Interactions between carboxymethyl cellulose and cationic surfactants 1. Phase equilibria and surface tensions

Colloids and Surfaces A:Physicochemical and Engineering Aspects 89 (1994) 59-69

ELSEVIER

COLLOIDS AND SURFACES

A

Interactions between carboxymethyl cellulose and cationic surfactants 1. Phase equilibria and surface tensions Mari Barck *, Per Stenius Laboratory of Forest Products Chemistry, Department of Forest Products Technology, Helsinki University of Technology, Vuorimiehentie 1 A, 02150 Espoo, Finland

Received 6 January 1994; accepted 11 March 1994

Abstract The surface tensions and phase equilibria of carboxymethyl cellulose (CMC)/surfactant/water systems were investigated as a function of the charge density of the polymer, the chain length of the surfactants and the ionic strength. No interactions were observed between CMC and non-ionic surfactants. In systems of CMC and cationic surfactants (tetraalkylammonium bromides) critical association concentrations are observed at concentrations well below the critical micelle concentrations of the surfactants. Associative phase separation occurs in extremely dilute systems when the charge ratio between the surfactants and the polymers is close to zero. The separated phase is a viscous gel phase containing 40-60% of water. The properties of the systems can be qualitatively understood assuming that (a) the driving force for association of the surfactants with the polysaccharide chains increases as the hydrophobic chain length of the surfactant or the charge density of the polyelectrolyte increases, and (b) there is an associative interaction between the surfactant aggregates and the polysaccharide which decreases with increasing ionic strength. Keywords: Carboxymethyl

cellulose; Cationic surfactants; Interactions;

1. Introduction

The structure and dynamics of systems containing water, ionic or non-ionic surfactants and watersoluble polymer are of considerable importance, in view of the use of such systems in technical dispersions. Several extensive reviews and monographs on the subject have been published recently (see for example Refs. [ 1,2]). A large part of the published literature has focused on determining binding constants and aggregate structure in dilute aqueous solution. A commonly used starting point

is to describe the interaction in terms of a cooperative binding reaction, in its simplest form for interactions between ionic surfactant and polyion described by the equation [3] mS+nC+P+PC,S, Equilibrium constant K,

author.

0927-7757/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO927-7757(94)02865-P

(1)

where S is the surfactant, C is the counterion and P is the polymer; this reaction is considered to compete with micellisation:

pc + qs *Corresponding

Phase equilibria; Surface tension

5 c,s,

Equilibrium constant K, (2)

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M. Barck, P. Stenius/Colloicis Surfaces A: Physicochem. Eng. Aspects 89 (1994) 59-69

As has been pointed out by Lindman and Piculell [4] another approach is to consider directly the interaction between polymer and micelle. As a first approximation, the latter may then also be considered as a polymer and it is found that polymer/ micelle systems show many similarities to mixtures of two different polymers. In particular, the interaction between polyions and ionic micelles appears to lead to an associative phase behaviour analogous to that of polycation/polyanion systems. This approach appears useful in that it leads to a better understanding of more concentrated systems, in particular the formation of gel phases. A polymer that has been relatively little studied in spite of its technical importance is carboxymethyl cellulose (CMC). The binding of dodecylammonium bromide and tetradecylammonium bromide to CMC was investigated by Hayakawa et al. [S]. Gupta et al. [6] measured binding isotherms for hexadecyltrimethylammonium bromide and a non-ionic surfactant. Polymers of somewhat similar structure were investigated by Malovilova et al. [7] who determined the binding constants for a cationic surfactant to polygalacturonate and partially esterified polygalacturonates. Interactions of surfactants with cationised cellulose were investigated by Goddard and co-workers [8,9]. Although these studies have to some extent clarified the association behaviour in dilute aqueous solutions, little is known about the phase behaviour, in particular with regard to the formation of gel phases. In this and following papers we present a systematic study of the interactions of carboxymethyl celluloses with different degrees of substitution with tetraalkylammonium surfactants, with emphasis on adsorption, phase behaviour and viscosity. In this paper we present studies of surface tensions of aqueous solutions and phase equilibria; we show that these can be qualitatively well understood in terms of polycation-polyanion interactions. We have also made some preliminary studies of the interactions of CMC with non-ionic surfactants. These investigations indicated that the interaction is very weak and we therefore focus on CMC/cationic surfactant systems.

