Ellipsometry Studies of Nonionic Surfactant Adsorption at the Oil

In the presented study we have developed and implemented a methodology for ellipsometry measurements at liquid interfaces that makes it possible to ...
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Langmuir 2005, 21, 149-159

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Ellipsometry Studies of Nonionic Surfactant Adsorption at the Oil-Water Interface Jan-Willem Benjamins, Krister Thuresson, and Tommy Nylander* Department of Physical Chemistry 1, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received January 16, 2004. In Final Form: October 15, 2004 In the presented study we have developed and implemented a methodology for ellipsometry measurements at liquid interfaces that makes it possible to determine the amount adsorbed without assumptions of refractive index or thickness of the adsorbed layer. It was demonstrated that this is possible by combined measurements from different aqueous phases, H2O and D2O, which were shown to have sufficiently different refractive indices. The methodology was tested by studying adsorption of two types of nonionic poly(ethylene glycol) alkyl ether surfactants, CnH2n+1(OC2H4)mOH or CnEm at the decane-aqueous interface, where C12E5 was adsorbed from the oil phase and C18E50 from the aqueous phase. The observed plateau values of the adsorbed amounts were 1.38 and 0.93 mg/m2 for C12E5 and C18E50, respectively, which is in agreement with the corresponding values of 1.49 and 1.15 mg/m2 obtained from applying the Gibbs equation to interfacial tension data for the same systems. We will briefly discuss the adsorption behavior in relation to the molecular structure of the surfactant and the phase behavior of the oil-surfactant-aqueous systems in relation to our experimental results.

Introduction Nonionic poly(ethylene glycol) alkyl ethers, CnH2n+1(OC2H4)mOH (often designated as CnEm), are a widely used class of surfactants. Their phase behavior has been studied extensively, and typical properties are a low monomeric solubility and a reversed aqueous solubility, i.e., solubility decreases with temperature. Since the relative size of the hydrophobic and hydrophilic blocks can easily be varied, by altering n and/or m values in the manufacturing step, the hydrophilic/lipophilic balance (HLB value) and therefore phase behavior can be conveniently tuned. In addition to decreased solubility, an increased temperature promotes aggregate structures that have a lower tendency to curve toward oil. Another typical property of this class of surfactants is that the nonionic headgroup gives a low sensitivity to variations in ionic strength of the aqueous media. For these reasons CnEm surfactants are commonly used as stabilizers and detergents, where they are expected to adsorb at apolar-polar interfaces. Here the behavior at the interface is crucial for performance, and several studies have shown that structural variations of the CnEm surfactants, important for phase behavior, also influences their behavior at solid surfaces.1-6 Adsorption of surfactants can be followed by ellipsometry, which is a well-established, nondestructive optical method to characterize ultrathin films.7 Due to experimental difficulties, relatively few studies have been published that concern surfactant adsorption at the liquid-liquid interface.8-13 Ellipsometry measurements at air-liquid and liquid-liquid interfaces require special * To whom correspondence should be addressed. E-mail: [email protected]. (1) Tiberg, F.; Lindman, B.; Landgren, M. Thin Solid Films 1993, 234, 478. (2) Tiberg, F.; Jo¨nsson, B.; Lindman, B. Langmuir 1994, 10, 3714. (3) Tiberg, F.; Jo¨nsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (4) Brinck, J.; Jo¨nsson, B.; Tiberg, F. Langmuir 1998, 14, 1058. (5) Brinck, J.; Jo¨nsson, B.; Tiberg, F. Langmuir 1998, 14, 5863. (6) Brinck, J.; Tiberg, F. Langmuir 1996, 12, 5042. (7) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland Publication: Amsterdam, 1979. (8) Nylander, T.; Hamraoui, A.; Paulsson, M. Int. J. Food Sci . Technol. 1999, 34, 573.

arrangements, which for instance ensure a well-defined angle of incidence.8,9,14 Another difficulty with measurements at the air-liquid and liquid-liquid interfaces is the lack of contrast between a formed transparent layer and a substrate that does not absorb light at the used wavelength. In a previous study we used ellipsometry to study the adsorption of soybean-PC (sb-PC) from caraway oil (containing limonene and carvone in a 1:1 ratio) and olive oil (mainly triolein) at the oil-aqueous interfaces.9 We found monolayer adsorption at the caraway oil-aqueous interfaces but multilayers at the olive oil-aqueous interface. The lower solubility of sb-PC in the olive oil was suggested to drive the lipid to the aqueous interface, where it eventually can form a liquid crystalline phase. The main objective of the presented study is to develop and implement a methodology for ellipsometry measurements at liquid interfaces that combine measurements from different aqueous (or oil) phases with disparate refractive indices. Neutron reflectivity measurements are very powerful in this approach,15 but to our knowledge ellipsometry has not yet been tried. Although the potential benefit is expected to be less than when using neutrons, the number of parameters that can be determined is expected to increase,10,16 which would make it possible to determine the amount adsorbed without any assumption of refractive index or thickness of the adsorbed layer. This (9) Bylaite, E.; Nylander, T.; Venskutonis, R.; Jo¨nsson, B. Colloids Surf., B 2001, 20, 327. (10) Russev, S. C.; Arguirov, T. V.; Gurkov, T. D. Colloids Surf., B 2000, 19, 89. (11) Kapilashrami, A.; Malmsten, M.; Eskilsson, K.; Benjamins, J. W.; Nylander, T. Colloids Surf., A 2003, 225, 181. (12) Lei, Q.; Bain, C. D. Phys. Rev. Lett. 2004, 92, 176103. (13) Schulz, J.; Bowers, J.; Findenegg, G. H. J. Phys. Chem. B 2001, 105, 6956. (14) Hutchison, J.; Klenerman, D.; Manning-Benson, S.; Bain, C. D. Langmuir 1999, 15, 7530. (15) Penfold, J.; Richardsson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (16) Antippa, A. F.; Leblanc, R. M.; Ducharme, D. J. Opt. Soc. Am. A 1986, 3, 1794.

