

ing foam instability reported are; (i) coarsening caused

by inter-bubble gas transport, (ii) gravitational drain-

age from the films, and (iii) coalescence of adjacent

bubbles due to rupture of inter-bubble lamellae [4].

Drainage rate is often used to characterize the degree

of water retention ability of foam [5, 6].

Sodium lauryl sulfate is a commonly used surfac-

tant in detergent industry. This has also been used in

the concentration range of 0.1 to 0.4% for the

production of foamed gypsum of density less than

1,000 kg/m3 [7]. Studies were made on the effect of

additives namely sodium carboxyl methyl cellulose

and Galonol PBD, foam generation pressure and

bubble size distribution and temperature, on drainage

of aqueous foam produced with surfactant Sodium

lauryl sulfate [8–10]. Sarma et al. [10] observed that a

more uniform bubble size distribution and high initial

gas fraction resulted in stable foam. When sodium

lauryl sulfate is ethoxylated it forms sodium lauryl

ether sulfate with enhanced foaming properties [11].

Surfactant mixture of 2% sulfanol as foaming agent

and 0.3% bone glue hydro-solution as stabilizer in the

ratio 1:0.15 is reported to produce a stable foam for

which stability was assessed by the time taken for

surfactant breakdown [12].

In comparison to air aspiration method, compressed

air mode of foam generation is reported to result in

foam having uniform bubble size distribution [13]. At

low pressures (\30 kPa), the physical properties of

solutions, density, viscosity and dynamic surface

tension determine the size of bubble being formed.

However, as the pressure and hence the flow rate of the

air increases, solution effects are negated and the

bubble diameter is determined by the generation

pressure [14–17]. Aqueous foams used in fire fighting

applications are mainly classified by their expansion

ratio. For foam concrete applications, the expansion

ratio is defined in terms of foam density. Foam which

is over-expanded (say expansion ratio greater than

50:1) and thus of lower foam density may collapse and

increase the concrete density.

As a first step, the authors studied the effect of

foam generation parameters on foam characteristics

of one typical synthetic surfactant [18]. Based on

encouraging results of this study, it was decided to

undertake a systematic investigation on relative

performance of four commonly available and afford-

able synthetic surfactants on the foam characteristics.

The relative characteristics of foam produced with

four synthetic surfactants has been studied through a

systematic experiment design based on Response

surface methodology and to check their suitability

for use in foamed concrete production as per ASTM

C 796-97 [19]. The surfactant concentration and foam

generation pressure required to produce stable foam

was determined first. As a next step, the behaviour

of the foam in cement slurry was studied, which

established the stability of the foam in the mix.

2 Experimental investigations

2.1 Materials and equipment used

Foam was produced by aerating four commercially

available synthetic surfactants viz; sodium lauryl

sulfate, sodium lauryl ether sulfate, sulfanol (ionic

surfactants) and cocodiethanolamide (non-ionic sur-

factant). Table 1 shows an overview of their chemical

classification. A laboratory-scale foam generator

designed and developed at IIT Madras was used

wherein the foam was generated by mixing com-

pressed air and foaming solution in high density

restrictions.

2.2 Parameters and properties studied

For evaluating the relative characteristics of foam

produced with these surfactants, a range of surfactant

concentration (dilution ratio) from 0.5% (1:200) to 10%

(1:10) and foam generation pressure 98–294 kPa,

were adopted. The initial foam density was measured

immediately after its generation while the stability of

foam was assessed by free drainage test prescribed by

Def Standard 42–40 [20]. A drainage pan of 1612 ml

nominal volume with the centre of the conical base

rounded to accept externally a 12.7 mm bore by

25 mm long polymethyl methacrylate tube with a

1.6 mm bore brass cock at its lower end was used

(Fig. 1). The pan was filled with foam and the weight

of foam was measured at various time intervals after

foam generation. The small variations in the height of

foam with time were accounted for in the density

calculation. Response surface methodology (RSM)

using a two factor central composite design (CCD)

1318 Materials and Structures (2010) 43:1317–1325

with rotatability or equal precision was employed to

study the effect of surfactant concentration and foam

generation pressure on initial foam density (IFD)

[21, 22]. For each surfactant, 13 experimental treat-

ments were assigned based on the CCD with two

independent variables at five levels of each variable

using Statistical Analysis Software (SAS Release

8.02) [23]. The quadratic response surface model is

presented in Table 2, which were used to assess the

relative performance of foam produced with the four

surfactants.

