J. Phys. Chem. B 2001, 105, 1185-1190
1185
Effect of Fructose, Sucrose, and Dimethyl Sulfoxide on the Equilibrium Thickness of DMPC Foam Films N. Krasteva,* R. Krustev, H. J. Mu1 ller, D. Vollhardt, and H. Mo1 hwald Max Planck Institute of Colloids and Interfaces, Am Mu¨ hlenberg 1, 14476 Golm, Germany ReceiVed: July 23, 2000; In Final Form: NoVember 22, 2000
Foam films stabilized by phospholipids are applied for model studies of interactions between lipid layers. The thickness of the free-standing foam films, obtained from aqueous lipid dispersions of DMPC in the presence of low molecular weight sugars fructose and sucrose, and dimethyl sulfoxide (DMSO), was measured by means of the microinterferometric technique. The equilibrium film thickness is determined by the interplay of the different components of the disjoining pressure and it is influenced by the composition of the film forming dispersion. In the absence of solutes and electrolyte DMPC forms Newton black films. The addition of sugars and DMSO results in an increase in the film thickness, which is attributed to an increase in the surface potential. In this case, the electrostatic disjoining pressure determines the film thickness, while the van der Waals attraction is not significantly influenced by the solutes. The effective Hamaker constants are obtained from the velocity of film thinning. The double layer potential φo of the thick films is calculated according to the postulations of the DLVO theory. Stable black films are obtained at high salt concentration in the presence of sugars and DMSO. In these thin films, the short-range structural forces control the equilibrium thickness, which increases in the presence of sugars and decreases in the presence of DMSO. The changes in the film thickness caused by the solutes are attributed to modified hydration interactions. The observed thickness changes correspond to inclusion or removal of several layers of water molecules in the film aqueous core.
Introduction Phospholipid bilayers are the major structural elements of biological membranes. In this respect, studying the interaction of lipids with water and with water-soluble solutes delivers information about the processes that occur at the membrane/ water interface. Addition of certain solutes, such as small carbohydrates and dimethyl sulfoxide (DMSO) influence membrane stability. Soluble sugars stabilize liposomes,1 intact membranes,2 and whole cells3 against irreversible desiccation and fusion caused by freezing or freeze-drying. The incorporation of sugars into the hydrophilic region of the membrane prevents tight packing of the aliphatic chains.4 The influence of the DMSO on membrane structure and integrity appear to be controversial. In some cases, DMSO acts as a membrane cryoprotector, while in others it destabilizes the membranes and promotes fusion and leakage.5,6 We showed that sugars and DMSO affect the strength of the lateral interactions between the lipid molecules in a monolayer spread on the air/aqueous solution interface.7 Foam films stabilized by phospholipids are applied for the study of interactions between lipid layers in the present study. These films represent a suitable model for studying the DLVO and nonDLVO forces operating between two fluid surfaces.8-11 Foam films consist of two adsorbed monolayers of amphiphilic molecules separated by a solvent layer. There are three equilibrium types of films - thick common films (colored), thinner common black films (CBF) and the thinnest Newton black films (NBF). * Corresponding author. Tel: 0049 331 567 9461. Fax: 0049 331 567 9222. E-mail:
[email protected]. Mailing address: Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany.
