Langmuir 1994,10, 1584-1591
1584
Refractive Index of Thin Aqueous Films Confined between Two Hydrophobic Surfaces Patrick Kbkicheff and Olivier Spallat Department of Applied Mathematics, Research School of Physical Sciences, The Australian National University, Canberra, ACT 0200, Australia Received January 3, 1994" Multiple-beam interferometry is used to determine the refractive index profile of the aqueous medium aa a function of the separation between two molecularly smooth mica surfaces rendered hydrophobic by surfactant adsorption from bulk solution. Within experimental accuracy,no deviation from the bulk value is found. The consequences of this observation are discussed in view of recent proposals aa to the origin of the hydrophobic attraction. 1. Introduction
The hydrophobic effect,' or the attraction between small, nonpolar solute molecules in aqueous solutions, is one of the driving interactions in the self-assembly mechanism of amphiphilic compounds's2 and in determining the conformation of proteins or biological species in water:3 such compounds attempt to minimize the contact of their hydrophobic regions with water either by aggregating or by adjusting their conformation. It is also crucial to a wide range of phenomena, such as adhesion, wetting, metastability of water films in the vicinity of hydrophobic surfaces,4*5cavitation,6v7or coagulation.%" Many industrial technologies take advantage of the destabilization of aqueous colloidal suspensions when the macroscopic surfaces of the particles are rendered hydrophobic. Surfactant aggregation is involved in many processes such as detergency, oil recovery, food emulsification, paint technology, coating, waterproofing of surfaces and fabrics, etc. In mineral separation techniques, the degree of hydrophobicity of the mineral particles is the key to their floatability as the mineral particle-bubble attachment is one of the decisive processes in flotation.12 The existence of a long-range attraction between two macroscopic hydrophobic surfaces immersed in aqueous solutions has been known for at least 30 years.4 Over the last decade, most of the experimental investigations which have dealt with this interaction have focused on the direct measurement of the force-distance profile between a variety of hydrophobed surfaces immersed in different t On leave from Service d e Chimie MolBculaire, Bht 125, Commissariat B 1'Energie Atomique, CEN Saclay, 91191 Gif-sur-Yvette CBdex, France. Abstract published in Aduance ACS Abstracts, April 15,1994. (1) Tanford, C. The hydrophobic effect, 2nd ed.; Wiley: New York, @
1973. (2) Franks, F. Water: a comprehensioe treatise; Franks, F., Ed.; Plenum: New York, 1975; Vol. 4, pp 1-94. (3) Hopfinger, A. J. Intermolecular interactions and biomolecular organization; Wiley: New York, 1977. (4) Blake, T. D.; Kitchener, J. A. J. Chem. SOC.,Faraday Trans. 1 1972,68, 1435-1442. (5) Shulze, H. J. Developments in mineral processing; Elsevier: Amsterdam, 1984. (6) Yaminsky, V. V.; Yushchenko, V. S.; Amelina, E. A.; Shchukin, E. D. J. Colloid Interface Sci. 1983, 96,301-306. (7) Yushchenko, V. S.; Yaminsky, V. V.; Shchukin, E. D. J. Colloid Interface Sci. 1983, 96,307-314. (8)Usui, S.; Imamura,I.; Barouch, E. J.Dispersion Sci. Technol. 1987, 8, 359. (9) Xu,Z.; Yoon, R.-H. J. Colloid Interface Sci. 1989,132, 532-541. (10) Skvarla, J.; Kmet, S. Int. J.Miner. Process. 1991, 32, 111. (11) Skvarla, J. J. Colloid Interface Sci. 1993, 155, 506-508. (12) Ralston, J. Ado. Colloid Interface Sci. 1983, 19, 1-26.
