The interaction of cetyltrimethylammonium bromide and sodium dodecylbenzene sulfonate with polyvinyl alcohol. adsorption of the polymer—surfactant complexes on silica

The Interaction of Cetyltrimethylammonium Bromide and Sodium Dodecylbenzene Sulfonate with Polyvinyl Alcohol. Adsorption of the Polymer-Surfactant Complexes on Silica T H . F. T A D R O S Jealott's Hill Research Station, Imperial Chemical Industries Limited, Bracknell, Berkshire, RG12 6EY, England Received December 12, 1972 ; accepted August 30, 1973 The interaction of cetyltrimethylammonium bromide (CTABr) and sodium dodecylbenzene sulfonate (NaDBS) with polyvinyl alcohol (PVA) (12% acetate groups, _~r,=42,000) has been investigated using surface tension, viscosity, conductivity and cloud point measurements. The results showed that the polymer-surfactant "complex" behaves as an association polyelectrolyte. The adsorption of PVA, CTABr, NaDBS and "complexes" of CTABr + I?VA and NaDBS + PVA on silica was studied in aqueous solution at low and high pH. The adsorption of PVA was higher at pH 3.6 than at pH 9.1, because at low pH there are more undissociated silanol groups which act as adsorption sites for PVA. CTA+ ions are adsorbed at low pH (3.6) in a more or less vertically oriented layer. At high pit (9.1) a bilayer of CTA+ ions form at the silica-solution interface. The adsorption of NaDBS was low, increasing as the pH was lowered, and the adsorption isotherms deviated from the Langmuir equation. In the presence of preadsorbed CTA+ ions at high pH, the PVA adsorption increased significantly relative to the value in absence of surfactant. It is believed that the CTA+ ions act as "anchors" between the dissociated -SiO- sites on the surface and PVA chains. At low pH, CTA+ ions are adsorbed to a greater extent in presence of preadsorbed PVA. It is assumed that under these conditions PVA acts as the "anchor" between the undissociated SiOH groups and CTA+ ions. On a silica surface with preMsorbed DBS- ions at low and high pH PVA is adsorbed to a greater extent than in the absence of DBS- ions, but to a lesser extent than in the presence of CTA÷ ions. The adsorption of DBS- ions on a silica surface with preadsorbed PVA at low pH is complex, especially at high PVA concentration. I. INTRODUCTION I t is now fairly well established t h a t ionic surfactants interact with nonionic polymers, p r o b a b l y b y " h y d r o p h o b i c bonding" and t h a t the resulting p o l y m e r - s u r f a c t a n t "complexes" behave as "association polyelectrolytes" 0 - 1 2 ) . T h e polyelectrolyte nature of these complexes has been investigated using viscosity (1, 2, 7, 10, 11), surface tension (7, 10), conductivity (7, 9), dye solubilization (1, 10), dialysis (11), electrophoresis (2) and ion activity (3, 5) measurements. A t t e m p t s have been m a d e to

calculate the number of surfactant ions bound per polymer molecule or per segment (3, 5, 7). T h e adsorption and orientation of p o l y m e r surfactant complexes has only previously been studied a t the a i r / w a t e r interface (7, 10). Similar studies a t the solid/liquid and l i q u i d / liquid interface would, however, be relevant in interpreting both the stabilization and the flocculation of dispersions and emulsions b y such complexes, their role as detergents, etc. T h e object of the present p a p e r was first to investigate the interaction between (a) C e t y l t r i m e t h y l a m m o n i u m bromide (CTABr)

528 Journa~ of Colloid and Interface Science, Vol. 46, No. 3, M a r c h 1974

C o p y r i g h t ~ 1974 b y A c a d e m i c Press, Inc. All rights of reproduction in a n y f o r m reserved.

POLYMER-SURFACTANT INTERACTION

and (b) sodium dodecylbenzene sulfonate (NaDBS), and polyvinyl alcohol (PVA) in bulk solution, using surface tension, viscosity, conductivity and cloud point measurements. The adsorption of the polymer-surfactant complexes on silica from aqueous solution was then studied. Silica is a convenient substrate since its surface charge density can be controlled by varying the pH of the solution (13). Moreover, the adsorption of cationic surfacrants on silica is well established (14). A previous investigation (15) showed that PVA itself adsorbs on silica at low pH, provided that there are sufficient "free" silanols to hydrogen bond with the PVA chains. The stability of dispersions and emulsions in the presence of these polymer-surfactant complexes will be reported in subsequent papers. II. E X P E R I M E N T A L METHODS

Materials Cetyltrimethylammonium bromide (B.D.H.) was recrystallized from ethanol-acetone mixtures (1:1 by volume). The surface tension (3,)-log concentration (C) curve showed no minimum after double recrystallization (see Fig. 2). The critical micelle concentration (CMC) at 25°C obtained from the break in the y-log C curve was 7.5 X 10-~ mole/liter; that obtained from the break in the equivalent conductivity A - (C) ~ plot (see Fig. 6) was 8.3 X 10-4 mole/liter. The latter value agrees well with that of Barry, Morrison, and Russel (16) namely 8.2 X 10-4 mole/liter. Sodium dodecylbenzene sulfonate was a specially pure material obtained from K. and K. Laboratories (Lot. 60959, Plainview NY; Hollywood, CA). The material showed no minimum in the 7-log C curve (see Fig. 3); the CMC at 25°C was 1.61 X 10- 3 mole/liter in excellent agreement with the value reported by Ludlum (17) for the p-isomer. However, the material would seem to contain an inorganic salt impurity as was apparent from its rather high conductivity. Attempts to remove

