J. Phys. Chem. 1996, 100, 3775-3782
3775
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 Jianxi Zhao and Wyn Brown* Department of Physical Chemistry, UniVersity of Uppsala, Box 532, 751 21 Uppsala, Sweden ReceiVed: September 19, 1995X
The adsorption of the nonionic surfactants Triton X-100 and C12E7 on predominantly hydrophobic polystyrene latex particles has been examined as a function of surfactant concentration by measuring the hydrodynamic radius by dynamic light scattering and adsorption isotherms by a surface tension technique. The adsorption behavior of these materials differs significantly and is related to the structures of the surfactant “tails”. Adsorption of TX-100 most probably occurs in an approximately perpendicular geometry with contact between its methyl end groups and the partially extended polymer chains at the latex surface and is accompanied by association between the aromatic nuclei of the adsorbed surfactant molecules. The latter leads to cooperative adsorption and an S-type isotherm. The perpendicular adsorption mode leads to expansion of the polymer chains for steric reasons, giving a pronounced increase in the hydrodynamic radius in the initial adsorption stage. There is a larger adsorbed amount of TX-100 compared with the C12E7. C12E7 has a straight dodecyl chain and an extended molecular conformation. This favors bridging of neighboring polymer chains through interactions both between the alkyl tails and the polymer chains and between the POE chain and the terminal sulfate groups. Bridging leads to aggregation of the polymer chains and reflects the strong interaction between C12E7 and the polymer chain which gives an L-type adsorption isotherm. The adsorption isotherm is twostepped in TX-100/latex solutions but is single-stepped for C12E7/latex suspensions, except in 100 mM added NaBr and at 35 °C. The characteristic types of adsorption isotherms observed with the two surfactants remain unchanged on altering other parameters, for example, the ionic strength or the temperature. NaBr augments C12E7 adsorption partly owing to salting-out of water from the hydration shell of the POE chain. On the other hand, TX-100 adsorption in 100 mM NaBr is complicated by competition between adsorption and micellization in aqueous solution since there is a pronounced tendency for TX-100 to aggregate in bulk solution at higher ionic strengths. Enhanced adsorption of both surfactants occurs on increasing the temperature, owing to a decrease in surfactant solubility in aqueous solution deriving from a combination of changes in the POE chain conformation and a reduction in the amount of structured water around the surfactant head group.
Introduction The adsorption of nonionic surfactants at the solid/aqueous interface has been the subject of many experimental and theoretical studies.1-17 Besides industrial applications such as the stabilization of dispersions, the structure of the adsorbed layer of nonionic surfactants is also of considerable fundamental interest. Triton X-100 is a commercial, polydisperse preparation containing an average of 9.5 oxyethylene units per molecule which has been widely used as a dispersing agent for colloidal suspensions. Most previous studies have dealt with the adsorption of TX-100 at the solid/aqueous interface but have focused on hydrophilic substrates such as silica.1-3,8-10,13 Levitz et al.,1 for example, studied the adsorption of TX-100 on spherosil using the fluorescence decay technique. It was shown that, at low coverage, the adsorption was micellar in nature while, at high surface coverage, steric interaction caused the micelles to coalesce and form a continuous bilayer-like structure. Gu et al.8 suggested, from a kinetic treatment of adsorption isotherms, that the adsorption of TX-100 on both narrow- and wide-pore silica media occurs by the single-step, cooperative formation of aggregates at the surface. Rudzinski et al.13 also found that the adsorption isotherm of TX-100 on silica has a sigmoidal X
Abstract published in AdVance ACS Abstracts, January 15, 1996.
0022-3654/96/20100-3775$12.00/0
shape, similar to that obtained by Gu et al.8 This characteristic shape of the isotherms may be ubiquitous for the adsorption of nonionic surfactants with short and medium length POE chains on polar hydrophilic solid surfaces.1,3,8,11,18,19 Poly(ethylene glycol) monoalkyl ethers (CmEn) are another widely used family of nonionic surfactants with molecular structures containing a linear alkyl chain. The adsorption behavior of CmEn surfactants at polar hydrophilic surfaces is apparently quite similar to that of TX-100. Lee et al.5 studied the adsorption of C12E6 on a quartz surface using neutron reflection and proposed a similar scheme to that suggested by Levitz et al.,1 in which “defective bilayers” at low coverage fuse into full bilayers at high coverage with a hydrocarbon layer having the thickness of a single alkyl chain. Cummins et al.20 examined the adsorption of the same nonionic surfactant (C12E6) on ludox TM silica sol using small-angle neutron scattering and observed a constant adsorbate thickness above a minimum coverage. They proposed that adsorption occurs as “islands” of bilayers. The results obtained by Rutland et al.16 using atomic force microscopy showed that the adsorption of C12E5 on a silica substrate was sufficient to produce a sparse, weakly hydrophobic, layer at very low concentrations (≈1/3 of the critical micelle concentration). A further increase in concentration led to a reduction in adhesion and screening of this layer by the adsorption of small aggregates which expose the hydrophilic © 1996 American Chemical Society