2. Experimental 2.1. Materials 2.1 .l. Carboxymethyl cellulose CMC samples with three different degrees of were supplied by Mets%-Serla substitution Chemicals OY (Aanekoski, Finland). The composition of these samples (Table 1 and Fig. 1) was determined by gas chromatography. The analysis was performed at the laboratory of Mets%Serla OY using the method developed by Niemela and Sjijstriim [ lo]. The degree of substitution increases from sample A to sample C. Another difference worth noting between the three samples is the distribution of carboxymethyl substituents over glucose units. Samples A and B are mostly monosubstituted whereas for sample C the distribution is more uneven. In addition to the cellulose derivative, the samples are expected to contain some salt and possibly some surface-active impurities. To check the influence of these, sample C was further purified by dialysis. The surface tensions of CMC/surfactant solutions were systematically lowered after dialysis, but there is no qualitative change in the shape of the curve (Fig. 2). The reduction is most probably due to concentration changes during dialysis. Because of the minor effects of dialysis the other samples were used without further purification. 2.1.2. Surfactants The cationic surfactants (C,H2,,+ ,N(CH,),Br, n= 10, 12, 14 and 16, denoted ClOTAB, C,,TAB, C14TAB and (&TAB respectively) were from Tokyo Kasei Kogyo Co. and were used without further purification. Table 1 Properties

of carboxymethyl

Sample

Degree of substitution

Mean molecular

A B C

0.56 0.71 1.22

6.6 x 10’ 6.3 x lo3 6.5 x lo3

cellulose

“According to the manufacturer. bFrom viscosities.

samples” weightb

61

M. Barck, P. Stenius/Colloids Surfaces A: Physicochem. Eng. Aspects 89 (1994) 59-69 60% T

Fig. 1. The composition of the CMC samples, as determined by gas chromatography.

mJ/m’ 707

mJ/m’

‘OT

m

65.. 60.. 55.. 50..

SO--

45..

45 -.

40..

40 ..

35..

35

J”-

I

30 -3

-2.5

-2

-1.5 -1 -0.5 log c (mM C,,TAB)

