Kinetic investigations at the cloud point of nonionic surfactants

Kinetic Investigations at the Cloud Point of Nonionic Surfactants H. HOFFMANN, H. S. KIELMAN, 1 D. PAVLOVIC, 2 G. PLATZ, AND W. ULBRICHT Lehrstuhl fiir Physikalische Chemie der Universiti~t Bayreuth, D-8580 Bayreuth, West Germany

Received April 23, 1980; accepted July 18, 1980 Kinetic relaxation measurements usirtg the T-jump technique, electric birefringence, and lightscattering measurements were carried out with aqueous solutions of the neutral detergents C10H21(OC2Ha)4OH, C12H25(OC2H4)4OH, CsHlr(OC2H4)4OH, C~H19Ph(OC2H4)5OH, CgHtgPh(OC~H4)sOH, and Triton X-100 (Ph stands for the phenylen group) above and below the cloud point T~. Two relaxation processes could be detected in the clear solutions below To. These processes could be associated with the change of the aggregation number and concentration of micelles according to the theory of Aniansson and Wall. The C12E4 system shows electric birefringence below T~ from which it can be concluded that the miceUes are nonspherical. In the turbid solutions above Tc up to six relaxation times could be observed by recording the turbidity as a function of time. These processes could be associated with the normal micellar effects in clear solutions, with the transport of matter to the droplets of the dispersed new phase that is formed above Tc and with the equilibration of the droplets, respectively. The aggregation number of the micelles and the dimensions of the droplets are calculated from the relaxation times. These values agree fairly well with the values obtained from light-scattering measurements.

vestigation was to find out what happens to the micelles in a solution if the cloud point is approached and surpassed. Similar measurements on binary mixtures have been reported by Jost and Schneider (2). At the start of the investigation it was imagined that the spherical micelles that are normally present in the solution could start to grow to rodlike micelles that could arrange themselves into a new phase when they reach a certain length. The micelles were to be characterized by kinetic and electric birefringence measurements and by quasielastic fight-scattering measurements. The last two kind of measurements are especially well suited to obtain information on the shape of micellar aggregates (3).

INTRODUCTION

One of the characteristic features of nonionic surfactants is their cloud point To. When solutions of neutral detergents are heated they turn cloudy at a specific temperature that is characteristic for the given detergent. The cloud point is dependent on the concentration of the detergent and other additives. The appearance of the turbidity can be looked upon as a phase separation process in a binary system with a critical point which is located close to one axis of the phase diagram. In accordance with a binary mixture above the phase boundary, solutions of neutral detergent with a temperature higher than Tc separate into two liquid phases both of which contain a considerable amount of water (1). The more aqueous phase still contains some detergent molecules and micelles while the other phase usually contains most of the detergent in the form of an isotropic or structured phase (hexagonal, lamellar) (1). The objective of the present in1 Unilever Research, Vlaardingen, The Netherlands. On leave from the University ofZagreb, Yugoslavia.

EXPERIMENTAL

The first measurements were carried out with solutions of Triton X-100, which is a commercial, polydisperse product of p(1,1,3,3-tetramethylbutyl)phenoxypolyoxyethylene glycols, containing an average of 9.5 oxyethylene units per molecule. As the 237

Journal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

0021-9797/81/030237-18502.00/0 Copyright© 1981by AcademicPress, Inc. All rightsof reproductionin any form reserved.

238

HOFFMANN TABLE

following C8E4, C10E4, C12E4, C9PhEs, and C9PhE8, respectively (Ph stands for the

Ia

V a l u e s f o r t h e C M C a n d t h e S u r f a c e T e n s i o n 3' a t t h e CMC for Some Neutral Detergents T (°C)

CMC (mole/liter)

ycMc (N/m ~)

Triton X-100

25

3.0 x 10 -4

3.3 x 10 -2

C 9 P h E s + 4.8 x 10-2MKCI

23

4.5 x 10 -5

3.1 x 10 -2

C9PhEs:Triton X-100 = 7:3 a

23

5 . 7 × 10 -5

3.1 × 10 -2

CgPhEs:Triton X-100 = 2 . Y' C1oE4 (15) C12E4 (15) C12E4 (15) CgPhE5 (15)

23 20 5 25 25

8.5 6.4 7.8 4.0 5.7

3.1 x 10 -~ -----

System

ET AL.

x x x × ×

10 .5 10 -4 10 -~ 10 -5 10 -5

a S o l u t i o n c o n t a i n s 4 . 8 × 10 -2 M KC1.

compound did not have a defined composition and also because the turbidity point of Triton X-100 is around 70°C where it is difficult to carry out kinetic measurements, some other pure compounds were also used that had much lower turbidity points. These well-defined compounds were C8H~r(OCeH4)4OH, C10H21(OC2H4)40 H, C12H25(OC2H4)4OH, C9H~oPh(OC2H4)5OH, a n d C 9 H 1 9 Ph(OC~H4)8OH, which shall be called in the

phenylen rest C 6 H 4 ) . They have been kindly synthesized for us by Unilever Research, Vlaardingen, The Netherlands. In Tables Ia and Ib the compounds used for the measurements are listed together with their characteristic constants. (CMC, cloudpoints and surface tension at the CMC.) All these systems have been described before in the literature (4). Table Ib also contains the cloudpoints of various mixtures of the nonionic detergent CgPhE8 and the ionic detergent cetyltrimethylammoniumbromide (CTAB) which were also used for the investigation in order to shift the cloud point of the pure CoPhE8 from the value that was close to the freezing point of water to higher values. The binary phase diagrams of some of the systems with water have been studied before (1, 5). As the diagrams are important for the interpretation of the kinetic measurements, the diagram of Ci2E4-water is given in Fig. 1.3 The kinetic measurements

TABLE

3 T h e d i a g r a m h a s b e e n k i n d l y g i v e n t o u s f o r inc l u s i o n in t h i s m a n u s c r i p t b y D r . G . J. T. T i d d y .

