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Control of Surface Crystallization of 1-Alcohol Monolayers by pH Changes in the Water Subphase Francesca Bonosi, Anne Renault, and Bruno Berge* Laboratoire de Spectrome´ trie Physique, UA 08, Universite´ Joseph Fourier, BP 87, 38402 Saint Martin d’ He` res, France Received July 31, 1995. In Final Form: October 20, 1995X By surface tension and ellipsometric measurements, we studied how 2D crystallization of fatty alcohol monolayers is influenced by the presence in the subphase of a short chain carboxylic acid, whose surface activity is dependent upon pH. The surface crystallization of the alcohol monolayer was appreciably perturbed for pH values smaller than the pKa of the acid, that is when the protonated form of the acid was predominant in the bulk solution. On the contrary, for pH values above the pKa of the acid, the monolayer was not influenced by the presence of the acid in the subphase. The effect was greatly dependent on the acid concentration in the subphase, at a fixed pH. We also studied the effect of the acid chain length (C6-C10) on a decanol monolayer, the effect of the alcohol chain length with octanoic acid in the subphase, and the effect of the lateral surface pressure of the monolayer.
Introduction Recently a lot of work has been done to understand the molecular organization in monomolecular interfacial layers, as it plays a crucial role in the macroscopic properties of the film.1 The goal is to be able to control the architecture of the films at a molecular level, in order to get the optimum functionalization. Usually monolayers at the liquid-air interface are studied with the Langmuir technique, by varying the surface area of the film and measuring the corresponding variation of the surface pressure. This method is often limited to long chain amphiphilic molecules, which, after spreading from a volatile solvent, are completely insoluble in the underneath liquid phase. In the last few years it was shown that dense monolayers of short chain alcohols can be studied by spreading a liquid drop of the alcohol on a clean water surface.2 A monomolecular layer is spontaneously formed at the water surface, which is in a thermodynamic equilibrium with the excess bulk liquid collected as a lens. This lens acts as a reservoir of molecules for the monolayer, compensating the eventual losses of material from the surface, due to evaporation or dissolution in the subphase, thus permitting the study of short chain molecules. For short chain fatty alcohols the monolayer crystallization temperature is about 15 °C higher than the bulk crystallization temperature. The bulk phase fixes the chemical potential of the molecules in the monolayer and thus the lateral surface pressure.3 Previous studies from our laboratory have demonstrated that surface tension and ellipsometric measurements are able to detect the first-order phase transition corresponding to the 2D melting-crystallization in the monolayer.2,3 It is manifested as a change in the slope of the surface tension versus the temperature curve and as a sharp discontinuity in the ellipsometric angle ∆ versus the temperature curve. * To whom correspondence should be addressed. Telephone number: 33-76514333. Fax number: 33-76514544. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. M.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Berge, B.; Renault, A. Europhys. Lett. 1993, 21, 773 . (3) Renault, A.; Schultz, O.; Konovalov, O.; Berge B. Thin Solid Films 1994, 248, 47.
0743-7463/96/2412-0784$12.00/0
In this paper we report on the influence of short chain carboxylic acids, dissolved in the subphase, on the surface crystallization of alcohol monolayers, at different pH values of the subphase. Indeed, it is well-known that the surface adsorption of fatty acids with relatively short chains (C6-C10) is greatly dependent on the pH of the medium. In basic media, sodium carboxylates which are well soluble in water are obtained, while in the acid range the protonated form of the fatty acid gives Gibbs monolayers.4 This chemical equilibrium is shifted in one of the two directions simply by acting on the pH of the medium, by adding to the solution containing the soluble fatty acid different amounts of HCl or NaOH. We then expected that the protonated form of the acid, adsorbed at the surface, would interact with the alcohol monolayer, thus affecting its surface properties. In fact, we observed a strong decrease in temperature for the solid-liquid phase transition of the alcohol monolayer, with increasing concentration of the acid in the subphase, at acid pH. We also studied the effect of the acid chain length, in the range C6-C10, on the transition temperature of a decanol monolayer, thus determining that the perturbing effect is also dependent on the relative difference between the chain lengths of the acid and the alcohol. On the other hand, by varying the alcohol chain length (C10C14), we established that the effect of the acid was greater when the alcohol monolayer was at lower surface pressure. The same result was obtained by studying the effect of the acid on alcohol monolayers with different lateral pressures. These monolayers were obtained by spreading a solution of the alcohol in tetradecane, instead of the pure alcohol. Experimental Section All fatty alcohols and fatty acids, used in this work, were obtained from Aldrich SA and used without further purification. The water was ultrapure, from the ELGASTAT UHQII system. The subphases were freshly prepared by dissolving the appropriate amount of the carboxylic acid and adjusting the pH value by the addition of the necessary volume of HCl or NaOH stock solutions. The monolayer was formed by placing a drop of pure alcohol, or of alcohol diluted in tetradecane (when it is necessary to control the lateral surface pressure), on the water surface. The concentration of the diluted alcohol solutions ranged from 10% to 50% by weight. The system was allowed to equilibrate for (4) Matjevic, E.; Pethica, B. Trans. Faraday Soc. 1958, 54, 1400.
