1088
Langmuir 1990, 6, 1088-1093
Microelectrophoresis as a Probe of the Surface Charge of an Expandable-Layer Latex A. A. Morfesist and R. L. Rowell' Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003 Received January 15, 1990 Measurements of the electrophoretic mobility of an expandable-layer latex have been carried out as a function of pH and pX, the negative log of the specific conductance. We find a self-consistent mobility surface (SCMS) which is a three-dimensional representation of the measured mobility as a function of the measured pH and pX. The SCMS can be used to obtain mobility profiles along any path in the pH-pX domain. Also, in a phenomenological approach, the mobility data are combined with earlier independent measurements of hydrodynamic radius to display an electrophoretic titration of the system giving end points that are sensitive to added acid and electrolyte.
Introduction Carboxylic acid comonomers such as acrylic acid have been included in latex polymerizations to provide specific properties of mechanical stability to latex particles and latex films. The copolymerization process increases stability, improves adhesion properties, provides functional crosslinking sites for interparticle thermosetting reactions, and controls the viscosity of the latex formulations. Copolymerization makes the latexes more useful in a wide range of industrial applications, especially as adhesives and coatings on Through control of the means of acid addition during the polymerization, the location of the acid groups and therefore the surface characteristics of the particles are i n f l u e n ~ e d . ~The , ~ incorporation of acrylic acid monomers is carried out so that the ionization of the carboxylic acid groups will increase the surface charge of the particles and increase the electrostatic repulsion forces to produce stable suspensions.'+12 Surface layers, whether adsorbed, solvated, or ionic, are important in controlling the stability and rheologi+ Present address: PPG Fiber Glass Research Center, P.O.Box 2844, Pittsburgh, PA 15230. (1)Bassett, D. R.; Derderian, E. J.; Johnston, J. E.; MacRury, T. B. In Emulsion Polymers and Emulsion Polymerization; Bassett, D. R., Hamielec, A. E., Eds.; ACS Symposium Series 165; American Chemical Society: Washington, DC, 1981; p 263. (2) Fordyce, D. B.; Dupre, J.; Toy, W. Ind. Eng. Chem. 1959,51, 115. (3) Muroi, S.;Hosoi, K.; Ishikawa, T. J. Appl. Polym. Sci. 1976, 11, 1963. (4) Muroi, S. J. Appl. Polym. Sci. 1966, 10, 713. (51 Verbrupee. C. J. J. ADDLPolvm. Sci. 1970. 14.897. (6) Verbruiie; C. J. J. A y p l . PoGm. Sci. 1970,141 911. (7)Nishida, 5.; El-Asser, M. S.; Klein; Vanderhoff, J. W. In Emulsion Polymers and Emulsion Polymerization; Bassett, D. R., Hamielec, A. E., Eds.; ACS Symposium Series 165; American Chemical Society: Washington, DC, 1981; p 291. (8) Bassett, D. R.; Hoy, K. L. In Emulsion Polymers and Emulsion Polymerization; Bassett, D. R., Hamielec, A. E., Eds.; ACS Symposium Series 165; American Chemical Society: Washington, DC, 1981; p 371. (9) Johnston, J. E.;Bassett, D. R.; MacRury, T. B. In Emulsion Polymers and Emulsion Polymerization; Bassett, D. R., Hamielec, A. E., Eds.; ACS Symposium Series 165; American Chemical Society: Washington, DC, 1981; p 389. (10) Overbeek, J. Th. G. In Colloidal Dispersions; Goodwin, J. W., Ed.; Special Publication No. 43, The Royal Society of Chemistry; Burlington House: London, 1981; p l. (11) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (12) Buscall, R.; Ottewill, R. H. Polymer Colloids; Buscall, R., Corner, T., Stagman, J. F., Eds.; Elsevier Applied Science: London, 1985; p 141.