Ib

V a l u e s f o r t h e C l o u d P o i n t s Tc a n d t h e P h a s e T r a n s i t i o n T e m p e r a t u r e s T~am a t V a r i o u s C o n c e n t r a t i o n s for the Investigated Neutral Detergents

System 9.4 3.0 2.1 3.3 1.9 3.3 2.1 7.2 6.5 5.8 5.0 4.3 3.6 2.9 7.0

× x × × × × × x × × × × × × x

10 -3 M 10 -3 M 10-3M 10-3 M 10 - 3 M 10 -3 M 10-3M 10 4 M 10 -4 M 10 -4 M 10-4M 10 -4 M 10 -4 M 10 -4 M 10 4 M

C8E4 + 2 x 10 -3 M KC1 C10E4 + 2 × 10 -3 M K C 1 C10E4 + 1.1 × 1 0 - 4 M C T A B + 2.8 × C12E4 + 2 × I0 -~ M K C 1 C~PhE5 + 7.5 × 10 -4 M C T A B + 5 × C g P h E s + 2 . 7 × 10 -4 M C T A B + 1.3 C g P h E s + 2.0 × 1 0 - 4 M C T A B + 1.7 C g P h E s + 4 . 8 × 10 -2 M K C 1 CgPhE8 + 7 × 10 -5 M T r i t o n X - 1 0 0 + C g P h E s + 1.4 x 10 -4 M T r i t o n X - 1 0 0 CgPhE8 + 2.1 x 1 0 - 4 M T r i t o n X - 1 0 0 CoPhE8 + 2.8 × 10 -4 M T r i t o n X - 1 0 0 C g P h E 8 + 3.5 × 10 -4 M T r i t o n X - 1 0 0 CgPhE8 + 4.2 x 10 -4 M T r i t o n X - 1 0 0 T r i t o n X - 1 0 0 + 4 . 8 x 10 -2 M K C 1

Journal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

10-3M KC1 10 - 2 M K C I × 10 -2 M K C 1 x 10-2M KC1 4.8 × + 4.8 + 4.8 + 4.8 + 4.8 + 4.8

10 - 2 M K C 1 x 10 -2 M K C 1 x 10-2M KC1 × 10 -2 M K C 1 × 10 -2 M K C 1 × 10 -2 M K C 1

Tc (°C)

Tiara (°C)

45 14.5 25 3.5 25 22 25 <0 <0 1 12 22 27 40 >40

-35 -15 ------------

239

K I N E T I C I N V E S T I G A T I O N S OF N O N I O N I C S U R F A C T A N T S TEMP. °C

/

80

WATER + L2

60

YL;QuID

LIQUID( L 3 ) ' - . ~ . ~ . ~ WATER+ L ~ ~ //// f L3 ~LAM, /I/ WATER ÷ LAMELLAR

CL2)

"~

LAMELLAR

40

20 //

WATER+ L O

~

~

~

L1eutD (LI) ( 20

// /

\\ I

\

40

I

l

GO

80

/

/~OLID ~00

WEIGHT % CI2E4

FiG. 1. Phase diagram of the system C,~E4/H~O (taken from Bostock, Donald, Tiddy and Waring (1)).

have been carried out at a concentration of 3 x 10-3 mole/liter between 0 and 25°C. At this concentration only one phase exists below 5°C. Above 5°C (the cloud point) the clear solution separates into two phases and becomes turbid. The continuous phase is water containing some detergent as monomers and micelles. The dispersed phase contains water and most of the surfactant. When standing for longer times the two phases separate completely. As can be seen from the diagram, the composition of the new phase changes with increasing temperature in such a way that the droplets of the new phase lose water. At 16°C another phase transition occurs; phase L1 disappears and a lamellar phase is formed. This temperature is indicated by Tiara. The lamellar phase is in equilibrium again with water that may contain a few detergent molecules. The composition of the lamellar phase seems to be independent of temperature. It has a high water content. When standing for long times, the two phases separate completely. The phase diagrams of the other compounds are,

at least in the investigated region, similar to the diagram of C,2E4. The cloud point is shifted to higher temperatures with decreasing chain length of the alkyl group which can clearly be seen from Tables Ia and Ib. Addition of KC1 which was necessary for the kinetic measurements does not change the characteristic features of the diagrams, but shifts the could point to lower values. A typical diagram of a recording of the turbidity as a function of temperature for a low concentration of C12E4 is shown in Fig. 2. From the cloud point on, the turbidity increases rapidly with temperature until Zla m is reached whereupon the turbidity drops sharply to a fairly low value and remains constant thereafter. Similar results were obtained for the C10E4 with the corresponding higher cloud point. The kinetic measurements were carried out using a T-jump equipment (Firma MeBanlagen, G6ttingen). For most of the measurements light at a selected wavelength that passed through the T-jump cell was recorded after the T-jump by a digital transient reJournal of Colloid and Interface Science,

Vol. 80, No. 1, March 1981

240

HOFFMANN

Transmission % 3.10-3m C12El' ÷ 2.10 -3 m KCt 100

80

60

~0

i

i

,

x

20 T[°Cl

0

1'0

2'0

310

FIG. 2. Plot of the transmission of light at wavelength 600 nm through a solution of 3 x 10-3 M C12E4+ 2 x 10-2 M KCI in water as a function of the temperature (d = 1 cm).

corder and the relaxation times were automatically evaluated in an H P 21 MX computer. For some measurements polarizers were introduced into the optical path in order to monitor the electric birefringence of the solution (6). Usually kinetic measurements of micellar equilibria are more difficult to carry out on neutral detergents than on ionic detergents. Often light detection cannot be used because the detergent molecules do not absorb light in the U V - V I S wavelength region. Such systems are usually investigated by the use of suitable dye indicators which are incorporated into the micelles and have different absorption spectra inside and outside of the micelles (7). It has to be pointed out, however, that the dynamic behavior of the micelles can be strongly affected by the incorporation of substrates (8); thus it is necessary to be certain that by adding the indicator the relaxation times of the system are not affected. Journal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

ET AL.

Fortunately, due to their phenyl group alkylphenolpolyglycolethers absorb light in the UV region. The absorption spectra change when the concentration is above the CMC and micelles are formed. This change can be used to monitor the relaxation process. F o r these systems, also eosin can be used as an indicator to study both relaxation processes. This is remarkable because most of the dye indicators interact only with the micelles but not with the monomers and are thus suitable for monitoring processes which change only the concentration of the micelles while processes which change only the concentration of the monomers by increasing or decreasing of the aggregation number of the micelles cannot be detected. The absorption spectrum of eosin, however, changes already, when concentrations of Triton X- 100 are added that are far below the CMC; it changes once more when the concentration of the added Triton X-100 exceeds the CMC, which can be seen in Fig. 3. This shows that we have to distinguish between eosin molecules, the eosin complex with the Triton X-100 molecules, and the eosin m o l e c u l e s incorporated into the micelle. Therefore eosin is an ideal indicator to monitor processes which are connected either with a change in the m o n o m e r or in the micelle concentration. No extinction change can be observed of eosin when alkylpolyglycolethers are added. This makes it likely that the change in the absorption spectrum of eosin with alkylphenylpolyglycolethers is due to an interaction between the benzene ring and the aromatic ring system of eosin. As alkylpolyglycolethers do not absorb light in the wavelength region between 300 and 800 nm, it can readily be understood that relaxation processes due to a change in m o n o m e r or micelle concentration cannot be observed in clear solutions of such detergents. Fortunately, some detergents form nonspherical micelles that give rise to orientation effects in the electric field which can be observed with polarized light (6). Furthermore, at

