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Effect of the Electrostatic and Structural Surface Forces on the Contact Angles in Langmuir-Blodgett Systems of Cationic Surfactants. 1. Homogeneity and Electrostatic Properties of Mixed Monolayers of Methyl Arachidate and Dimethyldioctadecylammonium Bromide Jordan G . Petrov,*tt Dietmar Mobius,$ and Angelina Angelovat Bulgarian Academy of Sciences, Central Laboratory of Mineral Processing, P.O.Box 32, 1126 Sofia, Bulgaria, and Max-Planck-Institut f u r biophysikalische Chemie, Postfach 2841, D - 3400 Gottingen, FR G Received January 16, 1991.I n Final Form: August 22, 1991 For characterization of the gaslliquid and solidlliquid interfaces during Langmuir-Blodgett deposition of mixed monolayers of methyl arachidate and dimethyldioctadecylammoniumbromide, the miscibility of the components and thus the homogeneity of the interfaces have been studied. The negative change of the Gibbs free energy of mixing and the dependence of the collapse surface pressure on the monolayer composition lead to the conclusion that methyl arachidate and dimethyldioctadecylammoniumbromide form homogeneous mixed monolayers. Measurements of the AV potentials of the spread monolayer mixtures as a function of the molar ratio of the charged and uncharged components were performed. From these data and from the results of the previous experimental determination of the Gouy-Chapman potentials J.0 (by means of interfacial pH probes), the charge and dipole contribution to the electrostatic field at the interfaces were obtained. This discrimination allows better understanding of the relationship between the contact angles in the studied Langmuir-Blodgett systems and the monolayer composition that is a matter of investigation of the second part of this publication.
Introduction Present theories relate the contact angle hysteresis either to inhomogeneity and roughness of the interface~l-~ or to the interactions in the three-phase contact 20ne.~JIn spite of the fact that the first two effects can be excluded only in a few model systems, there are many investigations especially devoted to the third one. In all these cases the correctness of the conclusions strongly depends on the reliability of the interfacial characterization. Frumkins and latter Smolders? Morcos,10 Nakamura et al.,ll and Hato12made use of the polarizability and the smoothness of the interface mercurylaqueous solution to relate the equilibrium contact angles to the applied potential differences. The application of additional electrochemical techniques, such as differential capacity measurements, provided information on the electrodesolvent interaction and on the orientation of the solvent molecules near the solid surface. Ottewill et al.13 related the advancing and receding contact angles of a bubble
* To whom correspondence should be addressed. + Bulgarian Academy of Sciences. Max-Planck-Institut fur biophysikalische Chemie. (1)Johnson, R. E.; Dettre, R. H. Surface and Colloid Science; Matijevic, E., Eirich, E., Eds.; Wiley-Interscience: New York, 1969,Vol. 2, p 85. (2)Neumann,A. W.; Good, R. J. J. Colloid Interface Sci. 1972,38,341. (3)Eick, J. D.;Good,R.;Neuman,A. W.Proceedings of3rdSymposium on Dental Adhesiue Materials; Moskowitz, H. D., Ward, E. D., Eds.; Woolridge: New York, 1974;p 18. (4)Schwartz, L. W.; Garoff, S. J.Colloid Interface Sci. 1985,106,422. (5)De Gennes, P. G. Reu. Mod. Phys. 1985,57,827. (6)Scheludko, A. Ann. Uniu. Sofia, Fac. Chem. 1968,63,43. (7)Martynov, G.A.; Starov,V. M.; Churaev, N. V. Colloid J. USSR (Engl. Transl.) 1977,39,406. (8)Frumkin, A. N.; Gorodetzkaya,A. Acta Physicochim. USSR 1938, 9,327. (9)Smolders, C. A. Recl. Trau. Chim.Pays-Bas 1961,80,699. (10)Morcos, I. J. Colloid Interface Sei. 1971,37,410. (11)Nakamura, Y.; Kamada, K.; Katoh, Y.; Watanabe, A. J. Colloid Interface Sei. 1973,44,517. (12)Hato, M. J. Colloid Interface Sci. 1989,130,130.
