Adsorption and Aggregation of C8E4 and C8G1 Nonionic Surfactants

for C8E4, the deviation increases to 1 order of magnitude in the aggregative adsorption region. The observed .... water/air and water/oil interfaces h...
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Langmuir 1997, 13, 3308-3315

Adsorption and Aggregation of C8E4 and C8G1 Nonionic Surfactants on Hydrophilic Silica Studied by Calorimetry Z. Kira´ly,† R. H. K. Bo¨rner, and G. H. Findenegg* Iwan-N.-Stranski-Institut fu¨ r Physikalische und Theoretische Chemie, Technische Universita¨ t Berlin, Strasse des 17 Juni 112, 10623 Berlin, Germany Received December 6, 1996. In Final Form: April 1, 1997X The critical micelle concentrations and the enthalpies of micelle formation of n-octyl tetraethylene glycol monoether (C8E4) and n-octyl β-D-monoglucoside (C8G1) in water have been determined at 298.15 K by titration microcalorimetry. The adsorption and thermal behavior of these nonionic surfactants at the hydrophilic silica glass/aqueous solution interface have been investigated by simultaneous measurements of the adsorption isotherm and the calorimetric enthalpies of displacement in a flow system. Both surfactants display pronounced cooperative adsorption behavior (S-shaped isotherms). While in the low-affinity region the amount adsorbed and the integral enthalpies of displacement are only slightly smaller for C8G1 than for C8E4, the deviation increases to 1 order of magnitude in the aggregative adsorption region. The observed difference in the adsorption behavior is interpreted in terms of the different hydrophilic head groups of the two amphiphiles. The differential molar enthalpies of displacement are comparable in magnitude in both the low-affinity (exotherm) and aggregative (endotherm) regimes. The molar enthalpies of aggregate formation at the silica/water interface were found to be close to those in the bulk solution, indicating that surface aggregation and bulk micellization are very similar phenomena. The enthalpy/entropy balance of the formation of the adsorption layer has been described in terms of thermodynamic potential functions.

Introduction

Poly(ethylene glycol) alkylphenol monoethers (CmPEn),1-5, poly(ethylene glycol) alkyl monoethers (CmEn),3,4,8,11-16,21-25,28,29 or alkylmethyl sulfoxide (AMS)5-7 nonionics have been selected for use in most academic work. While earlier studies were based mainly on the analysis of the adsorption isotherms,1-9 in recent studies the measurement of the isotherms have been supplemented by a variety of experimental techniques. Among others, contact angle measurements,6,8-10 X-ray diffraction,8 neutron scattering and/or reflection techniques,11-16 1H or 13C NMR spectroscopy,14,17 fluorescence decay spectroscopy,18-20 ellipsometry,21-24 atomic force microscopy,25,26 adsorption calorimetry,27-38 and stability measurements of silica sols12,13,37 provided valuable information about the mechanism and thermodynamics of the formation as well as on the structure of the adsorption layer. Various theoretical models have been introduced which correlate well with experimental obser7,9,10,18-20,27,30-38

The adsorption of nonionic surfactants from aqueous solutions on hydrophilic silica surfaces has been the subject of much experimental1-38 and theoretical research.23,39-42 * To whom correspondence may be addressed: e-mail, findenegg@ chem.tu-berlin.de. † Permanent address: Department of Colloid Chemistry, University of Szeged, Aradi Vt. 1, H 6720 Szeged, Hungary. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Koganovskii, A. M.; Klimenko, N. A; Tryasorukova, A. A. Kolloidn. Zh. 1975, 37, 560. (2) Furlong, D. N.; Ashton, J. R. Colloids Surf. 1982, 4, 121. (3) Clunie, J. S.; Ingram, B. T. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic: London, 1983; Chapter 3, p 105. (4) Partyka, S.; Zaini, S.; Lindheimer, M.; Brun, B. Colloids Surf. 1984, 12, 255. (5) Zhu, B.-Y.; Zhao, X., Gu, T. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3951. (6) Zhu, B.-Y.; Zhao, X. J. Colloid Interface Sci. 1988, 125, 729. (7) Gu, T.; Zhu, B.-Y. Colloids Surf. 1990, 44, 81. (8) Gellan, A.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2235. (9) von Rybinski, W.; Schwuger, M. J. In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Surfactant Sci. Series 23; Marcel Decker: New York, 1987; Chapter 2, p 45. (10) Gonzalez, G.; Travalloni-Louvisse, A. M. Langmuir 1989, 5, 26. (11) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. (12) Cummins, P. G.; Staples E.; Penfold, J. J. Phys. Chem. 1990, 94, 3740. (13) Cummins, P. G.; Staples E.; Penfold, J. J. Phys. Chem. 1991, 95, 5902. (14) Bo¨hmer, M. R.; Koopal, L. K.; Janssen, R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 2228. (15) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204. (16) Penfold, J.; Staples, E.; Tucker, I.; Cummins, P. J. Phys. Chem. 1996, 100, 18133. (17) So¨derlind, E.; Stilbs, P. Langmuir 1993, 9, 1678. (18) Levitz, P.; El Miri, A.; Keravis, D.; Van Damme, H. J. Colloid Interface Sci. 1984, 99, 484. (19) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1984, 88, 2228. (20) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 1302. (21) Tiberg, F.; Landgren, M. Langmuir 1993, 9, 927. (22) Tiberg, F.; Jo¨nsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (23) Tiberg, F.; Jo¨nsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 3714. (24) Brinck, J.; Tiberg, F. Langmuir 1996, 12, 5042. (25) Rutland, M. W.; Senden, T. J. Langmuir 1993, 9, 412.