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corresponds well to the temperature value of the jump in the ellipsometric ∆ angle. Previous X-ray diffraction studies6-8 performed on a decanol monolayer spread on a pure water surface have shown that it undergoes a bidimensional solid-liquid phase transition at 14.5 °C, which is manifested as a slope change of the surface tension versus temperature curve and a sudden change in the ellipsometric angle. The crystalline structure of the monolayer is hexagonal with the long axis of the molecules assuming on average a vertical orientation with respect to the water surface. At pH 1.8, in the presence of octanoic acid in the subphase, we thus had a strong decrease in the surface crystallization temperature of the decanol monolayer. At this point, it is necessary to remember that octanoic acid, as all carboxylic acids, being a weak acid, is subjected in a bulk water solution to a chemical equilibrium between the protonated uncharged form and the unprotonated charged form: Ka
CH3(CH2)nCOOH {\} CH3(CH2)nCOO- + H+
Figure 1. Surface tension (a) and ellipsometric angle ∆ (b) as a function of temperature for a decanol monolayer spread on subphases containing octanoic acid at a concentration of 1.9 mM and at two different pH values: (b) 3.4 mM NaOH (pH 11.1) and (O) 10 mM HCl (pH 1.8). about 20 min before starting the experiment, mainly waiting for thermal equilibrium. It was necessary to work with a small excess drop. The trough was homemade with Teflon (surface, 160 × 160 mm; depth, 8 mm) and inserted in a closed metallic container regulated by circulating water. The cover was equipped with three small holes for the optical and surface tension measurements. The temperature was measured with a Pt thermometer dipped into the trough. The thermal stability was about 0.01 K over 1 h. The surface tension was measured using a platinum Wilhelmy plate with a Sartorius balance. A He-Ne laser was the light source for the ellipsometer. The laser light, polarized with the aid of a Glan-Thompson polarizer, after reflection on the water surface, passed through a l/4 retardation plate, a Glan-Thompson analyzer, and a photomultiplier. The incidence angle of the light on the surface was 1° away from the Brewster angle. Through small variations of the computer-controlled polarizer and analyzer rotations, the null ellipsometer5 continuously followed the zero of intensity . In this configuration the analyzer angle was half the phase difference (∆) between the two incident polarizations.
Results Figure 1 reports the variation of surface tension (1a) and of ellipsometric angle ∆ (1b) as a function of temperature for a decanol monolayer. The data are relative to two different subphases obtained by dissolving 1.9 mM octanoic acid and the necessary amounts of HCl or NaOH in order to obtain the reported pH values. The surface tension measurements exhibit a slope change, which (5) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and polarized light; North-Holland Personal Library: Amsterdam, 1977.