Film thickness and stability are determined by the balance of the attractive van der Waals component Πvw and the repulsive electrostatic term Πel and/or hydration and steric repulsion terms of the disjoining pressure.8 The thick films and CBF are stabilized by the long-range Πel. Their thickness depends on the electrolyte content in the film forming solution. NBF have a bilayer structure where the two monolayers of amphiphilic molecules are separated by only few layers of trapped water molecules. These films are stabilized by the short-range hydration and steric repulsion.12-14 In a series of papers we investigated the influence of the electrolyte, ethanol and the temperature on the thickness and the free energy of film formation of foam films made from phospholipids.14-16 The aim of the present work is to study the influence of the biologically important solutes like sugars and DMSO on the short and the long-range interactions between the phospholipid monolayers comprising the thin liquid film. The thickness of the films is obtained as a function of the solute concentration or electrolyte concentration. Conditions for formation of the thinnest NBF are found. The role of the van der Waals attraction and electrostatic repulsion between phosphatidylcholine lipid layers are revealed. Experimental Section The zwitterionic phospholipid 1,2-dimyristoyl-sn-glycerophosphocholine (DMPC) (SIGMA, purity > 99.9%) was used without further purification. Dimethyl sulfoxide (DMSO) and sugars fructose and sucrose were obtained from Sigma and used as supplied. Sodium chloride (NaCl) (Riedel-de Haaen) was roasted at 600 °C for 5 h to remove surface-active contaminations. Ultrapure Milli-Q-filtered water (Millipore Co.) with a
10.1021/jp002611j CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001
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specific resistance 18.2 MΩ. cm and pH ) 5.5 was used in preparing the solutions. The foam films were formed from suspensions of lipid vesicles. The degradation of the vesicles delivers the material for densely packed monolayers of DMPC molecules at the air/ aqueous solution interface that ensures high stability of the foam films.15,17-20 The suspensions were prepared by adding water/ sugar or water/DMSO solutions with different concentrations to the desired amount of DMPC. The concentration of DMPC in the suspensions was constant 0.4 mg/mL in all experiments. The lipid dispersions were kept overnight at room temperature. They were sonicated in a sonicating bath (Sonorex RK52, BANDELIN Electronic, Berlin, Germany) for 60 min at 45 °C before the measurements. Experiments were performed using a Scheludko ring cell.10,21 The cell was situated in a closed vessel on an inverted reflected light microscope. The volume of the vessel was saturated with the vapors of the film forming solution. The cell was thermostated at 26 °C and the temperature was kept constant with an accuracy of (0.1 °C. This temperature is above the temperature of the main phase transition of DMPC and allows formation of stable and homogeneous foam films.15,22,23 The thickness of the films was measured with the microinterferometric method as described elsewhere.10,21 A small central part of the film is illuminated with monochromatic light. The equivalent solution thickness hw was calculated from the intensity of the reflected light, assuming a homogeneous optical film structure.24 The three-layer model was used to calculate the actual film thickness h.25 According to it, the film consists of two adsorbed monolayers (thickness h1 and refractive index n1) and an aqueous core (thickness h2 and refractive index n) that includes the headgroups of the lipid molecules. The foam film thickness h and the water-core thickness h2 were calculated from the experimental values of hw using the relation26
h ) 2h1 + h2 h2 ) hw - 2h1
n12 - 1
(1)
n2 - 1
The value of h1 ) 1.29 nm27 and n1 ) 1.4290, the refractive index of tetradecane,28 was used for the calculations. Each value of h, presented hereafter, is an average of at least 10 single measurements and the sample standard deviations are shown as error bars in the figures. The film begins to thin under the action of the meniscus capillary pressure PC and the film disjoining pressure Π(h) until it reaches its equilibrium thickness or ruptures. The dynamic method29 connects experimentally measured velocity of film thinning with the disjoining pressure operative in the film. For surfactant stabilized liquid films, the relation between the film thinning velocity V and the thickness h is given by30
(
) (
)(
)
hs hs Π(h) V )β 1+b+ +β 1+b+ 3 h h PC h
(2)
where b and hs account for bulk and surface diffusion, respectively, and β is a factor comprising all thickness independent quantities. The capillary pressure PC ) 2σ/R where R ) 2 mm is the radius of the film holding capillary. The surface tension σ of the DMPC suspension is shown to be 27 mN/m.15 Our measurements showed that σ does not vary on addition of sugars or DMSO. In this case, the capillary pressure PC is equal to 27 N/m2.
Figure 1. Illustration of the application of the dynamic method for film thinning studies. The DMPC stabilized film is formed from a suspension that contains 0.07 M NaCl and 0.5 M sucrose. The points represent the experimental V/h3 vs 1/h dependence; the solid line is only a guide for the eye. The dashed line is the best fit to the experimental data for thick films, where the disjoining pressure is zero.