KINETIC INVESTIGATIONS

Absorption

OF NONIONIC

241

SURFACTANTS

spectre x

E

/"'\

1.10-5mEosin

/41

0,9

(alone

1.10-5m Eosin÷l'lO-lrn NaCI + different conc. of Trifon X100

o

o

1.10-4m T

,~ 4.10-4m T

x

-4 m

0,8

-3 m T 1-3 rn )-3rn

0,7

)-9 .,, T

0,6

0,5

0,4

0,3

0,2

480

500

520

540

~ [nm]

FIG. 3. Absorption spectra of solutions of l0 -~ M eosin + 10-1 M NaC1in water with various amounts of Triton X-100. x, Eosin alone; ©, with 10-4 M Triton; (~, with 4 × 10-4 M Triton; O, with 6 x 10-4 M Triton; II, with 1.5 x 10-3 M Triton; Z3, with 4 x 10-3 M Triton; A, with 8 x 10-3 M Triton; A, with 3 × 10 2 M Triton.

t e m p e r a t u r e s near to or a b o v e the cloud points, the solution shows a significant turbidity. As T-jump experiments in these solutions change the equilibrium conditions for all existing particles and phases, the relaxation p r o c e s s e s can be monitored optically b y measuring the light intensity of the transmitted light. Usually a wavelength of 470 nm was used for the m e a s u r e m e n t s . THEORETICAL CONSIDERATIONS F o r a detailed discussion of the processes in the turbid solutions it would be helpful

to k n o w the exact p h a s e diagrams of the investigated s y s t e m s in order to be able to calculate the amount of material that is transferred f r o m one phase to the other w h e n the cloud point is surpassed for a given concentration of detergent. While several phase diagrams of nonionic detergents h a v e been reported in the literature, it is v e r y unfortunate that the exact p h a s e line at the e x t r e m e left of the phase diagram is not known. As in Fig. 1, the phase line is usually drawn straight up to the w a t e r axes which would suggest that, a b o v e the cloud p o i n t , w a t e r should be in equilibrium with the new phase. Journal o f Colloid a n d Interface Science,

Vol. 80, No. 1, March 1981

242

HOFFMANN

This is not the case because the detergent has a certain solubility and therefore at least a monomeric solution of the detergent should be in equilibrium with the dispersed phase. The present experimental data even suggest that in most cases there are even micelles in equilibrium with the new phase which means that the phase boundary that is usually drawn to cut the H20 axis must curve upward at a concentration that is even higher than the CMC. The exact position at which this occurs might vary from system to system. While the structure of some of the hexagonal or lamellar phases that exist in binary phase diagrams of nonionics has been determined with X rays (9), the phase that precipitates out at the cloud point of the investigated systems has not been studied in great detail. One fact that follows from the phase diagram is that most of the phase consists of water. It is also certain that under the experimental conditions the new phase is dispersed as small particles in the continuous aqueous phase. The size of the particles seems to be similar to the wavelength of the visible light or a little smaller. So, one hypothesis for the appearance of the new phase could be that the micelles can agglomerate to large aggregates in which the micelles keep their original identity. While it is not known exactly what happens at the cloud point, it seems safer to conclude what does not happen. The strong dependence of the cloud point on the length of the alkyl group and on the number of the glycol groups seems to rule out the possibility that the glycol groups are desolvated. Since we do not know the structure of the new phase it is also not possible to say anything about the particles that might serve as nuclei for the formation of the new phase. We really seem to have to rely on our kinetic information only for the interpretation of the results. F o r the clear micellar solution the theory of micelle formation can be used to analyze the two relaxation processes that are due to the change of the aggregation number (process 1) and the change of the micelle concentration (process 2). These two processes Journal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

ET AL.

have mostly well-separated time constants so that they can easily be evaluated. According to the theory of Aniansson and Wall (10) the faster process is due to a shift o f the mean aggregation number n of the micelles; during this process the n u m b e r of the micelles remains constant. The size of the micelles changes by the incorporation or release of monomers and therefore the monomer concentration changes also. F o r the dependence o f the relaxation time rl on the detergent concentration the following expression was derived: 1/~-1 = k - / o -z + ( k - / n ) × (Co - CMC)/CMC.

[1]

Here k- is the rate constant for the dissociation of a m o n o m e r out o f the micelle, n the mean aggregation number, o- the variance of the micellar distribution curve, and Co the total concentration o f the detergent, F r o m a plot of 1/rl as a function of Co the parameters k - / o 2 and k - / n can be obtained. k - I n is the reciprocal lifetime of a m o n o m e r in the micelle. The slow relaxation process is due to a change of the micellar concentration. This change can take place via the micellar nucleus, which is defined as that aggregate of monomers with the lowest concentration. As this concentration is in many cases below 10-1° M, it is understandable why the "rz process is rather slow. Aniansson and Wall derived for this process expression [2]: k-'cr l/r2 = - CM

CMC + n2CM CMC + o-2cM

[2]

Here Cr is the concentration of micellar nuclei and CM the concentration of micelles, which can be e x p r e s s e d b y CM = (Co -- CMC)/n. As was recently shown by Hoffman (11), all parameters for the micelle, i.e., n, k-, o- can be calculated from the measurement o f the two relaxation times as a function of the total concentration. Up to six distinct different processes could be detected when the light intensity that passed through the turbid solutions above

KINETIC INVESTIGATIONS

OF NONIONIC SURFACTANTS

Tc was monitored. The processes have time constants ranging from a few microseconds to several seconds. Two of these six processes can be associated again with the normal processes that are present in clear micellar solutions if we assume that micelles still exist in the turbid solutions and are in equilibrium with particles of the dispersed new phase. By anology two more of the six processes can be associated with the large aggregates, because we can imagine them to be a special kind of giant micelles. So, without making any specific assignment, we can account for four of the six observable processes and we are left with two more to explain. In the following discussion the processes will be indexed with the consecutive numbers, 1, 2 . . . 6. In turbid solutions where an amplitude for a fast process is observable and the time constant for this process could be resolved, it has similar values as the ~'1 process in Triton X-100 solutions below re. This makes it likely that this process is indeed the normal Zx process of micellar systems and that in some turbid solutions micelles still exist in equilibrium with particles of the new phase even above T~. In some of the solutions of the nonionic detergents small amounts of CTAB were added in order to raise the cloud point. Mixed micelles are formed under these conditions and the two types of detergent molecules can exchange with different rates from the mixed micelles that results in two fast relaxation processes. The details of such a coupled system have recently been worked out by E. A. G. Aniansson and the equation for the two fast relaxation times are given by Eqs. [3] and [4] (12).

1 _ kT~ (1 + ~2 CM] 1"A

0"2 .

