Adsorption Properties of C10E8 at the Water− Hexane Interface

J. Phys. Chem. B 1998, 102, 10521-10527

10521

Adsorption Properties of C10E8 at the Water-Hexane Interface Michele Ferrari, Libero Liggieri, and Francesca Ravera* Istituto di Chimica Fisica Applicata dei Materiali, CNR, Via De Marini 6, 16149 GenoVa, Italy ReceiVed: June 23, 1998; In Final Form: September 9, 1998

A study of the equilibrium and dynamic adsorption properties of octaethylene glycol n-decyl ether (C10E8) has been done by using different tensiometric techniques. The adsorption isotherms have been characterized in the range from 15 to 35 °C at both water-air and water-hexane interfaces. In the same temperature range, the partition coefficient in the water-hexane system has been measured, and this allows the estimation of the standard enthalpy of transfer for C10E8. Moreover, the adsorption kinetics at a water-hexane interface has been studied by a capillary-pressure tensiometer. The results indicate that the adsorption properties, at both water-air and water-hexane interfaces, can effectively be described in the framework of a model in which the possibility of different orientations for the adsorbed molecules is considered. This is also in agreement with the results of the adsorption-kinetics experiments at the water-hexane interface, which cannot be described by a diffusion-controlled model with a Langmuir isotherm.

Introduction When the adsorption processes in liquid-liquid systems are studied, some special aspects have to be considered because, in most cases, these systems are more complex than those of liquid-air systems. The principal characteristic of such systems is the solubility of the surfactant in both phases, which is practically never negligible. This implies that the theoretical description for the treatment of the data is more complicated and needs to consider the transfer of surfactant across the interface during dynamic processes. Furthermore, in most cases, the volumes of the bulk phases may become important parameters.1,2 For these reasons, another important parameter is the partition coefficient which, besides characterizing the dynamic behavior of the system, is mandatory for an adequate evaluation of the equilibrium adsorption properties. With regards to the experimental techniques used to measure the dynamic interfacial tension of liquid-liquid systems, some restrictions in the choice of the method compared with those of liquid-air systems can be introduced. In fact, the methods providing the shortest time scale, i.e., the dynamic maximum bubble-pressure method and the dynamic drop-volume method, even in their upgraded versions,3,4 are not applicable to the liquid-liquid systems because of the flow-rate limitation coming from the viscosity properties. The drop-shape methods are applicable to liquid-liquid systems if the density difference is large enough to warrant the deformation of the drop by gravity, which, for example, is the case of the water-hexane system studied in this work. However, such techniques allow the adsorption dynamics to be studied only if the characteristic time is at least on the order of ca. 10 s. Capillary-pressure methods are particularly useful for studying the dynamic adsorption properties of liquid-liquid systems.5 In fact, these kinds of techniques, in particular the expandeddrop method,6 allow adsorption times on the order of tenths of a second to be studied, as proved by the experimental data presented here. * To whom correspondence should be addressed. Fax: +39 106475700. E-mail: [email protected].

The principal aim of this work is the characterization of the equilibrium adsorption properties of the octaethylene glycol n-decyl ether, C10E8, in the water-hexane system. An experimental study is presented in which a systematic evaluation of the partition coefficient of this surfactant in water-hexane is reported together with the adsorption isotherms as functions of the temperature. Recent works about polyoxyethylene glycol n-alkyl ethers, CiEj, show that both the equilibrium and dynamic properties of adsorption of these surfactants cannot be explained by the classical approaches of the diffusion-controlled adsorption and by the Langmuir or Frumkin equilibrium models. By the analysis of the apparent diffusion coefficient, Eastoe et al.7 have found that the adsorption kinetics for some CiEj is compatible with mixed-kinetics models involving potential barriers to adsorption on the order of 3-4RT, where R is the gas constant and T the absolute temperature. Lin et al.8 have observed for C12E8 a shift of the adsorption-controlling mechanism from diffusion controlled to mixed kinetics when the bulk concentration is increased. A similar behavior also has been observed for C10E8 at the water-air interface.9 More recently, models considering the orientation of the adsorbed molecules have been applied to explain the observed properties of this surfactant.10 Thus, polyoxyethylene glycol n-alkyl ethers seem to be ideal candidates for testing the existing theories for mixed kinetics and for developing new models for the description of the equilibrium properties. Materials Hexane was spectrophotometric-grade (Uvasol, Merck) and was used after purification as explained in the next section. The water used for the surfactant solutions was produced by a Millipore MilliQ purifier fed with distilled water. C10E8 (Sigma) was used as received without further purification. All parts entering in contact with measurement liquids were cleaned with chromic acid and rinsed several times with distilled water and finally with MilliQ water.