3 Precision and reliability of models

The adequacy of response models were determined

using model analysis, coefficient of determination

(R2) analysis, and by comparing the experimental

data with values predicted by response surface

models [20]. Validation of the second order polyno-

mial regression models carried out through additional

experimental data were observed to be highly

adequate to interpret a reliable relationship between

the independent variables (surfactant concentration

and foam generation pressure) and response variables

(foam density at various time intervals) with a

satisfactory coefficient R2 ([0.9) for most of the

regression models (Table 3). As a next step, the

relative assessment of density and stability of foam

produced with four different surfactants are

discussed.

4 Initial foam density (IFD)

The effects of surfactant concentration and foam

generation pressure on the initial foam density are

plotted in Figs. 2, 3 and 4, respectively. For ionic

surfactants (i) the foam density is maximized when

the surfactant concentration and foam generation

pressures are at lower and higher levels, respectively,

and (ii) the initial foam density decreases with an

increase in surfactant concentration of up to 4% after

which there is only a marginal increase (Fig. 2a). For

non-ionic surfactant (cocodiethanolamide), (i) the

initial foam density increases with an increase in

surfactant concentration at lower foam generation

pressure (Fig. 2a), and (ii) this trend is reversed at

Table 1 An overview of foaming agents used for the present study

Name of foaming

agent

Chemical synonyms General group name Chemical

formula

Classification

based on charge

Sodium lauryl sulfate Sodium dodecyl sulfate Alkyl sulfates C12H25NaO4S Anionic

Sodium lauryl ether sulfate Sodium laureth sulfate Alkyl ether sulfate C16H33NaO6S Anionic

Sulfanol Sodium dodecyl benzene sulfonate Linear alkyl benzene sulfonate C18H29NaO3S Anionic

Cocodiethanolamide Coconut diethanolamide,

Cocamide DEA

Alkanolamides C16H33NO2 Non-ionic

200 mm

100 mm internal diameter

12.7 mm internal diameter x 25 mm long Polymethyl methacrylate tube

1.6 mm Bore brass cock

11°

Fig. 1 Experimental setup

for foam drainage study

(Def Standard 42-40

(2002))

Materials and Structures (2010) 43:1317–1325 1319

higher foam generation pressure (Fig. 2b). This is

attributed to the entry of more foaming solution into

foam due to turbulence at lower surfactant concen-

tration, which is not significant at lower foam

generation pressure unlike ionic surfactants.

For foam concrete, ASTM C 796 specifies the

foam unit weight range of 32 to 64 kg/m3 with a

remark that this range could be adjusted to manufac-

turer’s recommendation based on foam chemical and

generator used. For the surfactant concentrations and

Table 2 Response surface models for foam density at various time intervals for different surfactants

Foam property Surfactant Response surface models

IFD SLS IFD = 20.12124 - 2.05373 * SC ? 0.08349 * FGP - 0.0038 * SC * FGP

? 0.184089 * SC2 - 9.7 * 10-5 * FGP2

SLES IFD = 40.96705 - 9.17408 * SC ? 0.09647 * FGP ? 0.671866 * SC2

SULF IFD = 17.5775 - 0.91786 * SC ? 0.02379 * FGP - 0.00565 * SC * FGP

? 0.132912 * SC2 ? 0.0002 * FGP2

CDA IFD = 68.63874 ? 6.673567 * SC - 0.20224 * FGP - 0.03073 * SC

* FGP - 0.00825 * SC2 ? 0.000757 * FGP2

FD at 5th minute SLS FD = 18.39425 - 1.68899 * SC ? 0.066825 * FGP - 1.17893 * 10-3 * SC

* FGP ? 0.15216 * SC2 - 1.32224 * 10-4 * FGP2

SLES FD = 36.70574 - 5.80818 * SC ? 0.019059 * FGP ? 0.41971 * SC2

SULF FD = 20.66503 ? 0.65285 * SC - 0.086648 * FGP - 3.57686 * 10-3 * SC * FGP

? 7.9726 * 103 * SC2 ? 3.67456 * 10-4 * FGP2

CDA FD = 59.94294 ? 10.83776 * SC - 0.24160 * FGP - 0.027712 * SC * FGP

- 0.31490 * SC2 ? 7.29971 * 10-4 * FGP2

FD at 10th minute SLS FD = 7.51031 - 0.19505 * SC ? 1.00621 * 10-4 * FGP - 2.64309 * 10-3 * SC