I

k~

k~" CM

"rs

o-~

B1

UA A 1 }

kg(1 - -

O'~B 0"~

[3]

CM) -1 +

[4]

A1

'

where the subscripts A and B stand for the

243

species A and B andAx a n d B t are the concentrations of the monomers A and B. For concentrations not too high above the CMC both reciprocal relaxation times increase linearly with the total micelle concentration CM. Processes 2 and 3 are present only in turbid solutions. They cannot be observed if the temperature is raised above the cloud point, starting from a perfectly clear solution and ending with a turbid solution. Processes 2 and 3 therefore have to be assigned to the presence of the turbid solution. Furthermore the amplitudes of 2 and 3 oppose each other. The turbidity is lowered during r2 and is increased again during ~'~. The time constant of step 2 is the RC time constant of the discharge of the condenser through the solution; in most cases the condenser had a capacity of 10-8 nF and in some cases of 5 x 10-s nF and the resistance of the solution was in the region of some kfL In every case the evaluated time constant for step 2 was within an experimental error of 10% equal to the calculated RC value of the system. The time constant for step 3 is rather insensitive to resistance of the solution and capacitance of the highvoltage condenser and also is not very dependent on temperature. It seems that the dispersed phase is modified by the discharge (passage of the current) and relaxes back in step 3 to its original state. The time constant of the process is usually around one or few milliseconds. The time constant of step 3 is usually close to the time constant of step 4 in which the main turbidity increase occurs. It is clear that the turbidity increases by the transfer of matter from the micellar state to the dispersed state of the new phase and it is likely that this transfer proceeds in a stepwise fashion by the diffusion of monomers which are in equilibrium with existing micelles to the dispersed particles. As mentioned it is conceivable that the adjustment of equilibrium does not proceed only by steps in which monomers are involved but also by agglomeration or coalescence of micelles. Journal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

244

HOFFMANN

If the changes in temperature during the jump experiments are small enough, this process can also be treated as a relaxation experiment and we can use the Aniansson expression of z~ also for this process. Because we deal with very broad distribution curves, the term k - / o -2 can be neglected and the equation can be rewritten into the form 1/z4 = k +'co,

[5]

where co is the concentration of particles of the new phase and k + is the rate constant for the diffusion o f monomers toward these particles. The similarity of the zz and ~4 time constants makes it likely that z3 comes about also by a diffusion of molecules from the bulk to the droplets. These molecules must have been pulled out during the z~ process from the dispersed phase by the electric current. Keeping this in mind and furthermore knowing that most of the new phase consists of water in which supporting electrolyte is dissolved made us conclude that electrolyte is lost during r2 from the large particles of this new phase and diffuses back into the phase in step 3. The loss of electrolyte with its solvating water could lead to a change of the size of the particles and hence to a change of the turbidity of the solution that can be monitored. In some solutions the turbidity decreases again slightly after step 4. It is conceivable that this decrease has to do with the redistribution of matter between the scattering particles. The reason for the redistribution could be that few particles grow v e r y large during the growth period because not many particles were initially present and that with more material present in the new phase a redistribution has to take place. This step 5 is sometimes followed by step 6 in which the turbidity increases again somewhat. This process has a time constant that is close to the r2 time constant of normal micellar solutions, which makes it likely that even this process can still be monitored in turbid solutions. Micelles are dissolved during process 6 and their detergent moleJournal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

ET AL,

cules are added to the material in the new phase that leads to an increase in turbidity. Because all fast processes are coupled to the slow processes it is clear that an exact derivation of the mathematical expressions for these processes becomes v e r y complicated; for this reason these calculations have not been carried out in this work. F o r assigning the various processes that are observed in the turbid solution it was helpful to estimate times that would be required for the conversion of micelles to the new phase. If we assume again that this process proceeds in a stepwise fashion like the micelle formation where the aggregates grow by the incorporation of single monomers it is possible to write for the passage of monomers into the scattering droplets of the new phase dCA1/dt = k + "CAt "Cd.

[6]

The concentration CA~ is buffered by the existing micelles and we can therefore assume that it remains fairly constant as long as micelles exist. The micelles would have to b e c o m e smaller when monomers are used up. If we furthermore assume that the backreaction o f monomers from the droplets of the new phase can be neglected we can calculate the flux of monomers per unit if we know the concentration o f the particles cd. If we take as the highest possible value for cd, the micelle concentration itself, we obtain A t = A A ~ / ( A ~ ' k +'co).

[7]

Estimating valuesA, ~ 5 x 10-5 mole/liter, AA ~ 2.5 × 10 -3 mole/liter, k + ~- 109 liter/ mole. sec, and c0 ~ 2.5 × 10-5 mole/liter (for Co = 3 × 10-8 mole/liter), one obtains At > 1 msec. This clearly shows that it would take at least 1 msec to transport the material from the micelles to the new phase and consequently we can safely assume that all processes that are shorter than 1 msec cannot be caused by a major mass transport from one phase to the other. T o w a r d the end of the flow process, the system again approaches a state o f pseudo-

KINETIC

INVESTIGATIONS

OF NONIONIC

equilibrium, where the monomers are in equilibrium with the particles of the new phase but the number of particles might not have reached its equilibrium value. The rate constant k + is the rate constant for the diffusion of monomers to the large particles with radius rd. Assuming that k ÷ is given by k + = 47r(Da + Da)ra and rd is known from the diffusion coefficient that can be obtained from quasielastic light-scattering measurements from the Stokes-Einstein equation kT D d -

-

-

[8]

67r~/"rd

the concentration Cd may then be obtained from the relaxation time r4. A different approach may be taken if Dd is not available. In this case we can also express the concentration ca by the radius r a and the volume ratio q~of surfactants in the solution cd = 3. qb/ 41rr~; from the phase diagram we know that the new phase contains, depending upon the temperature, from at least 98% water at 5°C to about 72% water at 16°C. As an estimate we have taken into account that the phase contains about 90% water and increased • accordingly. We obtain 1

r4

10.qb.3 4~r-r~

- k +'cd = 47r(DA + Dd)-rd" -

and, as Do can be neglected compared with Da 1 DA'3qb'10 [91 T4 r~ The equation permits the calculation of the radius from kinetic measurements. The same equation may be used for the process with time constant ra because during this process, the supporting electrolyte equilibrates between the bulk and the new phase. We now have to use the diffusion coefficient of KCl instead of the one forA1. The given description of step 4 makes it clear that the time constant for this process should depend on

