3395
Langmuir 1994,lO, 3395-3401
Interactions between Sodium Dodecyl Sulfate and Styrene-Butadiene Copolymer Latex Particles with Carboxyl Groups Studied Using Dynamic Light Scattering and Electrophoretic Mobility Measurements Wyn Brown* and Jianxi Zhao Department of Physical Chemistry, University of Uppsala, Box 532, 751 21 Uppsala, Sweden Received September 7, 1993. In Final Form: May 6, 1994@ The adsorption of the charged surfactant SDSto hydrophobic styrene-butadiene copolymer latex particles, surface-modified with carboxyl groups, has been examined as a function of the SDS concentration by determination of the particle hydrodynamic radius (RH) using dynamic light scattering and the particle mobility by electrophoretic mobility measurements. Adsorption isotherms were also determined by using a surface tension method. The observed maxima in RH and the mobility as a function of added salt, SDS concentration,and pH are the result of competing processes sited at the surface "hairy" layer of the latex particle. Adsorption is strong at zero and low levels of added salt (1mh4 of low molecular weight electrolyte) on the partly extended polymer chains at the surface of the latex particle. Close to the critical micelle concentrationof SDS, micelles are formed which are anchored on these chains. At a higher salt level (32 mM), a further increaseofthe size ofthe primarily adsorbed SDS micelles is observed. Increasedadsorption was also observed at pH < 4. A possible explanationis hydrogen bonding between protons of the carboxyl groups at the latex surface and the polar groups of the SDS molecules with the formation of a double surfactant layer. In order to facilitate interpretation of the adsorption phenomena, the properties of the latex particle in the absence of SDS were also examined.
Introduction
produced by varying the degree of dissociation of the carboxyl groups by changing the pH. The hydrodynamic radius, and therefore the effective The adsorption of surfactants at a solifliquid interface thickness of a n adsorbed surfactant layer at the particle/ is an important aspect coupled to surfactant applications solvent interface, can be determined with high accuracy and has therefore been widely studied. Recent reviews by measurement of the latex diffusion coefficient a t high are in refs 1 and 2. dilution in the presence, and the absence, of surfactant. Monodisperse, spherical latex particles are frequently Illustrations of the usefulness of this approach are used as model substrates in investigations of colloid provided by previous papers dealing with the adsorption stability. The surface of a polystyrene latex particle is of polymers to latex particles (e.g. refs 13 and 14). A more dominantly hydrophobic and thus provides a suitable comprehensive interpretation of the data, however, also material for investigating hydrophobic interactions berequires information on the total amount of adsorbed tween surfactants in solution and a polymerholid intermaterial. For this reason the adsorption isotherms have face. Earlier reports have mostly dealt with adsorption determined as before using a surface tension method isotherms and changes in the zeta p~tential/mobility.