Dynamic surface tensions, foam and the transition from micellar solution to lamellar phase dispersion

COLLOIDS

AND ELS EV I ER

Colloids and Surfaces A: Physicochemical and Engineering Aspects 103 (1995) 127-145

A

SURFACES

Dynamic surface tensions, foam and the transition from micellar solution to lamellar phase dispersion P . R . G a r r e t t *, P . L . G r a t t o n

Unilever Research Port Sunlight Laboratory, Wirral, Merseyside, UK Received 8 February 1995; accepted 21 April 1995

Abstract

If the packing behaviour of surfactants in close-packed layers is sufficiently modified then micelles are not formed. Solutions can become turbid at a critical aggregation concentration where a dispersion of liquid crystalline particles is found. Examples of this type of behaviour considered here include dodecyl polyethyleneglycol ethers with low degrees of ethoxylation. The precipitation of liquid crystalline particles either from mixtures of such compounds with micelle forming anionic surfactants or from mixtures of micelle forming zwitterionics with certain micelle forming anionics are also included. Rates of transport of surfactant to air-water surfaces may be monitored by measuring dynamic surface tensions. Solutions containing dispersed liquid crystalline phase, rather than micelles, exhibit diminished rates of transport to air-water surfaces which implies relatively low levels of dynamic adsorption at these surfaces. This leads to diminished foamabilities. However the stability of the resulting foam may be enhanced. This may be caused by diminished bulk foam drainage rates due to accumulation of liquid crystalline material in Plateau borders. It may also derive from diminished foam film drainage rates caused by suppression of marginal regeneration.

Keywords: Dynamic surface tension; Foam; Lamellar phase dispersion; Micellar solution

1. Introduction

The role of the dynamic surface behaviour of adsorbed surfactant layers in determining foam behaviour is likely to be complex. However the effect on foam behaviour of the extremes of possible dynamic behaviour appears to be clear if we consider surface tension gradients. It is often argued that surface tension gradients are necessary if foam films are to withstand external stress [ 1 ]. If surfactant transport to surfaces is sufficiently rapid then ¢~ Paper presented at the 10th International Symposium on Surfactants in Solution held in Caracas, Venezuela, 26-30 June, 1994. * Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03218-5

surface tension gradients are eliminated and foam films may thin catastrophically by plug flow [-1,2]. However, even with rapid surfactant transport to surfaces it is possible that surface tension gradients may be established by depletion effects as films become sufficiently thin. Thus surface tension gradients can exist as a result of differential stretching of film elements of different thicknesses (i.e. where different film elements exhibit different Gibbs elasticities) if the elements are so thin that they exhibit depletion of intralamellar surfactant upon dilation. Eventually films may thin until disjoining forces dominate. Here any positive contribution to the disjoining force from adsorbed surfactant may dominate over effects due to minimisation of surface tension gradients by rapid transport of surfac-

128

P.R. Garrett, P.L. Gratton/Colloids Surfaces A. Physicochem. Eng. Aspects 103 (1995) 127-145

tant to air-water surfaces. We therefore have effects due to dilational depletion (i.e. Gibbs elasticity) and disjoining forces which can mean foam film stability is assured even though surfactant transport to surfaces is extremely rapid. Nevertheless there are examples in the literature where the latter dominates and foamability is low [3]. At the other extreme, where surfactant transport is slow, the air-water surface may be relatively denuded of surfactant during foam generation if the rate of air-water surface generation is sufficiently rapid. Surface tension gradients will then, of necessity, be of limited magnitude because surface tensions will be everywhere high. If transport is sufficiently slow then stretching a film will mean de/dh > 0 where e is the "dynamic" Gibbs elasticity and h is the film thickness [1,2]. This will mean that thinner parts of the film will offer less resistance to stretching in response to any applied stress. Any positive contribution to the disjoining forces will also be low if surfactant adsorption is low under dynamic conditions. We therefore find a relatively unambiguous situation where slow transport to the ai~water surface is expected to produce low foamability. We may investigate the rate of surfactant transport to air-water surfaces by measuring dynamic surface tensions. This should, however, be measured using a method which affords access to the surface ages characteristic of the foam generation methods under consideration. Here we consider foam generation by the Ross Miles and shaking measuring cylinder techniques for which surface ages in the range 0.05-1 s are apparently relevant [4]. Such surface ages are conveniently accessed by the maximum bubble pressure method for measurement of dynamic surface tensions [4]. It is known that the foamability of micellar solutions is often largely determined by the properties of the micelles (especially if the CMC is low relative to the total surfactant concentration). The relevant properties are the micelle diffusion coefficient and the rate of micelle breakdown. It has been shown, for example, that foamabilities may decline in micellar solutions of homologous anionic surfactants as the CMC declines. Somewhat tentatively this behaviour has been ascribed, not to

declining rates of micelle diffusion, but to declining rates of micelle breakdown [4]. Here we consider the effect of the transition from micellar solution to dispersion of lamellar phase. The transition is affected by changes in the packing behaviour of the surfactant. This is achieved by either changing the degree of ethoxylation of alkyl polyethyleneglycol monoethers, addition of a lamellar phase forming alkyl polyethyleneglycol monoether to an anionic surfactant or mixing of zwitterionic surfactants with anionic surfactants. All of these systems form turbid solutions due to the presence of particles of lamellar phase. It is well known that such materials can stabilise existing foam (see for example Refs. [5-7]). However, it is also possible that transport to air-water surfaces is slow because of slow diffusion and breakdown of lamellar phase particles which are large relative to the size of normal micelles. The transition from micellar solution to lamellar phase dispersion may therefore be expected to produce diminished foamability (where foamability means initial foam volumes just after aeration has ceased). The stability of the resulting foam may however be enhanced. We therefore present a study of equilibrium and dynamic surface tension and foam behaviour in order to compare these predictions with experiment.

2. Experimental 2.1. Materials n-Dodecyl polyethylene glycol mono ethers, C12E3, C12E4, C12E5, C12E6 and C12E7 were used as received from Nikko Chemicals Co. The sample of sodium 5-phenylundecyl-benzene para-sulphonate (5~-CHLAS) had a purity of 97.3% and that of sodium 6-phenyldodecyl-benzene-para-sulphonate (6~Z~-C12LAS) had a purity of 94.4%, with the major impurity water in both cases. A sample of N,N-dimethyldodecylamine butyrate (Cx2 betaine; C12H25 N + (CH3)2 (CH2)3 CO2) of purity>97% by NMR was used. The sample of N,N-dimethyltetradecyl propane-sulphonate (TDPS; C14H29N+(CH3)2(CH2)3503) had purity 95% by chromato-

P.R. Garrett,P.L. Gratton/ColloidsSurfaces A: Physicochem.Eng. Aspects103 (1995) 127 145 graphy and was subsequently recrystallised twice from acetone/propanol. Sodium tripolyphosphate (STP, NasP3Olo ) was recrystallised twice as the hexahydrate from water to give material of purity > 98%. Sodium carbonate and calcium chloride were Analar grade materials ( B D H ) and used as received. Unless otherwise stated all surfactant solutions were prepared at 5 x 10 .3 M in a solution of an electrolyte mixture of 2.5 x 10 .3 M STP, 3.8 x 10 .3 M Na2CO 3 and 2 x 10 .3 M CaC12. This electrolyte solution had a p H of about 10 and an ionic strength of 0.055 M. Turbid solutions containing dispersed liquid crystalline phase were prepared by first cooling in ice to 0°C for about 4 h then warmed to 25°C (the temperature used throughout for all measurements) and held in a thermostat bath for at least 12 h. Filtration of these solutions was done with Millipore surfactant-free filters (type GS) of pore size 0.22 gm.