529

the salt impurity by recrystallization from isopropyl alcohol (17) were not successful. Sodium lauryl sulfate was a specially pure B.D.H. sample which was used without further treatment. Polyvinyl alcohol 88/10 (Revertex Ltd., London) was used without further purification (the first number refers to the degree of hydrolysis, i.e., the polymer contains 12% acetate groups, while the second number refers to the viscosity in centipoise of a 4% aqueous solution at room temperature). The intrinsic viscosity I-n] of the polymer was measured at 25°C (see Fig. 4) and was found to be 0.52 dl/g. The viscosity average molecular weight, 21~r~, was calculated from the Mark-Houwink equation [~] = K1]llv ~, using the K1 and a values quoted by Fleer (18) (8.7 X 10-4 and 0.6, respectively); this gave 21~r~= 42,000. Silica was a precipitated, acid washed sample supplied by B.D.H. Methylene blue (B.D.H.) was used as received. Water, double distilled from all glass apparatus, and having a specific conductivity K < 2 X 10- 6 fl-1 cm-1 at 25°C, was used for the preparation of all the solutions. The solutions were not kept for longer than 1 wk.

Viscosity and Density Measurements For the viscosity measurements, an Ubbelohde suspended level viscometer was used. The flow time for water at 25°C was 265.8 sec and therefore kinetic energy corrections could be neglected. A U-shaped pyknometer with two graduated capillary arms was used for the density measurements. It was calibrated by filling with fresh doubly distilled water to various levels. A least-squares method was applied to calculate the necessary constants.

Surface Tension Measurements The drop volume method was used following the technique of Parreira (19). A 0.5 ml Agla syringe (Burroughs Welcome and Co., London) having a flat tip (radius 0.269) was used. Adc motor (9 V) was used to drive the micrometer and the speed of the motor was adjusted

Journal o.f Colloid and Interface Science, Vol. 46, No. 3, March 1974

530

TADROS

to control the time (4 min) required for the formation of the drop. In order to eliminate evaporation from the drop, it was formed in a dosed vessel at the saturation vapor pressure of the solution under investigation. For that purpose a 1 cm spectrophotometer cell containing a small portion of the solution was fixed to the capillary tip by a brass cover containing a hole in the middle for the tip, and stainless steel inlet and outlet tubes. All connections were made water tight with Araldite (Ciba Geigy). The whole glass syringe and cell were immersed in a thermostat bath at 25 4- 0.05°C. A lamp was used for illumination and the detachment of the drop was followed by a traveling microscope. Care was always taken to avoid vibrations. Usually 7-9 drops had to be measured before agreement between the volume of two successive drops was obtained. However, such agreement does not imply that true equilibrium has been achieved. The rate of polymer adsorption at the air/water interface is probably much slower than the rate of creation of fresh interface (20). However, if the time of formation of the drop is kept fixed, the calculated surface tension values of the various solutions m a y be compared. Surface tensions were computed from the drop volume and the measured density of the solutions, incorporating the Harkins and Brown correction (21), as calculated from the quadratic equation of Lando and Oaktey (22). The reproducibility in 3' was better than 0.5%.

Conductance Measurements A Tinsley conductivity bridge (Type 4896; H. Tinsley and Co. Ltd., London) incorporating a Wagner earth was used. Measurements at 1 kHz were made using a Mullard cell (Type 7591/B). This had a cell constant of 1.415 4- 0.001 at 25 4- 0.05°C, having been determined using standard KC1 solutions (23). The specific conductance K of PVA solutions was appreciably greater than that of water, e.g. a 0.1% w / v solution had a ~ value of 1.461 M 10-5 f~-i cm-1 at 25°C. This was due to the presence of some ionizable groups, e.g.

sulfate or carboxyl (18, 24). To account for this the conductance of the polymer solution was subtracted from that of the PVA + surfactant mixture. Similarly the conductance of water was subtracted from that of the aqueous surfactant solution. The accuracy of the conductance measurements was better than

0.05%. Cloud Point Measurements The cloud point of 1% (w/v) PVA was measured as a function of CTABr or NaDBS concentration. A turbidity method was applied using a Pye-Unicam SP 1800 fitted with a temperature programming unit (Pye Unicam SP 876). The latter was connected to a specially designed metal block (SP 877) located in the cell house of the spectrophotometer. The solution in question was placed in a 1 cln cell fitted with a fight lid and placed in position in the metal block. The heating rate was adjusted to l°C/min (a linear temperature rise of 0.25, 0.5, or l°C/min could be applied with an accuracy of 4-3%) and the chart speed to 1 cm/min. The turbidity at an arbitrary wavelength of 500 nm was read on the Y-axis of the chart paper and the temperature on the X-axis. When the cloud point of the solution was reached, there was a sudden rise in turbidity; the temperature at which this occurred could be recorded to within 4-0.2°C.