3776 J. Phys. Chem., Vol. 100, No. 9, 1996 surface to the solution. Above the cmc the aggregates have the character of closely packed regions of intercalated bilayers. Monodisperse, latex spheres are frequently used as model substrates in investigations of colloid stability. In general, the surfaces of such latex particles have dominantly hydrophobic properties and thus provide good substrates for studies of hydrophobic interaction between the hydrocarbon moieties of a nonionic surfactant and the latex surface. Adsorption of a nonionic surfactant onto a latex surface is apparently more complex than that on a hydrophilic surface. First, there exists a “hairy” layer of dangling polymer chains which are exposed at the latex surface; i.e., the latex surface is comprised of flexible polymer chains terminating in polar entities such as sulfate groups.21-25 Our prior work indicates that the pendant polymer chains at the latex surface with polar groups are the key feature in determining the adsorption pattern of surfactants and the overall hydrodynamic dimensions of the latex particle.26-28 Second, the hydrophobicity of the particle surface will strongly influence the adsorption characteristics of surfactants.29-33 Third, chemical modification of the latex surface may impart additional amphiphilic character to the surface, for example, by carboxylation or amination of the essentially hydrophobic latex particles. The carboxyl or amino groups readily hydrogenbond with the ether oxygen of the poly(ethylene oxide) units of nonionic surfactants under suitable conditions.28a,34-36 Relatively few studies exist concerning the adsorption of nonionic surfactants to hydrophobic latex particles compared to those on hydrophilic surfaces. Most investigations have described the adsorption isotherms and/or changes in the electrophoretic mobility (zeta potential). A “Langmuir” form for the adsorption isotherms is usually observed, and it was suggested that both hydrophobic interaction and hydrogen bonding may be involved in the adsorption.34,36-38 Dynamic light scattering (DLS) allows precise determination of the hydrodynamic radius of a colloidal particle in suspension at high dilution and consequently permits estimation of the changes in RH during adsorption. Typical studies39-43 have been concerned with the adsorption of polymers such as poly(ethylene oxide) or poly(propylene oxide). This technique has been surprisingly little used, however, in the investigation of the adsorption of common surfactants. In the present study we use DLS to examine the adsorption behavior of TX-100 on polystyrene latex particles. Adsorption isotherms have also been determined in order to allow a more comprehensive interpretation of the adsorption behavior. To provide a comparison of the effects of different compositions and structures in the hydrocarbon tail on adsorption, we chose another nonionic surfactant, heptaethylene glycol mono-n-dodecyl ether (C12E7), having a straight chain of 12 carbon atoms but with closely similar POE segments to TX-100. Variations in the hydrodynamic radius and the adsorption isotherms of C12E7 have been traced under various conditions. Experimental Section Materials. Triton X-100 (p-(1,1,3,3-tetramethylbutyl)phenyl)poly(oxyethylene) from Merck, 99.5%) and C12E7 (heptaethylene glycol mono-n-dodecyl ether from Nikko Chemicals, Tokyo) in crystalline form have been used. In surface tension measurements on aqueous solutions of both materials, no characteristic dip prior to the cmc “knee point” was observed (see, for example, Figure 1), which would indicate the presence of impurities. TX-100 and C12E7 were used without further purification. The surface activities as obtained by a surface tension method for their aqueous solutions at different concentrations of added NaBr are shown in Table 1. The water used
Zhao and Brown
Figure 1. Semilogarithmic plots of surface tension determined by the drop-volume method (γ) versus TX-100 concentration at (a, top) three ionic strengths and (b, bottom) three temperatures. The “knee” corresponds to the critical micelle concentration (cmc) of TX-100. Such plots are used as calibration curves for determination of adsorption isotherms.
TABLE 1: Characteristics of TX-100 and C12E7 Surfactant Aqueous Solutions with Added NaBr at 25 °C70 surfactant
NaBr (mM)
cmc (10-4 M)
C20 (10-5 M)
cmc/C20
TX-100
0 1 100 0 32 100
3.1 3.3 2.9 2.1 2.0 2.0
5.6 5.2 6.1 4.2 3.9 3.8
5.5 6.4 4.8 4.9 5.3 5.3
C12E7
was Milli-Q grade from a Millipore apparatus. The latex sample, which was synthesized by emulsion polymerization,44 was purchased from Serva AG, Heidelberg, Germany. This latex was further purified by ion exchange using equal weights of AXR (anion exchange resin) and CXR (cation exchange resin). The latex solids were 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 preparations. The content of sulfate groups was estimated as ≈10 mmol/kg of latex. DLS experiments were in general made using a constant concentration of latex, C ) 5 × 10-5%. All solutions were filtered through 0.45 µm Millipore filters 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
Adsorption of Nonionic Surfactants
Figure 2. Hydrodynamic radius of latex (RH) from dynamic light scattering measurements at high dilution (latex concentration ≈ 10-5 g/mL) shown as a function of surfactant concentration for a latex suspension in aqueous solution without added salt at 25 °C. Vertical arrows indicate each cmc.
by centrifugation (20 000 rpm for 30 min). Typical calibration curves are shown in Figure 1, where the cmc is given by the knee point. The steeply rising initial part of the curve was always used for determining the surfactant concentration by surface tension measurement at that temperature, after appropriate dilution of the solution following centrifugation. Dynamic light scattering measurements were made in the homodyne mode using a Spectra Physics 50 mW, 633 nm HeNe laser as the light source. The scattering cells (10 mL cylindrical ampules) were immersed in a large diameter (10 cm) thermostated bath of index-matching liquid (trans-decalin) of n ) 1.479 at 25 °C. The detector system was comprised of 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 cumulants and inverse Laplace transformation (ILT) using the algorithm REPES contained in the analysis package GENDIST.45 The autocorrelation functions were close to single exponential with low values of the second cumulant (