The procedure of calculating the disjoining pressure from the experimental thinning curve is to plot the dependence of V/h3 vs 1/h. Such dependence for a film prepared from DMPC suspension in the presence of 0.5 M sucrose is presented on Figure 1 as an example. The points represent the experimental values and the solid line is only a guide for the eye. Equation 2 predicts a linear dependence of V/h3 on 1/h in thick films where both film surfaces do not interact (Π ) 0). The linear fit to the experimental data for thick films is shown as a dashed line on Figure 1. The quantities βb and βhs are determined from the slope and the intercept of this fit. At smaller film thickness, when Π * 0, a deviation from linearity arises. The value of the disjoining pressure Π at given film thickness h is calculated from the difference between the experimental data and the linear dependence. Thus, a Π(h) isotherm is constructed. If the electrostatic double layer repulsion is suppressed the experimental Π(h) isotherms can be fitted with the equation
Πvw ) -
AH 6πh3
(3)
where AH is the Hamaker constant of the system. This procedure allows AH to be estimated experimentally. Results and Discussion Effect of Solutes at Low Ionic Strength. The thicknesses of films prepared from suspensions of DMPC in DMSO or sugar aqueous solutions with different concentrations were measured in the absence of electrolyte. The concentration of sugars was varied up to 0.5 M. Films formed from lipid suspensions with DMSO concentration in the range up to 2 M were studied. The dependence of the film thickness on the sugar concentration in the film forming dispersion is presented on Figure 2a. The effect of both fructose and sucrose on the film thickness is similar. At low sugar concentrations Newton black films about 7 nm thick are formed. The film thickness increases steadily
Thickness of DMPC Foam Films
J. Phys. Chem. B, Vol. 105, No. 6, 2001 1187
Figure 3. Equilibrium foam film thickness vs NaCl concentration: (a) in the presence of 0.5 M fructose, and (b) in the presence of 2 M DMSO in the film forming lipid dispersion.
Figure 2. Equilibrium foam film thickness vs concentration of the solute in the DMPC dispersion: (a) in the presence of sugars (9) fructose, (b) sucrose; (b) in the presence of DMSO. The lipid dispersions contain no electrolyte. Note the breaks in the thickness axes.
with increasing sugar concentration up to about 0.35 M. After this point the film thickness increases jump wise and thick films (h g 50 nm) are observed. Above 0.5 M sugar films become very thick and their thickness cannot be measured with our equipment. The dependence of film thickness on the DMSO content is presented on Figure 2b. At concentrations of DMSO lower than 1 M Newton black films about 6-7 nm thick are formed. The film thickness varies nonmonotonically with the DMSO concentration in this range. At about 1.5 M DMSO the film thickness increases with a jump. On further increase in DMSO concentration above 2 M the stability of the film decreases and the film ruptures during the thinning. For both solutes two regions in thickness vs solute concentration dependencies are distinguished: one at lower amount of
the additive where NBF are formed and the film thickness changes slightly, and another at higher solute concentrations where thick films are formed. Observed small changes in the film thickness at low solute concentrations are probably caused by modified steric-hydration repulsion. Observed strong increase in the film thickness at high solute contents could be created by an increased electrostatic repulsive contribution to the disjoining pressure. Effect of Electrolyte Concentration at High Solute Content. When the formation of stable thick films is caused by increased long-range electrostatic double layer repulsion between the film surfaces, one should expect a dependence of the film thickness on the electrolyte concentration in the suspension. Such dependencies for DMPC films in the presence of 0.5 M sucrose or 2 M DMSO on the NaCl concentration are shown in Figure 3. These are the concentrations of the solutes at which the thickest films without electrolyte were obtained (Figure 2). The film thickness gradually decreases with increasing electrolyte concentration. Formation of CBF is observed in both cases at electrolyte concentration of around 0.02 M. In the presence of sugar CBF exist within a wide range of salt
1188 J. Phys. Chem. B, Vol. 105, No. 6, 2001