solution conditions (for a recent review see ref 13). These works have shown that the very peculiar feature of this force remains its long-range behavior. Furthermore its strength appears to be larger than any conceivable van der Waals attraction. The microscopic origin of this long range attraction is currently not understood, although there have been several theoretical attempts.l6lg The phenomenological mean-field analysis involving an enhancement of the hydrogen bond network of the fluid14 appears unrealistic in view of the rapid decay of surfaceinduced effects.20 Electrostatic correlation forces between similar surfaces have been considered independently.l"17 However, as pointed out by Podgornik17the anomalous dielectric behavior of the solvent close to the surface assumed by Attard16 cannot give rise to an attraction stronger than the dispersion forces calculated within the Lifshitz theory. More recent proposals involve the metastability of the film due to its confinement between two hydrophobic walls.18J9 Conceptually, the capillary evaporation or cavitation which will occur if the spinodal of the confined fluid is approached18 is equivalent to a capillary condensation from an undersaturated bulk gas.21 Similarly, it has been recently argued from microscopic nucleation theory that the increased attraction may be a result of the lateral enhancement of density fluctuations in the aqueous film between two hydrophobic ~ a l 1 s . l ~ Further, the presence of dissolved gas in the aqueous medium has been speculated as to the origin of the hydrophobic a t t r a ~ t i o n . ~ ~ ~ ~ ~ At first sight, it could seem rather unusual that such different explanations are claimed for a unique phenomenon, but this is mostly due to the very few experimental properties reported for the interaction between hydrophobic surfaces. Further, it appears that conventional criteria of surface hydrophobicity such as the advancing (13) Christenson, H. K. In Modern approaches to wettability: theory and applications; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1992; Chapter 2; pp 29-51. (14) Eriksson, J. C.; Ljunggren, S.; Claesson, P. M. J. Chem. SOC., Faraday Trans. 2 1989,85, 163-176. (15) Attard, P. J. Phys. Chem. 1989,93, 6441-6444. (16) Podgornik, R. Chem. Phys. Lett. 1989,156, 71-75. (17) Podgornik, R. J. Chem. Phys. 1989,91, 5840-5849. (18) Bbrard, D. R.; Attard, P.; Patey, G. N. J. Chem. Phys. 1993,98, 7236-7244. (19) Yaminsky, V. V.; Ninham, B. W. Langmuir 1993,9,3618-3624. (20) Lee, C. Y.; McCammon, J. A.; Rossky, P. J. J. Chem. Phys. 1984, 80, 444a-4455. (21) Evans, R.; Marini Bettolo Marconi, U. J. Chem. Phys. 1987,86, 7138-7148.
0743-7463/94/2410-1584$04.50/00 1994 American Chemical Society
Refractive Index of Aqueous Films
contact angle of water on the surface or the adhesion between two such surfaces (related to the interfacial energy) cannot on their own yield reliable predictions of the range or the strength of the hydrophobic interaction.24125The force-distance profile of the attraction appears to be very sensitive to the exact surface preparation procedure (e.g. surface pressure during Langmuir-Blodgett d e p o ~ i t i o nplasma , ~ ~ ~power ~ ~ ~during ~ activation of mica for subsequent ~ i l y l a t i o netc.), , ~ ~ to the electrical charging of the surface^,^^^^^-^^ to the presence of additives,37-40 and, in a very crucial way, to the stability of such surfaces in the presence of dissolved e l e ~ t r o l y t e . 3 3 ~The ~ ~ influence ~~~~0 of t e m p e r a t ~ r e or ~ ~polarity l~~ of the solvent43has also been reported but remains controversial (see respectively ref 44 and ref 45). Whether such confusion is the result of complexity due to the very nature of hydrophobic surfaces or is a consequence of insufficient accuracy or care in either of the experimental techniques involved remains an open question. Clearly resolution of this question requires further experimental investigations dealing with very well characterized surfaces. In search of physical parameters associated with the hydrophobic attraction, the density of the aqueous film confined between two hydrophobic walls is here investigated via the determination of its refractive index. This study appears very timely in view of the recent proposals that the attraction may be a result of the metastability of the film18J9 or due to the effect of dissolved gas.22~23In any of these situations, the density of the solvent medium is expected to deviate from its bulk value. Multiple-beam interferometry which employs fringes of equal chromatic order (FECO fringes)46 can be used to measure the refractive index profile as a function of the separation between two hydrophobic surfaces. To our knowledge no systematic investigation of the refractive index has been reported yet in this situation. A surface force apparatus (24)Claesson, P. M.; Christenson, H. K. J.Phys. Chem. 1988,92,16501655. (25)KBkicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989,40,31-41. (26)Claesson, P. M.;Blom, C. E.