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Langmuir 1999, 15, 3250-3255
Competitive Adsorption of Sodium Dodecyl Sulfate on Two Polymer Surfaces within Latex Blends Jeffrey M. Stubbs, Yvon G. Durant, and Donald C. Sundberg* Polymer Research Group, Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824 Received October 9, 1998. In Final Form: January 11, 1999
Experiments were performed to determine how the partitioning of surfactant between two different polymer surfaces in a latex is dependent on the nature of the polymers and their relative surface areas. Titrations of individual latices and latex blends with sodium dodecyl sulfate (SDS) were carried out while continuously measuring the conductivity of the latex. Polymers studied were polystyrene, poly(n-butyl methacrylate), poly(methyl methacrylate), and poly(methyl acrylate) and the individual adsorption areas for SDS on these surfaces at saturation, As, were found to be 44.0, 58.6, 96.0, and 163.3 Å2/molecule, respectively. For blends of these latices, the average adsorption area at saturation was found to be a linear function of the individual polymer surface area fractions in the blend. It was also possible to predict the behavior of a titration of a latex blend based on the individual titrations for the pure latex. These results confirm the hypothesis that SDS adsorption on multiple polymer surfaces at a fixed temperature is determined by the activity of the surfactant in the water phase.
Introduction
µw ) µ0 + RT ln aw
(1)
Adsorption of surfactants on polymers is important for a variety of reasons. The most obvious is related to maintaining colloidal stability of a polymer latex. Another reason is to control particle nucleation in emulsion polymerization, and knowledge of adsorption characteristics is necessary to prevent secondary nucleation in a seeded emulsion polymerization. The adsorption area of surfactant at saturation, As, is required in order to determine the amount of surfactant that is needed to saturate the latex particle surface. This, plus the amount of surfactant in the water at the critical micelle concentration (cmc), will be the maximum amount of surfactant that can be used in the initial recipe for a seeded polymerization, since above this amount micelles will form and secondary nucleation will be likely. Surfactant adsorption characteristics are also very important in contributing to the polymer/water interfacial tension at the particle surface. Interfacial tension of polymers against water determines the equilibrium morphology of composite particles produced by seeded emulsion polymerization.1-4 Since the presence of surfactant on the particle surface greatly affects this interfacial tension,5 knowledge of how much surfactant adsorbs on a given polymer surface is equally important. Thermodynamic analysis leads to the conclusion that the packing of surfactant on a particular polymer surface at a fixed temperature is a function only of the concentration of the surfactant in the water phase. This conclusion is obtained by writing the chemical potentials of the surfactant in the water (µw) and on the polymer surface (µi). These chemical potentials are expressed as
µi ) µ0 + RT ln ai
(2)
* To whom correspondence may be addressed. (1) Sundberg, D. C.; Cassasa, A. J.; Pantazopoulos, J.; Muscato, M. R.; Kronberg, B.; Berg, J. J. Appl. Polym. Sci. 1991, 41, 1425. (2) Waters, J. A. Colloids Surf., A 1994, 83, 167. (3) Winzor, C. L.; Sundberg, D. C. Polymer 1992, 18, 3797. (4) Durant, Y. G.; Sundberg, D. C. J. Appl. Polym. Sci. 1995, 58, 1607. (5) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982.