Analysis of PVA and Surfactants The concentration of PVA was determined colorimetrically. The optical density of the PVA complex with iodine formed in the presence of boric acid and potassium iodide was measured at 670 nm (25, 26). CTABr was analyzed by titration with standard sodium lauryl sulfate solution using the method of Barr, Olivier and Stubbings (27). The concentration of NaDBS was determined using the uv adsorption band at 224 nm. A BeerLambert relationship was obtained in the concentration range 2 )K 10-5 to 3 X 10-4 mole/liter. In the presence of PVA, the titration pro-

Journal of Colloid and Interface Science, Vol. 46, No. 3, M a r c h 1974

POLYMER-SURFACTANT INTERACTION cedure of CTABr with sodium lauryl sulfate was unsuccessful. For the measurement of the adsorption of CTABr in the presence of PVA, a conductometric method was therefore used. The calibration curves for conductance versus CTABr concentration were measured in the presence of the same amount of PVA remaining in the equilibrium solution after adsorption. Since adsorption of CTABr into PVA was measured at low concentration of PVA (20-125 ppm), then the contribution of the PVA to the total conductance was small. The green-blue color of the complex formed between PVA, boric acid and iodine was also affected by the presence of appreciable amounts of CTABr. Therefore, analysis of PVA in the equilibrium solution was only possible provided that most of the CTABr was adsorbed by silica. This was always checked before PVA was added. It was also checked that no CTA + ions were desorbed after the addition of the PVA, by comparing the conductance of the equilibrium solution with that initially. If there was any release of CTA + ions, the conductance of the equilibrium solution should have increased. For the PVA-NaDBS system, similar problems did not arise, and each component could be determined in the presence of the other by the methods indicated above. Measurement of the Surface Area of Silica

The surface area was determined by the methylene blue dye adsorption method (28) and by the nitrogen B E T method. In the first method, adsorption of methylene blue was determined at pH 10.1. A Langmuir isotherm was obtained and the surface area obtained from the maximum adsorption at the plateau was 47.4 m~/g, using a coverage factor of 2 and an area of 120 A~/molecule of methylene blue (28). For the nitrogen gas adsorption, the silica sample was outgassed at 110°C for 2 hr. The sample was shown to be microporous and using Sing's analysis (29) an a, plot was established. The micropore volume was 0.96 cma/g and the external surface area was

531

72 m2/g, a value much higher than that obtained by the methylene blue method. However, since the adsorption and orientation of the methylene blue molecules depend on surface charge, salt concentration, nature of solvent, etc. (30), the results obtained by this method are always doubtful (31). Therefore, we preferred to use the external surface area obtained by nitrogen adsorption for analysis of the adsorption results of polymer and surfactants. We do not believe that the presence of micropores affects the results since both the surfactant ions and the polymer molecule are larger than the dimensions of the micropores. Adsorption Measurements

A silica suspension of the known concentration was prepared by stirring the appropriate amount of powder into a given volume of distilled water; the pH of the suspension was adjusted using standard HC1 or NaOH solutions. A given volume of the resulting suspension, which was kept stirred, was added to PVA, CTABr, NaDBS, CTABr + PVA or NaDBS + PVA solutions covering a wide concentration range. The whole mixture was rotated end-over-end in stoppered tubes overnight at room temperature (20 ± 2°C), centrifuged at 3500 rpm and the supernatant liquid was analyzed as described above. III.

RESULTS

Surface Tension

Figure 1 shows the -),-log C for PVA solutions; a transition at 0.7% (w/v) indicates some sort of association above this concentra~ tion. Recent results on solubilization of terbacil (32) (3-t-butyl chloro-6-methyl uracil) showed a sharp rise in solubility at ---1%

(w/v) PVA. The influence of the addition of PVA on the "hlog C curves of CTABr and NaDBS is shown in Figs. 2 and 3, respectively. Marked changes in the "y-log C curves are observed, especially at high PVA concentration. At a PVA concentration of 0.005% (w/V), the

Journal of Colloidand InterfaceScience,VoL46, No. 3, March 1974

532

TADROS

q

T

60

~0

E

0.5

~so

3C

~5 10-3

I

i

I

10-2

10~

I00

CpvA (%w/v)

~0'

FIO. 1.7-log C curve for PVA.

shape of the ~,-log C curve (Fig. 2) is maintained but the CMC is shifted to a higher value, and the magnitude of ~, is lowered. For CTABr in 0.02 and 0.05% (w/v) PVA, the "/-log C curve resembles that obtained by ~ones (7) and Arai et al. (10), namely with two transitions. Moreover, the surfactant solutions become less surface active as the PVA concentration increases. These transitions become ill defined at relatively high PVA concentrations and eventually [-PVA concentration > 0.1% (w/v)] 3' merely decreases slowly with increasing surfactant concentration. The latter behavior starts at an earlier

35

~ , 0,I

.0.01 0.3

0 3÷1°9 CNaDBS

FIG. 3. "r-log C curves for N a D B S at various PVA concentrations.

concentration of PVA in the case of NaDBS (Fig. 3).

Viscosity Measurements The influence of the addition of CTABr or NaDBS to PVA solutions is shown by a rapid increase in ~p/CpvA, where ~ = ( ~ r e l - 1), with decrease of CpvA(g/lO0 ml) at low PVA concentration (see Figs. 4 and 5). A similar increase was observed by Isemura and Imanishi

3

'

~0

o

5 3 2

•.c

30

~

0

5

o.,

08

o~

~.IOgCcTAB r

,o

FIG. 2. ~,-log C curves for CTABr at various PVA concentrations. Journal of Colloid and In~erface Scignce,

o

r

(20I~'~

~ o o o-

i

f

oL2xlO -3 olxlO -3 uSxlO''( ..~SxlO-4

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FIG. 4. Variation of reduced viscosity with PVA concentration. The Y scale shown is for (1, 2) ; (3, 4, 5) have been displaced upwards 0.2, 0.4, and 0 . 6 scale units, respectively.

