FOAMSCAN - EXAMPLE 4

INTRODUCTION AND PRINCIPLE OF THE METHODS :

This note compares the influence of an antifoaming agent (here called antifoam) on the behaviour of three foams generated from three aqueous liquids containing respectively C12E6, SDS and OVALBUMIN.

Two general methods can be used with the FOAMSCAN for the characterisation of antifoam:

First method : when the foam has been created, the antifoam is dropped with a pipette on the free surface of the foam. The instrument records the variations of its characteristics (volume, quantity of liquid, etc).

Second method : the antifoam dissolved in the liquid sample. A comparison of its foaming power and behaviour of the foam can be done.

This second method makes it also possible to start again the bubbling after the collapse of the foam and to check the efficiency of the antifoam with time.

 

The instrument used is the FOAMSCAN (description - measurement)

 

1/ Liquid samples :

• C12E6 : non-ionic surfactant, C24 H50 O7
  Hexaethylen glycol monododecyl ether (Molar weight=450.66).
  liquid sample : C12E6 (from Sigma), concentration 5g/l, in a phosphate-citrate buffer (pH 6.4),

• SDS : anionic surfactant, CH3 (CH2)11 OSO3 Na
  Sodium dodecyl sulfate (Molar weight=288.38).
  Liquid sample : SDS (from Sigma), concentration 5g/l, in 0.1M NaCl,

• Ovalbumin : Egg protein (Molecular weight=45000 d).
  Liquid sample : Ovalbumin (from Chimie-Plus), concentration 2g/l, in a phosphate-citrate buffer (pH 6.4).

2/ Antifoam :

Silicon based agent 10 ppm in the liquid sample (24 cm3).

3/ Measurement conditions :

• Temperature : 25°C

• Gas : azote U

• Gas flow : 30 ml/min

• Initial liquid volume in the cuvette : 24 ml

• Glass fritt : porosity 16-40 µm

RESULTS :

1/ Influence of the antifoam dropped on the free surface of the foams (first method),

2/ Influence of the antifoam on the foaming power of the liquids (second method),

3/ Variation of the antifoam efficiency with time (C12E6 and SDS).

1/ Influence of the antifoam dropped on the free surface of the foams (first method),

Protocol :

The foams are generated by bubbling during 150 seconds. They final volume is about 100 cm3. We let them collapse during 450 seconds, and then antifoam is dropped on their free surface in the glass tube of the FOAMSCAN. Their volume variations with time are measured by image analysis.

Results :

The figure 1 shows these variations :

Comments :

A) After the end of bubbling, one can observe the decrease of the foam volumes. The decrease rates have been calculated in the example 2 (see note above).

B) As soon as the antifoam has been dropped, the foams formed from C12E6 and SDS collapse instantaneously. However, this agent has no significant influence on foams generated from ovalbumine.

 

Recapitulation :
Decrease rates of the foam volumes (in cm3/min)

	Decrease without antifoam	Decrease with antifoam
C12E6 (5g/l)	5.1	720
SDS (5g/l)	2.6	600
OVALBUMINE (2g/l)	0.6	1

2/ Influence of the antifoam on the foaming power of the liquids (second method)

a/ Foam from SDS

Protocol :

When the previous experiment has been over, when the foam has totally collapsed, the antifoam is totally in the liquid sample.
The bubbling has started again up to the generation of 100 cm3 of foam, then has been stopped.

Results :

The figures 2 and 3 show the variations of the foam volume and quantity of liquid in the foam versus time (green lines) and that without antifoam (black lines).

Comments :

A) The foam formation rates (with and without antifoam) are quite different. One can observe that the quantity of liquid dragged into the foam is higher when the antifoam is in the liquid sample.

B) When the bubbling is stopped, the decrease rate of the foam volume is higher with the antifoam and the drainage rate is higher.

b/ Foam from C12E6

Protocol :

As with the SDS, the bubbling has starting again just after the total collapse of the foam at the end of the previous experiment.

Results :

The figures 4 and 5 show the variations of the foam volume and quantity of liquid in the foam versus time (green lines) and that without antifoam (black lines).

Comments :

A) In this experiment, it has not been possible to generate again 100 cm3 of foam, because its volume has stabilised around 80 cm3. This steady state can be characterised by a BIKERMANN index of 170 seconds.

B) As for SDS after the bubbling has been stopped, the decrease rate of the foam volume and the drainage rate are higher with the antifoam.

c/ Foam from Ovalbumin

Protocol :

Differently from the previous experiments with the SDS and C12E6, a new liquid sample containing Ovalbumin (2g/l) and antifoam (10 ppm) has been run with the FOAMSCAN. The bubbling has been conducted during 150 seconds.

Results :

The figures 6 and 7 show the variations of the foam volume and quantity of liquid in the foam versus time (green lines) and that without antifoam (black lines).

Comments :

The foam volume generated during the same time is lower with the antifoam. However its stability and drainage rates are similar.

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3/ Variation of the antifoam efficiency with time (C12E6 and SDS)

These experiments have been implemented with the SDS and C12E6 based foams. The bubbling has started a second time after the previous experiments described in the paragraph 2 above.

a/ Foam from SDS

Protocol :

The bubbling has started again up to the formation of 100 cm3 of foam again.

Results :

The figures 8 and 9 show the variations of the foam volume and quantity of liquid in the foam versus time (blue lines) and that from the previous experiments (black and green lines).

Comments :

The differences are :

• foam formation duration :

- without antifoam: 150 seconds
- first bubbling with antifoam: 800 seconds
- second bubbling with antifoam: 160 seconds


• liquid quantities in the foam

- without antifoam: 9.7 cm3
- first bubbling with antifoam: 16.2 cm3
- second bubbling with antifoam: 10.5 cm3


• decrease rates of the foam volumes during the collapse phases (see chart at the end) which have become higher and higher as the texture of the foam was changing.

These results show that the performances of the antifoaming agent have changed.

b/ Foam from C12E6

Protocol :

As it has not been possible to generate 100 cm3 of foam with the antifoam, the bubbling has lasted up to a steady state and has been stopped.

Results :

The figures 10 and 11 show the variations of the foam volume and quantity of liquid in the foam versus time (blue lines) and that from the previous experiments (black and green lines).

Comments :

Only 80 cm3 of foam can be formed with the antifoam while 100 cm3 can be formed during 150 seconds without this agent.

The decrease rates of the foam volume do not significantly change between the first and second bubbling with the antifoam (see chart under). One can assume that the performances of the antifoam do not change significantly with this kind of foam.

 

Recapitulation :
Decrease rates of the foam volumes (in cm3/min)

CONCLUSION

The FOAMSCAN makes it possible to characterise the influence of antifoam agents on the behaviour of foams (either by dropping it on their surface or by mixing it them in the liquids). It also makes it possible to show any variation of their performances with time.