10.1021/jp9827429 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/25/1998

10522 J. Phys. Chem. B, Vol. 102, No. 51, 1998

Figure 1. Equilibrium interfacial tension of water-hexane interfaces vs the number of purification cycles. The first point corresponds to the hexane used as received, the dashed line indicates the value achieved by the column purification, and the last point indicates the value achieved after a further washing with MilliQ water.

Experiments A. Purification of Hexane. When surfactants at water-oil interfaces are studied, an important aspect which has to be considered is the purity of the oil phases. In the present work, the criterion previously proposed11 to judge the “surface chemical purity” of hexane has been used. This is based on the comparison of the two apparent equilibrium values of the interfacial tension achieved after a contraction and expansion of the interface. Moreover, an experimental procedure for the purification of alkanes similar to that previously proposed12 has been adopted. This technique consists of absorbing impurities contained in the alkane onto solid, basic alumina. This is achieved by several passages of the oil through an alumina column. In Figure 1, the interfacial tension measured after each purification cycle is reported and shows the effectiveness of the treatment. The first data point corresponds to the solvent as received from Merck without purification, and the last one is the value found after a final washing with the measurement water, which was intended to dissolve any water-soluble impurities. As is clear from Figure 1, this last treatment provides a further increase of γ compared to the plateau value achieved by the column purification. B. Measurement of the Partition Coefficient. The partition coefficient of C10E8 between water and hexane, kp, is here assumed to be the ratio between the concentration in hexane, ch, and that in water, cw, and it has been measured by using the experimental procedure described previously.13 This method is based on the determination of the bulk concentration of an aqueous solution by using the surface-tension value. Obviously, this is possible only for submicellar solutions because only in this case is there a mutual correspondence between surface tension and bulk concentration. In practice, two samples of hexane and aqueous surfactant solution at known concentration cw0, with known volumes, Vh and Vw, respectively, are kept in contact in a controlled-

Ferrari et al.

Figure 2. Equilibrium interfacial tension by ASTRA. In particular, dynamic surface tension γ(t) at water-air interfaces at T ) 25 °C for different concentrations of C10E8: (a) 0.2, (b) 0.8, (c) 2.0, (d) 4.0, (e) 8.0, (f) 20, (g) 40, and (h) 80 (10-8 mol/cm3).

temperature environment long enough to obtain the partition equilibrium. After that, the concentration of the aqueous depleted solution, cw, is evaluated by using the surface γeq-cw isotherm of the system at the water-air interface as a calibration curve. Then, the partition coefficient is calculated as

kp )

(

)

Vw cw0 -1 Vh cw

(1)

The error on this measure is

∆k )

[

(

) (

)]

Vw (cw0 - cw) ∆Vw ∆Vh cw0 ∆cw0 ∆cw + + + Vh cw Vw Vh cw cw0 cw

(2)

where ∆Vw, ∆Vh, and ∆cw0 are the errors of Vw, Vh, and cw0, respectively, and the error of cw is given by

∆cw )

| |

∂cw ∆γeq ∂γeq

(3)

which is calculated by the best-fit γeq-cw isotherm. The meaning of all of the terms present in eqs 2 and 3 has been thoroughly discussed.13 One of the points resulting from this discussion is that, to minimize this error, experimental parameters such as the volumes, Vw and Vh, and the concentration range must be suitably chosen. C. Measurement of the Equilibrium Interfacial Tension. The equilibrium surface-interfacial tensions, for water-air and water-hexane systems, have been determined by using a dropshape analysis technique based on ASTRA (Automatic Surface Tension Real-Time Acquisition).1,2,14,15 These data, obtained by waiting long enough to warrant the adsorption equilibrium values (Figure 2), are used to evaluate the surface isotherms for both systems. In particular, the surface tension for the water-air system has been measured by using a pendant drop of aqueous solution.

