Pulsed-Flow Microcalorimetric Study of the Template-Monolayer

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Pulsed-Flow Microcalorimetric Study of the Template-Monolayer Region of Nonionic Surfactants Adsorbed at the Graphite/Water Interface Zolta´n Kira´ly*,†,‡ and Gerhard H. Findenegg† Stranski Laboratory of Physical and Theoretical Chemistry, Technical University of Berlin, Strasse des 17 Juni 112, D-10623 Berlin, Germany, and Department of Colloid Chemistry, University of Szeged, Aradi Vt. 1, H-6720 Szeged, Hungary Received December 5, 2004. In Final Form: February 13, 2005 The formation of half-cylindrical surfactant aggregates at the graphite/aqueous solution interface is templated by an ordered monolayer of molecules disposed parallel to the graphite basal plane. Beyond a critical alkyl chain length, monolayer formation is effectively irreversible. Since enthalpic interactions in this template-monolayer region cannot be resolved with adequate accuracy by the traditional adsorption calorimetric methods, we applied a novel method, pulsed-flow calorimetry, for simultaneous measurement of the material balance and the enthalpy balance in this high-affinity region. For the three nonionic surfactants studied, n-octyl β-D-glucoside (C8G1), dimethyl-n-decylamine oxide (C10DAO), and n-octyl tetraethylene glycol monoether (C8E4), the adsorption was found to be strongly exothermic and effectively irreversible at low adsorbate densities, and the differential heat of adsorption markedly decreased with increasing surface coverage in this region. This deviation from the ideal adsorption behavior was attributed to intermolecular interactions within the adsorption layer rather than to surface heterogeneity of the graphite basal planes. A thermodynamic consistency test clearly demonstrated that pulsed-flow calorimetry is a unique experimental method for the study of nonreversible adsorption phenomena at solid/solution interfaces, representing an excellent tool to complement traditional methods, e.g., frontal-flow and titration adsorption calorimetry. Studies by the frontal-flow method revealed that aggregation on top of the surfactant monolayer was endothermic and reversible.

Introduction The regular patterns of atomic force microscopy (AFM) images reveal that a majority of ionic1-5 and nonionic5-9 surfactants self-assemble into half-cylindrical aggregates at the graphite/water interface. This structure is templated by a preformed ordered monolayer, in which the alkyl chains are adsorbed in their all-trans conformation to maximize contact with the hydrophobic surface of graphite. High-resolution scanning tunneling microscopy (STM) images have provided evidence that these flat-lying surfactant molecules are packed parallel to each other in a series of rows and that the molecules in neighboring rows are paired with their headgroups facing one another.10 The results of calorimetric studies have indicated that surfactant aggregation on top of the underlying monolayer is completely reversible. It is moderately * To whom correspondence should be addressed. E-mail: zkiraly@ chem.u-szeged.hu. † Technical University of Berlin. ‡ University of Szeged. (1) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (2) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (3) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 32073214. (4) Liu, J.-F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558-8567. (5) Kawasaki, H.; Ban, K.; Maeda, H. J. Phys. Chem. B 2004, 108, 16746-16752. (6) Kawasaki, H.; Syuto, M.; Maeda, H. Langmuir 2001, 17, 82108213. (7) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349-4356. (8) Grant, L. N.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294. (9) Holland, N. B.; Ruegsegger, M.; Marchant, R. E. Langmuir 1998, 14, 2790-2795. (10) Xu, S.-L.; Wang, C.; Zeng, Q.-D.; Wu, P.; Wang, Z.-G.; Yan, H.K.; Bai, C.-L. Langmuir 2002, 18, 657-660.

exothermic for ionic surfactants11,12 but endothermic for nonionic surfactants,13,14 reminiscent of the micelle formation in free solutions. In the template-monolayer region, the adsorption is always exothermic.11-20 This exothermic feature increases with increasing length of the alkyl chain.15,16,19,20 Above a critical chain length (ccl), the stability of the ordered monolayer becomes so high that the surfactant film cannot be leached away, even if the graphite surface is exposed to a stream of pure water for an extended period of time.5,9,13,19,20 Experimental studies are rather difficult to perform in the submonolayer region. Determination of the differential heat of adsorption as a function of surface coverage would be a plausible way to gain insight into the thermodynamics of monolayer formation. For high-affinity adsorption, however, the surface coverage cannot be resolved with adequate accuracy by traditional (immersion, titration, frontal-flow) adsorption calorimetric methods.20 When the surface becomes saturated with the solute at zero equilibrium concentration, or at solution concentrations that are finite, (11) Kira´ly, Z.; Findenegg, G. H. J. Phys. Chem. B 1998, 102, 12031211. (12) Kira´ly, Z.; Findenegg, G. H.; Klumpp, E.; Schlimper, H.; De´ka´ny, I. Langmuir 2001, 17, 2420-2425. (13) Kira´ly, Z.; Findenegg, G. H. Langmuir 2000, 16, 8842-8849. (14) Kira´ly, Z. In Thermal Behavior of Dispersed Systems; Garti, N., Ed.; Marcel Dekker: New York and Basel, 2001; pp 335-356. (15) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday Soc. 1966, 62, 979-986. (16) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday Soc. 1967, 63, 2264-2269. (17) Hey, M. J.; MacTaggart, J.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 699-707. (18) Gellan, A.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1985, 81, 1503-1512. (19) Findenegg, G. H.; Pasucha, B.; Strunk, H. Colloids Surf. 1989, 37, 223-233. (20) Kira´ly, Z.; Findenegg, G. H.; Mastalir, AÄ . J. Phys. Chem. B 2003, 107, 12492-12496.

