Emulsion particle size

JOURNAL OF COLLOID SCIENCE 15, 76--82

(1960)

EMULSION PARTICLE SIZE I. THE SOAP TITRATION OF ACRYLIC EMULSIONS 1 J. G. Brodnyan and G. L. Brown Research Laboratories, Rohm & Haas Company, Philadelphia, Pennsylvania Received June 29, 1959; revised September 15, 1959 ABSTRACT In soap titration experiments the soaps are usually calibrated by titrating emulsions characterized using an electron microscope. In this report an independent method was used to determine the effective surface areas of the soaps used. The interfacial tension between'water solutions of the surface-active materials and n-hexane, an environment similar to the polymer phase, as a function of concentration is determined and used to calculate the effective surface area from the Gibbs adsorption isotherm. Then the soap titration method is used to determine the surface average radius of several acrylic emulsions using not only two anionic soaps, sodium lauryl sulfate and an alkyl aryl polyether suifonate (Triton X-202), but also one nonionic surface-active agent, an octylphenoxy polyethoxy ethanol, OPE. The validity of this technique is shown by the agreement between the radii de~ermined by the soap titration method and by the use of an ultracentrifuge. INTRODUCTION

The soap titration method has been used extensively for the determination of surface area and particle size of synthetic latices. References to its use in the Government Synthetic Rubber Program (1-6) and for characterizing polystyrene latices (7) are found in the literature. The principles of the method have been described in detail by Maron et al. (7). They calibrated several fatty acids by characterizing several polystyrene emulsions using an electron microscope, titrating these emulsions with the fatty acids, and carrying out the calculations assuming coverage of the emulsion particles at the critical micelle concentration (c.m.c.). The average error reported was 5 %. This method seems to be the one favored by most investigators. In this report a simpler technique of determining the effective surface area of the soaps is described, and then the soap titration method is used to determine the surface-average radii of several acrylic emulsions using not only the usual anionic soaps but also one nonionic surface-active agent. 1 Presented at the 135th American Chemical Society Meeting, Boston, April, 1959.

76

EMULSION

PARTICLE

SIZE.

I

77

M~TERIALS USE])

Soaps. The sodium lauryl sulfate (NaLS) was crystals of reported 99.5 % purity from American Alcolac Corporation. The Triton X-202 is the sodium salt of an alkyl aryl polyether sulfonate and the OPE is an octylphenoxy polyethoxy ethanol, a nonionie surface-active agent. Both these were commercial grade materials as supplied by the Rohm & Haas Company. Emulsions. Three latices were emulsions of poly(n-butyl methacrylate) prepared with 2 % sodium lauryl sulfate based on the solids content. (I) was polymerized with potassium persulfate as initiator at 30°C. (II) also was prepared with potassium persulfate but the reaction was carried out at 50°-57°C. ( I I I ) was polymerized with a redox system of potassium persulfate, Lykopon, and a trace of Fe ++. (IV) was a copolymer of ethyl acrylate and methyl methacrylate prepared by a suspension polymerization technique, i.e., no soap was initially on the surface. The n-hexane was reagent grade. EXPEmM~NTAL METHODS Any method that will indicate the c.m.c, can be used to follow the progress of a soap titration. However, in M1 the experiments described here a DuNouy ring tensiometer was used. It was found that reproducible measuremerits could be made almost immediately after the addition of the soap solution provided that there was sufficient agitation during the addition. All the experiments were carried out in a constant-temperature (23.2 ° 3= 2°C.), constant-humidity (54 3= 4%) room.

6O

g

~o-

F..

3o 0

I

I

I

I

I

to

zo

30

40

50

ml. of soap added

Fro. 1. Typical curve of surface tension (~) versus milliliters of soap.

78

BRODNYAN AND BROWN

20--

x

*= I0 4" ...

= O E

5

O

0

I

I

I

I

~

t

I0

20

30

40

50

60

m ( g liter';) FIG. 2. T y p i c a l c u r v e of c v e r s u s m.