SURFACTANTS

245

the amplitude of the perturbation (AT for a T-jump experiment). For small perturbations we can use the relaxation equation and 74 and 7a should be given by 1/k + "cd. Since k ÷ does not vary an order of magnitude for KC1 and the detergent monomers, both processes should have about the same time constant and the two steps would merge to one. If the temperature jump is made larger, the relaxation equation can no longer be used for step 4. The time constant would now be approximately given by ~xA /A1 "k ÷ "c d where zXA is the monomer concentration in the micellar phase that has to be transformed to the droplets. The ratio G 4 / A 1 can be larger than 10 and therefore r4 can be at least an order of magnitude longer than r3. This could be observed in several experiments. From the given arguments the observable consecutive steps in the turbid solutions can be assigned to the following process: Step 1. Normal fast relaxation process in micellar solutions. Adjustment of monomer concentration to the electrical field, rl = 1050/xsec. Step 2. Due to the change in scattering power of the large particles that is caused by the passage of electrical current through the solution, r2 -- time constant of the voltage discharge R C (R is the resistance of the solution, C the capacity of the high-voltage condenser of the T-jump equipment). Step 3. Due to the increase of the scattering power of the large particles by the back-diffusion of the electrolyte with solvated water molecules into the droplets which leads to an increase in size of these particles. The concentrations of micelles and scattering particles have not yet changed. The process is not dependent on temperature and hence is not influenced by the higher temperature after the T-jump, thus the turbidity of the solution relaxes back to the initial value before the jump. Step 4. Mass transport of matter from micelles to the scattering droplets. Step 5. Equilibration of concentrations of large particles. Journal of Colloid and Interface Science, Vol. 80° No; 1, March 1981

246

HOFFMANN ET AL. 1/"[ 1 [ s e c ' l ]

TritonX

100

w10~

25oc

o a ~ v o * 1 Io~lmKcI •

•

J

÷1 lO-1mKCI.510-4mEosin

f

6

o~

°~

2°°c

4

2

~ " ~ o

,

,

4

.102i 1/%2 [sec-11

C [mot/I]

- ~----~"

r-f?

i

,

,

8

,

8

10

• 10 -~

T r i t o n X 100

i

./ 6' .....

0 1 0. . .o. c

110mKc10oc

B + 1.10~2m KCI 20eC • ÷1 lo-lm KC[+ 5.10J*mEosin 20~

v ÷ 1 10-2mKCI 1O°C • + 110-1m KCI 10°C + 5 10-4m Eosin

o

/~,~_~_ I

~

,

"'--4

C [mot/l] '

6

'

8

'

10

.10-4

FIG. 4. (a) Plot of the reciprocal values of the short relaxation time for Triton X-100as a function of the total concentration at different temperatures and with various additives: OFq/~VO, with 10-1 M KC1; ®[]~/Z~, with 10-2 M KCI; liT, with 10-1 M KC1 + 5 x 10-4 M eosin. (b) Plot of the reciprocal values of the long relaxation time for Triton X-100 as a function of the total concentration at different temperatures and with various additives: E3, with 10-I M KC1 at 20°C; [], with 10_2 M KC1 at 20°C; II, with 10-1 M KC1 + 5 x 10-4 M eosin at 20°C; ~7, with 10-1 M KC1 at 10°C; ~, with 10-2 M KC1 at 100C; T, with 10-1M KC1 and 5 x 10-4 M eosin at 10°C.

Step 6. C h a n g e o f m i c e l l e c o n c e n t r a t i o n ( n o r m a l slow r e l a x a t i o n p r o c e s s in m i c e l l a r solutions). RESULTS AND DISCUSSION

a. Triton X-IO0 T h e r e c i p r o c a l r e l a x a t i o n t i m e s for T r i t o n X-100 with a d d e d KC1 a n d s e v e r a l a m o u n t s o f a d d e d e o s i n are p l o t t e d i n Figs. 4a a n d b as a f u n c t i o n o f the total c o n c e n t r a t i o n Co. As p o i n t e d o u t earlier, b o t h r e l a x a t i o n procJournal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

esses c o u l d be o b s e r v e d with a n d w i t h o u t e o s i n ; in the first case the optical d e t e c t i o n was m a d e with U V light at the w a v e l e n g t h 283 n m , while visible light at the w a v e l e n g t h 530 n m was u s e d in the s e c o n d case. T h e T r i t o n X-100 s o l u t i o n s w e r e a l w a y s clear in the t e m p e r a t u r e r a n g e w h e r e the m e a s u r e m e n t s w e r e c a r r i e d out. I n all exp e r i m e n t s o n l y two p r o c e s s e s c o u l d be detected. T h e r e l a x a t i o n t i m e s agree fairly well with d a t a that C h a n et al. (13) h a d r e p o r t e d r e c e n t l y . I n t h e s e i n v e s t i g a t i o n s a fluores-

KINETIC INVESTIGATIONS OF NONIONIC SURFACTANTS

247

TABLE II Values for the parameters k-/n, k-/o-2, k-, k ÷, n, and cr for micelles of Triton X-100 Calculated from the Two Relaxation Times at Different Temperatures T

k /o-~

k-/n

k-

k+

(°C)

(sec -1)

(sec -~)

(sec 1)

(liter/mole'sec)

5 10 15 20 25

2.8 x 4.0 x 6.2 x 8.3 x 1.2 x

103 103 108 103 104

6.0 8.4 1.1 1.5 2.2

x x x x x

10~ 103 104 104 105

4.0 5.1 5.7 7.4 1.1

x x x x x

l0s 105 105 l0s 106

1.3 x 1.7 x 1.9 x 2.5 x 3.7 x

n

109 109 109 109 109

n~

o-

(from Ref. 13)

66 61 52 49 48

12 11.5 9.5 9.4 10

100-144 b

a Average values obtained from calculations with r = 5 and r = 10. b Depending on the amount of added salt.

cence indicator has been used to monitor the relaxation times. It can be seen from the Fig. 4a that the relaxation times of the fast process are not significantly influenced by the addition of additives like KC1 or eosin. With different samples of Triton X-100 fairly well reproducible values were obtained, too. That means that even the number of glycolethers in the hydrophilic head group does not affect the fast relaxation process, because it is very likely that different samples of Triton are somewhat different in the average number of polyglycolethers per molecule. The short relaxation time is strongly dependent on the total concentration while its temperature dependence is only relatively small as can be seen in Fig. 4a. This behavior has also been observed for ionic detergents so that there is no doubt that the fast relaxation time of the Triton is due to the shift of the micellar distribution curve as described above. Figure 4b shows that the long relaxation time (which is called here only for the Triton X-100 system r2 and corresponds to step 6 in the above scheme) is highly sensitive to all kinds of impurities; it also shows significant deviations for different samples of Triton. This and the concentration and temperature dependence of the effect which has also been often observed in solutions of ionic detergents leads to the conclusion that the slow process is due to the change of the concentration of the micelles. Thus the

relaxation effects could be evaluated according to the theory of Aniansson and Wall. As the aggregation number of the micellar nucleus is unknown, it was assumed to have the same value as for ionic detergents with a corresponding chain length of the alkyl group. A value f o r r between 5 and 10 monomers per nucleus was postulated. The calculation showed that the obtained values were not very sensitive to r; the value for r = 5 is only 10% lower than for r = 10. In Table II the parameters k - / n , k - / o "2, n, k - , and o- are listed for Triton X-100 at different temperatures. It has to be pointed out that k ÷ values, the rate constants for the insertion of a m o n o m e r into the micelle, are in the region of diffusion controlled reactions as had been found for the ionic detergents. In most cases the k + values for ionic detergents are slightly smaller than what is reasonable for nonionic ones, because the insertion of an ionic detergent molecule into the micelle is hindered by electrostatic repulsion. Table II also contains n values for Triton X-100 from the literature (13). The agreement of these values with the n values determined from the kinetic data is satisfactory considering the fact that the experimental conditions were different.