This equilibrium, which is determined by the value of Ka, the equilibrium constant, can be altered by varying the solution pH. At pH values lower than pKa, which for octanoic acid is equal to 4.9,9 the prevalent form is the protonated one, CH3(CH2)6COOH, while at pH values above 4.9 the prevalent form is the charged one, that is CH3(CH2)6COO-. Previous studies4 using surface tension measurements have demonstrated that the protonated form of octanoic acid gives Gibbs monolayers at the airwater interface, while the charged form gives a sodium salt that is well-soluble in water, which is not adsorbed at the water-air interface. We also checked the adsorption of octanoic acid at the water-air interface by measuring the surface tension as a function of the octanoic acid concentration in a solution of 10 mM HCl. With the concentration of octanoic acid from 0.315 to 1.9 mM, we obtained a good linear relationship between surface tension and the logarithm of the concentration, thus demonstrating the increasing adsorption of molecules at the interface with increasing concentration. This behavior, which is typical of Gibbs monolayers, also excludes the formation of micellar aggregates in solution for this range of concentration. The decrease in transition temperature of the decanol monolayer is therefore mainly due to the adsorption of octanoic acid which results in a mixed monolayer film with interactions between alcohol and acid molecules. Figure 2 shows the surface crystallization temperature of decanol as a function of pH in the presence of 1.9 mM octanoic acid in the subphase. The curve of pH versus transition temperature is sigmoidal, and the midpoint corresponds roughly to the pKa. For the sake of comparison, surface crystallization temperatures of decanol on water subphases of different pH, in the absence of octanoic acid, are also shown. The alcohol monolayer is completely insensitive to pH changes of the subphase, and the presence of octanoic acid is necessary in determining the decrease in transition temperature. Furthermore, the magnitude of the effect, being dependent on pH, is determined by the amount of acid which is present in the protonated form. (6) Renault, A.; Legrand, J. F.; Goldmann M.; Berge B. J. Phys. II 1993, 3, 761. (7) Legrand, J. F.; Renault, A.; Konovalov, O.; Chevigny, E.; AlsNielsen, J.; Gru¨bel, G.; Berge, B. Thin Solid Films 1994, 248, 95. (8) Berge, B.; Konovalov, O.; Lajzerowicz, J.; Renault, A.; Rieu, J. P.; Vallade, M.; Als-Nielsen J.; Gru¨bel G.; Legrand J. F. Phys. Rev. Lett. 1994, 73, 1652. (9) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics.; CRC Press: Boca Raton, FL 1980-1981.
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Bonosi et al. Table 1. Effect of Alcohol Chain Length on the Depression of the Melting-Crystallization Temperature of the Monolayer, Due to the Presence of Octanoic Acid in the Subphasea Tc (°C) alcohol
pH > pKa
pH < pKa
∆Tcb (°C)
decanol dodecanol tetradecanol
15 ( 0.5 40.5 ( 0.5 55.9 ( 0.5
4.5 ( 0.5 32 ( 0.5 54.4 ( 0.5
10.5 ( 1 8.5 ( 1 1.5 ( 1
a Subphase content: 1.9 mM CH (CH ) COOH, 10 mM HCl (pH 3 2 6 1.8). b Equal to Tc(pH > pKa) - Tc(pH < pKa).
Figure 2. Surface crystallization temperature of decanol as a function of bulk pH for different subphase contents: (O) 1.9 mM CH3(CH2)6COOH on heating and (b) on cooling, (0) water on heating and (9) on cooling, (solid line) fitting of experimental data using eq 2 in the text.
Figure 3. Dependence of the surface crystallization temperature (a) and surface tension (b) of a decanol monolayer on the octanoic acid concentration in the subphase: (b) and (9) on cooling, (O) and (0) on heating.
