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Langmuir 1997, 13, 6274-6278
A Critical Comparison of Adsorption Models for Soluble Surfactants S. Bae, M. Harke, A. Goebel, K. Lunkenheimer, and H. Motschmann* Max-Planck Institute of Colloids and Interfaces, Rudower Chaussee 5, D-12489 Berlin, Germany
D. Prescher Institut fu¨ r Du¨ nnschichttechnologie und Mikrosensorik e.V, Kantstrasse 55, D-14513 Teltow, Germany Received August 8, 1997X The adsorption isotherm of a soluble anionic surfactant was measured and discussed within the framework of established thermodynamical models. The adsorbed amount derived by these models was compared with the results obtained by second harmonic generation, SHG. SHG can be used as a highly surface specific probe which allows the determination of orientation and number density. Since no thermodynamic quantities are involved in the interpretation of SHG measurements, the data can be used to assess the validity and underlying assumptions of thermodynamic adsorption models. The SHG experiments reveal a good agreement with the Frumkin model. No indication of a subsurface layer can be found.
I. Introduction Adsorption layers of soluble surfactants at the airwater interface and the oil-water interface play an important role in daily life and industrial processes. As a result of their importance, a lot of research has been undertaken over the years to understand the nature of these adsorption layers and their static and dynamic properties.1,2 A challenging aim is still the understanding of the macroscopic properties of the adsorption layer on the basis of the molecular structure of the surfactant. This knowledge would provide the means to deliberately tune surface properties according to requirements imposed by the application. In addition it would express a deep level of understanding of the underlying processes. Unfortunately, the nature of liquid interfaces imposes severe restrictions on the choice of experimental methods. The most powerful surface analytical methods are based on the scattering of charged particles at the surface and cannot be applied at air-liquid or liquid-liquid interfaces. Consequently, the investigation of soluble surfactants is mainly based on measurement of the equilibrium surface tension, σe, as a function of the bulk concentration, c, of the surfactant. The isotherm σe(c) is discussed within the framework of thermodynamic theory. Several models have been suggested and are used in order to retrieve relevant data of the adsorption layer like area per molecule, adsorbed amount, and interaction energy from the equilibrium σe(c) isotherm. However, there is still some discussion about the validity of the different surface equations of state.3 The simplest description is provided by the LangmuirSzyskowski model which uses elements of the ideal gas theory. The model predicts that the maximum surface coverage, Γ∞, is only dependent on the cross sectional area of the surfactant. Any interactions between the molecules within the adsorbed layer are neglected.4 The Frumkin model introduces an interaction parameter a, to take the * Corresponding author: electronic mail:
[email protected]. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Mo¨hwald, H. Rep. Prog. Phys. 1993, 56, 653. (2) Miller, R.; Joos, P.; Fainermann, V. Adv. Colloid Interface Sci. 1994, 49, 249. (3) Lunkenheimer, K.; Hirte, R. J. Phys. Chem. 1992, 96, 8683.
S0743-7463(97)00100-5 CCC: $14.00
behavior of real systems into account.5 In the limit of low bulk concentration c, both models lead to the HenryTraube regime where experimentally a linear relation π ) KHc between surface pressure π and bulk concentration c is found. The slope KH should then be KH ) RTΓ∞. However, it turns out that this relation is quite frequently not fulfilled and that model fits and experimental isotherms show quite frequently systematic deviations at intermediary surface coverages. In these cases a better description of the experimental σe(c) isotherm was obtained by a two-state model, which was recently introduced.3 It assumes two distinct states of the adsorption layer which differ for instance in the orientation of the surfactant. In order to assess the assumptions, validity, and conclusion of these models, independent measurements are necessary which are not based on an interpretation of surface tension measurements. This paper should provide a contribution in this direction. In order to measure directly orientational order and number density within the adsorption layer second harmonic generation (SHG) is used. SHG is a nonlinear optical χ(2) process; it is the result of the interaction of an intense laser pulse within a noncentrosymmetric medium. It has been used for decades to extend the frequency range of laser light sources using noncentrosymmetric crystals. SHG, as a surface specific tool, exploits the fact that there is no generation of SHG light in centrosymmetric media (strictly speaking this is only valid within the dipole approximation). At the interface of two isotropic media the centrosymmetry is broken and SHG light is generated within the transition region of both adjacent media. Since optical techniques can be applied to any reflecting interfaces, SHG is a suitable tool for investigating interfacial order in liquidair, liquid-liquid, or liquid-solid interfaces.6,7 In favorable cases, the analysis of polarization dependent SHG measurements allows the determination of the number density N and orientation of the molecules in the interfacial layer. Depending on the hyperpolarizability, (4) Adamson, A. W. Physical Chemistry of Surfaces; Wiley & Sons: New York, 1993. (5) Stauff, J. Kolloidchemie; Springer: Berlin, 1960. (6) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327. (7) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107.