where aw and ai are the activities of the surfactant in the water and on particle surface i, respectively. The activity of the surfactant in the water phase is defined in a way that is analogous to the definition of the activity of organic solvents and monomers in water,6 but using the cmc as the saturation concentration of the surfactant as follows
aw ) [SDS]w/cmc
(3)
where [SDS]w is the surfactant concentration in the water and cmc is the critical micelle concentration. Therefore, the activity of the surfactant in the water is simply equal to the fraction of the cmc in the water. The activity of the surfactant on particle surface i is given by
ai ) f(yi);
yi ) Xi/Xisat
(4)
where i ) {1, 2, 3, ..., N} with N being the number of different surfaces present, Xi is the amount of surfactant on surface i, Xisat is the amount on surface i at saturation, and yi is therefore the fraction of saturation on the particle surface. Equation 4 shows that, unlike the situation in the water, the activity of the surfactant on the particle surface is not equal to the fraction of saturation on the particle surface. Instead, it is some function of the fraction of saturation on the particle surface. At equilibrium the chemical potentials of the surfactant in the water and on the surfaces must be equal. Thus from eqs 1, 2, and 3 we have
aw ) ai ) f(yi)
(5)
Since the activity in the water (fraction of the cmc) is a function of the fraction of saturation on the particle surface, it can also be said that the inverse of this is true, i.e., that (6) Maxwell, I. A.; Kurja, J.; Van Doremaele, G. H. J.; German, A. L.; Morrison, B. R. Makromol. Chem. 1992, 193, 2049.
10.1021/la981410s CCC: $18.00 © 1999 American Chemical Society Published on Web 04/01/1999
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Conductometric Titrations Surfactant titrations have been previously performed to study surfactant adsorption on latex particles, and in most cases these have monitored the surface tension of the latices, which is related to the surfactant concentration in the water.7-9 It is also possible to titrate water or latex with ionic surfactant and to record the fluid conductivity as a function of the added surfactant.10,11 Conductivity provides a convenient way to monitor the titration because it is easy to continuously obtain data while surfactant is being continuously added to the latex. The conductance of an ion in solution is a function of both the charge and the size (or mobility) of the ion. The total conductivity of a solution of SDS is given by Figure 1. Representation of surfactant adsorption on a composite particle of PMMA and polystyrene.
the fraction of saturation on the particle surface is a function of the fraction of saturation in the water. This is given by
yi ) g(aw) ) f -1(aw)
(6)
This result shows that the fraction of saturation on a polymer surface, yi, is determined by the activity of the surfactant in the water phase, and this is known as the adsorption isotherm. As an example we can take a latex with two different polymer surfaces present (N ) 2), as is the case with all the latex blends studied in this work. Based on this result, it should be possible to predict the total amount of surfactant present in a composite latex of two polymers, P1 and P2, as a function of the surfactant concentration in the water phase based on (1) the total surface area of P1 in the latex, (2) the total surface area of P2 in the latex, and (3) the adsorption isotherms for the individual latices. This should also be possible for latices with any number of different polymer surfaces present. We have used a latex with only two polymer surfaces as an example, and this is the case for all of the latex blends studied in this work. As depicted in Figure 1 the more polar polymer should adsorb less surfactant than the less polar polymer at a given surfactant concentration in the water. In fact this idea has been used to estimate composite particle morphology by titrating with sodium dodecyl sulfate (SDS) and relating the amount of SDS adsorbed at saturation to the relative surface areas of the two polymers in the composite latex.7 Perhaps the most interesting aspect of this analysis is the possibility to study the competitive adsorption of surfactant on two polymer surfaces in a composite latex with a certain particle morphology by monitoring the surfactant concentration in the water phase. However, the difficulty involved with actually using a composite latex for these studies is that it is very difficult to obtain an accurate measure of the particle surface area that is made up of each of the two polymers. We propose that a blend of two single polymer latices should mimic the behavior in a composite latex while allowing control over the total surface area of each polymer in the blend. Experiments were performed to test the hypothesis that the packing on each polymer surface in a composite latex or latex blend is a function only of the surfactant concentration in the water at the temperature of the experiment. (7) Okubo, M.; Yamada, A.; Matsumoto, T. J. Polym. Sci.: Polym. Chem. Ed. 1980, 16, 3219.
