Langmuir 1993,9, 556-560
Visualization of First- and Second-Order Phase Transitions in Eicosanol Monolayers Using Brewster Angle Microscopy Gernot A. Overbeck,* Dirk Honig, and Dietmar Mobius Max-Planck-Inatitut fiir biophysikalische Chemie, Postfach 2841,D-3400 Gattingen, Germany Received June 1,1992. In Final Form: November 16,1992
In the intermediate temperature and the upper pressure regions, the phase diagram of an eicoeanol monolayer consists of four phases: (1)one normal condensed high-pressure phase, A’, (2) and (3) two high-pressure superliquids, Rot I and Rot II, and (4) one low-pressure phase, C. The phase transitions A’/C and Rot II/C are of second order and correlated with a collective tilting of the molecules. The phase transition C/Rot I, however, is of firat order and correlated with formation and growth of nuclei consisting of tilted or normally oriented molecules, depending on the direction of the transition. The kinetics of the phase transitions between the tilted and untilted phases were visualized with a Brewster angle microscope (BAM). After C A’ C compreesion/decompression cycles a memory effect for the shape of the domains in the C phase was observed.
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I. Introduction Research on phase transitions in two-dimensional systems is at the present moment an exciting exercise. The reduced dimensionalitysimplifies model calculations and is the basis for physical phenomena which are not known in three-dimensionalsystems. Monolayers at the air/water interface are interesting systems for studying two-dimensional phase transitions, althoughan enormous complexity is introducedby the one-dimensionalcharacter of the molecules and the larger variety of intermolecular interactions including those with the aqueous subphase. Up until recent years, the techniquesfor the investigations of phase transitions were restricted to isotherm and viscosity measurements. The interpretation of the kinks and sometimes very narrow plateaus in terms of phase transitions of the amphiphilic material being examined was quite controversial and confused by the alternative possibility of contamination.lI2 X-ray diffraction measurements on floating monolayers gave the first information about the microscopic structure, tilt angle, and translational order.3-8 Polarizedfluorescence microscopy visualized tilt orientational order in mesophases! but the influence of the probe molecules is still controversialand the diffusion of the dye molecules complicates the interpretation of the phase transition kinetics. Brewster angle microscopy (BAM)1@13provides information on the lateral structure of monolayers without requiring fluorescent probes. It has been demonstrated that BAM is an excellent (1) Peterson, I. R. Ber. Bumen-Gee. Phys. Chem. 1991,95,1417. (2) Bibo, A.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991,67, 5591.
( 3 ) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zachack, J.; Dutta, P. J. Chem. Phys., to be published. (4) Lin, B.; Shih, M. C.; Bohanon, T. M.; Ice, G. E.; Dutta, P. Phys. Rev. Lett. 19SO,65, 191. (5) Kenn, R. M.; Bbhm, C.; Bibo, A. M.; Peterson, I. R.; Mbhwald, H. J. Phys. Chem. 1991,96,2092.
(6) Shih, M. C.; Bohanon, T. M.;Mikrut, J. M.; Zechack, P.; Dutta, P. Phya. Rev. A 1992,45,5734. (7) Barton, 5.W.; Thomas, B. N.; Flom, E. B.; Rice, St. A,; Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. J. Chem. Phys. 1988,89,2267.
(8) Schlwman, M. L.; Schwarz, D. K.; Pershan, P. 5.;Kawamoto, E. H.; Kellogg, 0. J.; Lee, S. Phys. Rev. Lett. 1991,66, 1599. (9) Qiu, X.; Rub-Garcia, J.; Stine, K. J.; Knobler, C. M.; Selinger, J. V. Phys. Rev. Lett. 1991, 67, 703. (10) HBnon, 5.; Meunier, J. Rev. Sci. Imtrum. 1991, 62, 936. (11) Hbnig, D.; Mbbius, D. J . Phys. Chem. 1991,95,4590. (12) Hbnig, D.; Mbbius, D. Thin Solid Film 1992,210/211,64. (13) Hanig, D.; Overbeck, G. A.; Mbbius, D. Adv. Mater. 1992,4,419.