© 1997 American Chemical Society
Adsorption Models for Soluble Surfactants
β, of the molecules within the adsorption layer, a submonolayer sensitivity to within 1/50 - 1/100 of a monolayer can be achieved. It is possible to study the adsorption layer at low surface coverage. SHG discriminates between the adsorbed monolayer and subsurface layer. SHG data can thus be used to assess the quantities derived by thermodynamic models. SHG has been applied to adsorption layers of soluble surfactants. However, the results obtained by different groups are contradicting. Vogel et al. investigated the adsorption of a soluble C6 and insoluble C18 sodium n-alkyl Cn naphthalenesulfonate at the air-water interface.8 They demonstrated that soluble surfactants form a densely packed monolayer close to the critical micelle concentration. The detecteed SHG signal of this adsorption layer was comparable to the SHG response of a highly compressed insoluble monolayer C18 at the air-water interface. In a sequence of experiments comparing insoluble and soluble monolayers, experimental evidence was provided that there is no subsurface layer with polar ordering. Furthermore surface pressure and SHG data were compared on the basis of the Gibbs equation.9 It was found that the ratio of the surface activity coefficient to the bulk activity coefficient deviates significantly from unity. Furthermore both activity coefficients turned out to be nearly independent with respect to the bulk concentration of the solution. Lehmann et al. investigated a nonionic surfactant by SHG and surface tension measurements.10 The adsorption isotherm σe(c) was analyzed in order to retrieve the adsorbed amount Γ. A comparison of Γ with the number density N retrieved from SHG showed major deviations. The authors attributed these deviations to the existence of a subsurface layer contributing to the surface tension but not to the SHG signal. The aim of the present publication is to clarify these discrepancies. In order to fully utilize the potential of SHG, a soluble surfactant containing an NLO-chromophore with a high hyperpolarizability, β, was designed and synthesized. The hyperpolarizibility of this compound is at least a factor 20 higher than the one used in the studies mentioned above. The recorded SHG signal is thus completely dominated by the adsorbed molecules and contributions arising from the bare air/water interface including quadrupolar contributions from the bulk phase can be neglected. For the sake of simplicity it is desireable that the hyperpolarizability tensor is dominated by a single component. In this case, the analytical expressions relating molecular properties with the measurable components of the susceptibility tensor components χ(2) are significantly simplified. The ambiguity in the data interpretation is minimized and also the orientational order can be directly monitored with a high accuracy covering a broad concentration range. Polarization dependent SHG measurements were not feasible in the studies of Vogel et al., possibly due to the low hyperpolarizability, and information on the orientational order was derived in an indirect manner. The present contribution pays special attention to preparation of the samples. In general, due to the peculiarities of surfactant synthesis, all surfactants contain trace impurities, which possess a stronger surface activity than the main component. These trace impurities have no impact on any bulk properties. However, on the surface the impurities are enriched and features of the (8) Vogel, V.; Mullin, C. S.; Shen, Y. R. Langmuir 1991, 7, 1222. (9) Vogel, V.; Mullin, C.; Shen, Y. R.; Kim, M. W. J. Chem. Phys. 1995, 95, 4620. (10) Lehmann, S.; Busse, G.; Kahlweit, M.; Stolle, R.; Simon, F.; Marowsky, G. Langmuir 1995, 11, 1174.