+
-
λ ) Λ0Na [Na+] + Λ0ds [ds-] + λx
(7)
where λ is the total conductivity, Λ0Na+ and Λ0ds- are the equivalent ionic conductivities at infinite dilution for the sodium ion and the dodecyl sulfate ion, and [Na+] and [ds-] are the sodium and dodecyl sulfate concentrations, respectively. The last term in eq 7, λx, refers to the contribution to the conductivity from all other ions that may be present in the water such as H+ or OH-. We have dealt with this term by expressing conductivity as λ - λ0, where λ0 is the conductivity of the water or latex at an SDS concentration of zero. Since λ0 is equivalent to λx, it should remain constant throughout the titration. It is also very small to begin with due to the low concentrations of H+ or OH-. From eq 7 it is clear that for a titration of pure water with SDS the conductivity should increase linearly with the surfactant concentration until the cmc is reached. At this point the dodecyl sulfate molecules will start to form micelles, and these will contribute negligibly to the conductivity due to their larger aggregate size and decreased mobility. Above the cmc the increase in conductivity with increasing SDS concentration will be almost entirely due to the sodium ions which will all be in the water phase. Therefore, a plot of the conductivity versus concentration will show a change in slope as the cmc is crossed. This allows the cmc to be determined by monitoring the conductivity in a titration with ionic surfactant. For a titration of latex with ionic surfactant the situation is slightly more complicated. Here it is assumed that the surfactant that is adsorbed on the particle surfaces does not contribute to the conductivity due to the very large size of a particle on a molecular scale. Therefore, the conductivity in a latex titration with SDS will be due to the total concentration of sodium ions added to the latex plus the concentration of only the “free” surfactant in the water phase. In a latex the conductivity will be given by +
λ ) Λ0Na [Na+] + Λ0ds [ds-]w + λx -
(8)
where [ds-]w is the concentration of dodecyl sulfate that is free in the water phase, which will be less than the total amount of SDS present in the latex. To describe the conductivity in a latex titration it is therefore necessary to have knowledge of the adsorption isotherm of the surfactant on the polymer of interest. Combining the isotherm with the mass balance: (8) Paxton, P. R. J. Colloid Interface Sci. 1969, 31, 19. (9) Piirma, I.; Chen S. J. Colloid Interface Sci. 1980, 74, 90. (10) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (11) Shanks, P.; Franses, E. J. Phys. Chem. 1992, 96, 1794.
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[ds-] ) [ds-]w + [ds-]p
Stubbs et al.
(9)
allows one to determine [ds-]w and [ds-]p, where [ds-]p is the concentration of dodecyl sulfate that is present on the surface of the particles. To describe the adsorption on a polymer surface we have assumed a Langmuir-type isotherm12 of the form
( ) ( ) ( )
[ds-]w b 1 cmc X ) 1 + sat b X [ds-]w 1+b cmc
(
)
(10)
where b is a coefficient related to the ratio of the rate coefficient for adsorption to the rate coefficient for desorption. The term (1 + 1/b) was included so that the surface would be saturated at the same point that the cmc was reached in the water. Conceptually, the value of b for a particular polymer can be determined by adjusting b to obtain the best agreement between the predicted and experimental graphs of conductivity versus SDS concentration from titrations of the homopolymer latex with SDS. In practice we have found that the accurate determination of b using the conductivity method is rather difficult, as described later. For a titration of a latex, as with a titration of water, there will be a change in the slope of the plot of conductivity versus SDS concentration at the point where the polymer surface is