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TABLE 1: Effective Hamaker Constants AH for the Foam Films Prepared from DMPC/Water and DMPC/Solute/ Water Dispersions in Presence of 0.07 M NaCl AH, J 3.1 × 10-20 1.4 × 10-20 0.7 × 10-20 1.5 × 10-20
DMPC DMPC+0.5 M fructose DMPC+0.5 M sucrose DMPC+2 M DMSO
DMPC DMPC+0.5 M fructose DMPC + 0.5 M sucrose DMPC+2 M DMSO
concentration up to 0.05 M (Figure 3a). Their thickness slightly decreases with increasing NaCl concentration. Abrupt transition to NBF with constant thickness of about 7 nm occurs when the NaCl concentration reaches 0.05 M. In the case of DMSO, formation of CBF is observed only in a very narrow salt concentration region (Figure 3b). Instead, NBF with constant thickness of about 6 nm are formed already at 0.02 M NaCl. The dependence of the film thickness on the electrolyte concentration depicted above confirms the electrostatic origin of the forces that stabilize these films. The behavior of such films is successfully described by the DLVO theory, when only the long-range attractive and repulsive forces acting in the film are taken into consideration.31 In this case, the short-range interaction terms can be omitted. In the present study, the Πvw(h) isotherms are determined using the dynamic method29 studying the process of film thinning at high electrolyte concentrations where the electrostatic repulsion is suppressed. The film drainage was investigated at 0.07 M NaCl, which ensures negligible double layer repulsion. The experiments were carried out at a sugar concentration of 0.5 M or DMSO concentration of 2.0 M. The addition of solutes in the lipid suspension leads to a decrease in the film thinning velocity. The effect of the DMSO is less pronounced than that of the sugars. The observed decrease in the film thinning velocity cannot be attributed to the increased viscosity of the film forming suspension, since the bulk viscosities of the sugar and DMSO solutions increase by less than 5% in the investigated concentration range.32,33 The obtained values of the Hamaker constants are presented in Table 1. Actually, these are effective values of the constant, as other attractive interactions, not included in our treatment, might exist in the film.12,34 The values of the constant slightly decrease with addition of the investigated solutes. According to the Lifshitz theory of van der Waals forces, the Hamaker constant depends on the dielectric constants and polarizabilities of the interacting media.8 Addition of sugars or DMSO only slightly changes these properties. That is why at the conditions of our experiment the Hamaker constant should not vary appreciably with the addition of solutes. The obtained AH values are in good agreement with the published data for similar systems.8 At equilibrium, the disjoining pressure in the film is equal to the capillary pressure PC in the film surrounding meniscus. Thus, the electrostatic double layer repulsion component Πel of the disjoining pressure can be obtained knowing the attractive term Πvw and the capillary pressure. The term Πel in the case of constant surface potentials φ0 in the limit of the weak overlap approximation is given by8,31
( )
Πel ) 64RTCelth2
zFφ0 exp(-κh) 4RT
with
κ)
x
8πz2F2Cel RT
TABLE 2: Film Thickness h and the Double Layer Potentials φo for the Foam Films Prepared from DMPC/ Water and DMPC/Solute/Water Dispersions in the Presence of 0.005 M NaCl
(4)
h, nm
φ0, mV
7 39 42 37
21 58 62 59
where Cel is the concentration of the electrolyte in the film forming suspension; z, the valence of the electrolyte; F, the Faraday number; R, the gas constant; T, the absolute temperature; and , the dielectric permittivity of the medium. In our range of solute concentrations does not deviate appreciably from the dielectric permittivity of pure water. The potential φ0 was determined through eq 4 from the experimental data of the film thickness vs electrolyte concentration, presented in Figure 3. The thicknesses of the films measured at the lowest available electrolyte concentration, Cel ) 5 × 10-3 M, were taken. The obtained φ0 values are listed in Table 2. The addition of solutes increases φo from 21 mV for DMPC/water suspensions to about 60 mV in the films obtained from lipid suspensions in the presence of sugars or DMSO. We should mention that the obtained φo values might be overestimated since not all possible repulsive interactions that may exist in the film are considered in the calculation.35,36 These values are similar to those obtained for films stabilized with