*
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pressed against a silver iodide surface to the electrostatic potential $0 of the liquidlsolid interface utilizing the reversibility of the AgI/Ag+ electrode and the dependence of $0 on the Ag+ concentration. In a series of papers Whitesides and co-workers14applied contact angle measurements in studying organic polymer surfaces and self-assembled monolayers of alkanethiols and their w derivatives, chemisorbed on gold. Powerful spectroscopic and optical methods, such as electron spectroscopy for chemical analysis, attenuated total reflectance infrared spectroscopy, scanning electron microscopy, and ellipsometric determination of the monolayer thickness, were used for surface characterization, and thus direct relationships between the contact angles (their hysteresis) and the molecular composition of the solidlliquid interface were obtained. In this study mixed monolayers of methyl arachidate (with a neutral hydrophylic group) and dimethyldioctadecylammonium bromide (with a positively charged head group) have been transferred by means of the Langmuir Blodgett technique from aqueous subsolutions of NaCl onto hydrophobic solid substrates. By variation of the molar ratio of the charged and the uncharged components, the interfacial charge density and electrostatic potential were monitored in a well-defined manner. The homogeneity and the electrostatic properties of the spread monolayers as well as the deposition ratios, characterizing the transferred monolayer and the solidlliquid interface, are studied in part 1 of this paper. In part 2, the static advancing and receding contact angles were determined as a function of monolayer and subsolution composition. The relationships obtained were interpreted on the basis of the characterization of the solidlliquid and the gas/ liquid interfaces performed in part 1. (13)Ottewill, R. H.; Billet, D. F.; Gonzalez, G.; Hough, D. B.; Lovell, V. M. Wetting, Spreading and Adhesion; Paddey, J. F., Ed., Academic Press: London, 1978;p 183. (14)Whitesides, G.M.; Laibinis, P. E. Langmuir 1990,6, 87.
0 1992 American Chemical Society
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F [~'/molecule] Figure 1. Surface pressure-area isotherms for pure methyl arachidate (MA), dimethyldioctadecylammonium bromide (DOMA),and their mixtures DOMA/MA on 0.001 M NaCl subsolution: 1,MA; 2, 1/10; 3,3/10; 4, 1/2; 5, l / l ; 6,7/3; 7, DOMA. 1
Methods a n d Materials
An automatic Langmuir-Adam balance with a Teflon trough was used for surface pressure-area measurements. The dynamometric system was a transducer measuring the force exerted on a vertical strip of filter paper (2 cm wide),completely wettable by the monolayer-covered aqueous subsolution. Its sensitivity corresponded to 0.2 dyn/cm. The isotherms were recorded at two compression speeds of 7 and 15cmZ/min giving equal results. The stability of the mixed monolayers, characterized by the change of the monolayer area A at constant surface pressure of 30 dyn/cm, was also followed. The relative change, AAIAo, for the first 2 min after the initial compression was taken as a quantitative parameter. A V potentials have been measured parallel to the surface pressure-area isotherms by means of the vibrating plate method. The circular plate with diameter of 1.5cm was positioned 0.1 cm above the liquid surface in order to minimize the condenser field losses. A variation of this distance in the limits 0.05-0.15 cm did not change the compensation voltage. The deposition ratios during the first dipping and withdrawal, giving the change of the monolayer density during the transfer from the liquid onto the solid substrate, have been determined at 30 dyn/cm. These quantities can be obtained if the decrease of the area of the spread monolayer, AAL,accompanying the dipping and the withdrawal of the solid substrate through the gas/liquid interface, has been recorded and this change has been divided by the geometrical area of the solid, A,, passing through the interface. Both AAL and A, have been measured with an accuracy of 1%. Polystyrene-coated glass tubes were used as solid substrates. The samples were cleaned in hot chromic acid, dried at 110 OC, and dipped and withdrawn from a solution of polystyrene in xylene. After the evaporation of the solvent under an infrared lamp the hydrophobized substrates were stored in a desiccator. Methyl arachidate for chromatographic purposes (Merck)and dimethyldioctadecylammonium bromide, produced by Kodak, were used as 1 X M chloroform solutions. Mixtures of particular molar ratios were prepared from these stock solutions before spreading. Water from a Milli-Q system and sodium chloride of supra pure quality (Merck) were used for preparation of the subsolutions. Results Homogeneity of t h e Spread Monolayer Mixtures. Figure 1 illustrates the surface pressure-area isotherms for pure methyl arachidate, dimethyldioctadecylammonium bromide, and for several mixed monolayers on 0.001 M NaCl subsolution. They and the isotherms recorded on 0.1 M NaCl subsolutions show that a t 30 dynlcm (the constant surface pressure a t which the Langmuir-Blodgett
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Figure 2. Mean area per molecule in the spread monolayer F m at 30 dyn/cm versus molar part of the charged component XD for two subsolutions: A, OOO.1 M NaC1, 0,O.l M NaC1.