saturated, since after this point all of the dodecyl sulfate will go into micelles. The amount of surfactant required to saturate a latex, combined with knowledge of the cmc in the water, the particle size, and solid content allows the value of As to be determined. Since this value is determined by a single point where the latex is saturated, it is independent of the shape of the isotherms and can be determined accurately even if the isotherms are subject to uncertainty. Experimental Section All latices were produced by emulsion polymerization at 70 °C with potassium persulfate (Acros Organics) as initiator and sodium dodecyl sulfate, 99% pure, as surfactant (Acros Organics). Four monomers were used: styrene (Aldrich), butyl methacrylate (Polysciences), methyl methacrylate (Aldrich) and methyl acrylate (Acros organics). Inhibitor was removed by distillation. The latices were made with diameters less than 100 nm in order to maximize the surface area for adsorption of surfactant. Particle size was measured by QUELS using a Coulter Nanosizer, and solid content was determined by gravimetry. Surfactant and residual initiator were removed from the latices by repeatedly passing them over anionic and cationic resins (Barnstead/Thermolyne Corp.) until the conductivity of the latex no longer decreased with successive passes over the resins. The latex was then neutralized to the isoelectric point by adding 0.01 M NaOH. Titrations were conducted by adding an SDS solution of 50 g/L to the cleaned latex at a rate of 6.7 mL/h using a motordriven syringe. The latex was contained in a jacketed beaker with stirring and the temperature held constant at 20 °C. Before titrating, the latex was purged with nitrogen to remove CO2 and O2, which could form complexes and may affect the conductivity. The conductivity of the latex was monitored over the duration of the titration using an Accumet model 50 pH/ion/conductivity meter connected to a PC, with data points being recorded every 2 s. Titrations usually took 1-2 h to complete depending on the solid content of the latex and the type of polymer. (12) Jirgensons, B.; Straumanis, M. E. 2nd ed.; The Macmillan Company: New York, 1962; p 78.
Figure 2. A typical titration curve for a polystyrene latex.
Figure 3. Example of a titration of deionized water with SDS. Table 1. Particle Sizes and Solid Contents for Latices Studied latex
PS
PBMA
PMMA
PMA
diameter (nm) solid content (%)
86 4.28
90 5.51
97 3.93
94 4.28
Results and Discussion Latex Titrations. Polymer latices of poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), poly(n-butyl methacrylate) (PBMA), and polystyrene (PS) have been studied. Particle sizes and solid contents of these latices are given in Table 1. A typical titration curve is shown in Figure 2 for a PS latex. We have used the square root of the SDS concentration as the x axis variable to make the change in slope at saturation more pronounced. Titrations were also performed for SDS in pure distilled, deionized (DI) water plotted as normalized conductivity versus the SDS concentration. The normalized conductivity is given by
λn )
λ - λ0 λcmc - λ0
(11)
where λ is the conductivity and the subscripts n, 0, and cmc refer to the normalized conductivity, the conductivity at a surfactant concentration of zero, and the conductivity at the cmc, respectively. An example of a water titration is shown in Figure 3. Two pieces of information were obtained from this water titration and later used to fit the latex titrations. The first was the value of the cmc of SDS in deionized water, which was determined to be 2.43 g/L (0.0084 M) at 20 °C. The second was related to the curvature of the graph in Figure 3. On the basis of eq 7, one would expect this graph to be linear. However, Figure 3 shows that it is not perfectly linear, although it is very close. The reason for this slight nonlinearity is not clear.