2.2. Methodology Foam measurements by the Ross Miles technique and equilibrium surface tension measurements by the Wilhelmy plate technique have both been previously described [4]. F o a m behaviour was also determined by vigorous shaking of 25 cm 3 solution in a 100 cm 3 graduated cylinder for 10 s. The experiments were done at ambient temperature (21.5°C) using solutions which had been thermostatted at 25°C. Dynamic surface tensions were measured by the maximum bubble pressure technique as described previously [ 8 ]. For solutions which exhibited high turbidity the technique was modified so that bubble frequencies and the "dead time" could be estimated from measurements of the pressure fluctuations accompanying bubble formation. The pressure fluctuations were measured using an electronic micromanometer (Furness Controls, type FC001). Videomicroscopy was done using a Zeiss photomicroscope III fitted with a differential interference optical system. A H a m a m a t s u C2400 Newvicon camera was used to obtain images which were processed using a suitable image analyser. Absorbance measurements were made using

129

either a P y e - U n i c a m SP8-100 or a L a m b d a 2 UV-visible double beam spectrophotometer at 500 nm wavelength with glass cells of either 10 m m or 40 m m path length. Foam films were drawn using a glass frame in a manner previously described [9]. The movement of the distinct black/silver film boundary was observed using a cathetometer. All measurements were made at 25°C.

3. Results and discussion

3.1. Effect of degree of ethoxylation of dodecyl polyethyleneglycol ethers Here we consider aqueous solutions, at 25°C, of 5 x 10 . 3 M dodecyl polyethyleneglycol monoethers, C1/E, where 3 ~15 these solutions are clear, isotropic and micellar. However turbid solutions are found for surfactants with n ~<4. For C12E3 and C12E4 the first liquid crystalline phase formed at 25°C as the surfactant concentration is increased is lamellar (L~) [10]. The turbid solution therefore consists of dispersed lamellar phase particles and dissolved surfactant monomer. A complete summary of the relevant properties of all these surfactants is given in Table 1. Here we see that the effect of the electrolyte mixture on the critical micelle and critical aggregation (CAC) concentrations is, not unexpectedly, small for these solutions of total ionic strength 0.055 M. A difficulty with lamellar phase dispersions concerns stability and the attainment of equilibrium. Solutions were held at 25°C for at least 12 h in an attempt to ensure equilibrium between m o n o m e r and lamellar phase before measurement of absorbance, surface tension and foam behaviour. Absorbance at 500 nm as a function of degree of ethoxylation n is presented in Fig. 1. Turbid solutions were agitated immediately before measurement to avoid effects due to sedimentation. This problem was more pronounced with solutions of C12E 3 than with solutions of C12E4. it probably

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127 145

130

Table 1 Physical properties of dodecyl polyethyleneglycol ethers Ether

Cloud point in water (~C)

CAC a or CMC in water at 25°C (M)

CAC a or CMC in electrolyte b at 25°C (M)

Solution state (either water or electrolyte b at 5 x 10 -3 M, 25°C)

C12E 3 ClzE4 C12E5

<0 5 10c 26 d

3.2 × 10- s 4.3×10 s

C12E6

50 d

C12E7

66d

5.5× 6.4× 6.4 x 7.9x 8.2 ×

Lamellar (LcO Lamellar (Let) Micellar (L 0 Micellar (L1) Micellar (L~)

10 5 10 -5 10 -5 10 -5 10 -5

jill [121 [121 Jill [12l

6.0 × 10 5

aCritical aggregation concentration (CAC) for lamellar phase dispersions. bElectrolyte is 2.5 x 10 3 M STP, 3.8 × 10 3 M NazCO 3, 2 x 10 -3 M CaCI2. CMeasured for 0.1% solutions. aMeasured for 1.0% solutions.

2,0

Abeotbence 75

S u r f a c e Tension / mN m ' l ...........

1.8

70-

1.6

\

65

1.4

dispersed

elle

Surface Tension of water

~ ~2S

6O

1.2 ~

55

1.00.00.60.430

0.2 0.0

, 3

, t~ ~ 4 5 6 Ethoxylatlon number (n)

0 7

equilibrium

25 8

2

3 4 6 6 E t h o x y l a t i o n n u m b e r (n)

7

8

Fig. 1. Plot of absorbance (500nm, 10mm path length at 25°C) against ethoxylation number n for ClzE, at 5 x 10 -3 M surfactant (in solutiorl of electrolyte mixture, see experimental section).

Fig. 2. Dynamic and equilibrium surface tensions against ethoxylation number n for ClzE, at 5 x 10 3 M surfactant (in solution of electrolyte mixture, see experimental section).

correlates with the higher absorbance found for the C12E 3 solution. This means more intense light scattering which is in turn unlikely to be due to differences in volume fraction of dispersed lamellar phase because the CAC values of these two surfactants are negligible compared with the solution concentration. More intense light scattering is therefore likely to be due to large particle sizes which would also account for the greater sedimentation rate in C12E3solutions. Equilibrium and dynamic surface tensions are presented in both Fig. 2 and Table 2. Here we see

a monotonic decrease in equilibrium surface tension with decreasing degree of ethoxylation n. Dynamic surface tensions at surface ages of 1 s and 0.2 s are seen to increase dramatically for lamellar dispersions of C12E3 and C l z E 4. By contrast the dynamic surface tensions for the micellar solutions (where n~>5) are seen to be relatively weakly dependent upon surface age. The micellar solution concentrations considered here significantly exceed the C M C (or CAC) so that about 99% of the surfactant is present in the form of micelles or dispersed lamellar phase.

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127-145 Table 2 Equilibrium and dynamic surface tensions for solutions of dodecyl polyethyleneglycol ethers

")'o-')'e/mN

l 3l

m "1

50 46

Ether

Equilibrium Dynamic surface tension surface (mN m 1) tension ( m N m ~) at l s ~ at 5s I

Absorbance 500 nm, 10 mm path length

Theory 40 /J

35 30 26

CI2E 3 C12E4

C12E5 C12E6

C12E7

27.45 28.40 30.39 32.09 33.12

48.5 37.1 31.7 32.2 34.0

~72 49.5 34.1 33.1 35.4

1.730 0.379 0.004 0.002 0.004

20 15-

Diffusion and breakdown of either micelles or dispersed lamellar phase is therefore potentially an important aspect of transport of surfactant to airwater surfaces. The relevance of such processes in for example the case of C 1 2 E 6 solutions is illustrated by calculation of the dynamic surface tension behaviour to be expected when the presence of micelles is ignored. Thus for a sub-micellar single component surfactant solution we can write, after Hansen [ 13], for the dynamic surface tension rD

T

x/1/t

( 1t

provided is small and assuming that surfactant transport rates are determined only by diffusion. Here ~e is the equilibrium surface tension, F is the equilibrium surface excess, R is the gas constant, T is the temperature, D the surfactant monomer diffusion coefficient and t is the time. C is the concentration which in this context is equal to the CMC. Eq. (1) may be directly applied to C12E 6 solutions using published values for D (3.5 x 10 lO m 2 S 1) [11] and F (3.0 x 10 -6 m o l m 2) [11] for electrolyte-free solutions. In using these values we therefore assume a negligible effect for the electrolyte present in the solutions considered here. A plot of ri~ against x / ~ , calculated using Eq. (1), is compared with experiment for 5 x 1 0 - 3 M ClzE6 solutions in Fig. 3. The gradients of plots of rD - re against ~ are more