S0743-7463(96)02076-8 CCC: $14.00

(26) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (27) Denoyel, R.; Rouquerol, F.; Rouquerol, J. In Adsorption from Solution; Ottewill, R. H., Rochester, C. H., Eds.; Academic: London, 1983; p 225. (28) Gellan, A.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1985, 81, 3109. (29) Gellan, A.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1986, 82, 953. (30) Partyka, S.; Lindheimer, M.; Zaini, S.; Keh, E.; Brun, B. Langmuir 1986, 2, 101. (31) Thomas, F.; Bottero, J. Y.; Partyka, S.; Cot, D. Thermochim. Acta 1987, 122, 197. (32) Denoyel, R.; Rouquerol, F.; Rouquerol, J. In Proceedings of the 2nd Engineering Foundation Conference on Fundamentals of Adsorption, Santa Barbara, CA, May 4-9, 1986; Liapis, A. I., Ed.; Amer. Inst. Chem. Eng.: New York, 1987; p 199. (33) Lindheimer, M.; Keh, E.; Zaini, S.; Partyka, S. J. Colloid Interface Sci. 1990, 138, 83. (34) Denoyel, R.; Rouquerol, J. J. Colloid Interface Sci. 1991, 143, 555. (35) Partyka, S.; Lindheimer, M.; Faucompre, B. Colloids Surf. 1993, 76, 267. (36) Giordano, F.; Denoyel, R.; Rouquerol, J. Colloids Surf. 1993, 71, 292. (37) Giordano-Palmino, F.; Denoyel, R.; Rouquerol, J. J. Colloid Interface Sci. 1994, 165, 82. (38) Noll, L. A. Colloids Surf. 1987, 26, 43. (39) Zhu, B.-Y.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3813. (40) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1990, 6, 1478. (41) Levitz, P. Langmuir 1991, 7, 1595. (42) Cases, J. M.; Villieras, F. Langmuir 1992, 8, 1251.

© 1997 American Chemical Society

C8E4 and C8G1 Nonionic Surfactants

vations.7,23,39-42 The major conclusions of these experimental and theoretical studies can be summarized as follows. The adsorption isotherms of nonionic surfactants from aqueous solutions on hydrophilic silica surfaces are sigmoidal in shape, indicative of a cooperative adsorption mechanism. Well below the critical micelle concentration (cmc), isolated surfactant molecules are adsorbed weakly on polar surface sites (by hydrogen bonding to surface silanol groups, etc.) represented by a low-affinity regime on the isotherm. The physically anchored amphiphiles induce an incipient surface aggregation process as the “critical surface aggregation concentration” (csac) (typically, csac ) 0.6-0.9 cmc22-24) is reached, after which the extent of adsorption dramatically increases. The aggregates are formed from single surfactant molecules coming from the bulk solution. Direct deposition of micelles from the aqueous phase can safely be excluded. Clearly, the driving force of the aggregative adsorption is similar in nature to that of the formation of micelles in the bulk solution (entropically driven hydrophobic interactions). Slightly above the cmc, the adsorption levels off and a plateau is reached, attributable to the stabilization of the monomer surfactant chemical potential as the cmc is exceeded. In general, the shape of the ascending section of the isotherm becomes sharper, its position is shifted to lower concentrations and the plateau value increases with decreasing pH, with increasing temperature and salinity, and with increasing hydrophobicity in a surfactant homologue series.4,34,35 Depending on the systems studied, a variety of surface aggregate structures have been proposed, ranging from hemimicelles to spherical or ellipsoidal aggregates, to interconnected network of micellar aggregates, to patchy bilayers, or to uniform bilayers. In general, lipophilic nonionic surfactants tend to form large surface aggregates while the hydrophilic ones adsorb as small micelles. Since a number of different phases have been identified for bulk surfactant/water systems (see ref 43 for alkyl poly(oxyethylenes) and ref 44 for alkyl polyglucosides, for relevance to the present work), it seems reasonable to assume the existence of various surfactant self-assemblies on solid surfaces. Among the various experimental techniques, calorimetry is a most sensitive tool for elucidating the thermodynamic behavior of surfactants both in the bulk solution45-49 and at the solid/liquid interface.27-38 In particular, the cmc and the aggregation number,45,46 the heat of micelle formation,45-48 and the heat of solution of monomeric surfactants in water45,47,49 can be determined in the bulk liquid phase. On the other hand, sorption calorimetry has been used to measure the enthalpy balance of adsorption of CmPEn and CmEn nonionic surfactants on hydrophilic silica at ambient temperature.27-38 The main feature of the (integral) enthalpy isotherms is, that while the adsorption is an exothermic process well below the cmc, it turns toward the endothermic direction at the csac and approaches a plateau beyond, but close to the cmc. The calorimetric evidence of the formation of surface aggregates is that, in the cooperative adsorption region, (43) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (44) Balzer, D. Langmuir 1993, 9, 3375. (45) Andersson, B.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1 1988, 84, 4087. (46) Paula, S.; Su¨s, W.; Tuchtenhagen, J.; Blume, A. J. Phys. Chem. 1995, 99, 11742. (47) Antonelli, M. L.; Bonicelli, M. G.; Ceccaroni, G.; La Mesa, C.; Sesta, B. Colloid Polym. Sci. 1994, 272, 704. (48) Corkill, J. M.; Goodman, J. F.; Tate, J. R. J. Chem. Soc., Faraday Trans. 1 1964, 60, 996. (49) Corkill, J. M.; Goodman, J. F.; Tate, J. R. J. Chem. Soc., Faraday Trans. 1 1967, 63, 773.