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sci. 1986,114,234-242. (27)Christenson, H. K.; Claesson, P. M. Science 1988,239,390-392. (28)Kurihara, K.; Kato, S.; Kunitake, T. Chem. Lett. 1990, 15551558. (29)Kurihara, K.; Kunitake, T. J. Am. Chem. SOC. 1992,114,1092710933. (30)Parker, J. L.; Claesson, P. M.; Cho, D. L.; Ahlberg, A.; Tidblad, J.; Blomberg, E. J. Colloid Interface Sci. 1990,134,449-458. (31)Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985,229,1088-1089. (32)Rabinovich, Y. I.; Dejerguin, B. V. Colloids Surf. 1988,30,243251. (33)Christenson, H. K.; Claesson, P. M.; Berg, J.; Herder, P. C. J. Phys. Chem. 1989,93,1472-1478. (34)Herder, P. C. J. Colloid Interface Sci. 1990,134,336-345. (35)Christenson, H. K.; Fang, J.; Ninham, B. W.;Parker, J. L. J.Phys. Chem. 1990,94,8004-8006. (36)Christenson, H. K.; Claesson, P. M.; Parker, J. L. J.Phys. Chem. 1992,96,6725-6728. (37)Claesson, P. M.; Kjellander, R.; Stenius, P.; Christenson, H. K. J. Chem. Soc., Faraday Trans. I 1986,82,2735-2746. (38)Herder,C.E.;Claesson,P.M.;Herder,P.C. J.Chem.Soc.,Faraday Trans. 1 1989,85,1933-1943. (39)Herder, P. C. J. Colloid Interface Sci. 1990,134,346-356. (40)Tsao, Y.-H.; Evans, D. F.; WennerstrBm, H. Langmuir 1993,9, 779-785. (41)Tsao, Y.-H.;Yang, S.X.;Evans,D. F.; Wennerstrom, H. Langmuir 1991,7,3154-3159. (42)Rabinovich, Y. I.; Guzonas, D. A.; Yoon, R.-H. Langmuir 1993,9, 1168-1170. (43)Parker, J. L.; Claesson, P. M. Langmuir 1992,8,757-759. (44)Christenson, H. K.; Parker, J. L.; Yaminsky, V.V.Langmuir 1992, 8,2080. (45)Tsao, Y.-H.; Evans, D. F.; Wennerstrom, H. Science 1993,262, 547-550. (46)Tolansky, S.An introduction t o Interferometry, 2nd ed.; Long mans: London, 1973;251 pp.
Langmuir, Vol. 10, No. 5, 1994 1585
is used to vary the separation of the aqueous medium between two molecularly smooth mica surfaces rendered hydrophobic by surfactant adsorption from a bulk solution. This choice, at the sake of LangmuirBlodgett deposition which usually gives a longer attraction range, was made on purpose since deposition by adsorption from the bulk remains of more general relevance in colloid science. The cationic surfactant cetyltrimethylammonium bromide (CTAB) was chosen because of its availability in a very pure state and its previously thorough investigation in the SFA.25y48At equilibrium, a full monolayer coverage on muscovite mica can be obtained by adsorption from a bulk solution of 4 X lV M CTAB.25 Our results indicate that no significant variation of the refractive index of the aqueous medium from its bulk value has been detected even for very short separations where the magnitude of the hydrophobic attraction is large. The consequences in view of the recent p r o p o s a l ~ ~ for ~ Jthe ~ ~origin ~ ~ ~ of ~ 3the hydrophobic interaction are discussed. 2. Experimental Section 2.1. Materials. Cetyltrimethylammonium bromide (CTAB or n-hexadecyltrimethylammonium bromide CH3(CH2)l6Nt(CH3)sBr) was purchased from Kodak and was used without further purification. Water was prepared as previouslydescribed, with the water being passed through a Millipore UHQ unit in the final stage. To avoid nucleation of macroscopic air bubbles upon injection through the apparatus inlet, the water which is saturated with nitrogen after removal from the MilliQ unit was degased for half an hour under vacuum beforehand. The chamber was then filled with extreme care, by gently pushing the water using a filtered nitrogen stream (without bubbling) evaporated from a liquid nitrogen tank. As the SFA is not perfectly insulated against the external environment, in addition to a very rapid gas diffusion (the duration of experiments range from a few hours to a few days) the results reported here can be considered to have been obtained for solutions which are in equilibrium with ambient air. This is evidenced in particular by the solution having a pH of about 5.7 indicating equilibrium with atmospheric COz. After measurements in water, the surfaces were separated about 0.2 mm apart and the chamber was emptied, making sure that a drop of water was left between the mica surfaces. The surfactant solution was then injected into the chamber and the surfaces were separated and then moved back and forward several times without reaching contact to ensure that the liquid residing between the surfaces was properly mixed. The measurements were carried out in a temperature-controlled room at 25.0 0.2 "C above the Krafft temperature of the solutions. The mica was best quality, optically clear Muscovite green mica, obtained from Mica Supplies Ltd. (England). 