/

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//

5-

Measured at concentration of 5 x 1 0 - 3 M surfactant in 2.5x10 3M STP, 3.8x10 3M NaaCO3, 2 x 1 0 3M CaC12 at 2YC.

rD - re -

/

10-

/

/

/

/

/ //

0[ 0

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Experiment

/

[] []

, rnl:~D 1

21

[]

[]

D

~ 0 0 [] [] 2

3

4

5

(1/t)1/2/ 8-1/2 Fig. 3. Comparison of theory and experiment in plots of'~'D - )'e against t , ~ for 5 x 10 -3 M solution of C12E6 at 25~C (in solution of electrolyte mixture, see experimental section). Theory calculated using Eq. (l) and ignoring the presence of micelles.

than an order of magnitude higher than those found experimentally. This must represent strong evidence that micelles participate in the transport process. We now take the opposite extreme view of transport behaviour in this C12E 6 solution by ignoring the contribution of monomers and assume that the rate determining process in surfactant transport to the air-water surface is micellar diffusion. Here we suppose that micelles diffuse to the region of the air-water surface and instantaneously dissociate to form monomers which then adsorb. We can therefore write for the characteristic time of this process l ' 2 / C Z D m where Cm is the total micellar surfactant concentration and Dm is the micelle diffusion coefficient. Taking a value of D m ~ l 0 lo m 2 s 1 f r o m Ref. [14] we find a characteristic time for attainment of equilibrium of ~0.004 s. Examination of Fig. 3 however reveals significant, but small, deviations from equilibrium for surface ages of ~0.06 s (i.e. at x / q ~ 4 s 1/2). Nevertheless this represents much better agreement with experiment than is found when the presence of micelles is ignored. The conclusion that transport of C12E6 t o airwater surfaces may involve micelles in consistent with the findings of Lucassen [ 14]. Here measure-

132

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127-145

ments of dynamic dilational moduli indicated enhanced rates of transport in micellar solutions. Lucassen interpreted this behaviour using a quantitative model based upon a simplified description of micelle breakdown due to Kreschek et al. 1-15]. In this treatment it is supposed that micelles do not adsorb but first break down to surfactant monomers in regions near the air-water surface where depletion by adsorption produces monomer concentrations below the CMC. We must now consider the dynamic surface tension behaviour of lamellar dispersions of C12E3 and C12E 4. Again we may use Eq. (1) to calculate ~2D- }'e' For this we use F = 4 . 6 × 10 - 6 m o l m 2 [16] and 3.78 × 10 - 6 m o l m 2 [17] for C I z E 3 and C12E 4 respectively. Both values were obtained from neutron reflection measurements [-16,17] on electrolyte-free solutions so that again we neglect the effect of the electrolyte present in the solutions considered here. The diffusion coefficient of C12E3 monomer is reported [11] to be 4 x 10 -1° m 2 s 1 and we may reasonably, with sufficient accuracy, give the same value to C12E4 (the diffusion coefficient enters into Eq. (1) as the square root). These values of F and D, together with the critical aggregation concentrations given in Table 1, may be substituted into the Hansen equation (1) to give predicted values of 7D --?e assuming no contribution from the dispersed lamellar phase. The plots are compared with experiment in Figures 4 and 5 for C12E4 and C12Ea respectively. Here we see that the calculated gradients of plots of "~D-- )'e against x/Ut (as x / ~ 0 ) are within a factor of about two of the experimental gradients which compares with more than an order of magnitude difference in the case of C12E 6. Clearly then the theoretical and experimental gradients in the case of C12E4 and C12E3 are sufficiently close to suggest that the dispersed lamellar phase makes a much smaller contribution to the overall process of transport of surfactant to air-water surfaces under these dynamic conditions than is found in the case of micelles in C12E 6 solutions. Lower dynamic surface tensions found for C12E 4 with respect to C l z E 3 may simply concern a relatively larger contribution from transport of lamellar phase, higher monomer concentrations (see Table 1) and lower values of F. That the contribution from transport of lamellar

")'D-'Ye/ mN m "1 60

/

45

Theory

40 •

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// 35

//

30 •

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25

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20

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Experiment

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// 16

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[2~ []

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10

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5

[~

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[~

0

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1

3 (1/t)1/2/ S 1/2

Fig. 4. Comparison of theory and experiment in plots of), D - ~e against t x ~ for 5 x 10 3 M solution of C12E4 at 25°C (in solution of electrolyte mixture, see experimental section). Theory calculated using Eq. (1) and ignoring the presence of dispersed lamellar phase.

"(D-'Ye/ mN m -1 50 ~

4s-

Theory /

40-

/

36-

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30-

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2520-

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15 -

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10 -

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6 ~r//

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o~ 0

1

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( l / t l V 2 / s-1/2

Fig. 5. Comparison of theory and experiment in plots of),o - ~,¢ against t~fi~r for 5 × 10 -3 M solution of ClzEa at 25°C (in solution of electrolyte mixture, see experimental section). Theory calculated using Eq. (1) and ignoring the presence of dispersed lamellar phase.

phase could be greater with C12E 4 is suggested by smaller particle sizes (as revealed by turbidities) and therefore larger diffusion coefficients. It seems possible that any transport of surfactant from particles of dispersed lamellar phase to the air-water surface does not concern breakdown to

P.R. Garrett, P.L. Gratton/Colloids Surfaces A. Physicochem. Eng. Aspects 103 (1995) 127-145

monomers in the manner suggested by Lucassen [14] (and others [18]) for micelles. It may occur by a combination of direct emergence of particles into the air-water surface followed by spreading as described by Veer and van den Tempel [19] for aqueous suspensions of "mesomorphous" long chain alcohol particles. Ross Miles foam behaviour is presented in Fig. 6 for lamellar phase dispersions and in Fig. 7 for micellar solutions. The foamabilities (i.e. foam heights immediately after foam generation has ceased) of the lamellar phase dispersions are seen

2.5-

133

to be low relative to those of the micellar solutions. However the stability of the resulting foam appears to be higher. This is revealed more clearly if we consider Table 3 where the relative foam heights after 30 min are compared. Foamability measured by cylinder shaking is compared in Fig. 8 with that measured by the Ross Miles technique (where results are presented relative to the foamability of C 1 2 E 7 solutions). The behaviour is essentially similar with the exception that the foamability peak at C 1 2 E 5 is not present. It is tempting to correlate the low foamabilities of the lamellar dispersions with slow transport of

Foam height / cm Table 3 Relative foam stabilities of solutions a of dodecyl polyethyleneglycol ethers

C12E4 2.0

Ether

Structure

Relative foam height after 30 min b

Cl2Ee C12E 4 C12E 5 C12E6 C 1zE7

Lamellar dispersion Lamellar dispersion Micellar Micellar Micellar

0.75 0.70 0.23 0.22 0.10

1.6

1.0

~

,~

C1=E3

0.5

0.0 0

200

400

600

800

1000

Time / e Fig. 6. Ross Miles foam height vs. foam age for CI2E 3 and C12E a (5 × 10-3 M surfactant in solution of electrolyte mixture at 25 'C).

