Langmuir 1995,11, 764-766
764
Surface-Induced Shift of the Hexagonal-to-IsotropicPhase Transition in a Lyotropic System Studied by X-ray Reflectivity C. Braun, P. Lang,* and G. H. Findenegg Iwan-N.-Stranski Institute of Physical and Theoretical Chemistry, Technical University Berlin, Strasse des 17. Juni 112, 0-10623 Berlin, Germany Received October 7, 1994. In Final Form: December 5, 1994@
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X-ray reflectivity measurements of the free surface of a C12E5 water mixture at a mass fraction of the surfactant of 0.4 are presented. This system forms a hexagonal LC-phase at ambient temperatures. At a temperature of T = 23.82 "C the typical Bragg pattern for hexagonal symmetry is no longer observed when the angle of incidence of the beam is larger than the critical angle of total external reflection, a+ If the angle of incidenceis smaller than ac,well-pronounced reflexes are observed at the same temperature. This shows that the anisotroDic Dhase is stable to higher temperatures at the surface as compared to the bulk. I
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Introduction Aqueous mixtures of nonionic surfactants of the oligo(oxyethy1ene)mono-n-alkyl ether (C,E,) type exhibit a rich phase behavior which has been studied extensively over the past 30 y e a r ~ . l -The ~ higher members of this family are known to form lyotropic mesophases. The system water forms a hexagonal phase a t a volume fraction of the amphiphile (&) around 0.4 and ambient temperature. This phase, which consists of cylindrical micelles, transforms into an isotropic micellar liquid at T x 23 "C for #s = 0.4. In the present study we investigate the possibility of a surface-induced shift of this hexagonalto-isotropic transition. Such phenomena4 have been observed a t the free surface of pure mesogenic compound^,^ LC polymer melts,6and even for higher n-alkanes.' In all these cases it was found that the surface stabilizes the more ordered state of the fluid. In this paper we report evidence that such pretransitional surface ordering may also occur in a relatively dilute lyotropic system. A useful tool for such investigations is X-ray reflectivity.8 This technique is sensitive to variations of the electron density perpendicular to the reflecting surface. If the reflecting surface represents a n ideal step function profile, the specular reflectivity (R)decays with q4for q >> qc according to the Fresnel law, where q is the wave vector transfer and qc= 2k sin a,with k being the wave number of the incident beam and a, the critical angle of total external reflection. The thermal roughnessgof the surface,
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@Abstractpublished in Advance A C S Abstracts, February 15, 1995. (1)Mitchell, D. J.;Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. SOC.Faraday Trans. I1983, 79, 975. (2) Tiddy, G. J. T. Phys. Rep. 1980, 57, 1. (3)Sjoblom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactants Physical Chemistry-Surfactant Science Series 23; Schick, M. J., Ed.; Marcel Dekker: New York and Basel, 1987. (4) See, for example: Proceedings of the Discussion Meeting of the Deutsche Bunsen-Gesellschaft fur Physikalische Chemie, Bad Herrenalb, Germany, September22-24,1993; Phase Transitions at Interfaces. Ber. Bunsenges. Phys. Chem. 1994,98 (3). (5) Immerschitt, S.; Koch, T.; Stille, W.; Strobl, G . J. Chem. Phys. 1991,96,6249. (6)Uzman, M.; Song, B.; Runke, T.; Cackovic, H.; Springer, J. Makromol. Chem. 1991,192, 1129. (7) Earnshaw, J. C.; Hughes, C. J. Phys. Rev. A 1992,46, 4494. (8) See for example: Als-Nielsen, J. In Structure and Dynamics of Surfaces II; Schommers, W.; von Blankenhagen, P., Eds.; Springer: Berlin, 1986. (9)Als-Nielsen, J. 2.Phys. B-Condens. Matter 1985, 61, 411.
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Figure 1. Image plate picture from the hexagonal phase of a water mixture with a volume fractionof the surfactant 9, = 0.4. The rectangle indicates the position of the PSD for the off specular measurements (see Figure 3 ) .