2.2. Apparatus. The surface force apparatus Mark IV used in the present study is the latest modification in this series and has been described elsewhere.49 As previously," all operations (cleaning, assembly of the instrument, mica cleaving and gluing) were performed in a laminar flow cabinet under essentially dustfree conditions. The apparatus was suspended from springs to minimize the influence of external vibrations. The separation between the mica surfaces was controlled by the expansion of a piezoelectric crystal. Since one surface is suspended at the end of a cantilever spring, regions where the gradient of the force dF/aD is larger than the spring constant K are inaccessible: at the distances where such intrinsic instabilities occur, the surfaces will jump to the next mechanically stable regime (dF/dD C K ) . In order to carry out measurements over the largest possible surface separation range (in particular in the case of long-range
*
(47)Israelachvili, J. N.; Adams, G. E. J. Chem. SOC.,Faraday Trans. 1 1978,74,975-1001. (48)Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988,126,569-578. (49)Parker, J.L.;Christenson,H. K.;Ninham,B. W.Reu.Sci.Instrum. 1989,60,3135-3138. (50)Shubin, V. E.;KBkicheff, P. J. Colloid Interface Sci. 1993,155, 108-123.
1586 Langmuir, Vol. 10, No. 5, 1994
Kkkicheff a n d Spalla
attractive forces), a double-cantilever variable spring was used whose stiffness could be varied by a factor of lo00 during the course of an experiment. When divided by the radius of curvature R (-1-2 cm) of the mica surfaces, the normalized stiffness K/R ranges from 2 X l @ to 5 X 10s N/m2. 2.3. Optical Thickness Measurements. As previously, the optical thickness of the medium between the mica surfaces was measured with multiple-beam interfer0metry.61*~~ The two partly silvered mica sheets, glued silvered side down onto the surface of two glass cylinders, form an optical cavity into which white light is directed. Multiple reflections are undergone between the two metallic surfaces (silver 50 nm thick; reflectivity >98% in the green region of the visible spectrum). Only certain wavelengths of the transmitted light have appreciable intensity: the spectrum is comprised of a series of fringesof equal chromatic order (FEC0)M whose wavelengths are related to the thickness and the refractive index of each layer in the interferometer. The transmitted light is directed to the entrance slit of a spectrometer, and finally dispersed by a grating (-32 A wavelength/"), directed to an exit port, which serves as a window for direct observation (by the eye) or for record on videotapes using an intensified video camera (DAGE MTI, Model 65), where fringe shifts, etc., can later be measured from the tapes with a Colorado video micrometer, Model 305. Each fringe exhibits a finite width, correspondingto an intensity wavelength profile. Here X is taken to be the wavelength of the center of a FECO, and A changes continuously as the two surfaces are moved relativeto one another. Since the wavelength of the FECO is measured (by eye or by analysis of its recorded image on videotapes) by aligninga moving graticule with the center of the fringe, the width of the entrance slit of the spectrometer was kept constant during an experiment. This ensures a minimal scatter in the measurements due to variations in the appearance of the FECO at the exit port (broadening and brightness) upon widening or narrowing of the entrance slit while approaching the two surfaces during an experiment. With an entrance slit 50 pm wide, the wavelength of a FECO is measured within 1 A. In the case where no material is deposited or adsorbed on the, mica sheets the interferometer is comprised of three layers: mica (thickness Y = 1-3 pm)/intermediate medium (&..)/mica. For mica rendered hydrophobic by adsorption of surfactant (our case of interest here) the interferometer is made of five layers: mica/ surfactant monolayer (thickness 2 = 5-20 &/aqueous medium/ surfactant monolayer/mica. It is worth noting that the sandwiched layer constituted of solvent may be inhomogeneous. Indeed, one goal of this work was to seek an eventual change in the solvent density and hence of its refractive index. Standard matrix multiplication methods can be usedm to determine the coefficients of reflection and transmission for any combination of layers of specified optical properties a t varying wavelengths, and consequently fringe positions can be deduced. Ignoring for the moment the possible inhomogeneity of the sandwiched solvent layer, the resonant wavelengths Xi emerging from the symmetrical five-layer interferometer, whose characteristics are summarized in Figure 1, are given byS1