Adsorption Properties of C10E8 In this case, because of the small drop volume, the depletion due to the adsorption has to be considered to correctly evaluate the values of the bulk concentration. For this aim, an iteration procedure involving two steps has been used. At first, the correction is done by using the adsorption values calculated from the theoretical isotherm corresponding to a set of estimated parameters, and then the corrected concentrations are introduced in the fitting procedure of the isotherm to get a new set of parameters. This is iterated until the parameter values stabilize. The interfacial tensions of water-hexane systems, have been measured by using an emerging drop of hexane inside each aqueous solution. In this case, because of the large amount of surfactant available in the water phase, the depletion due to the adsorption is completely negligible; however, the impoverishment of the solution due to the transfer of surfactant into the hexane has to be considered. For this aim, the previously determined values of the partition coefficient have been utilized to calculate the actual concentration in water. D. Measurement of the Dynamic Interfacial Tension. Although the pendant-drop method is very effective for studying adsorption kinetics on large time scales and for evaluating equilibrium γ(c), because of intrinsic limitations, it does not allow a perfectly fresh interface (Γ ) 0) to be obtained at t ) 0. Thus, a difference exists between the effective surface age and the time since the formation of the interface. This is a common problem for several measurement methods of dynamic surface tension; therefore, these methods have to be chosen according to the time scale of the phenomena under study. In practice, the pendent-drop method is suitable for measuring surface ages longer than a second, so that, as shown in Figure 2 for C10E8, the dynamics of adsorption can be resolved fully only for low concentrations. In fact, only in this case can the initial surface coverage be neglected. At intermediate concentrations, the interpretation of γ(t) data deserves some particular consideration, because when the critical micelle concentration is approached, the method is in practice ineffective. For these reasons, the adsorption process of C10E8 at a waterhexane interface has been studied using a time scale on the order of a second and the expanded-drop (ED) method.6 This method, which belongs to the capillary-pressure class, is based on the utilization of the Laplace equation as a relationship between the capillary pressure and the curvature radius of a drop. In practice, after a large and rapid expansion of the drop interface, which makes the drop interface similar to a fresh surface, the interfacial tension is calculated by the simultaneous measurement of the capillary pressure and the drop radius. Actually, the tensiometer used in this work has been largely upgraded compared to that described previously.6 The major improvement has been the use of a piezoelectric rod embedded in the liquid forming the drop; this allows for a more flexible and efficient control of the interfacial area. In fact, by using an electric signal, it is possible to vary the rod length, which results in variations in the drop volume. Thus, the experimental sequence of surface-area variation characterizing the ED method can be easily imposed via software. At first, the formation of a hexane drop inside the aqueous solution is provided by a slow dilation of the piezoelectric rod until a hemispherical shape is reached. Then, its expansion of about half of its full range in 0.2 s provides a relative variation of the surface area, ∆A/A, of about 50. After that, the interfacial area is kept constant while the adsorption process evolves. The large extent of the interface expansion largely dilutes the adsorbed layer so that with this

J. Phys. Chem. B, Vol. 102, No. 51, 1998 10523

Figure 3. Equilibrium surface tension vs C10E8 concentration in water. Measured values for T ) 15 °C (b), T ) 20 °C (9), and T ) 25 °C (2) and theoretical surface isotherms.

TABLE 1: Partition Coefficient of C10E8 in Water-Hexane as a Function of Temperature T (°C)

kp

15 20 25 30 35

0.49 ( 0.05 0.85 ( 0.08 1.5 ( 0.1 1.7 ( 0.2 2.3 ( 0.3

method one can effectively simulate the process of adsorption on a fresh interface. Results and Discussion A. Partitioning Properties. In Figure 3, the surface tension versus the surfactant concentration data for the water-air system are reported for three different temperatures, T ) 15, 20, and 25 °C, together with the best-fit isotherms. These have been used as calibration curves to calculate the concentration of the depleted solution for the partition coefficient determination. The measurement of kp has been done for some values of temperature from 15 to 35 °C (see Table 1) and, as shown in Figure 4 in the considered temperature range, a linear increase of kp(T) is observed with a slope of 0.093 K-1. The results obtained for the partition coefficient as a function of the temperature can be better interpreted by analyzing in more detail the meaning of kp. If the solutions in the two phases are very diluted, which is in general true for monomeric surfactant solutions, kp can be written as

kp )

xih Vw xiw Vh

(4)

where xih and xiw are the molar fractions of the surfactant i in the hexane and water phases, respectively, and Vw and Vh are the molar volumes of the two solvents.