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adsorbed surfactant monolayers,1° or insoluble lipid monolayers" cause systematic deviations in R(q) from the Fresnel reflectivity [ R ~ ( q ) lLaterally . ordered structures cause diffraction patterns in the form ofBragg peaks. In the case of hexagonal ordering of the bulk sample, a Bragg pattern occurs as shown in Figure 1for the ClzE5 water HI mesophase. This pattern is caused by a random orientation of ordered domains which consist of hexagonally packed cylindrical micelles with quasi-infinite extension along the cylinder axis. To prove the random orientation, the sample was rotated around its surface normal. This procedure did not cause changes in the diffraction pattern.
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(10) Penfold, J.;Thomas, R. K. J. Phys.; Condens. Mutter 1990,2, 1369. (11)Kenn, R. M.; Bohm, C.; Bibo, A. M.; Peterson, I. R.; Mohwald, H. J. Phys. Chem. 1991,95,2092.
0743-746319512411-0764$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 3, 1995 765
Hexagonal-to-IsotropicPhase Transition
0
10
5
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20
15
25
q z qc Figure 2. Reflectivity curves of the free surface of a C1&
sample with = 0.4 at a temperature below (20 "C)and above (25"C)the bulk phase transition. The reflectivity ( R )is plotted vs qplqcwhere qz is the wave vector transfer perpendicular to the surfaceand qcis the criticalvalue of total external reflection. The transition temperature was estimated t o Thi = 23-24 "C by polarization microscopy.
Experimental Section Reflectivity measurements were performed at the beam line D4 at HASYLAB/DESY, Hamburg, Germany. Details of the mechanical setup of this instrument have been described elsewhere.12 A monochromatic beam of wavelength1= 0.13328 nm was extracted from the white synchrotron source by Bragg reflection at the (111)plane of a Ge monochromator crystal. The angular dependence of the specular reflectivity was recorded with a NaI(T1)scintillationcounter with a horizontal slit aperture mounted directly in front of it. The off-specular diffraction was recorded using a PSD 50M position-sensitive detector by MBraun, Munich, Germany, of 50 mm active length,with the slit aperture removed. The surfactant C&5 was purchased from Nikko Chemicals, Tokyo,Japan, and used without further purification. Water was purified by use of a Milli-Q water purification system from Millipore-Waters. The liquid sample was spread on a sandblasted steel plate of 80 mm diameter surrounded by a Teflon ring and mounted on a steel socket. To avoid solvent evaporation the sample was covered with a steel lid with two Kapton windows to allow beam entrance and exit. The temperature of the lid and the socket was controlled by a Lauda RS6 water bath thermostat and it was ensuredthat the temperatureof the lid was always slightly higher than that of the socket. Temperature was measured with a standard Pt-100 resistor element. Generally the temperature was constant during the measurements to f O . O 1 K.
Results and Discussion
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Reflectivity curves [R(q)]of a C12E5 water mixture of a volume fraction of surfactant GS= 0.4 a t temperatures below and ca. 2 K above the hexagonal-to-isotropic transition are displayed in Figure 2. From the peaks in the lower curve the lattice constant of the hexagonal phase ~~
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(12)Als-Nielsen, J.; Pershan, P. S. Nucl. Instrum. Methods 1983, 208,545.