10.1021/la049848h CCC: $30.25 © 2005 American Chemical Society Published on Web 12/07/2004

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approach will be compared with the currently used way which makes use of a complementary method, such as interfacial tension measurements or neutron reflectivity, to correlate the ellipsometry readings to the adsorbed amount.11,17-21 Russev et al. studied the kinetics of adsorption of a protein, β-casein, at the oil-aqueous interface and were able to get quite reasonable adsorbed amounts and layer thicknesses by combining with measurements from the air-aqueous interface.10 In a way their approach is therefore analogous to the one adopted in the present study as it is built on the strategy to vary the refractive index of one of the media. We have applied the new methodology to investigate the adsorption characteristics of two different nonionic surfactants that greatly differ in structure: C12E5, which has a relatively small polar headgroup compared to its apolar tail, and C18E50, where the proportions are opposite. This means that C12E5, in its monomeric form, is soluble in oil, and in two-phase systems with oil (here decane) and water, the monomeric concentration in the oil phase is orders of magnitude higher than in the aqueous phase.22-24 C18E50 shows the reverse behavior and its monomeric form is comparatively more water soluble than C12E5, while it is practically insoluble in decane. The choice of these two surfactants allows us to test the methodology for adsorption from the oil phase (C12E5) as well as from the aqueous phase (C18E50). In addition we will briefly discuss the adsorption behavior in relation to the molecular structure of the surfactants, and the phase behavior of the oilsurfactant-aqueous systems in relation to our experimental results. In a larger perspective, a deeper understanding of the relation between interfacial properties and bulk phase behavior will make it possible to predict interfacial properties from the vast knowledge on phase behavior. It might also provide insight in phenomena such as emulsification and the formation of microemulsions, which are important in technical applications. It should also be noted that the formation of microemulsions in the C12E5-alkane-aqueous systems has been studied extensively.22-25 Theory Ellipsometry allows for nondestructive, in situ studies of adsorption processes. The technique is based on measuring a change in polarization of a light beam that is reflected at the sample surface. This change is described by two parameters, ∆ and Ψ. These angles are related to the reflection coefficients, Rp and Rs, parallel and perpendicular to the plane of incidence, respectively, F being the ellipticity coefficient.

Rp/Rs ) F ) tan Ψei∆ ) tan Ψ(cos ∆ + i sin ∆) (1) The presence of a film at the interface will affect the (17) Battal, T.; Shearman, G. C.; Valkovska, D.; Bain, C. D. Langmuir 2003, 19, 1244. (18) Manning-Benson, S.; Bain, C. D.; Darton, C. R. J. Colloid Interface Sci. 1997, 189, 109. (19) Manning-Benson, S.; Parker, S. R. W.; Bain, C. D.; Penfold, J. Langmuir 1998, 14, 990. (20) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143. (21) Beaglehole, D.; Lawson, F.; Harper, G.; Hossain, M. J. J. Colloid Interface Sci. 1997, 192, 266. (22) Kabalnov, A.; Olsson, U.; Thuresson, K.; H., W. Langmuir 1994, 10, 4509. (23) Burauer, S.; Sachert, T.; Sottmann, T.; Strey, R. Phys. Chem. Chem. Phys. 1999, 1, 4299. (24) Ravera, F.; Ferrari, M.; Liggeri, L. Adv. Colloid Interface Sci. 2000, 88, 129. (25) Sottmann, T.; Strey, R. J. Chem. Phys. 1997, 106, 8606.

Benjamins et al.

Figure 1. Reflection of light, incident through the oil phase, at a thin film with a thickness d1 and a refractive index n1, on an aqueous phase. The upper (oil) and lower (water) phases have a refractive index of n2 and n0, respectively.

reflectivity coefficients. A schematic drawing of light reflected at a film at the oil-water interface is given in Figure 1, where we also define some of the parameters used in this work. If we assume that all interfaces are planar and represent discontinuities in the dielectric functions and that the film does not adsorb light at the used wavelength, we can now express F as7

R21p + R21pR10pe-iD F ) tan Ψei∆ )

1 + R10pR21pe-iD R21s + R21sR10se-iD

(2)

1 + R10sR21se-iD where

D)

4π d (n 2 - n22 sin2 φ)1/2 λ 1 1

Here λ is the wavelength of light and Rxyp and Rxys are the reflection coefficients for the interface between media x and y, for components p and s, respectively. This expression can be inverted to determine n1 and d1 from ∆ and Ψ.1-3,7,26,27 More intricate refractive index profiles would require additional parameters. The adsorbed amount, Γ (mg/m2), can be calculated from n1 and d1 (in Å) using eq 3, which was derived by Cuypers et al.,26 based on the Lorenz-Lorenz equation.

Γ)

0.3d1(n12 - n02) (n12 + 2)(r(n02 + 2) - v(n02 - 1))

(3)

In this equation, r (in mL/g) is the specific refractivity of the adsorbed molecules and v (in mL/g) is their partial specific volume. Alternatively and more simply the adsorbed amount can be calculated from n1 and d1 (Å) by assuming that the refractive index varies linearly with the concentration according to de Feijter et al.28

Γ)

0.1d1(n1 - n0) dn/dc

(4)

Here dn/dc is the refractive index increment (in mL/g). Both eq 3 and eq 4 give within 10% the same value of the (26) Cuypers, P. A.; Corsel, J. W.; Janssen, M. P.; Kop, J. M. M.; Hermens, W. T.; Hemker, H. C. J. Biol. Chem. 1983, 258, 2426. (27) McCrackin, F. L.; Passaglia, E.; Stromberg, R. R.; Steinberg, H. L. J. Res. Natl. Bur. Stand. 1963, 67A, 363. (28) DeFeijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759.

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Approach 1. In the first approach we used surface tension measurement on the same system to get an independent measure of Γ in one point of the adsorption isotherm, by using the Gibbs equation

ΓGibbs )

1 dγ RT d ln c

(5)

Here γ is the interfacial tension, c is the concentration of the adsorbing species (assuming concentration ) activity), R is the molar gas constant, and T is the absolute temperature. Battal et al.17 found that ImF ) tan Ψ sin ∆ (see eq 1) obtained for the adsorption of a homologous series of surfactants varied linearly with the surface excess, Γ. They also found that the effect of capillary waves on ImF is minor under the conditions they used. Thus we can obtain

Γ ) ImF(c)

Figure 2. The ellipsometer ∆ (a) and Ψ (b) calculated from eq 2 is plotted versus the refractive index of the film, n1, at various film thickness for the interface between decane (n2 ) 1.4125) and water (n0 ) 1.3348) at an angle of incidence of φ ) 50°. Note the extended scale for Ψ.

adsorbed amount,29 although the dependence on the refractive index of the film is different. In Figure 2, ∆ and Ψ are plotted versus the refractive index of the film, n1, at various film thickness for the decane-aqueous interface at an angle of incidence of φ ) 50°. Not unexpected we note that the large changes in ∆ are found when n1 is midway between the refractive index of oil and water. At this value of n1 it can also be seen that a variation in thickness has no effect on Ψ. In general we note that thickness and refractive index of the film only slightly affect Ψ. It is a well-known problem with measuring very thin, transparent layers on a substrate that does not absorb light at the used wavelength that only ∆ changes when a layer is formed, while Ψ remains virtually constant.16 This means that the thickness of the adsorbed layer (d1) and its refractive index (n1), cannot be calculated from a single measurement. It is therefore not possible to calculate Γ according to eqs 3 or 4 from the only affected variable, ∆, without assuming either a value of d1 or n1. In this investigation we have used two approaches to circumvent this problem: (29) Engstro¨m, S.; Ba¨ckstro¨m, K. Langmuir 1987, 3, 568.