0

~~~

-2

.

-1.5

.

.

-1

-0.5

0.5

Fig. 2. The effect of dialysis (viscose membrane, several days dialysis against pure water) on the surface tension of solutions of CMC (sample C, 0.001%) and hexadecyltrimethylammonium bromide (&TAB).

The purity of the surfactants was checked by measurement of the surface tensions. The results are shown in Fig. 3. Since there is no marked minimum in the surface tension below the critical micelle concentration (c.m.c.) the amounts of impurities were considered to be so small that the surfactants could be used without further purification. The c.m.c. values were found to agree with values reported in the literature [ 111.

Fig. 3. Surface solutions.

tensions

0 0.5 log c (mM)

1

1.5

of tetraalkylammonium

2

2.5

bromide

2.1.3. Other chemicals The water was ion exchanged and distilled. Its conductivity was 1.9 uS and its surface tension was 71.4( f 0.5) mJ m-’ (T=25”C). All other chemicals were of analytical grade and used as delivered from the manufacturer. 2.2. Methods 2.2.1. Surface tension measurements These measurements were performed using a ring tensiometer (KSV Instruments Sigma 70).

62

M. Barck, P. Stenius/Colloia% Surfaces A: Physicochem. Eng. Aspects 89 (1994) 5969

This instrument is computer controlled and makes it possible to pass the ring repeatedly through the weight maximum by lowering and raising the vessel containing the solution. The correction factors calculated by Huh and Mason [12] were used to calculate surface tensions. All solutions were prepared at least 24 h before measurements. The greatest possible care was taken to avoid the introduction of surface-active impurities. All vessels were washed in aqua regia and rinsed carefully with deionised water and distilled water and heated in a Bunsen flame before use. The surface tension measurements were carried out at room temperature (25 “C). Although this is below the Krafft temperature of &TAB, which is reported to be 27°C for a 1% solution [13], the concentrations used were so dilute that we are confident that the measurements were done at temperatures above the Krafft boundary of C&TAB, as is also evident from the phase diagram given in Ref. [ 131. The samples were allowed to equilibrate for 30 min in the measuring vessel before carrying out the actual measurement. In spite of these precautions the precision of the surface tension values was of the order +0.5 mJ me2 on repeated measurements of the same sample. This is probably because the ring becomes slightly hydrophobic when the solution of cationic surfactant repeatedly passes over it. This is known to cause some uncertainty in surface tension measurements by the ring method [ 143. The reproducibility of parallel tests (measurements of parallel solutions) was of the order + 2.5 mJ m-‘, probably because the polymer concentration was so low.

that of water), leaving the highly viscous gel phase at the bottom of the test tube. The compositions of the two phases were determined by weighing both phases and analysing the solids and nitrogen contents of the gel phase.

3. Results 3.1. EfSect of charge density

The effect of the degree of substitution, i.e. the charge density of the polymer on the polymersurfactant interaction is shown in Fig. 4. The CMC concentration is constant (0.001%). The break points in the curves of surface tension vs. Ci6TAB concentration suggest critical concentrations at which the cooperative interaction between surfactant and polymer leads to a constant surface activity until the polymer is “saturated” with surfactant (critical association concentrations (c.a.c.)). The c.a.c. values are indicated with arrows. It can be concluded from this figure that the interaction starts at lower concentrations of surfactant the higher the degree of substitution. At surfactant concentrations about five times higher than the concentration corresponding to charge equivalence

E

mJ/m’

* C, DS=1.22

65 60 55

2.2.2. Phase equilibria Phase equilibria at 30°C were determined by mixing appropriate amounts of surfactant, water and CMC in tightly closed test tubes. The samples were mixed by turning end over end for 7 days. To enhance the phase separation, the samples were centrifuged for 30 min at about 1600g. After this the samples were left to equilibrate for a further 7 days. The temperature (30°C) was chosen in order to be well above the Krafft point of &TAB. The two phases were separated by gently decanting the supernatant phase (that had a viscosity close to

50 45 40 35 301 -3

I -2.5

-2

-1.5

-1

-0.5

0

0.5

log c (mM &TAB) Fig. 4. The surface tension of 0.001% CMC/C,,TAB solutions as a function of the degree of substitution of CMC compared with pure C&TAB.

M. Barck, P. SteniusJColloids

Surfaces A: Physicochem.