Vol. 66, No. 3, March 1974

POLYMER-SURFACTANT INTERACTION

533

CNaDB~9rno~ /,~

.~2.0xlO ~

"'"

~8 x 10-3 t.?x 10-3

T

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0.6 I

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I

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0.11 012 0.13 0.4 0/5 0.6 Q7 0.8 09 CpvA (% w/v)

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1.0

FIG. 5. Variation of reduced viscosity with PVA concentrations. The Y scale shown is for (1) ; (2, 3, 4, 5, 6, and 7) have been displaced upwards 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 scale units, respectively. (2) and by Lewis and Robinson (11) with other systems. Conductance Results

As shown in Fig. 6, the progressive addition of PVA to aqueous CTABr solutions, gives rise to more than one transition in the A--(CcTABr)½plots, below and above the CMC. The same behavior was also found for the PVA-NADBS system. 1 Similar transitions were found by Jones (7) for polyethylene oxide (PEO) + sodium dodecyl sulfate (NaDS), at polymer concentrations lower than 0.07% (w/v). However, above the second transition this author did not find any difference between of PEO + NaDB and that of NaDS at the same surfactant concentration. Above 0.07% (w/v) polymer there was only one transition in the K-C curve. Gravsholt (9) found more than one break in the K-C plot for NaDS + polyvinyl pyrrolidone (PVP) or polethylene glycol (PEG). However, above the second transition K for the polymer-surfactant solution was higher than that of the surfactant solution alone. The author did not apply any correction for the conductance of the polymer solution. For CTABr + P E G or PVP, i For that system the magnitude of A was apparently high, probably due to the presence of an inorganic impurity. For that reason the results are not shown.

|

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l

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20

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32

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3.~

102/CcrAsr Fro. 6. A-(C)~ curves for CTABr at various PVA concentrations. Gravsholt (9) found only one transition with a small decrease in K. Cloud Points

The cloud point (CP) of 1% (w/v) PVA increases rapidly with increase in NaDBS or CTABr concentration (Fig. 7); NaDBS being more effective than CTABr. Recently, Saito et al. (1) found that the anionic surfactants sodium actyl sulfate and guanidinium octyl sulfate raise the CP of polyvinyl alcoholpolyvinylacetate copolymer significantly. % w / v C T A B r ---* 0.01 0.02 0.03 0.04 0.05 i

i

i

i

i

~to7 0

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0,005 0.01 % w/v NaDBS -'-'-*

FIG. 7. Variation of cloud point of 1% (w/v) PVA with NaDBS or CTABr concentration.

Journal oJ Colloid and Interfac¢ Saienae, Vol. 46, No. 3, March 1974

534

TADROS (mole/g] lu moledu~

5

,o,

%,c

°6

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o. s

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3.s

2

4

6 8 I0 C2[mg/1OOmt)

The saturation adsorption, a, is much higher at the lower pH. The increase of adsorption with decrease of p H has been found before

6.5 2.8

__1

(15) for adsorption of PVA with 2% acetate

Fro. 8. Adsorption isotherms of PVA on silica at low and high pH. (x is the amount adsorbed by m grams of silica and C~ the equilibrium concentration). Adsorption

Isotherms

Figure 8 shows the adsorption isotherms of PVA on silica at p H 3.6 and 10.1; the results are also in the linearalized Langmuir form.

I

i

groups (21~ = 55,000) on preheated (16 hr at 350°C) precipitated silica (B.D.H., B E T N2 surface area 56 m~/g). Maximum PVA adsorption was obtained at the point of zero charge (pzc) of silica (pH 2-3). Moreover, adsorption increased when the silica sample was preheated [i.e., on removing physically adsorbed water (33)].

C2(mole/ I x 106)----,, 2 3 ~ 5 I

i

i

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i

I0 ~20 C2 (mole/Ix 10°) , 4,

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3.6 3 d 92 125 2 3 4 C2(mole/t xlO u)

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*

FIG. 9. Adsorption isotherms of CTABr on silica at low and high pI-I.

10.5

FIO. 11. Ads9rption isotherms of PVA on silica in presence and absence of CTABr at p H 10.1.

Journal of Colloid and Interface Science, Vol. 46, No. 3, March'1974

535

POLYMER-SURFACTANT INTERACTION

Figure 9 shows the adsorption isotherms of CTA + ions on silica at pH 3.6 and 9.1 ; adsorption increases with increase of pH as predicted for the adsorption of a cationic surfactant on a negative silica surface (34, 35). Figure 10 shows the results for NaDBS adsorption on silica at pH 3.6 and 10.1. The S-shaped isotherm at pH 10.1 is similar to that reported by Tamamashi and Tamaki (36) for sodium dodecyl sulfate on alumina. At pH 3.6, the adsorption isotherm still deviates from the Langmuir type. The influence of addition of CTA + ions on the adsorption of PVA on silica is illustrated in Fig. 11. The adsorption of PVA on a silica surface with preadsorbed CTA + ions increases considerably relative to its adsorption in absence of CTA + ions. In this experiment, the low concentration of CTA + ions in bulk solution made it possible to analyze for the PVA (see Experimental section). On progressively increasing the amount of CTA + ions adsorbed at the surface, the PVA adsorption increases in proportion as shown in Fig. 12. In the presence of preadsorbed PVA at low pH (3.3) CTA + ions were adsorbed to a greater extent compared to their adsorption in absence [

