Sequential Adsorption of Triton X-100 and Sodium Dodecyl Sulfate onto Positively and Negatively Charged Polystyrene Latexes

Journal of Colloid and Interface Science 239, 568–576 (2001) doi:10.1006/jcis.2001.7598, available online at http://www.idealibrary.com on

Sequential Adsorption of Triton X-100 and Sodium Dodecyl Sulfate onto Positively and Negatively Charged Polystyrene Latexes ´ R. Porcel, A. B. J´odar, M. A. Cabrerizo, R. Hidalgo-Alvarez, and A. Mart´ın-Rodr´ıguez1 Biocolloid and Fluid Physics Group, Department of Applied Physics, Faculty of Sciences, University of Granada, 18071 Granada, Spain Received November 28, 2000; accepted April 2, 2001

Individual and sequential adsorption of the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic surfactant Triton X-100 on cationic and anionic polystyrene latexes has been examined. Both latex samples, although charged differently (−16.2 and +11.0 µC cm−2 ), must be considered hydrophobic polymer colloids. The adsorbed amounts of both surfactants on cationic and anionic latexes were found to be different as a consequence of the dissimilar interfacial properties of these two surfactants. In addition, a comparison between the two surfactants showed that SDS is more easily replaced than Triton X-100 when sequential adsorption on both latexes was studied. It was found that the electrophoretic mobility of surfactant latex complexes depends on the addition sequence order of both surfactants. In relation to colloidal stability, when a layer of a nonionic surfactant is adsorbed on the surface of latexes, the electrosteric mechanism explains the experimental results. If SDS is adsorbed the stabilization or unstabilization is a consequence of the changes in the electrical repulsion between particles. However, when both surfactants are adsorbed, the assumption of additivity is not correct; that is, the electrostatic repulsion (VR ) and the steric repulsions (Vosm and Vvr ) are not totally independent. °C 2001 Academic Press Key Words: cationic and anionic polymer colloids; nonionic and anionic surfactant adsorption; sequential adsorption; colloidal stability.

INTRODUCTION

Surfactants have widespread industrial and technological applications. Detergency, pharmaceutical processes, foaming, emulsification, and lubrication are just a few examples of processes in which surfactants are used (1). Most of their applications are related to colloidal science, in particular, with the adsorption of the surfactants at the solid–liquid interface, and with their effects on the colloidal stability of the complexes so formed. It is well known that, when the surfactant has been adsorbed in the appropriate amount and orientation, it modifies the interparticle interaction and can produce the desired aggregation or 1

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stabilization of the system. Non-ionic surfactants adsorbed onto colloidal particles can act as steric stabilizers (2–4), and polyelectrolytes or ionic surfactants adsorbed cause electrostatic and steric effects, i.e., electrosteric stabilization (5, 6). It is commonly accepted in the field of surfactant science that mixtures of surfactants often perform better than the individual components (7–10), and there are studies about micelle composition and co-adsorption in mixed surfactant solutions (11–13). However there is a lack of both theoretical studies and experimental results in the aggregation of colloidal particles when two surfactants are adsorbed below the CMC. This is why we have also tried to supply a theoretical analysis of this problem and to relate the theoretical predictions to those experimental results obtained by turbidimetry. With this aim a nonionic and an anionic surfactant, Triton X100 and sodium dodecyl sulfate (SDS), respectively, have been used to modify the interparticle interactions and, therefore, the colloidal stability of polymer colloids. Both surfactants were chosen mainly because, although charged differently (uncharged and negatively charged), they present a similar molecular area at ˚ 2 /molecule) and they are widely the air/water interface (∼ =50 A used in industrial applications of colloidal science. Also, recent advances in the preparation and characterization of polymer colloids have been applied, using latexes with different sign surface charge (cationic and anionic). As an extension of the individual adsorption, the sequential adsorption of both surfactants onto the same latexes was also studied. Sequential surfactant adsorption is a two-step process. First, one type of surfactant is adsorbed onto latex particles. It is left there for a certain time after which a second surfactant is added to this surfactant–adsorbent complex. Adsorption of the second surfactant may involve partial or complete displacement of the surfactant pre-adsorbed. Finally to fully understand the stability a quantitative description of the distribution of charges and potentials around the particles is essential (14). Adsorption experiments have been combined with microelectrophoretic mobility measurements, allowing us to obtain information on the electrical double-layer structure as a function of the adsorbed amount of anionic and nonionic surfactant.

