5908
J. Phys. Chem. 1996, 100, 5908-5912
Dynamic Light Scattering Study of Nonionic Surfactant (C12E25) Adsorption on Polystyrene Latex Particles: Effect of Poly(ethylene oxide) Chain Size Jianxi Zhao and Wyn Brown* Department of Physical Chemistry, UniVersity of Uppsala, Box 532, 751 21 Uppsala, Sweden ReceiVed: October 11, 1995; In Final Form: December 12, 1995X
The adsorption of polyethylene glycol dodecyl ether (C12E25) on hydrophobic polystyrene latex particles of low surface charge density has been examined as a function of the surfactant concentration by determination of the particle hydrodynamic radius (RH) using dynamic light scattering and of the adsorption isotherms by a surface tension method. The adsorption is driven by hydrophobic interaction between the alkyl tails of C12E25 and the pendant polymer chains at the latex particle surface. The extended poly(oxyethylene) (POE) chains coil to form a hydrophilic surface around the latex particles. The conformation of the POE chains dominates the structure of the adsorbed layer, and this leads to similar adsorption behavior in both the RH trace and the adsorption isotherm, independent of added salt or the temperature.
Introduction The adsorption behavior of nonionic surfactants at a solid/ aqueous solution interface is influenced by various factors, one of which is the structure of the surfactant molecule. With a hydrophilic solid surface such as silica, the initial driving force for adsorption derives mainly from interaction between the POE chain of the nonionic surfactant and the surface. Increasing the length of the POE chain strongly influences the adsorption. Adsorption isotherm measurements by Bo¨hmer et al.1 displayed a transformation from the S-type (with C12E6) to the L-type (with C12E25). A similar phenomenon was also observed by Levitz et al.2 who studied the adsorption of the octylphenol poly(oxyethylene) family to a silica/aqueous solution interface. The S-type isotherm corresponds to the adsorption of surfactants with a shorter POE chain such as TX-100 (a member of the p-(1,1,3,3-tetramethylbutyl)phenyl) poly(oxyethylene) nonionic surfactant family, with an average of 9.5 oxyethylene (OE) units) and TX-102 (12.5 OE units), whereas L-type isotherms were observed in the cases of TX-305 (30 OE units) and TX-405 (40 OE units). This behavior suggests enhanced interaction between the surfactant molecules and the surface on increasing the contact area, since an L-type isotherm typifies strong interaction between adsorbate and adsorbent. However, it is expected that the adsorbed amount will decrease with increasing length of the POE chain as shown in refs 1 and 2. With a hydrophobic solid surface, on the other hand, for example, polystyrene latex particles, hydrophobic interactions between the alkyl tail of the surfactant and the surface have been suggested as the main driving force.3,4 In a previous paper,3 the influence on adsorption of different structures in the hydrophobic moieties of two surfactant molecules (C12E7 and Triton X-100) was examined and quite different adsorption behavior was observed. Thus an S-type (two-stepped) isotherm is found with TX-100, while C12E7 yields an L-type (singlestepped) isotherm. A layer of flexible polystyrene chains terminating in sulfate groups is considered to exist at the present latex surface,5 and surfactant adsorption will predominantly occur at these pendant polystyrene chains which sterically inhibit the surfactant molecules from close approach to the surface.3,4,6 Dynamic light scattering (DLS) measurements of the hydrodynamic radius show different characteristic changes in the X
Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-5908$12.00/0
extension of the surface hairy layer during adsorption. Bridging through the adsorbed C12E7 molecules can link neighboring polystyrene chains both through the alkyl tail and through the POE chain, leading to aggregation of the polystyrene chains and giving a strong initial decrease in RH. The TX-100 molecules, having a shorter molecular length due to the benzene nucleus and the branched hydrocarbon chain, tend to orient perpendicularly at the latex polymer chains, i.e., with their methyl end-groups in contact with the polystyrene chain and with the POE chains extending into the solution. This orientation favors interaction between the benzene nuclei of the adsorbed TX-100 molecules and leads to an initial extension of the polystyrene chains for steric reasons and an S-type adsorption isotherm as predicted by Giles et al.7 Kronberg et al.8 studied nonylphenol poly(oxyethylene) ether nonionic surfactants (NP-En) with 10, 20, and 50 OE units, respectively, adsorbing to polystyrene latex and poly(vinyl chloride) latex particles. They observed L-type isotherms in both cases. The adsorbed amount was found to decrease