nonionic surfactants containing polar sugar groups.37 The appearance of high surface potential in a noncharged lipid film is rather unexpected. Studies of the elechtophoretic mobility of lipid vesicles have shown the existence of a surface potential in DMPC multilamellar liposomes as well.38 The DLVO theory gives the possibility for determining only the absolute φo value, but does not explain the origin of this potential. One possible explanation could be the adsorption of hydroxyl ions at the film interfaces,10,37 which will result in a negative φo value. However, the appearance of stronger repulsion between the film interfaces in the presence of sugars and DMSO, which we correlate with increased φo potentials, remains unclear. In our treatment only the pure van der Waals attraction and the double layer repulsion forces are considered. We are aware that this is a rather simplified model of the interaction in thin film system. It is difficult to explain how the addition of solutes will increase the double layer potential φo. It might be possible that φo has the same magnitude at both small and high concentrations of the solute. However, an additional unidentified attractive component of the disjoining pressure, which is canceled at high solute concentrations, might exist in the thin films. The film forming suspensions, used in our study, consist of large lipid vesicles. Our thickness measurements show that these vesicles are excluded from the aqueous cores of the thin films. This exclusion may create a kind of two-dimensional osmotic suction, thereby establishing an additional attractive interaction in the film.39 At high concentrations of the solutes this osmotic suction is compensated by an enrichment of the solute in the film. It should be mentioned, however, that the exclusion of the vesicles, which are dispersed in the bulk of the solution, could not create such depletion force because of the much too small concentration of the vesicles. Effect of Solutes at High Ionic Strength. At high electrolyte concentration, formation of NBF is observed (the lowest plateau regions on Figure 3). The thickness of these NBF depends on the kind of the solute in the film forming dispersion, about 7 nm thick films are formed in the presence of sugar, while the
Thickness of DMPC Foam Films
J. Phys. Chem. B, Vol. 105, No. 6, 2001 1189 DMSO. Taking into account the diameter of the water molecule of about 0.2 nm, the observed changes of h2 correspond to addition or removal of several water layers caused by the presence of fructose or DMSO, respectively. Hence, the observed changes in the DMPC film thickness in the presence of solutes in the film forming suspension can be attributed to altered hydration repulsion between the film surfaces. We showed in a previous paper7 that sugars and DMSO influence the phase behavior and the condensed phase structure of insoluble lipid DPPC monolayers. Both DPPC and DMPC possess the same choline headgroups and differ only in the alkyl chain length. Since the polar lipid heads are mainly responsible for the interactions between the solutes and the phosphocholines, the results of the DPPC monolayer study can be used to explain the observed variation of the thickness of the black films. The sugar molecules penetrate into the polar headgroup region. The binding of the strongly hydrated sugars increases the hydration of the lipid. This results in stronger repulsion between the PC headgroups. The same phenomenon can be responsible for the observed increase in the film thickness on addition of sugar. In contrast to this, addition of DMSO into the subphase causes condensation of the DPPC monolayer and reduces apparently the amount of the water molecules in the vicinity of the polar lipid headgroups. Moreover, the hydrated lipid molecules and the DMSO molecules compete for the same limited amount of water at the interface. Thus, certain hydration water is extracted from the region around the lipid headgroups. The deficiency of hydration water decreases the lipid headgroup repulsion and diminishes the effective volume of the PC moiety. The DMSO induced dehydration can cause the observed decrease in the lipid film thickness. The effect of DMSO is similar to the dehydration effect of ethanol on phosphocholine monolayers and foam films.14 Conclusions
Figure 4. Film thickness h and aqueous core thickness h2 of Newton black films of DMPC at 0.05 M NaCl as a function of the (a) fructose and (b) DMSO concentration in the film forming dispersion.