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Figure 1. Structure of sodium 2-[4-((4-trifluoromethylphenyl)azo)phenoxy]ethanesulfonate.
adsorption layer can be dominated by trace impurities. This was first recognized by Mysels,11 and a purification scheme using foam fractionation was proposed.12 Trace impurities are a major concern, and the impact of trace impurities on the adsorption properties of soluble surfactants was investigated by Lunkenheimer et al. in detail.13 There is experimental evidence that some peculiarities, as for instance the frequently reported linear dependence of σe versus ln c, is simply caused by impurities (see ref 14 and references therein). Using samples as supplied, even if of a 99.99% purity, can lead to results of doubtful quality. A special purification setup was used here to ensure a complete removal of any trace impurities, a tedious but mandatory requirement for the purpose of this study. II. Experimental Section Materials. The chemical formula of the soluble anionic surfactant sodium 2-[4-((4-trifluoromethyl-phenyl)azo)phenoxy]ethanesulfonate used in this study is presented in Figure 1. The SHG activity is provided by the azobenzene chromophore with CF3 acting as an electron acceptor. Diazotation of trifluoromethylaniline (Aldrich) and subsequent coupling with phenol yields 4-(trifluoromethyl-4′-hydroxyazobenzene) (THA).15 THA (2 g, 7.5 mmol) was heated in 50 mL of dried acetone at 60 °C under vigorous stirring for 32 h together with 1.6 g sodium salt of 2-bromoethanesulfonic acid, 3.9 g (37 mmol), Na2CO3 (free of water), and a small amount of NaJ. The precipitate was filtered and purified by recrystallization. A total yield of 1.4 g (48%) of yellow colored crystals of the desired amphiphile was obtained. The crystals are soluble in water and form foams. Analysis: C15H12F3N2O4SNa: F, 14.38%, Found: F, 14.36 % 1H-NMR: δ 8.2 (2H, H-Ar, m, J ) 9 Hz); 7.94 (4H, H-Ar, m, J ) 9 Hz); 7.16 (2H, H-Ar, m, J ) 9 Hz); 4.34 (2H, CH2-O, t, J ) 7.6 Hz); 2.95 (2H, CH2-S, t, J ) 7.6 Hz) ppm; 13C-NMR: δ 162, 154, 146, 130, 127, 125, 123, 115 (C-Ar); 124 (CF3, q, 1J(CF) ) 280 Hz); 65 (CH2-O); 50 (CH2-S) ppm. Sample Preparation. An aqueous solution of the surfactant at a concentration close to its solubility limit was prepared using bidistilled water. This solution was then purified using an apparatus described in ref 16. This applied purification scheme removes all surface active impurities by repeated cycles consisting of (a) compression of the surface layer, (b) its removal with the aid of a capillary, (c) dilation to an increased surface, and (d) formation a new adsorption layer. This procedure ensures a solution of what we refer to as surface chemically pure grade.17 All solutions were prepared by diluting the stock solution. Surface Tension Measurement. Surface tension was determined by a Lauda tensiometer (Model TE 1C) with a slightly modified arrangement in order to meet the requirement imposed by surfactant solutions.18 Surface tension was recorded until a constant equilibrium value, σe, was established. Optical Characterization. Second harmonic generation experiments were carried out in reflection mode at a fixed angle of incidence of 53°. The fundamental (λ ) 1064 nm) of an active(11) Elworthy, P. H.; Mysels, K. J. J. Colloid Interface Sci. 1966, 331. (12) Mysel, K.; Florence, A. J. Colloid Interface Sci. 1973, 43, 577. (13) Lunkenheimer, K. J. Colloid Interface Sci. 1989, 131, 580. (14) Lunkenheimer, K.; Wedler, Ch. Tenside, Surfactants, Deterg. 1993, 30, 342. (15) Prescher, D.; Thiele, T.; Ruhmann, R.; Schulz, G. J. Fluorine Chem. 1995, 74, 185. (16) Lunkenheimer, K.; Pergande, H. J.; Kru¨ger, H. Rev. Sci. Instrum. 1987, 58, 2313. (17) Miller, R.; Lunkenheimer, K. Tenside Deterg. 1980, 17, 288. (18) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1989, 131, 580.
