Langmuir 1994,10, 3853-3856
3853
Energy Dispersive X-ray Reflection from a Liquid-Liquid Interface S. J. Roser* School of Chemistry, University of Bath, Claverton Down, Bath, U.K.
R. Felicj Istituto di Struttura della Materia, via Enrico Fermi, Frascati, Italy
A. Eaglesham ICI C & P Ltd, The Heath, Runcorn, U.K. Received October 15, 1992. In Final Form: July 8, 1994@ The first measurement of an X-ray reflection profile from a liquid-liquid interface is presented in this paper. The details of constructionof a laboratory-basedsimple energy dispersiveX-ray reflectometer are given, and it is demonstrated that the technique has enough sensitivity and penetrating power to enable structural measurements to be made at buried interfaces.
Introduction Specular reflectionofx-rays and, more recently, thermal neutrons from interfaces can be used to gain structural information about interfaces a t angstrom resolution.' Simple specular reflection profiles yield detail perpendicular to the interface, such as the density and thickness of any adsorbed film. With the advent of X-ray and neutron reflectometers designed specifically for liquid surfaces, the field has grown rapidly, particularly in the study of insoluble monolayers,2 and the ions bound to them,3adsorbed polymers,4 and soluble surfactant^.^ With powerful synchrotron X-ray sources, the necessary flux is available to extend the specular experiment to study the in-plane structure of insoluble monolayer films.6 In this paper we extend the measurements made by X-rays to the liquid-liquid interface, using a novel X-ray reflectometer.
Figure 1. Geometryof scatteringfromliquid-liquid interfaces.
interface depends on the refractive index difference between the two layers. Using the formulation previously Introduction to Reflection Theory employed for X-ray reflection2gives the electron density of a layer related to its refractive index, n, by The theory ofX-ray and neutron specular reflection has been well described elsewhere in the l i t e r a t ~ r e .The ~~~~~,~ n = 1 - (A2/2n)Nzr, (2) basic principle of a reflectivity measurement is to measure the reflectivity, R(Q),as a function of scattering vector, where N is the number of molecules per unit volume, z the Q , with number of electrons per molecule, and re the classical radius of an electron (2.82 x A). The specular Q = 4n sin(O)/d (1) reflectivity R(Q)can thus be interpreted in terms of the when a well collimated beam of wavelength is reflected scattering density profile perpendicular to the surface, from a macroscopic surface at a grazing angle 8. For which for X-rays is related to the electron density as a X-rays, as well as neutrons, the scattering from an function of depth. Inherent in eq 1is that the scattering vector, Q,can be * Author for correspondence. varied by changingeither the incident angle or the incident Abstract published in Advance ACS Abstracts, September 1, wavelength of the X-ray or neutron beam. Previous 1994. (1)(a) Parratt, L. G. Phys.Rev. 1964,95,359.(b)Penfold, J.;Thomas, laboratory-based X-ray reflection measurements have R. K.J.Phys. Cond.Matter 1990,2,1369. worked a t fixed wavelength, typically using Cu Ka (2) Richardson, R. M.; Roser, S. J. Langmuir 1991,7 , 1458. radiation a t 1.54 A, whereas the principal UK neutron (3)Richardson, R. M.;Roser, S. J. Liq. Cryst. 1987,2,797. (4)Rennie, A.R.; Crawford, R. J.;Lee, E. M.; Thomas, R. K.; Crowley, spectrometer, CRISP, at the ISIS facilitygoperates at fixed T. L.; Roberts, S.;Qureshi, M. S.;Richards, R. W. Macromolecules 1989, angle with a polychromatic incident beam. The experi22,3466. ments described in this paper were performed using a (5)Lee, E. M.; Thomas, R. K.; Penfold, J.;Ward, R. C. J.Phys. Chem. 1989. ~ . 93. _ 381. . novel X-ray reflectometer with a variable energy detector (6jKJaer, K.; Als-Nielsen, J.; Helm, C. A.;Tippmann-Krayer, P.; which operates a t much lower wavelengths than usual, Mowald, H. J.Phys. Chem. 1988,93,3200. and a t a fixed angle. (7)Grundy, M. J.;Richardson, R. M.; Roser, S. J.;Penfold, J.;Ward, @
R. Thin Solid Films 1988,59,43.
( 8 )Lekner, J. Theory of Reflection; Nijhoff Publ.: Dordrecht, The Netherlands, 1987.