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the measured differential molar heats of sorption are remarkably similar in magnitude to the corresponding molar enthalpies of micelle formation in the bulk solution and are less dependent on the nature of the silica. In this work, we report on the calorimetric determination of the enthalpies of micelle formation of two nonionic surfactants, n-octyl tetraethylene glycol monoether (C8E4) and n-octyl β-D-monoglucoside (C8G1), and the enthalpies of displacement of water by these nonionics on a hydrophilic, wide-pore CPG silica at 298.15 K. The enthalpies of micellization of C8E4 45 and C8G1 46,47 have been published recently. Adsorption and enthalpy data for C8E4-water/silica28 and closely related systems27-38 are also available in the literature. To our knowledge, the sorption properties of C8G1 at the solid/liquid interface have not been investigated yet, although studies on the water/air and water/oil interfaces have been reported.50 Experimental Section Materials. Controlled-pore glass, CPG-10-240 (Fluka) had a sieve fraction of 120-200 mesh, an average pore size of 24 nm, and a BET surface area of 88.1 m2 g-1. Details about the surface chemistry and some applications of CPG have been given elsewhere.51 n-octyl tetraethylene glycol monoether, C8E4 (Bachem, purity >98%) and n-octyl β-D-monoglucoside, C8G1 (CalBiochem, purity >97%) were used without further purification. Water was distilled and passed through a Milli-Q purewater system. Stock solutions were prepared by weight and diluted volumetrically to the desired concentrations. For C8E4 solutions, slight turbidity was observed around the cmc (possibly due to the presence of nonpolar impurities) which disappeared upon gentle heating. This effect, which was also observed when using another batch of the chemical, did not cause appreciable disturbance in the instrumental response curves. The appearance of water-insoluble impurities near the cmc of C8E4 (purchased from the same supplier as in the present work) has also been reported in a light scattering study59 with a minor influence on the accuracy of the measurements. On the other hand, evidence has been given that the presence of hydrophobic (probe) molecules does not modify the adsorption isotherm of nonionic surfactants on hydrophilic silica surfaces.19,20 Methods. Titration Calorimetry. The measurements of the cmc’s and the enthalpies of micelle formation (∆micH) were performed at 298.15 ( 2 × 10-4 K by using a multichannel thermal activity monitor (TAM) isothermal heat-flow microcalorimeter (ThermoMetric LKB 2277, Lund, Sweden). A twin detector, supplied with a sample cell and a reference cell, was used. The 4 mL titration ampule was equipped with a stirring facility and a Hamilton Microlab M syringe. The ampule was loaded with 2 mL of water, and a stirring rate of ∼70 rpm was applied. Titrant (0.5 mL; surfactant concentration ∼10-fold that of the cmc) was delivered into the ampule through a fine-bore cannula in aliquots of 10 µL and at a plunger speed of 0.5 µL/s. For each step, the equilibration time was back-fed to the syringe to initiate the next titration step. The experiment was computer controlled by the Digitam software, including titration, data collection, and data analysis. Good baseline stability with a noise of less than (50) Kutschmann, E.-M.; Findenegg, G. H.; Nickel, D.; von Rybinsky, W. Colloid Polym. Sci. 1995, 273, 565. (51) Haller, W. In Solid Phase Biochemistry, Analytical and Synthetic Aspects; Scouten, W. H., Ed.; Chemical Analysis Series 66; John Wiley & Sons: New York, 1983; Chapter 11. (52) Mehrian, T.; de Keizer, A.; Korteweg, A. J.; Lyklema, J. Colloids Surf. 1993, 71, 255. (53) Wang, H. L.; Duda, J. L.; Radke, C. J. J. Colloid Interface Sci. 1978, 66, 153. (54) Findenegg, G. H. In Theoretical Advancement in Chromatography and Related Separation Techniques; Dondi, F., Guiochon, G., Eds.; NATO ASI Series C 383; Kluwer: Dordrecht, The Netherlands, 1992; p 227. (55) van Os, N. M.; Haandrikman, G. Langmuir 1987, 3, 1051. (56) Kira´ly, Z.; De´ka´ny, I. Prog. Colloid Polym. Sci. 1990, 83, 68. (57) Kira´ly, Z.; Findenegg, G. H. J. Phys. Chem. B, in press. (58) Degiorgo, V.; Corti, M. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 1, p 471. (59) Corti, M.; Minero, C.; Degiorgo, V. J. Phys. Chem. 1984, 88, 309.