with an increasing number of OE units. The effect of temperature was examined by the same group for NP-E20, and enhanced adsorption was observed on increasing the temperature.9 Thermodynamic analysis of the above experimental data suggested that the driving force of adsorption is the weak surfactantwater interaction in the bulk solution compared to the surfactant-water interaction in the surface phase.10 Electrophoretic studies were made by Kayes11 of cetylpoly(oxyethylene) ether C16En having 10, 18, 30, 45, and 60 OE units, respectively, on polystyrene latex particles modified with carboxyl groups at the surface. The decrease in mobility was found to be proportional to the increase in length of the ethylene oxide chain. The purpose of this paper is to examine C12E25 adsorption on polystyrene latex particles having low surface charge density. The data are compared with the previous results3 for C12E7 adsorption on the same latex sample so as to elucidate the effect of the length of the POE chain. Experimental Section Materials. Nonionic polyethylene glycol monododecyl ether with 25 OE units (C12E25) was obtained from Nikko Chemicals, Tokyo. Its purity was checked by surface tension measurements of C12E25 aqueous solutions, and a semilogarithmic plot of surface tension versus C12E25 concentration is shown in Figure © 1996 American Chemical Society
C12E25 Adsorption on Polystyrene
J. Phys. Chem., Vol. 100, No. 14, 1996 5909
a
Figure 1. Semilogarithmic plot of surface tension (γ) versus C12E25 concentration without added salt at 25 °C and pH 6. The “knee” corresponds to the critical micelle concentration (cmc) of C12E25 in the solution. The curve between the two inflection points is typically used as a calibration for determination of the adsorption isotherms by measurements of surface tension by the drop volume method.
1. No characteristic dip prior to the “knee point” was observed which would indicate the presence of impurities. The critical micelle concentration (cmc) was determined as 4.0 × 10-4 mol/ dm3. The C12E25 was used without further purification. Water was Milli-Q grade from a Millipore apparatus. The latex was purchased from Serva AG, Heidelberg, Germany. This material was synthesized by emulsion polymerization as described in ref 14. It is known that there is a layer of flexible polymer chains terminating in sulfate groups at the latex surface.5,14 This material was further purified by ionexchange using equal weights of AXR (anion exchange resin) and CXR (cation exchange resin). The latex suspension was gently stirred for 2 h and then exhaustively dialyzed against a large volume of distilled water to remove traces of surfactants and salts. Conductometric titration was performed on the purified latex preparation. The content of sulfate groups which derive from the initiator was estimated as ≈10 mmol/kg latex. DLS experiments were in general made using a constant concentration of latex; C ) 5 × 10-5% (w/w). All solutions were filtered through 0.45-µm Millipore filters directly into the light scattering cells. Adsorption isotherms were estimated by determining the concentration of free surfactant in the solutions by surface tension measurements using the drop volume technique. Latex particles were suspended in the surfactant solution of known concentration for 48 h after which time the latex was removed by centrifugation (20 000 rpm for 30 min. A precipitate of the latex was observed at the bottom of centrifugation tube). The calibration curve used in a semilogarithmic plot of surface tension versus surfactant concentration, where the cmc is given by the knee-point. The steeply rising linear part (between the two inflection points) of the curve was always used for determining the surfactant concentration by surface tension measurement at that temperature, after appropriate dilution of the solution following centrifugation. Dynamic light scattering (DLS) measurements were made in the homodyne mode using a Spectra Physics 50 mW, 633 nm He-Ne laser as the light source. The scattering cells (10 mL cylindrical ampoules) were immersed in a large diameter (10 cm) thermostated bath of index-matching liquid (transdecalin) of n ) 1.479 at 25 °C. The detector system comprised an ITT FW130 photomultiplier, the output of which was digitized by an ALV amplifier-discriminator. The signal analyzer was an ALV multibit, multisample time autocorrelator (ALV 5000) covering logarithmically approximately 8 decades in delay time. Analyses were made using both the method of
b
Figure 2. The hydrodynamic radius (RH) from dynamic light scattering measurements at high dilution shown as a function of surfactant (a) C12E25 and (b) C12E7 concentration normalized by the cmc for a latex suspension in the salt-free aqueous solution at 25 °C. The corresponding adsorption isotherm is also shown.