thickness of NBF in the presence of DMSO is about 6 nm. The film thickness varies with the concentration of the solute as well, Figure 4. Addition of sugar leads to increase in the thickness (Figure 4a). In contrary, increasing DMSO concentration decreases the film thickness (Figure 4b). In NBF the equilibrium, thickness depends mainly on the short-range steric and hydration repulsion between the lipid monolayers adsorbed on the both film surfaces.8 If the thicknesses of both monolayers at the film surfaces are not influenced by the presence of solutes, the thickness h2 of the film aqueous core is calculated according to eq 1. The obtained dependencies of h2 on the solute concentration in the film forming dispersion are shown also in Figure 4. One can see that h2 increases from about 4 nm for the DMPC/water system to about 11 nm in the presence of 1 M fructose, while the addition of DMSO reduces the h2 value to about 3 nm in the presence of more than 2 M
The thickness of foam films obtained from DMPC lipid dispersions with added sugars or DMSO is determined by the interplay of dispersion attraction, electrical double-layer repulsion, and short-range molecular interactions. These interactions are influenced by the composition of the film forming solution: electrolyte or solute (sugar, DMSO) concentrations. The comparison of results allowed an estimation of the values of the different components of the interactions in the film to be drawn. The dependence of the equilibrium foam thickness on the content of the solute in the film forming solution demonstrates the influence of the solutes on the interaction between the DMPC film monolayers. High concentration of solutes leads to a great increase in the film thickness. It is probably caused by modification of the electrostatic double-layer repulsion between film interfaces. The obtained values of the effective Hamaker constant at different concentration of added solute do not show strong influence of the solutes on the van der Waals attraction. At low electrolyte and high solute concentration, the electrostatic disjoining pressure governs the film thickness. At high electrolyte concentration, formation of NBF with thickness of several nanometers is observed. In these very thin films, the equilibrium thickness depends largely on the shortrange interactions between the lipid monolayers adsorbed on both film surfaces. The thickness of these NBF depends on the kind of solute and its concentration in the film forming dispersion. The presence of solutes causes a change in the film thickness that corresponds to addition or removal of several layers of water molecules. Addition of greater amount of sugar
1190 J. Phys. Chem. B, Vol. 105, No. 6, 2001 leads to increase in the thickness while the presence of DMSO decreases the film thickness. Acknowledgment. The authors acknowledge the financial assistance of the Deutsche Forschungsgemeinschaft through the projects Sfb 312 and Mu 1040/9-1, and the support from the Fonds der Chemischen Industrie. References and Notes (1) Crowe, L. M.; Crowe, J. H.; Rudolph, A.; Womersley, C.; Appel, L. Arch. Biochem. Biophys. 1985, 242, 240. (2) Crowem, J. H.; Crowe, L. M.; Jackson, S. A. Arch. Biochem. Biophys. 1983, 220, 477. (3) Hoekstra, F. A.; Crowe, J. H.; Crowe, L. M.; van Roekel, T.; Vermeer, E. Plant Cell EnViron. 1992, 15, 601. (4) Crowe, J. H.; Hoekstra, F. A.; Crowe, L. M. Annu. ReV. Physiol. 1992, 54, 570. (5) Ashwood-Smith, M. J. Ann. N. Y. Acad. Sci. 1967, 45. (6) Yu, Z. W.; Quinn, P. C. Biosci. Rep. 1994, 14, 259. (7) Krasteva, N.; Vollhardt, D.; Brezesinski, G.; Mo¨hwald, H. Langmuir 2000. In press. (8) Israelachvili, J Intermolecular and Surface Forces; Academic Press: London, New York, 1992. (9) Ivanov, I., Ed. Thin Liquid Films. Surfactant Science Series; Marcel Dekker, Inc.: New York, 1988. (10) Exerowa, D.; Kruglyakov, P. Foam and Foam Films. Studies in Interface Science; Miller, P., Ed.; Elsevier: Amsterdam, 1998. (11) Prud’homme, R.; Kahn, S. Foams; Marcel Dekker: New York, 1996. (12) Bergeron, V. J. Phys.: Condens. Matter 1999, 11, 215. (13) Sedev, R.; Exerowa, D. AdV. Coll. Int. Sci. 1999, 83, 111. (14) Toca-Herrera, J. L.; Mu¨ller, H. J.; Krustev, R.; Pfohl, T.; Mo¨hwald, H. Colloids Surf. A 1999, 152, 357. (15) Toca-Herrera, J. L.; Mu¨ller, H. J.; Krustev, R.; Exerowa, D.; Mo¨hwald, H. Colloids Surf. A 1998, 144, 319. (16) Toca-Herrera, J. L.; Krustev, R.; Mu¨ller, H. J.; Mo¨hwald, H. Coll. Polym. Sci. 2000, 278, 771.