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Figure 1. Calorimeter response curve for the titration of water (2.0 mL) by C8G1 micellar solution (303 mM, 10 µL aliquots) at 298.15 K. (0.1 µW was achieved during the measurements. The reproducibility of the titration experiment was within 3%. A typical titration curve is given in Figure 1. More details about the titration microcalorimetric technique by using the TAM calorimeter have been published previously.52 Flow Sorption Experiments. The enthalpies of displacement of water by the nonionic surfactants were measured by flow sorption experiments with the same TAM calorimeter as used for the titration experiments. The calorimeter was connected to a differential refractometer, which allowed a simultaneous measurement of the adsorption isotherm, on the principle of flow frontal analysis solid/liquid chromatography.53,54 The measuring system represents an improved version of the design of van Os and Haandrikman.55 The system consisted of two three-channel degassers, an electric seven-port valve, an HPLC Micropump (each unit Knauer, Berlin, Germany), a back pressure regulator (SSI, State College, PA), a six-way valve (Knauer), the sample vessel (designed by van Os55) attached to the holder of the calorimeter perfusion cell, a differential refractometer (Knauer), and a liquid microflowmeter (PhaseSep, Clwyd, England). These units were connected via stainless steel or Tefzel tubings. The calorimeter vessel was loaded with 0.1-0.2 g of solid, and a liquid flow rate of 0.1-0.2 mL/min was applied. The measurement was computer controlled and automated, allowing jumps between any two of six solutions with different concentrations, in an arbitrary direction and as many times as required. Typically, the measurement of the adsorption and enthalpy isotherms consisted of two to three sets of measurements (10-15 experimental points for each isotherm). Each measurement included the following routines: step-by-step adsorption (five steps with increasing concentration) followed by step-by-step desorption, and then repeated one-step adsorption/one-step desorption jumps, i.e., starting from, and returning to the first solution (e.g., water) for each step.56 The instrumental response curve (calorimetric peaks and breakthrough curves) of a set of one-step experiments is displayed in Figure 2, where the smallest heat effects have been selected to illustrate the sensitivity of the calorimeter. Adsorption producing fractions of 1 µW could be detected with high accuracy and reproducibility. The flow sorption experiments were conducted under reversible conditions so that the single-step and step-by-step methods yielded identical results within 4%. In other words, the Γ1 (strictly, Γ1(v) volume-reduced surface excess concentration) vs c1 adsorption isotherms and ∆21H vs c1 integral enthalpy isotherms of displacement were not dependent on the experimental method used. Furthermore, the adsorption and calorimetric isotherms were found to be reversible by adsorption/desorption runs. Technical details of the measuring system will be published shortly.57

Results and Discussion Titration Calorimetry. The step-by-step (differential) titration curves of water by concentrated micellar solutions of C8E4 and C8G1 are S-shaped step functions as displayed in Figures 3a and 4a. In the low-concentration region,

Figure 2. (a) Calorimeter response curve and (b) differential refractometer signal of a set of flow sorption experiments. Singlestep displacements of water by C8G1 (adsorption) and C8G1 by water (desorption) on CPG silica at 298.15 K. Column packing 0.0880 g, void volume 1.5455 mL, and flow rate 103.0 µL/min. The signals are unsmoothed and unfiltered.

Figure 3. Titration of water by a concentrated micellar solution of C8E4 at 298.15 K: (a) step-by-step (differential) titration curve as a function of equilibrium concentration; (b) first derivative of curve a; (c) cumulative (integral) titration plot of curve a.

the large exothermic heat effects are mainly due to the demicellization process upon titration, although heats resulting from a dilution of the resultant surfactant monomers and the micelles are also included. The differential enthalpies remain apparently constant until

C8E4 and C8G1 Nonionic Surfactants

Figure 4. Titration of water by a concentrated micellar solution of C8G1 at 298.15 K: (a) step-by-step (differential) titration curve as a function of equilibrium concentration; (b) first derivative of curve a; (c) cumulative (integral) titration plot of curve a.

the enthalpy functions increase sharply over a small range of concentration. As the cmc in the titrated solution is approached and exceeded, less and less micelles will dissolve upon titration. Instead, the enthalpies of dilution of the micelles become more and more dominant. In the third sections of the step functions, the enthalpies show little variation with surfactant concentrations. The cmc values in the transition concentration regions were calculated according to Paula et al.46 The step function was fitted by an empirical formula and differentiated with respect to the surfactant concentration by using a numerical differentiation routine. The first derivative exhibits a maximum (see Figures 3b and 4b), the position of which was regarded as the cmc. Alternatively, the cumulative (integral) titration curve can be constructed by summation of the step-by-step enthalpies,52 as shown in Figures 3c and 4c. In this representation, two straight lines with different slopes are connected with a bend. The position of the intersection point of the extrapolated lines (break point) was identified as the cmc. This method gives a result identical to within 3% of the aforementioned one. The heat of micelle formation ∆micH was calculated from the differential titration curve as the height of the step function at the cmc. It should be mentioned that Andersson and Olofsson defined the cmc as the position of the first break on the differential titration curve, and these authors used the (upper) asymptote of the curve for the calculation of the micellization enthalpies.45 For the present systems, this method would shift the cmc to lower concentrations and ∆micH in a positive direction. The ambiguity arises because micelle formation is not restricted to a single concentration but extends over a range of concentration. On the other hand, ∆micH may be defined either at the position of the cmc or with infinite dilution reference. For C8E4, the present work yields a cmc of 8.4 ( 0.2 mM and ∆micH ) 15.8 ( 0.7 kJ‚mol-1 as compared with 7.3 mM and 16.8 kJ‚mol-1, respectively, obtained by Olofsson and Andersson. The differences may be due partly to the different purities of the samples. These cmc values can be compared further with 7.2 mM obtained