cumulants and inverse Laplace transformation (ILT) using the algorithm REPES contained in the analysis package GENDIST.12 The autocorrelation functions were close to single exponential, with low values of the second cumulant ( 0.4, RH decreases to a constant value of 77 nm and this change is accompanied by further adsorption as shown in Figure 2a. Since the extended POE chains determine the observed RH, the decrease in RH may reflect a different mechanism for short POE chain surfactants such as C12E7: with the latter, RH decreases generally due to aggregation of the polymer chains.3,4 The earlier studies by Kayes16,17 indicate that, when there are carboxyl groups at the latex surface, the poly(oxyethylene glycol) ether nonionic surfactants with long POE chains adsorb in a looped molecular conformation, i.e., adsorption takes place both at the hydrocarbon end and with the POE chain. The latter is due to hydrogen-bond formation between the EO units and the carboxyl groups. In the present case, however, a looped conformation would seem unlikely owing to the absence of suitable groups at the surface which can associate with the ether oxygen atoms of the POE chain. The hydroxyl protons of the POE chains will also with difficulty form hydrogen bonds with the sulfate groups terminating the polymer chains owing to the unfavorable steric geometry with large POE chains. We suggest an alternative mechanism below. We note the results with pure C12E25 in aqueous solution as measured by surface tension and shown in Figure 1. Two inflection points (IFP) are observed, and the second of these clearly corresponds to the cmc. Calculation using the Gibbs equation gives two values for the molecular area of the adsorbed C12E25 at the air/water interface: 55 Å2 (below the first IFP) and 105 Å2 (in the linear partion above the first IFP). Earlier studies of the surface properties and micelle formation of long POE chain nonionic surfactants18-20 suggest that nonionic surfactants orient at the air/water interface with their hydrophobic groups above the interface and with the ethylene oxide chains coiling in the aqueous phase. The size of the coils increases with increasing number of chain segments. Schick18 gave values of the molecular area of 82 Å2 for C12E23 and 107 Å2 for C12E30, respectively, at π ) 20 mN/m and 25 °C. Our data for the second IFP is only slightly larger than Schick’s. With C12E25, even below the first IFP, the area of 55 Å2 is also larger than the cross-sectional area of the n-alkyl chain (22 Å2).21 This indicates that it is where the POE chain is coiled that determines the molecular area occupied at the air/water interface. The fact that the concentration dependence of the area change of the POE coil and that the molecular area becomes larger beyond the first IFP implies that there is interaction through dipoles on adjacent EO units between the POE coils. When more C12E25 molecules are adsorbed at the air/water interface, the distance between the POE coils becomes smaller and their mutual interaction will lead to an oblate coil conformation (expansion parallel to the surface). Although few results have been reported on the behavior of neutral polymers together with nonionic surfactants, Szmerekova´ et al. provided evidence from gel permeation chromatography that there is a weak interaction between PEO and nonylphenol polyglycol ether surfactants.24 Our studies23 also show that the area of the coiled POE chain has the slightly smaller value of 103 Å2 in the presence of 32 and 100 mM NaBr. This supports the above interpretation since added electrolytes generally weaken dipole interaction when these ions cannot associate with EO units. According to the above, one can understand the decrease in RH shown in Figure 2a since it is the conformation of the POE chain which determines RH. In the initial stage, a few C12E25 molecules adsorb on the latex surface and the extended POE chains coil freely. When the surface has a certain coverage, however, the POE coils become oblate through mutual interaction. This conformation of the POE chain favors an extended coverage of the surface and leads to a decrease in RH. Finally,