~-'~ been to determine the equilibrium level of free surfactant as A few (e.g. ref 12)have considered changes in particle well as mobility measurements which are related to dimensions. This group has recently studied the adsorpchanges in surface charge. A combination of these three tion of sodium dodecyl sulfate (SDS)on polystyrene latex procedures can yield a more complete description of SDS particles of very low surface charge using dynamic light adsorption on the latex particles. Hydrophobic interacscattering.'3 It was observed that the pronounced maxima tions between the surfactant and polymer segments at in RHand the zeta potential reflect complex interactions the particle surface will be the main driving force for in the surface layer of partly extended polymer chains. adsorption. The presence of extended polymer chains at The present purpose was to extend this investigation to the latex particle surface, however, introduces significant the adsorption of the same surfactant on a carboxylated differences between the adsorption behavior of surfactant substrate and to also examine the changes in adsorption molecules on latex particles and on other frequently used polar substrates such as silica gel. The interpretation of results in the former case is also complicated by the Abstract published inAdvance ACSAbstracts, August 1,1994. (1)Kawaguchi, M.;Takahashi,A. Adv. Colloid Interface Sei. 1992, ionizable groups (Sod-) which terminate the polymer 37,219. chains. It will be shown that, in general, the surfactant (2)Zhu,B. Y.;Gu,T. Adv. Colloid Interface Sci. 1991,37,1. first adsorbs at sites on the polymer chains through (3)Paxton, T. R. J. Colloid Interface Sci. 1969,31,19. hydrophobic interaction. Further cooperativeaggregation (4)Keyes, J. B.J. Colloid Interface Sci. 1974,56,426. (5)Piirma, I.; Chen, S.R. J. Colloid Interface Sei. 1980,74,90. occurs at these sites with the formation of micelles or (6)Keyes, J. B.; Rawlins, D. A. Colloid Polym. Sci. 1979,257,622. hemimicelles as the cmc of the surfactant in solution is (7)Kronberg, B.; Kiill, L.; Stenius, P. J . Dispersion Sci. Technol. approached. At higher salt levels, micellar growth occurs. 1981,2,215. (8)Kronberg, B. J. Colloid Interface Sci. 1983,96,55. In the present communication we describe the adsorp(9)Kronberg, B.; Stenius, P.; Thorssell, Y. Colloids Surf. 1984,12, tion of SDS to a well-defined latex of uniform particle size 113. with the same charge sign: these are styrene-butadiene (10)Ma, C.Colloids Surf. 1987,28,1. (11)Steinby, IC;Silveaton,R.; Kronberg, B. J . Colloid Interface Sei. copolymer latex particles having surface carboxyl groups 1993,155,70. in addition to the sulfate groups originating from initiator (12)Killmann, E.;Maier, H.; Baker, J. A. Colloids Sur$ 1988,31, @
51. (13)Brown, W.; Zhao, J. X . Macromolecules 1993,26,2711.
~~
(14)Brown, W.;Rymden, R. Macromolecules 1986,19,2942.
0743-7463/94/2410-3395$04.5Qf0 0 1994 American Chemical Society
Brown and Zhao
3396 Langmuir, Vol. 10, No. 10, 1994 80.0
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Figure 1. Semilogarithmic plot of surface tension ( y ) versus SDS concentration at three pHvalues. The "knee" corresponds to the critical micelle concentration (cmc)of SDS in the solution. Such plots are used as calibration curves for the determination of adsorption isotherms by measurement of surface tension by the drop-volume method. fragments. The experimental techniques are the s a m e as those used p r e v i ~ u s l y 'and ~ yield the hydrodynamic radius from dynamic light scattering measurements, particle mobility from electrophoretic light scattering, a n d adsorption isotherms by means of a surface tension method. The effects of altering the parameters, SDS concentration and simple electrolyte (NaBr) concentration, a r e examined. Variation ofthe p H determines t h e degree of dissociation of the carboxyl groups at t h e surface of the latex particles.