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from surface tension measurements8 and with 8.4 mM obtained from light scattering measurements.58 For C8G1, cmc ) 27.1 ( 0.2 mM and ∆micH ) 7.5 ( 0.1 kJ‚mol-1 were obtained in this work, as compared with 23.6 mM and 7.9 kJ‚mol-1 obtained by Paula et al.46 or with 24 mM and 6.4 kJ‚mol-1 measured by Antonelli et al.47 Our results are summarized in Table 1. Flow Sorption Experiments. Due to technical difficulties, only a few attempts for a simultaneous measurement of the material and enthalpy balance of adsorption at solid/liquid (S/L) interfaces have been reported in the literature. The pioneering work of Denoyel et al.27 and Noll38 involves the adsorption of a nonionic surfactant (Triton-X 100, C8PE9-10) from aqueous solution on hydrophilic silica. The adsorption isotherm of C8E4 from water on CPG at 298.15 K is given in Figure 5a. Although the isotherm is sigmoidal in shape, the initial part of the isotherm has a slightly convex curvature which reaches a value of ∼0.06 µmol‚m-2 at a critical surface aggregation concentration of csac ≈ 0.62 cmc (Table 1). After this, the isotherm begins to rise sharply over a narrow concentration range, due to cooperative adsorption leading to the formation of surface aggregates. The ascending section of the isotherm turns to a plateau (Γ1 ) 5.7 µmol‚m-2) close to, but slightly beyond, the cmc. As mentioned above, the cmc was determined by using the same C8E4 sample as used in the flow sorption experiments. If a smaller value, say 7.3 mM, were used (taken from the literature8,45), one would come to the false conclusion that the plateau of the adsorption isotherm is reached well beyond the cmc. This would imply that while the surfactant chemical potential in the bulk phase remains nearly constant, the composition and structure (chemical potential) of the adsorption layer is changing. The integral enthalpy isotherm of displacement of water by C8E4 is displayed in Figure 5b. The enthalpy isotherm is slightly exothermic up to the csac, after which it becomes strongly endothermic and finally reaches a high plateau value (∆21H ) 75 mJ‚m-2) as the adsorption levels off. Both the adsorption isotherm and the calorimetric isotherm presented in this work differ in trends from those of Gellan and Rochester, who also determined adsorption and calorimetric data for aqueous C8E4 solution on a hydrophilic silica.8,28 The extent of adsorption in the lowconcentration region was found by these authors to be comparable in magnitude with that in the high-concentration region so that the typical S-shaped character of the isotherm was absent. Furthermore, very large exothermic heats were measured at low surface concentrations, and no endothermic heat effects were detected at higher concentrations, in contrast with other alkyl poly(oxyethylene)s measured by the same authors.28,29 The present results compare much better in tendency with those of Denoyel et al. and Partyka et al., who used CmPEn-type nonionic surfactants in otherwise similar studies.27,30-37 We now return to the analysis of the adsorption isotherm but will discuss the calorimetric data for C8E4 together with the calorimetric results for C8G1 later. The distinct plateau value of the adsorption isotherm affords an apparent cross-sectional area of 0.29 nm2/ molecule in a hypothetical monolayer arrangement (which is not plausible); i.e., an area of 0.58 nm2 is available for a molecule if a bilayer is formed. In a bilayer arrangement, one anticipates that the oxyethylenic parts of the molecules in the first layer are attached to the surface via hydrogen bonding, while the oxyethylenic chains of the molecules in the second layer face the aqueous phase.4 On the assumption that the tetraethylene glycol block occupies

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Table 1. Thermodynamic Parameters Characteristic of the Aggregation and/or Adsorption Behavior of C8E4 and C8G1 Nonionic Surfactants in Aqueous Solutions and at the CPG Silica/Water Interface at 298.15 K surfactant

cmc/mM

∆micH/kJ‚mol-1

csac/cmc

Γmax/µmol‚m-2 (plateau)

∆21Hmax/mJ‚m-2 (plateau)

∆21h1/kJ‚mol-1 (c < csac)

∆21h1/kJ‚mol-1 (c > csac)

C8E4 C8G1

8.4 ( 0.2 27.1 ( 0.2

15.8 ( 0.7 7.5 ( 0.1

≈0.62 ≈0.71

5.7 0.45

75.0 2.30

-13.5 ( 3.0 -5.9 ( 1.6

14.5 ( 1.5 6.2 ( 0.6

Figure 5. (a) Adsorption isotherm and (b) integral enthalpy isotherm of displacement for the system C8E4(1)-water(2)/CPG silica glass at 298.15 K (solid lines are drawn to guide the eye).

an area of ∼0.36 nm2,4 the present results for the isotherm at 298 K suggest a fragmented bilayer for the structure of the surface aggregates. Such a fragmented bilayer is consistent with the observed strong temperature dependence of the plateau value of the adsorption isotherms for similar systems.4 Alternatively, assuming that the surface aggregates are quasi-spherical in shape having a hydrodynamic radius of rH ) 1.24 nm with an average aggregation number of n ) 23, like C8E4 bulk micelles at 298.15 K,45 then one C8E4 molecule would occupy an area of 0.23 nm2, according to a close-packed hexagonal arrangement of the micelles. If we take different literature values of rH ) 2.5 nm and n ) 80,59 the apparent area is 0.27 nm2/molecule. This is in good agreement with the plateau value of 0.29 nm2; i. e., the micelles are only slightly, if at all, spaced apart on the surface. It should be noted that C8E4 aggregates in the bulk tend to increase in size with increasing concentration, giving rise to random percolation clusters60 so that the formation of secondary aggregates may also occur in the adsorption layer. The approximation of Gao et al. offers another alternative for the analysis of the adsorption isotherm.61 If we assume that each adsorbed amphiphilic molecule in the first (pseudo) plateau is an active center for subsequent surface aggregation, then the average aggregation number is equal to the ratio of the adsorption of the two plateaus. Based on this model, an aggregation number of ∼95 is obtained, which is not inconsistent with a patchy bilayer and is not in contradiction with a close-packed arrange(60) Strunk, H.; Lang, P.; Findenegg, G. H. J. Phys. Chem. 1994, 98, 11557. (61) Gao, Y.; Du, J.; Gu, T.; J. Chem. Soc., Faraday Trans. 1 1987, 83, 2671.