(9) Penfold, J.; Ward, R.; Williams, W. G . J.Phys. E 1987,20,1411.
0 1994 American Chemical Society
Roser et al.
3854 Langmuir, Vol. 10,No. 10,1994
Figure 2. Schematic plan of Frascati X-ray reflectometer; (1)X-ray tube; (2) collimation slits; (3)sample position; (4) single crystal solid-state energy sensitive detector.
Reflection from Liquid-Liquid Interfaces Although the liquid-liquid interface is of fundamental importance in colloid and biological science, it is only recently that attempts have been made to study it by neutron reflection, and none have employed X-rays. As the interface is “buried”, either technique must be relatively penetrating. In order to reach the range of Q values in a typical experiment, the beam of X-rays or thermal neutrons must impinge on the sample a t low angles, usually less than l o , which requires a long illuminated length of sample, typically 5 cm. This in turn requires that to reach the interface through a bulk phase, the beam must pass through a similar amount of that phase (Figure la), unless the thickness of the upper lessdense phase is substantially reduced (Figure lb). The relatively high values for X-ray absorption a t 1.54A imply that access through a thin upper layer (Figure lb) is the most viable technique, but the interface between air and the upper layer will of course also lead to significant reflection,unless the scatteringdensity difference between air and the upper layer is reduced close to zero. With neutrons, this can be achieved by the well-known technique of contrast-variation, using for example a mixture of deuterated and hydrogenated liquid to give an upper phase scattering length density of zero. This method is not available for X-ray reflection, where any profile will be dominated by the big step in electron density from air to liquid. A further problem arises in making the upper film thin enough to avoid significant absorption or incoherent scattering. In the first measurement of reflection from a liquid-liquid interface,1° the authors controlled the thickness of a very thin wetting layer ( (20 pm) in a saturated atmosphere of hexane using a set of Peltier coolers working at constant current. This paper demonstrates the feasibility of X-ray reflection from a liquid-liquid interface using the relatively short wavelength Brehmstrahlungfrom a tungsten X-ray source, and an energy dispersive detector. The experimental technique is simple, and the data collection rate relatively rapid. Instrumentation An X-ray reflectometerspecificallydesignedto work withliquid surfaces and with energy sensitive detection has been constructed at the Istituto di Struttura della Materia, Frascati. A schematic plan of the machine is shown in Figure 2. The machine is mechanically very simple, with two arms pivoting about a central drive sh&, and the X-ray optical path defined by Huber 50 pm (10)Cosgrove,T.;Phipps, J.;Richardson, R. M. Colloids Surf 1992, 62,199.
