Hydrodynamic radius of hydrophobic and hydrophilic polystyrene latex particles

LETTERS TO THE EDITOR Hydrodynamic Radius of Hydrophobic and Hydrophilic Polystyrene Latex Particles The measured differences in the diffusion coefficients of polystyrene latex particles in water, with and without an adsorbed nonionic surfactant layer, could be attributed to the increase in the radius of the surfactant-coated particles by a value corresponding to the size of the polar group of the surfactant used. The conclusion is that either the hydrophobic/hydrophilic nature of the particle surface does not affect the viscosity of the adjacent water layer or the relaxation time of "surface water" formation is much longer than the characteristic time for the Brownian motions of the particles. © 1991AcademicPress,Inc.

INTRODUCTION

EXPERIMENTAL

Numerous reports claiming that the properties of water in thin films or boundary layers differ from those of bulk water have caused a long-lasting controversy. For example, Churaev et al. ( 1 ) have recently discussed changes in the viscosity of water in a boundary layer adjacent to a hydrophobic or hydrophilic inner surface of a thin capillary. Based on previous measurements of water viscosity in thin capillaries (2) the authors postulated that the viscosity of the boundary water layer is "elevated for highly hydrophilic surfaces and reduced for hydrophobic surfaces" (1). It was tempting to check whether the postulated viscosity changes are reflected in diffusion coefficients of polystyrene latex particles with hydrophobic and hydrophilic surfaces. If the nature of the surface indeed affects the viscosity of the adjacent boundary water then the diffusion coefficient

Alkyl polyoxyethylene ethers (CnE,,) of the general formula CH3 - ( C H 2 ) , - 1- ( O - C H 2 -CH2 ),~ - O H were supplied by Sigma and used without further purification. A monodisperse polystyrene latex suspension manufactured by Dow Chemical Co. for the "Seradyn" Particle Technology Division was used for all the measurements reported herein. The manufacturer reported the particle diameter to be 2a = 38 nm, and the standard deviation a = 7.5 nm. This latex has been extensively studied in our laboratory (9) and our own electron microscopy experiments showed that 2a = 39.7 nm with a = 6.25 nm. When both water and latex suspension were filtered through a 0. l-#m Millipore filter, the size measured with PCS was 45 nm, independent of the scattering angle in the range 30°-150 ° (9). In the measurements reported here, the size obtained with a photon correlation spectrometer (PCS) was 47 nm with a reproducibility of 1.2 nm. The difference is probably due to the presence of a small number o f doublets, since a filter size of 0.47 um was used. The PCS diameter of 45 nm corresponds closely to the one calculated from

D =

kT

6~-rla

[11

of small latex particles should be altered accordingly; i.e,, it should be smaller for latex particles with hydrophilic surfaces than for those with hydrophobic surfaces. In the formula above k is the Boltzmann constant, T is the absolute temperature, n is the dynamic viscosity of the medium, and a denotes the particle radius. An untreated polystyrene latex surface is hydrophobic. It can be made hydrophilic by adsorption of a suitable nonionic surfactant (3), e.g., alkyl polyoxyethylene ethers (C,Em). Surfactants of this type are widely used as dispersants and emulsifiers. Stenius et al. performed extensive studies of the adsorption of nonionic surfactants on latexes (4-6) and on mica surface made hydrophobic by the adsorption of a doublechain cationic surfactant (7), showing that polyoxyethylene ethers adsorb on hydrophobic surfaces with oxyethylene groups facing the aqueous phase. Ottewill and Walker (8) investigated the adsorption of C~2E6 on polystyrene latexes and found that the surfactant stabilizes the latex at high coverages.

(a 6)

1 q- 15o.2 + 450. 4 + 1 5 a 6

aPcs= ( a S ) = ( a )

1 + 10a 2 + 150.4

[2]

(obtained by assuming a Gaussian particle size distribution), which predicts 2aPcs = 44.2 nm. Mobilities (in terms of diffusion coefficients) were measured using a Brookhaven Instruments PCS at 25°C for bare and surfactantcoated latex particles. Two surfactants, C12E4 and fleE20, were used for the studies; their concentrations were varied from zero to slightly above the CMC value. It was assumed that the surface coverage attains the maximum value at the CMC. Deionized water was used throughout for the preparation of all solutions. The latex concentration was kept constant at 3 • 108 particles/ml in the final dilutions, a concentration low enough for particle interaction to be absent (9). No flocculation of the latex after addition of