(calculated from the degree of substitution) between the surfactant and the polymer the surface tension increases significantly. This is due to the formation of highly insoluble surfactant-polymer complexes (see Discussion). Phase separation is also observed as formation of slightly turbid solutions. The concentrations at which phase separation occurs increase with the degree of substitution: A < B < C. When an excess of surfactant is added, the surface tension remains almost constant (around 37 mJ m-‘), as expected for a solution containing micellar surfactant (cf. Fig. 2).

Eng. Aspects 89 (1994) S9-69

63

phase separation occurs increase in the order &TAB x C,,TAB < &TAB < (&TAB. The concentration of charged groups on the polymer in 0.01 and 0.001% CMC C is 0.47 mM and 0.047 mM respectively. Note that although the lower phase boundary increases with increasing degree of substitution, it does not correspond to charge neutralisation and it increases rapidly with decreasing chain length of the surfactant. Table 2 gives the observed c.a.c. values and phase boundaries, as well as the difference between the c.a.c. and the c.m.c. values of the surfactants. The c.m.c. and c.a.c. values are shown in Fig. 7.

3.2, EfSect of surfuctant chain length 3.3. EfSect of electrolyte Figs. 5 and 6 show surface tensions at two different CMC concentrations as a function of the surfactant chain length. The c.a.c. values decrease for 0.01% CMC in the order Cr,TAB < &TAB < (&TAB. A c.a.c. cannot be observed for C16TAB in 0.01% CMC, probably because phase separation takes place at very low concentrations. Phase separation is indicated as a region of increased surface tension for C16TAB. Phase separation now occurs at concentrations considerably below that corresponding to charge neutralisation. At the lower CMC concentration (Fig. 6), the c.a.c. values increase in the order C&TAB < C&TAB < C,,TAB (no c.a.c. point is observed for C,,TAB), but the concentrations at which

l

mJ/m’ 65 60

sol -3.5

C,,TAB

A C,,TAB .

C,,TAB

‘\

-2.5

-1.5

-0.5 log c (mM)

0.5

Fig. 5. Surface tensions of CMC solutions (sample tration 0.01%) to which have been added surfactants chain lengths.

1.5

C, concenof different

Figs. 8 and 9 show the effect of simple electrolyte on the surface tensions. In the case of Cr,TAB + CMC C, addition of electrolyte up to 0.1 M NaBr strongly lowers the surface tension and suppresses phase separation. Also, a slight increase in the c.a.c. occurs. Fig. 9 shows the effect of electrolyte at a higher CMC concentration with a shorter-chain surfactant. The concentration of ionised groups at this CMC concentration is 0.47 mM. At low electrolyte concentrations (1 mM NaBr) the surface tension at low surfactant concentrations is lowered; at high surfactant concentrations there is no electrolyte effect. However, at higher electrolyte concentrations the surface tensions (for low surfactant concentrations) are higher than for electrolyte-free solutions. The surface tension shows a pronounced minimum at 0.006 mM and a maximum at 0.4 mM. 3.4. EfSect of polymer concentration Figs. 10 and 11 show the effect of the concentration of polymer on the surface tensions of solutions to which have been added surfactants of different chain lengths. In the case of C,,TAB, polymer concentration does not seem to play an important role. The differences in c.a.c. points are within 0.3 mM. The only difference is the occurrence of phase separation for the lowest CMC concentration. For C&TAB the surface tensions of 0.01% solutions are slightly lower than those of 0.001% solutions; for 0.1% solutions a strong

M. Barck, P. Stenius/Colloids Surfaces A: Physicochem. Eng. Aspects 89 (1994) 59-69

64 mJ/m’ 70

mJ/m’

65 T

65..

707

60.55..

A

50.. 45.. 40..

-3.5

-1.5

-2.5

(a)

-0.5 log c (mM)

0.5

1.5

2.5

35.. 307 -3.5 @)

I

-2.5

-1.5

-0.5 0.5 logc (mM)

-2.5

-1.5

-0.5 0.5 log c (mM)

1.5

2.5

mJ/m’ 70-

mJ/m’ 70 65 60

40 i-

h

35 304 -3.5

-2.5

-1.5

(c)

-0.5 log c (mM)

0.5

1.5

, 2.5

Fig. 6. Surface tensions of CMC solutions (sample C, concentration (b) &TAB; (c) &TAB; (d) &TAB.

-3.5 (d)

1.5

2.5

0.001%) to which have been added surfactants of different chain

le&ths: (a) &TAB;

decrease in surface tension is observed. For 0.001% and 0.01% CMC, phase separation is clearly observed, but it cannot be seen for the highest concentration. 3.5. Phase diagrams Figs. 12 and 13 show partial phase diagrams for the three-component systems Ci,TAB/CMC/water and C,,TAB/CMC/water. Only the formation of gel phases in equilibrium with dilute aqueous solutions is shown. In Fig. 13 a few tie lines have been drawn to show the trend in the two-phase area. The two-phase area of &,TAB is wider than that of C&TAB, but the water contents of the gel phases are similar (65-70 wt.%) in both cases. As can also be deduced from the surface tension measurements,