10000
I
I
I
I
through air and Si filter
1000
100
through liquid
10
1
I
I
0.4
0.8
1.2
I
1.6
wavelength (A)
Figure 3. Comparison of transmission of X-rays through air and 7 cm of water. variable slitsmounted on the arms. The sourceis a 2 kW tungsten tube, driven by an Ital-Structures X-ray generator. X-ray detection is accomplished by an EG&G high-purity germanium solid-state detector connected to a PC via ADCAM hardware and MAESTRO software, which perform the necessary analogue to digital conversions and amplification. The energy resolution of the detector is ca. 1.5%, with a maximum count rate of approximately 10000 cps. Inherent in the functioning of germanium solid-state detectors is that, for an incident photon of energy E, there is a finite possibility of counting an event at (E - E),where E is the energy of the Ge Ka transition. This phenomenon leads to echoes appearing in the spectrum which can, however, be numerically removed. The echo intensity as a function of incident energy for these experiments was calibrated using Bragg reflections from silicon single crystals. Samples were contained in a specially constructed Teflon trough, with the X-ray beam entering through the thin side wall in configuration a of Figure 1. The path length through the sample was 7 cm, and the wavelength dependences in the transmitted beam for this configuration and for a beam passing through air are shown in Figure 3. It can be seen that, at wavelengths greater than 1A,the beam transmitted through the liquid has virtually reached the background level by being absorbed, but there is a window of wavelengths from 0.25 to 0.8 A where the intensity transmitted is sufficiently high for the
Langmuir, Vol. 10, No. 10, 1994 3855
X-ray Reflection from a Liquid-Liquid Interface
Table 1. Parameters Used in Liquid-Liquid Interface FiP
~
angle (deg) 0.044 0.085 0.193 0.297
1
2
G
1
:I
a
scaling factor 7.26 513 2360 20600
background 7.38 x 10-3 f 3.41 10-3 2.27 x f 1.08 x 10-2 5.58 x loT2& 0.29 x 3.60 x 10-1 f 0.03 x 10-1
Interfacial roughness = 6.58 x
A.
1OD
i
lo-'
0 1
0 01
0 0
0.125
0.25
10-5
9 (A-1) Figure 4. Data from cyclohexane/water interface, normalized to transmission and corrected for echoes' incident angles: (a) 0.044"(b) 0.085" (c) 0.193' (d) 0.297'. reflection experiments, particularly bearing in mind that the strongest scattering in a reflection experiment is at lowest Q, or highest wavelength. Normalization of a measured raw reflection profile involves dividing the data through by a measured white beam profile through the same length of liquid. The data are then numerically corrected for the measured echoes from the germanium detector.
1o
0 0
Figure 6. Reflectivity profile of cyclohexane-water interface and model fit. 1
oo
1
o->
System under Study The two systems chosen to demonstrate the technique are based on mutually insoluble water and cyclohexane. The raw "scattering length densities", Nzre, for water and cyclohexane are 0.940x A-2 and 0.757 x A-2, respectively, and in order to increase the difference between them, and hence the reflectivity at higher angles, calcium chloride hexahydrate was added to the aqueous phase at a concentration of approximately 7 m o m of HzO. The resulting scattering length density, taking into account volume changes, was 1.14x 10-5A-2.In the second experiment, a modified polyester surfactant consisting of PEG 800 hydrophiles with fatty acid (predominantly unsaturated CIS) residues and a molecular mass (M,) of greater than 2500 gmol-l was added to the aqueous phase a t a concentration of 0.5g dm-3. These very concentrated subphase conditions are unusual for studying surfactant behavior, but the increase in contrast given by the calcium ions facilitates the object of this paper, which is merely to demonstrate that the technique is feasible. Further experiments at more realistic concentrations of subphase ions will be carried out.
Results and Discussion Figure 4 shows t h e data, divided by the transmitted beam, taken at four different angles from the interface between cyclohexane a n d the water/CaClz solution. The d a t a are on different scales due to different counting times a n d slit configurations b u t can be overlapped to give relative scaling factors. The d a t a were fitted using the suite of FITLAY programs described elsewheree2An initial fit to the d a t a is calculated using the single-layer Fresnel formula, which is valid even
- ~
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1o
- ~ \
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io
- ~
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lo-;
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I
'5
Figure 6. Reflectivity profile for cyclohexane-water interface with added surfactant, and single layer model fit.
Roser et al.