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Figure 2. Purification characteristics of a 3 mM aqueous solution of sodium 2-[4-((4-trifluoromethylphenyl)azo)phenoxy]ethanesulfonate. The equilibrium surface tension σe is plotted versus the number j of purification cycle. The surface of the surfactant solution is periodically compressed and the surface layer is removed with the aid of a small capillary. As the desired surface state was not achieved at j ) 465 the purification procedure had to be continued with a slightly diluted solution due to experimental reasons. The desired surface state is achieved when dσe/dj ) 0. passive mode locked Nd-YAG laser (PY-61, continuum), with a pulse width of τ ) 30 ps and a repetition rate of 12.5 Hz, was used as a light source and focused with a lens on the sample. All spurious SHGs created by the optical components were removed by a visible cutoff filter (RG630, Schott) placed just in front of the sample. The frequency doubled light generated at the interface was separated from the fundamental using an IR-cutoff filter (BG39, Schott) in conjunction with a narrow band interference filter (532 BP, Instruments S.A.) and subsequently detected by a photomultiplier (C83068, Burle) with a quantum efficiency of 15%. The signal was amplified (V5D, Fa. Seefelder Metechnik) and processed by a 500 MHz, 2Gs/s digitizing oscilloscope (HP 54522 A, Hewlett-Packard). A computer controls all vital elements of the experiment and performs also the integration of the waveform. The SHG signal of a quartz crystal was used as a reference in order to eliminate experimental errors due to intensity fluctuations. The plane of polarization of the incident beam can be rotated by a Glan laser polarizer (extinction ratio 10-6, PGL, Halle) and a low-order quartz half-wave plate (∆λ ) 0.001, RLQ Halle) mounted on motor-driven, computer-controlled rotary stages (M-445.21, Physik Instruments). The polarization of the reflected SHG light is analyzed using a Glan-Thomson prism (extinction ratio 10-6, Typ K, Fa. Steeg & Reuter).
III. Results and Discussion For all experiments special care was taken in the preparation of the solutions. The result of the purification scheme is illustrated in Figure 2. The equilibrium surface tension, σe, is presented as a function of the number of purification cycles consisting of the formation and subsequent removal of the adsorption layer. The desired surface state is achieved if the equilibrium surface tension, σe, does not change with the number j of the purification cycles.
(dσe/dj) ) 0
(1)
As the desired surface state was not achieved at j ) 465 the purification procedure had to be continued with a slightly diluted solution due to experimental reasons. The purification of the stock solution required approximately 3 days. It is remarkable that in this particular case about 7 mN/m is due to surface active trace impurities. According to our experience, this finding holds also for many commercially available surfactants.
Figure 3. Equilibrium surface tension of a special purified aqueous sodium 2-[4-((4-trifluoromethylphenyl)azo)phenoxy]ethanesulfonate salt solution as a function of the bulk concentration.
The equilibrium surface tension versus the concentration of the solution is depicted in Figure 3. The slope of the σe(c) isotherm increases monotonously with the bulk concentration c. According to the Gibbs relation this is due to an increase in the surface excess Γ.
Γ∝
∂σe ∂lnc
(2)
A quantitative relation describing the experimental data, σe, c in terms of the adsorption parameters Γ∞ (saturation adsorption), surface activity aF, and surface interaction Hs is provided by either the Langmuir of the Frumkin equation of state. The Langmuir model holds for systems with a negligible interaction of the surfactant within the adsorption layer whereas the Frumkins equation of state (eq 3) refers to a regular surface behavior with an interaction of the molecules within the adsorption layer.
(
σw - σe ) -RTΓ∞ ln 1 -
)
( )
Γ Hs Γ Γ∞ RT Γ∞
Γ c ) exp(-2HsΓ/RTΓ∞) af Γ∞ - Γ
2
(3)
Here, σw and σe denote surface tension of water and the equilibrium surface tension of the solution, respectively, Γ being the surface excess. The experimental σe(c) isotherm is best described within the framework of the Frumkin model by the following set of adsorption parameters: Γ∞ ) 4.99 × 10-6 mol/m2, af ) 9.80 × 10-3 mol/ dm3, Hs ) 4.79 kJ/mol. The experimental surface tension was fitted to within (0.2 mN/m; the experimental accuracy in the determination of the surface tension measurements is (0.1 mN/m. The interaction parameter Hs is higher than the value found normally for classical soluble amphiphiles with regular surface behavior and is even close to the limit where theoretical considerations predict a phase separation.19 The Frumkin model which is derived from pure thermodynamic considerations describes the interfacial region as a separate phase and introduces a chemical potential for all components. Thermodynamic considerations do not provide any information about dimensions and internal structure such as orientation within the adsorption layer. (19) Lucassen-Reynders, E. H. Prog. Surf. Membr. Sci. 1976, 10, 253.