10524 J. Phys. Chem. B, Vol. 102, No. 51, 1998

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Figure 4. Measured partition coefficients (b) and calculated differences in standard chemical potential (2) vs temperature.

After the thermodynamic equilibrium condition for ideal solutions has been established, the ratio between the molar fractions is found in terms of the standard chemical potentials µ0ih and µ0iw, and eq 4 becomes

kp )

(

)

( )

Vw µ0ih - µ0iw Vw ∆µ0i exp ) exp Vh RT Vh RT

(5)

This provides an expression for ∆µ0i , which is an interesting thermodynamic parameter, as a function of measurable quantities. From eq 5, ∆µ0i has been evaluated for the present system by the experimental data of kp and by use of known values of the molar volumes. The results of this calculation are reported in Figure 4. Another thermodynamic quantity which can be determined by this kind of measurement is the molar standard enthalpy of transfer.16 In fact, considering eq 5 and assuming that the exponential term is much more sensitive to the temperature change than the molar volumes, one has

(

)

0 0 ∂ ln kp 1 ∂ µih - µiw )∂T R ∂T T

(6)

Moreover, from basic thermodynamic arguments, the following relationship holds between the chemical potential and the molar enthalpy,

Hi ∂ µi )- 2 ∂T T T

(7)

hexane and water, respectively, and then ∆Hi0 is the molar standard enthalpy of transfer. Equation 8 can also be written in a more useful way

∆H0i )R ∂(1/T)

∂ ln kp

(8)

where H0ih and H0iw are the molar enthalpies of the species i in

(9)

As shown in Figure 5, in the range of temperature studied, a linear relationship exists between the ln(kp) data and 1/T; thus, it is possible to calculate ∆H h 0i from the slope of the best-fit straight line, which in this case gives ∆H0i ) 5.7 × 104 J/mol. This is the variation of enthalpy corresponding to the transfer of 1 mol of surfactant from water to hexane. The value of ∆H0i , as well as the behavior of ∆µ0i (T), shows that in the range of temperature studied in this work the surfactant transfer from water to hexane is thermodynamically favored. B. Equilibrium Adsorption Properties. In Figure 6 the γeqcw isotherms for the water-hexane systems are shown for different temperatures. As in the case of the water-air system reported in Figure 3, the theoretical curves have been calculated based on the adsorption model developed by Fainerman et al.17 which assumes the coexistence of two different states for the adsorbed molecules. These states are characterized respectively by the ω1 and ω2 molar surface areas, which correspond to two possible orientations for the adsorbed molecules, and by the parameters b1 and b2, which are related to the surface activities. The relationship

()

b2 ) b1

Thus, eq 6 becomes

∂ ln kp H0ih - H0iw ∆H0i ) ≡ ∂T RT2 RT2

Figure 5. Logarithm of the measured partition coefficients vs 1/T and best-fit straight line. The slope of this curve is the molar standard enthalpy of transfer of C10E8 from water to hexane.

ω2 ω1

R

(10)

is also assumed, which means that for R ) 0 the surface activity is independent from the surface area of the adsorbed molecules and for R > 0 the molecules with larger surface area are more surface active.

Adsorption Properties of C10E8

J. Phys. Chem. B, Vol. 102, No. 51, 1998 10525

TABLE 2: Best-Fit Parameters ω1, ω2, b2, and r of the Surface Isotherm for C10E8 at the Water-Air Interface and Surface Tensions of the Pure System γ0 as a Function of Temperature T (°C)

γ0 (mN/m)

ω1 (cm2/mol × 109)

ω2 (cm2/mol × 109)

b2 (cm3/mol × 109)

R

15 20 25

73.35 72.50 71.97

9.2 9.9 11

3.8 4.0 4.0

2.2 2.9 3.6

3.5 3.3 4.2

TABLE 3: Best-Fit Parameters ω1, ω2, b2, and r of the Surface Isotherm for C10E8 at the Water-Hexane Interface and Surface Tensions of the Pure System γ0 as a Function of Temperature T (°C)

γ0 (mN/m)

ω1 (cm2/mol × 1010)

ω2 (cm2/mol × 109)

b2 (cm3/mol × 109)

R

15 25 35

51.73 50.62 49.19

0.85 1.0 1.1

3.7 4.3 4.6

1.4 3.6 5.6

7.0 6.6 6.3

Figure 6. Equilibrium interfacial tension vs C10E8 concentration in water for the water-hexane system. Measured values for T ) 15 °C (b), T ) 25 °C (9), and T ) 35 °C (2) and theoretical surface isotherms.

