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Adsorption Enthalpies of Cationic and Nonionic Surfactants on Silica Gel. 2. Microcalorimetric Determination of Adsorption Enthalpies J. Seidel,* C. Wittrock,† and H.-H. Kohler† Institute of Physical Chemistry, Freiberg University of Mining and Technology, D-09596 Freiberg/Sa., Germany, and Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany Received April 15, 1996. In Final Form: August 2, 1996X There are two common methods for the experimental determination of adsorption enthalpies of surfactants on solid surfaces in aqueous solution, either directly by calorimetry or indirectly by using a ClausiusClapeyron equation. The present paper is the second part of a two-part contribution dealing with a comparison of both methods. In this part calorimetric results on the adsorption of a nonionic (n-octyltetraoxyethylene) and a cationic (cetylpyridinium chloride) surfactant on a wide pore silica gel are presented for different temperatures between 25 and 40 °C. For the fast adsorbing nonionic surfactant good quantitative agreement between the isosteric and calorimetric differential molar heats of adsorption (13.5 ( 1 kJ/mol) is found above a relative surface coverage of 0.1. For the cationic surfactant the agreement between the isosteric and the calorimetric enthalpy data is more qualitative. The reason is probably that the adsorption rate is too slow to reach equilibrium in the calorimetric experiments (flow calorimetry). The differential molar enthalpies of adsorption of the cationic surfactant are exothermic. Advantages and limitations of the calorimetric and isosteric method are discussed.
Introduction The adsorption of surfactants on solid surfaces is a very complex process influenced by a large number of parameters. Therefore, scientific understanding is still limited although extensive experimental investigations have been performed by means of different experimental techniques1-17 and although a number of new theoretical models have been developed.18-25 It has been shown recently that the differential molar enthalpy of adsorption, * To whom correspondence should be addressed. † University of Regensburg. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) Levitz, P. Langmuir 1991, 7, 1595. (2) Esumi, K.; Nagahama, T.; Meguro, K. Colloids Surf. 1991, 57, 149. (3) Seidel, J. Thermochim. Acta 1993, 229, 257. (4) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (5) Leimbach, J.; Sigg, J.; Rupprecht, H. Colloids Surf., A 1995, 94, 1. (6) Wangnerud, P.; Berling, D.; Olofsson, G. J. Colloid Interface Sci. 1995, 169, 365. (7) Noll, L. A.; Gall, B. L. Colloids Surf. 1991, 54, 41. (8) Partyka, S.; Keh, E.; Lindheimer, M.; Groszek, A. Colloids Surf. 1989, 37, 309. (9) Partyka, S.; Lindheimer, M.; Zaini, S.; Keh, E.; Brun, B. Langmuir 1986, 2, 101. (10) Gellan, A.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2235. (11) Sivakumar, A.; Somasandaran, P.; Thach, S. J. Colloid Interface Sci. 1993, 159, 481. (12) Giordano, F.; Denoyel, R.; Rouquerol, J. Colloids Surf., A 1993, 71, 293. (13) Soderlind, E.; Stilbs, P. Langmuir 1993, 9, 1678. (14) Mehrian, T.; De Keizer, A.; Korteweg, A. J.; Lyklema, J. Colloids Surf., A 1993, 73, 133. (15) Desnoyers, J. E.; Bouzerda, M.; Cote, J. F.; Perron, G. J. Colloid Interface Sci. 1994, 164, 483. (16) Thomas, F.; Bottero, J.; Partyka, S.; Cot, D. Thermochim. Acta 1987, 122, 197. (17) Tiberg, F.; Jonsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (18) Lajtar, L.; Narkiewicz-Michalek, J.; Rudzinski, W. Langmuir 1994, 10, 3754. (19) Zhu, B.-Y.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3813. (20) Schwarz, R.; Heckmann, K.; Strnad, J. J. Colloid Interface Sci. 1988, 124, 50. (21) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251.