ΓGibbs(cref) ImF(cref)

(6)

Here ImF(c) is the imaginary part of F recorded from ellipsometry at any given concentration c, and ImF(cref) is the corresponding value at concentration cref, where the adsorbed amount ΓGibbs is determined from interfacial tension data using eq 5. Approach 2. Another approach to increase the number of determined parameters is to perform ellipsometric measurements in media with different refractive index, such as in normal and deuterated water or by using different oils. This assumes that the adsorption behavior of the surfactant is the same at the different interfaces irrespective of the media. As shown by Antippa et al. consecutive measurements where either n2 or n0 is changed will allow determination of both of the unknown parameters.16 In their approach they were suggesting to measure at different wavelengths, which changes the refractive index of the media. Here we have performed measurements in both normal and deuterated water as a means to obtain a variation in the refractive index. To our knowledge this is the first time this approach is applied to evaluate ellipsometry data from measurements at the oil-aqueous interface. As evident from Figure 2b only slight changes are expected in Ψ, and we will therefore only use the measured value of the change in ∆, δ∆, due to the adsorption of the surfactant at the interface. As we are only considering thin films (d1 , λ), we can facilitate the calculations using the so-called thin film approximation of the Drude equations16

δ∆ ) qd1(n12 + (n02n22/n12) - n02 - n22)

(7)

where

q ) (4π/λ)(n0 sin(φ) tan(φ)/(n22 - n02)(1 (n0/n2)2 tan2(φ))) Note that δ∆ in the equations is in radians. We will first consider the simplest case, where n1 and d1 is the same in H2O (n0 ) nH2O) as in D2O (n0 ) nH2O). Here we will use q[nH2O] and q[nD2O] to the denote parameter q in H2O and D2O, respectively, and δ∆[nH2O] and δ∆[nD2O], the corre-

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Benjamins et al.

sponding measured change in ∆. We can now write eq 7 as

(n12 - nH2O2)(n12 - n22)

q[nH2O] δ∆[nH2O]

) (n12 - nD2O2) ×

(n12 - n22)

q[nD2O]

(8)

δ∆[nD2O]

with the solutions

n1 ) n2 and

n1 )

(

)

δ∆[nH2O]nD2O2q[nD2O] - δ∆[nD2O]nH2O2q[nH2O] δ∆[nH2O]q[nD2O] - δ∆[nD2O]q[nH2O]

1/2

(9)

The value of d1 can then be obtained from eq 7 and Γ from eq 3 or 4. Figure 3a shows δ∆[nH2O], obtained from eq 7, versus the adsorbed amount calculated according to de Feijter (eq 4) (with H2O as the solvent in the film) for various film thickness. Figure 3b shows the corresponding simulation for the expected difference between the δ∆ recorded in H2O and in D2O, δ∆[nH2O] - δ∆[nD2O], versus the adsorbed amount calculated according to de Feijter (eq 4) (with H2O as the solvent in the film), when n1 and d1 are assumed to be the same in H2O (n0 ) nH2O) as in D2O (n0 ) nD2O). In the second case we consider d1 and Γ to be the same in H2O (n0 ) nH2O) as in D2O (n0 ) nD2O). Applying eq 7 we then need to solve

(n12[nH2O] + (nH2O2n22/n12[nH2O]) - nH2O2 q[nH2O]

) (n12[nD2O] + (nD2O2n22/n12[nD2O]) n22) δ∆[nH2O] q[nD2O] (10) nD2O2 - n22) δ∆[nD2O]

Figure 3. (a) shows a simulation of δ∆[nH2O] versus the adsorbed amount calculated for different thickness from eq 7. (b) The expected difference between the δ∆ recorded in H2O and in D2O, δ∆[nH2O] - δ∆[nD2O], versus the adsorbed amount. Both n1 and d1 are assumed to be the same in H2O (n0 ) 1.3348) as in D2O (n0 ) 1.3298), n2 ) 1.4125, φ ) 50°, and the adsorbed amount was calculated according to de Feijter (eq 4) (with n0 ) 1.3348 and dn/dc ) 0.148 mL/g).

Considering Γ to be the same in H2O and D2O, eq 3 or eq 4 can be used to express n1 measured for the H2O case, n1[nH2O], as a function of n1 measured for the D2O case, n1[nD2O]. From eq 3 we get

From eq 4 we get

n1[nH2O] )

For both cases eq 10 has to be solved numerically.

(

2

2

2

2

)

2a(n1 [nD2O] - nD2O ) + bnH2O (2 + n1 [nD2O]) a(nD2O2 - n12[nD2O]) + b(2 + n12[nD2O])

where

a ) r(nH2O2 + 2) - v(nH2O2 - 1) and

b ) r(nD2O2 + 2) - v(nD2O2 - 1)

1/2

(11)

n1[nH2O] ) n1[nD2O] - nD2O + nH2O

(12)

Figure 4a shows a simulation of δ∆[nH2O], obtained from eq 7, versus the adsorbed amount calculated according to de Feijter (eq 4) for several thicknesses of the adsorbed film, when Γ and d1 are assumed to be the same in H2O (n0 ) nH2O) as in D2O (n0 ) nD2O). Figure 4b shows the corresponding simulation for the expected difference between the δ∆ recorded in H2O and in D2O, δ∆[nH2O] δ∆[nD2O], obtained by applying eq 12, versus the adsorbed amount calculated according to de Feijter (eq 4). Capillary Waves. Not only will adsorption of the surfactant at the oil-aqueous interface contribute to the observed changes in ∆ but the interface will be roughened by thermal capillary waves. Under some conditions this

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Figure 5. A schematic picture of the ellipsometry setup: Q, quarter wave plate; P, polarizer; C, compensator; A, analyzer; the two light guides and the specially designed sample cell.

Figure 4. (a) Simulation of δ∆[nH2O] versus the adsorbed amount, using eq 7 for different thickness. (b) Expected difference between the δ∆ recorded in H2O and in D2O, δ∆[nH2O] - δ∆[nD2O], versus the adsorbed amount, using eq 7 and eq 12. Γ and d1 are assumed to be the same in H2O (n0 ) 1.3348) as in D2O (n0 ) 1.3298), n2 ) 1.4125, φ ) 50°, and the adsorbed amount was calculated according to de Feijter (eq 4) (with dn/ dc ) 0.148 mL/g).

effect can be measured by ellipsometry.30-32 Capillary waves depend on the interfacial tension, and in the limiting case of an interface without rigidity they will add to the ImF obtained from ellipsometry measurements, by ImFcw as discussed by Meunier33 and Schulz et al.34

ImFcw )

2 2 3π (n0 - n2 ) πkT 2λ (n 2 + n 2)1/2 6γ 0

2

1/2

( )

(13)