10.1021/la047006c CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005

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but close to the detection limit of the analytical device used for the measurement of the adsorption isotherm, the measured heat divided by the amount adsorbed is an integral molar rather than a differential molar quantity. Consequently, all the thermodynamic information on the details of monolayer formation remains unavailable, as in the previously reported immersion15-18 and frontalflow11-14,19 adsorption microcalorimetric studies. To overcome such difficulties, we applied a novel method, pulsedflow calorimetry, which provided differential enthalpy data with fairly high resolution of the monolayer region for the adsorption of long-chain cationic surfactants on graphite.20 The method, which is an improved version of that proposed by Groszek,21 can be extended to the investigation of a variety of nonreversible adsorption phenomena at solid/ liquid interfaces. In the present study, we supplement the analysis of our frontal-flow calorimetric data on the adsorption of three nonionic surfactants at the graphite/ water interface.13,14,19 We show that pulsed-flow calorimery per se is a powerful method, and additionally an efficient tool with which to complement traditional adsorption calorimetry. Experimental Section Materials. The substrate Vulcan 3G is a granulated carbon black of Cabot Corp., Inc., graphitized by Sigri Elektrographite GmbH. The graphite pearls had a specific surface area of 68 m2‚g-1 to N2 at 77 K. n-Octyl β-D-monoglucoside (designated C8G1) was purchased from CalBiochem, and N,N-dimethyl-n-decylamine N-oxide (C10DAO) and n-octyl tetraethylene glycol monoether (C8E4) were purchased from Fluka. These compounds were used as received. Surfactant solutions were prepared with deionized water passed through a Milli-Q reagent water system. The cmc values of C8G1, C10DAO, and C8E4 at 298.15 K were found to be 27.1, 19.7, and 8.4 mM, respectively.13,22 Frontal-Flow Calorimetry.11-14,20,22 The adsorption experiments were performed by using a ThermoMetric LKB 2277 multichannel isothermal heat-flow microcalorimeter connected to an HPLC system (Knauer), as described in previous studies.11,14,22 The adsorption vessel, a small chromatographic column,23 was loaded with ca. 0.1 g of graphite and placed inside the measuring cylinder of the calorimeter. Degassed water and surfactant solutions with increasing concentrations were introduced into the column via a multiport electric switching valve connected to an HPLC micropump operated at a flow rate of 12 cm3‚h-1. For each concentration step, sufficient time was allowed for thermal equilibrium to be attained before the next step. The heat of adsorption was continuously recorded in the form of a series of calorimetric peaks. The concentration fronts passing through the column were continuously monitored by a differential refractive index detector (RID) connected to the outlet port of the calorimeter. For each step, the amount adsorbed was calculated from the retention time positioned at approximately the midsection of the ascending branch of the concentration front.11-14,21-23 The differential or the integral molar enthalpies of adsorption were calculated from the simultaneously measured adsorption and calorimetric data. Pulsed-Flow Calorimetry.20 The adsorption column was loaded with 0.1 g of graphite, and a permanent flow of water was applied at a rate of 12 cm3‚h-1. A 1 mM surfactant solution in portions of 1 cm3 was periodically injected into the eluent before the column inlet, and the heat evolved upon adsorption was measured for each pulse. Nonadsorbed surfactant residues passing through the loosely packed column without contact with the adsorbent bed were monitored by the RID in the form of small outlet peaks. For calibration purposes, the magnitude of the uniform inlet peaks was separately determined in the absence of the adsorbent. This procedure enabled us to calculate the (21) Groszek, A. J. Stud. Surf. Sci. Catal. 1999, 120A, 143-175. (22) Kira´ly, Z.; Bo¨rner, R. H. K.; Findenegg, G. H. Langmuir 1997, 13, 3308-3315. (23) Van Os, N. M.; Haandrikman, G. Langmuir 1987, 3, 10511056.

Kira´ ly and Findenegg amount irreversibly adsorbed by taking the difference in area of the inlet peak and the outlet peak recorded by the RID. For each pulse, the differential molar enthalpy of adsorption was obtained directly by dividing the enthalpy change by the increment of the adsorbed amount which caused that enthalpy change. Titration Calorimetry.14,24,25 The sample vessel of the calorimeter was loaded with 0.1 g of graphite and filled with 2 cm3 of water. The reference vessel was filled with 2 cm3 of water only. Both systems were kept under gentle stirring in two separate channels of the calorimeter. A 1 cm3 volume of surfactant solution in 0.1 cm3 aliquots was injected into the two vessels by using a motor-driven twin microsyringe (Lund). The concentration of the feed solution was chosen to be well below the cmc to eliminate heats of dilution originating from the decomposition of micelles. The differential molar enthalpies of adsorption were calculated as for the pulsed-flow method with the assumption that, for each titration step, all the surfactant molecules injected into the vessel are irreversibly adsorbed in the submonolayer region. Definitions of the Enthalpies of Adsorption at the Solid/ Solution Interface. Adsorption from solution onto a solid surface can be regarded as a displacement reaction between the solute (component 1) and the solvent (component 2) at the solid/liquid interface:11,26,27