For two fixed pH values, 1.8 and 11.1, we also measured the dependence of the transition temperature on the octanoic acid concentration. The results are reported in Figure 3a. The transition temperature is strongly dependent on acid concentration at pH 1.8, while it showed no dependence at pH 11.1. This behavior is not surprising because only in acid media did adsorption of octanoic acid at the surface occur.4 The curve at pH 1.8 is linear with a negative slope:
dTc d[CH3(CH2)6COOH]
) -6000 ( 500 °C l/mol (1)
where Tc is the transition temperature of the decanol monolayer. The transition temperature of the decanol monolayer on pure water is 14.5 °C, and [CH3(CH2)6COOH] is the concentration of the protonated form of octanoic acid in the subphase. It is easy to relate [CH3(CH2)6COOH] to the pH by the following:
[CH3(CH2)6COOH] ) Cacid10-pH/(10-pH + 10-pKa) (2)
where Cacid is the total concentration of the acid that is imposed. Equation 2 allows the variation of Tc with pH to be calculated, which has been reported in Figure 2 (solid line). It is then manifested that the surface crystallization of a decanol monolayer is fully regulated by the acidbase bulk equilibrium of the octanoic acid, and that the assumption of the linearity between Tc and [CH3(CH2)6COOH] is roughly valid. Figure 3b shows the change in surface tension with concentration of octanoic acid, at pH 1.8, in the liquid (18 °C) and solid (3 °C) phases of the decanol monolayer. In both phases we obtained an increase in surface tension with increasing concentration of the protonated form of octanoic acid, thus indicating that the presence of the acid in the subphase leads in some way to a desorption of molecules from the surface. This behavior is quite unusual for solutions of pure surfactants. It may be due to particular intermolecular interactions established in the mixed monolayer film. On the contrary, at basic pH, where the charged soluble form of the acid is prevailing, no dependence of surface tension on acid concentration was observed. The perturbing effect of the octanoic acid on the surface crystallization temperature of an alcohol monolayer was also greatly dependent on the chain length of the alcohol. Table 1 reports the difference (∆Tc) between the phase transition temperature of an alcohol monolayer on a subphase containing 1.9 mM octanoic acid, at pH . pKa and at pH 1.8. We investigated decanol, dodecanol, and tetradecanol monolayers. Table 1 shows that the dissolved octanoic acid more easily perturbs decanol superficial layers than tetradecanol layers. It could be related to the chain length difference between the alcohol monolayer and the acid which has to penetrate into this layer or to a temperature dependence of the surface adsorption of the acid.4 We also investigated the effect of chain length of the acid (C6-C10) on the transition temperature of a dodecanol monolayer. We decided to study the effect of the acid chain length on dodecanol, and not on decanol, because the phase transition temperature of a dodecanol monolayer on water is 39 °C, while the phase transition temperature of decanol is 14.5 °C. With the last one it would be possible to have, in presence of the acid in the subphase, a shift of the phase transition temperature under 0 °C, and this would have led to some experimental difficulties, with the necessity of using subphases containing salts, thus making the studied system more complex. We observed that for a dodecanol monolayer a decrease of the acid chain length produced the greater decrease of transition temperature. The shift of the transition temperature of the dodecanol monolayer was doubled on going from decanoic acid to hexanoic acid in the subphase. Figure 4 reports the surface tension-temperature phase transition diagram for dodecanol on a subphase containing 10 mM HCl and 1.9 mM octanoic acid. The solid line is the solid-liquid coexistence line. The different curves reported in the figure have been obtained by dissolving different amounts of dodecanol in tetradecane and mea-
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Figure 4. Surface tension-temperature phase diagram for dodecanol/tetradecane mixtures. Subphase content: 10 mM HCl and 1.9 mM CH3(CH2)6COOH (pH 1.8).