the concentrations of CMC in the solutions that are in equilibrium with the gels is extremely low (precipitation takes place even in 0.001% solutions); addition of an excess of surfactant results in the formation of homogeneous solutions (not shown in the phase diagrams).

4. Discussion 4.1. General considerations

As is well established from many previous studies, the main forces governing surfactant-polymer interactions in ionic systems are (i) the electrostatic interactions due to the dissociated groups on the polymer and the ionic end-groups on the

65

M. Barck, P. Stenius/Colloids Surfaces A: Physicochem. Eng. Aspects 89 (1994) 59-69

Table 2 Critical association concentrations (sample C) Surfactant

and phase boundaries in systems containing cationic surfactants and 0.01 and 0.001% CMC

c.a.c. (mM)

0.001% CMC Cl0 TAB C12 TAB C14 TAB C,, TAB

0.3 0.02 0.003

0.01% CMC Cl0 TAB C,, TAB Cl4 TAB C,, TAB

6.3 0.2 0.02

Lower phase boundary” (mM)

Upper phase boundary” (mM)

7.1 1.6 0.13 0.13

10 2.5 0.2 0.3

log(c.m.c.) log(c.a.c.)

1.5 2.25 2.2

0.9 1.7 2.25 0.01

0.03

“Horizontal arrows in Figs. 5 and 6(a)-6(d) mJ/m’

log c (mM) 2-

1 .-

Qf ._I_ f

0 .-

1

-1 --

mm

10

12

14

16

Surfactent chain length

304 -3.5

4

-2.5

-1.5

log c (mM

-0.5

0.5

C,,TAB)

Fig. 7. The c.m.c. values for pure surfactants and the c.a.c. values for two different concentrations of CMC (sample C) as a function of surfactant chain length.

Fig. 8. Surface tensions of 0.001% CMC/C,,TAB solutions at different concentrations of added simple electrolyte (NaBr); CMC sample C (DS = 1.22).

surfactant, (ii) solvation of the polymer and the polar end-groups of the surfactant, and (iii) hydrophobic interactions due to the hydrophobic chains of the surfactants and hydrophobic moieties in the polymer chain. In the present case, we consider highly watersoluble CMC molecules with a fairly high degree of substitution, and their interactions with fully dissociated surfactants. The CMC solutions do not show any phase separation on heating. Thus we

may assume that the most important interactions in the C,TAB/CMC systems are electrostatic interactions as well as the hydrophobic interactions between the surfactant chains. Hydrophobic surfactant-polymer interactions (which are prominent in many other systems containing modified polysaccharides) are probably of minor importance. The electrostatic interactions are very complex. Schematically, they may be classified as follows. (i) Interactions between the charged groups on

M. Barck, P. Stenius/ColloidF Surfaces A: Physicochem. Eng. Aspects 89 (1994) 59-69

66 mJ/m’ 7065..

A CMC + 1 mM NaBr

60..

l

55..

CMC + 10 mM NaBr

50-m 45 .. 40.. 35 .*

I

304 -3

-2

-1

0

30 -3.5

I -3

-2.5

-2

1

log c (mM C,,TAB)

-1.5 -1 -0.5 log e (mM C,,TAB)

0

0.5

1

Fig. 11. Surface tensions of CMC/C,,TAB solutions at different concentrations of CMC (CMC sample C (DS = 1.22)).

Fig. 9. Surface tensions of 0.01% CMC/Cr,TAB solutions at different concentrations of added simple electrolyte (NaBr); CMC sample C (DS = 1.22).

mJ/m’ 70

l

0.001 %

A 0.01 %

65

+ 0.1 %

60

I

60%

/ /

10% .J”

I

two phases,

*

20%

/ \

’

20%

\

30%

40%

CMC

.

-2

-1.5

-1

-0.5 0 0.5 1 log c (mM C ,zTAB)

1.5

2

Fig. 10. Surface tensions of CMC/C,,TAB solutions at different concentrations of CMC (CMC sample C (DS = 1.22)).

the polymer; these will largely govern the configuration of the free polymer in solution. The most important molecular parameter affecting these interactions is the charge density (DS) of the polyion. (ii) Interactions between the ionic end-groups of the surfactant; these limit the growth of the micellar aggregates. For a homologous series of surfactants, the most important molecular parameter affecting these interactions is the size of the hydrocarbon

Fig. 12. Partial phase diagram of the system CMC (sample C)/C,,TAB/water at 30°C. The crosses indicate the experimental compositions investigated, the dots represent analyses of the separated gel phase.

moiety which determines the packing conditions of the monomers into micellar aggregates and the balance between hydrophobic interactions and electrostatic repulsion. (iii) Interactions between the polyelectrolyte and the ionic end-groups of the surfactants. As suggested by Lindman and Piculell [4], it is useful to consider this interaction as somewhat analogous to the interaction between two differently charged polyelectrolytes, i.e. the polysaccharide and the aggregate formed by the surfactant.

M. Barck, P. Stenius/Colloids Surfaces A: Physicochem. Eng. Aspects 89 (1994) 59-69

10%

20%

30%

40%

CMC