(component 1)l + r(component 2)s a (component 1)s + r(component 2)l (1) where the molar ratio of displacement r is the number of moles of solvent displaced from the surface layer (superscript s) by 1 mol of solute from the bulk liquid phase (superscript l) at solute concentration c. The differential molar enthalpy of displacement ∆21h˙ 1 for the above reaction is given by11-13,26,27

∆21h˙ 1 ) (h1s - h1l) - r(h2s - h2l)

(2)

which depends on the reference states chosen for the two components. Below we use the term enthalpy of adsorption, instead of enthalpy of displacement, when the bulk solution in equilibrium with the adsorbed phase is taken as the reference state. With this choice, the enthalpy of adsorption ∆ah˙ is a direct experimental quantity, provided that ∆Γ is sufficiently small:

∆ah˙ ) ∆ah1 - r(∆ah2) )

( ) ∂∆aH ∂Γ

T,p,A

≈

∆(∆aH) ∆Γ

(3)

where ∆ahi is the partial molar enthalpy of adsorption of component i at solution concentration c in equilibrium with the surface concentration Γ of the solute and ∆aH is the integral enthalpy of adsorption per unit surface area of the solid. In a flow replacement experiment, ∆aH is the enthalpy change of the adsorbed phase measured when pure solvent initially filling the adsorption space (Γ ) 0 at c ) 0) is displaced by the solute, itself provided by a solution of concentration c replacing the solvent in a single step from c ) 0 to c; in parallel with this, the surface concentration increases from 0 to Γ. Alternatively, ∆aH is the enthalpy change of the adsorbed phase measured when pure solvent bound to the surface is displaced step by step by a set of solutions, in small steps of ∆c, in the range from 0 to c; in parallel with this, the surface concentration successively changes from 0 to Γ in small increments of ∆Γ. In accordance with eq 3, ∆ah˙ can be calculated from a combination of the adsorption isotherm (Γ vs c) and the integral enthalpy isotherm of adsorption (∆aH vs c) measured separately. Higher accuracy can be achieved when ∆(∆aH) and ∆Γ are simultaneously measured in small steps over a wide range of surface coverage. The link between the differential molar enthalpy of adsorption and the integral molar (24) Mehrian, T.; de Keizer, A.; Korteweg, A. J.; Lyklema, J. Colloids Surf. 1993, 73, 133-143. (25) Zajac, J.; Chorro, M.; Chorro, C.; Partyka, S. J. Therm. Anal. 1995, 45, 781-789. (26) Kira´ly, Z.; De´ka´ny, I.; Klumpp, E.; Lewandowski, H.; Narres, H. D.; Schwuger, M. Langmuir 1996, 12, 423-430. (27) Denoyel, R.; Rouquerol, F.; Rouquerol, J. J. Colloid Interface Sci. 1990, 136, 375-384.

Template-Monolayer Region of Nonionic Surfactants

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Figure 1. Frontal-flow experiment: (A) refractive index detector signal and (B) calorimeter signal of the stepwise adsorption of C10DAO from aqueous solutions onto graphite at 298.15 K (column packing 0.0893 g of graphite, flow rate 6.25 cm3‚h-1, concentration sequence 0, 0.5, 1, 2, 3, and 5 mM). Adsorption steps are followed by desorption steps. Adsorption is seen to be nonreversible in the monolayer plus transition region (c < 2 mM) and reversible in the surface aggregation region (c > 2 mM), up to the cmc ) 27 mM (not shown). enthalpy of adsorption can be expressed by two relations for the mean molar enthalpy of formation of an adsorption layer, ∆ah h:

∆ah h) ∆ah h)

1 Γ

∆aH Γ

∫ ∆ h˙ dΓ Γ

0

a

(4a) (4b)

We successfully applied eqs 4a and 4b to the present systems to check whether the frontal-flow method (measurements of ∆ah h) and the pulsed-flow method (measurements of ∆ah˙ ) provided thermodynamically consistent results. It may be noted that, for the special case of ideal adsorption behavior, the differential enthalpy of adsorption is independent of the surface coverage: ∆ah˙ ) ∆ah h ) constant at any Γ. Although both ∆ah˙ and ∆ah h are, at least in principle, experimentally accessible quantities, difficulty in the calorimetric determination of ∆ah˙ may arise when Γ is large at very low c. In fact, for high-affinity adsorption it is only the mean molar enthalpy of the formation of the adsorption layer, ∆ah h , which can be measured by traditional calorimetric methods. In the thermodynamics of adsorption phenomena, however, it is the variation of ∆ah˙ with Γ which is of particular interest. In the present work we demonstrate that pulsed-flow calorimetry is a very efficient method for the measurement of ∆ah˙ with high resolution in the nonreversible adsorption region. Further, we show that pulsed-flow calorimetry, frontal-flow calorimetry, and titration calorimetry provide mutually consistent results.