Experimental Section Materials. Two types of nonionic poly(ethylene glycol) alkyl ether surfactants, CnH2n+1(OC2H4)mOH or CnEm, were used in the present study. The C18E50 (Mw ) 2450) that was used in this study was produced by Akzo Nobel Surface Chemistry AB, Stenungsund, Sweden. Before use it was purified by dialysis of a solution with ca. 5 wt %, C18E50 against Millipore water for (30) Beaglehole, D. Phys. Rev. Lett. 1987, 58, 1434. (31) Beaglehole, D. J. Phys. Chem. 1987, 91, 5091. (32) Braslau, A.; Pershan, P. S.; Swislow, G.; Ocko, B. M.; Als-Nielsen, J. Phys. Rev. A 1988, 38, 2457. (33) Meunier, J. J. Phys. (Paris) 1987, 48, 1819. (34) Schulz, J.; Hirtz, A.; Findenegg, G. H. Physica A 1997, 244, 334.

several days, followed by freeze-drying. C12E5 ((Mw ) 411, lot no. 9051) was purchased from Nikko Chemicals Co., Ltd. (Tokyo, Japan), and used as received. Water was deionized and passed through a Milli-Q water purification system (Millipore Corp., Bedford, MA). Deuterated water (D2O, 99.8% D) was purchased from Dr. Glaser AG, Basel, Switzerland. We found that this water did not have the same purity as the Milli-Q water. The water was therefore filtered using a Sartorius Minisart syringe filter, with a pore size of 0.2 µm, and then degassed under vacuum. Methods. Ellipsometry. The ellipsometry measurements were performed using an Optrel Multiskop ellipsometer (Optrel, Berlin, Germany, www.optrel.de).35 A schematic picture of the setup is given in Figure 5 and an extensive description of the setup is given by Benjamins et al.36 Here we will only highlight some of the features relevant to the present study. The instrument consists of two adjustable arms, the laser arm and the detector arm, mounted on a goniometer. The laser arm contains the laser (Nd:YAG, λ ) 5320 Å), a quarter-wave plate, a polarizer, and a compensator, while the detector arm contains an analyzer and a photodetector. To be able to perform measurements on interfaces between two liquids, the instrument was fitted with light guides submersed in the top phase (decane). The light guides consist of glass tubes with very thin (0.15 mm) windows of optical glass. They are so designed that the effect on polarization state of the incident and reflected light is minimal. Further, the measuring cell is designed to ensure a planar liquid surface. Thus the lower part is made of glass and a stainless steel rim, which ensure wetting of water, while the top was covered by a Teflon ring (5.5 cm inner diameter) that was wetted by the oil, but not by water. The area of the oil-aqueous interface is about 24 cm2. To reduce risk of contamination and minimize evaporation, a plexiglass lid with holes for the light guides was applied to the measuring cell during measurements and equilibrations. To isolate the instrument from ambient vibrations, it was mounted on a damped optical table (RS4000 sealed hole tabletop) with tuned damping placed on stabilizers (high-performance laminar flow isolator, Newport, Irvine, CA). At the Brewster angle, which for decane-water is 43.3°, the formation of an interfacial layer would cause the largest changes in ∆. On the other hand, the intensity of the reflected light is at a minimum at the Brewster angle, increasing the risk for inaccuracies in the detector. Therefore, ellipsometry measurements were performed at an angle of incidence (φ) of 50°, being a compromise of these considerations. In addition, the measurements in the H2O-system were carried out at two additional angles of incidence, where one was further above the Brewster angle (φ ) 60°) and the other below the Brewster angle (φ ) 40°). Before a measurement was started, the measuring cell was filled with an excess D2O and allowed to equilibrate. After 1 h, the surface layer (including any surface-active impurities) was removed using a suction pump. The measuring cell was then placed in the ellipsometer and aligned according to the procedure described by Benjamins et al.36 The cleaning process was repeated several times, until the surface appeared clean as determined by constant reading from the ellipsometry. The volume of H2O or D2O was then about 27 mL. About 37 mL of pure decane (99+%, Lot No. A013827101), purchased from Acros Organics (New Jersey, USA) and used without further purification, was then added on top of the aqueous phase. At this time the alignment (35) Harke, M.; Teppner, R.; Schultz, O.; Orendi, H.; Motschmann, H. Rev. Sci. Instrum. 1997, 68, 3130. (36) Benjamins, J. W.; Jo¨nsson, B.; Thuresson, K.; Nylander, T. Langmuir 2002, 18, 6437.

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of the ellipsometer was fine-tuned so that the ∆ reading was 0. Therefore the data are presented as the δ∆ value, which is also the parameter used in the evaluation according to Antippa et al.,16 employed in the present study as described further in the theory section. The Ψ reading was, within the experimental error, close to the ones expected for the decane-H2O (13.775) and decane-D2O (14.060) interfaces. It should be noted that no changes in the ellipsometer reading with time were observed for the pure oil-water interface, which demonstrated that no adsorbed layer of contaminants is formed. A concentrated stock solution was prepared by dissolving C18E50 in water. Small amounts of this stock solution were added in situ to the aqueous bottom phase in the ellipsometer cell by using a syringe. After addition, a magnetic stirrer was used to obtain a thoroughly mixed solution. The added amounts were always so small that any change in the position of the reflecting interface could be neglected. For the measurements of C12E5 adsorption, the pure surfactant was injected in the top phase (decane), and the top phase was stirred gently, so as not to disturb the interface. Since quite low surfactant concentration was used, the presence of the surfactant did not affect refractive index enough to be taken into account in analyzing the ellipsometry data. In the present study we have given the C12E5 concentration as the concentration in the oil phase because the partition between oil and the aqueous phase is strongly shifted toward the oil phase. Kabalnov et al.22 reported the concentration of C12E5 in an aqueous phase in equilibrium with decane to be below 7% of that in the oil phase. As will be discussed further below all measurements with C12E5 were done below the critical micelle concentration (cmc), which has been reported to be 56 µM as determined by surface tension measurement at the air-liquid interface.37 The adsorption process for each surfactant concentration was followed for 1 h, and data presented represent the plateau value of the adsorption as indicated by no change in the ∆ reading. It should be noted that the plateau value was reached after a maximum of 30-45 min. The data evaluation was done with the help of Mathematica 5.0 (Wolfram Research Inc, Champaign, IL). The refractive index data for oil, water, and deuterated water at λ ) 5320 Å was recalculated from the λ ) 5893 Å data in ref 38 by use of the method described by Mahanty and Ninham39 and Bo¨ttcher and Bordewijk.40 The obtained values for the H2O and D2O were 1.3348 and 1.3298, respectively, and the value for decane was 1.4125. The values of r and v used for the studied surfactants are as follows: C12E5, r ) 0.288 mg/mL, v ) 0.833 ( 0.009 mg/mL; C18E50, r ) 0.254 mg/mL, v ) 0.833 ( 0.009 mg/mL. The values for r were determined from the atomic composition of the surfactants as described by Cuypers et al.,26 and the values for v were those recorded for poly(ethylene glycol) derivates by Tziatzios et al.41 A value for dn/dc of 0.148 mg/mL was used for both surfactants.42 The values of ∆ an Ψ could be determined with an accuracy of (0.01° or better. Interfacial Tension Measurements. Interfacial tension measurements were performed using the automated drop tensiometer (Tracker, LT-Concept, France) based on axisymmetric drop shape analysis (ADSA) of an oil droplet in water. A detailed description of the technique is given elsewhere.43