Experimental Section Materials. SDS was obtained ("special purity" grade) from BDH. The SDS purity was checked by surface tension measurements to determine the cmc. The latter gave a cmc value (7.8 mmollL) in good agreement with literature values. There was no characteristic dip prior to the cmc "knee point" (see, for example, Figure 1)which would have indicated the presence of impurities. The SDS was used without further purification. Water was deionized and distilled. Latex samples were purchased from Serva AG, Heidelberg, Germany (Latex 14). The latex particles used in this paper are uniform spheres having a nominal diameter of 213 nm. They are prepared by emulsion polymerization of styrene and butadiene monomers (4:l) and contain sulfate and carboxyl groups at the particle surface. The latex was further purified by exhaustive dialysis against a large volume of distilled water and by ion-exchange using equal weights of AXR (anion exchange resin), C X R (cation exchange resin), and latex solids and gently stirring for 2 h to remove surfactant and traces of salts. The sulfate groups, deriving from initiator fragments, are about 16 mmol per 1kg solid content, and carboxyl 79 mmol/kg latex from conductometric titration (ratio of carboxyl to sulfate about 5:l). Details of the experimental technique are given in ref 13. The zeta potential was -36.1 mV for the salt-free latex suspension. Adsorption Isotherms. Adsorption isotherms were estimated by determining the concentration of fiee SDS in the solutionsby surface tension measurements using the drop volume technique. Latex particles were suspended in the SDS solution of known concentration for 48 h after which time the latex was removed by centrifigation (20000 rpm for 30 min). Typical calibration curves for SDS in different pH solutions are shown in Figure 1,where the cmc is given by the knee-point. The steeply rising initial part of the curve was always used for determining the SDS concentration by surface tension measurement at that temperature, after appropriate dilution of the solution following centrifugation. Dynamic Light Scattering. Measurements were made in the homodyne mode using a Spectra Physics 50-mW 633-nm He-Ne laser as the light source. The scattering cells (10 mL cylindrical ampules) were immersed in a large diameter (10 cm) thermostated bath of index-matching liquide (transdecalin) of n = 1.479 at 25 "C.The detector system comprised at I'IT FW130
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Figure 2. (a, top) Mobility (u)shown as a function of pH for a latex suspension in aqueous solution without added salt at 25 "Cand without surfactant. (b,bottom) Hydrodynamic radius (RH)from dynamic light scattering measurements at high dilution (concentration g/mL) shows as a function of pH for a latex suspension in aqueous solution without surfactant. photomultiplier, the output of which was digitized by an ALV amplifier-discriminator. The signal analyzer was an ALV multibit, multisample time autocorrelator (ALV5000) covering approximately 8 decades in delay time. Analyses were made using both the method of cumulants and inverse Laplace transformation using the algorithm REPES. The autocorrelation functions were close to single exponential, with low values of the second cumulant ( c0.05). Hydrodynamic radii were calculated from the diffusioncoefficientusing the Stokes-Einstein equation together with the water viscosity at the appropriate temperature. RH is estimated to have an uncertainty of xl%.
Results and Discussion As discussed in the Experimental Section, Figure 1 shows typical surface tension ( y ) versus SDS concentration linear-log curves used in the determination of t h e concentrations for the adsorption isotherms (see below). The interaction with H+ions is relevant in this connection. In SDS solutions, y decreases strongly with increasing H+ concentration, passes through a narrow minimum, a n d then increases strongly again.40 This effect is probably due to a strong interaction of H+ions with the negative SDS groups in the adsorbed layer. Parts a a n d b of Figure 2 show the mobility, u, a n d the hydrodynamic particle radius,RH, respectively, as a function of p H in the absence of SDS in t h e solution without added salt. At p H 6-8, where t h e medium is almost neutral, the mobility remains approximately constant at about -2/(lO-* cm2 8-l V-l) (Figure2a). Above p H 8,u gradually increases and finally attains a plateau at about -3 above p H 11,at which p H value the carboxyl groups a r e fully dissociated. A t p H 9 was constant mobility noted. At pH < 5 there is full association of both positive and carboxyl groups with protons, while pH > 9 corresponds to full dissociation of both types of groups. This latex, having both positive and negative groups, is thus more sensitive to pH change than a substrate having only negative groups. In the latter case, constant mobility over the intermediate pH range was also observed by Kame1 et al.I6 Similarly they fo+d a decrease in u at pH < 4 and an increase at pH > 92 These variations in u were interpreted as being due, on the one hand, to the adsorption of protons a t low pH and, on the other, to the adsorption of hydroxyl ions a t the higher pH. With only strong acid groups, for example sulfate groups, a decrease in u in a strongly acid medium and a corresponding increase in strongly basic solution was also observed.16 These results suggest that the interpretation of the adsorption of protons or hydroxyl ions is the most likely mechanism. Figure 2b shows that the hydrodynamic radius of the predent latex remains approximately constant at about 105 nm over the range pH 4-9. This implies that the size of the latex particle is insensitive to partial dissociation ofthe carboxyl groups. Goossens et al.17observed an increase in the size of the latex particle (a carboxylate styrene-butadiene latex similar to that used here) below pH 3, while at pH * 0 the latex was completely destabilized. We also note that RH starts to increase below pH 4. At pH < 2 the suspension of latex particles becomes unstable with aggregate formation and measurements by dynamic light scattering are very difficult. On increasing the concentration of NaOH above pH 9,however, the hydrodynamic radius decreases to an approximately constant value of 94 nm. The concentration of Na+ ions deriving from the added base (NaOH) on adjusting the pH of the solution to pH 12is about 10 mM. The latex particle conformation in neutral pH aqueous solutions is very sensitive to the Na+ ion concentration as shown by the data in Figure 3 illustrating the changes in the particle size and the mobility on increasing the concentration of NaBr at constant pH 7. Thus at very low concentrations of NaBr ( < 5 “om), RH decreases sharply to about 93 nm (compared to 107 nm in zero added salt). The mobility correspondingly decreases to about -1. Above 5 mmoVL NaBr, u changes (15) Kandori, K.; Ishiguro, H.; Kon-no, K.; Kitahara, A. Langmuir 1989, 5, 1258.