Results The instrumental response curves for a frontal-flow experiment are reported in Figure 1. The concentration fronts (in terms of refractive index variation) and the calorimeter heat-flow signals are given for an adsorptiondesorption cycle for a series of aqueous solutions of C10DAO adsorbed on graphite in the region c , cmc. The surfactant concentration of the flowing solution was increased step by step from 0 to 5 mM, after which it was successively changed in the reverse direction. The adsorption is strong and exothermic at high dilution: Γ ) 1.836 µmol‚m-2 and -∆aH ) 43.82 mJ‚m-2 were measured for the concentration step 0/0.5 mM, and not much lower values (Γ ) 1.708 µmol‚m-2 and -∆aH ) 43.26 mJ‚m-2) for the step 0/0.25 mM in a repeated experiment, as a

Figure 2. Pulsed-flow experiment: (A) refractive index detector signal and (B) calorimeter signal of the successive saturation of the surface of graphite with C10DAO in small portions from water at 298.15 K (column packing 0.0950 g of graphite, flow rate 11.70 cm3‚h-1, pulse concentration 1 mM, pulse volume 0.980 cm3). The input pulses were uniform throughout and are represented by the first peak in (A). Subsequent peaks in (A) are output pulses. The differences between the magnitudes of the input and output pulses are proportional to the change in the amount adsorbed.

consequence of the high affinity of the surfactant for the hydrophobic graphite basal plane. A further reduction of the concentration to attain lower Γ was not feasible because the RID signal approached the detection limit of the device (a concentration step of 0.25 mM produced a signal of 5.7 mV with a resolution of 0.25 mV), and the retention time became unreasonably long (>10 h) under the experimental conditions (column packing, flow rate) applied. The long retention time originates from the continuous adsorption of surfactant molecules from the feed solution of very low concentration until a high surface coverage is reached. The high-affinity adsorption of C10DAO is further evidenced by the irreversible nature of the adsorption at c < 1 mM (Figure 1B); the surfactant layer remained stable on the surface of graphite even in a stream of pure water. This is indicated by the large difference between the magnitudes of the calorimeter peaks for the first adsorption step (0/0.5 mM) and the last desorption step (0.5/0 mM). The adsorption turns into the endothermic direction at c > 1 mM, and the process becomes reversible at higher concentrations, up to a cmc of 27 mM.13 The reversal in sign of the enthalpy of adsorption occurs in a relatively narrow transition region between the exothermic formation of the ordered monolayer and the subsequent, endothermic surface aggregation induced by this monolayer.13 Parts A and B of Figure 2 depict the RID signal and the calorimeter signal, respectively, for a pulsed-flow experiment. A series of small pulses with the same volume (0.98 cm3) and the same concentration (1 mM) were injected into water as the eluent liquid just before the solution entered the adsorption column. ∆Γ was calculated from the difference between the areas of the input pulse (first peak in Figure 2A) and the output pulses (subsequent peaks). In this way, the first adsorption step in the frontalflow method in Figure 1 (Γ ) 1.836 µmol‚m-2 and -∆aH ) 43.82 mJ‚m-2) was resolved into 10 small steps by the pulsed-flow method; ∆Γ was about 0.14 µmol‚m-2 for each pulse, during which -∆aH gradually decreased from 4.8 to 2.5 mJ‚m-2. In a repeated experiment, an even higher resolution was achieved: ∆Γ) 0.011 µmol‚m-2 was introduced into the freshly packed column, eight times in

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Figure 3. Thermometric titration of an aqueous graphite dispersion (0.1020 g in 2 cm3) with 10 mM C10DAO solution in aliquots of 0.1 cm3. The calorimeter peaks are proportional to the differential heats of adsorption, provided that the adsorption is completely irreversible.

succession, which produced -∆aH values from 0.44 to 0.38 mJ‚m-2. At high monolayer coverages, as the surface aggregation region was approached, the injected solute continued to adsorb selectively. However, as the mode of adsorption became reversible, the pulsed-flow method was no longer applicable: while the RID signal displayed strongly elongated peaks (long tailings) with an ill-defined area, the calorimeter peaks were composed of overlapping negative and positive portions. For a fully reversible adsorption step, the opposite deflections of the composite calorimeter peak approximately canceled each other out, and the area of the input RID peak nearly coincided with that of the output RID peak. However, the signals were not suitable for a quantitative analysis. The present pulsed-flow calorimetric method is therefore restricted to the nonreversible adsorption region. The calorimeter signal for the thermometric titration of an aqueous suspension of graphite with an aqueous solution of C10DAO is given in Figure 3. For titration calorimetry, the amount adsorbed cannot be measured simultaneously with the enthalpy change; a separate adsorption experiment is required. This is the major disadvantage of titration calorimetry as compared with the other two methods discussed above. In view of the results of the flow adsorption experiments, however, it seemed reasonable to assume that all the surfactant species injected (0.15 µmol‚m-2 per injection) are adsorbed until saturation of the irreversibly adsorbing surface sites is attained. Accordingly, thermometric titrations were carried out in the nonreversible adsorption region, and the differential molar enthalpies of adsorption were calculated in accordance with eq 3. The enthalpogram in Figure 3 depicts a reduced baseline stability (drift), a higher level of noise, and a rather long equilibration time as compared with those in the flow methods. The instrumental response curves for all the systems studied were qualitatively similar to those in Figures 1-3. For C10DAO, the adsorption isotherm and the calorimetric enthalpy isotherm of adsorption are given in parts A and B of Figure 4, respectively. For C8G1, the isotherms are given in Figure 5. In each case, the ascending branch of the isotherm is located at zero equilibrium concentration, reflecting the nonreversible nature of the adsorption in the submonolayer region. In this high-affinity region, the cumulative isotherm Γ vs c and ∆aH vs c were determined by summation of the ∆Γ and ∆(∆aH) data acquired during the pulsed-flow experiment, which are indicated by the open symbols in Figures 4 and 5. The solid symbols in the figures are ascribed to a narrow overlap region and a broad surface aggregation region determined by the frontal-flow method. A less steep increase in Γ is observed throughout the reversible adsorption region until a plateau is reached at the cmc. In parallel with this, ∆aH turns from exothermic