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Figure 3. Change of the Gibbs free energy by mixing of MA and DOMA versus the molar part of DOMA for two subsolution: A, 0.001 M NaC1; 0,O.l M NaC1. deposition was performed) all monolayers were in solid condensed state. Figure 2 shows the negative deviations from the ideality of the mixtures a t 30 dynlcm, presented in the scale mean area per molecule, F ~ oversus , molar part of the charged component, XD. Such a behavior, characterizing stronger interaction between the charged and uncharged molecules in the mixtures compared to the ion-ion or dipole-dipole interactions in the monocomponent monolayers, favors the miscibility. A more definite conclusion about the miscibility of the monolayer components can be drawn from the negative values of the change of the Gibbs free energy by mixing-Figure 3. These data have been obtained through graphical integration of the surface pressure-area isotherms according to the equation of Goodrich15
ACmi," = f F dII - XMJonFM dII - XDfFD dII + kT
+
(X, In X, XD In XD) (1) Here FM,FD,and F are the areas per molecule in the (15)Goodrich, F. C . Proc. Int. Cong. Surf. Activity, 2nd 1957, 85.
Effect of Surface Forces on Contact Angles
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Figure 5. Relative decrease AA/Ao of the monolayer area at 30 dyn/cm versus molar part XD of DOMA.
Figure 4. Dependence of the collapse surface pressure IIc on the molar part XDof DOMA A, 0.001 M NaC1; 0 , O . l M NaC1.
pure methyl arachidate, dimethyldioctadecylammonium bromide, and the mixed monolayers, respectively, and XM and XD are the corresponding molar parts of the components in the mixtures. Figure 4 presents the dependence of the collapse surface pressure II, on the molar part of the dimethyldioctadecylammonium bromide, XD.As was shown by CrisplGwho applied the Gibbs phase rule to monolayer mixtures, such a dependence indicates a homogeneous mi~ing.'~J'In the case of phase separation, nc should be invariant with respect to the monolayer composition. Equation 1 and the thermodynamic analysis of Crisp are strictly applicable below the equilibrium spreading pressures of either pure components. Nevertheless they have been often used as criteria for miscibility even for rather high surface pressures.18 Sometimes parallel determination of the stability of the mixed monolayer was performed. Such a measurement for our mixtures showed a very small decrease of the monolayer area at 30 dynlcm, 0.1-0.5 A2/molecule during the first 2 min after compression. These changes are comparable with those obtained under the same conditions for the similar systems. For the 5/ 1methyl stearate/eicosyltrimethylammoniumbromide mixture on 0.1 M NaCl, studied in ref 19, this change was 0.2 A2/molecule. We used the relative change of the monolayer area AAI Ao during the first 2 min as a stability characteristic at 30 dynlcm. This quantity, plotted versus XD in Figure 5, shows the same trend as the collapse pressure IIc and the change of the free energy of Gibbs by mixing AGmix-all three dependences pass through a shallow extrema in the range of XD = 0.2-0.4. This correlation could mean that the changes of AAIAo, IIc, and AG,i, are due to variation of one and the same monolayer property determining the interactions in the mixtures and the spread monolayer with the liquid substrate. Electrostatic Properties of the Spread Monolayers. Figure 6 shows the AV potential versus the mean area per molecule for the pure components and several mixtures spread on 0.001 M NaCl subsolutions. From these and similar data on 0.1 M NaCl subsolutions the dependences illustrated in Figure 7 have been extracted. They give the surface potentials a t the area per molecule
Figure 7. AVpotential at the area per moleculeF ~correspondo ing to 30 dynlcm versus molar part XDof the charged component (DOMA); A, 0.001 M NaCl; 0,0.1 M NaCl subsolutions.
(16) Crisp, I. S.Research, Suppl. (London) 1949, 17, 23. (17)Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966; p 286. (18) Costin, I. S.;Barnes, G. T. J. Colloid Interface Sci. 1975,51,106. (19) Fromherz,P.;Masters, B. Biochem. Biophys. Acta 1974,356,270.