may be calculated from Bragg's equation, which yields a cross sectional diameter of the cylinders d,, = 5.26 nm, in very good agreement with the value calculated from small angle scattering experiments.13 The corresponding cross sectional radius of the cylinder is only slightly smaller than the length of the completely stretched all-trans configuration of a C12E5 molecule (I x 2.7 nm). In the upper curve in Figure 2 there are small maxima a t about the same q values as the Bragg peaks in the curve from the anisotropic system. The upper curve was recorded a t 25 "C, which is well above the transition of the bulk phase which had been estimated by polarization microscopy to be Thi = 23-24 "C. It is tempting to ascribe these maxima to a pretransitional ordering effect, but this is not the only explanation as we will show later. Furthermore, in view of the uncertainty of an absolute measurements of the surface temperature, and the possibility of slow shifts of T h i due to solvent evaporation, a n in-situ measurement of the bulk transition temperature is more reliable thana-situ measurements by polarization microscopy or small-angle scattering, etc. It is therefore desirable to obtain information from the surface and the bulk in the same experimental setup within the briefest possible time interval. This requirement can be met in reflectivity experiments since the penetration depth (A) of the reflected X-ray beam is very sensitive to the angle of incidence (ai). In the limiting case of angles much smaller than the critical angle of total external reflection (ai> G,the penetration depth is proportional to a and governed solely by the linear absorption coefficient $1 i.e.,
For example, a t the experimental wavelength d = 0.133 nm, eq 2 yields a n effective penetration depth of L x 15 pm a t a n angle of incidence ai = 5%. Thus it is possible to obtain structural information from a very thin surface layer, only about 2 times the length of a stretched surfactant molecule, by observing the reflected intensity in grazing incidence geometry. On the other hand the response a t larger angles of incidence may be assigned unambiguously to the bulk properties of the sample. The reflexes which are indicated by arrows in Figure 1lie off the direction of the specularly reflected beam and are solely due to the hexagonal symmetry of the sample. Accordingly these Bragg peaks must vanish above the temperature Thi. We have monitored these peaks with a position-sensitive detector (PSD) as a function of temperature and a t different angles of incidence. In Figure 3 the PSD spectra are displayed for ai= 0.8% (where the signal results only from surface ordering) and ai = 3% (where the spectra are dominated by the bulk phase behavior). At the lower temperatures (23.46,23.55,23.74 "C in Figure 3) Bragg peaks are observed a t both angles of a,.The full width half-maximum (FWHM)of the peaks, which is related to the thickness of the ordered layer at ai= OB&, is constant for all temperatures. This indicates that the thickness of the hexagonally ordered surface layer exceeds the penetration depth L , which equals to about 50 nm in this case. At T = 23.82 "C the Bragg peaks are (13)Olsson, U.; Wiirz, U.; Strey, R. J . Phys. Chem. 1993,97, 4535.
766 Langmuir, Vol. 11, No. 3, 1995
Braun et al. a,= 3.0a,
ai = 0.8a,
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23.82"C
23.74"C
23.55"C
23.46"C 0
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af "f Figure 3. PSD spectra from the off specular Bragg peaks; the detector was in the position.indicatedby the rectangle in Figure 1 (28 = 1.22"). Left, angle of incidence smaller than the critical angle of total external reflection (ai = 0.8%); right, ai = 3%.
still well pronounced a t ai/&= 0.8 but have disappeared a t ai/& = 3.0, suggesting that this temperature is above Thi but still below the surface transition temperature. A rough estimation assuming a n exponential dependence of the scattered intensity on the penetration depth reveals that the surface scattering contributes only by 0.5%to the total scattered signal at a n angle of incidence of ai/% = 3.0, and therefore the surface peaks will disappear in the data noise if the angle of incidence is changed to ai/& = 3.0. If the sample temperature is increased further to 23.93 "C, the Bragg peaks also disappear a t ai/& = 0.8. Hence from Figure 3 we may conclude that the surface transition temperature is ca. 0.1-0.2 K above the bulk transition temperature. The surface transition appears to occur a t a fairly sharp temperature between 23.82 and 23.93 "C in view of the fact that the Bragg peaks are still quite sharp (signal-to-noiseratio ofabout 4:l)a t the former temperature but have entirely disappeared a t the latter temperature. The characteristic feature of all spectra recorded a t ai/ a,= 3 is the broad maximum a t small angles, i.e. a t small q vectors. This maximum is attributed to the diffuse
scattering of cylindrical micelles or aggregates of micelles. In view of this observation, the meaning of the small maxima in the upper curve in Figure 2 has to be reconsidered. It is not possible to distinguish whether they are caused by the mentioned diffuse scattering or by the reflectivity of a very thin layer of the hexagonal phase. This uncertainty stresses again the importance of an insitu measurement of the bulk transition. In conclusion, the present study shows unambiguously that the hexagonal phase of a C12ES water mixture is stable to higher temperatures at the free surface as compared to the bulk. A quantitative analysis of this finding by fitting theoretical models to the experimental reflectivity curves is in progress.
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Acknowledgment. We thank H. Dorsch and A. Moller for their help with the experiments and appreciate the assistance ofH. Rhan and the HASYLAB staff. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) in the Sonderforschungsbereich 335. LA9407830