b. C9PhEs In clear solutions of C9PhE8 that had small amounts of CTAB added in order to raise the cloud point three relaxation processes Journal o f Colloid a n d Interface Science, Vol. 80, No. 1, March 1981

248

H O F F M A N N ET AL.

o°1

~oo

I I -~

=~

=

Y. ~

¢-q I---

I~

O

CN

I~

uC

.O

m

lzk

[-, "~"~ 0-~

I~

~!~ ~'~

el

=

e~

E

"~

I

~[

~

I

~

~

I

I

,.,a

0 0

"~ &

I

Z

=

<

-'~

I

I

A

V

~1

I

= . 0~

V v ~ ~ A

V

~ ~

~ A

v

< r..)

+ >

44-

4-

,--,

..~ l~

?

x

i'

x

X

4-

X

4-

:Z 4r.)

Journal of Colloid and Interface Science, Vol. 80, No. 1, M a r c h 1981

~ A

KINETIC INVESTIGATIONS OF NONIONIC SURFACTANTS

¢xl

~-.~

I ©

O

~.a.,

d "6

"8

.-

",'7~ ~

az

= x o=~.~ ~

~

,.~ ~

.= E . . .

.,..~

~ ~

(,-q (,-q

~

~,~ ~1

~ ~ @

o

•-~

o .,~

x ' ~ -~-' ~~

t'q ¢'q

I I I

V

~

U

A

A

V

g T

+

X + r..)

x

?

+

+

d

~

A

~~. ~~- .~. ~

~~

249

were detected, when the UV absorption was monitored. The processes are called r01, r~l, and ~02. The index 0 is introduced in order to distinguish between these processes in the clear solutions which are due to changes in micellar size and concentration and the processes in the turbid solutions above To. The fastest process ~01 was sometimes too fast to be resolved in the T-jump apparatus. The existence of the amplitude which decayed with the time constant of the equipment (1/zsec) can be used as an indication for the presence of the effect. This effect is believed to be due to the same process as the fast one in the Triton X-100 solutions. Here the solutions contain mixed micelles and we have to expect two fast relaxation processes according to the Aniansson theory, one for the CgPhE8 monomers (~01) and one for the CTA ions which is likely to be associated with ~1. The concentration of the mixed micelles changes again during a relatively slow process G02). If the temperature in such a clear solution is raised to the cloud point T¢ so that the solution becomes slightly turbid, several new relaxation processes are detectable that were not present when the temperature was below To. These processes were already characterized in the theoretical part. All the relaxation times are summarized in Table III for different experimental conditions. Some of the data were used to calculate the concentration and the radii of the droplets that are present in the new phase. The results are given in Table IV. This table also contains values for radii of droplets (rd) in turbid solutions calculated from diffusion coefficients which were obtained by quasielastic light-scattering measurements. These values do not agree with the radii calculated from the kinetic data. The disagreement could be caused by the fact that the prerequisite for the usage of Eq. [8] for the calculation of Stokes' radii is not fulfilled. The particles could be polydispers and the quasielastic light-scattering technique is sensitive to the large aggregates while the Journal of Colloid and Interface Sc&nce, V o l . 80, N o . 1, March 1981

250

H O F F M A N N ET AL. T A B L E IV Values for the Concentrations c and the Radii r of the Particles in the Different P h a s e s Existing in Solutions of Nonionic Detergents in Wateff r~" (A)

System

l~mb (h)

r~]' (A)

r~ '~ (A)

c~' e (drops/cm ~)

--

No rods

--

660

3 x 10 -3 M Ca0E4 q- 2 x 10 -3 M K C I

104 e

No rods

10,000

1100

C,0E4 + K C 1 + C T A B a

-420 e 930±

No rods

--

1200

8.3 >< 10 la 1.8 >< 10 a' 1.5 x 10 '1

1100

10,000

1800

4.1 x 10 TM

C g P h E ~ + 5 × 10 -2 M K C I d

--

No rods

--

1900

CgPhE~ + KC1 + CTAB d

--

No rods

--

2300

--

No rods

--

3200

--

No rods

--

3200

3.5 2.0 7,3 7,3

10 -2 M C8E4 + 2 X 10 3 M K C 1

3 × 10 -3 M C l z E 4 + 2 x 10 -2 M K C I ~

CaPhEs

+ KC1 a

CoPhE8 + KC1 + CTAB a

x x x ×

r~} (A)

1800

10 TM 10 a° 10a lO9

Calculated from orientation (upper index 0), light scattering (uppei" index 1) and kinetic (upper index 2) m e a s u r e m e n t s (lower index I, clear solution below T¢; lower index II, turbid solution b e t w e e n Tc and Tiara; lower index III, turbid solution above T~am). b Length of the anisotropic aggregates. c Values calculated from ~'3. See footnote c in Table III. e Value calculated from the angular d e p e n d e n c e of the scattered light intensity. s Value calculated from d y n a m i c light-scattering m e a s u r e m e n t s (Stokes' radii).

kinetic method is more sensitive to the particles that are smaller but present in higher concentrations. C.

C12E4

The solution below the turbidity point gives rise to electric birefringence. F o r the measurements the plane of the polarized light and the vector of the electric field formed an angle of 45°; the analyzer was oriented to minimum light intensity on the detector. The relaxation time for this process is called z~ in order to distinguish this orientation effect which can be detected only with polarized light in clear solutions below Tc and also in turbid solutions not far above Tc from the described other processes. If rodlike particles are assumed for the micelles, the measurements permit the calculation of the length of the particles from the orientation time. The birefringence decays with the time constant of the condenser discharge through the solution. No effect could be detected without the polarizers if the T-jump is small and the temperature after the T-jump is lower Journal o f Colloid and Interface Science, Vol. 80, No. 1, March t981

than the cloud point. If the final temperature after the jump is higher than Tc, the building up of the turbidity can be observed (process 4) (Fig. 5). It is n o t e w o r t h y that the light intensity for this system begins to change right after the temperature rise with practically no delay time. The new phase is formed immediately, which means that there are enough particles present that can act as nuclei for the formation of the new phase. Most of the change in turbidity occurs in about 2 sec but even after 10 sec the process is still incomplete. It is conceivable that the slow changes are related to a change in the

T- jump

a

b

FIG. 5. Recordingsof the light intensity behind a solution of 3 × 10-3M C,2E4 + 2 × 10_3 M KC1 in water at 0.5°C (solution is below To) after different T-jumps; sweep time t = 10 sec; (a) Temperature after the Tjump is still below To; (b) temperature after the T-jump is above To.

