FOAMSCAN - EXAMPLE 2

INTRODUCTION :

This note makes a comparison between the behaviour of three foams, during their collapse phase, formed from three liquids containing respectively C12E6, SDS and OVALBUMIN.

In order to do this comparison easier, the surfactant concentrations have been determined so that these three liquids successively tested with the FOAMSCAN form, with the same bubbling conditions, the same volume of foam (about 100 cm3) during the same time (150 seconds).

When the bubbling is stopped, the collapse phases take place which are quantified by the FOAMSCAN by recording the variations versus time of the three following parameters:

• volumes,

• drainage,

• conductivity at different heights.

These three parameters allow the differentiation between these three foams.

 

The instrument used is the FOAMSCAN (description - measurement)

 

Formation of the foams:

The foams are formed by bubbling of nitrogen through the liquid samples. The gas flows through a glass fritt then the liquids themselves. Foams are formed on the surfaces of the liquids and rise in the glass tube of the instrument. The user defines the gas flow and the duration of the bubbling.

Physical quantities :

The physical quantities, which are measured versus time, are:

• the foam volumes by image analysis,

• the liquid volumes dragged into the foams, by measuring the remaining liquid volumes in the cuvette with its electrodes

• The conductivity at four different levels of the glass tube, which give information relative to the drainage of the liquids through the foams and to their density gradients.

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/ Measurement conditions :

• Temperature : 25°C

• Gas : azote U

• bubbling duration :150 seconds

• Gas flow : 30 ml/min

• Initial liquid volume in the cuvette : 24 ml

• duration of the collapse phase : 450 seconds, after the bubbling has been stopped

• Glass fritt : porosity 16-40 µm

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RESULTS :

1/ Results related to the variations of the foam volumes

2/ Results related to the quantities of liquids contained in the foams

3/ Results related to the variations of the foam conductance

+ Note about the liquid quantities in the foams

The FOAMSCAN software makes it possible to compare several measurement results, as it is shown in the following screen copy.

It also makes it possible to export the results in order to process the data with other software (text files). The following results are presented in this form.

1/ Results related to the variations of the foam volumes

The figure 1 shows these variations.

Comments :

A) When the bubbling is stopped, the foam volumes reached by the foams are about 100 cm3, as expected from the measurement protocol.

B) The behaviour differentiation during the collapse phase can be quantified by the collapse speed of the foams

• C12E6 -> 5.1 cm3/min

• SDS -> 2.6 cm3/min

• Ovalbumine -> 0.6 cm3/min

2/ Results related to the quantities of liquid contained in the foams

This behaviour differentiation is also shown with the variations of the liquid volumes in the foams (figure 2).

Comments :

A) Also the foams have been created in the same conditions and the same speeds, the figure 2 shows that they can drag very different quantities of liquid.

B)During the collapse phase, they also very different drainage speeds. That of the C12E6 foam is much higher than the other ones, that of the ovalbumin is the lowest.

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3/ Results related to the variations of the foam conductance

The figures 2 and 4 show the variations of the foam conductance measured with the pairs of electrodes n° 1 and n° 2 (see picture above; the foams did not reached the pair of electrodes n° 3).

These results show the different liquid volume in the foams and the different drainage speeds with time at each levels of the glass tube (see following note).

The figures 5 and 7 show the different conductance relative to the foams with the heights of the electrodes and give an information about the foam density gradient with the height.

Note about the liquid quantities in the foams

The FOAMSCAN measures the overall quantity of liquids in the foams, as it is shown in the figure 2. However the conductance measurements at different heights give indications relative to the liquid quantity gradients.

Reminder :

The conductivity of the foams between two electrodes of the glass tube (in micro-siemens/cm) can be determined from their conductance. They are a function of the conductivity and quantity of liquid between these electrodes.

The relative conductivity of foams is defined by the following ratio:

which is a representation of the liquid fractions in the foams ([1] and [2]).

The conductivities of liquids in foams are not known. Indeed, when the liquids are dragged into the foams, the number of surfactant molecules is shared between the interfaces and the liquids in the foam lamella. Then the surfactant concentrations in the lamella are different from that in the liquid samples. However one can use the liquid conductivity of the liquid samples in order to get indication of the liquid quantities in the foams.

[1] P.J. Wilde and D.C. Clark
Methods of testing protein functionnality / Chapter 5 : Foam formation and stability.

[2] A. Einstein The Physiscs of Foams / Oxford / Chapter 9 : Electrical conduction in a foam

[3] D. Exerowa, P. Kruglyakov Foam and foam films / Elsevier / Chapter 4 : Foam structural parameters and related properties : techniques for determination

Interpretation of the results got with the FOAMSCAN :

The measurement protocol here applied makes it possible to assume that the conductivity of the liquids in the foams are the same as those of the liquid samples poured in the cuvette.

Indeed, as the C12E6 and the ovalbumin have been prepared in buffers, the conductivity of the liquid phases are those of the buffers and do not depend on the surfactant concentrations. By another way, as the SDS is an ionic surfactant, the behaviour of liquid is similar to that of buffer liquids concerning conductivity.

Then one can assume that the relative conductivity of foams in the above results is the liquid fractions at each pair of electrodes, as usually defined.

For information,
the liquid fractions calculated when the bubbling are stopped are :

Surfactant	Liquid fractions (électrode 1)	Liquid fractions (électrode 2)
C12E6 (5g/l)	5%	2.50%
SDS (5g/l)	3%	1.30%
OVALBUMIN (2g/l)	2.50%	1.00%

CONCLUSION

The FOAMSCAN makes it possible to differentiate the behaviour of foams by measuring versus time the volume of the foams, the liquid volumes dragged into them, their drainage, and their conductance at different heights and liquid fractions.