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EXPERIMENTAL

Latex Particles All of the latexes used were synthesized in our laboratory, using styrene as a monomer. The styrene (Merck) was previously distilled under low pressure (10 mm Hg and 40◦ C). Negatively charged polystyrene latex, PS[−], was prepared by using 4,40 azobis(4-cyanopentanoic acid) (ACPA) (from Aldrich) as initiator; therefore, there are only carboxyl groups on the surface of these colloidal systems (15). Positively charged polystyrene beads, PS[+], were synthetized by polymerization of styrene and the initiator azo-N ,N 0 dimethylene isobutyramidine hydrochloride (ADMBA) (Bayer) as previously described (16). Two latexes were prepared by emulsifier-free emulsion polymerization in a discontinuous reaction. The reactor was a spherical vessel (volume of 1 L), and solutions were stirred with a Teflon palette located 1 cm over the bottom of the vessel. The latexes were first cleaned by centrifugation at 15000 rpm three times and redispersed in water at acidic or basic pHs (PS[+] and PS[−], respectively) followed by serum replacement until the specific electrical conductivity of the supernatant was similar to that of the DDI water. A complete characterization of both latexes was carried out. The main results, such as size, polydispersity index (PDI), and surface polar groups, are shown in Table 1. Surface charge densities were determined by conductometric and potentiometric titration. Particle size was obtained by using two different methods: (i) transmission electron microscopy (TEM) and (ii) photocorrelation spectroscopy (PCS). Results taken from both methods were very similar. Surfactants SDS and Triton X-100 were supplied by Merck with a high grade of purity (for tenside test and gas chromatography-grade material, respectively). Due to the aromatic ring on the Triton X-100 molecules, the detection of this surfactant in aqueous media is possible by means of a direct spectrophotometry method. The concentrations of Triton X-100 were determined by using a UV spectrophotometer (Milton-Roy Spectronic 601) at 275 nm (17). The concentration of SDS was determined by the wellknown methylene blue method; a cationic dye (methylene blue) was added to the supernatant liquid, the non-ionic surfactant–

TABLE 1 Main Features of Bare Latexes Sample

Surface polar groups

σ0 (mC/m2 )

D (nm)

IPD

PS(−) PS(+)

Carboxyl Amidine

−162 ± 3 +110 ± 3

268 ± 8 369 ± 12

1.0039 1.0087

569

cationic dye complex was extracted with chloroform, and the color in the CHCl3 phase was measured by spectrophotometry at 652 nm. All of these analyses were carried out automatically by means of a flow injection analyzer (FIA) system (18). Furthermore, the liquid extraction was also made automatically. The critical micelle concentration (CMC) of Triton X-100 was determined by measuring the absorbance at different concentrations. The change in the slope of the absorbance versus the concentration curve corresponds to the CMC. The CMC of the SDS was determined by measuring the conductivity at different concentrations. The change in the slope corresponds once again to the CMC. The CMC values obtained were 9.2 ± 0.3 × 10−4 M (Triton X-100) and 9.8 ± 0.2 × 10−3 M (SDS). Adsorption Isotherms Adsorption isotherms were determined by using the depletion method. Known volumes of polystyrene latex, surfactant, and pH = 7 buffer standard solutions were mixed. The amount of latex was chosen so that the overall area represented by the polystyrene particles was 0.3 m2 . The adsorption cells were kept in a thermostatic bath. Previously, the kinetics of adsorption was examined to determine the best conditions for the adsorption experiments. The latexes were separated by centrifugation, and the surfactant concentrations were determined before and after the adsorption. Electrophoretic Mobility The electrophoretic mobilities were measured at 25◦ C under different ionic strengths, using KBr as electrolyte, and the pH conditions were determined by using a Zeta-Siser IV (Malvern Instrument). The electrophoretic mobility values were obtained by taking the average of at least five measurements at the stationary level in a cylindrical cell. For the data shown throughout this paper the standard deviation was always lower than ±0.2 × 10−8 m2 V−1 s−1 . In these measurements the latex particle concentration was 0.03 mg ml−1 . Colloidal Stability The stability of the dispersions was evaluated with a Spectronic 601 spectrophotometer (Milton Roy, USA) by measuring the turbidity as a function of time for different electrolyte concentrations. In a typical coagulation experiment, 2.4 ml of a buffered latex solution (pH = 7) was put into the spectrophotometer cell and the optical absorbance was measured. Then, 0.6 ml of a potassium chloride solution at a given concentration was quickly added, and the resulting solution was mixed automatically. The final particle concentration in the cell was 1010 part/ml. The optical absorbance was measured immediately and recorded continuously via a computer for a period of 30 s. The initial slope of these curves is directly proportional to the initial coagulation rate. Therefore, the stability can be expressed in terms of the stability factor W , obtained as the ratio of the rate constants for rapid and slow coagulation.