251

K I N E T I C I N V E S T I G A T I O N S OF N O N I O N I C S U R F A C T A N T S

particle concentration or the micelle concentration or both. If the turbidity measurements are carried out above Te, the signal looks the same if the light intensity is monitored with or without polarizers. It is therefore evident that the micelles have disappeared completely and the particles of the new phase have no anisotropy. The increase and consecutive decrease of the light intensity can be explained as mentioned for the results on CgPhE8 (processes 2 and 3). The intensity of the light is almost the same after the two processes are completed as before the T-jump. If the temperature is increased to slightly below the temperature Tlarn and then a T-jump is carried out the signal changes again from the previous response pattern. It starts the same way as at lower temperatures. This means the first two processes 2 and 3 are still present. But before process 3 can take the signal back to the equilibrium value, a new process comes in that lowers the turbidity (Fig. 6). This process must be due to the transformation of the dispersed phase to the dispersed lamellar phase. The main amplitude change for this process occurs in about 200 msec but as in the case for the formation of the new phase the process is not yet complete in 10 sec. The relaxation time for this process is called r; in analogy to the similar process at To. Finally if measurements are made above this phase transition point the situation changes again. There are differences in the signal when polarizers are used in the light beam indicating anisotropic particles. When the turbidity is measured directly, it decreases again quickly and then recovers to its original value in a process that consists of several time constants. Some of the obtained data are given in Table III and the results that were calculated are given in Table IV. On this system a few measurements were carried out in the clear solution with the quasielastic light-scattering method. The data also indicate the existence of rather large particles, which agrees very well with the conclusions from

a

b

d

e

c

f

FIG. 6. Recordings of the light intensity behind a solution of 3 × 10-3 M C12E4 + 2 x 10-3 M KC1 in water at 12°C (solution is above T0 and below Tlarn) after different T-jumps. Upper series: Temperature after the T-jump is still below/'tam. (a) Sweeptime t = 10 msec; (b) t = 100 msec; (c) t = 1 sec; lower series: temperature after the T-jump is above Ttar,. (d) Sweeptime t = 100 msec; (e) t = 1 sec; (f) t = 10 sec.

the electric birefringence measurements. The data from both methods are included in Table IV. It is noteworthy to point out that rodlike particles were already observed in these solutions at much higher concentrations by monitoring the streaming birefringence (1). The present data make it clear that these aggregates exist already in very dilute solutions. d.

C10E4

This system behaves very similar to C~2C4. It is remarkable that the 71 process cannot be detected in the clear solution below Te but is clearly visible in the turbid solution with T _-> Te. It is also worth mentioning that the Zl process is detectable under the condition that the time constant for the voltage discharge is longer than 71, which clearly indicates that the micellar equilibrium is not shifted by the temperature but must be effected by the electrical field. There seems to be a reequilibration process of the monomers in the field during the course of which Journal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

HOFFMANN ET AL.

252

a

b

c

d

e

f

FIG. 7. Recordings of the light intensity behind a turbid solution of 3 x 10-a M CloE44- 2 X 10-3 M KC1 at 16.5°C after a constant T-jump (a signal change upward corresponds to an increase of the turbidity). (a) Sweeptime t = 200 #sec; (b) t = 1 msec; (c) t = 10 msec; (d) t = 50 msec; (e) t = 200 msec; (f) t = 1 sec.

m o n o m e r s are transferred to particles of the Contrary to the solutions of C12E4 no large new phase which leads to an increase in the aggregates could be o b s e r v e d for C10E4 in turbidity. I f the new phase is not yet pres- the clear solutions below Tc. It is very reent (T < To), the process is not detected. markable that the difference of two CH2 This process can be treated in the same way groups in the chain m a k e s such a big difas the fast step in the Aniansson theory ference in the capacity to form rods. because it represents a shift in the m e a n agIt could be that the p r e s e n c e or absence gregation n u m b e r of particles whose con- of rods has something to do with the rate centration does not change during the proc- with which the new phase is formed when ess; the disturbing p a r a m e t e r is in this case Tc is crossed in a T-jump experiment. In the electric field. All the processes that have C10E4 solutions it took up to several seconds been mentioned for CgPhE8 are clearly vis- until the new phase a p p e a r e d after a T-jump ible in solutions of C,0E4 (Fig. 7). Further- from the clear solution into the two-phase more the data indicate that the turbidity af- region (Fig. 8). These data suggest that the ter the completion of all six p r o c e s s e s is a p p e a r a n c e of the new phase is controlled higher than before the T-jump, which shows by a nucleation p h e n o m e n o n and the mithat the amount of dispersed p h a s e grows celles at T~ cannot act as nuclei. As menduring the relaxation e x p e r i m e n t and clearly t i o n e d in the discussion on ClzE4, this sysindicates the existence of micelles in equi- tem b e h a v e d differently in this respect. librium with the new phase. This result is in agreement with turbidity m e a s u r e m e n t s e. CgPhE~ as a function of t e m p e r a t u r e where it was M a n y m e a s u r e m e n t s were carried out on o b s e r v e d that the turbidity keeps increasing the system CgPhE5 which has a cloud point up to several degrees a b o v e T~. Contrary to the C12E4 systems, no bire- below 0°C. All the solutions therefore were fringence effects could be detected on the turbid when no ionic detergent was added. C10E4 and also on the C8E4 system. The mi- The signal in these solutions was hardly celles for these systems must therefore be affected when the t e m p e r a t u r e was varied small and spherical, otherwise they would between 5 and 45°C. The turbidity in the have been aligned in the electric field. This solution after the T-jump was always slightly shows that the existence of rodlike particles lower than before the T-jump when all the is not a prerequisite for the formation of relaxation processes had decayed. This can the new phase. The experimental results and be taken as evidence that there was no mass the calculated data are given again in Tables transport f r o m the micellar solution to the I I I and IV. These conclusions f r o m the ki- new phase after the T-jump. It is therefore netic data were confirmed again by data from likely that only v e r y few micelles are present the quasielastic light-scattering method. in these turbid solutions. As a c o n s e q u e n c e Journal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