S0743-7463(96)00357-5 CCC: $12.00
∆Had, is of great value for studying directly the different stages of adsorption of surfactants from solution3,26-31 because of its sensitivity to changes in the mechanism. Often remarkable changes in the adsorption mechanism are not as clearly reflected in adsorption isotherms as in the corresponding enthalpies of adsorption. The knowledge of both the adsorption and the enthalpy data suggests that we use these data simultaneously within new thermodynamically consistent models of adsorption as developed and published recently.32 The success of these new approaches depends not only on the reliability of the model but also on the consistency of the experimental data. An open problem is the comparability of the directly measured differential molar heats of adsorption with the so-called isosteric heats of adsorption obtained from the temperature dependence of the adsorption isotherms. In the past the consistency of these data has been discussed controversially14,33 because of the simplifying assumptions entering into the Clausius-Clapeyron equation used in the form
∆Had ) -RT2
∂ ln c2 | ∂T p,Γ2
(1)
This equation is only exact for one-component adsorption. Γ2 is the surface excess concentration of the surfactant, and c2 is the bulk concentration. Some obvious sources of errors are (i) adsorption of additional components and (22) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463. (23) Woodbury, G. W.; Noll, L. A. Colloids Surf. 1988, 33, 301. (24) Cases, J. M.; Villieras, F. Langmuir 1992, 8, 1251. (25) Van Os, N. M.; Rupert, L. A. M.; Smit, B.; Hilbers, P. A. J.; Esselink, K.; Bo¨hmer, M. R.; Koopal, L. K. Colloids Surf., A 1993, 81, 217. (26) Trompette, J. L.; Zajac, J.; Keh, E.; Partyka, S. Langmuir 1994, 10, 812. (27) Mehrian, T.; De Kaizer, A.; Lyklema, J. Langmuir 1991, 7, 3094. (28) Partyka, S.; Rudzinski, W.; Brun, B.; Clint, J. H. Langmuir 1989, 5, 297. (29) Rouquerol, J. Thermochim. Acta 1985, 96, 377. (30) Partyka, S.; Lindheimer, M.; Faucompre, B. Colloids Surf., A 1993, 76, 267.
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(ii) use of the concentration instead of the activity of the surfactant in solution. Unfortunately, the few available experimental data dealing with the problem of comparability are contradictory.14,34 This situation gave reason for a detailed theoretical and experimental study of the comparability of calorimetric and isosteric heats of adsorption in our present work. In an earlier paper, the authors have published a generalized Clausius-Clapeyron equation for systems with several adsorbing components based on a detailed thermodynamic analysis of the adsorption process.35 In continuation of this theoretical work, we report in this contribution and in part 1 of this series36 isosterically and calorimetrically determined heats of adsorption of two model systems. The adsorptions of a common nonionic surfactant (n-octyltetraoxyethylene, C8E4) and of a common cationic surfactant (n-cetylpyridinium chloride, CpCl) on silica gel are used as model cases. In this paper attention is focused on the calorimetric study by means of a liquid flow microcalorimetric technique. The adsorption isotherms, necessary for the calculation of the differential molar heats of adsorption, have been determined by means of an equivalent flow method. In addition, the temperature dependence of the adsorption and the micellization of the surfactants in solution have been investigated in the temperature range from 25 to 40 °C. The results will be discussed with respect to the properties of the surfactants. Conclusions will be drawn concerning the usability and consistency of the two methods used for the determination of the adsorption enthalpies. Materials and Experimental Techniques Adsorbent. For both the static and the flow adsorption experiments a well-defined high-purity silica gelsFractosil 1000sdelivered by Merck (Darmstadt, Germany) was used. This silica gel is characterized by a narrow particle size distribution (200-500 µm) and an average pore radius of 60 nm. These relatively wide pores help to reduce diffusion problems of adsorbing surfactants. The small content of fines in the Fractosil sample has been removed by sedimentation and decantation of the aqueous suspension. The remaining fraction was dried at 105 °C. The specific surface area measured by a N2 BET method (ASAP 2000, Micromeritics) is 21.1 ( 0.4 m2/g. Surfactants. The cationic surfactant CpCl was provided by Merck (Darmstadt, Germany) as monohydrate with a purity of >97%. The sample was recrystallized from an ethanol/acetone mixture several times for further purification. The surface tension vs concentration curves (Laser Scanning Tensiometer, LASDA, Ferstl GmbH, Hemau, Germany) did not show a minimum near the cmc, which proves the purity of the sample. The cmc at 25 °C was 0.85 mmol/L, in good agreement with the value of 0.9 mmol/L at 20 °C reported in the literature.37 The nonionic surfactant C8E4 was a high-purity sample (>99%) from Bachem Biochemica (Heidelberg, Germany) with a defined chain length. Further purification has been carried out using a recently described extraction procedure.38,39 The cloud point of an aqueous (31) Tan, X.; Yan, H.; Hu, R.; Hepler, L. G. Chin. J. Chem. 1992, 10, 200. (32) Narkiewicz-Michalek, J.; Rudzinski, W.; Keh, E.; Partyka, S. Colloids Surf. 1992, 62, 273. (33) Denoyel, R.; Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1990, 136, 375. (34) Killmann, E.; Eckart, R. Makromol. Chem. 1971, 144, 45. (35) Wittrock, C.; Kohler, H.-H. J. Phys. Chem. 1993, 97, 7730. (36) Wittrock, C.; Seidel, J.; Kohler, H.-H. Langmuir, preceding paper in this issue. (37) Heckmann, K.; Schwarz, R.; Strnad, J. J. Colloid Interface Sci. 1987, 120, 114. (38) Schubert, K.-V.; Strey, R.; Kahlweit, M. J. Colloid Interface Sci. 1991, 141, 21. (39) Schubert, K.-V.; Strey, R.; Kahlweit, M. Prog. Colloid Polym. Sci. 1991, 84, 103.