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(2) DLVO Theory Considering Ion Size

THEORETICAL BACKGROUND

(1) DLVO Theory The stability factor W has been used extensively in the literature to characterize the stability of hydrophobic colloids. The relationship between the net interaction energy (V ) and the W factor was obtained by Fuchs (19), Z W = 2a 0

∞

exp(V /kB T ) d H, (2a + H )2

[1]

µ

where H is the distance between the boundary of two spheres and V , according to the DLVO theory, is determined by two particle interaction energies, a repulsive (VR ) one and an attractive (VA ) one. • VR is due to the overlap between the diffuse double layers of the particles and decays exponentially with the distance of separation. For moderate surface potentials VE is given by µ

4kB T γ VR = 2πεr ε0 a zie

¶2

e−κ H,

[2]

where κ is the Debye parameter, z i is the valence of the ion, e, ε0 , εr , kB , and T have their usual meanings, and γ is given by µ

¶ z i e9d γ = tanh , 4kB T

[3]

where 9d is the diffused potential. The diffused potential is related to the surface charge density (σ0 ) by µ ¶ κσ0 2kB T 9d = arcsin zie 4n 0 z i e

[4]

and n 0 is the concentration of ions in the bulk for an 1:1 electrolyte. • VA is due to the London–van der Waals (dispersion) energy and decays according to a power law, µ ¶ A H (4a + H ) 2a 2 2a 2 + ln + VA = − , 6 H (4a + H ) (2a + H )2 (2a + H )2 [5] where a is the particle radius and A is the Hamaker constant for particles interacting in the medium. Thus, V is given by µ V = 2πεr ε0 a −

A 6

µ

4kB T γ ze

¶2

If the Stern layer thickness is considered, Eq. [6] must be modified since in the original DLVO theory the reference planes for attractive and repulsive energy coincide. However, Vincent et al. (20) refined this idea by shifting the reference plane for the repulsive energy outward over a distance corresponding to the thickness (1) of the Stern layer. Taking into account this correction, the electrical double-layer repulsion between two equal spheres of radius a is given by

e−κ H

¶ H (4a + H ) 2a 2 2a 2 + ln + . [6] H (4a + H ) (2a + H )2 (2a + H )2

VR = 2π εr ε0 (a + 1)

4kB T γ zie

¶2

e−κ(H −21) ,

[7]

which should be introduced in Eq. [6]. (3) Electrosteric Stabilization Vincent et al. (21) made a quantitative study of the steric stabilization effect including two contributions: osmotic and coil compression. If there are polymeric chains covering the external surface of a particle, the average thickness of such coils being δ, then an osmotic effect will appear when the two particles are closer than a distance equal to 2δ. The osmotic pressure of the solvent in the overlap zone will be less than that in the regions external to it, leading to a driving force for the spontaneous flow of solvent into the overlap zone, which pushes the particles apart (22). In that case the osmotic potential of repulsion (Vosm ) can be considered to be Vosm =

4πa (φ2 )2 (1/2 − χ)(δ − H/2)2 , v1

[9]

where v1 is the molecular volume of the solvent, ϕ2 is the effective volume fraction of segments in the adlayer, and χ is the Flory–Huggins solvency parameter. However, if the two particles are closer than a distance equal to δ, at least some of the polymer molecules will be forced to undergo elastic compression. Thermodynamically, this compression corresponds to a net loss in the configurational entropy. This effect gives rise to a new repulsion potential (Vvr ) related to the restriction of the movement of the hydrophilic coils extended toward the solvent. This elastic–steric repulsion is given as " µ ¶ # ¶Ã h 3 − H/δ 2 2πa H 2 φ2 δ ρ2 ln Vvr = MW δ δ 2 ¸ ¶ · 3 − H/δ + 3(1 + H/δ) , − 6 ln 2 µ