Figure 4. (A) Adsorption isotherm and (B) enthalpy isotherm of adsorption for C10DAO-water/graphite at 298.15 K: solid symbols, frontal-flow method;13 open symbols, pulsed-flow method (present work). To avoid overcrowding, only a reduced number of pulsed-flow data are indicated.

Figure 5. (A) Adsorption isotherm and (B) enthalpy isotherm of adsorption for C8G1-water/graphite at 298.15 K: solid symbols, frontal-flow method;13 open symbols, pulsed-flow method (present work). To avoid overcrowding, only a reduced number of pulsed-flow data are indicated.

to endothermic in the transition region, i.e., between monolayer formation and surface aggregation, far below the cmc. After this, -∆aH decreases smoothly until a constant value is reached at the cmc. The figures clearly show the advantage of the combination of pulsed-flow calorimetry with frontal-flow calorimetry to obtain evenly distributed adsorption and calorimetric data over the entire range of c of interest. Discussion Determination of the differential molar enthalpies of adsorption as a function of surface coverage is a major

Template-Monolayer Region of Nonionic Surfactants

Figure 6. Enthalpies of adsorption for the system C10DAOwater/graphite at 298.15 K, plotted against the surface coverage as determined by different adsorption calorimetric methods. It should be noted that ∆ah h measured by frontal-flow calorimetry13 (first open circle) agrees well with ∆ah h (dashed line) calculated from the pulsed-flow data ∆ah h (solid line) by using eq 4b.

Figure 7. Enthalpies of adsorption for the system C8G1-water/ graphite at 298.15 K, plotted against the surface concentration as determined by different adsorption calorimetric methods. It should be noted that ∆ah h measured by frontal-flow calorimetry13 (first open circle) agrees well with ∆ah h (dashed line) calculated from the pulsed-flow data ∆ah˙ (solid line) by using eq 4b.

goal in the experimental thermodynamics of interfacial h are plotted phenomena. In Figures 6 and 7, ∆ah˙ and ∆ah against Γ for the systems C10DAO-water/graphite and C8G1-water/graphite, respectively. The two modes of adsorption, i.e., monolayer formation and surface aggregation, are clearly distinguished in this representation: the heat of adsorption steeply decreases and changes sign from negative to positive within a relatively narrow region of surface coverage, 1.6 < Γ < 1.9 µmol‚m-2. At this stage, the adsorbate density and the degree of ordering in the monolayer are sufficiently high to induce surface aggregation: the surfactant concentration in the equilibrium solution is called the critical surface aggregation concentration, csac.13,20,28 For the hydrophobic surface of graphite, csac , cmc, as predicted by theory28 and confirmed experimentally.5,11-13 It may be noted that completion of the template monolayer does not imply that the graphite surface is fully covered by horizontally adsorbed surfactant molecules; a significant portion of the graphite surface may remain covered by water molecules. In fact, adsorption measurements have indicated that Γ is always less than that for a perfect, closepacked monolayer predicted by geometric considerations.11-13,20,29 These findings of the analysis of the adsorption isotherms are in accordance with the AFM images, which indicated surface periodicities larger than (28) Johnson, R. A.; Nagarajan, R. Colloids Surf. 2000, 167, 21-36. (29) Groszek, A. J. Proc. R. Soc. London 1970, A134, 473-497.

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Figure 8. Structure of the interfacial layer for the adsorption of surfactants at the graphite/water interface (schematic). The surfactant molecules are adsorbed in their all-trans conformation on the surface hexagons, and along one of the three axes of symmetry of the basal planes. There is a reasonable lattice fit between the hydrogen atoms attached to one side of the chain and the centers of the graphite hexagons.29 The molecules assemble in a closely packed, ordered monolayer in a headto-head arrangement and parallel to each other. The formation of half-cylindrical aggregates occurs on top of this template monolayer. The periodic distance between the rows of aggregates is slightly more than twice the contour length of a surfactant molecule. The number of surfactant molecules in the crosssection of a (loosely packed) half-cylinder is in the range 4-7.3,5,11-13