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Table 2. Adsorption Areas per Molecule of SDS at Saturation on Different Polymers polymer PS PS (average) PBMA PBMA (average) PMMA PMMA (average) PMA PMA (average) PBA PEMA PEA PVAc
χ (decane/ polymer)14 1 1.17 2 2.5 1.2 1.3 1.6 2
One possibility is that the dodecyl sulfate ions start to partially associate with each other as the cmc is approached, and since the size and mobility of two or more ions associated with each other is less than if they were free, the conductivity would increase less rapidly. Other possibilities are experimental difficulties such as slight errors in the conductivity and, possibly, variation of this error as the magnitude of the conductivity increases. Nevertheless, this slight curvature was accounted for by fitting the line in Figure 3 to a function that allows the conductivity of the dodecyl sulfate to be a slightly nonlinear function. The conductivity results for the single polymer latex titrations were fit by assuming an initial value of b in eq 9 and combining this with the mass balance to determine the concentrations of the SDS on the particles and in the water as a function of the total SDS concentration during the titration. This was done using the cmc determined from the pure water titration and with the conductivity of the dodecyl sulfate being described by the slightly nonlinear function determined from the pure water titration. Setting the cmc determines As through the mass balance, since the particles become saturated at the same point that the cmc is reached in the water. Equation 8 was then used to calculate a predicted conductivity as a function of the total SDS concentration during the titration. The predicted and experimental conductivities (both normalized) were then compared, and the value of b was varied so as to minimize the sum of the squares of the error. The values for As determined for several titrations of each latex are shown in Table 2 along with various values for different polymers reported by Vijayendran,13 which were compiled from various sources. It is clear that the adsorption area per molecule of SDS increases with increasing polymer polarity. The values we have obtained for both PS and PBMA agree very well with those reported by Vijayendran.13 Those reported for PMA in the literature varied widely between 146 and 175 Å2/molecule. The values we have obtained for PMA, 167.8 and 158.7 Å2/ molecule, lie right between the literature values with much better consistency. The value of 79 Å2/molecule reported for PMMA is much lower than we have obtained (94 and 97.9 Å2/molecule for PMMA). However, the values we have determined are reproducible and seem to agree better when comparing to the other polymers and considering their relative polarities. The Flory-Huggins interaction parameters (χ) for decane against the different polymers14 are shown in Table 2, and we have used them to correlate the adsorption areas of the different polymers as shown in Figure 4. The χ parameter for PMMA is closer to that for poly(vinyl acetate) (PVAc) or PMA than it is to PS or (13) Vijayendran, B. J. Appl. Polym. Sci. 1979, 23, 733.
As, this work (Å2/molecule)
As, lit.13 (Å2/molecule)
41.4, 42.4. 44.2, 44.7, 47.1 44.0 60.7, 56.4 58.6 94, 97.9 96.0 167.8, 158.7 163.3
49, 45 47 54, 58.5 56.25 79 79 175, 146 160.5 62, 56 77 86 110
Figure 4. Correlation of adsorption areas for SDS on different polymers as a function of the Florry-Huggins interaction parameter between the polymer and decane.
PBMA, so one would expect it to have an adsorption area closer to PVAc or PMA. The values we have obtained for PMMA are closer to those for PMA or PVAc than the literature value. This combined with the reproducibility of the values leads us to the conclusion that the values we have obtained for PMMA are more likely to be accurate than those reported by Vijayendran.13 Although Figure 4 shows significant scatter in the data when plotted versus the χ parameter, it shows that the adsorption area does increase as the χ parameter increases and thus provides a way to obtain a rough estimate for the value of As of SDS on a certain polymer if it is not otherwise available. It is also noted that there will be sulfate end groups on the polymer chains derived from the potassium persulfate initiator, some of which will be present on the particle surface,15 and that these may affect the adsorption characteristics of the SDS. We minimized this effect by using low concentrations of potassium persulfate in the polymerizations (less than 0.5 g/L). The concentration of end groups on the surface of the particles was measured for each latex by titrating the fully ion exchanged latices with NaOH until the isoelectric point was reached. By this method the charge densities of all the latices were measured in the range of 0.3-2.7 µC/cm2, from which we can calculate packing densities for the end groups. If we assume that the effectiveness of sulfate end groups on the surface is the same as an SDS molecule on the surface, then it follows that the true packing density of SDS on the “clean” polymer surface (no end groups) is the sum of the measured packing density of the SDS molecules plus the packing density of the end groups. We can use this idea (14) Barton, A. F. M. Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters; CRC Press: Boca Raton, FL, 1990. (15) Durant, Y. G. Ph.D. Thesis, Universite Lyon Claude Bernard, France, 1994.