298 0021-9797/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science. Vol. 145, No. I, August 1991

299

LETTERS TO THE EDITOR 60

the surfactants was observed (as reflected in second-moment PCS results) as expected (cf. (8)). c

RESULTS AND DISCUSSION

i

Figure 1 shows the measurement results for C~2E4. The results are expressed in terms of hydrodynamic particle diameter calculated from the measured diffusion coefficient according to Eq. [ 1] under the assumption that the dynamic viscosity of the boundary water layer is equal to that of bulk water at 25°C, i.e., n = 0.8902 mPa s (10). In the figure, the dots represent single experimental points and the × symbols represent the means of a series of measurements performed on the same solution. There were usually five measurements taken for any given solution. Finally, the arrows on the right margin denote averages over all experimental points for (a) bare latex suspensions (at a zero surfactant concentration) and (b) particles totally coated with a surfactant, i.e., surfactant concentrations >t CMC. Although the data are noisy, an increase of the apparent hydrodynamic diameter of the panicles is clearly visible. It increases from 47 nm for bare panicles (no surfactant added) to 49 nm for surfactant-coated latex• The difference, which is statistically significant at the 0.99 confidence level, is equal to 2 nm; i.e., the increase in the radius was 10 A. Since our sample has a polydispersity (standard deviation in particle size) of 15.7%, this apparent increase in radius must be corrected. From Eq. [2 ] and assuming that with PCS one measures the radius a ~ s + ~pp, ~appbeing the apparent increase in radius, while the true radius a increases with ~ and the standard deviation decreases by a factor of 1 + 6 / ( a ) , one obtains -= ~app

1 +

50 "2-

350- 4 .

[3]

Here terms of order a 6 and ~2 have been neglected. For cr = 0.157 one obtains 6 = 1.10~pp and thus the corrected 54

52

50

: ®

48

x

t

; x

i~

44 0.00

i 0.01

k

b

• • ..2_ ~ CMC (prea.)

46

,

0.02

CMC6) (ref.

i

0.04

0.05

1 0.03

55

Concentration [ m m o l e / I ]

FIG. 1. Experimental data for CI2E4. ( ' ) single measurement result, ( × ) mean of a series. Arrows on the right margin denote the mean of all measurements (a) for pure water suspensions and (b) for all measurements at or above the CMC.

X

-~ 50

i

~t

:

b

¥

i

:

a

1

,5

0.00

x

.~ CMC

l

i

1

J

0.01

11,02

0.03

0.04

Concentration [ m m o l e / I ]

FIO. 2. Experimental data for Cl6Ezo. The meanings of the symbols are given in Fig. I. increase in radius is 11 ~,, which is more or less the size of four (O-CH2-CH2) groups. Figure 2 shows the hydrodynamic panicle diameter of Ct6E20. AS in the case of Ct2E4, (cf. Fig. 1 ), the dots rely resent single experimental points, × symbols represent the means for the same solution, and the arrows denote averages over all experimental points for bare (in pure water) and surfactant-coated panicles at or above the CMC. For CI2E4 the CMC value was reported as 0.04 m M ( 11 ), while the CMC for ClrE2o was estimated by interpolation from the data for Cl6El0, C16E18, C16E30, and CI6Ero reported by Guveli et al. ( 12 ). There is substantial scattering of the data for concentrations just below the CMC for both surfactants studied. In the case of C,2E, (cf. Fig. 1 ) the scattering decreases markedly at 0.03 m M , while the reported CMC value is 0.04 m M . We determined the CMC for our sample of C~2E4 by surface tension (DeNouy ring method) and PCS light scattering measurements. The results, presented in Fig. 3, suggest that the CMC for our C12E4 sample may be somewhat lower, i.e., about 0.03 m M , in agreement with the picture emerging from Fig. 1. We cannot provide a proven explanation for the observed scattering of the data below the CMC. It is, however, likely that small amounts of impurities (polymers with markedly longer chains than those of the surfactant used?) adsorb on the latex panicles, causing an increase in their apparent hydrodynamic diameter. Above the CMC the impurities undergo solubilization in the micelles formed, and free surfactant molecules, which are available for adsorption at latex panicles, have a more uniform size distribution as reflected in the measurement results. If this interpretation is correct, the present technique may be utilized as a new method for a relatively fast CMC determination or as a check for surfactant purity. Above the CMC the data behave decently again• The apparent increase in hydrodynamic diameter of C~6E2ocoated particles is 4.8 nm, significant at the 0.99 confidence level. The apparent increase in the panicle radius is thus equal to 24 A. Correcting for the effects of polydispersity (Eq. [3]) yields 6 = 26/~. Journal of Colloid and Interface Science, Vol.