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SHG was used to provide structural information. In our case SHG is determined by the dipole contribution and measures the components of the macroscopic susceptibility tensor χ(2) which is related to the molecular quantities by the oriented gas model20
χ(2) )
β ) N〈β〉 ∑ mol
(4)
It states that the susceptibility χ(2) is the sum of the hyperpolarizabilities β of all molecules. This can also be expressed in terms of their number density, N, and their corresponding orientational average 〈β〉 as denoted by the brackets. The macroscopic susceptibility tensor χ(2) is defined in the laboratory frame of reference (I, J, K) as given by the plane of incidence; the hyperpolarizability β is defined in the molecular frame of reference (i, j, k). Both third rank tensors can be transformed using an Euler transformation21
βIJK ) UIi(φ,θ,ψ)βijkUJj-1(φ,θ,ψ)UKk-1(φ,θ,ψ) (5) with U ) RcRbRa and the Ri are describing the rotation around three axis a, b, c.
(
)
cos ψ sin ψ 0 Rc(ψ) ) -sin ψ cos ψ 0 , 0 0 1
(
cos φ sin φ 0 Rb ) -sin φ cos φ 0 0 0 1 cos θ 0 -sin θ Ra(θ) ) 0 1 0 sin θ 0 cos θ
(
)
)
Figure 4. Polar diagram of the SHG intensity I2ω versus the plane of polarization of the linear polarized fundamental beam. The squares represent the SHG data as measured, the solid line refers to a model fit according to eq 9 and eq 10 yielding the unknown elements of the susceptibility tensor. Part a was measured at a fixed analyzer setting of A ) sˆ , Part b shows the corresponding measurement for an analyzer setting of A ) pˆ . The concentration of the solution was 1.4 × 10-3 mol/cm3.
With the aid of Maxwell’s equations a relation between the intensity I2ω and Iω can be derived. For an analyzer setting at pˆ the following equation holds
(6)
I2ω ) D|(A cos2 P + C sin2 P) χzxx(2) + B cos2 Pχzzz(2)|2I(ω)2 (9)
(7)
Evaluation of this equation is a tedious procedure involving lengthy expressions relating the individual tensor components. The explicit calculation can be found in ref 22 or can be conveniently generated with the aid of algebraic computer math packages (e.g., Mathematica, Wolfram Research). From our experimental arrangement these equations are significantly simplified. The chromophore used in this study is dominated by its βzzz component with its value being orders of magnitude greater than any other element of the tensor. Thus the β-tensor can be treated as a scalar quantity. In addition the number of independent tensor elements is further reduced by the symmetry in the arrangement of the molecules. In our experiment a C∞v symmetry with an isotropic azimuthal distribution of the molecules is observed. The remaining tensor elements are
χzzz ) Nβzzz〈cos3 θ〉 χxxx ) χxxx ) χzxx ) χyyz ) χzyy ) 1/2Nβzzz〈cos θ sin θ〉 (8) 2
Thus an SHG analysis allows the determination of the number density and the orientation of the chromophore. The most accurate determination of the tilt angle can be achieved by a continuos rotation of the plane of polarization P of the incident beam at fixed settings of the analyzer A. (20) Prasad, P.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (21) Goldstein, H. Classical Mechanic; Addison Wesley: London, 1981. (22) Hirose, C.; Akamatsu, N.; Domen, K. Appl. Spectrosc. 1992, 6, 1051.