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cal properties of colloidal systems. Latex particles and other materials acquire surface electric charge when brought into contact with a polar medium by ionization, ion adsorption, or ion dissolution processes.13 In the case of acrylic acid latex particles, the particle surface charge is derived from the dissociation of the carboxylic acid groups. The surface charge is intentionally created by adding acrylic acid during the emulsion polymerization procedure to improve the bulk stability of the suspension. The consequences on the molecular level can be understood by observing particle diameter, viscosity, and surface charge changes while the acrylic acid layer expands with increasing pH. It has been shown in previous work that the low acid acrylic acid latexes increase in hydrodynamic size with increasing pH14J5 and show a maximum around pH 10. Past work supports the accepted mechanism of electrosteric stabilization.lJl As the charge of acrylic acid chains increases with increasing pH, the molecular chains repel each other and begin to expand. The expansion layer has a refractive index very close to that of water and is probably made up partially of bound water. Therefore, the surface charge density is a variable quantity in this system. In this work, we used the measured electrophoretic mobility in dilute solution as a probe of the hydrodynamic surface electrical charge. Our intent was to use electrophoresis to study aging. To our surprise, we found that aging effects were small and independent of pH. We also found two new interesting ways to interpret electrophoretic data. First, we found that the electrophoretic mobility should be represented as a function of two characteristic state variables: pH and PA, a new index, which is defined as the negative log of the specific conductance (S/m). The second point was that a t the extreme dilutions and rapid measurements, the dispersed system could be measured without coagulation occurring. Because of that, the data could be combined with measurements of hydrodynamic particle size and plotted to give a sensitive electrophoretic titration curve. (13) Shaw, D. J. Electrophoresis; Academic Press: London, 1969. (14) Ford, R. J.; Morfesis, A. A.; Rowell, R. L. J. Colloid Interface Sci. 1985, 105 (20), 516. (15) Ford, J. R. Wide Angle Light Scattering Study of the Internal Structure of Expandable Layer Latices. Ph.D. Dissertation, Chemistry Department, University of Massachusetts, Amherst, May 1984.
0 1990 American Chemical Society
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Microelectrophoresis as a Probe of Surface Charge
Experimental Section Materials. An acrylic acid polymer latex, 28BRD26, from Union Carbide was studied. The latex was prepared1 by a semibatch technique in which a monomer mix was continuously fed into a stirred reactor at 80 "C in the presence of ammonium persulfate and an anionic surfactant, Aerosol OT. The standard polymer composition was 40 parts methyl methacrylate, 58 parts ethyl acrylate, and 2 parts acrylic acid. The polymerization took place in two stages. Stage I included the above ingredients, and stage I1 included the addition of acrylic acid. Preparations, The latex was diluted for measurement by using doubly distilled water. The optimum concentration was found to be 1 pg/mL, where the turbidity was linear with concentration.16 After dilution and adjustment of pH with NaOH, latexes were equilibrated for 24 h before measurements were obtained. Solutions prepared independently and measured on different days gave self-consistent results. Immediately after each mobility measurement was made, a pH measurement was made on the solution which had just been measured. Ionic strength was adusted by diluting the latexes with 1 mM NaCl, 10 mM NaCl, and 10 mM KCl. The pH of the solutions was adjusted by using NaOH in the NaCl solutions and KOH in the KCl solutions. The equilibration time of 24 h was chosen as an optimum amount of time by diluting latex 28BRD26 solutions to 1hg/mL and watching the pH drift to quasi-equilibrium as in earlier work.16 Measurements. The electrophoretic mobility, UE, was measured with a commercially available PenKem System 3000 automated electrokinetics analyzer.l6 The instrument illuminates particles with a 2-mW He-Ne laser. The sample chamber is an 8-pL (1-mm-diameter by 10-mm-long capillary) silica sample chamber with plated palladium electrodes at each end. The chamber is a circular capillary cell mounted horizontally inside a constant-temperature water bath, which for the measurements was maintained at 25 OC (rt0.l "C). The He-Ne laser that illuminates the cell is focused into a sheet of light which illuminates only those particles in the plane in which the microscope is focused. In an actual measurement, the