Fig. 13. Partial phase diagram of the system CMC (sample C)/C,,TAB/water at 30°C. The crosses indicate the experimental compositions investigated, the dots represent analyses of the separated gel phase. Because of uncertainties in the analysis, the tie lines indicate only roughly the compositions of the equilibrium phases.

Finally, one has to consider the screening effect of added simple electrolyte on all electrostatic interactions. 4.2. Adsorption at the air/liquid interface As shown by Figs. 4-10, surfactant concentrations far below the c.m.c. in CMC solutions give rise to a strong reduction in the surface tension of the solutions. The effect of added electrolyte on the surface tension and micelle formation of alkyltrimethylammonium bromides was investigated by Mukerjee [ 151. His data show that there is a linear dependence between log(c.m.c.) and log[Br-1: log(c.m.c.) = A + I3 log [ Br -1

(3)

For Cr,TAB, B= -0.70 and A= -4.11 [16]. The ion concentration added with 0.01% CMC (sample C) is about 0.47 mM. According to Eq. (3) this concentration of 1: 1 salt would lower the c.m.c. of C1,TAB from 3.6 mM to 0.017 mM. As is seen in Fig. 9, the lowering of the surface tension indicates that aggregation actually does occur at concentrations that are lowered by about this

61

amount. Although CMC from an entropic point of view certainly is very different from the corresponding number of small counterions, it is noteworthy that the screening of electrostatic repulsion between the cationic end-groups of Ci4TAB is apparently very similar to what would be expected for small counterions. Inspection of Figs. 4 and 5 shows that the strong decrease in the surface tension when the CMCs are added is largely similar to the shift in the log c vs. y curves that would be expected if the ionic strength were increased by the same amount of small counterions. The break points giving rise to the horizontal parts of the curves in Figs. 4-6 show that the surfactant does indeed interact with the polymer at very low concentration by cooperative formation of aggregates. The increased adsorption at the air/liquid interface seems to be mainly due to screening of electrostatic repulsion since the surface tension depends on the electrolyte concentration, as expected for free monomers in a low molecular weight electrolyte solution of the same ionic strength. 4.3. Critical association concentrations The difference between log c.m.c. and log c.a.c. increases with increasing chain length (Fig. 7 and Table 2). The effect is very similar to the results observed previously for hyaluronic acid and alkylammonium bromides [ 171. The polymer concentration seems to have little effect on the c.a.c. Although the concentration changes, the distance between the charges in the polymer remains the same, so that the conditions for formation of aggregates at the polymer chain remain the same. The addition of simple salt increases the c.a.c. This is caused by a reduction of the electrostatic interaction between the polymer and the surfactant. Salt also screens the repulsion between the positively charged head-groups, thus favouring the formation of micelles. Although the difference in DS between samples B and C is quite high the surface tension values do not differ as might be expected. This could be explained by the difference in the distribution of charges; the distribution for sample C is more uneven (Table 1) and because of this it behaves as

68

M. Barck, P. Stenius/Colloi& Surfaces A: Physicochem. Eng. Aspects 89 (1994) 59-69

if the degree of substitution were lower (locally, the binding of simple counterions will be quite high). 4.4. Phase separation The phase diagrams for C,,TAB/CMC/water and C,,TAB/CMC/water were determined for systems to which no electrolyte was added. Thus they actually represent planes in the quaternary system C,TAB/CMC/water/NaBr, where the NaBr concentration varies depending on the C,TAB/CMC ratio. The phase diagrams are similar to those found for other ionic surfactant/polymer systems [4,18], i.e. phase separation occurs because the two solutes associate (the tie lines in the two-phase area run from the water corner, roughly in a direction represented by a constant C,TAB/CMC ratio). As pointed out by Lindman and Piculell [4] it is useful to describe the formation of gel phases in such systems in terms of attractive interactions between the polymer and surfactant aggregates, similar to the interaction between oppositely charged polyelectrolytes. Thus the attraction is of entropic origin, i.e. it is due to the increase in entropy as counterions are released when the polyelectrolyte associates with the surfactant aggregate. The gel phases have a very high viscosity, and as shown in Figs. 6 and 8, they separate even at extremely low concentrations of surfactant. The phase separation shows