Results The results from interfacial tension measurements for C12E5 and C18E50 at the decane-water interface are shown (37) Olofsson, G. J. Phys. Chem. 1985, 89, 1473. (38) Handbook of Chemistry and Physics; CRC Press: Cleveland, OH, 1974. (39) Mahanty, J.; Ninham, B. W. Dispersion forces; Academic Press: London, 1976. (40) Bo¨ttcher, C. J. F.; Bordewijk, P. Theory of electric polarization. Volume 2. Time-dependent fields; Elsevier: Amsterdam, 1978. (41) Tziatzios, C.; Precup, A. A.; Weidl, C. H.; Schubert, U. S.; Schuck, P.; Durchschlag, H.; Ma¨chtle, W.; Van den Broek, J. A.; Schubert, D. Prog. Colloid Polym. Sci. 2002, 119, 24. (42) Holmquist, P.; Nilsson, S.; Tiberg, F. Colloid Polym. Sci. 1997, 275, 467. (43) Benjamins, J.; Cagna, A.; Lucassen-Reynders, E. H. Colloids Surf., A 1996, 114, 245.

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Figure 6. Interfacial tension (γ) as a function of the surfactant concentration. Note that the concentration of C12E5 is for the oil phase and C18E50 is for the aqueous phase as discussed in the methods section.

in Figure 6. For C18E50, a steep initial decrease of the interfacial tension is followed by a more moderate decrease at higher concentration, and the break point in the curve is located approximately at 0.5 µM, which also suggests that the cmc is located at this point. However, no reliable surface tension or cmc data for C18E50 have been published. A linear fit to the data in Figure 6 below 0.5 µM give a slope, dγ/d ln c, of 7.235 mN/m, which inserted in the Gibbs equation (eq 5) would give an adsorbed amount of 7.15 mg/m2. This is significantly higher than the value reported for adsorption of C18E50 on a hydrophobized silica surface of 1.56 mg/m2.11 It should be noted that the breakpoint in the interfacial tension in Figure 6 is not defined as usually observed for smaller surfactants. This can partly be attributed to the polydispersity of the sample. In fact, a wider transition region or the presence of two breakpoints has been observed for other diblock and triblock copolymers of rather high molecular weights.44,45 Linse and Hatton45 has explained this phenomenon based on mean-field lattice calculations for ethylene oxide and propylene oxide containing homopolymers and triblock copolymers at the air-water interface. They propose that the low concentration break point, which is often observed when the surface tension of block copolymers is plotted against the logarithm of the concentration, is due to depletion of the copolymers in solution. The use of the slope of the surface tension curve below this low concentration break point to calculate the surface excess from the Gibbs equation therefore gives values that are too high. Furthermore they showed that the adsorbed amounts below cmc and above the low concentration break point are dominated by the longest and most surface active species. The correct value for the surface excess calculated from the Gibbs equation is therefore obtained if the slope of the surface tension versus logarithm of concentration in this concentration regime is used to calculate the adsorbed amount. From Figure 6 we get a value of dγ/d ln c of 1.169 mN/m for a C18E50 concentration above 0.5 µM but below 5 µM. Inserted in the Gibbs equation (eq 5), (44) Rippner, B.; Boschkova, K.; Claesson, P. M.; Arnebrant, T. Langmuir 2002, 18, 5213. (45) Linse, P.; Hatton, T. A. Langmuir 1997, 13, 4066.

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Figure 7. δ∆ (in degrees) as a function of the concentration C12E5 for both the H2O and the D2O system. Note that the concentration of C12E5 is the one in the oil phase as discussed in the methods section.

mg/m2

this would give an adsorbed amount of 1.15 . This value agrees much better with the value reported for C18E50 adsorption on hydrophobic silica. For C12E5 the crossover region from steep to a less accentuated decrease is located at about 120 µM of surfactant in the oil phase (Figure 6). A linear fit to the data in Figure 6 for concentrations immediately below the inflection gives a slope, dγ/d ln c, of 8.967 mN/m, which when inserted in the Gibbs equation (eq 5) would give an adsorbed amount of 1.49 mg/m2. Aveyard et al. reported a partitioning value of C12E5 between n-heptane and water of 273 at 20 °C,46 and Kabalnov et al. found that the maximum concentration of C12E5 in the aqueous phase is 7% of that in the decane phase.22 This suggests that the inflection point in Figure 6 would correspond to a cmc of at the most about 8 µM. This is significantly lower than the value of the cmc reported to be 56 µM as determined by surface tension measurement at the air-liquid interface.37 Furthermore, the plateau value of the surface tension in the decaneC12E5-water system has been reported to be very much lower, 0.2 mN/m,25 than the value of 7 mN/m in the present study. This shows that the plateau value corresponding to the cmc value has not been reached and suggests that only monomers of C12E5 exist in the oil and in the aqueous phase under the experimental conditions used in the present study. The reason for the plateau observed in Figure 6 is not clear, but we noted that the aqueous solution becomes opaque as the plateau is reached. We also noted that the ellipsometry readings became more unstable. In fact it was not possible to perform reliable ellipsometry or interfacial tension measurements with the ADSA technique at higher C12E5 concentration. One plausible explanation is formation of emulsion droplets at the oilaqueous interface. The ellipsometric angles Ψ and ∆ were measured for the two investigated surfactants, C12E5 and C18E50, as a function of the surfactant concentration, and the changes in ∆ due to surfactant adsorption, δ∆, recorded at an angle of incidence of 50° are plotted in Figures 7 and 8, respectively. As was discussed already in the theory section above, Ψ recorded for an interface between two dielectric media is virtually unaffected by the presence of a thin transparent layer. On the other hand changes in ∆, δ∆, (46) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1990, 86, 3111.