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in.4t?senserepresent, ofNaDBS otiV-aDBS 2ta~

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Z

6

8

10

C2 (mg/lOOml)

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I

12

i0

ld

Fie. 13. Adsorption isotherms of PVA on silica in

of PVA. For example, in the presence of 20-125 ppm PVA, adsorption from 4 X 10-5 mole/liter CTABr using 2 g/liter silica showed complete adsorption of CTA + ions. In the absence of PVA, adsorption of CTA + ions from this solution is also high but not complete

Toi

{ 0

6~

presence and absence of NaDBS at pH 3.6.

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/ ~

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in absence in presence of NaDB5 dfNaDBS

~

~l[- ~

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rpH:,ol;

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2

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z:

5

C2 (mg/lO0 ml)

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FIn. 12. Adsorption isotherm of pVA on silica in presence of 4 X 10 -4 moles/liter CTABr (2 X 10 -4 moles/g silica) at p H 8.8.

~pH=96~

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]

2

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Fro. 14. Adsorption isotherm of PVA on silica in presence and absence of NaDBS at high pH.

Journal oJ Colloid and Interface Science, VoL 46, No. 3, March 1974

536

TADROS adsorption does not follow the Langmuir equation), in the presence of PVA, there is a tendency towards a "second" adsorption step as indicated by the broken line on the isotherm. At higher PVA concentrations (500 ppm) the adsorption isotherm is of a more complex nature showing a maximum (see Fig. 15).

/o 5"

~

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/

/

?

.p.rnPVA

•~ 0 . 6

oJ

IV. DISCUSSION

1

55

1. The Interaction Between C T A B R or N a D B S and P V A in Bulk Solution

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o =n,£V%~o

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Fro. 15. Adsorption isotherms of NaDBS on silica at pH 3.6 in presence of 50 and 500 ppm PVA.

The present results (Figs. 2-7), which are similar to those previously reported in the literature (1-12), point towards the formation of a polymer-surfactant "complex" or polymer nucleated micelle. This complex can form by oriented adsorption of the surfactant ions through some sort of "hydrophobic" bonding between the hydrophobic end of the surfactant ion and the hydrophobic part of the polymer chain. The solution properties of the polymersuffactant "complex" show the typical behavior of a polyelectrolyte. Firstly, at high polymer concentration (> 0.1%, w/v), where all surfactant ions are "adsorbed" by the PVA chains, 3' decreases slightly with increase in concentration (see Figs. 2 and 3) with no break in the 3"-log C curve (no CMC). On the other hand, at low (<0.1%, w/v) polymer concentrations not all the surfactant ions are adsorbed and it is the presence of these surfactant ions together with the polyelectrolyte and micelles which may be used to explain why more than one transition in the 3"-logC curve is observed. Mixed micelles between surfactant and polymer may be formed as has been recently suggested by Fishman and Eirich (4). Furthermore, the rapid increase in ~=p/CvvA with decrease in CpvA at low polymer concentration (see Figs. 4 and 5) resembles the typical behavior of a polyelectrolyte. The rapid increase is due to the appearance of the first electroviscous effects (deformation of the

(Fig. 2). In other words, in the presence of PVA, the adsorption isotherm of CTA + ions would be initially steeper than in its absence. However, in view of the lengthy procedure of analysis we did not measure the whole isotherm. Figure 13 shows the influence of NaDBS on the adsorption of PVA at pH 3.6, whereas Fig. 14 shows the results at pH 9.6. In both cases addition of 5 >( 10-4 mole/liter NaDBS increased the adsorption of PVA on silica. The influence of addition of PVA on the adsorption of NaDBS on silica at pH 3.6 is shown in Fig. 15. In the presence of 50 ppm PVA, the isotherm is of Langmuir type at low concentration. C2 in this case represents the total concentration of DBS- in the equilibrium solution including the amount adsorbed on PVA. However, since the concentration of PVA in the equilibrium solution is small (< 50 ppm), the amount of DBS- adsorbed on the PVA chains is small and therefore C~ is approximately equal to the equilibrium concentration. This justifies the linear plot of 2 Isemura and Imanishi(2) attributed the rapid inFig. 15 (lower half). Although the amount of crease in w ~ , / C at low poIymer concentration to an DBS adsorption at the plateau is comparable anomalous viscosity behavior arising from adsorption with that obtained in absence of PVA (where of polymer moleculesby the capillary wall of the visd o u r n a l o f Colloid and I n t e r f a c e Science,

Vol. 46, No. 3, March 1974

POLYMER-SURFACTANT INTERACTION double layer around the "polyelectrolyte" chain under shear). One of the referees suggested that the rapid increase in rl,p/CpvA at low polymer concentration could be due to swelling of the polymer coil as a result of the acquired charge. This swelling must be reflected in an increase in the intrinsic viscosity of the polymer on addition of surfactants. For polyelectrolytes, the intrinsic viscosity can be determined using the Fuoss-Strauss equation (39).