(16)Kamel, A. A.; Ma, C. M.; El-Aasser, M. S.;Micale, F. J.; Vanderhoff, J. W. J. Dispersion Sci. Technol. 1981,2, 315. (17) Goossens, J. W. S.; Zembrod, A. Colloid Polym. Sci. 1979,257, 437.
little over a wide range of 5-50 mmol/L NaBr. The initial decrease in RH and u is attributed to the adsorption of electrolyte ions leading to compression of the latex surface diffise double layer. As would be anticipated, the hydrodynamic radius is about 95 nm at 10 mmol/L NaBr which is in good agreement with the radius at pH 12.The decrease in RH with increasing pH is thus also attributable primarily to the compression of the diffuse double layer of the latex particles by Na+ ions. Several papers exist which deal with the effect of inorganic electrolytes on the mobility (zeta potential) of latex particles.18-22 The model mostly used to interpret the electrophoretic mobility behavior of latex particles at different ionic strengths postulates ion adsorption of coiondcounterions, although refs 23 and 24 invoke changes in surface roughness (“hairiness”)to explain the observed maximum in the zeta potential. Voegtli et a1.21suggest that ion adsorption is a general feature of polymer colloids having hydrophobic surfaces with a low density of titratable surface charges. The adsorbed ions probably play an important role in determining the particle surface properties. Ion adsorption will move the shear plane away from its original site which leads to changes in the mobility and the hydrodynamic radius. With latex particles having negative surface groups, Zukoski et a1.18 assumed that cations adsorb at the negatively charged sites, while anions adsorb at neutral sites by interaction with the hydrophobic regions of the surface. Such a mechanism was postulated as early as 1934 by Abramson2s who reviewed experimental data gathered for adsorption on solid paraffin waxes in simple salt solutions. In our case, an increased negative charge (carboxyl plus sulfate groups) favors counterion adsorption in the initial stage. We consider that the latter is the main reason for the abrupt decrease in RH and u at low concentrations of NaBr. At higher NaBr concentrations, co-ion adsorption becomes appreciable and leads to an increase in RHas shown in Figure 3. At both pH 4 and pH 12 the effect of salt on the hydrodynamic radius of the latex particle is small (see Figure 4). At pH 12,the adsorption ofNa+ions is inhibited since there is a 10 mmol/L concentration of Na+ deriving from the NaOH used in adjusting the pH. Therefore, a constant value of RH is reasonable over a wide range of NaBr concentration. At pH 4,all carboxyl groups at the latex surface will be associated with protons and the surface charge density is thus low. Again the salt-effect will be small. Flexible Chains at the Latex Surface. Several authors19,20,24,26,27 have suggested that a surface roughness (“hairiness”) of the latex particles is the main reason for the low particle mobilities observed and this model has been used successfully to interpret electrophoretic phenomena. Hydrodynamic studies also support this inter(18) Zukoski, C. F., IV.;D. A. J.Colloid Interface Sci. 1986,114,32. (19) Chow, R. S.; Takamura, K. J . Colloid Sci. 1988,125, 226. (20) Elimelech, M.; O’Melia, C. R. Colloids Surf. 1990,44, 165. (21) Voegtli, L. P.; Zukoski, C. F., IV J.Colloid Interface Sci. 1991, 41,92. (22) Chabalgoity-Rodriguez, A.; Martin-Rodriquez, A.; GalisteoGonzalez, F.; Hidalgo-Alveraz,R. Prog. Colloid Polym. Sci. 1991,84, 416. (23) Hidalgo Alvarez, R.; De Las Nieves, F. J.; Van der Linde, A. J.; Bijsterbosch, B. H. Colloids Surf. 1986,21, 259. (24) Midmore, B. R.; Hunter, R. J. J.Colloid Interface Sci. 1988,122, 521. (25) Abramson, M. D. Electrokinetic Phenomena and Their Application to Biology and Medicine; ACS Monograph Series No. 66; Chem. Catalog. Co.: New York, 1934. (26) van der Put,A. G.; Bijsterbosch, B. H. J. Colloid Interface Sei. 1983,92,499. (27) van den Hoven, Th.J. J.;Bijsterbosch,B. H. Colloids Surf. 1987, 22, 187.
Brown and Zhao
3398 Langmuir, Vol.10,No.10,1994 I
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Figure 4. Hydrodynamic radius (RH) shown as a function of NaRr concentration for a latex suspension at the pH values shown.