particles move back and forth, changing direction approximately every 10 s, as the potential across the electrodes is reversed. The light scattered from the particles is passed through a rotating grating and focused on a photomultiplier which generates an electric signal which is fed to a frequency tracker. The frequency shift of the light passing through the cell is a function of the electrophoretic mobility, the microscope magnification, and the periodicity of the grating. The frequency spectrum of many particles is obtained by a real time fast Fourier transform analyzer which is capable of averaging from 2 to 1024 spectra in order to obtain a representation of the electrophoretic mobility distribution. The following measurements were an average of 32 Fourier transformed spectra. This was the smallest number of spectra that could be averaged to obtain a signal to noise ratio of 3:l. The value of specific conductance (S/m) is obtained from the PenKem 3000 on each measured sample. The PenKem 3000 applies a square-wave current and measures the voltage across the electrodes to obtain the specific conductance values. The electrode separation value is calibrated by using 10 mM KCl solution to set the internal instrument constant. The data that are presented here were obtained by making four independent measurements of each sample preparation and averaging the average mobility measurements that were obtained. Since the PenKem 3000 instrument calculates specific conductivity for each sample being measured, the conductivity data also were an average of four independent measurements. The scatter in the data was small, comparable in size to the symbol used in the graphs, so that the averaging procedure was justified. All measurements on the latex system without indifferent electrolyte also being added are defined as system-limited ionic strength, SLIS, measurements. Since no indifferent elec(16)The PenKem 3000 Automated Electrokinetics Analyzer Reference Manual; 1983; PenKem Inc., 341 Adams St., Bedford Hills, NY. Gcetz, P. J. Automatic Electrophoresis Apparatus. US.Patent 4,154,669, May 15,1979.
250
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Figure 1. Comparison of the core diameter of latex 28BRD26 from angular intensity light scattering (open triangles) with the hydrodynamic outer diameter from photon correlation spectroscopy (shaded circles), sedimentation (open circles), and viscosity (shaded triangles).
trolyte was added, the ionic strength of the system was controlled by the particles themselves and the base, NaOH, which was used to adjust the pH.
Results and Discussion pH Dependence. In the previous paper, angular intensity light scattering (AILS), photon correlation spectroscopy (PCS), and sedimentation measurements were compared to study the expansion mechanism of the acrylic acid latex 28BRD26.14 A comparison of that data with viscosity measurement^,'^ shown in Figure 1,further corroborates that there is an expandable layer which increases with pH until a maximum at approximately pH 10 occurs. It is important to note that the viscosity data shown in Figure 1 were carried out at a much higher particle concentration than either the present electrophoresis work or our previous particle size work.14J5 In the electrophoresis work, the viscosity of the system was constant and essentially that of the viscosity of pure water. This was so because we made measurements at very low mass concentration, where the volume fraction of the dispersed phase was of the order of lo+ and the mean separation between particles was on the order of 80 core diameters. The particle size measurements by AILS were carried out at still lower concentration as discussed below. Potentiometric titrations of the latex and comparison systems were obtained from Union Carbide17 and are shown in Figure 2. The titration of the 2% acrylic acid latex acts as a strong buffer so that the system does not reach its pK, until pH 10. The new titration data also support the expansion mechanism that the carboxylic acid groups dissociate with increasing pH until a maximum dissociation is reached. An increase in pH beyond the maximum increases the shielding of the interparticle electrostatic repulsion. The increased shielding gives a relaxation in hydrodynamic size at pH 10 as shown in Figure 1. Therefore, the equilibrium of the carboxylic acid ionization is important to the delicate balance of expansion and compression in this system. Electrophoretic mobility measurements were carried out in order to obtain results sensitive t o the changes in the surface charge density of the expandable-layer latex. System-limited ionic strength conditions (SLIS) were used in order to compare these results to previous light-scattering measurements. The concentration used in these (17) Bassett, D. R., private communication.