Figure 8. δ∆ (in degrees) as a function of the concentration of C18E50 in water for both the H2O and the D2O system.

are larger. For C18E50, ∆ initially decreases sharply with the surfactant concentration, while at concentrations higher than 0.4 µM it approaches a plateau and only slightly decreases with concentration, Figure 8. For C12E5 the decrease in δ∆ with surfactant concentration seems to be constant in the investigated regime, Figure 7. For both surfactants the results from measurements in H2O and D2O are similar, where δ∆ is lower in D2O than in H2O, with a difference of about 0.2° at high surfactant concentration. The data from the measurements were used to calculate the adsorbed amount, the layer thickness, and the refractive indices with the different methods to evaluate the data as discussed in the theory section. A summary of the results is provided in Table 1. The adsorption isotherm, resulting from applying the Gibbs equation on the surface tension data (in Figure 6) and the ellipsometry data from the H2O system following approach 1 (eq 6) are shown in Figures 9 and 10 for C12E5 and C18E50, respectively. The plateau value of the ellipsometry reading in terms of ImF was obtained by linear fit of the data at the plateau, or for C12E5 the values at a concentration of 400 µM or below. The obtained value at 400 and 4 µM, for C12E5 and C18E50, respectively, was then used as the value for ImF(cref) in eq 6. For C12E5 the corresponding value used for ΓGibbs(cref) was 1.49 mg/m2, which was obtained, using eq 5, from the slope at the surfactant concentration just below the inflection point in Figure 6 as discussed above. The corresponding value of ΓGibbs(cref) for C18E50 was 1.15 mg/m2, obtained from the slope in Figure 6 at the surfactant concentration between 0.5 and 5 µM as discussed above. The corresponding isotherms, calculated by combining the results from ellipsometric measurements in the decane-H2O and decane-D2O system, are also inserted in Figures 9 and 10. Here n1 and d1, obtained from eqs 9and 7, are assumed to be the same in both systems. The obtained values were used to calculate Γ, according to de Feijter (eq 5), with n0 ) 1.3348. As evident from the figures, the two methods to obtain the amount adsorbed give basically the same results, where the Gibbs approach gives a slightly higher value. For C12E5 no clear plateau in the adsorbed amount versus concentration is evident. This indicated as pointed out above that the cmc in the aqueous phase has probably not been reached. The value (1.38 mg/m2) obtained at a high concentration corresponds to an area per molecule of about 52 Å2. This

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Table 1. Summary of the “Plateau” Values of the Ellipsometer Data C12E5 (400 µM) evaluation condition data approach 1 Gibbs approach 2 n1[nH2O] ) n1[nD2O] d1[nH2O] ) d1[nD2O] Γ[nH2O] ) Γ[nD2O] d1[nH2O] ) d1[nD2O] Γ[nH2O] ) Γ[nD2O] d1[nH2O] ) d1[nD2O]

parameter

C18E50 (4 µM)

H2O data

D2O data

H2O data

D2O data

-0.736

-0.926

-0.527

-0.722

∆ (deg) Γ from eq 6 (mg/m2) (mg/m2)

Γ from eq 4 n1 from eq 9 d1 from eq 7 (Å) Γ from eq 4 (mg/m2) n1 from eqs 10, 12 d1 from eq 7 (Å) Γ from eq 3 (mg/m2) n1 from eqs 10, 11 d1 from eq 7 (Å)

1.49

1.15

1.38 1.34845 149 6.61 1.39867 153 5.6 1.40112 180

1.34845 149 6.61 1.39367 153 5.6 1.39725 180

0.933 1.34505 135 6.33 1.40214 139 9.13 1.40537 194

1.34505 135 6.33 1.39714 139 9.13 1.40219 194

Figure 9. Adsorbed amounts as a function of the concentration C12E5 in the oil phase. The data were obtained by using: approach 1 (Gibbs), using eq 6, with ImF(cref) ) -0.00316 extrapolated from Figure 7 at a concentration of 400 µM and ΓGibbs(cref) ) 1.49 mg/m2 calculated from the slope of Figure 6 as described in the text; approach 2 (H2O/D2O), using measured values of δ∆ in both H2O and D2O. Both n1 and d1 are assumed to be the same in H2O (n0 ) 1.3348) as in D2O (n0 ) 1.3298) and were obtained from eqs 9 and 7, respectively, using n2 ) 1.4125 and φ ) 50°. The adsorbed amount was calculated according to de Feijter (eq 4) (with n0 ) 1.3348 and dn/dc ) 0.148 mL/g).

Figure 10. Adsorbed amounts as a function of the concentration C18E50 in the aqueous phase. The data were obtained by using: approach 1 (Gibbs), using eq 6, with ImF(cref) ) -0.00221 extrapolated from Figure 8 at a concentration of 4 µM and ΓGibbs(cref) ) 1.15 mg/m2 calculated from the slope of Figure 6 as described in the text; approach 2 (H2O/D2O), using measured values of δ∆ in both H2O and D2O. Both n1 and d1 are assumed to be the same in H2O (n0 ) 1.3348) as in D2O (n0 ) 1.3298) and were obtained from eqs 9 and 7, respectively, using n2 ) 1.4125 and φ ) 50°. The adsorbed amount was calculated according to de Feijter (eq 4) (with n0 ) 1.3348 and dn/dc ) 0.148 mL/g).

agrees well with the plateau value of the adsorption isotherm at the air-aqueous interface, which corresponds to an area per C12E5 molecule of 50 Å2, as determined by ellipsometry and neutron reflection.47,48 Also on the hydrophobic solid-aqueous interface similar values were reported, including 53 Å2 on the hydrobized silica surface using ellipsometry49 and 51 Å2 on poly(methyl methylacrylate) surfaces.50 It is indeed interesting that our study, where the surfactant is adsorbed from the oil phase, gives the same results as these reported values, which are from studies where the surfactant is adsorbed from the aqueous phase. It should also be noted that the area per C12E5 molecule for the bicontinuous microemulsion in the C12E5decane-aqueous systems has been reported to be about 51 Å2, based on neutron scattering data.51

The adsorption isotherm for C18E50 features a marked plateau with Γ values of about 1 mg/m2, which corresponds to an area per molecule of about 407 Å2. This seems reasonable considering the much larger size of this surfactant compared to C12E5. C18E50 has so far not been very much studied. However, Kapilashrami et al. reported plateau values of Γ ) 1.56 mg/m2 on hydrophobized silica surface.11 They used this value to evaluate the adsorption on different silicon oil-aqueous interfaces. However, no independent measurement of the adsorption on the silicone oil-aqueous interface was presented. Table 1 gives a summary of the data resulting from the different evaluation methods. We note that the values for the adsorbed amount are higher when we apply evaluation models where Γ and d1 are assumed to be the same in D2O systems as in H2O systems. This is a consequence of a higher refractive index of the film than when n1 and d1 are assumed to be the same in the D2O system as in the H2O system. We also note from Table 1 that the calculated layer thickness is always quite high. For example d1 values are 149 and 135 Å for C12E5 and C18E50, respectively, when we assume that n1 and d1 are the same in the D2O system as in the H2O system. This is much higher than what has been reported from other studies. In a neutron reflectivity

(47) Binks, B. P.; Fletcher, P. D. I.; Paunov, V. N.; Segal, D. Langmuir 2000, 16, 8926. (48) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Binks, B. P.; Crichton, D.; Fletcher, P. D. I.; McNab, J. R.; Penfold, J. J. Phys. Chem. B 1998, 102, 5785. (49) Dedinaite, A.; Bastardo, L. Langmuir 2002, 18, 9383. (50) Gilchrist, V. A.; Lu, J. R.; Staples, E. J.; Garrett, P.; Penfold, J. Langmuir 1999, 15, 250. (51) Sottmann, T.; Strey, R.; Chen, S. H. J. Chem. Phys. 1997, 11065, 6483.