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0 0
I
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Figure 5. Hydrodynamic radius (RH) (left),and the mobility (u)(right)shown as a function of SDS concentration for a latex suspension (a, top) in the system without added salt and (b, bottom) with added salt, 1mM (pH 6).
pretation.17 The hairy layer is considered to be comprised of flexible polymer chains terminating in polar entities such as carboxyl or sulfate groups. Repulsion between the charged ends will increase with decreasing ionic strength causing this layer to expand and the particle mobility to decrease; i.e. the shear plane moves outward causing the hydrodynamic radius to increase. Adsorption of surfactants at the latex surface is considered to occur at suitable sites on these flexible chains (through hydrophobic interactions). This conclusion is drawn since the observed thickness of the adsorbed layer substantially exceeds the typical dimension of the surfactant molecule itself (see below). The pendant polymer chains at the latex surface are consequently the key feature in determining the adsorption of surfactants. Hydrodynamic Dimensions and Mobility. Figure 5a shows the variation in RH and the corresponding changes in u for the latex particle in the system without added salt on changing the SDS concentration. The
00
20
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120
Figure 6. Absolute adsorptionisothermsfor a latex suspension versus concentration of SDS in the system without added salt and with 1mM NaBr at 25 "C (pH 6). Vertical arrows indicate the cmc at each salt level. increase in u reflects the increasing negative charge as SDS is adsorbed a t the latex interface. The pattern ofRH differs, however, from that found for SDS adsorption on the earlier studied latex of low charge density.13 Here a decrease is observed in the initial adsorption stage (C< 2 mmoVL) in contrast to the previously found initial increase in RH. There are two maxima in the RHtrace: the first at 3-3.3 mmoVL and the second at about 6.5 mmoVL which is close to the cmc of SDS, followed by a final decrease to a constant level above 8 mmoVL SDS. Complementary data are provided by the adsorption isotherms in Figure 6. They show that the second maximum in Figure 5a is close to saturated adsorption. The maxima observed in RH and the mobility at about CSDS= 7 mM correspond approximately to the peak positions for analogous measurements on the latex of low charge density.ls Figure 5b, corresponding to a low level of added salt (1 mM), is virtually identical to Figure 5a except that the decreasein the initial adsorption stage is not evident and the first maximum in RHis more pronounced than that in the salt-free system. The source of differences in the RH plots recorded in Figure 5 is mostly the extreme sensitivity of RH to the presence of salt ions through screening even when salt is present at a level as low as 1 mM. This is clear from the decrease in RH shown in Figure 3 at the lowest salt concentrations. We place the followingtentative interpretation on the data summarized in Figure 5: (i) At low SDS concentration, single SDS molecules adsorb on the exposed chains a t the latex surface. The fact that SDS does not adsorb significantly to silica gel having a similar negative charge densityz8suggests that hydrophobic interaction is the main driving force for adsorption at these sites. A favorable orientation of the adsorbed molecule is necessary for the negatively charged molecule to adsorb onto a negatively charged surface. &yes4 suggests that the alkyl chain of the surfactant molecule associates with the hydrophobic regions of the latex surface with the charge group orientated toward the diffuse layer. This orientation of the adsorbed molecules will increase the zeta potential (mobility) at the shear plane and also increase the hydrodynamic radius. The latter change arises owing to the extension of the surface chains due to both electrostatic repulsion and steric interaction. As mentioned above, the surface layer of the latex studied here is very sensitive to the concentration of added Na+ ions. As Na+ is added (from the SDS),the conformation ofthe surface chains is strongly (28) Gao, Y.; Yue, C. L.; Lu,S.;Gu,W.J . Colloid Interface Sei. 1984,