In Fig. 1 a typical curve of surface tension (~,) versus milliliters of soap added is found. The usual soap concentrations were approximately 0.01 M. The surface tension is uncorrected but this does not interfere with the results since only the milliliters of soap added at the c.m.c, is needed. Because of the curvature in the plot it is difficult to obtain the break in the curve which determines the c.m.e, accurately. However, the errors can be assumed to be random and will cancel out in the plot of concentration of soap at the e.m.e. (c) versus the concentration of emulsion at that point (m) if a sufficient number of concentrations of emulsions are titrated. Figure 2 is a typical curve of c versus m. Only three concentrations of emulsions were needed because there is very little scatter. The slope of this line is equal to Sa, the grams of adsorbed soap per gram of emulsion solid, and the intercept equals the e.m.e, for the soap in the emulsion solution. If Si is the grams of soap initially on the surface of a gram of emulsion solid and the emulsion is titrated with the original soap, then the surface area per gram of solid is a. (Sa -}- Si), where a is the effective area of 1 gram of soap. From this area and the weight of polymer the surface-average radius, rs, is obtainable (7). This radius is defined by Eq. [1].

~i Tbi ri g r~ -

[1]

~i Tbi Ti2" Here ni is the number fraction of particles of radius r l . The particle size distributions obtained by the use of the Spinco Model L ultracentrifuge depend on the use of Stokes' law, etc. The applicability of

EMULSION PARTICLE SIZE. i

79

this technique has been shown by a number of investigators (8, 9). This technique has also been extensively investigated in our laboratories and found to give a possible error in determination of the radius of approximately 8 %. DETERMINATION OF THE EFFECTIVE SURFACE AREA OF EMULSIFIERS The Gibbs adsorption isotherm was used to obtain the effective surface area of the soaps. The following equation defines the isotherm: -d~,

_ 2.303RTF.n

d log C

[2]

where: 7 P R T 1

= the interfacial tension at a concentration of soap C; = surface concentration; = the gas constant, 8.3 X 107; = the absolute temperature; =< n -<_ 2, n dependent on concentration of ions other than the surface-active agent being investigated. The area (A) is calculated from the relationship: 1026 A

-

r X N'

[3]

where N is Avogadro's number. The interpretation of this type of data using the Gibbs adsorption isotherm has been described by Pethica (10) and Cockbain (11). To remove any ambiguity in interpreting the plot of 7 versus the logarithm of the concentration, the soap solutions were made 0.2 M with respect to NaC1. In this situation of an excess of salt n = 1. To have an environment for the soap similar to the polymer particles the interracial tension between water and n-hexane (Fig. 3) was measured and not that between water and air. This gave an effective surface area of 61 A. 2 for sodium lauryl sulfate, in good agreement with the value of 58 A. 2 found by T a r t a r (12) from light scattering. The air environment gave an area of 38 A. 2, which was much too low. The alkyl aryl polyether sulfonate, X - 2 0 2 , was treated in the same manner yielding an effective surface area of 62 A. 2. In determining the effective surface area of the nonionic OPE it was not necessary to buffer the soap solution with salt. However, this experiment was tried and no difference was observed in the effective area obtained with or without the salt. This effective area was found to be 88.5 A. 2. I~:EPRODUCIBILITY OF THE T E C H N I Q U E

The reproducibility was checked by determining the surface-average radius of (IV) three times with sodium lauryl sulfate solutions, three concentrations of emulsion being titrated each time. The results (Table I) have

80

BRODNYAN AND BROWN

40

30

2O (3 Cn C "ID

I0

o

-7

I

-6

-5

-4

..3

ol

"2

log C FIG. 3. I n t e r f a c i a l t e n s i o n b e t w e e n n - h e x a n e a n d s o a p s o l u t i o n s .

TABLE

I

Reproducibility of Soap Titration .Experiment

r,(~)

~1 #2 ~3

0.26 0.23 0.29

~ ( ~ ) = 0.26 =t= 0.02 ~,

r, -

~s

0.00 0.03 0.03

(r, -

~,)~

0.00 X 10-4 9 . 0 X 10 -4 9 . 0 )< 10-4

~ = 0.03.

a probable error of approximately 7.7 %, which is reasonable considering the small number of experimental points entering into each determination. V A L I D I T Y OF T H E SOAP T I T R A T I O N R E S U L T S