[10]

where ρ2 and MW are the density and the molecular weight of the adsorbed polymer. This modifies the osmotic potential,

SEQUENTIAL ADSORPTION OF TRITON X-100 AND SODIUM DODECYL SULFATE

571

which is now given by Vosm

4πa = (φ2 )2 (1/2−χ)δ 2 v1

·µ

¶ µ ¶¸ 1 H H − − ln . 2δ 4 δ

[11]

To consider the electrosteric stabilization mechanism, both effects (electrostatic repulsion and steric stabilization) must be combined. Conventionally, the total interaction energy is assumed to be the sum of all of the attractive and repulsive potentials: V = VR + VA + Vosm + Vvr . RESULTS AND DISCUSSION FIG. 1. Adsorption of SDS on PS[−] in the presence of preadsorbed Triton X-100: amount of SDS adsorbed (- - -) and Triton X-100 desorbed (——).

(1) Adsorption Table 2 shows the results of the individual adsorption of SDS and Triton X-100. The amount of nonionic surfactant adsorbed on cationic latex is higher than that adsorbed on the anionic one. We can conclude that the cationic latex is more hydrophobic than anionic latex, as it is well known that the packing of the nonionic surfactant becomes more dense as the hydrophobic character of the surface increases (23, 24). When the surface and the surfactant have opposite sign charges (SDS on cationic latex), there is an important electrostatic contribution in the adsorption process. The maximum amount adsorbed (9.3 µmol/m2 for the adsorption of SDS) corresponds to a charge density eight times higher than the charge on the latex surface. It suggests that small associates of surfactant molecules are formed on the surface by hydrophobic interaction between the hydrocarbon chains (25). Thus, hydrophobic interaction plays an important role even when the surface and surfactant have opposite charges. When the surfactant and surface have the same charge (SDS on PS[−]), the adsorption also occurs by hydrophobic interaction with the surface. The confirmation of the adsorption of aggregates on the surface can be observed in Table 2, since the areas occupied by SDS and Triton X-100 obtained in this study are smaller than those obtained at the air/water interface (26). Figures 1–4 show the sequential adsorption of SDS and Triton X-100 on both latexes. In relation to the sequential adsorption, the results showed that the adsorption of a second surfactant on a latex–surfactant complex is higher when the amount of the first adsorbed surfactant is low and the replacement occurs gradually.

The experiments show that, when Triton X-100 is adsorbed on PS[−] first, the pre-adsorbed surfactant causes “blocking” of the adsorption of SDS added later (Fig. 1). However, when SDS is added first it does not block the adsorption of Triton X-100; moreover, the non-ionic surfactant replaces the ionic SDS on the surface of the anionic latex ( Fig. 2). The results confirm the previous hypothesis, which establishes that the adsorption of SDS on PS[−], as well Triton X-100, is mainly due to hydrophobic interaction. Similar results were found regarding the adsorption on PS[+] latex. When Triton X-100 is adsorbed first, the electrostatic interaction SDS–latex appears, and a considerable amount of Triton X-100 is now desorbed (Fig. 3). When SDS is preadsorbed, again, the replacement of SDS by Triton X-100 occurs; even the complete replacement of SDS is not attained (Fig 4). (2) Electrokinetic Characterization The presence of both surfactants on the latex surface, obtained by sequential adsorption, has also been confirmed by electrophoretic mobility measurements. Table 3 shows the µe of bare latex, SDS–latex, and Triton–latex complexes and the sequentially adsorbed Triton–SDS and SDS–Triton on both latexes at two different degrees of coverage.

TABLE 2 Cross-Sectional Molecular Area at Different Interfaces

Sample

Air/water interface ˚ 2 /molecule) (A

Surface/water interface ˚ 2 /molecule) (A

PS (+)-SDS PS (−)-SDS PS (+)-Triton X-100 PS (−)-Triton X-100

53 ± 3 53 ± 3 48–54 ± 4 48–54 ± 4

18 ± 2 37 ± 2 29 ± 1 43 ± 2

FIG. 2. Adsorption of Triton X-100 on PS[−] in the presence of preadsorbed SDS: amount of Triton X-100 adsorbed (- - -) and SDS desorbed (——).