Seidel et al. solution of mass fraction 1%, which is very sensitive to impurities, has been determined to be 42.3 °C, in very good agreement with the 42.1 °C reported in the literature.38 Concentrated stock solutions were prepared by weight from the purified surfactants and ultrapure water (Milli-Q Plus, Millipore Corp.). Additional details about the substances and the pretreatment procedures were reported in part 136 of this series. Liquid Flow Microcalorimetry. All adsorption enthalpies were measured using a home-made highly sensitive flow adsorption calorimeter based on a Calvet-type microcalorimeter. The calorimeter is characterized by a minimal detectable heat power of 1 µW and a good long-term stability of the baseline. The flow system consists of several heat exchangers and a special adsorption column which can hold a maximum volume of 0.5 cm3 of adsorbent. Details concerning the construction, properties, and efficiency of the calorimetric system have already been described in refs 3, 40, and 41. All measurements were carried out at a constant flow rate of 20 cm3/h using a cumulative procedure; i.e., the concentration of the surfactant solution was increased in a stepwise manner after reaching the apparent equilibrium state. Dilution effects were negligible up to the cmc of the surfactant. In addition, the surfactant solutions had to be degassed carefully (vacuum degassing) to avoid the formation and accumulation of air bubbles in the adsorption cell, especially at temperatures above room temperature. Air bubbles would cause an increasing signal noise and drift. Differential molar heats of adsorption have been calculated using the approximation
∆Had )
∆(∆had) ∆Γ
(2)
where ∆(∆had) is the increment of the experimental integral heat of adsorption measured over a small adsorption step and ∆Γ is the corresponding change in the surface excess of the surfactant. ∆Γ is obtained from the corresponding adsorption isotherm. Adsorption Isotherms. For calculation of the differential molar heats of adsorption from the calorimetrically determined integral heats of adsorption the corresponding amounts of adsorbed surfactant must be known. Therefore, the adsorption isotherms have been measured by means of a flow method working under the same experimental conditions (temperature, flow rate, geometry of the adsorption cell, weight of adsorbent, measuring time) as for the flow calorimeter. UV (for CpCl) or RI (for C8E4) detectors (KNAUER variable wavelength monitor, KNAUER differential refractometer, KNAUER Berlin, Germany) were used to record the surfactant concentration at the outlet of the adsorption column. The amounts of surfactant adsorbed were calculated from the retention volumes, as described in refs 42-44. The calculated values can be distinctly smaller than those obtained from static equilibrium methods which may be due to transport limitations in the flow system.45 Batch Calorimetry. Differential molar heats of solution of the surfactants added to solutions of different concentrations were measured by means of a precision reaction calorimeter LKB 8700 (LKB Producter AB, Sweden) using a 100 cm3 reaction cell and standard 1 cm3 glass ampules. The reaction cell was filled with surfactant solution (concentrations below and above the cmc). The heats of solution were determined by breaking the glass ampule containig a small amount of pure surfactant. The temperature change was recorded by a computer-controlled data acquisition system, and the appropriate corrections were made by standard methods.46,47 The experimental uncertainties of the differential molar heats of solution were less than 0.5 kJ/mol. (40) Seidel, J. J. Therm. Anal. 1988, 33, 317. (41) Seidel, J. Prog. Colloid Polym. Sci. 1992, 89, 176. (42) Seidel, J. In Thermische Analysenverfahren in Industrie und Forschung; University Jena Press: Jena, 1990; p 149. (43) Noll, L. A.; Burchfield, T. E. Colloids Surf. 1982, 5, 33. (44) Van Os, N. M.; Haandrikman, G. Langmuir 1987, 3, 1051. (45) Kwok, W.; Nasr-El-Din, H. A.; Hayes, R. E.; Sethi, D. Colloids Surf. A 1993, 78, 193. (46) Wadso¨, I. Sci. Tools 1966, 13, 1. (47) Hemminger, W.; Hoehne, G. Calorimetry-Fundamentals and Practice; Verlag Chemie: Weinheim, Deerfield Beach, Basel, 1984; Chapter 6.
Adsorption Enthalpies of Surfactants on Silica Gel
Figure 1. Adsorption isotherms for C8E4 on Fractosil determined by the flow method at different temperatures. The error bars indicate the typical standard deviation in Γ. (Solid lines are fits from the Gu model equation.)
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Figure 2. Integral heats of adsorption for C8E4 on Fractosil at different temperatures. The error bars indicate the typical standard deviation of the flow calorimetric experiments. (Solid lines are drawn to help the eye.) Table 1. Fitted Gu Parameters for C8E4 and CpCl Adsorption Isotherms (Flow Method) Θ (°C)
Results and Discussion Calorimetric Measurements. C8E4. Adsorption isotherms and integral adsorption enthalpies determined by the described flow methods are shown in Figures 1 and 2 for different temperatures. The adsorption of the nonionic surfactant is characterized by a two-step isotherm. At low concentrations (