Thus, the isotherm resulting from this model depends on the parameters ω1, ω2, b2, and R, which can be found by a best fit procedure. It is important to notice that in this context the classical adsorption Langmuir model is found as a special case in which ω1 ) ω2; i.e., there is only one possible orientation, that with molar surface area 1/Γ∞ (Γ∞ being the saturation adsorption). For both the water-air and water-hexane systems, the theory fits well with the experimental data for R > 0 and with the values of the other parameters reported in Tables 2 and 3, respectively. The values of R at the water-hexane interface are about twice those at the water-air interface. Thus, the molecules adsorbing in the state with larger molar area, compared to the other state, are more surface active at the water-hexane interface than at the water-air interface. A possible justification for this behavior can be given by considering the partial lipophilic character of the polyoxyethylene chain,18 which is also consistent with the partitioning properties of this surfactant. This promotes the adsorption of the C10E8 molecules inclined along the water-hexane interface. The molar areas ω1 and ω2 increase with the temperature,

and as expected, they are practically unchanged for the waterair and the water-hexane interfaces. In the framework of this theoretical approach, for linear molecules such as C10E8, these two areas are expected to correspond to the normal and longitudinal orientations with respect to the surface. To verify this hypothesis, the ω1 and ω2 values found with this indirect evaluation have been compared with the maximum and minimum surface areas obtained by a computer simulation of the surfactant molecule. To this end, the geometry of the C10E8 molecule has been evaluated by computing with the Cerius2 program (Molecular Simulation, San Diego, CA) the Connely surface with a probe radius of 1 Å. The geometry was optimized by the semiempirical molecule orbital program MOPAC, with the AM1 parametrization.19 By this calculation, the area occupied by a molecule oriented along the surface is 180.4 Å2, corresponding to ω1 ) 1.1 × 1010 cm2, and for the other normal orientation, it is 67.4 Å2, corresponding to ω2 ) 4.1 × 109 cm2, results which are in very good agreement with the best fit parameters of the isotherms. Moreover, in some literature studies,20 the adsorption of C10E8 at the water-air interface has been interpreted according to the Langmuir model. The results found for 1/Γ∞ were 4.4 × 109, 4.2 × 109, and 4.1 × 109 cm2/mol for T ) 15, 20, and 25 °C, respectively. These values agree with the ω2 values reported in Tables 2 and 3. Thus, the model adopted here can be considered a refinement of the Langmuir model and shows the amount of molecules adsorbed in the state with larger surface area decreasing with increasing surface coverage.10 As shown in Figure 7, the two-state model remarkably improves the description of the equilibrium experimental data with respect to the Langmuir model. The surface activity of CiEj at the water-hexane interface is generally high, and even at c ) 10-9 mol/cm3, the surface coverage is quite large. Because there are many operational and technical difficulties in the evaluation of the γ(c) isotherms for concentration below about 10-9 mol/cm3, the study of the equilibrium properties in this range is problematic for many systems, because the behavior at small surface coverage has to be extrapolated from measurements done at large coverage. Because the two-state approach explicitly takes into account the features of the system at small coverage, it increases the suitability of this extrapolation, which is particularly useful for interpretation of dynamic adsorption properties. C. Dynamic Adsorption Properties. The dynamic interfacialtension data shown in Figure 8, as measured by the expandeddrop method, correspond to the adsorption of C10E8 at the water-hexane interface with a transfer of surfactant from water to the hexane drop at 25 °C. In fact, these measurements have been made while making sure that the hexane phase was initially

10526 J. Phys. Chem. B, Vol. 102, No. 51, 1998

Figure 7. Best-fit isotherms of the equilibrium interfacial tension vs C10E8 concentration in water for the water-hexane system at 25 °C [Langmuir (dashed line) and two-state (solid line) models]. The bestfit Langmuir parameters are Γ∞ ) 1.7 × 10-10 mol/cm3 and a ) 3 × 10-11 mol/cm3. Those for the two-state model are reported in Table 3.