100,583.
Interaction between SDS and Latex Particles affected. On the one hand, electrolyte addition tends to compress the flexible polymer layer, while on the other hand the adsorbed SDSmolecules lead to a more extended conformation for steric reasons. These competing trends together yield an overall weak decrease in RHin contrast to the influence of NaBr alone. In the range of low SDS concentration, the minimum value of RH (about 100 nm) is reached at 2.5 mM SDS. This is close to the radius found in NaBr a t 1 mM concentration (see Figure 5b). There is a similar gradual decrease in u;this trend differs from SDS adsorption in 32 mM NaBr in which medium the surface polymer layer is already saturated by the electrolyte added prior to the surfactant. (ii) The first maximum in RH appears at about 3.3 mmol/L SDS. Above this SDS concentration, the decrease in RHcontinues up to about 5 mmol/L SDS. The variation in RHover this SDS concentration range may be the result of competition between the adsorption of SDS and the influence of Na+ ions since the concentration of Na+ is in the most sensitive range (see Figure 3). With further increase of SDS concentration, giving greater surface coverage, SDS molecules start to adsorb cooperatively. Above about 5 mmol/L SDS,the steric role of the adsorbed surfactant molecules determining the thickness of the adsorbed layer becomes dominant since the influence of NaBr is almost constant in this range. Thus RHincreases. However, above 6.5 mmol/L SDS, a further increase in the amount of SDS adsorbed a t the interface moves the surfactant molecules into a more vertical orientation so that the head groups are preferentially directed toward the aqueous phase. The alkyl chains can then interact more effectively with the surface favoring the formation of micelles or hemimicelles at surface adsorption sites as the cmc for the surfactant in solution is approached. At above 7-8 mmol/L SDS,the latex radius becomes abruptly smaller and there is only a small increase in the particle mobility. This concentration is close to the cmc of free SDS micelles in aqueous, salt-free solution a t which free micelles are formed. Any further addition of SDS (and thus excess Na+ counterions), functions to screen the interactions between the anchored SDS micelles and will result in a more compact adsorbed layer. The envisaged competition between compression of the polymer layer by Na+ ions and extension of the polymer chains by SDS adsorption is substantiated by’ the adsorption isotherms. Figure 6 shows absolute adsorption isotherms in the system without added salt and at a salt level of 1 mM. In general, electrolyte ions screen the interactions between the polar end-groups of the adsorbed SDS molecules which promotes an increase in the adsorbed amount of surfactant. This is the case at saturation in Figure 6. At low SDS concentration, however, the adsorbed quantity of SDS in the salt-free system is greater than that in 1 mM NaBr. Evidently extension of the polymer chains at the particle surface (i.e. the surface swollen layer)yields further exposed hydrophobic surface enhancing adsorption of surfactant. The adsorbed quantity of SDS in the system without added salt is 25 mmovg, and that of polymer chains with sulfate end-groups is 16 mmol/kg. For the latex of low charge density, the corresponding quantities are, respectively, 15 mmoVg and 9.7 mmoI/kg.l3 The ratios of (a)the adsorbed amounts at saturation and (b)the quantities of exposed polymer chains are 1.67 and 1.65, respectively. This consistency supports the view that the adsorption of SDS mainly occurs at the polymer chains terminated by so4- groups. The hydrophilicityhydrophobicityof the particle surface will clearly determine the adsorption characteristics of surfactants. Various aspects of this are discussed in refs
Langmuir, Vol. 10, No. 10, 1994 3399 120.0 110.0
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Figure 7. Analogous plot to those in Figure 5 for the same latex suspension but with 32 m M added salt (pH 6).