with
+
A = sin{2k(n8Z n,Y*))
- 2r2sin(2kn$) + r z sin(2k(nJ
- n,Y*))
(lb)
and
The propagation of light inside the mica medium is dispersive (nM = nM(Ai)) and the phase changes on reflection at the silveF(51) Israelachvili, J. N. Colloid Interface Sci. 1973,44, 259-272. (52) Horn, R. G.; Smith, D. T. Appl. Opt. 1991,30,59-65. (53) Born,M.; Wolf, E. Principles of0ptics;Pergamon Press: Oxford, 1959.
si I
0
Y Z
T
Z Y
I *z Figure 1. Schematic illustration of a five-layer interferometer formed between highly reflecting silver coatings a t z = 0 and z = 2Y + 2 2 + T. Each mica sheet (thickness Y) is dispersive nM(X), and the phase change on reflection a t the mica/silver interface is equivalent to a decrease in the apparent thickness by q(A). Asurfactmt monolayer (refractiveindex&) ofthickness, 2,coats each mica surface. Here, the polar heads of CTAB are turned toward the mica surface exposing the hydrophobic tails in a liquid-like state to the aqueous medium. The refractive index n,of the aqueous film may depend on the separation, T, between these two hydrophobic surfaces, i.e. n,= n,(7'). At the surfactant/water interface and a t the surfactant/mica interface the coefficients of reflection are rl and r2, respectively. mica interfaces can be expressed as an equivalent thickness, $(Ai), added to the bare mica thickness, Y. For muscovite mica, the representations given by Horn for both the dispersion in refractive index and the effect of phase change, which decreases the apparent thickness of the mica (Y* = Y + $(Xi) with $(Xi) < 0), were used.52 For a given set of values Y, 2,and &, every resonant wavevector ki is directly related to the thickness, T,and the refractive index of the sandwiched medium, n,. Conversely, the limits of resolution of multiple beam interference fringes (seeabove) make it possible to measure the thickness T to about 1A and this sets the effective limit to which 7' and the refractive index n, may be obtained from the experimental measurement of the positions of different ki.5' Thus the refractive indices may be measured to hO.01 for films 100 A thick, the accuracy increasing with the film thickness. In the presence of an adsorbed layer coating each mica surface, the FECO pattern can only be analyzed in terms of a five-layer system and can never be replaced by an effective three-layer interferometer. In particular the optical thickness of the sandwiched medium will be in error if one uses an artificial contact as defined when the two adsorbed layers are brought into molecular contact. Only for each of the special cases T = 0 and 2 = 0 does the five-layer system become a threelayer equivalent interferometer. 2.4. Determination of the Refractive Index of the Sandwiched Medium and Accuracy. As noted previously,'*51*52comparison of the situation where two bare mica surfaces are in contact (T = 0, 2 = 0) and where a film is sandwiched shows that the shifts in wavelengths, AA, of the odd and even fringes will in general be different, depending on the optical properties of the intermediate medium. As the measurement of the shift in position of one unique FECO only leads to the product hT,two FECO of different parity are required to obtain T and n, simultaneously. The refractive index n, is thus determined by matching two different determinations of the optical thickness. Thus, any precise measurement of n,will require a simultaneous accurate measurement of the position of two FECO. To achieve a high degree of accuracy (as quoted in section 2.3), careful attention must be paid to the method, as it is highly sensitive to any systematic error in the measured shift AX of one of these two fringes. Errors mainly arise either from an incorrect determination of the contact value or from the occurrence of a thermal drift of the surfaces during the measurements. These effects on the accuracy of the measurements are discussed below. (i) Effects due to the Use of Incorrect Contact Values. Errors in the contact determination originate from the finite precision
Langmuir, Vol. 10, No. 5,1994 1587