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TABLE 3 Effect of the Sequential Adsorption on the Electrophoretic Mobility PS[+]

PS[+] SDS 7.2 µmol/m2

PS[+] TX-100 1.7 µmol/m2

PS[+] SDS, TX-100

PS[+] TX-100, SDS

4.5

−1.0

4.5

−0.60

−0.40

PS[+]

PS[+] SDS 4.9 µmol/m2

PS[+] TX-100 2.7 µmol/m2

PS[+] SDS, TX-100

PS[+] TX-100, SDS

4.5

−1.0

4.5

−0.40

0.05

PS[−]

PS[−] SDS 2.1 µmol/m2

PS[−]TX-100 2.4 µmol/m2

PS[−] SDS + TX-100

PS[−] TX-100 + SDS

−4.15

−3.5

−3.9

−3.97

−4.15

PS[−]

PS[−] SDS 1 µmol/m2

PS[−] TX-100 2.4 µmol/m2

PS[−] SDS + TX-100

PS[−] TX-100 + SDS

−4.15

−4

−4

−4.24

−4.15

µe 108 m2 /Vs

µe 108 m2 /Vs

µe 108 m2 /Vs

µe 108 m2 /Vs

As expected the results show that the presence of SDS on the surface of both latexes is the most important factor determining the electrophoretic mobility of the complexes. However, the most striking result was that the electrophoretic mobility of surfactant latex complexes depends on the order during the sequential adsorption; i.e., even when the final adsorbed amount of both surfactants is identical, the electrophoretic mobility is a function of the order of the adsorption. Thus, we can conclude that the conformation (and, therefore, the adsorption mechanisms) of both surfactants at the solid–liquid interface is quite different, depending particularly on the addition sequence. (3) Colloidal Stability Bare latexes stability. The Hamaker constant (A) and the diffuse potential (9d ) are usually obtained by fitting the theoretical expression of W to the experimental values. The main problem with this treatment is that the experimental A values for latexes are far from the theoretical value (14). In this paper we have decided to use 9d as a fitting parameter. An expression for A given by Bowen et al. (27) was taken for the polystyrene–polystyrene interaction in water. This expres-

FIG. 3. Adsorption of SDS on PS[+] in the presence of preadsorbed Triton X-100: amount of SDS adsorbed (- - -) and Triton X-100 desorbed (——).

sion was obtained as follows. Initially, A = Av=0 + Av≥1 = 0.965 × 10−20 J, where Av=0 (Keesom and Debye contributions) was equal to 0.266 × 10−20 J and Av≥1 (London contribution) was equal to 0.699 × 10−20 J. However, at all separations both retardation and screening effects are very important to the van der Waals interaction, as reported recently by Bevan and Prieve (28). Therefore, these effects must be also included. Retardation has been taken into account, multiplying Av≥1 by the following function: " F(h) = 1 +

µ

π D(H ) √ 4 2

¶3/2 #−2/3 ,

[12]

where the D(H ) function is equal to 6.044 × 107 h. The screening effect was considered, multiplying Av=0 by (1 + 2κ H )e−2κ H . In addition, by considering the ion size as a correction of the DLVO theory, the surface potentials were fitted to 9d = 27 mV for PS[+] and 9d = −29 mV for PS[−] by considering the potassium radius 1 = 0.133 nm (29). By using the above

FIG. 4. Adsorption of Triton X-100 on PS[+] in the presence of preadsorbed SDS: amount of Triton X-100 adsorbed (- - -) and SDS desorbed (——).