Figure 6. (a) Adsorption isotherm and (b) integral enthalpy isotherm of displacement for the system C8G1(1)-water(2)/CPG silica glass at 298.15 K (solid lines are drawn to guide the eye).

ment of quasi-spherical (more strictly, ellipsoidal) aggregates. Although the first plateau is rather ill-defined, so that it makes little sense to apply this model, the result of the calculation is not unreasonable. We may estimate the mean thickness of the adsorption layer at the plateau as MwΓ1,max/F, where Mw is the molar mass and F is the density of the surfactant4 to give a value of ∼2.0 nm. This thickness is consistent with an intercalated bilayer or a fragmented bilayer structure. The discussion above illustrates that the analysis of the adsorption isotherm alone cannot provide sufficient information on the structure and composition of the adsorption layer. Either a bilayer with defect sites or an ensemble of quasi-spherical micelles can be ascribed for the morphology of the C8E4 surface aggregates. It is worth mentioning that, in a recent study, the image of cationic surface aggregates (by using noncontact atomic force microscopy, AFM) revealed that spherical micelles do exist on hydrophilic silica, having a well-defined nearestneighbor distance between the spheres, but with little crystalline symmetry.26 The spacing of the micelles was found to be dependent on the density of the surface anchor sites. Closely packed micelles often appeared to be interconnected by a thin neck, giving rise to coalescence. This picture is also consistent with the present results. The application of AFM to our systems may provide further evidence concerning the morphology of the surface aggregates. The adsorption isotherm and the integral enthalpy isotherm of displacement of the system C8G1-water/CPG are given in panels a and b of Figure 6, respectively. The two isotherms run parallel and are similar in shape to the corresponding isotherms for C8E4. In the low-affinity region, the extent of adsorption and the associated

C8E4 and C8G1 Nonionic Surfactants

exothermal heat effects are comparable in magnitude with those of C8E4, although somewhat lower values are obtained for C8G1. As the cmc is approached, the isotherms begin to rise steeply at csac ≈ 0.71 cmc and turn to a plateau just above the cmc. It is most striking, and rather peculiar, that the adsorption isotherm and the enthalpy isotherm of displacement reach plateau values (Γ1 ) 0.45 µmol‚m-2 and ∆21H ) 2.3 mJ‚m-2) 1 order of magnitude smaller than the corresponding ones for C8E4 (Table 1). Such a dramatic, 12-13-fold decrease in adsorption was not foreseen on the basis of a 3-fold increase in the cmc. For a surface aggregate with a limited number of molecules in the adsorption layer, the surface micelle is certainly the more stable configuration;34 i.e.; the surface possesses small aggregates. One can speculate that if the conditions for the Gao model are met, an average surface aggregation number of ∼11 is obtained at the plateau. For comparison, C8G1 forms large ellipsoidal micelles in aqueous solutions, with rH ) 2.35 nm and n ) 87.62,63 The S-type isotherm equation derived by Gu and Zhu for the adsorption of nonionic surfactants at the silica gel/water interface7 was not applicable to the present systems. The linear representation of the proposed equation yielded two straight lines with markedly different slopes with a bend around the csac. In the aggregative regime, this model gave surface aggregation numbers of 10.5 and 14 for C8E4 and C8G1, respectively, while values close to unity were obtained in the low-affinity region. However, the model assumes that adsorption occurs by a single-step cooperative formation of aggregates on the surface; i.e., there is no initial monomeric nucleation, in contrast with the model proposed by Gao et al.61 The large difference between the aggregative adsorption behavior of C8E4 and C8G1 must be related to the different polar head groups. Previous adsorption studies on alkylphenol poly(oxyethylene) homologues at the hydrophilic silica/water interface support this statement. For example, in the series of C8PEn surfactants, the value of the adsorption plateau was found to be smaller by a factor of ∼7 as n increased from 10 to 30.2,4,9,20,33 The number of molecules per surface aggregate also decreased with increasing hydrophilic character of the head group, from ca. 200 to 32.20,34 It should be noted, however, that the gradual decrease of the plateau with increasing number of ethylene oxide segments is always accompanied by a broadening of the rising (sigmoidal) section of the adsorption isotherm.2,4,9,20,33 For C8PEn surfactants possessing a long poly(oxyethylene) block, S-type adsorption isotherms transform into L-type isotherms.34 However, such behavior is completely absent for C8G1; for this material, the ascending part of the isotherm remains steep near the cmc. The spectacular difference between the adsorption behavior of the alkyl glucoside and the ethoxylate surfactant may also arise from the unique water-bonding capacity of the multiple OH groups of the sugar head.44,62-66 Hydration of the sugar moiety rules the solubility of alkyl glucosides much stronger than hydration of the ethoxy groups does for alkyl poly(oxyethylene)s. Hence, the affinity for the silica surface is expected to be higher for (62) Kameyama, K.; Takagi, T. J. Colloid Interface Sci. 1990, 137, 1. (63) Mo¨ller, A.; Lang, P.; Findenegg, G. H.; Keiderling, U. Ber. Bunsenges. Phys. Chem., submitted. (64) Cecutti, C.; Focher, B.; Perly, B.; Zemb, T. Langmuir 1991, 7, 2580. (65) Drummond, C. J.; Warr, G. G.; Grieser, F.; Ninham, B. W.; Evans, D. F. J. Phys. Chem. 1985, 89, 2103. (66) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 3382.