The validity of the radii determined by soap titration can be shown by comparison with the surface-average radii determined from the particle size distributions obtained from the ultracentrifuge experiments. These radii are also defined by Eq. [1] as are those determined by soap titration. The comparisons are given in Table II. The agreement found among the results is seen to be quite good. In the comparisons the question arises whether the sodium lauryl sulfate present initially on the surface of the emulsion particles of (I), (II), and (III) is displaced by either the OPE or the X-202 when they are used as titrants. The fact that the agreement between the three soaps and the ultracentrifuge isnot different for (I) and (IV) indicates that displacement

EMULSION

PARTICLE

81

SIZE. I

TABLE II Comparison of Radii Determined by Different Soaps and Methods

Ultracentrifuge NaLS OPE X-202

(I)

(II)

(III)

~s (,)

r, (,)

ro (~)

0.070 0.072 0.069 0.070

0.045 0.051 ---

0.022 0.026 ---

(IV) (~)

r.

0.28 0.26 0.32 0.28

does not take place. Moreover, in one experiment emulsion (I) was mixed with a mixed-bed ion-exchange resin to remove the original sodium lauryl sulfate, and the remaining suspension was titrated with the OPE. The surface-average radius obtained in this manner was the same as that reported in Table II, which was obtained by assuming no displacement. CONCLUSIONS

The assumption of Maron et al. (7) that a monolayer was formed on the particles in the course of the soap titration seems self-evident. However, they essentially calibrated the soap and would get correct particle sizes even if multilayers were formed. The fact that the effective surface areas look reasonable for monolayers strongly indicates that the assumption is correct. In the work reported here an independent method, i.e., the use of the Gibbs adsorption isotherm, was used to obtain the effective surface areas of the soaps. Since it has been shown (13) that a monolayer is present at such an interface, correct particle sizes would be calculated only if a monolayer is present on the particles at the completion of the soap titration. Therefore the good agreement shown in Table II leads to the conclusion that the latices must have been covered with a monolayer. The soap titration technique is relatively fast and reasonably accurate and requires only inexpensive equipment provided the effective surface areas of the soaps used are known. In this report a method of obtaining this parameter which does not require an electron microscope or other elaborate equipment is demonstrated. Therefore it is possible to titrate any emulsion with the same soap that is initially on the surface, taking only a short time to determine its effective surface area. This use of the same soap will eliminate any problems of equilibrium between the two soaps on the surface, etc. However, there must be a reasonable differential between the surface tension of the original emulsions and that of the soap past its c.m.c, point for accurate determinations. It has also been shown that acrylic emulsions can be soap titrated not only with the usual anionic soaps such as sodium lauryl sulfate but also with nonionics such as OPE.

82

BRODNYAN AND BROWN ACKNOWLEDGMENTS

The authors wish to thank Dr. B. E. Larsson for preparing the emulsion samples and Mr. J. E. Mark for making many of the measurements. The advice of Dr. H. Greenwald is also gratefully acknowledged. ~:~EFERENCES 1. •LEVENS, N. B., J. Colloid Sci. 2, 365 (1947). 2. BORDERS, A. M., AND PIERSON, R. M., Ind. Eng. Chem. 40, 1473 (1948). 3. WILLSON, E. A., MILLER, J. R., AND ROWE, E. H., J. Phys. & Colloid Chem. 53' 357 (1949). 4. MARON, S. H., MADOW, B. P., AND KRIEGER, I. M., J. Colloid Sci. 6, 584 (1951). 5. MORTON, M., SALATIELLO,P. P., AND LANDFIELD, H., J. Polymer Sci. 8, 279 (1952). 6. MORTON, M., CXLA, J. A., AND ALTIER, l~V[.W., J. Polymer Sci. 19, 547 (1956). 7. MARON, S. H., ELDER, M. E., AND ULEVITCH, I. •., J. Colloid Sei. 9, 89 (1954). 8. NISONOFF, A., AND HOWLAND, L., Anal. Chem. 26, 856 (1954). 9. BROWN, C., J. Phys. Chem. 48, 246 (1944). 10. PETHICA, B. A., Trans. Faraday Soc. 50, 413 (1954). 11. COCKBAIN,E. G., Trans. Faraday Soc. 50, 874 (1954). 12. TARTAR,H. V., J. Phys. Chem. 59, 1195 (1955). 13. ALEXANDERAND JOHNSON, "Colloid Science." Oxford Uuiversity Press, London, 1950.