3856 Langmuir, Vol. 10,No. 10,1994 h
2e-005,
5M
le-005
I
I
I
c
a,
li
1
2 l 3
k Q,
4
5e-006
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4
water
id
0
m
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1
- 40
I
-20
I
I
1
0
20
40
60
distance from interface (A) Figure 7. Fitted profile to surfactant data.
at low Q values. Through the use of a least squares method, the program then calculates the best fit to the experimental data. The parameters used in the initial model fit can be varied for each data set, tied together, or fixed as desired. In this experiment, the scatteringlength densities of the aqueous and cyclohexane layers are k e d at their calculated values, the relative scaling factors between runs are fxed by their regions of overlap, and the interfacial roughness, assumed to have an error function profile, between the phases is tied to be the same for all of the data sets. This leaves a single scaling factor to convert to absolute reflectivity, the interfacial roughness, and the individual backgrounds for each set as the only parameters in the data fit. Table 1summarizes the parameters used for the final fit, and Figure 5 shows the data sets normalized to their linked scaling factors, and with their fitted background subtracted. The features to note are that the critical angle, which is a measure of the step in scattering length density a t the interface, is well reproduced by the calculated values, and that the interfacial roughness indicated by the data is virtually zero (less than 1A). To test the sensitivity of the technique to changes in the interface, we have measured the reflection protile after addition of a small quantity of polymeric surfactant. Figure 6 shows the data and best fit. These data fall off significantly faster with Q than from the clean interface, indicating that the addition of surfactant has made the interface considerably more diffuse. However, it was not possible simply to use an error function profile to describe the excess electron density at the interface. To fit the data, it is necessary t o invoke the single block model scattering length density profile shown in Figure 7 and parameterized in Table 2. There is clearly an excess of electron density associated with the surfactant at the interface. This is almost certainly due to an interfacial excess of ions binding to the surfactant layer rather than the surfactant molecules themselves. Given the structure of the surfactant used, one might imagine that at an oillaqueous interface the PEG chains would extend into the aqueousphase and the CISaliphatic chains into the cyclohexane. On the aqueous side of the interface, the data appear to be broadly consistent with this picture-the slight depression in electron density could conceivably reflect the presence of PEG chains. However, from Figure 7, it can be seen that the interfacial region is very diffuseand that most of the excess electron density
Table 2. Parameters Used in Surfactant Fit angle (den) scaling factor background 0.050 0.151 0.299
133 30000 30900
9.91 10-4 & 1.05 10-2 8.30x f 2.73 x 3.34 x 10-1 f 0.10 x 10-1
Surfactant Layer thickness of layer scattering length density roughness hexandsurfactant waterisurfactant
15.4& 0.25 8, 1.46 x 10-6A-2 18.1iz 0.48, 5.93 & 0.83 A
is to the nonaqueous side of the interface. To account for this, it is necessary to assume that a substantial amount of water and/or electrolyte is pulled over to this side of the interface, perhaps by some of the surfactant forming aggregated structures. Also, since the volume fraction of watedelectrolyte decays very rapidly (over ca. 60 A) as a function of distance from the interface, then a single layer of small aggregates would be sufiicient to explain this excess density. It is not possible to assign a precise structure to the adsorbed surfactant from these preliminary experiments, and further evaluation will be undertaken using classical surface science techniques. However, this initial experiment clearly demonstrates the exciting potential of the technique.
Conclusions We have demonstrated that it is possible to obtain structural detail of liquid-liquid interfaces using a relatively simple in-house X-ray reflectometer. No complicated sample environment facilities are required, and the sensitivity of the measurements is sufficient to detect the presence and infer some structural detail of a surfactant at the interface. Although we have used a very concentrated aqueous phase, simple extrapolative calculation from these results shows that the experiment is indeed possible at the ‘natural” contrast of water. In future experiments we intend to demonstrate that the technique may also be applied to the liquid-solid interface. Acknowledgment. S.J.R. acknowledges financial support for travel costs from the Nufiield Foundation, IC1 C & P Ltd and CNR,Italy. In addition, we acknowledge Fabio Zuccaro for help with constructing the instrument.