Langmuir, Vol. 13, No. 13, 1997 3313

C8E4 than for the C8G1 molecules. In fact, the extent of adsorption at high dilutions was found to be smaller for the C8G1. A further difference between the solution properties of the two classes of nonionic surfactants is that the micellar microenvironment of alkyl polyglucosides is more aqueouslike than the micellar microenvironment of alkyl poly(oxyethylene)s.44,62-66 In addition to hydrophobic interactions restricted to the hydrocarbon core, the strong water structuring around the interfacially located sugar may contribute to the stabilization of large aggregates. The structure of the aggregates is strongly influenced by the stereochemistry of the sugar head.64,65 Although a monoglucoside like C8G1 will differ in detail from alkyl polyglucosides, they share some common properties. In a recent study of the micellar behavior of β-alkyl maltoside, whose properties proved in many respects to be similar to β-alkyl glucosides,44,50,65 it was concluded that conformational or orientational changes of the sugar head group are induced upon interaction of the head with any other moiety which, in turn, drastically decreases the size of the micelles.64 Such an effect, if it also prevails for C8G1, would result in the formation of surface micelles on the silica substrate with a low aggregation number. Moreover, micelles formed by technical alkyl polyglucosides have been shown to carry a net negative charge in water, in contrast with alkyl poly(oxyethylene) micelles which are uncharged or slightly positively charged.44 The adsorption of polydisperse alkyl glucosides on titanium dioxide also resulted in a negative charge of the particles over a wide range of surfactant concentration.67 The reason for the charge on the micelles is as yet unexplained; it may simply be due to the presence of impurities originating from the preparation technology. However, if C8G1 micelles, like the micelles of commercial alkyl polyglucosides, were also negatively charged (which is not known at present), then the formation of large surface micelles would be accompanied by an increase in the net negative charge of the aggregates. This process would be energetically unfavorable, since the surface of hydrophilic silica is to some extent also negatively charged.4,34,37 As for C8E4, the analysis of the adsorption isotherm of C8G1 in relation to the structure of surface aggregates is not conclusive, although there are arguments in favor of the formation of a large number of small surface aggregates (micelles), rather than a small number of larger aggregates. In any case, it is conjectured that the striking difference in the sorption behavior of C8E4 and C8G1 must be related to the less hydrophilic but more flexible poly(oxyethylene) group of C8E4 as compared with the more hydrophilic but less flexible glucoside head. To analyze further the thermal behavior of the adsorption of C8E4 and C8G1 at the silica/water interface, the differential molar enthalpies of displacement ∆21h1 ) δ(∆21H)/δΓ1 are plotted as a function of surface coverage Γ1/Γmax, and on a reduced concentration scale, c/cmc, in Figure 7. The characteristic enthalpy data are included in Table 1. It should be emphasized that the points indicated in the figure are raw experimental data, obtained directly from simultaneous adsorption and calorimetric measurements and were not calculated from smoothed curves. Although the scatter of our data appears to be larger than in respective plots reported in the literature,30-37 the present results are free from systematic errors as will arise from a combination of separately measured adsorption and calorimetric enthalpy isotherms. This holds particularly for the low-affinity region, where (67) Smith, G. A.; Zulli, A. L.; Grieser, M. D.; Counts, M. C. Colloids Surf. 1994, 88, 67.

3314 Langmuir, Vol. 13, No. 13, 1997

Kira´ ly et al.

Figure 8. Enthalpy (∆21H), entropy (T∆21S), and Gibbs free energy (∆21G) functions of the displacement of water(2) by C8E4(1) on CPG silica at 298.15 K. Reference state equilibrium bulk solution. (Solid lines are drawn to guide the eye.)

Figure 7. Differential molar enthalpies of displacement of water by C8E4 (b) and C8G1 (O) on CPG silica glass at 298.15 K as functions of (a) surface coverage Γ1/Γmax and (b) reduced concentration c/cmc.