Measured by the Ross Miles technique at 25°C. aAll solutions 5 x 1 0 - 3 M surfactant in 2.5x 10 3 M STP, 3.8 x 10 3 M NazCO3, 2 x 10 -3 M CaC12. bFoam height after 30 min divided by initial foam height.

Foamebility Relative to CI2E7 1.4-

Foam height / cm

1.2

C12E6

14

Ross-Miles S

1.0

~

-

~

_

0.8-

Shaking

0.60,40.2-

"

C12 Er

2

~

0.0 3

4

5

6

7

Ethoxylation number (n) 200

400

600

800

1000

Time / s Fig. 7. Ross Miles foam height vs. foam age for Ct2Es, C12E6 and C12E 7 (5 x 10 3 M surfactant in solution of electrolyte mixture at 25~C).

Fig. 8. Plot of foamabilities (relative to those for C12E7) against ethoxylation number n for C12E, at 5 x 10 -3 M surfactant (in solutions of electrolyte mixture, see experimental section). Foamabilities measured using both Ross Miles and cylinder shaking techniques.

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127-145

134

surfactant to air-water surfaces. This is of course the origin of the relatively high dynamic surface tensions for such solutions. Slow transport will mean low dynamic adsorption levels which in turn will mean low resistance to dilational stress and low contributions to disjoining forces. These factors should conspire to enhance the probability of foam film rupture and thereby diminish foamability. An alternative explanation must concern the possibility that lamellar phase particles may adsorb directly onto the air-water surfaces of foam films to produce Marangoni shocks and cause film rupture. That such antifoam effects are unlikely to occur is readily shown if we compare the foamability by cylinder shaking of C12E3 and C l z E 4 solutions, at their respective critical aggregation concentrations, with the foamability of the lamellar phase dispersions at 5 × 10 -3 M. Results are presented in Table 4. The foamabilities of the monomeric surfactant solutions are seen to be lower than those of the lamellar phase dispersions. This would suggest that the low foamability of the lamellar phase dispersion does not derive from any antifoam effect. A similar conclusion is made by Aveyard et al. [20] for the decline in foamability of Aerosol O T (sodium bis-(2-ethylhexyl) sulphosuccinate) solutions in aqueous NaC1 when a dispersed liquid crystalline phase is formed as the NaCI concentration is increased. It is of interest to speculate about the cause of the relatively high stability of foam formed from lamellar phase dispersions. This could be due to accumulation of the dispersed phase at Plateau border junctions in the same manner as suggested Table 4 The effect of dispersed lamellar phase on foamability (by cylinder shaking) of solutions of dodecyl polyethyleneglycol ethers Ether

CIEEa C12E4

Volume of air in foam (cm3) at CAC

at 5 x 10 3 M surfactant

3.5 5.5

4 17

All solutions in 2.5x 10 3M STP, 3.8x 10 3M Na2CO3, 2 × 10 3 M CaCI 2 at 25'C (air, 21.5 C).

by Koczo et al. [21] for hydrocarbon emulsion droplets. Drainage rates for the foam would then be reduced and the time taken to thin films to any putative critical rupture thickness increased. An alternative explanation may concern the effect of the presence of dispersed lameltar phase on the process of drainage within foam films. We will consider this possibility in more detail below for other lamellar dispersions.

3.2. Effect of mixing surfactant

C12E 3 with

an anionic

As we have seen, C12E3 does not form micellar solutions at 25°C, but yields turbid dispersions of lamellar phase. It is therefore of interest to mix this surfactant with a micelle forming anionic surfactant in order to assess the effect of mixing ratio on equilibrium and dynamic surface tensions and on foam behaviour. In Fig. 9(a) we plot the turbidities of 5 x 10 -3 M solutions of mixtures of C12E3 and 5~-C11LAS as measured by absorbance at 500 nm. The solutions were again prepared in a background electrolyte mixture of 3.8 × 10 -3 M Na2CO3, 2.5 x 10 3 M STP and 2 × 10 -3 M CaC12. Clear isotropic micellar solutions were found for mole fractions C12E 3 ~<0.3 ( C M C o f 5~3-CI~LAS under these conditions is 1.14 x 10 -3 M and for a mixture of 0.3 mole fraction C12E 3 it is 6.8 × 10 5 M). However for higher mole fractions of C12E3 the solution becomes progressively more turbid. It seems probable that this turbidity is due to dispersed liquid crystalline material. The nature of this material was shown to be lamellar phase by microscopic examination, with crossed polarisers, of the material centrifuged from a 1.5 × 10 - 2 M solution of the mixture (of 0.7 mole fraction CtzE3 in the same background electrolyte). As for pure C12E 3 increasing turbidities probably correlate with increasing particle size at essentially constant volume fraction of dispersed material. It is tempting to suggest that particle size may derive from decreasing spontaneous curvature of mixed lamellar phase as the proportion of anionic surfactant declines with a concomitant decline in electrostatic contribution to the curvature. Equilibrium surface tensions are given in

135

P.R. Garrett, P.L . Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127-145

Absorbance 0.4.

0.3. 0.2

,

micelles

dispersed lamellar phase

/ ~

r~

0.1

0.00.0 (a)

011

0[2

0"7.3 6.4 0.5 0.6 Mole Fraction C12E3

0'.7

o18

Equilibrium surface tension measurements at 25 °C were 27.35 m N m - 1 and 27.37 mN m - x before and after storage respectively. Absorbances were respectively 0.022 and 0.028. These differences are small. They suggest that equilibrium between the liquid crystalline phase and solution is established in at most a few hours in these systems. Constant surface tensions for the lamellar phase dispersions could imply identical composition of both lamellar phase and air-water surface. This can be illustrated if we develop an argument outlined by Hall [-22]. We can write the GibbsDuhem equation of the air-water surface as F~- d/z + + F [ - d / t [

(plus remaining terms for non-surfactant

7dmN m- l 31 30

~

m

icelles

electrolytes) = - dTo

dispersed lamellar phase

~

~

_ [] H

27 0.0 (b)

011

0.'2

--

[]

u

013 014 015 0'.6 Mole Fraction Ct2E3

(2)

where F~- and /~+ are the surface excess and chemical potential of Na ÷ ions, F;- and # are the surface excess and chemical potentials of the surfactant anion and F2 and #2 are the surface excess and chemical potential of C~2E3. All remaining terms refer to the non-surfactant electrolyte (excepting Na+). It is obvious from Eq. (2) that we may write

2928-

+ F2-d/.t 2

o17

o18

Fig. 9. Plots of: (a) absorbance (500 nm, 40 mm path length); (b) equilibrium air-water surface tension '/e at 25°C against mole fraction C12E3 for mixtures of 5~-CnLAS and C12E3. Total surfactant concentration 5 x 10-3 M (in solutions of electrolytemixture, see experimentalsection).

Fig. 9(b) where they may be compared with absorbance measurements (Fig. 9(a)). Here we see that increasing proportions of C12E3 reduce the airwater surface tension until it becomes essentially constant at the same mole fraction of C~2E3 (0.3) at which the turbidity increases and dispersed lamellar phase is present. As a check on the attainment of equilibrium in the systems containing liquid crystalline phase, measurements of surface tension and absorbance were repeated on solutions of mole fraction C12E 3 of 0.45 after one week of storage at I°C.