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Figure 11. Experimentally determined values of δ∆ (filled symbols, in degrees) as a function of the concentration C12E5 for the H2O system at φ ) 40, 50, and 60°. Note that δ∆ was recorded consequently at the different angles in the same experiment. The values at φ ) 50° and the corresponding δ∆ values from the D2O measurements were used to calculate n1 and d1, which are assumed to be the same in H2O (n0 ) 1.3348) as in D2O (n0 ) 1.3298) and were obtained from eqs 9 and 7, respectively. On the basis of these n1 and d1 values, the δ∆ values expected at φ ) 40 and 60° were calculated from eq 7 and are shown as unfilled symbols.

study, Lu et al.48 reported a total layer thickness of C12E5 adsorbed at the air-aqueous interface of 27.5 Å, where the hydrocarbon chain contributed with 14 Å. From neutron reflectivity studies of the hydrophobic poly(methyl methylacrylate)-aqueous interface Gilchrist et al.50 reported a total thickness of the adsorbed C12E5 layer of 20 Å, with only about 4 Å taken up by the hydrocarbon chain. This suggested that the hydrocarbon chain in this case oriented parallel to the interface. The adsorption of C18E50 on hydrophobized silica surfaces has been found to result in a layer thickness of 48 Å,11 which is lower than the values in the present study (145 Å, Figure 8). As described in the materials and methods section the used ellipsometer setup allowed for measurements at different angles of incidence within the same experiment. To test our experimental approach, the measurements in the H2O system were carried out at two additional angles of incidence, where one was further above the Brewster angle and the other below the Brewster angle. The data are shown in Figures 11 and 12 for C12E5 and C18E50, respectively. Below the Brewster angle, δ∆ is positive, and as expected the absolute value of δ∆ is larger closer to the Brewster angle. We also used the data from Figure 7 and Figure 8 recorded at φ ) 50°, which were evaluated by approach 2, assuming that n1 and d1 are the same in the D2O system as in the H2O system, to calculate the expected δ∆ values at φ ) 40° and φ ) 60°. The obtained values are shown as unfilled symbols, and we note that they agree very well with the experimentally determined δ∆ values (filled symbols) at these angles of incidence. However as discussed by Antippa et al.,16 it should be noted that measurements at different angles of incidence do not give additional information about the interfacial layer when using the thin film approximation according to eq 7. Discussion We will start our discussion with the ellipsometry data evaluation. In Figures 13 and 14, ImF from C12E5 and C18E50 is plotted versus Γ. Γ was calculated from eq 4,

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Figure 12. Experimentally determined values of δ∆ (filled symbols, in degrees) as a function of the concentration C18E50 for the H2O system at φ ) 40, 50, and 60°. Note that δ∆ was recorded consequently at the different angles in the same experiment. The values at φ ) 50° and the corresponding δ∆ values from the D2O measurements were used to calculate, n1 and d1, which are assumed to be the same in H2O (n0 ) 1.3348) as in D2O (n0 ) 1.3298) and were obtained from eqs 9 and 7, respectively. On the basis of these n1 and d1 values, the δ∆ values expected at φ ) 40 and 60° were calculated from eq 7 and are shown as unfilled symbols.

Figure 13. The imaginary part of the ellipticity (ImF) for decane-H2O or decane-D2O as a function of the amount of C12E5 adsorbed at the oil-aqueous interface. The effect of capillary wave ImFcw, calculated according to eq 13 and the surface tension data from Figure 6, is inserted. The solid and the dashed lines (based in Figure 3) are ImF from the δ∆ value given by eq 7 for a thickness of 150 and 50 Å, respectively, at a given adsorbed amount. Ψ is set to 13.775° and 14.060° for H2O and D2O, respectively. Both n1 and d1 are assumed to be the same in H2O (n0 ) 1.3348) as in D2O (n0 ) 1.3298), n2 ) 1.4125, φ ) 50°, and the adsorbed amount was calculated according to de Feijter (eq 4) (with n0 ) 1.3348 and dn/dc ) 0.148 mL/g).

assuming that n1 and d1 are the same in the D2O as in the H2O system according to approach 2. These two figures show that Γ is indeed proportional to ImF as was observed by Battal et al.17 for the adsorption of a homologous series of cationic surfactant at the air-water interface. For an ideal Fresnel interface, the lines in Figures 13 and 14 should intersect the y axis at 0°. The effect of capillary waves will roughen the interface and will give rise to an apparent layer at the interface, which can be measured

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Figure 14. The imaginary part of the ellipticity (ImF) for decane-H2O or decane-D2O as a function of the amount of C18E50 adsorbed at the oil-aqueous interface. The effect of capillary wave ImFcw, calculated according to eq 13 and the surface tension data from Figure 6, is inserted. The solid and the dashed lines (based in Figure 3) are ImF from the δ∆ value given by eq 7 for a thickness of 150 and 50 Å, respectively, at a given adsorbed amount. Ψ is set to 13.775° and 14.060° for H2O and D2O, respectively. Both n1 and d1 are assumed to be the same in H2O (n0 ) 1.3348) as in D2O (n0 ) 1.3298), n2 ) 1.4125, φ ) 50°, and the adsorbed amount was calculated according to de Feijter (eq 4) (with n0 ) 1.3348 and dn/dc ) 0.148 mL/g).

by ellipsometry.30-32 Even for a clean air-water surface, values for ImF of between 0.33 × 10-3 and 0.42 × 10-3 have been reported.17,52-54 The effect of capillary waves as estimated from eq 13 and based on the data in the figures is introduce in Figures 13 and 14. The maximum effect of the capillary waves, at the lowest interfacial tension, is calculated to be ImFcw ) -0.00055. It should here be noted that if we reach the plateau value of the interfacial tension in the decane-C12E5-water system, reported to 0.2 mN/m,25 eq 13 suggests ImFcw ) -0.0032. In Figures 13 and 14, we have also expressed the data from Figure 4 in terms of ImF, for d1 values of 50 and 150 Å. In both cases the data show that the thickness remains almost constant over the entire adsorption isotherms. As discussed above a layer thickness of C12E5 of about 150 Å is unreasonable high. As pointed out by Bain,55 it is often difficult to resolve the thickness data from ellipsometry measurements at liquid interfaces. Therefore one has to be careful when interpreting thickness data. However, as we notice from Figures 13 and 14, the 50 and 150Å lines are reasonable well separated. An important factor is that we here have applied a very simple slab model, where we assume that the adsorbed layer has a set refractive index. More sophisticated models with a refractive index profile would probably give a different thickness. We also do not know if there is any decane in the film, but assume that it consists of water. Such a factor might affect the thickness calculation as well as the calculation of the adsorbed amount. It also turned out that the simplest approach to combine the H2O and D2O ellipsometry data according to approach 2 was the one that gave reasonable results in terms of adsorbed amount. This model lies on the assumption that (52) McBain, J. W.; Bacon, R. C.; Bruce, H. D. J. Chem. Phys. 1939, 7, 881. (53) LordRayleigh. Philos. Mag. 1892, 33. (54) Bouhet, C. Ann. Phys. 1931, 15, 5. (55) Bain, C. D. Curr. Opin. Colloid Interface Sci. 1998, 3, 287.