* FGP ? 0.096673 * SC2 ? 3.67349 * 10-5 * FGP2

SLES FD = 16.60996 - 2.10015 * SC - 0.035515 * FGP ? 1.25622 * 10-3 * SC * FGP

? 0.14902 * SC2 ? 6.54138 * 10-5 * FGP2

SULF FD = 14.5659 ? 0.55116 * SC - 0.11302 * FGP ? 5.94545 * 10-4 * SC

* FGP - 0.036546 * SC2 ? 2.83675 * 10-4 * FGP2

CDA FD = 14.68048 ? 22.21186 * SC - 0.22063 * FGP - 0.026326 * SC

* FGP - 1.03357 * SC2 ? 6.25567 * 10-4 * FGP2

SLS sodium lauryl sulfate, SLES sodium laureth sulfate, SULF sulfanol, CDA cocodiethanolamide, IFD initial foam density, FD foam

density, SC surfactant concentration, FGP foam generation pressure

Table 3 R2, adjusted R2, probability values and F values for the response surface models

Foaming agent Variables R2 R2 adj Regression P value F value

SLS Initial foam density 0.9599 0.9312 \0.0001 33.48

SLES 0.9392 0.9189 \0.0001 46.34

SULFANOL 0.9247 0.8709 0.0008 17.19

CDA 0.9529 0.9192 0.0002 28.307

SLS Foam density at 5th minute 0.9633 0.9371 \0.0001 36.74

SLES 0.9512 0.935 \0.0001 58.52

SULFANOL 0.9397 0.8966 0.0004 21.80

CDA 0.9945 0.9907 \0.0001 255.64

SLS Foam density at 10th minute 0.916 0.86 0.0012 15.26

SLES 0.9327 0.8847 0.0006 19.41

SULFANOL 0.9109 0.8472 0.0015 14.31

CDA 0.9762 0.9592 \0.0001 57.50


foam generation pressures adopted, the range of

initial foam density produced with sodium lauryl

sulfate, sodium lauryl ether sulfate, sulfanol and

cocodiethanolamide, respectively, are 20–35, 20–65,

20–40 and 40–100 kg/m3. For all the four surfactants,

the initial foam density obtained is satisfying the

ASTM requirements at lower surfactant concentra-

tion and higher foam generation pressure. But such

foam with higher initial foam density was observed to

contain foaming solution entrapped with the foam

due to turbulence resulting in foams with lower

stability. This aspect is confirmed by higher drop in

density with time as discussed in the next section.

The foam generation pressure controls the mixing

of air with foaming liquid and hence the foam density

varies with foam generation pressure. For a given

surfactant concentration the initial foam density

increases with an increase in foam generation

pressure for all surfactants except in the case of

cocodiethanolamide. It is observed from Fig. 3 that

the effect of foam generation pressure on the initial

foam density is lower for sodium lauryl sulfate and

sulfanol irrespective of the surfactant concentration.

Hence the densities of the foam produced using

sulfanol and sodium lauryl sulfate are the lowest. The

ASTM specified range of initial foam density was not

achieved when foam is produced at lower foam

generation pressure for surfactants sodium lauryl

sulfate and sulfanol. In the case of sodium lauryl

ether sulfate at higher surfactant concentration the

effect of foam generation pressure is significant.

Cocodiethanolamide produces foam with highest

10

20

30

40

50

60

70

80

90

Initi

al fo

am d

ensi

ty (

kg/m

3)

Surfactant concentration (%)

Sodium lauryl sulfate Sodium lauryl ether sulfate Sulfanol Cocodiethanolamide

0 2 4 6 8 10

0 2 4 6 8 1010

20

30

40

50

60

70

80

90

Initi

al fo

am d

ensi

ty (

kg/m

3)



(a)

(b)

Fig. 2 Variation in initial foam density with surfactant

concentration. a FGP 110 kPa, b FGP 294 kPa

20

40

60

80

100

120

140

160

180

Initi

al fo

am d

ensi

ty (

kg/m

3)

Foam generation pressure (kPa)


100 150 200 250 300

100 150 200 250 30010

20

30

40

50

60

70

80

90

Initi

al fo

am d

ensi

ty (

kg/m

3)

Foam generation pressure (kPa)


(a)

(b)

Fig. 3 Variation in initial foam density with foam generation

pressure. a SC 0.5%, b SC 10%


initial foam density irrespective of the foam gener-

ation pressure. For cocodiethanolamide, the foam

generation pressure has significant effect on initial

foam density, i.e., at higher surfactant concentration

an increase in foam generation pressure results in a

reduction in initial foam density and vice versa at

lower surfactant concentration.