Figure 8. Dynamic interfacial tension at the water-hexane interfaces by the expanded-drop method, at T ) 25 °C, for different initial C10E8 concentrations in water: (a) 0.2, (b) 0.4, (c) 0.8, (d) 1.0, (e) 2.0, and (f) 4.0 (10-8 mol/cm3).

free of surfactant, which can be achieved by discarding several drops before starting the experiment. The drop was formed at the inner edge of a glass capillary tube with an internal radius of 0.213 mm, and the relative area change ∆A/A was in the range from 41 to 45. The interpretation of these dynamic data deserves particular discussion. Unlike many other surfactants, C10E8 seems not to

Ferrari et al.

Figure 9. Experimental data of dynamic interfacial tension obtained at T ) 25 °C with initial concentration in water c ) 4 × 10-8 mol/ cm3, and theoretical diffusion-controlled adsorption curves calculated using the Langmuir model with Γ∞ ) 1.0 × 10-10 mol/cm2, a ) 1.06 × 10-12 mol/cm3, diffusion coefficient D ) 3.1 × 10-5 cm2/s (solid line) calculated using the short-time data for the fitting, and D ) 3.7 × 10-6 cm2/s (dashed line) calculated for extrapolation on the basis of ref 21. This particular Langmuir isotherm, in the time scale considered, is a good approximation of the two-state isotherm.

follow a diffusion-controlled adsorption process. In Figure 9 the theoretical dynamic surface-tension curves obtained by the diffusion-controlled model are compared with the experimental data. The theoretical curves have been calculated with the algorithm described previously2 by using a Langmuir adsorption isotherm and considering the transfer of surfactant from water to hexane. The saturation adsorption Γ∞ ) 1/ω1 has been used, with the value of ω1 reported in Table 3. The utilization of this Langmuir isotherm is not in contrast with the results found for the equilibrium properties. In fact, in the time range considered, the sublayer concentration as well as the surface coverage, remains very low. In the framework of the equilibrium model with two adsorption states, this corresponds to a situation in which most of the molecules adsorb in the larger molar-area state. Such a situation, therefore, can be well approximated by a Langmuir model. The two curves reported in Figure 9 have been calculated by using two values of the surfactant diffusion coefficient, D. The lower one has been extrapolated from the value obtained for C8E4 by PFGSE-NMR7,21 by assuming that the diffusion coefficients of CiEj scale as M-1/2, where M is the molecular weight. The other value of D has been calculated by fitting the short-time approximation for diffusion-controlled adsorption

(Dtπ )

Π ) 2Rtc

1/2

(11)

to the experimental data, where Π is the surface pressure and c is the initial bulk concentration. These values of diffusion coefficients lie in the realistic range expected for this surfactant. Thus, it is evident that whatever the value of D, adsorption evolves in a different way than that predicted by the diffusion-controlled model.

Adsorption Properties of C10E8 From all these considerations, it follows that the diffusioncontrolled-adsorption theory is not adequate to describe the kinetics of adsorption of C10E8 at a water-hexane interface because the observed deviation from this model seems not to be ascribable to a mistaken definition of the isotherm or of the diffusion-coefficient value. A wider treatment of the mixed adsorption dynamics involving the reorientation processes was given previously.10 The application of this treatment to the adsorption dynamics of C10E8 gives results which are consistent with the conclusion given here.

J. Phys. Chem. B, Vol. 102, No. 51, 1998 10527 Acknowledgment. The authors gratefully thank Dr. V. Buscaglia from CNR-ICFAM for his useful calculation of the geometric data of the C10E8 molecule. The authors also thank Dr. V. Fainerman (Institute of Technical Ecology, Donetsk, Ukraine), Dr. R. Miller (Max-Planck Institute, MPIKG, Berlin), Dr. A. Passerone (CNR-ICFAM), and the other members of the “Equilibrium and Dynamic Properties of Adsorbed Layers” Topical Team of the European Space Agency for some helpful discussions. M.F. acknowledges the Italian Space Agency and the Italian National Research Council for their financial support. This work was partially supported by the CNR-ASI Research Contract Number ARS-98-43.