F ~ corresponding o to 30 dynlcm, A V n versus the molar part of the charged component, XD. In the framework of the concept of Schulman and Hudges20 and Cassy and Palmer21 the AV potential can
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Figure 8. Interfacial Gouy-Chapman potential $0 as a function of the surface charge density uo for monolayer mixtures at 30 dyn/cm; A, 0.001 M NaCl subsolution (data from ref 19); 0 , O . l M NaCl subsolution (data from ref 22). The dashed lines represent the Gouy-Chapman equation.
be divided into two parts, related to the contributions of the dipoles and the charges in the interface
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Figure 9. Dipole contribution AV-J/o to the electrostatic field at the interface versus the monolayer composition represented by the molar part XD of DOMA. The data for XD > 0.50 are obtained by assuming a plateau in the $O/UO dependence above this value ( X D= 0.50 corresponds to 20 pC/cm*). T w o subsolutions are considered: A, 0.001 M NaCl; 0 , O . l M NaCl.
4a P L AV = -- $o
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F30
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Here pI is the mean normal molecular dipole moment, F30 is the mean area per monolayer molecule a t 30 dynl cm, E is the dielectric constant of water in the interfacial region, and $0 is the Gouy-Chapman potential in the plane of the monolayer head groups. Experimental dependences $oluo (UOis the surface charge density) for the same and similar positively charged monolayer mixtures, spread on 0.001 and 0.1 M NaCl subsolutions, were obtained in previous investigation^'^^^^ by means of spectroscopic titration of an interfacial pHindicator, Figure 8. From the relationship between a0 and XD the corPesponding +o/XDdependences can be found, and after subtracting the two sets of data from Figures 7 and 8, the dipole contribution AV-$o to the electrostatic field a t the interface can be determined as a function of the monolayer composition. The result of this procedure is illustrated in Figure 9. It shows that the intensity of the dipole field a t the gaslliquid interface varies with the rise of X Dbut it does not depend on the ionic strength of the subsolution. Monolayer Transfer and Characterization of the SolidILiquid Interface. The monolayer transfer is usually characterized by the deposition ratios, a,that give information about the change of the density I’ of the monolayer during its transfer from the liquid to the solid substrate a = TIs/FL = FL/Fs
(3)
Here rs and r L are the mean monolayer densities (moll cm2) on the solid and liquid substrates, respectively, and FS and FL are the corresponding mean areas per molecule. Figure 10 represents the dependence of the deposition ratios during dipping, CY*,and withdrawal, ar,versus the molar part of the charged component XD for a 0.001 M NaCl subsolution. Both arand a, remain constant up to XD = 0.23, decreasing steeply above X D = 0.30. On both subsolutions 0.001and 0.1 M NaC1, the mixtures with X D < 0.23 show practically the same (within the (20) Schulman, J. H.; Hughes,A. H.Pr0c.R. Soc.London, A 1932,138, 430. (21) Cassie, A. B. D.; Palmer, R. C. Tram. Faraday SOC.1941,37,156. (22) Petrov, J. G.; Mbbius, D.Langmuir 1990, 6, 746.
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Figure 10. Dependence of the deposition ratios during dipping aa (V)and withdrawal ar(A)on the molar part of DOMA for a subsolution containing 0.001 M NaC1.
scattering) deposition ratios of dipping and withdrawal. The values of these ratios are close to 1 (rs and rL are almost equal), and therefore, the gadliquid and the solid/ liquid charge densities and Gouy-Chapman potentials have almost the same values. It is difficult to make a direct conclusion about the homogeneity and the roughness of the solid/liquid interface only on the basis of the methods applied here. However, in many early studies the deposition ratios of 1 were considered as a demonstration of the fact that the closepacked monolayer being transferred bridges over the microroughnesses of the solid surface.23 Therefore it could be expected that the monolayer-covered solid/liquid interface would be much more homogeneous and smooth than the bare one, resembling closely the molecular structure of the gaslliquid interface.
Discussion Homogeneous miscibility of charged and neutral insoluble surfactants in a monolayer on a gadliquid interface has already been observed for similar systems-long chain (23) Gaines, G. L. See ref 14, p 330.