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to estimate the effect that the end groups may have on the adsorption of SDS. Upon doing this we have determined that the error experienced by neglecting the effect of the end groups is in the range of 1.6-7.0% for all of the latices investigated. From Table 2 it is clear that this is comparable to the experimental error in the measurements, so it is judged that it is a reasonable approximation to neglect the effect of initiator end groups on the polymer surface. Furthermore, the experimental values in Table 2 agree well with values reported in the literature13 even though no attempts were made there to produce latices with similar charge densities in the different experiments. This further suggests that the effect of the end groups on the adsorption of SDS should be a minor one. When fitting the experimental data using eqs 8, 9, and 10, it was possible in all cases to obtain good agreement between the predicted and experimental normalized conductivities using positive values of b. However, it was not possible to obtain reproducible values of b for different titrations of the same latex. The reason for this appears to be due to the nature of the conductivity titration as a method to study the adsorption of SDS on polymer latices. To determine the isotherm, one needs to measure the partitioning of the dodecyl sulfate between the water, which contributes to the conductivity, and on the surface of the particles, which does not contribute to the conductivity. The equivalent ionic conductances at infinite dilution, Λ0, of sodium and dodecyl sulfate are 50.1 × 10-3 m2 S/mol and 24 × 10-3 m2 S/mol, respectively.16 Therefore, even if all of the dodecyl sulfate was in the water (none on the particles), it will only be responsible for one-third of the total conductivity of the latex. Since much of the dodecyl sulfate will be on the surface, it will actually contribute even less to the total conductivity. If the adsorption isotherm (yi vs aw) was such that it was slightly above the diagonal (diagonal represents equal fractions of saturation on the surface and in the water at all times), then the change in the conductivity is likely to be only a few percent more than what it would be if the isotherm did lie on the diagonal. Essentially, for a titration of a latex with SDS, the small effect on the conductivity due to the partitioning of the dodecyl sulfate between the surface and the water is hidden by the presence of higher concentrations of more conductive sodium ions in the water phase. Therefore, even the smallest amount of error or variation in the conductivity measurements make accurate and repeatable measurements of b difficult. Due to this problem, the actual adsorption isotherms of SDS on the different polymers have not been reported here. Fortunately, the measured As values are not subject to this problem since they are taken from the single point where there is a change in the slope of the titration curve, corresponding to the saturation point, which can be accurately determined. Latex Blends. Latices of two different types of polymers (referred to hereafter as P1 and P2) were blended in order to give a desired fraction of the total particle surface area that is P1 or P2 (referred to hereafter as surface fraction). The blends that were studied were PS/PMA, PS/PMMA, PS/PBMA, and PBMA/PMA. The PMA/PS and PMMA/ PS systems represent polar/nonpolar pairs and the PS/ PBMA represents a pair with similar polarities. The PMA/ PBMA system was chosen because it represents a polar/ nonpolar pair of two acrylic polymers in order to study at least one system that did not include PS. In each system blends were made at various surface fractions, and titrations with SDS were performed on these blends. (16) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley: New York, 1989.
Stubbs et al.
Figure 5. Surfactant packing at saturation, 1000/As, for blends of PMA and PS plotted versus the surface fraction of PMA.
Figure 6. Surfactant packing at saturation, 1000/As, for blends of PMMA and PS plotted versus the surface fraction of PMMA.
Figure 7. Surfactant packing at saturation, 1000/As, for blends of PBMA and PS plotted versus the surface fraction of PBMA.
Blends Saturated with SDS. If in a blend of two latices the packing of surfactant on one polymer surface is independent of the presence of the other polymer, one would expect the average number of molecules per particle surface area in the blend to be a linear function of the surface fraction of the two polymers. These results for the blend titrations are shown in Figures 5-8, plotted as the number of molecules per 1000 Å2 versus the polymer surface fraction in the blends. Clearly these plots are indeed linear, supporting the hypothesis that adsorption on P1 is not affected by the presence of P2. These results are particularly conclusive since they are obtained for latices at saturation and are therefore not dependent on the uncertainty involved with determining the adsorption isotherms.