Krasteva et al. (17) Vassilief, C.; Panaiotov, I.; Manev, E.; Proust, J.; Ivanova, Tz. Biophys. Chem.. 1996, 58, 97. (18) Nikolova, A.; Exerowa, D. J. Stat. Phys. 1995, 78, 147. (19) Lawrie, G.; Barnes, G.; Gentle, I. Colloids Surf. A 1999, 155, 69. (20) Lawrie, G.; Gunton, K.; Barnes, G.; Gentle, I. Colloids Surf. A 2000, 168, 13. (21) Scheludko, A. AdV. Coll. Int. Sci 1967, 1, 391. (22) Nikolova, A.; Exerowa, D.; Lalchev, Z.; Tsonev, L. Eur. Biophys. J. 1994, 23, 145. (23) Toca-Herrera, J. L. Wechselwirkungskra¨ fte und Struktur in Phospholipid-Schaumfilmen. Dissertation UniVersity Potsdam; GCA-Verlag: Herdecke, Germany, 2000. (24) Vasicek, A. Optics of Thin Films; North-Holland: Amsterdam, 1960. (25) Den Engelsen, D.; Frens, G. J. Chem. Soc., Faraday Trans. 1 1974, 70, 237. (26) Frankel, P.; Mysels, K. J. Appl. Phys. 1966, 37, 3725. (27) Cuvillier, N.; Millet, F.; Petkova, V.; Nedyalkov, M.; Benattar, J. J. Langmuir 2000, 16, 5029. (28) Handbook of Chemistry and Physics; CRC Press: Boca Raton, 2000. (29) Scheludko, A. Proc. K. Ned. Akad. W. 1962, B65, 76. (30) Ivanov, I. Pure Appl. Chem. 1980, 52, 1241. (31) Derjaguin, B. V. Theory of Colloids and Thin Films; Consultants Bureau: New York, 1989. (32) Vazquez Una, G.; Chenlo Romero, F.; Alvarez Dacosta, E.; Moreira Martinez, R.; Padro Calvo, P. J. Chem. Eng. Data 1994, 39, 87. (33) Areas, E. P. G.; Areas, J. A. G.; Hamburger, J.; Peticolas, W. L.; Santos P. S. J. Coll. Int. Sci. 1996, 180, 578. (34) Bibette, J. J. Colloid Interface Sci. 1991, 147, 474. (35) Honig, E. P.; Mul, P. M. J. Colloid Interface Sci. 1971, 38, 532. (36) Gregory, J. J. Chem. Soc., Faraday Trans. 2 1973, 69, 1723. (37) Bergeron, V.; Waltermo, A.; Claesson, P. M. Langmuir 1996, 12, 1336. (38) Alonso, J. M.; Llacer, C.; Vila, A. O.; Figueruelo, J. E.; Molina, F. J. Colloids Surf. A 1995, 95, 11. (39) Schindler, H. Biochim. Biophys. Acta 1979, 555, 316.