J. Phys. Chem., Vol. 100, No. 14, 1996 5911 a more hydrophilic surface is formed at saturation, giving a stable dispersion of the latex particle in water. In an earlier study of the adsorption of polyethylene glycol alkyl ether nonionic surfactants having long POE chains (n ) 18, 48, 98, respectively) on arsenic trisulfide sols,27 Glazman et al. also suggested that a hydrate shell is formed at the surface of the colloidal particle. The further observation that RH reaches almost the same value of 77 nm at saturation in spite of changing experimental parameters, for example, adding NaBr or changing the temperature (see Figures 3 and 4), demonstrates that the conformation of the extended POE chain is the main factor determining RH and thus supports the above interpretation. Influence of Added NaBr. Adding NaBr generally enhances hydrophobic interaction between the alkyl tails of the surfactant and the polymer chains because of the increased polarity of the solvent22 and therefore greatly promotes adsorption, for example, as has been shown in the case of C12E7.3 However, the role of NaBr is not significantly reflected in the present case. Viewing the data as a whole, in the presence of NaBr both the RH trace and the adsorption isotherm are similar to the value in the saltfree system (compare Figures 2a and 3). Both with and without NaBr, hydrophobic interaction dominates the adsorption pattern. The C12E25 molecules thus have the same orientation as in the salt-free system, and the structure of the adsorbed layer is determined by the extended POE chains. Our studies23 have shown that the effect of NaBr on the surface properties of pure C12E25 in aqueous solution is insignificant and there is only a very small difference in the area of the adsorbed molecule in the absence/presence of NaBr (103 Å2 in 32 or 100 mM added NaBr in comparison with 105 Å2 in the salt-free system). The similar behavior shown in Figure 3 (compared to Figure 2a) is thus reasonable. A small difference in the RH trace on addition of NaBr lies in the slightly larger increase in the initial maximum (see Figure 3). This may derive from steric effects of the adsorbed molecules since the polymer layer is substantially compressed by adsorption of counterions (Na+) prior to surfactant adsorption. Influence of Temperature. The water solubility of the R-(EO)n types of nonionic surfactants derived from the weak interaction between the ether oxygen of the ethylene oxide unit and water. Kronberg et al.9,25 concluded from thermodynamic analysis that the main factor determining the temperature dependence of the adsorption of nonionic surfactants is the poly(ethylene oxide)-water interaction. An interpretation in terms of the poly(ethylene oxide) chain conformation suggests that the temperature dependence of the poly(ethylene oxide)-water interaction is due to a higher population of unfavorable states at higher temperature.9,26 This leads to a diminished solubility of nonionic surfactants in aqueous solution with increasing temperature and thus an enhanced adsorption at the solid/water interface. However, a temperature effect is not observed in the present case and almost the same adsorbed amount at saturation is observed from 15 to 35 °C. This result is apparently related to the large size of the POE chain. Surface tension data23 show that the change in the area of the coiled POE chain at the air/ water interface is small on increasing the temperature: 102, 105, and 106 Å2 at 15, 25, and 35 °C, respectively. At 15 °C, a greater affinity of the POE chain for water leads to deeper penetration into the aqueous phase and thus a smaller cross section. Here extended POE coils still cover the latex surface and determine the observed RH. Similar behavior in the RH traces and the isotherms therefore seems reasonable.