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technique for visualizing anisotropy in monolayer phases.12J3 With the H6nig-type microscope kinetic changes can be followed at a video frame rate. We have used it to investigate the firsborder C/Rot I as well as the secondorder C/A’ and C/Rot I1 phase transitions in eicosanol monolayers. Long-chain alkanol monolayers were first investigated by W. D. Harkins et al.l4JS They presented a phase diagram consisting of three phases, L2, LS, and S. As deduced from isotherm measurements, the high-temperature phase transitions between L2 and LS and the lowtemperature phase transitions between S and LS are of second order, represented by a kink in the isotherm. In contrast, over an intermediate temperature region the phase transition between L2 and LS is of first order, represented by a plateau in the isotherm. The S/LS phase transition could be detected with viscosity measurements showing the LS phase as a kind of superliquid (i.e., the viscosity is independent of the surfacepressure). Further evidence for this transition was obtained from an estimation of the thermodynamic coefficient @ = ( b r / b T ) = ~ (SS/IA)T( A = surface pressure, T = temperature, S = entropy, and A = area) as a function of the temperature, calculated from an isochore. and Shih et aL3 in their studies on Barton et monolayers of heneicosanol (C21HUOH)identified the L2 phase with a C phase consisting of tilted molecules and the S phase with a distorted hexagonal A’ phase. In fatty acidss as well as in alcohols,3 Shih et al. were able to show that the LS phase consists of a distorted hexagonal Rot I and a hexagonal Rot I1 phase.l6 (14) Harkins, W. D.; Copeland, L. E. J. Chem. Phys. 1942, 10, 272. (15) Copeland,L. E.; Harkins,W. D.;Boyd, G. E. J. Chem. Phys. 1942, 10, 357. (16) It may be argued that a short-range herringbone order in Rot I
establishes electron densitieswhoee two-dimensional projections are no circles. Analogous to the observation of Levelut,” thia may give r k to additional reflection spots of a hexagonal phase. In this case,the Rot I and Rot I1 phases observed by Shih et al.336 would belong to the same phase and the additional diffraction spots in Rot I would be induced by short-range herringbone order whose coherence length decreaecw with increasing temperature, being approximately zero in Rot 11. But in contrast to the observations of Levelut1’ the reflection spota of Shih et al.3,6cannotberelatedsimultaneouslytothe(11)apd(21) linesofaplane hexagonal lattice. Additionally there in no evidence explaining why the behavior of the alcoholsshould be similarto that of the highlyasymmetric PBBA (N-@-phenylbenzylidene)-4-n-butylaniline)and not to that of the alkanes where Rot I and Rot I1 were observed as two different
1993 American Chemical Society
Overbeck et al.
556 Langmuir, Vol. 9, No. 2, 1993 monitor
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Bib0 et aL2 have applied the nomenclature of smectic crystal phases to monolayer phases, but the difference between Rot I and Rot I1 is not included. At the moment, it is unknown whether the C phases does or does not have a herringbone order corresponding to the SH or the SF smectic phase, respectively. Therefore, we will stay with the nomenclature of Shih et al. although we think that the introduction of the smectic crystal nomenclature is an essentialstep for the characterizationof monolayerphases. Although Harkins and Copeland worked mainly with octadecanol (C18H370H) and Shih et al. with heicosanol (C21H430H),the qualitative results are not changed since our experiments with alcohols (C19H390H to C21HaOH) (vide infra) show that an increase in chain length by one C atom corresponds approximately to a decrease in transition temperature by about 5 K. Similar results are found for fatty acids by, e.g., Akamatsu et al.19 11. Experimental Section The principle of Brewster angle microscopy and the setup used (shown in Figure 1)have been described in detail in ref 13, with the only difference being that we used a 10-mW polarized HeNe laser (Siemens)to get a higher contrast instead of a 5-mW polarized HeNe laser. Some pictures were made with the commercialBrewster angle microscope BAMl manufactured by NFT (Giittingen, Germany). We used a PTFE (poly(tetrafluorethylene))trough (50 cm X 18cm X 0.7 cm) with a POM (poly(oxymethylene))barrier. The spreading solution had a concentration of 10-3 mol/L, and the spread volume was 150 pL. The compression velocities were (slow) l/, cm/min and (fast) 4 cm/ min. The main problem was the temperature control of the trough; due to temperature gradients in the trough, the temperature values had a precision of 10.7 K. We used eicosanol (Larodan, 99+%) without further purification and water from a Milli-Q system (Millipore). The pictures shown have been filtered, and the contrast has been increased (framegrabber card, DT2862, Data Translation; image processing software, DTOptima, Data Translation).
111. Results Figure 2 shows surface pressure/area isothems of eicosanolfor differenttemperatures, The isotherms have a plateau for a limited temperature range, indicating a phase transition of first order. In the following we will concentrate on the surface pressure interval 2 mN/m < ?r (17) Levelut, A. M. J. Phys. (Paris) 1976,37, C34251. (18) Doucet, J.; Denicolo, I.; Craievich, A. J. Chem. Phys. 1981, 75, 5125. Doucet, J.; Denicolo, I.; Craievich, A. J. Chem. Phys. 1981, 75, 1523. (19) Akamatsu, S.; Rondelez, F. J. Phys. ZZ 1991, I , 1309.
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Figure 3. Phase diagram of eicosanol. A', Rot I, Rot 11,and C are the abbreviations of Dutta et al. S, LS, and L2 are the abbreviationsof Copeland and Harkins. The dots are determined from isotherm measurements.
< 35 mN/m of the isotherm and the correlated states of the monolayer. Figure 3 shows a phase diagram analogous to that of Harkins et al.14but with the additional phase Rot I whose existencewas proved by Shih et and Barton et al.7 The dots are determined from isotherm measurements. In the following we discussBAM imagesof several phase transitions and the related change of the molecular polar ((p) and azimuthal (8) tilt angles (Figure 4). 1. Second-OrderPhase TransitionsC A' C.In the low surface pressure part of the C(L2) phase, the monolayer displays at T = 18"C a mosaic texture (Figure 5) with molecules of constant, nonzero polar angle (p and constant tilt azimuth 8. We will refer to them in the future as "TO domains" (domainswith tilt order). In generalthe molecules in different TO domains have different tilt azimuths, which generates the optical anisotropy visible in Figure 5, top. The contrast of the structures is inverted
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Phase Transitions in Eicosanol Monolayers Y polar tilt angle