up in the measurements of surface tensions as a jump. In the interval where the jump occurs the solutions become slightly turbid. The jump occurs because the systems were investigated by increasing the surfactant concentration at constant concentration of CMC; this implies that the line of investigation crosses the tie lines in the two-phase area so that the chemical potential of the components in the equilibrium phases changes. It is noteworthy that as the line of investigation passes through the two-phase area from low to high total surfactant concentrations, the surface tensions always increase. Thus the chemical potential of the surfactant on the “surfactant-rich” side of the two-phase area is always lower than on the “CMC-rich” side, indicating strong binding of the surfactant to the polymer. As more surfactant is

added, the surface tension decreases until it reaches the value characteristic for solutions containing free surfactant micelles. The main driving force for phase separation evidently is the interaction between the oppositely charged polyelectrolyte and micelles. As shown in Figs. 8 and 9, phase separation can be completely suppressed by the addition of screening electrolyte. However, phase separation does not occur at the point of charge equivalence. The lower phase boundary occurs at higher concentrations of surfactant the higher the charge density of the polymer, so that the charge ratio is indeed important. However, it is also observed that the phase gap occurs at lower concentrations the longer the surfactant chain length (Table 2), i.e. the occurrence of phase separation runs parallel to the c.a.c. and is clearly also related to cooperative interactions between the hydrocarbon chains. This is strong evidence that the formation of the gel is indeed due to interactions between the two “polyelectrolytes” CMC and cationic micelles: as the hydrocarbon chain length increases the micelles are formed at lower concentrations. At the higher CMC concentration, phase separation is observed only for C&TAB. Clearly at these concentrations not enough surfactant can be bound to the polyelectrolyte to induce the cooperative formation of a gel phase containing micellar surfactant aggregates before free micelles are formed in the aqueous solution. For C,,TAB, the cooperative hydrophobic interaction between the chains is strong enough to cause phase separation already at very low concentrations, even before the c.a.c.

5. Conclusions The interaction between CMC and cationic surfactants leads to strong adsorption at the air/liquid interface at concentrations far below the c.m.c. of the surfactant. This interaction can be explained as a salt effect. Similar behaviour can be expected whether simple salt (e.g. NaBr) or CMC is added. The interactions lead to the cooperative formation of surfactant aggregates associated with the polyelectrolyte at the c.a.c.; this aggregation occurs at

M. Barck, P. Stenius/Colloids Surfaces A: Physicochem. Eng. Aspects 89 (1994) 5969

surfactant concentrations around, but not exactly at, those required for neutralisation of the charges on the polymer. The driving forces for the aggregation are the electrostatic screening of head-group repulsion by the polymer and the hydrophobic interactions of the surfactant chains; aggregation is presumably still limited by electrostatic repulsion. At concentrations around charge equivalence, CMC and cationic surfactants form a gel phase in equilibrium with very dilute aqueous solution. The main driving force for this gel formation is electrostatic interaction between the polyanion and the cationic surfactant aggregates. However, hydrophobic interactions between the surfactant chains are also of importance. Among the many practical applications of CMC/ surfactant systems, two cases that are highlighted by the results presented in this paper may be mentioned. (i) The very strong enhancement of adsorption at the air/liquid interface at very low surfactant/polymer concentrations is of obvious interest in the use of these compounds as emulsifiers or stabilisers of foams. (ii) The formation of gels by surfactant/polymer systems has many applications; one that may be of particular interest in these systems is the possible precipitation of such gels as colloidal particles in the process waters where CMC is used as a stabiliser, e.g. in the manufacture of coated paper.

Acknowledgements We thank Metsa-Serla Chemicals OY, Aanekoski, Finland, for donating the CMC samples. This work was partially supported by the program “Biopolymers” of the Finnish Technology Development Centre (TEKES). The skilful experimental assistance of Ms. Ritva Kivela in the surface tension measurements is gratefully acknowledged.

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