Benjamins et al.

the n1 and d1 are the same in the H2O and D2O systems. If we assume that the amount of surfactant in the film is the same in the H2O and D2O systems, this implies that the solvents in both layers have the same refractive index. The other models in approach 2, where Γ and d1 were set to be the same in the H2O and D2O systems gave far too high Γ values. This implies that the composition of the film in these models needs to be further refined. Ellipsometry has quite extensively been used to study surfactant adsorption at the air-liquid interface, often in combination with other techniques such as surface tension measurements and neutron reflectivity measurements.14,17-21,28,47,55-57 The studies of similar phenomena at liquid-liquid interfaces are significantly fewer as mentioned in the Introduction. Although, ellipsometry is complex to apply to liquid-liquid systems, quite a number of the studies have managed to go beyond simple adsorption isotherms. This demonstrates the potential of the technique. Lei and Bain used ellipsometry to detect surfactant-induced surface freezing at the alkane-water interface.12 Harke and Motchmann used imaging ellipsometry to study monolayers of dipalmitoylphosphatidylcholine (DPPC) at the air-aqueous interface when exposed to a hydrocarbon (hexane, cyclohexane, dimethylbutane) gas phase.58 By changing the hydrocarbon partial pressure, they were able to tune between a gaseous-water and oil-water interface. Schulz et al.34 found from studies of near-critical liquid-liquid interfaces that for systems including weak amphiphiles, such as n-C4E1 and i-C4E1, and water the obtained values of ImF could be fitted to theories describing interfacial thickness including capillary waves.33 However, if a stronger amphiphile, such as C10E4, was used, the obtained value of ImF deviated substantially from the value predicted by theory, which was attributed to the formation of a surfactant layer at the interface. In later study13 they used ellipsometry to investigate a system where the aqueous amphiphile phase contained oligo(ethylene glycol) monoalkyl ethers (CnEm) with different amphiphilicity and the inert oil phase was a perfluoroheptane. For the shortest and least amphiphilic compound, C4E1, they found that the adsorption of the amphiphile at the fluorocarbonaqueous interface increases as the phase separation temperature is approached. This was interpreted as an initial wetting of the interface by a surfactant-rich phase. They observed the opposite effect for the more amphiphilic C5E2 and C6E3, where instead an incipient wetting of a water-rich phase at the interface was observed. This indicated depletion of the surfactant-rich phase from the interface and was attributed to the formation of a surfactant monolayer at the fluorocarbon-aqueous interface. This made the interface quite hydrophilic, due to the more hydrated headgroup of this surfactant, which prevented wetting of the more hydrophobic surfactantrich phase. If the perfluoroheptane was replaced with octane, this depletion effect did not occur for C5E2, since this system can form a microemulsion phase. This phase is then expected to “wet” the oil-aqueous interface. The decane-C12E5-water system, investigated in the present study, shows indeed a microemulsion phase in the C12E5-decane-aqueous systems, which has been studied extensively.22,24,25,51,59 We note that based on these and numerous other reports on the same ternary system, (56) Reitner, R.; Motschmann, H.; Orendi, H.; Nemetz, A.; Knoll, W. Langmuir 1992, 8, 1784. (57) Teppner, R.; Haage, K.; Wantke, D.; Motchmann, H. J. Phys. Chem. B 2000, 104, 11489. (58) Harke, M.; Motchmann, H. Langmuir 1998, 14, 313. (59) Bagger-Jo¨rgensen, H.; Olsson, U.; Mortensen, K. Langmuir 1997, 13, 1413.

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we expect only monomers to be present in the oil and aqueous phase at the conditions employed in the present study. Despite this fact we observed considerable instability of the oil-aqueous interface at high surfactant concentration at about 500 µM in the oil phase. At this concentration the aqueous phase became turbid. We also note the plateau in the interfacial tension isotherm below the cmc in water (Figure 6). This might indicate emulsification of the oil by the surfactant in the aqueous phase. This can also be the explanation to the large d1 value observed for the adsorbed layer of this surfactant. Another interesting issue is the partitioning of the surfactant between the oil and the aqueous phase. Here it should be noted that the portioning of C12E5 between the aqueous and the alkane phase is very sensitive to the temperature46 as is the phase behavior of the C12E5-decane-aqueous system in general.22,24,25,51,59 In principle it should be possible to measure the partitioning from the change in Ψ following from changes in refractive indices. However with the present measuring cell it is not possible to have enough control of the temperature and alignment of the oil-aqueous interface for such measurements. The partitioning of a surfactant with higher cmc would be easier to analyze with this method as one then could work at higher monomer concentrations and hence larger changes in refractive indices. The results for the C18E50 surfactant are more straightforward to interpret, as this surfactant is only soluble in the aqueous phase, and suggests monolayer formation close to an apparent cmc of this surfactant. It is also likely that the large headgroup of the surfactant leads to a less densely packed layer. An interesting question to address is to what extent the C18-hydrocarbon chain is solubilized in the oil or if it is orientated parallel to the interface as was observed by Gilchrist et al.50 for C12E5 at the hydrophobic poly(methyl methylacrylate)-aqueous interface. Such a determination would require far more

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sophisticated models than the simple slab model used in the present studies. Such models have been presented by the group of Thomas et al. in Oxford20,48,50 for a whole series of the poly(ethylene glycol) alkyl ether surfactants, based on neutron reflectivity measurements using partially deuterated surfactant and contrast matching. Applying and verifying these models for adsorption at the oil-aqueous interface requires determination of more parameters. One possibility is to also use an oil phase with different refractive index or use spectroscopic ellipsometry and utilizing the wavelength dependence of the refractive indices.16 Conclusion The presented study of combining ellipsometry measurements in H2O and D2O has demonstrated theoretically and experimentally that the refractive index difference between H2O and D2O is enough to extract additional information from ellipsometry data. Quite a number of the studies using ellipsometry have managed to go beyond simple adsorption isotherms, and also the present study has demonstrated the potential of the technique. This also requires further development of models as well as a measuring cell which can adequately control the temperature and the interface. Even without sophisticated models, changes in reflectivity can help identify under which conditions phase changes in liquid-liquid systems occur, e.g., to identify when a two-phase system is transformed into a three-phase system. Acknowledgment. We are grateful to Professor Bengt Jo¨nsson for his generous help and many clever suggestions and to Dr. Alexey Kabalnov for fruitful discussions. The financial support from The Swedish Research Council and Swedish Foundation for Strategic Research (SSF) is gratefully acknowledged. LA049848H