5912 J. Phys. Chem., Vol. 100, No. 14, 1996 Conclusion With C12E25 adsorption to polystyrene latex particles of low surface charge density, hydrophobic interaction between the alkyl tails and the polymer chains is the main force for adsorption. The long coiled POE chains extend into the solution and lead to the formation of a hydrophilic surface about the latex particle. The influences of adding salt and changing temperature on adsorption are insignificant since the long POE chain dominates the structure of the adsorbed layer. Acknowledgment. This work has been supported by the Jacob Wallenberg Fond, Stora Kopparbergs AB, and the Swedish Technical Research Council (TFR); this support is gratefully acknowledged. References and Notes (1) Bo¨hmer, M. R.; Koopal, L. K.; Janssen, R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 2228. (2) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 130. (3) Zhao, J.; Brown, W. Comparative study of the adsorption of nonionic surfactants: Triton X-100 and C12E7 on polystyrene latex particles using dynamic light scattering and adsorption isotherm measurements. J. Phys. Chem., in press. (4) Zhao, J.; Brown, W. Dynamic light scattering study of a non-ionic surfactant (C12E7) adsorption on polystyrene latex particles: Effects of aromatic amino groups and surface polymer layer. J. Colloid Interface Sci., in press. (5) Zhao, J.; Brown, W. Surface characteristics of polystyrene latex particles and comparison with styrene-butadiene copolymer latex particles using dynamic light scattering and electrophoretic light scattering measurements. J. Colloid Interface Sci., in press.
Zhao and Brown (6) (a) Brown, W.; Zhao, J. Macromolecules 1993, 26, 2711. (b) Brown, W.; Zhao, J. Langmuir 1994, 10, 3395. (7) (a) Giles, C. H.; Smith, D. J. Colloid Interface Sci. 1974, 47, 755. (b) Giles, C. H.; D’Silva, A. P.; Easton, I. A. J. Colloid Interface Sci. 1974, 47, 766. (8) Kronberg, B.; Ka¨ll, L.; Stenius, P. J. Dispersion Sci. Technol. 1981, 2, 215. (9) Steinby, K.; Silveston, R.; Kronberg, B. J. Colloid Interface Sci. 1993, 155, 70. (10) Kronberg, B. J. Colloid Interface Sci. 1983, 96, 55. (11) Kayes, J. B. J. Colloid Interface Sci. 1976, 56, 426. (12) Schille´n, K.; Brown, W.; Johnsen, R. M. Macromolecules 1994, 27, 4825. (13) Robson, R. J.; Dennis, E. A. J. Phys. Chem. 1977, 81, 1075. (14) Bangs, L. B. Uniform Latex Particles; Seragen Diagnostics Inc.: Indianapolis, IN, 1984. (15) (a) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (b) Cabane, B.; Duplessix, R. J. Phys. 1982, 43, 1529. (16) Kayes, J. B. Kolloid-Z. Z. Polym. 1972, 250, 939. (17) Kayes, J. B. J. Colloid Interface Sci. 1976, 56, 426. (18) Schick, M. J. J. Colloid Sci. 1962, 17, 801. (19) Barry, B. W.; El Eini, D. I. D. J. Colloid Interface Sci. 1976, 54, 339. (20) El Eini, D. I. D.; Barry, B. W.; Rhodes, C. T. J. Colloid Interface Sci. 1976, 54, 348. (21) Harkins, W. D.; Florence, R. T. J. Chem. Phys. 1938, 6, 847. (22) Johnson, P. Langmuir 1993, 9, 2318. (23) Zhao, J. To be published. (24) Szmerekova´, V.; Kra´lik, P. J. Chromatogr. 1984, 285, 188. (25) Kronberg, B.; Silveston, R. Progr. Colloid Polym. Sci. 1990, 83, 75. (26) Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4962. (27) Glazman, Y.; Blashchuk, Z. J. Colloid Interface Sci. 1977, 62, 158.
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