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Figure 8. Analogous plot to that in Figure 6 for the same latex suspension but with 32 mM added NaBr (pH 6). The vertical arrow indicates the cmc at this system.
29-36. There is some literature regarding the influence of the surface polarity on the adsorption of surfactants.29-33 Illustrating this point, it has been shown that surfactant adsorption at a latedwater interface decreases with increasing polarity ofthe polymer.29~30~33~35~36 For example, Urban30 found that the adsorbed amount of potassium oleate adsorbing onto M e r e n t polymer surfaces decreases in the order polystyrene polybutadiene polystyreneco-acrylonitrile. This observation supports the view that SDS will mainly adsorb at the surface polystyrene chains. It is also possible however, that the protons associated with the carboxy groups interact with the sulfate endgroups of the SDS molecules, through hydrogen bonds leading to double layer formation. The saturated adsorption amount of SDS will then be due to both types of interaction. Influence ofAdded Salt. While the adsorption levels a t a low level of added salt (1mM) are similar to those in the system without added salt, raising the salt level to 32 mM leads to qualitatively different behavior (Figure 7). The first broad maximum in RH (Figure 7) at a concentration of about 3-4 mmol/L is close to the cmc (3.0 mmol/L) for SDS in an aqueous solution of the same salt concentration, the cmc being lower in the presence of salt than in the salt-free system. Thus, as was discussed above and in the previous communication^,'^ we associate the
-
-
(29)Vijayendran, B. R. J . Appl. Polym. Sci. 1979,23,733. (30)Urban, P.C.J.Dispersion Sci. Technol. 1981,2,233. (31)Kronberg, - B.: Stenius, P. J. Colloid Interface Sci. 1984,102, 410. (32)Kronberg, B.;Stenius, P.; Igeborn, G . J . Colloid Znterfuce Sci. 1984,102,418. (33)Vijayendran, B. R. In Surfactants in Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1986;Vol. 5. (34)Yeliseyeva, V.Acta Chim. (Budapest) 1972,71,465. (35)Zuikov, A.V.;Vasilenko, A. I. Colloid J . USSR 1976,37,640. (36)Sutterlin, N.;Kurth, H. J.; Markert, G .Makromol. Chem. 1976, 177,1549.
Brown and Zhao
3400 Langmuir, Vol. 10, No. 20, 1994 120.0
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.100.0
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Figure 10. Hydrodynamic radius (RH) (left)and the mobility (u)(right)shown as a function of SDS concentrationfor a latex suspension in (a, top) pH 4 and (b, bottom) pH 12 aqueous solution without added salt at 25 "C.
(37) Brown, W.; Fundin, J.; de Grapa Miguel, M. Macromolecules 1002,25,7192. (38)Connor.. P.:. Ottewill, R. H. J . Colloid Interface Sci. 1971, . 37. .
642.