145, N o . I , A u g u s t 1991

300

LETTERS TO THE EDITOR 10

6O •

\ ~

cue

,

SURFAGE TENSION

• L,aHT~C,TTE,,,O a z~

g_ u~

~

6 40

•

~g ",9 uJ~

~o

20 -6

I

I

I

-5

-4

-3

LOGARITHM

0 -2

OF CONCENTRATION

FIG. 3. Surface tension (A) and light scattering ( e ) for aqueous CI2E4 solutions. The CMC for our CIzE4 sample is about 3.10 -5 M. Schefer et al. have recently studied ClrE20 micelles using small-angle neutron scattering (13). They proposed a twosphere model of a C~6E20 micelle consisting of a central hydrocarbon core and an outer shell of more or less stretched head groups, intermixed with water. The calculated thickness of the outer shell was about 25/~ ( 13 ), in perfect agreement with our observations. CONCLUSIONS The changes in the measured diffusion coefficients can be attributed to the increase in the hydrodynamic radius of the latex particles by a value equal to the dimensions of the polar part of the surfactant molecules used for making the latex hydrophilic. This suggests that the surfactants adsorb on the latex surface with their alkyl chains lying fiat on the surface and with their oxyethylene groups protruding into the aqueous phase. Our results can thus be interpreted without assuming that the viscosity of the boundary water layer depends on whether the nature of the latex surface is hydrophobic or hydrophilic. Therefore, there are two possibilities: one, that the nature of the surface has no influence on the viscosity of the adjacent water layer and, two, that the relaxation time for "surface water" structurization is much greater than a2 t

2D

3rr/a 3 -- 150 us, kT

[4]

the characteristic time for Brownian movements of the latex particles studied. ACKNOWLEDGMENTS The authors are grateful to Dr. Per Stenius for discussion and for providing the latex sample used for preliminary

Journal of Colloid and Interface Science, Vol. 145, No. 1, August 1991

tests, to Dr. Jim Tyerman for C~2E4 surface tension measurements, and to Gerhard Schumacher for the C~2E4CMC determination by light scattering. REFERENCES 1. Muller, V. M., Sergeeva, I. P., Sobolev, V. D., and Churaev, N. V., Kolloidn. Zh. 48, 718 (1986). 2. Kiseleva, O. A., Sobolev, V. D., Starov, V. M., and Churaev, N. V., Kolloidn Zh. 41,245 (1979). 3. Stenius, P., private communication. 4. Kronberg, B., K~II, L., and Stenius, P., J. Dispersion Sci. Technol. 2, 215 ( 1981 ). 5. Kronberg, B., and Stenius, P., J. Colloid Interface Sci. 102, 410 (1984). 6. Kronberg, B., Stenius, P., and Igeborn, G., J. Colloid Interface Sci. 102, 418 (1984). 7. Claesson, P. M., Kjellander, R., Stenius, P., and Christenson, H. K., J. Chem. Soc., Faraday Trans. 1 82, 2735 (1986). 8. Ottewill, R. H., and Walker, T., Kolloid Z. Z. Polym. 227, 108 (1968). 9. Schumacher, G., Ph.D. thesis. McGill University, Montreal, 1990. 10. Kaye, G. W., and Laby, T. H., "Tables of Physical and Chemical Constants," 15th ed., p. 36, Longman, London/New York, 1986. 11. Mukerjee, P., and Mysels, K., "Critical Micelle Concentrations of Aqueous Surfactant Systems," p. 150. National Standard Reference Data System. U.S. Department of Commerce, Washington, DC, 1971. 12. Guveli, D. E., Davis, S. S., and Kayes, J. B., J. Colloid Interface Sci. 86, 213 (1982). 13. Schefer, J., McDaniel, R., and Schoenborn, B. P., J. Phys. Chem. 92, 729 (1988). J. C Z A R N E C K I |

T. G. M. VAN DE VEN

Paprican and Department of Chemistry Pulp and Paper Research Centre McGill University Montreal, Quebec, Canada H3A 2/t 7 Received January 9, 1991

t To whom correspondence should be addressed at present address: Syncrude Research Centre, P.O. Box 5790, Edmonton, Alberta, Canada T6C 4G3.