1(

d#;-

dp-

dp2~

F+ -d~l + F[ ~ ( + r2 dY,/

d#+ d#[ d#2 = x+ dy~ + X[ -d- y l + X2 - -

1 dTe FT dY1

(3)

for a change in total mole fraction Y1 of anionic surfactant at constant concentration of background electrolyte (if we ignore the change of the activity coefficients of the background electrolyte components). Here x~-, xi- and x2 are the mole fractions of Na + , surfactant anion and nonionic surfactant in the surface and Fx is the total surface excess ( = F+ + F1 + F2 + ...). Similarly we may write for the lamellar phase dp~d/~ d/~2 2+ ~ y ~ [ + 2 , ~ - 1 +22 dY,l=0

(4)

where 2 [ , 2 [ and 22 are the mole fractions of Na ÷ , surfactant anion and nonionic surfactant in the lamellar phase. Clearly if we have identical

P.R. Garrett, P.L. Gratton/Colloids SurJaces A: Physicochem. Eng. Aspects 103 (1995) 127-145

136

compositions for the surface and lamellar phase then x~- = 2~, xl = x l , and x 2 = "~2 which means that we must have =0

dY1

Foamheight/cmRelativeto 100%LAS Cylinder

~ ~ a k i n g

1.01=

0.9

Dynamic surface tensions ~D for mixtures of C ~ 2 E 3 and 5 ~ - C l l L A S are shown in Fig. 10. Here ?'D is seen to rise dramatically with increasing proportions of C 1 2 E 3 in the region where lamellar phase is present. This presumably correlates with declining contributions of dispersed lamellar phase to the overall rate of transport of surfactant to the air-water surface as the particle size increases (as revealed by increasing turbidities). initial foamabilities measured by the Ross Miles and cylinder shaking techniques are shown in Fig. 11. The foamabilities are plotted relative to values found with mole fraction unity of 5 ~ CHLAS (where Ross Miles foamability was 16 cm and shaking gave a full cylinder of foam c o n t a i n i n g ~ 9 5 c m 3 air). Although drainage and bubble disproportionation due to diffusion were apparent, the foam volume was essentially stable for > 600 s for all solutions considered. Declining foamabilities are clearly apparent in Fig. 11 at compositions where dispersed lamellar phase is present. This is similar behaviour to that

DynamicSurface TensionI mN m- ]

surfaceage • 0.05s

85

x ~ -

so-

~

dispersed

4550-56 i micelles

lamellar x/ phase , / 0.5s

,o-

: / x×

86- _ _

~× _ ~ u ~ D

3oi

0.0

1.1

°

~ O

[]

O

[]

equilibrium []

25

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Mole Fraction C12E3

Fig. 10. Plot of dynamic surface tension against mole fraction C12E3 at 25°C for mixtures of 5~-CI~LAS and C~2E3 . Total surfactant concentration 5 x 10 3 M (in solution of electrolyte mixture, see experimental section).

0.8

Ross Miles

~ ~

o.r 0.6

0.5

dispersed lemellar micelles phase

0.4 o.ao

!

\

)

i

i

0.1

0.2

0.3

i

i

i

i

0.4

0.5

0.6

0.7

0.8

Mole fraction C12E3 Fig. 11. Plots of foamabilities (relative to those for solutions of pure 5 ~ - C l l L A S ) against mole fraction C12E3 for mixtures of 5 ~ - C H L A S and CIzE 3. Total surfactant concentration 5 x 10 -3 M (in solution of electrolyte mixture, see experimental section). Foamabilities measured using both Ross Miles and cylinder shaking techniques.

found with the pure dodecyl polyethyleneglycols. Millipore filtration to remove the dispersed lamellar phase did not increase foamability. Antifoam effects due to lamellar phase would therefore appear to be absent. It is therefore probable that relatively low foamabilities in the presence of dispersed lamellar phase correlate with the high dynamic surface tensions and concern relatively low dynamic adsorption levels due to slow transport of surfactant to air-water surfaces. The presence of dispersed lamellar phase is seen in Fig. 11 to have a more pronounced effect on the foamabilities measured by cylinder shaking than on those measured by the Ross Miles technique. This could concern more rapid rates of air-water surface generation with cylinder shaking which would, in turn, mean lower surfactant adsorption levels and lower foam film stabilities under the relevant dynamic conditions. However no such differences between Ross Miles and cylinder shak-

PmR. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127 145

ing in the presence of lamellar phase were observed with the pure dodecyl polyethyleneglycols. That the presence of dispersed lamellar phase causes a decline in foamabilities would appear to contradict claims by, for example, Friberg [-7] and Roberts et al. [-5] that liquid crystalline phases can stabilise foams. We should stress however that such claims usually concern highly concentrated solutions. Here the characteristic time for adsorption by diffusion, FZ/CZD, is likely to be small despite low values of D because of extremely high values of C. The claims of these authors also usually concern the stability of foam with respect to drainage and/or diffusional disproportionation. Foam film drainage in the system considered here is in fact inhibited when dispersed lamellar phase is present. This is illustrated in Fig. 12 where we plot the height as a function of time of the black/ silver film boundary for liquid films drawn on a glass frame from an aged air-water surface.

3.5

Boundary Height / cm

~ ,

~

Ic phase present - rigid film

(C)

1.5

~

s

t

s

here

(b)

0.5

U

o

i

~

500

looo Film age / s

i

15oo

i

2000

Fig. 12. Plot of the rate of movement of the black/silver film boundary in liquid film for (a) pure 5~-CllLAS (micellar solutions, mobile film), (b) mole fraction ClzE3 of 0.3 (micellar solution, mobile film) and (c) mole fraction of C12E3 of 0.7 (dispersed lamellar phase present, rigid film). Mixtures are 5~C]ILAS and C12E3at 25°C and total surfactant concentration is 5 × 10 3 M (in solution of electrolyte mixture, see experimental section).

137

For solutions from which lamellar phase is absent relatively rapid film drainage is observed, indicated by a relatively rapid rate of decline of the height of the black/silver film boundary. In the case of a film drawn from a solution containing pure 5 ~ - C l I L A S this leads to film collapse after about 15 min. The presence of ClzE 3 appears to stabilise the film against such a collapse. These relatively rapidly draining films formed from solutions from which liquid crystalline phase is absent are in fact "mobile" films [23], that is they exhibit the phenomenon of marginal regeneration which is the main drainage process in such films. This process occurs at the margins of the films where thick film elements, formed by random fluctuations, are sucked into the Plateau borders as thin elements are drawn out [23]. Films drawn from solutions at 0.7 mole fraction ClzE3, which therefore contain liquid crystalline phase, exhibit markedly different drainage behaviour. Soon after the black/silver film boundary has developed the process of marginal regeneration ceases and the film becomes "rigid". In this situation film elements do not move with respect to one another and the only possible drainage mechanism is Poiseuille flow through a film of thickness everywhere < 1 lam which is negligibly slow over the time of the experiment. As we see from Fig. 12 the black/silver film boundary height then ceases to decline. The onset of such rigid film behaviour is usually associated with high surface dilational or shear storage moduli. High values of surface dilational moduli could be expected if levels of surfactant adsorption are high and transport to and from the air-water surface is slow. As we have seen the compositions of the surface and lamellar phase are similar in this system. It is hard to see how this could be reconciled with significant differences in molecular area. Therefore if molecular areas at the air-water surface are similar to those in the lamellar phase then this would mean relatively high levels of adsorption (based upon consideration of molecular packing in the lamellar phase.) Slow transport is of course demonstrated by the dynamic surface tension measurements shown in Fig. 10. Extremely high values of surface dilational moduli (>103 m N m 1) have in fact been reported for aqueous suspensions of long

138

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem, Eng. Aspects 103 (1995) 127-145

chain alcohols where the particles are probably gel phase (L~) liquid crystals.