537

t~

~ee oJ

"~8z,

~1

0.2

0.3

0.4

CpvA (%w/v)

FIG. 16. Variation of Walden product with PVA concentration.

n~,/c = A / O + BC~). where A and B are constants, A being the intrinsic viscosity. However, since the polyelectrolyte, formed as a result of adsorption of surfactant ions on the polymer, does not have a constant charge when the polymer concentration is varied at constant surfactant concentration (or vice versa), the Fuoss-Strauss equation (and of course the Huggin's equation rl~p/CpvA = En~ + K'En~2Crva) cannot be applied to obtain [-n]. More information for binding of the surfactant ions to the polymer is obtained from the transitions of the A--(CcTABr)~ plots (Fig. 6). At low surfactant concentration (prior to the first transition), A decreases more than would be expected from the increase of viscosity. This is clearly shown from the decrease of Walden product, An, with increase in PVA concentration (Fig. 16 for 5 X 10-4 mole/liter CTABr). Binding of surfactant ions to individual sections of the polymer as proposed by Fishman and Eirich (4) accounts for the decrease in A. The more rapid decrease of A with increase in surfactant concentration, in the intermediate region (between first and last transition) must be due to a more rapid uptake of surfactant ions, with the formation of mixed micelles as the result of the cooperative action of bound surfactant ions and the polymer chain segments (4). However, the "onset" of this region does not show any regular trend with increascometer (37, 38). However, since the effect is absent in absence of surfactant ions and the increase in n,p/C is dependent on surfactant concentration, we favor the explanation in terms of the electroviscouseffect.

ing polymer concentration. The slower decrease in A with increase in surfactant concentration in the high surfactant concentration region (after the final transition) might be due to change in binding of the counterions and/or formation of true and mixed micelles. In this region An decreases more slowly with increasing PVA concentration (curve for 1.6 N 10-a mole/liter CTABr, Fig. 16) than it does at low surfactant concentration. The increase in the CP of PVA on the addition of either CTABr or NaDBS must be due to a stronger interaction of the polymersurfactant complex with the solvent relative to the interaction of the polymer alone with the solvent. This is the result of the stronger ion-dipole interaction between the "polyelectrolyte" and the water molecule compared to the dipole--dipole interaction between nonionic PVA and water molecules (probably by hydrogen bonding).

2. Adsorption of PVA, CTABr, NaDBS and Their Mixtures on Silica The increase in adsorption of PVA on silica by lowering the p H (Fig. 8) suggests that the main force responsible for adsorption is hydrogen bonding between the OH groups of the PVA chain and the free silanol groups on the silica surface. The same mechanism has been suggested by Groit and Kitchener (40) for the adsorption of polyacrylamide on "Aerosil." These authors had direct evidence for H bond formation from the disappearance of the

Journal of Colloid and Interface Science, VoL 46, No. 3, M a r c h 1974

538

TADROS

2.74 /~m bond of silica (attributed to free or isolated OH groups) on adsorption of polyacrylamide. The adsorption of PVA on silica differs from its adsorption on a hydrophobic surface such as AgI (18), where the main forces responsible for adsorption are van der Waals attraction or hydrophobic bonding. The irreversibility of adsorption and the high affinity isotherms obtained on AgI are not shown on silica. Desorption of PVA can occur from the silica surface if the pH is raised. Assuming an area of 26-31 A2/segment of PVA for a close-packed monolayer (41), the amount of PVA adsorption at the plateau should be 20-17.3 mg/g silica. These values are significantly higher than those obtained even at low pH (see Fig. 8). This shows that most of the silanols are not available for PVA adsorption, due to the presence of physically adsorbed water. The adsorption of CTA + ions on silica at pH values higher than the pzc (see Fig. 9) takes place by an ion-exchange mechanism (34, 35)

+ -SiO- + R(CHa)aN + -" ,-- SiO- - [L~(cm)~]. R

The area/CTA + ion at pH 3.6 is 33 A2 which suggests vertical orientation of the adsorbed molecule. Robb and Alexander (42) found an area of 35 k 2 at monolayer saturation on polyacrylonitrile latex. More recently, Conner and Ottewill (43) found an area of 47 A2 in 5 X 10-3 mole/liter KBr and of 35 A2 in 5 X 10-2 mole/liter KBr, on polystyrene latex at pH 8.0. Vertical orientation with positive groups directed towards the surface and hydrophobic end pointing into solution was suggested. It should be mentioned that in our experiment the ionic strength was not controlled. At pH 9.1, the area of 10 A2/CTA+ (see Fig. 9) suggests the formation of at least a bilayer of surfactant ions on silica. The first layer forms with the polar groups pointing towards the SiO- sites on the surface. The second layer forms by hydrophobic bonding