Summary and Conclusions The equilibrium and dynamic adsorption of C10E8 at liquid interfaces can be effectively described by assuming a model17 with two possible adsorption states for the molecules, which are characterized by different orientations and by different molar areas. This result is also supported by the good agreement of these areas with the values calculated by the geometry of the molecule. The differences between the isotherms at water-air and water-hexane interfaces are consistent with the partial lipophilic character of the polyoxyethylene chain and with the partitioning properties of the surfactant. Moreover, the dynamic behavior of C10E8 at the water-hexane interface cannot be explained in the framework of the classical diffusion-controlled model. The equilibrium characteristics of the system lead one to expect that the adsorption dynamics are driven by different processes, among them the orientation of the adsorbed molecules. This makes a mixed-kinetics approach more suitable to describe the system. The characterization of the adsorption properties described in this paper has been undertaken in the framework of a larger project aimed at studying the dynamic aspects of adsorption in conditions of reduced gravity (microgravity). In fact, in these conditions, the convective motions due to the temperature and the concentration gradients are strongly reduced, making bulk transport purely diffusive. Therefore, microgravity provides a unique environment in which to perform adsorption dynamic experiments in very well-defined transport conditions. The adsorption kinetics of C10E8 at water-hexane interfaces will be extensively studied by the authors, with expanded-drop experiments using FAST-SH (Facility for Adsorption and Surface Tension studies on SpaceHab), an automatic capillarypressure tensiometer built by the European Space Agency, that will be carried onboard the STS-95 NASA Shuttle mission in October 1998.

References and Notes (1) Ferrari, M.; Liggieri, L.; Ravera, F.; Amodio, C.; Miller, R. J. Colloid Interface Sci. 1997, 186, 40. (2) Liggieri, L.; Ravera, F.; Ferrari, M.; Passerone, A.; Miller, R. J. Colloid Interface Sci. 1997, 186, 46. (3) Fainerman, V. B.; Miller R. In Drops and Bubbles in Interfacial Research; Mo¨bius, D., Miller, R., Eds.; Studies in Interface Science Series; Elsevier: Amsterdam, 1998; Vol. 6, p 279. (4) Miller, R.; Fainerman, V. B. In Drops and Bubbles in Interfacial Research; Mo¨bius, D., Miller, R., Eds.; Studies in Interface Science Series; Elsevier: Amsterdam, 1998; Vol. 6, p 139. (5) Liggieri, L.; Ravera, F. In Drops and Bubbles in Interfacial Research; Mo¨bius, D., Miller, R., Eds.; Studies in Interface Science Series; Elsevier: Amsterdam, 1998; Vol. 6, p 239. (6) Liggieri, L.; Ravera, F.; Passerone, A. J. Colloid Interface Sci. 1994, 169, 226. (7) Eastoe, J.; Dalton, J. S.; Rogueda, P. G. A.; Crooks, E. R.; Pitt, A. R.; Simister, E. A. J. Colloid Interface Sci. 1997, 188, 423. (8) Lin, S.-Y.; Tsay, R.-Y.; Lin, L.-W.; Chen, S.-I. Langmuir 1996, 12, 6530. (9) Chang, H.-C.; Hsu, C.-T.; Lin, S.-Y. Langmuir 1998, 14, 2476. (10) Miller, R.; Aksenenko, E. V.; Liggieri, L.; Ravera, F.; Ferrari, M.; Fainerman, V. B. Langmuir 1998, in press. (11) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987, 120, 176. (12) Goebel, A.; Lunkenheimer, K. Langmuir 1997, 13, 369. (13) Ravera, F.; Ferrari, M.; Liggieri, L.; Miller, R.; Passerone, A. Langmuir 1997, 13, 4817. (14) Liggieri, L.; Passerone, A. High Temp. Technol. 1989, 7, 82. (15) Liggieri, L.; Ravera, F.; Passerone, A. J. Colloid Interface Sci. 1994, 169, 238. (16) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1992; Vol. 1, p 269. (17) Fainerman, V. B.; Miller, R.; Wu¨stneck, R.; Makievski, A. V. J. Phys. Chem. 1996, 100, 7669. (18) Bailey, F. E.; Koleske, J. V. In Nonionic Surfactants; Schick, M. J., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1987; Vol. 23, p 958. (19) Stewart, J. J. P. J. Comput.-Aided Mol. Des. 1990, 4, 1. (20) Van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physico-Chemical Properties of Selected Anionic, Cationic, and Nonionic Surfactants; Elsevier: Amsterdam, 1993; p 123. (21) Faucompre, B.; Lindmann, B. J. Phys. Chem. 1987, 91, 383.