twice the length of a fully extended surfactant molecule, the remaining space being filled by surface-bound water molecules.1-5,11-13,20 The arrows in Figures 6 and 7 indicate that the positive enthalpy of surface aggregation is close to that of micelle formation in bulk solution for both C10DAO and C8G1. The similarity of these values indicates that surfactant aggregation at surfaces is due mainly to hydrophobic interactions, just as in the free solutions. The proposed structure of the surface layer of surfactant molecules is sketched in Figure 8: the molecules are disposed side-by-side in ordered domains which are oriented along one of the three axes of symmetry of the graphite basal plane. This monolayer in turn acts as a template for the surface hemicylinders in such a way that the aggregates grow perpendicular to the horizontally adsorbed alkyl chains. For a detailed thermodynamic analysis of the aggregative adsorption region we refer to our previous studies on the present13 and related systems.11,12,19,20 Of particular interest in the present study are the calorimetric data measured in the high-affinity templatemonolayer region. The first open circle symbols in Figures 6 and 7 were determined by frontal-flow calorimetry. These enthalpy data are mean values averaged over a wide range h carries little of Γ, according to eq 4a. Consequently, ∆ah information concerning the progressive development of the ordered monolayer. In contrast, a large number of experimental data, in small increments of Γ, were collected in the submonolayer region by the pulsed-flow method: -∆ah˙ decreases from 40 to 15 kJ‚mol-1 for C10DAO and from 45 to 22 kJ‚mol-1 for C8G1 (open symbols in Figures 6 and 7, respectively). To check the mutual consistency between the results obtained by pulsed-flow calorimetry and frontal-flow calorimetry, the experimental data (solid lines in Figures 6 and 7) were evaluated by using eq 4b. The upper limit of the integral in eq 4b was taken as the surface coverage at which the first enthalpy data were h was then measured by the frontal-flow method, and ∆ah calculated from the ∆ah˙ vs Γ plot determined by the pulsedflow method. The result of the numerical integration is

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Figure 9. Enthalpies of adsorption for the system C8E4-water/ graphite at 298.15 K, plotted against surface concentration as determined by different adsorption calorimetric methods. It should be noted that ∆ah h measured by frontal-flow calorimetry19 (solid circle) agrees well with ∆ah h (dashed line) calculated from the pulsed-flow data ∆ah˙ (solid line) by using eq 4b.

represented by the dashed line running parallel to the Γ axis. Mathematically, this straight line passes through the -∆ah˙ vs Γ enthalpy decay curve, thereby creating two equal areas joined at around the common intersection point (Figures 6 and 7). For both C10DAO and C8G1, the good agreement between the integral molar enthalpy data obtained directly and indirectly is regarded as attested documentation that pulsed-flow calorimetry and frontalflow calorimetry provide mutually consistent results. While the frontal-flow method is a versatile tool for the study of reversible adsorption phenomena, the pulsedflow method is a unique tool for, but restricted to, the study of nonreversible adsorption phenomena. Further, one of the two methods can advantageously be supplemented by the other. For the adsorption of C8E4 at the graphite/water interface, a mean value of -∆ah h ) 32 kJ‚mol-1 was earlier reported in the monolayer region up to Γ ) 1.3 µmol‚m-2.19 This value was calculated from a combination of the adsorption isotherm and the enthalpy isotherm, measured separately by the depletion method and the frontal-flow method, respectively. For more details in the templatemonolayer region, we applied pulsed-flow calorimetry and measured ∆ah˙ with increasing Γ. The experimental data were analyzed in terms of eq 4b. As for C10DAO and C8G1, the thermodynamic consistency test proved to be positive: Figure 9 illustrates the excellent agreement between h value reported earlier19 and that determined from the ∆ah the ∆ah˙ vs Γ plot in the present study. A striking common feature of the three amphiphiles is the pronounced decrease in the exothermic character of the adsorption in the course of the formation of the monolayer. At infinitely low surface coverage -∆ah˙ is about 40, 45, and 90 kJ‚mol-1 for the adsorption of C8G1, C10DAO, and C8E4, respectively, and the respective values of -∆ah˙ at completion of the monolayer are only 15, 22, and 18 kJ‚mol-1 (Figures 6, 7, and 9). The very high initial value of -∆ah˙ for C8E4, as compared with those of the other two surfactants, indicates that the E4 chains of the molecules are lying flat on the surface and thus the number of surface contacts is significantly greater than for C8G1 and C10DAO. The pronounced decrease in -∆ah˙ with increasing Γ in the monolayer region cannot be attributed to energetic heterogeneity of the surface, as the hydrophobic surface of graphitized carbon black is energetically quite homogeneous.30 Indeed, an opposite trend, i.e., increasing -∆ah˙ with increasing Γ, has been reported for (30) Zettlemoyer, A. C. J. Colloid Interface Sci. 1968, 28, 343-369.