between the hydrocarbon chains of the surfactant molecules. The polar groups now point towards the solution. This picture is supported by the contact angle measurements reported by Elton (44). The contact angle of water on a fused silica plate increased with increase in CTABr concentration, until a maximum at 1 X 10-5 mole/liter CTABr was reached and then decreased again. During adsorption of the first layer, the surface of silica became more hydrophobic and the contact angle increased. When the second layer began to build up, the surface regained its hydrophilic character and the contact angle decreased. The low adsorption of DBS- ions on silica, especially at high pH (see Fig. 10) is to be expected for an anionic surfactant on a negative surface. At low pH values (pH 3.6), it is reasonable to assume that the main force responsible for adsorption is the dispersion force between the benzene ring of NaDBS and the silanol groups of the silica surface. The area/DBS- suggests flat orientation. The increase in adsorption of PVA onto a silica surface having preadsorbed CTA + ions at high pH (see Figs. 5, 11, and 12) can be explained in the light of the interaction between CTA + ions and PVA chains. The CTA + ions, which neutralize some of the SiO- groups create "hydrophobic" sites on which PVA can adsorb. In this case hydrophobic attraction between the alkyl groups of the surfactant ions and the hydrophobic part of the PVA chains takes place in the same manner as occurs in bulk solution. Thus, one can visualize the CTA + ions acting as "anchors" between the dissociated silanol groups (at high pH) and the PVA chains. On increasing the number of hydrophobic sites (see Fig. 12 at pH 8.8), the PVA adsorption increases. The saturation adsorption level of PVA is higher than required for a close packed monolayer of PVA. In this case adsorption is of a "loop" nature with a proportion of PVA segments attached to the hydrophobized silica surface (45, 46). At low pH, a silica surface with preadsorbed PVA also adsorbs CTA + ions to a larger extent than in absence of polymer. In this case it is

Journal of Colloid and Interface Science, Vol. 46, No. 3, M a r c h 1974

POLYMER SURFACTANT INTERACTION

reasonable to assume that PVA acts as an "anchor" between the undissociated silanol groups on the silica surface and the CTA + ions. Thus the "polyelectrolyte complex" can form at the silica/solution interface in the same manner described for bulk solution. There is still the possibility of direct adsorption of CTA + ions on the silica surface between the PVA loops which can happen during desorption of some of the PVA segments. The same explanation can account for the increased adsorption of PVA in the presence of NaDBS (Figs. 13 and 14). Now, the PVANaDBS "polyelectrolyte complex" is adsorbed to a larger extent than PVA on silica. Thus, the adsorption of polymer at the silica/water interface can be increased by the addition of a short chain ionic surface. In the case of anionic surfactants this lead to the additional advantage of introducing negative charges on the polymer chain giving rise to an extra electrical repulsive force in addition to the "steric" force between silica particles. The effect of these polyelectrolyte complexes on dispersion stability will be reported in a subsequent paper. The complex nature of the adsorption of D B S - on silica in the presence of preadsorbed PVA (Fig. 15) is difficult to explain. At high PVA concentrations (500 ppm), there is probably competitive adsorption between the PVA, which is present in excess in solution, and the PVA-NaDBS complex. This could explain, why the adsorption of D B S - ions that are bound to PVA chains is lower at higher PVA (500 ppm) concentration, than at lower concentration (50 ppm). In the latter case all the PVA chains form complexes with DBS- ions. ACKNOWLEDGMENTS The author is indebted to Messrs. M. Goss and L. Newman for their assistance in the experimental part of this work. The BET surface area of silica was determined by Dr. M. A. Day, Imperial Chemical Industries, Petrochemical Division.

539

REFERENCES i. SAI~O,N., ANDSAITO,S.,Kolloid Z. 128, 154 (1952) ; SAITO, S. Kolloid Z. 133, 12 (1953); 137, 98 (1954) ; 143, 66 (1955) ; 154, 19 (1957) ; 158, 120, (1958); 168, 28 (1960); J. Colloid Sci. 15, 283 (1960) ; SAITO,S., TAN1GUCtII,T., ANDKITAMURA, K., J. Colloid Interface Sci. 37, 154, 346 (1971). 2. ISEMURA,T., AND KIMURA,Y., J. Polym. Sei. 16, 90 (1955); ISEMURA, T., K]MURA, ¥-., AND IMANISm, A., Mere. Fib. Inst. 9, 41 (1956); ISEMURA, T., AND Ilk~AN1SttI,A., J. Polym. Sci. 33, 337 (1958). 3. MARUTA,I., Y. Chem. Soc. Jap. 83, 782, 861 (1962). 4. BAR:KIN,S., FRANK,H. P., ANDEIRICH, F. R., Ric. Sci. Sez. A 25, 844 (1955); J. Phys. Chem. 61, 1357 (1957); FIStIMAN, M. L., AND EIRICH, F. R., Y. Phys. Chem. 75, 3135 (1971). 5. ]~OTRE, C., DE MARTIIS, F., AND SOLINAS, M., J. Phys. Chem. 68, 3624 (1964). 6. NAKACAKI,M., ANDNINOM~YA,Y., Bull. Chem. Soc. 37, 817 (1964). 7. JONES,M. N., Y. Colloid Interface Scl. 23, 36 (1967). 8. SCRWUOER, M. J., AND LANGE, H., PrOC. Congr. Surface Aetiv., 5th 2, 955 (1968). 9. GRAVSHOLT,S., Prac. Int. Congr. Surface Actlv., 5th 2, 993 (1968). 10. ARAI, H., AND HORIN, S., J. Colloid Interface Scl. 30, 372 (1969); HORIN, S., AND ARAb H., Y. Colloid Interface Sci. 32, 547 (1970) ; ARAI,H., MURATA,M., AND SmNODA,K., J. Colloid Interface Sci. 37, 223 (1971). i1. LEwis, K. E., AND ROBINSON, C. P., Y. Colloid Interface Sd. 32, 539 (1970). 12. GREE~, F. A., J. Colloid Interface Sd. 35, 481 (1971). 13. TADROS, Tm F. AND LYKLEMA, J. Eleclroanal. Chem. Interracial. Electrochem. 17, 267 (1968). 14. GAUDIN,M. A., AND FUERSTENAU,D. W., Trans. A I M E 202, 958 (1955); FUERSTENAU,D. W., HEALY, T. W., AND SOMASUNDARAN,P., Trans. A I M E 229, 321 (1964); SOMASUNDARAN,P., HEALS, T. W., AND FUERSTENAU, D. W., J. Phys. Chem. 68, 3562 (1964); TER-MINASSlANSARAGA, L., J. Chlm. Phys. 2, 1278 (1966). 15. TADROS,TH. F., unpublished data. 16. BARRY, ]~. W., MORRISON, J. C., AND RUSSEL, G. F. J., J. Colloid Interface Sci. 33, 554 (1970). 17. LUDLUM,D. ]~., J. Phys. Chem. 60, 1240 (1956). I8. FLEER, G. J., thesis, Agricultural Univ., Wageningen, Netherlands; Meded. Landbouwhogesch., Wageningen 71-20 (1971). FLEER, G. J., KOOPAL, L. K., AND LYKLE]~A,J., Kolloid Z. Z. Polym. 250, 689 (1972). 19. PARREIRA, H. C., J. Colloid Sd. 20, 44 (1965). 20. LAN~:VELD, J. M. G., thesis, Agricultural Univ. Wageningen, Netherlands; Meded. Landbouwhogesch., Wageningen, 70-21 (1970).