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SEQUENTIAL ADSORPTION OF TRITON X-100 AND SODIUM DODECYL SULFATE

FIG. 5. Stability factor (W ) versus KBr concentration for PS[+] at different degrees of coverage (r, 50%; n, 75%; d, 100%) of Triton X-100.

data the energy barrier disappears [V = (d V /d H ) = 0] at the CCC value for both latexes. Surfactant–latex complexes stability. Figures 5 and 6 show the experimental dependence of log W on log[KBr] for surfactant–latex complexes at different coverage degrees. Figure 7 shows the effect of the adsorption of SDS and Triton X-100 on the stability (CCC) of both latexes. As can be seen, the stabilization of latex suspensions was significantly affected by the adsorption of the surfactants. In this way, the adsorption of SDS on PS[+] made the dispersion unstable, while the adsorption of SDS on PS[−] and Triton X-100 on both latexes increased the dispersion stability (30, 31). The adsorption of SDS at least modifies the electrical interaction between particles. Thus, by considering the ion size, the surface potentials were calculated by fitting the theoretical expression of W to the experimental values. The results are shown in Table 4. The adsorption of SDS on PS[−] increases in absolute value the surface potential from −29 mV (bare latex) to −44 mV (4.5 µmol/m2 ) while the adsorption on PS[+] decreases 9d from 27 mV to −20 mV (2.3 µmol/m2 ). Higher adsorption of SDS on

FIG. 7. Effect of the degree of coverage on the CCC: Triton X-100 on PS[+] (n) and PS[−] (m); and SDS on PS[+] (s) and PS[−] (d).

PS[+] does not produce stabilization. The most striking result is that, even when the adsorbed amount increases fourfold, the potentials and the stability of the complexes are only slightly modified. When a layer of a nonionic surfactant adsorbed on the surface of latexes exists in addition to all the corrections mentioned above, we must apply the electrosteric mechanism to explain the stability results found for the non-ionic/PS complexes. To fit the theoretical expression of W to the experimental values there are three more parameters, that is, the Flory– Huggins solvency parameter (χ), the average thickness of surfactant chains covering the external surface of a particle (δ), and the effective volume fraction of segments in the external layer (ϕ2 ). In this work, we have decided to keep constant the Flory–Huggins solvency parameter, χ = 0.45 (32), and δ = 20 ˚ estimated from electrokinetic measurements (33). The effecA tive volume fraction ϕ2 was used as a fitting parameter in the W versus [KBr] plots. The results (shown in Table 4) indicate the TABLE 4 Surface Potentials (9d ) and Effective Volume Fractions of Segments in the External Layer (ϕ2 ) Obtained by Fitting Parameter in the W versus [KBr] Plots in the SDS and Triton X-100 Adsorption on Latexes Surfactant latex complex SDS-PS[−]

SDS-PS[+] Triton X-100-PS[−]

Triton X-100-PS[+] FIG. 6. Stability factor (W ) versus KBr concentration for PS[−] at different degrees of coverage (s, 25%; r, 50%; n, 75%; d, 100%) of Triton X-100.

Amount adsorbed (µmol/m2 )

9d (mV)

ϕ2

1.12 2.25 3.37 4.5 2.25 1 2 3 4 1.48 2.96 4.4 5.9

41 42 43 44 −20 −29 −29 −29 −29 27 27 27 27

0.049 0.050 0.051 0.052 0.040 0.041 0.043 0.045

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FIG. 8. Effect of the sequential adsorption on the CCC of surfactant/PS[+] complexes when Triton X-100 is adsorbed in presence of preadsorbed SDS.

FIG. 10. Effect of the sequential adsorption on the CCC of surfactant/PS[−] complexes when Triton X-100 is adsorbed in presence of preadsorbed SDS.

following:

Colloidal stability after sequential adsorption. Figures 8– 11 show the effects on the stability of the sequential adsorption of Triton X-100 and SDS on PS[+] and PS[−]. On PS[+] the stability of the dispersion is mainly due to the amount of SDS that is on the latex surface. It should be noted that this effect is independent of which surfactant is adsorbed first (Figs. 8 and 9).

On dispersions of PS[−] the adsorptions of both surfactants act in a different way. The stability of these surfactant-latex complexes is a consequence of the sequential adsorption of both surfactants; thus, when SDS is adsorbed first and the nonionic surfactant completely replaces the ionic SDS on the surface, the stability is due to the amount of Triton X-100 adsorbed. If the replacement is not total, the stability increases when the amount of SDS increases (Fig. 12). When Triton X-100 is adsorbed on PS[−] first, the preadsorbed surfactant causes “blocking” of the adsorption of SDS added later and the colloidal stability is mainly due to Triton X-100; therefore the stability increases when the amount of Triton increases (Fig. 11). In this paper we have applied the stability theories mentioned above to latex with SDS and Triton X-100 adsorbed. We have considered that the electrostatic repulsion (VR ) must be modified as a consequence of the presence of SDS on the surface. The adsorption of Triton X-100 affects the steric stabilization (Vosm + Vvr ). Thus, we have used the following:

FIG. 9. Effect of the sequential adsorption on the CCC of surfactant/PS[+] complexes when SDS is adsorbed in presence of preadsorbed Triton X-100.