Kira´ ly and Findenegg

the cooperative adsorption of longer chain alkanols from nonpolar solvents onto graphitized carbon.31 For the present systems, the deviation from ideal adsorption behavior must be related to intermolecular interactions, rather than to surface heterogeneity, during monolayer formation. A variety of such interactions may be thought to occur upon adsorption, among others, a rearrangement of the monolayer in which the extended molecules, or some of these molecules, are lifted away from the surface protruding into the aqueous phase, the stabilization of the monolayer via additional interactions between the adjacent alkyl chains, the stabilization of the monolayer via specific interactions (e.g., hydrogen bonding) between facing headgroups, the loss of hydration water associated with the headgroups in the aqueous phase, the loss of hydration water around the alkyl chains, incipient surface aggregation on top of the growing monolayer patches, etc. These possibilities will be considered in the further discussion. A gradual reorientation of the adsorbed surfactant molecules from a horizontal to a vertical position or a partial lifting away from the surface, giving rise to the adsorption of further incoming molecules, is unlikely to occur in view of the strongly exothermic and irreversible nature of the adsorption. The physical picture of reorientation has been ruled out by recent AFM1-10 and calorimetric11-13,19,20 studies for a variety of surfactants adsorbed on graphite. The stabilization of the ordered arrays of long-chain paraffinic molecules by lateral interactions between the alkyl chains adsorbed onto graphite from nonpolar solvents and the additional stabilization of the monolayer by specific interactions between the headgroups of fatty alcohols and fatty acids have been well established calorimetrically31,32 and by STM.33 Little is known about the occurrence of such interactions in aqueous systems. AFM studies have indicated that ordering of the monolayer is a prerequisite for the formation of half-cylindrical surfactant aggregates; otherwise a featureless topmost layer is formed.7-9 However, regular patterns on AFM images were observed only above a certain ccl. This ccl is strongly influenced by the nature of the headgroup: the less hydrophilic the headgroup, the lower the ccl.7-9 While ccl is n e 8 for alkyl monoglucosides CnG1,13 it is n > 8 for alkyl maltosides CnM, which possess a disaccharide head.9 Similarly, while ccl is n > 12 for CnEm at m ) 23,7 it is n e 12 at m < 107 and, more specifically, n ) 10 at m ) 5.8 It is not clear whether the ccl necessary to template the formation of surface half-cylinders coincides with the ccl at which monolayer formation tends to become nonreversible.20 The influence of the headgroup on ccl for a given chain length and the influence of the chain length on ccl for a given headgroup remain questions that are open for future systematic studies. For sugar surfactants, the hydroxy groups of the neighboring heads are certainly capable of hydrogen bonding. The same holds true for the oligoethylene glycol and dimethylamine oxide heads. Neutral alkylamine oxides are known to be extremely sensitive to the solution pH: the amine oxide head progressively transforms to a hydroxide salt as the pH is increased, or to a cationic conjugate acid as the pH is decreased.5,6 Even under slightly acidic conditions, hydrogen bonds are formed (31) Bien-Vogelsang, U.; Findenegg, G. H. Colloids Surf. 1986, 21, 469-481. (32) Liphard, M.; Glanz, P.; Pilarski, G.; Findenegg, G. H. Prog. Colloid Polym. Sci. 1980, 67, 131-140. (33) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600-1615.

Template-Monolayer Region of Nonionic Surfactants

between the cationic-cationic and the cationic-nonionic heads. Such interactions may also occur for C10DAO, at least in the bulk liquid phase. However, such a cooperative adsorption mechanism implies additional attractive interactions in the monolayer, accompanied by favorable enthalpic interactions,31,32 for which the present study provides no evidence. Unlike the situation for nonionic surfactants, ∆ah˙ was found to be constant over a wide range of surface coverage for a series of even-numbered alkyltrimethylammonium bromides CnTAB with n ) 6-16.20 For the monolayer adsorption of ionic surfactants with chain lengths relevant to the present nonionics, -∆ah˙ was reported to be 35 kJ‚mol-1 for n-octyltrimethylammonium bromide20 and 43 kJ‚mol-1 for both sodium n-decyl sulfate12 and n-decyltrimethylammonium bromide,20 independent of Γ. It may be argued that the major difference between the head-to-head adsorption of ionic surfactants and nonionic surfactants is the electrostatic repulsion between the charged headgroups, which is absent for the uncharged nonionic heads. In fact, surface periodicities significantly larger than twice the length of a fully extended molecule have been observed on the AFM images of ionic surfactants adsorbed on graphite, this characteristic distance decreasing with gradual screening of the charge of the headgroups by the addition of simple electrolytes.1-4 Close-packing or further stabilization of the adsorption monolayer by lateral interactions between adjacent alkyl chains may also be moderated by Coulombic repulsion between adjacent headgroups. Our preliminary results on the adsorption of anionic alkyl sulfates further support the working hypothesis that the ideal behavior observed during the formation of an ordered monolayer of ionic surfactants adsorbed on graphite can be attributed to Coulombic repulsion between the hydrated ionic heads. An alternative explanation of the enthalpy decay before ccl is reached is the gradual occurrence of preaggregation at the interface, in parallel with the adsorption in the monolayer, the latter being exothermic and the former endothermic. This incipient association into small aggregates before the beginning of the propagation of surface hemicylinders would cause a depression of the highly exothermic character of the adsorption proceeding in direct contact with the solid surface. Because aggregation proved to be completely reversible, preaggregation should have also been reversible, which, however, was not experienced in the present adsorption microcalorimetric study. A more plausible explanation for the nonideal behavior of the monolayer is a progressive change in the state of hydration accompanied by a gradual change of the conformation of the surfactant molecules in the course of monolayer formation. Since the hydrophobic parts of each of the surfactants studied are similar to one another, the major difference in the hydration behavior of these amphiphiles may be attributed to the properties and molecular geometries of the respective hydrophilic components. The E4 head has a high affinity for hydration water because four times two lone pairs of electrons on the oxygen atoms are available for hydrogen bonding with the hydroxy hydrogen of water. For the G1 head, four hydroxy groups and two ether oxygen atoms can participate in hydrophilic hydration. In practice, the hydration number per headgroup was estimated from the volumetric determination of the hydration state of the corresponding amphiphiles to be 6-8 for E434 and 12-15 for G1.35 The strength of the hydrogen bond is in the range -10 to -40 (34) Briganti, B.; D’Arrigo, G.; Maccarini, M. J. Phys. Chem. B 2004, 108, 4039-4045. (35) La Mesa, C.; Bonincontro, A.; Sesta, C. Colloid Polym. Sci. 1993, 271, 1165-1171.