with
A ) [Fz(2ω)Fx(ω) - 2Fx(2ω)Fz(ω)] Fx(ω) cos2 R B ) Fz(2ω)Fz2(ω) sin2 R C ) Fz(2ω)Fy2(ω)
()
D)4
µ0 0
3/2
ω2 tan2 R
where P is the angle denoting the polarization of the fundamental with respect to the plane of incidence, R is the angle of incidence, µ0 is the permeability constant, 0 is the permittivity constant, and Fi is the Fresnel factor as derived by the boundary conditions at the interfaces. The value is determined by the dielectric function of the individual layer and the angle of incidence R. The corresponding equation for an analyzer setting at sˆ reads
I2ω ) DFy2(2ω)Fy2(ω)Fz2(ω) sin2(2P)|χyzy(2)|2I(ω)2 (10) The unknown susceptibility components can be determined by a least-squares fit. Figure 4 shows a representative measurement of the SHG intensity I2ω of an aqueous surfactant solution of a concentration of 1.4 × 10-3 mol/cm3. The polar diagram shows the SHG intensity I2ω as a function of the plane of polarization P of the fundamental beam. The squares represent the SHG data as measured; the solid line refers to a fit according to eq 9 and eq 10 to determine the unknown elements of the susceptibility tensor. Figure 4a refers to an analyzer setting of A ) sˆ . The absence of sˆ -polarized second harmonic light irrespective of incident sˆ or pˆ -polarization
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of the fundamental is in agreement with an isotropic azimuthal distribution of the molecules. Figure 4b shows the corresponding data together with the model fit for an analyzer setting at pˆ -polarization. All measurements at different bulk concentrations c, possess the very same features and reveal unambiguously that the orientation of the surfactant stays constant in the whole concentration range. The tilt angle θ was determined to be 38° being defined in a way that θ ) 0 denotes a orientation parallel to the surface normal. A careful error analysis suggests an accuracy to within (5°. However, we would like to point out that SHG measurements are very sensitive to slight changes in orientation and the reader should not mix the accuracy in the determination of the absolute values with the sensitivity in recording slight relative changes. On a molecular scale it appears to be a rather peculiar finding that the orientational order is independent of the packing density. However, a constant tilt angle over a wide range of surface concentrations has also been reported for other soluble23 as well as for insoluble monolayers.24 The experimental fact that there is no detectable change in the orientational order simplifies the determination of the number density N of the surfactant within the adsorbed layer. SHG measurements with an analyzer setting at A ) sˆ depend only on the tensor element χyzy as can be read from eq 10. The susceptibility χyzy is related by the oriented gas model eq 4 with the number density N and the orientational average of the hyperpolarizability β of the adsorbed molecules. Since the orientation remains constant in the whole concentration range, the intensity reading at a selected polarization setting is proportional to the number density (I2ω(P)45,A)90))1/2 ∝ χyzy ∝ N. The tensor element χyzy is directly proportional to the number of oriented amphiphiles at the interface. Therefore it can be compared to the surface excess derived by the Frunkin model. The corresponding plot is shown in Figure 5. Γ obtained by the surface equation changes linearly with susceptibility element χyzy. SHG is only generated in the region with polar ordering. Hence, we conclude that the ordered region in the interfacial layer, which gives rise only to the SHG data, is identical to the region which determines the surface tension. Summary and Conclusion The present paper assesses the validity of the conclusion of the Frumkin model for soluble surfactants by means of a comparison with the results obtained by SHG. For this purpose a surfactant with a high hyperopolarizability and a sufficient solubility in water was designed and synthesized. Special care was taken to ensure that all surface active impurities were completely removed. Po(23) Hicks, J. M.; Kemnitz, K.; Eisenthal, K. B.; Heinz, T. F. J. Phys. Chem. 1986, 90, 560. (24) Berkovic, G.; Rasing, T.; Shen, Y. R. J. Opt. Soc. Am. B 1987, 4, 945.
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Figure 5. Surface concentration of the sodium 2-[4-((4trifluoromethylphenyl)azo)phenoxy]ethanesulfonate salt solution as obtained from the SHG measurements in dependence on the surface concentration Γ calculated by Frumkin equation.
larized SHG was then used to determine the number density and the orientational order of the surfactant as a function of the concentration of the solution. It was demonstrated that orientation of the molecules within the adsorption layer remain constant and is independent of the concentration of the solution. The NLO chromophore is accommodated upright with a tilt angle of 38°. The molecule is rather rigid which in turn favors a regular packing within the adsorption layer with a constant tilt. The experimental isotherm σe(c) could be described by the Frumkin model. The fitting procedure yields the interaction energy Hs and the adsorbed amount Γ. The latter thermodynamic quantity was compared to the number density of the surfactant as obtained by the SHG measurements. Both quantities match within experimental accuracy. Therefore we conclude that the oriented region within the adsorption layer is identical to the region which determines the surface tension of the system. The results confirm the suitability of thermodynamical models put forward by the pioneers of surface thermodynamics, in particular J. W. Gibbs, who attributed all excess properties of the interfacial region to a hypothetical two-dimensional layer of infinitesimal thickness. According to our findings it is not necessary to take into consideration any sublayer contribution for explaining the SHG data, at least for the equilibrium properties studied. Future investigations will be directed toward obtaining insight into the dynamic properties of the adsorption layers of similar surfactants systems by a combination of SHG and surface tension measurements. Acknowledgment. The authors thank Professor H. Mo¨hwald for stimulating and encouraging support, and Dr. Martina Bree for helpful discussions. Furthermore the financial support of the Deutsche Forschungsgemeincle (grant S. Bae) is greatly acknowledged. LA970100R