Morfesis and Rowell
1090 Langmuir, Vol. 6, No. 6, 1990
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Figure 2. Potentiometric titrations of latex 28BRD26 along
> t
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PH Figure 3. Average electrophoretic mobility (10s X SI units of m2/(V-s)) as a function of pH for latex 28BRD26 over a 3-year period: June 1983 (squares),May 1985 (circles),and March 1986
(triangles).
experiments was 1pg/mL (1 x lO-4% solids). This concentration was chosen because it fell within the linear region of turbidity as a function of concentration obtained by the PenKem 3000. Ford showed15 that there were two linear regions obtained from AILS. One linear region occured in the Rayleigh regime below 2 X lo4% solids, which accounts for molecular scattering. The other region occurred between 4 X lo4% and 3.2 X lo+% solids in the Mie regime, where particle scattering was prevalent. The electrophoretic measurements were 1order of magnitude higher than the AILS high linear regime. Therefore, our measurements detected particle scattering and were free of small molecule scattering. The electrophoretic mobility was measured a t SLIS of latex 28BRD26 as a function of pH. The results of the average mobility as a function of pH are summarized in Figure 3 for 47 independent preparations. Although there seems to be a great deal of scatter in these individual experiments, there are also definite trends in the data. Overall, the mobility decreases as the pH increases. There appears to be a slight leveling off of the slope of the mobility change a t the extremities of the pH range examined except for the jump in the May 1985 data a t very low pH. In a given run, the general trend of a decreasing mobility with increasing pH persists even with 13 independent preparations spanning 7 orders of magnitude in pH. The low-pH May 1985 data illustrate the large sensitivity of the electrochemistry of the particle surface at
Figure 4. Average electrophoretic mobility (10sX SI units of m2/(V-s)) as a function of pH for latex 28BRD26 in 10 mM NaCl (squares) or 10 mM KC1 (triangles).
low electrolyte concentration. We show below that aging effects were less significant than electrolyte concentration and the method of data analysis. It is difficult to discern the aging effects shown in Figure 1, but in general one can note that the mean mobility did decrease somewhat with time, which would be expected from a charged system. The aging effects become clearer when the data are interpreted as an electrophoretic titration below. The SLIS measurements indicate that the latex retains a negative charge throughout pH changes of 7 orders of magnitude. The particles therefore maintained a strong negative charge throughout the pH changes and never reached a point of zero charge. Such behavior corresponds to the potentiometric titration data for the 2% acrylic acid (AA) latex shown in Figure 2. pX as a State Variable. Mobility was also measured under higher salt conditions at near-constant ionic strength. The results in Figure 4 show mobility as a function of pH a t 10 mM NaCl and 10 mM KCl salt concentrations. The increase in ionic strength compresses the double layer and makes the mobility more sensitive to electrochemical changes on the surface. The sharp dependence of mobility on pH a t low pH explains the large jump in the May 1985 data of Figure 3. This data was obtained separately with two different salts, yet the data are selfconsistent. Striking differences in the mobility results are found from the SLIS case. The mobility increased sharply with pH until reaching a maximum at pH 5.4. The mobility then decreased sharply until approximately pH 7.5, where it remained constant at -2.5 X mz/ (V-s). The latex was also measured in 1 mM NaCl as shown in Figure 5. The trend is the same as in Figure 4, but the peak is broader. Figures 3-5 are the usual types of graphs prepared to display the mobility data. We show below that such graphs are special case profiles. In a broader understanding, we must recognize that mobility is in general a function of two variables: we shall take the acid-base variable as pH and in a similar fashion we shall take the electrolyte variable as pX, the negative log of the specific conductance (where specific conductance is in S/m). Figure 6 shows the locus of the data of Figures 3-5 in the pH-pX domain. The plot shows that the data a t high salt concentration were a t nearly constant pX while the data a t SLIS varied over many orders of magnitude of both pH and ph. The important point to note is that as the pH is changed in Figure 6, the conductivity, reported as ph, changes
Langmuir, Vol. 6, No. 6, 1990 1091
Microelectrophoresis as a Probe of Surface Charge
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Figure 5. Average electrophoretic mobility (1Oe X SI units of m*/(V.s)) as a function of pH for latex 28BRD26 in 1mM NaC1. O t
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greatly in the absence of added salt. This is shown by the 4 orders of magnitude change in ph under SLIS conditions (shaded circles, triangles, and squares). In many systems, it is desirable to measure the electrokinetic properties in the absence of salt, so this effect is very important. In fact, if the measurable electrophoretic mobility is to be used to explore the electrokinetic properties of an unknown system, it is necessary to measure the conductance in order to characterize the electrical state of the system. It is impossible to use the ionic strength because the ionic strength is a calculated quantity that requires a knowledge of the concentration and charge of each ionic species present. The use of pX to characterize the electrical state of the system is a logical complement to pH to characterize the chemical state of the system. The variables pH and pX are characteristic but not completely independent state variables. A further consideration of this point will be given later.ls Electrophoretic Topography. It is clear that the mobility can be more completely represented by topographic representations shown in Figure 7. In Figure 7a we show the topograph or three-dimensional representation of electrophoretic mobility as a function of pH and (18)Marlow, B. J.; Rowell, R. L., in preparation.