Refractive Index of Aqueous Films in the measurement of the wavelength of a FECO. An error in Aop, Aoq, the measured wavelengths of the pth and qth fringes when the bare mica are in molecular contact (i.e. T = 0,Z = 0) will affect subsequent measurements of the shifts AA,, A&, when the mica are separated by an intermediate medium and thus will affect the refractive index profile n,(7'). To illustrate this effect the simplest case of a three-layer interferometer (i.e. 2 = 0) has been simulated. Here the calculation is performed for fringes of order p = 18 and q = 17 which correspondsexplicitlyto the case of one of our experiments. For a thickness of each mica sheet Y = 15 772.2 A, the exact values of the wavelengths of the FECO fringes a t contact are calculated to be Aole = 5381.40 A and x017 = 5674.33 A. Taking a constant value (independent of the separation) for the refractive index of the water medium (n,= 1.3331,the exact pair A18, A179 is also calculated for every separation T ranging from contact to loo0 A. Then, one value (or both) of the contact wavelength is artificially shifted by fl A to simulate an experimental error. It results from this shift that a constant value of n,(i.e. n,= 1.333) can no longer be the solution of eq 1. Matching the predicted opticalthickness &?a's deduced either from thep fringe or from the q fringe makes the value n,deviate from the exact one, the effectbeing enhanced dramatically a t short separations. Typical deformations are shown in Figure 2a. In every case, the shift depends on distance and is dramatic at small separations. Eight possible situations can arise for "incorrect contacts" which lead to six typical deformations. These eight situations form two subgroups where the refractive index increases (situations a, b, and c) or decreases (situations a', b', and c') when the separation is reduced. The first group can be distinguished by the fact that the wavelength value of the even FECO at contact is underestimated. This rule is not general since the situation shown in c will lead to the opposite effect if the error made on the odd fringe is much larger than that made on the even fringe. The second group corresponds to the opposite situation. Finally, the two cases of a correct contact wavelength of the even fringe reduce to case b when the odd wavelength is overestimated (b-bis), and conversely to case b' when it is underestimated (b'-bis). Two remarks conclude this section. First, note that for a given uncertainty in the contact wavelength measurement (6AOpfl 7 fl A), the larger the error in the measured refractive index for higher fringe order, due to a stronger dependence of the shift of the fringe (Ax) with thickness. Secondly, this effect is independent of any given pair (A",, A",) when expressed in terms of the errors (axo,, axo,) made in the measurement of the even and odd fringes: the effect depends on the magnitude of the measured error and also on the relative error axo, - 6Aoq made in the measurement of the even order fringe compared to that made in the odd one. The decrease in the measuring accuracy a t small surface separations could explain anomalous high refractive indices of thin films or liquid condensates reported in the literat~re.~'.~ The magnitude of the effect is very important and far above the precision required in this work (typically a change of refractive index of around 0.02; see discussion). Thus, an error of only 0.5 A in the measurement of either the odd or even order fringe or in the wavelength a t contact will prevent accumulation of reliable measurements. As there is no way to improve the accuracy of direct measurement of the contact with the FECO analysis, a methodology has been developed in order to extract the exact experimental contact for each experiment (seethe result section). (ii) Effects Due to a Drift of the Surfaces. The direct measurement of the positions of two consecutive FECO cannot be performed a t the same time when visualizingthe fringes with the eye, but rather a duration of 10-20 s is required. With a typical thermal drift of the surfaces being 5-20 A/min (in the climatized room where the experiments are conducted) the data are collected for separations differing by 2-4 A. To illustrate how a thermal drift of the surfaces can effect the results, the same three-layer interferometer as above has been simulated. Only two situations can arise depending on the parity of the fringe to be measured first and on the direction of the drift: either the even order fringe is measured at a shorter (54) Christenson, H.K.J. Colloid Interface Sei. 1985,104,234-249.
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