1,0

3.3. Effect of mixing C12 betaine with 6~J-C12LAS

0,8

Mixtures of anionic and zwitterionic surfactants are known to exhibit marked deviations from ideal mixing as a result of strong unlike head group interactions, presumably of a largely electrostatic origin [24]. It is therefore of interest to explore the possibility that such mixtures may form solutions containing dispersed lamellar phase which exhibit similar foam and dynamic surface tension behaviour to that shown by dodecyl polyethyleneglycols and their mixtures with anionic surfactants. Both C~2 betaine and 6~-C12LAS form clear micellar solutions in 2.5 × 10 3 M STP, 3.8 × 10 -3 M NazCO3 and 2 x 10 -3 M CaCI2. It is well known also, for example, that in the absence of electrolyte 6~-C~2LAS forms small micelles of aggregation number 20 30 [25,26]. This implies that the micelle packing parameter V/aL~½ despite the high value of V/L for this two chain molecule (here a is the molecular area, L is the effective hydrocarbon chain length and V is the total volume of the hydrocarbon chain). In turn this will mean that a is correspondingly large and the micellar aggregate will therefore exhibit wide charge separation and a high degree of exposure of hydrophobe to the aqueous phase. The last factor dominates so that this arrangement is energetically unfavourable and small micelles are probably only found because of the relative magnitude of the entropic contribution to the overall chemical potential of micelles. However admixture with a zwitterionic like C, 2 betaine tends to reduce electrostatic repulsions so that the energetics of small micelles become so unfavourable that it overwhelms the entropic contribution'leading to large aggregates with high values of the packing parameter V/aL. This behaviour is illustrated in Fig. 13 where turbidity as measured by absorbance at 500 nm is plotted against composition for the system C12 b e t a i n e + 6 ~ - C l z L A S at 5 x 10 -3 M total surfactant in 2.5 × 10 -3 M STP, 3.8 × 10 -3 M Na2CO3 and 2 × 10 -3 M CaC12. Turbid solutions are found

| e / mN rn 1

Abaorbance

28

0.6

°°

0,4

0,2

-26

0.0 0.2

0.3

0.4

0,5

0.6

Mole Fraction

0.7

0.8

0.9

1.0

6~-CI~AS

Fig. 13. Plots of equilibrium surface tension and absorbance (500 nm, 40 mm pathlength) at 25°C against mole fraction 6 ~ C12LAS for mixtures of C12 betaine and 6~2~-CIzLAS. Total surfactant concentration 5 x 10 -3 M (in solutions of electrolyte mixture).

over the composition range ~0.4-0.95 mole fraction of 6~-C12LAS. That this turbidity is probably due to the dispersed lamellar phase adopting vesicle or liposome structures is revealed by videoenhanced microscopy, stills for which are reproduced in Fig. 14. Such structures are of course associated with high values of the packing group so that V/aL > ½. Also shown in Fig. 13 is the corresponding equilibrium surface tension behaviour for this system. It is clear that 7e is sensibly constant in the region where dispersed lamellar phase is found. This behaviour is of course obviously analogous to that shown by the system 5 ~ - C H L A S + C ~ 2 E 3 (see Fig. 9). Again it implies that the composition of the ai~water surface is essentially the same as that of the dispersed lamellar phase. In Fig. 15 we give plots of 7D -- 7e for this system. Extremely large deviations from equilibrium characterise the C12 betaine+6~-C12LAS system in the composition range where turbid solutions containing large aggregates are found. This then presumably reflects the slow diffusion rates and/or breakdown rates of the liposomes (or multi-layered vesicles). Initial foamabilities for Ross Miles and cylinder shaking are shown in Fig. 16 where values are

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127 145

(a)

139

(b)

Fig. 14. Video-enhancedmicrographs of liposomal aggregates in solutions of equimolar mixtures of C12 betaine and 6~-C12LAS. Total surfactant concentration 5 x 10 3 M (in solutions of electrolytemixture). Size bar 10.8gm.

plotted relative to those found with mole fraction unity of Cl2 betaine (where Ross Miles foamability is 11.5 cm and cylinder shaking gave foam containing ~65 cm 3 air). As with mixtures of 5 ~ - C H L A S and C 1 2 E 3 w e see declining foamabilities in the region where dispersed lamellar phase is present. Again we find that millipore filtration (0.22 lam filter) to remove the dispersed lamellar phase does not increase foamability and conclude that antifoam effects due to that material are absent. The decreases in foamability found in the presence of dispersed lamellar phase would therefore appear to be attributable to the same cause as the high dynamic surface tensions of these solutions. It would appear to suggest that the foamability declines due to relatively low dynamic adsorption

levels which in turn derive from slow rates of transport of surfactant to air water surfaces. As with mixtures of 5 ~ - C l l L A S and C12E3 we find that the decline in foamability in the presence of dispersed lamellar phase is more pronounced with cylinder shaking than with the Ross Miles technique. Again this could concern the competition between adsorption of surfactant and the relative rates of ai~water surface generation with the two techniques. 3.4. Effect of mixing C14 sulphobetaine with 6;ZJ-C12LAS

Not surprisingly the behaviour of 5 x 10 - 3 M solutions of mixtures of C14 sulphobetaine (TDPS)

P.R. Garrett, P.L . Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127-145

140

Foam relative to 100% Cl~Betaine 1.6-

mN m"1 45"

surface age = O.06e

40

1.4-

/ 1.3"

35

Ross-M~

/

-

1.2"

I/1'1

30 -

1.1

25-

~

C~.

surface age - ls

/ \

1.0

Shaking

0.9"

15- /

[]

iiiI

D~

0.8-

,

0.7

I' 0 0.0

0.2

0.4

dispersion l 0.6

'I J 0.8

dispersion

,

0.6

0.0

Fig. 15. Plots of 7 D - ~e at 25°C against mole fraction of 6 ~ C~2LAS for mixtures of C~z betaine and 6~-Ca:LAS. Total surfactant concentration 5 × 10 -3 M (in solutions of electrolyte mixture).

and 6~-C12LAS closely resembles that of C12 betaine + 6~-Cx2LAS. Again we find the formation of large aggregates (and dispersed lamellar phase) in solutions rich in 6~-C12LAS. A plot of turbidity, as measured by absorbance at 500nm, against composition is shown in Fig. 17. As with the C12 betaine system the equilibrium surface tensions (also shown in Fig. 17) are seen to be sensibly constant (despite some experimental scatter) in the region-where turbidity is found. In the case of equimolar mixtures video-enhanced microscopy suggests that the turbidity is due to the formation of hollow multi-layered vesicles and perhaps tubules. Typical photomicrographs are shown in Fig. 18. Solutions in the region of 0.3 0.4 mole fraction 6~-C12LAS exhibit viscoelastic behaviour. At 0.4 mole fraction 6~-C11LAS video-enhanced microscopy reveals the presence of long tubules (see Fig. 19). Solutions at 0.3 mole fraction 6 ~ C12LAS were however both clear and viscoelastic, presumably due to the presence of long rod micelles.