0
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Figure 6. Locus of the electrophoretic mobility data in the pH-ph domain for 10 mM KC1 (open triangles), 10 mM NaCl (open squares), 1 mM NaCl (open circles), and SLIS data of Figure 3 (shaded points); ph is the negative log of the conductivity in S/m.
2
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Figure 7. (a, Top) Mobility topograph giving mobility as a function of pH and pX for latex 28BRD26. (b, Bottom) Same data represented as an isornobility contour plot or electro-
phoretic fingerprint.
PA. The same data may also be represented as an isomobility contour plot. The contour plot has been termed a “fingerprint” by Marlow and Fairhurstl9 because it is a characteristic pattern. Both the topograph and the fingerprint are computer drawn from a sampling of mobility measurements in the pH-pX domain. Further discussion of the methodology is given in forthcoming publications involving the data-sampling schememand comparison with other approaches.18 Figure 7 gives an adequate representation of the three-dimensional interpolation of the data, but as noted in forthcoming work,18vmcertain roughly triangular regions a t low ph and the extremes of pH are experimentally inaccessible. The inaccessible high-pH domain is defined by the locus of the SLIS data shown in the pH-pX domain of Figure 6 and is easily seen to have no bearing on the theme of our presentation. The maximum in mobility as a function of pH shown in Figures 4 and 5 appears to undulate in both the pH coordinate and the pX coordinate as shown in the topography of Figure 7a and 7b. From the contour lines of the fingerprint in Figure 7b, we can pinpoint the peak mobility occurring a t the coordinates pH = 5.5 and pX = 0.90. From our analysis, we presume that the coordinates of maximum mobility and the detailed contour of the fingerprint in the pH-pX region of the maximum mobility (19) Marlow, B. J.; Fairhurst, D. Langmuir 1988, 4, 776-780. (20) Rowell, R. L.; Shiau, S. J.; Marlow, B. J., in preparation.
1092 Langmuir, Vol. 6, No. 6, 1990
Morfesis and Rowel1
are characteristic properties that may vary somewhat in different dispersions. Certainly, the topographic approach is a powerful tool to measure and compare such differences. The effect has had considerable consideration by several a ~ t h o r s , because ~ ~ - ~ ~classical double-layer theories predict only a continuous decrease in mobility with increasing ionic strength. Goff and L u n e P have fit the maximum with a three-parameter ion-exchange model while a theoretical treatment by Zukoski and Saville has introduced the dynamic Stern layer An experimental approach by Bijsterbosch et alaz2has suggested that the effect may be due to adsorbed polyelectrolyte. Our data are not inconsistent with any of the above. We would like to stress that the topographic or fingerprinting approach allows a self-consistent representation of the experimentally measured mobility in the experimentally measured pH-ph domain. From the selfc o n s i s t e n t m o b i l i t y s u r f a c e ( S C M S ) , o n e can mathematically take cuts at constant pH or along any desired path in the pH-pX domain.18p20 This is especially important in salt-free systems, where a change in pH by the addition of acid or base necessarily changes ph at the same time. To understand the system, one would like to have a mobility-pH profile at constant pX or a mobility-pX profile at constant pH to explore the effects of changing surface charge and changing double-layer thickness. The SCMS could be used to continue the study of work such as that of Zukoski and S a ~ i l l ewho , ~ ~reported mobility as a function of log [HCl]. The variable log [HCl] in the SCMS representation would follow a path in the pH-ph domain that would depend on the manner of addition of acid but in general would not be parallel to either the pH or pX coordinate. Electrophoretic Titration. The relationship between measurable quantities such as electrophoretic mobility U E or hydrodynamic radius ah and theoretical quantities such as the [ potential depends on the double-layer structure and has been approached from several theoretical points of view as reviewed by Hunter.26 We propose a phenomenological expression that is consistent with a general understanding of all of the available theories UE =