5 Foam stability

The foam stability is assessed through the variation in

foam density with time which is caused predomi-

nantly by the drainage of diluted foaming agent

entrapped along the walls of the bubbles and to a

minor extent due to breakage of foam bubbles.

Figure 4 shows the variation in foam density with

time for the effect of surfactant concentration at

lower and higher foam generation pressures. For all

the four surfactants, the drainage increases with an

increase in foam generation pressure resulting in

unstable foam. For the three ionic surfactants within

the range of surfactant concentration studied, the

drainage is proportional to the initial foam density at

different foam generation pressures.

The reduction in foam density is significantly

higher after 5 min. For the three ionic surfactants,

within 10 min more than 40% of foam density is

reduced. In the case of cocodiethanolamide, a con-

centration of 4% and above results in retention of

stability. The foam produced with cocodiethanola-

mide is more stable when compared to that produced

0

10

20

30

40

50

60

70

Initial foam density (kg/m3) Foam density at 5th minute (kg/m3) Foam density at 10th minute(kg/m3)

Solid line - FGP 110 kPa Dashed line - FGP 294 kPa

Foa

m d

ensi

ty (

kg/m

3)


0

10

20

30

40

50

60

70 Initial foam density (kg/m3) Foam density at 5th minute (kg/m3) Foam density at 10th minute(kg/m3)


Foa

m d

ensi

ty (

kg/m

3)


0

10

20

30

40

50

60

70 Initial foam density (kg/m3) Foam density at 5th minute (kg/m3) Foam density at 10th minute(kg/m3)


Foa

m d

ensi

ty (

kg/m

3)


0 2 4 6 8 10

0 2 4 6 8 10

0 2 4 6 8 10

0 2 4 6 8 100

10

20

30

40

50

60

70

80

90

Initial foam density (kg/m3) Foam density at 5th minute (kg/m3) Foam density at 10th minute(kg/m3)


Foa

m d

ensi

ty (

kg/m

3)


(a) (c)

(b) (d)

Fig. 4 Effect of SC and FGP on foam density with time for various surfactants. a Sodium lauryl sulfate, b sodium lauryl ether

sulfate, c sulfanol, d cocodiethanolamide


with ionic surfactants, exhibiting substantially lower

drainage at high surfactant concentration. This retar-

dation in drainage is attributed to the high viscosity

enhancing and foam stabilizing property of cocodie-

thanolamide. Also the surfactant concentration has

opposite effect at lower and higher levels of foam

generation pressure for this non-ionic surfactant. This

is because at lower foam generation pressure, the

effect of lower surfactant concentration on foam

stability is not significant unlike ionic surfactants as

explained earlier. This is confirmed by lesser drop in

foam density with time up to 5 min at lower foam

generation pressure when compared to higher pres-

sure for the non-ionic surfactant. As the usage of

higher surfactant concentration is not economical, the

selection of lower concentration is preferable for use

in foam concrete production. But at very low

surfactant concentration and higher foam generation

pressure, though the foam produced has high initial

density, the stability is poor. Hence it appears that

there is an optimal surfactant concentration and foam

generation pressure which can produce stable foam.

6 Optimization of response surface models

Having identified that the foam stability is an

important factor, a multiple optimization was carried

out by numerical optimization method using SAS

Release 8.02 to predict the optimum levels surfactant

concentration and foam generation pressure for the

following criteria; minimize percentage solution

drained, maximize foam density ratio (ratio of foam

density to initial foam density) at various time

intervals (to increase foam stability), minimize sur-

factant concentration (to reduce cost), and to achieve

a target foam output rate of at least 0.09 m3/h which

was observed to be the minimum requirement to get

uninterrupted foam production for the laboratory

foam generator used. Each response has been

assigned an importance value (weightage) relative

to the other responses. Percentage solution drained

and foam density ratio was assigned an importance of

4 while the other responses were assigned of 3 out of

5 scale. Hence higher weightage was assigned to

foam stability.

From this study, the optimum surfactant concen-

tration value is 2 and 5% when economy is considered

as one of the factors for ionic and non-ionic surfactants

respectively (Table 4). The optimal surfactant con-

centration values for non-ionic surfactant cocodie-

thanolamide were 5 and 8%. However for all the four

surfactants the optimum foam generation pressure

ranges between 110 and 120 kPa under which a stable

foam with drainage less than 12% in 300 s (by

considering economy as a factor) is achieved. This

drainage value is very low when compared to the value

of 25% drainage obtained in time not less than 210 s as

prescribed by Def Standard 42-40 (2002) for synthetic

aqueous film forming foam for fire extinguishing.