0.4

0.6

0.8

1.0

Mole Fraction 6~b- C12LAS

1.0

Mole Fraction 6~b - CI~.AS

0.2

Fig. 16. Plots of initial foamabilities (relative to those for C12 betaine) at 25 °C against mole fraction 6~-C12LAS for mixtures of Clz betaine and 6~-C12LAS. Total surfactant concentration 5 × 10 3 M (in solutions of electrolyte mixture). Foamabilities measured using both Ross Miles and cylinder shaking techniques.

0.6

Absorbance

)tel mN m "1

3O

29

0.4 ~ viscoelastic

28

27 0.2

.

o

26

0.1

26

0.0

24

, 0.2

0,3

0.4

0.6

0.6

0.7

0.8

Mole Fraction 6~-Cl~-AS

0.9

1.0

Fig. 17. Plots of equilibrium surface tension and absorbance (500 nm, 40 mm path length) at 25°C against mole fraction 6~-C12LAS for mixtures of TDPS and 6~5-C12LAS. Total surfactant concentration 5 x 10 -3 M (in solutions of electrolyte mixture).

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127-145

Fig. 18. Video-enhanced micrograph of liposomal aggregates in solutions of equimolar mixtures of TDPS and 6~-C12LAS. Total surfactant concentration 5 × 10 3 M (in solutions of electrolyte mixture). Size bar 11 ~tm.

In Fig. 20 we present CMC (or CAC if appropriate) values and molecdlar areas at the air-water surface as a function of overall solution composi-

141

tion. The molecular areas were obtained from (surface tension) Gibbs plots. Justification for this procedure is given in the Appendix. The plots of CMC and molecular areas are seen to exhibit pronounced minima which implies marked deviation from ideal mixing behaviour. Such behaviour is not of course unexpected for mixtures of a zwitterionic and an anionic surfactant [-24]. A plot of 7 D - 7e for various surface ages is shown in Fig. 21. As with the turbid solutions of C12 betaine+6~-C12LAS we find marked deviations from equilibrium at surface ages as low as 1 s when large aggregates such as rods, tubules or vesicles are present. Again this is consistent with slow transport of surfactant to air-water surfaces due to low diffusion coefficients and/or slow vesicle breakdown rates. The vesicles shown in Fig. 18, for example, make little contribution to such transport for surface ages < 1 s. Thus Stokes-Einstein diffusion coefficients are in the region of l0 14 m 2 s - t (for spheres of 1 ~tm radius) and F is in the region of 3 x 10 - 6 m o l m -2. The characteristic time for diffusion/adsorption of such entities is therefore about 9 s. The foam behaviour for mixtures of T D P S and 6~-C12LAS is presented in Fig. 22. Here both

S !

Fig. 19. Video-enhanced micrograph of turbid viscoelastic solution formed from mixtures of TDPS and 6~3~-C12LAS (at mole fraction 6~-C12LAS of 0.4). Total surfactant concentration 5 x 10 -3 M (in solutions of electrolyte mixture). Size bar 10.8 lam.

P.R Garrett, P.L. Gratton/Colloids SurJaces A." Physicochem. Eng. Aspects 103 (1995) 127 145

142

C M C or CAC ( x 10-4M)

6"

" "Ye mN m-1 44"

5

turbid • vesicles

40-

4-

3632282420"

0.0 (a)

0.2

1.0

0.4 0:6 0'.8 Mole Fraction 6di~CI2LAS

16128-

Molecular Area (nm 2)

0.55

4t

0[ 0.0

0.50

,

0.2

0.4

0.6

0.8

1.0

Mole Fraction 6 4 - C I ~ A S o

Fig. 21. Plots of )'O -- )'e at 25°C against mole fraction of 6~C12LAS for mixtures of TDPS and 6~-C12LAS. Total surfactant concentration 5 x 10-3M (in solutions of electrolyte mixturet.

0.45"

0.40"

0 o 0.35 0.0 (b)

0'.2

014 016 018 Mole Fraction 6t~-C12LAS

1.0

Fig. 20. Plots of: (a) CMC (or CAC); and (b) molecular areas at 25°C against mole fraction of 6~-C12LAS for mixtures of TDPS and 6~-C12LAS. Total surfactant concentration 5 x 10 3 M (in solutions of electrolyte mixture, see experimental ).

Ross Miles and cylinder shaking foamabilities are given relative to those found with mole fraction unity of the sulphobetaine (where the Ross Miles foam height was 10.2 cm and cylinder shaking gave foam containing ~68 cm 3 of air). Again we see declining foamabilities with turbid solutions where dispersed lamellar phase is present. Millipore filtration (0.22 Ilm filter) of the solution did not increase foamability. Therefore antifoam effects due to multi-walled vesicles (see Fig. 18) are absent. It is noteworthy that the clear viscoelastic solution (presumably containing cylindrical micelles at mole

fraction 0.3 of 6~-C12LAS ) also yields low foamabilities. All of this behaviour is clearly consistent with the high dynamic surface tensions found with those solutions. In the presence of large aggregates relative foamabilities as measured by the Ross Miles technique are in general higher than those found by cylinder shaking. We have found similar behaviour with all three mixed surfactant systems studied here. As we have suggested this could imply that the rate of air-water surface area generation is higher with cylinder shaking than with the Ross Miles technique (so that the "characteristic time" for foam generation is shorter than the 0.5-1 s suggested for the latter) [-4,27].

4. C o n c l u d i n g

remarks

We have considered the effect of the transition from micellar solution to lamellar phase dispersion upon foam and surface tension behaviour. A dis-

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 12 ~145

Foam Relative to 100% TDPS

1.8

1.6-

1.4-

tibid%L D//

1.2-

1.0

0.8-

0.6-

Shaking

0.4-

0.2-

visccoelastic

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Mole Fraction 64 - C12LAS Fig. 22. Plots of initial foamabilities (relative to those for TDPS) at 25~'C against mole fraction 6~-C,2LAS for mixtures of TDPS and 6~-C,/LAS. Total surfactant concentration 5 x 10 -3 M (in solution of electrolyte mixture, see experimental section). Foamabilities measured using both Ross Miles and cylinder shaking techniques.

persed lamellar phase may form liposomes or multi-walled vesicles with low diffusion coefficients and (presumably) low breakdown kinetics relative to the corresponding behaviour of normal micelles. Not surprisingly then we would expect slow transport to air-water surfaces and therefore high values for dynamic surface tensions. All of these features have been demonstrated with systems varying from pure dodecyl polyethyleneglycols to mixtures of zwitterionics with sodium alkylbenzene sulphonates. When dispersed lamellar phase is present in mixed surfactant solutions we find that equilibrium air-water surface tensions do not depend on overall solution composition. This implies that the compositions of the lamellar phase and surfactant layer at the air-water surface are identical. Increase in dynamic surface tensions when lamellar phase dispersion replaces micellar solution correlates with diminished foamability. This appears to concern reduction in surfactant adsorp-

143

tion under dynamic conditions. It is not implied that a simple relationship between foamability and (dynamic) surface tension is to be expected. It is simply that slow transport of surfactant to airwater surfaces will influence both dynamic surface tension and foamability. As we have discussed, low dynamic adsorption will mean diminished resistance of foam films to dilational stress, diminished disjoining forces and enhanced foam film drainage rates, etc. It is these factors that will conspire under dynamic conditions, in an as yet poorly understood manner, to diminish overall foamability. There is no evidence that the dispersed lamellar phase exhibits antifoam effects. Indeed we found evidence that vertical liquid films, drawn from the equilibrium air-water surfaces of solutions containing dispersed lamellar phase, exhibit no marginal regeneration. These films therefore drain more slowly and this should contribute to enhanced foam stability. This type of behaviour has been correlated elsewhere with high dilational moduli [ 1,28]. These in turn may be expected if equilibrium adsorption levels are high and transport away from surfaces is slow in response to the dilations which accompany the process of marginal regeneration [ 1,28]. Systematic study of the dynamic dilational behaviour of solutions containing dispersed lamellar phase is of course required if we are to put this explanation on a firm basis.