(QF/6raaJ(l+ S) (1) where Q is the total charge a t the hydrodynamic radius ah, 7 is the effective viscosity of the medium, S is a dimensionless double-layer structure factor, and F is a factor that includes both a constant of proportionality and the effect of double-layer changes. A t low concentration, 7 is approximately that of the solvent. The phenomenological approach allows separation of measurables such as ah and U E from unknowns such as Q , F , and S by simple rearrangement 6r7phuE= Q F ( 1 + S )
(2)
This differs from the classical treatment of electrokinetic properties,26 which preserves the ratio Q / a h , in essence, the { potential. In that case, interpretation of (21) Goff, J. R.; Luner, P. J . Colloid Interface Sci. 1984, 99, 468. (22) Bonekamp, B. C.; Alvarez, R. H.; De Laa Nieves, F. J.; Bijsterbosch, B. H. J . Colloid Interface Sci. 1987, 118, 366. (23) Zukoski, C. F.; Saville, D. A. J. Colloid Interface Sci. 1985, 107, 322. (24) Zukoski, C. F.; Saville, D. A. J. Colloid Interface Sci. 1986, 114, 32. (25) Zukoski, C. F.; Saville, D. A. J . Colloid Interface Sci. 1986, 114, 45. (26) Hunter, R. J. The Zeta Potential in Colloid Science; Academic Press: London, 1981.
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the [ potential requires the assumption of a model for the double layer. We have separated Q from ah so that the left-hand side of eq 2 contains measurable hydrodynamic transport properties while the right-hand side of eq 2 consists of the total charge Q and a modicum of structural parameters F and S in a form convenient for balancing dimensions and representing the structure and deformation properties of the double layer. We show below that this approach allows a simple interpretation in terms of titrating the total charge on the particle. The pH dependence of ah from previous work2’J4J7 was combined with the pH dependence of U E from the present work by using eq 2, and the quantity 6 q a h u ~ was plotted as a function of pH to obtain the striking results of Figure 8. The curvatures in the separate pH profiles of ah and U E become straight lines that are of the form of an electrophoretic titration at SLIS. Aging effects shift the ordinate in Figure 8 but have little effect on the end point at pH 9.5-9.7. The electrophoretic titration was carried out a t wt % solids whereas conductometric titrations are commonly carried out at 2% or higher solids concentration. The linear fits giving a titration-type curve are best over the pH range from 6 and higher shown in Figure 8. In fact, the data in Figure 8 below pH 6 for the squares and triangles could only be fit by a “turnover” of the curves. But we have shown in Figure 7b that a t pH 6 the contour lines are closely spaced giving a steep topography or sensitivity to pH. Moreover, we have noted above that the sensitivity to pH and pX is greatest around the peak mobility, which was found a t pH 5.5 and pX 0.90. That this is, in part, a sensitivity to ph is considered next. The electrophoretic titration is extremely sensitive to pX as shown in Figure 9, where a similar treatment gives an end point at pH 5.6 using either 10 mM NaCl or 10 mM KC1. Figure 9 shows data at a high concentration of added salt. The results obtained with either 10 mM NaCl or 10 mM Kcl are remarkably self-consistent. In 10 mM salt, the electrophoretic titration end point is much more like the behavior of acrylic acid shown in Figure 2. On the other hand, the titration end point a t pH 9.5-9.7 for no added salt (SLIS conditions) is what one would expect for an acrylic latex, 2% AA as also shown in Figure 2. Indeed, the conditions of added salt and acid (ph (27) Morfesis, A. A. M.S. Thesis, University of Massachusetts, Amherst, 1983.