suring the change of surface tension as a function of temperature. Indeed, when the reservoir drop contains the alcohol, diluted in an alkane, e.g. tetradecane, the chemical potential of the alcohol in the drop decreases. Because the excess drop fixes the chemical potential of the alcohol molecules in the monolayer, a decrease of this quantity gives rise to an increase in surface tension and thus to a decrease in surface pressure of the alcohol monolayer.3 In this way it is possible to study the dependence of the phase transition temperature on the surface tension of the monolayer. The presence of tetradecane does not affect the alcohol monolayer formation. In fact, neutron reflectivity measurements10 have demonstrated that the tetradecane does not spread at the water surface. From this curve we deduced that a decrease of surface pressure in the monolayer gives rise to a decrease of the surface crystallization temperature. This result was not unexpected; the same effect was also observed when octanoic acid was not in the subphase.3 The slope of the solid-liquid coexistence line is equal to -0.46 (mN/ m)/°C when octanoic acid is dissolved in the subphase, while it is equal to -1.24 (mN/m)/°C in the absence of octanoic acid,3 indicating that the dissolved acid perturbs the monolayer at low lateral pressure more than at high lateral pressure. Discussion These experimental results show a strong dependence of the surface alcohol monolayer crystallization upon the pH of the subphase. Here the soluble carboxylic acid plays the role of a “coupling agent” between the surface and the bulk; pure alcohol monolayers are completely insensitive to the bulk pH, and it is necessary to use the short fatty acid in order to get a monolayer sensitive to the pH. It should be very interesting also to study a pure acid monolayer, because in this case one expects an effect of the pH directly on the molecules constituting the monolayer. Unfortunately monolayers of fatty acids of comparable chain length (C10-C14) cannot be produced by the excess drop technique; before crystallization there is formation of lamellar phases at the neighborhood of the surface, thus rendering the ellipsometry measurement impossible. Nevertheless the pressure-area isotherms of long chain saturated fatty acids studied by conventional barrier techniques usually do not show a great dependence upon pH (of course, the surface potential exhibits a variation with the pH, due to the difference in the surface dipoles). If we suppose that it is the same for short chain (10) Legrand, J. F.; Renault, A.; Webster, J.; Bucknall D.; Penfold J. Private communication.
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fatty acids, this shows that the indirect influence of the pH through the surface activity of a short amphiphile destroys the monolayer ordering more efficiently than the direct influence of the pH on the surface charge. We have to address the question of the mechanism leading to a crystallization temperature decrease in the presence of octanoic acid at pH < pKa. One can expect that when the surface activity of the octanoic acid is increased, the amount which is present in the alcohol monolayer increases. The acid thus plays the role of an impurity, and as usual during crystallization, the solubility of the impurity is much higher in the liquid phase that in the solid phase. Nevertheless Figure 1b clearly shows that the ellipsometric angles are changed even when the monolayer is in the solid phase (for instance at T ) 3 °C). This indicates that there is also a penetration of octanoic acid in the monolayer solid phase. We plan to clarify this point by performing a neutron reflectivity experiment. Figure 4 shows that when the lateral pressure of the monolayer is decreased, the influence of octanoic acid on the alcohol monolayer is increased. It is probably due to the fact that the liquid alcohol monolayer can incorporate more octanoic acid when the lateral pressure is decreased, resulting in a lower freezing point. Table 1 shows that the main influence is the chain length difference between the acid and the alcohol; the influence of the dissolved fatty acid vanishes rapidly if the chain length difference exceeds six carbons, meaning that the penetration of the acid with a large chain length difference is more difficult, even in the liquid alcohol monolayer phase. A similar observation was found when we tried to incorporate fluorescent amphiphilic molecules into pure alcohol monolayers. Most fluorescent molecules did not incorporate, even in the liquid monolayer phase, forming three-dimensional aggregates. Table 1 should be taken with some care because the transition absolute temperature depends upon chain length. The adsorption of octanoic acid at the water surface has been shown to be temperature dependent,4 and it could be the same in the presence of a superficial alcohol monolayer. In conclusion, the system presented here exhibits large and reproducible variations of the monolayer crystallization with the bulk pH. This could be an example of controlling the mechanical properties of a monolayer, or a membrane, by the bulk pH. In this view, it would also be very interesting to study the dynamics of the phase transition, when the crystallization is controlled by the soluble impurity (here the octanoic acid); a lot of work has been done to shape the growth of crystals in two dimensions, when the growth is limited by the in-plane diffusion of an impurity.11,12 Here the impurity which controls the growth can escape in the third dimension, and probably the kinetics and the resulting shapes are strongly modified. Experiments in this direction could give interesting information. Acknowledgment. The original idea for doing this work came from a discussion with Joseph Lajzerowicz (University Joseph Fourier). We would like to thank him warmly for this and also for numerous discussion about the thermodynamics of complex surface systems in equilibrium with bulk phases. F.B. benefited from a C.E.E. postdoctoral fellowship. LA950647A (11) Miller, A.; Knoll, W.; Mo¨hwald, H. Phys. Rev. Lett. 1986, 56, 2633. (12) Akamatsu, S.; Bouloussa, O.; To, K.; Rondelez F. Phys. Rev. A 1992, 46, 4504.