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for the blend titration. This analysis was performed for numerous blends, and Figure 9 is representative of the results obtained. In many cases the agreement between experiment and prediction was much better than that for the example in Figure 9, with almost no difference between the experimental and predicted lines, and in some cases the difference was slightly larger than that in Figure 9. These additional graphs are not included here for the sake of brevity. Conclusions and Recommendations
Figure 8. Surfactant packing at saturation, 1000/As, for blends of PMA and PBMA plotted versus the surface fraction of PMA.
Figure 9. Predicted vs experimental titrations for a 50/50 surface fraction PMMA/PS blend.
Predicting Blend Titrations. For each blend it is possible to make predictions for the conductivity as a function of the total SDS concentration in the latex during a titration based on the individual latex titrations, using eqs 8, 9, and 10. There will be some error inherently involved in these predictions since the values of b determined from the pure latex titrations are not reliable. However, since the predicted conductivity of a latex is relatively insensitive to the value of b as discussed previously, this error is not large. It turns out that the most important information required to predict the conductivity as a function of SDS concentration for a latex blend titration is (1) the cmc in the water, (2) the values of As for both P1 and P2, and (3) the particle sizes, solid contents, and weight ratios of the two latices in the blend. Since all of these values are obtained independent of b and are accurately known, prediction of the latex blend titrations was possible. An example of these predictions is shown in Figure 9 for a 50/50 surface fraction blend of PMMA and PS. Lines on the figure depict predictions for the concentration of SDS that is present on the PMMA surface and the PS surface and in the water as a function of the normalized conductivity, based on the individual isotherms. The line labeled “predicted” is the sum of these three concentrations to give a prediction for the total concentration of SDS in the latex blend versus the normalized conductivity. This line can then be compared to the result of an actual titration of the latex blend, i.e., the line labeled “experiment”. Clearly, good agreement is obtained between the predicted and experimental results
It is clear that the adsorption area of SDS on polymer particles increases with increasing polymer polarity. The amount of SDS that is adsorbed on the surface of a polystyrene particle at saturation is more than three times the amount that is adsorbed on a PMA particle. Importantly, our experiments have also confirmed that the adsorption of surfactant on one polymer surface in a latex blend of two polymers is not affected by the presence of the other polymer. The packing of surfactant molecules on the polymer surfaces at saturation in a latex blend is a linear function of the polymer surface fraction. It is also possible to predict the conductivity of a surfactant titration of a latex blend based on titrations of the individual latices. While it is clear that using conductivity to monitor titrations of latex with SDS is an effective method to determine adsorption areas at saturation, it does not seem to yield high-quality adsorption isotherms. This is because the conductivity of a latex is dominated by the sodium ions from the SDS, while the ions of interest are those of dodecyl sulfate. A better method to determine adsorption isotherms may be to monitor the surface tension of the latex, since this is dependent on the dodecyl sulfate ion accumulation at the latex/air interface and not on the sodium ion concentration in the water phase. These results for both the adsorption areas of SDS on polymers and the information about the partitioning of surfactant between two polymer surfaces in a latex are useful in a variety of ways. One of these is to provide better estimates of the surfactant coverage on two polymer surfaces in a composite latex in order to obtain better estimates of the polymer/water interfacial tensions. This provides information essential for predicting the equilibrium morphology of the composite particles.1-4 The results also allow for more accurate calculation of surfactant loadings required to give a specified coverage on particles or a desired concentration in the water phase. This is useful in almost all areas of emulsion polymerization, including effects on nucleation, secondary nucleation in seeded polymerizations, and colloidal stability of latices. Further work is currently being conducted to investigate the effect of monomer concentration in a latex particle (conversion level), electrolyte concentration, and temperature on the surfactant adsorption. Acknowledgment. We are thankful for financial support from the members of the University of New Hampshire Latex Morphology Industrial Consortium (BASF, DSM Research, Elf Atochem, ICI Paints, Mitsubishi Chemical, Wacker-Chemie, Zeneca Resins). We would also like to thank Doris Garvey for performing much of the early work toward developing a reliable procedure for the conductometric titrations LA981410S