FIG. 11. Effect of the sequential adsorption on the CCC of surfactant/PS[−] complexes when SDS is adsorbed in presence of preadsorbed Triton X-100.

1. The volume fraction ϕ2 increases with the amount adsorbed. 2. The volume fraction ϕ2 is higher for PS[−] latex even when the amount adsorbed is lower than that on PS[+]; thus, on PS[+] the surfactant presents a more extended conformation (flat conformation) on the latex surface. The different conformation depending on the latex surface could explain the different colloidal stability of the latex–surfactant complexes with the same amount adsorbed.

SEQUENTIAL ADSORPTION OF TRITON X-100 AND SODIUM DODECYL SULFATE

575

added later. On PS[+] the electrostatic attraction SDS–latex appears, and a considerable amount of Triton X-100 is now desorbed.

FIG. 12. Net interaction energy (V /kB T ) as a function of the distance for complexes SDS–Triton X-100/PS[−] latex under CCC conditions. After sequential adsorption the final amounts of surfactants were the following: 1.7 µmol/m2 (SDS) and 1 µmol/m2 (Triton X-100) (m) and 0 µmol/m2 (SDS) and 3.9 µmol/m2 (Triton X-100) (s).

(1) DLVO theory corrected by the ion size (1 = 0.133 nm) and the hydrodynamic effect. (2) The surface potential obtained from bare latex or the values obtained by fitting parameter in the W versus [KBr] plots in the SDS adsorption (Table 4). (3) The effective volume fraction ϕ2 obtained by fitting parameter in the W versus [KBr] plots in the Triton X-100 adsorption (Table 4). (4) The Flory–Huggins solvency parameter, X = 0.45. (5) The average thickness of surfactant chains covering the ˚ external surface, δ = 20 A. Some results are shown in Fig. 12. In this figure the KBr concentration in the bulk was taken as being equal to the experimental CCC for the surfactant–latex complexes. When there are both surfactants on the latex surface, the interaction potential barrier between two particles does not disappear at the experimental CCC; i.e., theoretically the complexes should present higher CCC values. However, when SDS is completely replaced with Triton X-100, the CCC condition (V = (d V /d H ) = 0) is reached for the parameters obtained in the individual adsorption of the nonionic surfactant. In conclusion, when a nonionic and an anionic surfactant are adsorbed, the assumption of additivity made in this work is not correct; that is, the electrostatic repulsion (VR ) and the steric repulsions (Vosm and Vvr ) are not totally independent. SUMMARY

Sequential adsorption experiments showed the following: (a) When SDS is added first it does not block the adsorption of Triton X-100. The nonionic surfactant replaces the SDS. The replacement is enhanced in the case of PS[−] latex due to electrostatic repulsion between SDS and the latex surface. (b) When Triton X-100 is adsorbed first on PS[−], the preadsorbed surfactant causes “blocking” of the adsorption of SDS

Electrophoretic mobility of surfactant latex complexes depends on the addition sequence during the sequential adsorption. Thus, the conformation of both surfactants at the solid–liquid interface is quite different, depending particularly on the order of the adsorption. After adsorption of Triton X-100 and SDS on cationic and anionic latexes, two different colloidal stability behaviors were found for covered particles. The effect of the individual adsorption on colloidal stability can be explained by the extended DLVO theory. However, when both surfactants are adsorbed, the extended DLVO theory (by using the parameters obtained in the individual adsorption) and experimental values do not agree. ACKNOWLEDGMENTS This work was supported by the Commisi´on Interministerial de Ciencia y Tecnologia (CICYT), project MAT-99-0662-C03-02. A.B. J´odar thanks the Spanish Ministry of Education, Culture, and Sport for supporting her research in the University of Granada.