Langmuir 1990,6, 1093-1098
1093
(3) The end point arises at different pH because different conditions of pX (ionic strength) determine the balance between changing a h and changing UE. (4) For our system, q was constant a t the value of pure water, but we have retained it to preserve the separation of variables which leads to the electrophoretic titration curves. It would be of interest to try to relate our results to the acidification of soap solutions, which generally show two breaks in pH titrations. The breaks depend on the chain length of the soap and are thought to arise from transitions between acid-salt complexes and free acid form^.^*-^^ It would also be of interest to apply the phenomenological approach to other systems to test the generality of the results.
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Figure 9. Electrophoretic titration of latex 28BRD26 in 10 mM NaCl (triangles)and 10 mM KC1 (circles);6 T I ) ~ U in E units of 1017 X N.m/V.
and pH) profoundly affect the surface behavior of this interesting expandable-layer latex. The general trends of the titration curves shown in Figures 8 and 9 make sense with three simple principles: (1)The low-pH side of the end point (decreasing curve but increasing 6a7)ahu~)arises because of an increasing a h that dominates the decreasing UE. (At high salt, ah is constant at low pH and the decreasing U E dominates.) (2) The high-pH side of the end point (increasing curve but decreasing 6aqahu~)arises because of the reverse case: the decreasing mobility dominates over changes in a h .
Acknowledgment. We are pleased to acknowledge support from the Union Carbide Corp., the Atlantic Research Corp., and Pen Kem, Inc. Registry No. (Methyl methacrylate)(ethylacrylate)(acrylic acid) (copolymer),25135-39-1. (28) Force, C. G.Charleston Research Center, Westvaco, North Charleston, SC., private communication. (29) Lucassen, J. J. Phys. Chern. 1966, 70,1824-1830. (30) Rosano, H.L.;Christodoulou, A. P.; Feinstein, M. E. J . Colloid Interface Sci. 1969,29, 335-344. (31) Young, S.L.;Matijevic, E.; Meites, L. J.Phys. Chern. 1974, 78, 2626-2631. (32) Mino, J.; Matijevic, E.; Meites, L. J. Phys. Chern. 1976,80, 366369.
Ionic Interactions of Fatty Acid Monolayers at the Air/Water Interface Mehran Yazdanian,+ Hyuk Y u , ~and George Zografi*lt School of Pharmacy and Chemistry Department) University of Wisconsin-Madison, Madison, Wisconsin 53706 Received November 1, 1989 The static properties of stearic and arachidic acid monolayers at the air-water interface in the presence of various divalent metal ions (Mg2+, Ca2+, Ba2+,Co2+,Cd2+, and Pb2+) were examined by use of the ionizing electrode method for the measurement of surface potential coupled with the Wilhelmy plate method of surface tension measurement. At pH 6.0, the presence of all the divalent cations results in the condensation of the monolayer, with Pb2+ and Cd2+ having the most significant effects. Moreover, only Pb2+ has such an effect at and below pH 5.0. The surface potential of stearic acid at pH 6.0 and 20 A2/molecule changes with the concentration of the metal ion in the subphase, and the magnitude and direction of this change are dependent on the type of the metal ion and its respective counterion. The surface potential profiles relative to the metal ion concentration are shown to be concave upward with a ranked order of Pb2+ > Cd2+ > Co2+, whereas those of alkaline earth metals are respectively convex upward, flat, and slightly concave upward for Ba2+,Ca2+,and Mg2+. It is concluded that monolayer ordering induced by covalent binding of carboxylate-metal ion proceeds in the order Pb2+ > Cd2+ > Co2+,whereas alkaline earth ions exhibit no such ordering effect since their interactions with carboxylates are mainly ionic in nature. Introduction Lanpu+Bldgett (L-B) films are highly ordered arrays of surface-active compounds extending out as multimolecular layers on solid substrates. Such films, although
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first produced and studied earlier in this century,’ recently have attracted considerable attention and assumed renewed importance because of their potential applications in thintechno10gy.2’3 L-B are formed by transfer-
0 1990 American Chemical Society