Effect of Surface Forces on Contact Angles sulfates and alcohols. F ~ w k e studied s ~ ~ cetyl alcohol and sodium cetyl sulfate mixtures spread on a concentrated (1.2 M) NaCl subsolution in order to reduce the monolayer dissolution. Costin and Barnesl8 used octadecanol and a sulfate with a longer hydrocarbon chain ((222) to be able to perform the investigation a t lower salt concentration-on 0.1 M KC1 substrates. The last system is similar to ours-both pure components, as well as their mixtures, give condensed monolayers a t high surface pressures. Moreover the methyl arachidate/dimethyldioctadecylammonium bromide monolayer is condensed and stable a t 30 dyn/cm even on 0.001 M NaCl subsolutions. In this sense the conclusion about the homogeneity of the positively charged mixtures investigated here is in qualitative agreement with the earlier results. The miscibility of the monolayer components proves that the measured electrostatic parameters can be used as mean values characterizing the gadliquid interface. This enables relating them to theories discussing the effect of the electrostatic interactions in the three-phase zone on the contact angles and their hysteresis. All these considerations apply the Guoy-Chapman model of a charge smeared in a geometrical plane. The comparison of the $o/uo dependences for the gas/ liquid and the solid/liquid interfaces, performed in ref 22, showed that the corresponding Guoy-Chapman potentials do not differ a t equal values of cos and uoL. For this reason in this study we used the available $duo data to characterize both interfaces. The dipole field a t the solid/liquid interface can sometimes differ strongly, depending on the nature of the solid and on the solid surface hydrophobization-via siliconization, LB deposition of different monolayers, et^.^^ However, the relatively thick polystyrene coating (having the same Hamaker constant as the LB films26p27seems to be rather effective in screening the glass properties; the static contact angles do not depend on the number of deposited monolayers on it. (24) Fowkes, F. M. J.Phys. Chem. 1962,66,385. J.Phys.Chem. 1963, 67, 1982. (25) Petrov, J. G.; MBbius, D.Langmuir 1991, 7,1491. (26) Schulze, H. J.; Birzer, J. 0. Colloids Surf. 1987,24, 209. (27) Churaev, N. V. Physical Chemistry of Mass-transportProcesses in Porous Media; Khimia: Moscow, 1990 (in Russian).
Langmuir, Vol. 8, No. 1, 1992 205 Figures 7 and 8 show that both the values of AV and $0 for the two 0.001 and 0.1 M NaCl subsolutions differ by about 100 mV. At the same time the dipole field potential AV+o does not change with the ionic strength, Figure 9. Such an independence of the difference AV+o on the concentration of NaCl in the subsolutions (10-2-2.0 M) was obtained byDavis28formonolayers of C I & I ~ ~ N ( C H ) ~ + , if estimating the $0 potentials from the Guoy-Chapman theory. This good agreement between our results and Davis’ results is not surprising; it was found in both ref 19 and 22 that the experimental $ o / q data are in quantitative coincidence with the Gouy-Chapman theory up to a charge density of 5 pC/cm2. However, a t larger QO (i.e. for most of our mixtures) correct $0 values can be obtained only from the direct experimental determination. On the basis of the results of this investigation we can choose the experimental conditions under which the gas/ liquid and the solid/liquid interfacial properties are welldefied. These are mixtures With& 50.23 having dipping and withdrawal deposition ratios with values close to one. The latter are almost independent of the composition of the monolayer and of the ionic strength of the subsolution. Therefore, in this range of molar ratios, the values of the interfacial charge densities uoSand goL, Gouy-Chapman potentials $os and $oL, and dipole field potentials AV-$0 a t the two interfaces can be considered as equal. Under the same conditions the monolayer-coveredsolid/ liquid interface can be considered as homogeneous and smooth since the monolayers being transferred are condensed, i.e. close-packed, and their deposition ratios are close to 1. At the same time the constancy of the deposition ratios means that the smoothness and homogeneity of the solid/liquid interface do not vary for XD C 0.23 and therefore the changes of the contact angles in the corresponding Langmuir-Blodgett systems should result from a variation of the molecular interfacial properties. Registry No. Methyl arachidate, 1120-28-1;dimethyldioctadecylammonium bromide, 3700-67-2. ~
(28) Davis, J. T. Proc.
R. SOC.London, A
1951,208, 224.