REFERENCES 1. Myers, D., “Surfactant Science and Technology,” 2nd ed. VCH Publishers, Inc., 1992. 2. Hunter, R. J. “Foundations of Colloid Science,” Vol 1. Universities Press, Belfast. 3. Vincent, B., Edwards, J., Emmett, S., and Jones, A., Colloids Surf. 18, 261 (1986). 4. Weiss, A., Hartenstein, M., Dingenouts, N., and Ballauff, M., Colloid Polym. Sci. 276, 794 (1998). 5. Einarson, M. B., and Berg, J., J. Colloid Interface Sci. 155, 165 (1993). 6. Sung, A. M., and Piirma, I., Langmuir 10, 1393 (1994). 7. Ogino, K., and Abe, M., “Mixed Surfactant Systems,” Dekker (1993). 8. Bolze, J., H¨orner, K. D., and Ballauff, M., Colloid Polym. Sci. 274, 1099 (1996). 9. Kronberg, B., Lindstr¨on, M., and Stenius, P., in “Phenomena in Mixed Surfactant System.” (Scamehorn, J. F., Ed.), ACS Symp. Series 311. American Chemical Society, Washington, DC. 10. Rubingh, D. N., in “Solution Chemistry of Surfactants” (Mittal, Ed.) Plenum Press, New York, 1979. 11. Huang, L., Maltesh, C., and Somasundaran, P. J. Colloid Interface Sci. 177, 222 (1996). 12. Huang, L., and Somasundaran, P., Langmuir 12, 5790 (1996). 13. Poulin, P., and Bibette, J., Langmuir 15, 4731 (1999). ´ 14. Hidalgo-Alvarez, R., Mart´ın, A., Fernandez, A., Bastos, D., Martinez, F., and de las Nieves, F. J., Adv. Colloid Interface Sci. 67, 1 (1996). 15. Guthrie, W. H., Ph.D. dissertation, University of Lehigh, U.S.A., 1985. 16. Hidalgo-Alvarez, R., de las Nieves, F. J., van der Linde, A. J., and Bijsterbosch, B. H., Colloids Surf. 21, 259 (1986). 17. Chiming, Ma., Colloids Surf. 28, 1 (1987). 18. Ca˜nete, F., Rios, A., Luque de Castro, M. D., and Valc´arcel, M., Anal. Chem. 60, 2354 (1988). 19. Fuchs, N., Z. Phys. 89, 736 (1934). 20. Vincent, B., Bijsterbosch, B. H., and Lyklema, J., J. Colloid Interface Sci. 37, 171 (1970). 21. Vincent, B., Edwards, J., Emmett, S., and Jones, A., Colloids Surf. 18, 261 (1986). 22. Fischer, E. W., Z. Z. Polym. 160, 120 (1958).

576

PORCEL ET AL.

23. Kronberg, B., Stenius, P., and Igeborn, G., J. Colloid Interface Sci. 102, 418 (1984). ´ 24. Mart´ın Rodr´ıguez, A., Cabrerizo-V´ılchez, M. A., and Hidalgo-Alvarez, R., Colloids Surf. A 92, 113 (1994). 25. Rupprecht, H., and Gu, T., Colloid Polym. Sci. 269, 506 (1991). 26. Rosen, M. J., “Surfactant and Interfacial Phenomena,” 2nd ed. Wiley, (1989). 27. Bowen, W. R., and Jenner, F., Adv. Colloidal Interface Sci. 56, 201 (1995).

28. Bevan, M. A., and Prieve, D. C., Langmuir 15, 7925 (1999). 29. Israelachvili, J., “Intermolecular and Surface Forces,” Academic Press, 1992. 30. Romero-Cano, M. S., Mart´ın Rodr´ıguez, A., Chauveteau, G., and de las Nieves, F. J., J. Colloid Interface Sci. 198, 266 (1998). 31. Romero-Cano, M. S., Mart´ın Rodr´ıguez, A., Chauveteau, G., and de las Nieves, F. J., J. Colloid Interface Sci. 198, 273 (1998). 32. Einarson, M. B., and Berg, J., J. Colloid Interface Sci. 155, 165 (1993). 33. Sung, A. M., and Piirma, I., Langmuir 10, 1393 (1994).