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kJ‚mol-1.36 Thus, the picture of a gradual change in the degree of hydration of the ethylene oxide segments with an increase of the adsorbate density can account for the large enthalpy decrease from 90 to 18 kJ‚mol-1 (Figure 9). However, although the hydration number is larger for C8G1 than for C8E4, the initial decrease in the heat of adsorption, in which dehydration might be included, is less significant for the former (Figure 7). This apparent contradiction disappears if it is assumed that, while the E4 headgroups tend to release hydration water, the more hydrophilic G1 heads mostly retain it as the adsorption in the monolayer region proceeds. This explanation appears reasonable and is supported by the noteworthy difference in clouding behavior of these two surfactants. The cloud point temperature is known to be associated with dehydration of the polar headgroup. Clouding occurs for C8E4 but not for C8G1 within the accessible temperature range, presumably due to the more favorable interactions between water molecules and the hydroxy groups in saccharides as compared with the corresponding interactions between water molecules and the oxygen atoms in oligooxyethylenes.37,38 The high stability of the hydration shell around the sugar moiety was further confirmed by the low extent of adsorption of C8G1 on hydrophilic silica as compared to that of C8E4 on the same substrate.22 For the adsorption of C10DAO, the change in ∆ah˙ is similar in magnitude to that for C8G1, but 3 times less than that for C8E4 (Figures 6 and 7). The enthalpy of solution of a nonionic substance at infinite dilution in water is a measure of the hydration state of the substance. This enthalpy change is -37 kJ‚mol-1 for C8E4,39 i.e., a factor of 3 higher than that for C10DAO.40 Further, the enthalpy of solution at infinite dilution is -31 kJ‚mol-1 for tetraethylene glycol C0E4,41 suggesting that dehydration of the alkyl chain plays a far less significant role than dehydration of the headgroup. The present results are supportive (but not conclusive) of the situation that the enthalpy decay is predominantly attributed, among the various possible intermolecular interactions, to a gradual headgroup dehydration upon close-packing in the monolayer. Further systematic studies of other nonionic surfactants are needed to clarify the observed deviation from ideal behavior in the templatemonolayer region. Conclusions C10DAO, C8G1, and C8E4 form hemicylinders at the graphite/water interface templated by an ordered surfactant monolayer. Surface aggregation is endothermic and reversible. In this region, the differential enthalpy of adsorption as a function of surface concentration can be measured by traditional adsorption calorimetric methods such as frontal-flow calorimetry and titration calorimetry. Adsorption in the template-monolayer region is exothermic and nonreversible, and only the mean molar enthalpy can be determined by traditional calorimetric methods in this regime. Pulsed-flow calorimetry, however, is ideally suited for determination of the differential enthalpy of adsorption as a function of surface coverage, with high resolution in (36) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985; Chapter 8. (37) Zhang, T.; Marchant, R. E. J. Colloid Interface Sci. 1996, 177, 419-426. (38) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 33823387. (39) Aratono, M.; Ohta, A.; Ikeda, N.; Matsubara, A.; Motomura, K.; Takiue, T. J. Phys. Chem. B 1997, 101, 3353-3539. (40) Benjamin, L. J. Phys. Chem. 1964, 68, 3575-3581. (41) Ohta, A.; Takiue, T.; Ikeda, N.; Aratono, M. J. Phys. Chem. B 1998, 102, 4809-4812.

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the template-monolayer region. The differential molar heat of adsorption markedly decreases with increasing surface coverage. Gradual dehydration of the headgroup seems to play the predominant role in the deviation from ideal adsorption behavior. The integral molar enthalpy of adsorption, calculated from the differential molar enthalpy data of pulsed-flow calorimetry, agreed very well with the mean molar enthalpy of adsorption measured by frontalflow calorimetry. This thermodynamic consistency test

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affords evidence that the experimental results are reliable and that the two calorimetric methods are compatible. Acknowledgment. This work was supported by a research fellowship from the Alexander von Humboldt Foundation (Z.K.), the Deutsche Forschungsgemeinschaft (Grant FI 235/15-1), and the Hungarian Scientific Research Fund (Grant OTKA T042521). LA047006C