However by assigning higher importance to foam

stability (without considering economy) the solution

drained can be reduced further for ionic surfactants

when higher surfactant concentration say 4% is used.

7 Stability of foam in the mix

With the establishment of optimal surfactant concen-

tration and foam generation pressure, as a next step,

the suitability of these four surfactants for the

production of foam concrete, i.e., whether the

requirements of ASTM C 869 [24] with respect to

fresh density, strength and water absorption of

foamed cement paste are fulfilled need to be verified.

This especially is essential as the initial foam

densities of foam produced by two ionic surfactants

are marginally lower than the ASTM specified range.

Table 4 Optimized parameters and corresponding response goals

Foaming agent Surfactant

concentration (%)

Foam generation

pressure (kPa)

Initial foam

density (kg/m3)

Foam output

rate (m3/h)

Solution drained

in 5 min (%)

Foam density

ratio in 5 min

Sodium lauryl sulfate 2 117 25 0.274 11 0.88

Sodium lauryl ether

sulfate

2 117 38 0.172 12 0.86

Sulfanol 2 117 21 0.24 12 0.87

Cocodiethanolamide 5 122 70 0.09 0 1


ASTM C 796-97 furnished a way of measuring in the

laboratory, the performance of a foaming chemical to

be used in producing foam for making cellular

concrete through the following equations for arriving

at the foam volume required for achieving a cement

paste of known design density 641 kg/m3 and water-

cement ratio of 0.58:

Vf ¼ 1000 Va= 1000�Wufð Þ per m3 of cement paste� �

;

Va ¼ 0:359 �Wtw þ 0:7965 Wcð Þ=641 m3;

where Vf = volume of foam; Va = volume of air;

Wuf = unit weight of foam; Wtw = total weight of

water; and Wc = weight of cement.

Foam concrete was made by mixing the cement

slurry with a water-cement ratio 0.58 and preformed

foam produced from the surfactants at the optimized

economical surfactant concentration and foam gen-

eration pressure (Table 5). The stability of test mixes

was assessed by comparing the calculated and actual

quantity of foam required to achieve a plastic density

of foam concrete within ±50 kg/m3 of the design

value and is summarized in Table 5 along with

ASTM specifications. The foamed concrete made

with the foam produced with all the four surfactants,

at the optimized surfactant concentration and foam

generation pressure, meets the physical requirements

of ASTM, confirming the foam stability. If the foam

is unstable, slightly higher quantity of foam (to

compensate for the collapsed foam) than the volume

of foam calculated as per the equations listed above

would be required to attain the design density.

However, though the foam density of sodium lauryl

sulfate and sulfanol did not meet the minimum

criteria of 32 kg/m3 as specified by ASTM Standards,

the actual quantity of foam required to attain the

plastic density of 641 kg/m3 within ±50 kg/m3 of the

design value was the same as the calculated quantity

which again confirms the stability of the mix.

8 Conclusions

The conclusions drawn from this study and discussed

below are applicable to the characteristics of mate-

rials used and the range of parameters investigated.

• For all the synthetic ionic surfactants used, the

foam density increases with an increase in foamTa

ble

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generation pressure and decreases with an increase

in surfactant concentration up to a dosage of 4%.

• For the non-ionic surfactant cocodiethanolamide,

the initial foam density increases with an increase

in surfactant concentration at lower foam genera-

tion pressure with a reverse trend at higher foam

generation pressure. Also at higher surfactant

concentration, the foam density decreases with an

increase in foam generation pressure unlike ionic

surfactants.

• The effect of surfactant concentration has rela-

tively lesser effect on foam density for sodium

lauryl sulfate and sulfanol irrespective of foam

generation pressure adopted.

• The drainage is proportional to the initial foam

density for all the surfactant concentration for

ionic surfactants at different foam generation

pressures.

• For all the four surfactants under the optimum

foam generation pressure a stable foam with

drainage less than 12% in 300 s (by considering

economy as a factor) is achieved.

• From the foam stability test based on ASTM C

796-97, it is observed that all the four surfactants

are suitable for use in foamed concrete production

when optimized foam production parameters are

adopted.

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Documents

Relative Assessment of Four Synthetic Surfactants