the extent of adsorption and the associated heat effects are very small (Figure 2), and in the aggregation regime, where the isotherms rise sharply within a narrow concentration range. In the low-affinity region (isolated adsorbate molecules), the differential enthalpies of displacement are on average -13.5 and -5.9 kJ‚mol-1 for C8E4 and C8G1, respectively (Table 1). It appears that ∆21h1 decreases (in absolute value) with increasing concentration, hence, with increasing adsorption in this region, as to be expected due to the presence of surface heterogeneities. The observed tendency in the low-affinity region is in agreement with previously reported calorimetric results on closely related (CmPEn-water/silica) systems.30-37 Although the differential enthalpies of displacement of water by C8E4 on CPG are smaller than for the previously investigated alkylphenol ethoxylates on amorphous silica, this is mainly attributable to the difference in the number of ethylene oxide segments in the surfactant molecules under comparison. Adsorption and calorimetric study of the displacement of water by poly(ethylene glycol) polymers on a macroporous silica revealed that at low surface coverage the molecules are adsorbed in a flat conformation on the surface and the fraction of bound segments is close to unity.68 The interaction between surface silanol groups and ethylene oxide segments was found to be weak, ∼-3 kJ‚mol-1 per segment (corresponding to -12 kJ‚mol-1 for E4, which compares well with the present results). A comparison of the differential enthalpies of displacements for C8E4 and C8G1 indicates well the effect of the head group on the strength of the adsorption. Clearly, adsorption from water on the silica surface is energetically more favorable for the tetraethylene glycol block than for the glucoside moiety, the latter having a higher affinity for water than the former. It is seen from a further inspection of Figure 7, that the differential molar enthalpy functions undergo dramatic (68) Trens, P.; Denoyel, R. Langmuir 1993, 9, 519.

changes at the csac; the functions turn from an exothermic to an endothermic direction and remain apparently constant in the aggregation region. The enthalpies of surface aggregate formation are 14.5 ( 1.5 kJ‚mol-1 for C8E4 and 6.2 ( 0.6 kJ‚mol-1 for C8G1, which are close to the corresponding enthalpies of micelle formation ∆micH in the bulk solution: 15.8 ( 0.7 and 7.5 ( 0.1 kJ‚mol-1 for C8E4 and C8G1, respectively (indicated by the arrows in Figure 7) as obtained from titration calorimetry in this work. The close agreement is a further calorimetric evidence that bulk micellization and surface aggregation are very similar phenomena, as concluded in similar calorimetric studies.27-37 Obviously, the structure of the aggregates (micelles) in contact with the solid must be in some respect distorted in comparison with that in the bulk solution; this is reflected by the difference between the corresponding enthalpies of aggregate formation. On the basis of their calorimetric results, Partyka et al. suggested that in the very close vicinity of surface saturation the adsorption/aggregation process becomes apparently athermal.30,31,33,35 We also experienced some decrease in the differential enthalpy data somewhere above the cmc (not indicated in Figure 7), but this deviation is at present within our experimental uncertainty. An application of the Gibbs adsorption isotherm equation to the present systems allows ∆21G, the Gibbs free energy function of displacement, to be calculated.69,70 A combination of the enthalpy data with the Gibbs function gives access to the entropy term as

Γ

∫0c c11 dc1

T∆21S ) ∆21H - ∆21G ) ∆21H + RT

1

The thermodynamic potential functions of the displacement of water by C8E4 and C8G1 on the silica glass are displayed in Figures 8 and 9, respectively. In each case, the Gibbs free energy decreases with increasing concentration, hence, with increasing adsorption. However, the overall change in ∆21G is rather small, resulting from enthalpy/entropy compensation. In the low-concentration region, the displacement of water by the surfactant molecules is controlled by an exothermic enthalpy term, due to a direct contact between the head groups and the (69) Woodbury, G. W., Jr.; Noll, L. A. Colloids Surf. 1983, 8, 1. (70) Everett, D. H. Colloids Surf. 1993, 71, 205.

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Langmuir, Vol. 13, No. 13, 1997 3315

the respective equilibrium concentrations of the bulk solution.71,72 A rigorous thermodynamic analysis, in which the interfacial thermodynamic potential functions are calculated with respect to the pure water/silica interface, will be published in the future. Conclusion

Figure 9. Enthalpy (∆21H), entropy (T∆21S) and Gibbs free energy (∆21G) functions of the displacement of water(2) by C8G1(1) on CPG silica at 298.15 K. Reference state equilibrium bulk solution. (Solid lines are drawn to guide the eye.)

hydrophilic silica surface. This process is accompanied by some entropy loss. At higher concentrations, the entropy term becomes dominant, which is attributed to the formation of surface aggregates. Hydrogen-bonded water clusters around the hydrophobic alkyl chains of the monomers are broken endothermically accompanied by a large entropy production. Although Figures 8 and 9 illustrate well the delicate balance between the (large) enthalpy and entropy terms, it should be emphasized that ∆21H, ∆21S, and ∆21G refer to the adsorption process at

According to our isothermal calorimetric enthalpy data, the main driving forces of the aggregation of C8G1 and C8E4 nonionic surfactants from their aqueous solutions at the hydrophilic silica/water interface are entropically driven hydrophobic interactions, i.e., very similar in nature to those in aqueous bulk solution. The extent of aggregative adsorption of C8G1 is significantly smaller than that of C8E4, which is related to the difference between the less hydrophilic but more flexible tetraethylene glycol group and the more hydrophilic but less flexible glucoside head. The sorption behavior of C8G1 displays some pecularities as compared with C8E4, originating from the stereochemistry and unique water bonding capacity of the sugar head. The thermodynamic potential functions of the formation of the adsoption layer indicate a delicate balance between the large entropy and enthalpy terms resulting in a moderate decrease in the Gibbs free energy. Acknowledgment. Z.K. thanks the Alexander von Humboldt Foundation for a research fellowship. LA9620768 (71) Kira´ly, Z.; De´ka´ny, I.; Klumpp, E.; Lewandowski, H., Narres, H. D., Schwuger, M. J. Langmuir 1996, 12, 423. (72) Denoyel, R.; Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1990, 136, 375.