Acknowledgements The authors wish to acknowledge the assistance of J. Keen, J. Hope, Y. Plant, J. Roberts, L. Hilton and G. Pierre-Louis who did many of the measurements. The assistance of J. Conroy who synthesised most of the surfactants is also acknowledged. The authors are also grateful to E. Staples for pointing out the likely nature of the relationship between turbidities and dynamic surface tensions in Figs. 9 and 10.

Appendix: Calculation of area per molecule for multi-component mixtures of surfactants Consider a mixture of a blend of n species of sodium anionic surfactants i and a blend of m

144

P.R. Garrett, P.L. Gratton/ColloidsSurfaces A." Physicochem. Eng. Aspects 103 (1995) 127 145

species of zwitterionic or nonionic surfactants j in the presence of a relatively concentrated mixture of p species of non-surfactant electrolyte k. The last includes a high concentration of electrolyte with the same counterion as the anionic surfactant. If we define a Gibbs plane so that the surface excess of solvent is zero then we can write (using the Gibbs equation)

-dYe/RT:r~-dIAl+ P

-]- ~

~ l"jd~lj j=l

/'/-d~/-4-

i=l P

+ E r;, d , : + k~l

r;

contribution of the non-surfactant electrolyte so that d In fi-/d In CT~O) and that all f~= 1 (reasonable for a dilute solution) then we may write dye ~ d In C~R T d l n C T - 2i=1 ~ F/- d l n C x

+

~,

dlnC~

~

1=1

dlnC

- i=1

FF + ~ F j = F T

(A4)

j=l

Here FT is the total surface excess of surfactant species. The area per molecule can be readily calculated from FT.

k=l

+ terms for higher valent non-surfactant electrolytes

(A 1)

so that

-dye/gT=F~-dlnffC~- + ~ F:, dlnff CF + i=1 P

/=t P

x d l n f j C j + Z F[ d lnffkC~- + E F; k~l

k=l

x d lnffkkCk + higher valent terms

(A2)

where F£ ff~ and CF are the surface excess, bulk phase activity coefficient and bulk phase concentration of surfactant anions i. Cationic species 1 is sodium. All other symbols have their usual meanings. If Cr is the total concentration of anionic plus zwitterionic or nonionic surfactant, then we may deduce from Eq. (A2) that dye RTdlnCx

i=1

FF (d lnff~ CF/d in Cx )

+ ~ F i (d l n f F j d In CT )

(A3)

j=l

provided the non-surfactant electrolyte concentration is constant (so that for example d l n f~- Ck+/d In CT=0) and the non-surfactant electrolyte contains a large excess of sodium relative to that present from the anionic surfactant (so that d In fl+C+l/d In Cx'~O). If in addition we suppose that all fi-are determined only by the ionic strength (which is assumed to be dominated by the

References [1] J. Lucassen, in E.H. Lucassen-Reynders (Ed). Anionic Surfactants - - Physical Chemistry of Surfactant Action, Marcel Dekker, New York, 1981, p. 127. [2] P.R. Garrett, Chem. Eng. Sci., 48(2) (1993) 367. [3] K. Malysa, R. Miller and K. Lunkenheimer, Colloids Surfaces, 53 (1991) 47. [4] P.R. Garrett and P.R Moore, J. Colloid Interface Sci., 159 (1993) 214. [5] K. Roberts, C. Axberg, R. Osterlund and H. Saito, Nature, 255 (1975) 53. [6] E.D. Manev, S.V. Sazdanova, A.A. Rao and D.T. Wasan, J. Dispersion Sci. Technol., 3(4)(1982)435. [7] S. Friberg, Adv. Liq. Cryst., 3 (1978) 149. [8] P.R. Garrett and D.R. Ward, J. Colloid Interface Sci., 132 (1989) 475. [9] P.R. Garrett, J. Davis and H.M. Rendall, Colloids Surfaces, 85 (1994) 159. [10] J.D. Mitchell, G.J.T. Tiddy, L. Waring, T. Bostock and M.P. McDonald, J. Chem. Soc., Faraday Trans. 1, 79 (1983) 975. [ 11] J. Lucassen and D. Giles, J. Chem. Soc., Faraday Trans. l, 71 (1975) 217. [12] M.J. Rosen, A.W. Cohen, M. Dahanayake and X. Hua, J. Phys. Chem., 86 (1982) 541. [13] R.S. Hansen, J. Phys. Chem., 64 (1960) 637. [14] J. Lucassen, Discuss. Faraday Soc., 59 (1975) 76. [15] G.C. Kreschek, E. Hamori, G. Davenport and H.A. Scheraga, J. Am. Chem. Soc., 88 (1966) 246. [16] J.R. Lu, M. Hromadova, R.K. Thomas and J. Penfold, Langmuir, 9 (1993) 2417. [17] J.R. Lu, Z.X. Li, T.J. Su, R.K. Thomas and J. Penfold, Langmuir, 9 (1993) 2408. [18] E. Rillaerts and P. Joos, J. Phys. Chem., 86 (1982) 3471. [19] F.A. Veer and M. van den Tempel, J. Colloid Interface Sci., 42(2) (1973) 418. [20] R. Aveyard, B.P. Binks, P.D.I. Fletcher, T.G. Peck and C.E. Rutherford, Adv. Colloid Interface Sci., 48 (1994) 93.

P.R. Garrett, P.L. Gratton/Colloids Surfaces A: Physicochem. Eng. Aspects 103 (1995) 127-145 [21] K. Koczo, L.A. Lobo and D.T. Wasan, J. Colloid Interface Sci., 150(2) (1992) 492. [22] D.G. Hall, personal communication, 1989. [23] K.J. Mysels, K. Shinoda and S. Frankel, Soap Films; Studies of their Thinning and a Bibliography, Pergamon Press, New York, 1959. [24] M.J. Rosen and B.Y. Zhu, J. Colloid Interface Sci., 99(2) (1984) 427. [25] L.J. Magid, R.J. Shaver, E. Gulari, B. Bedwell and S. Alkhafaji, Symposium on Chemistry of Enhanced Oil

145

Recovery, American Chemical Society, Atlanta Meeting, 93, March, 1981. [26] L.J. Magid, R. Triolo, J.S. Johnson and W.C. Koehler, J. Phys. Chem., 86 (1982) 164. [27] K. Koczo, B. Ludanyi and Gy. Racz, Period Polytech. Chem. Eng., 31(1-2) (1987) 83. [28] A. Prins and F. van Voorst Vader, Proc. 6th International Congress on Surfaces Active Substances, Zurich, Verlag, Munich, p. 441.