Journal of Colloid and fn~erface Science. Vol. 46, No. 3, March 1974

540

TADROS

21. HARKINS, W. D., AND BROWN, F. E., J. Amer. Chem. Soc. 41, 499 (1919). 22. LANDO,J. L., AND OAKLEY,H. T., J. Colloid Interface Sci. 25, 526 (1967). 23. ROBINSON,R. A., AND STOKES,R. H., "Electrolyte Solutions," p. 462. BUTTERWORTtIS, London, 1959. 24. PRITC~ARD, ~. G., "PolyvinyI Alcohol, Basic Properties and Uses," Polymer monographs, Vol. 4, Gordon and Breach, London, 1970. 25. ZWlCI~, M. M., J. Appl. Polym. S t . 9, 2393 (1965); J. Polym. Sci. A-l, 4, 1642 (1966). 26. HORACEE,J. Chem. Prumysl. 12, 385 (1962) ; Chem. Abstr. 58, 10305 f. CnENE, M., MART1N-BARRET, O., AND CLEI~Y, M., Papeterie 3, 273 (1966); MONTE-BovI, A. J., J. Ass. Oflic. Anal. Chem. 52, 891 (1969). 27. BARR, T., OLIVER, J., AND STUBEINGS, W. V., Y. Soc. Chem. Ind. 67, 45 (1948). 28. GILES, C. H., AND TEIVEDI, A. S., Chem. Ind. 1426 (1969). 29. SING, K. S. W., Chem. Ind. 1520 (1968). 30. VANDEN HUL, H. J., thesis, State Univ. of Utrecht, 1966. 31. VAN DEN HUL, H. ~., AND LYIKLEMA,~., J. Amer. Chem. Soc. 90, 3010 (1968). 32. T~DEOS, T~I. F., SCI (Soc. Chem. Ind., London) Mono. 28, 221 (1973). 33. HocI~¥, J. A., AND PETHICA, B. A., Trans. Faraday

34.

35. 36. 37. 38. 39.

40. 41. 42.

43. 44. 45. 46.

Journal o] Colloid and InterflaceScience, VoL 46, No. 3, March 1974

Soc. 57, 2247 (1961); LANGE, K. R., J. Colloid Interface Sci. 20, 231 (1965). ALLIN~HAM,M. M., CULLEN, J. M., GILES, C. H., TAIN, S. K., Am) WOOD, J. S., Y. Appl. Chem. (London) 8, 108 (1958). TER-]V[INASSIAN-SARAGA,L., Y. Chim. Phys. 57, 10 (1960). TAMAM~TSHI,B., AND TAMAKI,K., Proc. Int. Congr. Surface Activ., 2nd 3, 449 (1957). CIILTER, M., AND KIMBALL, G. J. Polym. Scl. 7, (1951). (31IRN, O. E., J. Polym. Sci. 17, 137 (1955). Fuoss, R. M., Am) S~I~Alrss, U. P., Ann. N.Y. Acad. Sci. 51, 836 (1949); J. Polym. Sci. 3, 246 (1948). GROI% O., AND KITCt~ENER, J. A., Trans. Faraday Soc. 61, 1026, 1032 (1965). LANKVELD, J. ~VI.G., AND LYKLEMA,~., J. Colloid Interface Sci. 41, 466 (1972). ROBB, D. J. B., AND ALEXANDER, A. E., SCI (Soc. Chem. Ind., London) Monogr. 28, 292 (1968). CONNER, P., AND OTTEW1LL, R. H., Y. Colloid Interface Sai. 37, 642 (1971). ELTON, G. A., Proc. Int. Congr. Surface Activ., 2nd 3, 161 (1957). JENKEL,E., ANDRUMBACIt,B., Z. Electrochem. 55, 612 (1951). PATAT, F., KILLMAN, E., AND SCIcILIEBENER,C., Fortschr. Hochpolym. Forsch. 3, 332 (1964).