K I N E T I C I N V E S T I G A T I O N S OF N O N I O N I C S U R F A C T A N T S

O

d

b

e

c

f

FIG. 8. Recordings of the light intensity behind solutions of neutral detergents after T-jumps from temperatures below T¢ to temperatures above T¢. U p p e r series: 3 × 10-3 M C12E4 + 2 × 10-3 M KC1, T = 0.5°C waiting ca. 10 min between the T-jumps. (a) Sweeptime t = 10 msec; (b) t = 100 msec; (c) t = 20 msec; lower series: 3 × 10 -~ M C10E4 + 2 × 10 -3 M KCI, T = 13.3°C. (a) Sweeptime t = 10 sec, T-jump immediately after the preceding T-jump without waiting; (b) t = 1 sec, waiting ca. 1 min after the preceding T-jump; (c) t = 1 sec, waiting ca. 1 hr after the preceding T-jump.

of this, processes 4, 5, and 6 cannot be observed. It is noteworthy, however, that the increase in turbidity after step 2 does not proceed in a single relaxation process. At least two steps are involved in the reequilibration process. It seems likely therefore that the number of scattering particles changes after the T-jump. The fact that no mass transport takes place after the T-jump can possibly have its origin on the temperature dependence of the CMC. It is known that the CMC is lowered for the alkylpolyglycolethers with increasing temperature while it is little affected or even is increased for the alkylphenylpolyglycolethers. GENERAL OBSERVATIONS

All solutions that are slightly cloudy show a considerable reduction of their turbidity during the transport of electric current through the solution. The decrease of the turbidity follows the discharge of the condenser. In the solution with the highest ionic strength (5 x 10-2 M) the change in the turbidity is therefore very fast with a time constant of a few microseconds. After the con-

253

denser is discharged the temperature rise is complete and most of the previous turbidity bounces back within about 1 msec. This time constant varies very little with temperature or with the composition of the solution and is independent of whether the solution is at the turbidity point or very high above it. At temperatures around the turbidity point, the solution becomes more turbid after the Tjump while solutions with temperatures several degrees above the turbidity point, the turbidity goes back to the value before the T-jump. Sometimes it stays slightly below. How can we explain this phenomenon? A reduction of the turbidity could be caused by the disappearance of matter from the scattering phase or by the change of the polarizibility of the scattering phase. A redistribution of detergent molecules from this phase to normal micelles is practically impossible in the short times that are involved in the experiment. It is therefore more likely that the polarizibility in the dispersed form is changed. From the phase diagram it is evident that the scattering phase must contain up to about 90% of water and also of the supporting electrolyte that was added to allow the discharge. It is conceivable that some of the ions with their solvation spheres are removed from the scattering phase during the current pulse, which could give rise to a change in the polarizibility. This would explain why the decrease of the turbidity follows the discharge. After the current pulse, the concentration of the electrolyte in the new phase is therefore less than in the bulk water and has to reequilibrate. This can occur only by diffusion of the electrolyte from the bulk phase into the scattering phase. If the dispersion degree of the scattering phase is not very temperature dependent, the time constants that are involved for the process should also depend very little on temperature and this is experimentally observed. As was shown the model can be used to estimate the size of the scattering particles. The critical question that remains to be answered is, of course, why can the same Journal of Colloid and Interface Science, Vol. 80, No. 1, March 1981

254

HOFFMANN ET AL.

number of ions that leave the dispersed phase also not enter the dispersed phase during the discharge process? The answer to this problem could possibly be that the transference numbers of the ions in the two phases are different. Under such a condition the concentration of one kind of the ions would be depleted at the interface during the current pulse /~nd a potential would develop at the interface. The build up of the double layer potential could strongly affect the scattering power of the particles and could conceivably cause the described effects. More data are certainly needed in order to prove the given explanation. Preliminary measurements have already shown that the effects depend very much on the kind of electrolyte that was used for the measurements as should be the case for the proposed explanation. ACKNOWLEDGMENTS Financial support of this work by grant of the Deutsche Forschungsgemeinschaft(DFG) and the Fond der Chemischen Industrie and by the donation of the investigated nonionic surfactants by Unilever Research, Vlaardingen, The Netherlands, is gratefully acknowledged. REFERENCES I. Bostock, R. A., Donald, M. P., Tiddy, G. J. T., and Waring, L., Surface Active Agents, Soc. of Chem. Ind., Symposium Nottingham, 26.9.28.9, 1979.

Journal

of Colloidand Interface Science. Vol.80, No. 1, March1981

2. Jost, A., and Schneider, G. M., J. Phys. Chem. 79, 859 (1975). 3. Berne, B. J., and Pecora, R., "Dynamic Light Scattering." Wiley, New York, 1976. Fredericq, E., and Houssier, C., °'Electric Dichroism and Electric Birefringence." Clarendon Press, Oxford, 1973. 4. Schick, M. J., "Nonionic Surfactants." Marcel Dekker, New York, 1966. 5. Wennerstr6m, H., and Lindman, B., Phys. Rep. 52(1), 1- 86 (1979); Shinoda, K., J. Colloid Interface Sci. 34, 278 (1970). 6. Hoffmann, H., Platz, G., Reizlein, K., Rehage, H., and Ulbricht, W., MakromoL Chem. 182, (1981). 7. Tondre, C., Lang, J., and Zana, R., J. Colloid Interface Sci. 52, 372 (1975). 8. Folger, R., Hoffmann, H., and Ulbricht, W., Ber. Bunsenges. Phys. Chem. 78, 986 (1974). 9. Fontell, K., in "Liquid Crystals and Plastic Crystals" (G. W. Gray and P. A. Winsor, Eds.), Vol. 2. Wiley, New York, 1974. 10. Aniansson, E. A. G., and Wall, S. N., J. Phys. Chem. 78, 1024 (1974); Aniansson, E. A. G., and Wall, S. N., J. Phys. Chem. 79, 857(1975). I1. Hoffmann, H., Ber. Bunsenges. Phys. Chem. 82, 988 (1978). 12. Aniansson, E. A. G., "Techniques and Applications of Fast Reactions in Solution," p. 249. Reidel, Dordrecht, Holland, 1979. 13. Chan, S. K., Herrmann, U., Ostner, W., and Kahlweit, M., Ber. Bunsenges. Phys. Chem. 81, 396 (1977). 14. Corti, M., and Degiorgio, V., Optics Commun. 14,358 (1975); Robson, R. J., and Dennis, E.A., J. Phys. Chem. 81, 1075 (1977). 15. Mukerjee, P., and Mysels, K. J., in "Critical Micelle Concentrations of Aqueous Surfactant Systems." NSRDS-NBS 36, 1970.