(39)Ali, 5. I.; Steach,J. C.; Zollare, R.L. Colloids Suf. 1987,26,1. (40) Zhao,J. Unpublished results.
latex particle at pH 4 and pH 12 in the system without added salt. A salt effect is not apparent at these two pH values and RH increases in the initial adsorption stage as was the case in 32 mM NaBr (Figure 7). At pH 4, the strongly enhanced maximum (in comparison with Figure 5a) with a thickness of about 8 nm is considered to be mainly due to hydrogen bonding between protons associated with the carboxyl groups and the sulfate groups of the SDS molecules since the carboxyl groups are fully associated at pH 4. This causes double layer formation of surfactant (in which the exposed tail of the SDS associates with the corresponding tail of the free SDS molecule with orientation of the endgroup to the aqueous phase). Repulsion between SDS molecules associated with the carboxyl groups of the polymer chains leads to extension of the latter. This will in turn expose more hydrophobic sites enhancing SDS adsorption. A large increase in RH will result. Further increase in SDS will lead to cooperative binding on the polymer chains as the cmc is approached; i.e. micelles are formed. The smaller U-values then follow from a combination of the larger particle radius and reduced charge of the carboxyl groups. The postulated formation of hydrogen bonds at neutral and low pH is supported by the adsorption isotherms at three different pH values (Figure 11). On raising the pH so that the carboxyl groups are dissociated, the adsorbed amount of SDS markedly decreases. The two-step feature of the data at pH 12 is noteworthy. Here the adsorption is dominated by hydrophobic interactions (compare the isotherm in Figure 8 at 32 mM salt and the accompanying discussion). The initial step is visualized as involving cooperative formation of micelles close to the cmc followed by micellar growth. The overall adsorption level is, however, lower than that shown in Figure 8 due to repulsive forces between the dissociated carboxyl groups.
Interaction between SDS and Latex Particles 50.0 0
-
Langmuir, Vol. 10, No. 10, 1994 3401
I
OH-4
40.0
- 30.0 ‘M
P 20.0
2
10.0
0.0 0.0
2.0
4.0 C,
6.0
8.0
10.0
12.0
/ mmol d ~ n ’ ~
Figure 11. Absolute adsorption isotherms for a latex suspension obtainedthrough surfacetension measurement;(r)versus concentration of SDS at three pH values in aqueous solution at 25 “C. Vertical arrows indicate that cmc at each pH.
Conclusions The maxima arising in the observed hydrodynamic radius and particle mobility as a function of the variables examined (added simple salt; SDS concentration; pH) are the result of competing effects at the surface “hairy”layer of the latex particle. The complexity of the interactions, however, only allows a qualitative interpretation of the results which is summarized below. At low concentrations of SDS, single molecules of SDS adsorb on the polymer chains at the latex surface through
association of the alkyl moiety of the surfactant with the hydrophobic polystyrene chaidsurface. The charged groups will be oriented toward the diffusionlayer resulting in an increase of charge at the shear plane and thus an increase in the particle mobility. Screening effects arising from the excess Na+ counterions lead to a partial compression of surface polymer chains while the adsorbed SDS molecules promote extension of the surface polymer chains through a combination of electrostatic repulsion and steric interactions. When the SDS concentration is close to the cmc, micelles are formed on the polymer chains at the surface layer. On increasing the SDS concentration above the cmc at higher levels of simple salt, further surfactant molecules will adsorb through growth of the micelles and lead to further expansion of the polymer chains. Coalescence subsequently occurs between the micelles and a microcontinuous phase of surfactant molecules will be formed around the polymer chains. This yields a much more compact arrangement of the surfactant molecules.
Acknowledgment. This work has been supported by the Jacob Wallenberg Fond, Stora Kopparbergs Bergsslags AB, and the Swedish Technical Research Council (TFR); this support is gratefully acknowledged. We are greatly indebted to the Institute of Surface Chemistry, Stockholm, Sweden, for permission to use their electrophoretic light scattering instrument.
