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Arachidic Acid Monolayers at High pH of the Aqueous Subphase: Studies of Counterion Bonding V. B. Fainerman, D. Vollhardt,*,‡ and R. Johann‡ Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, D-14424 Potsdam/Golm, Germany, and International Medical Physicochemical Center, Donetsk Medical University, 16 Ilych Avenue, Donetsk 340003, Ukraine Received March 13, 2000. In Final Form: June 30, 2000 The thermodynamic and morphological properties of arachidic acid monolayers at high pH values (pH 11-13) and temperatures (10 °C e T e 35 °C) are studied. The two-phase coexistence region of the surface pressure (Π)-area (A) isotherms is shifted to higher surface pressures with increasing pH value, just as with increasing temperature. Accordingly, regularly shaped condensed phase domains are formed which have various textures depending on the pH of the aqueous subphase. The equation of state, derived recently using the generalized Volmer’s equation and the quasichemical equilibrium model of 2D aggregation, was developed for the case of dissociating monolayer material taking into account the effects caused by the increase in the number of kinetic entities from the dissociation of ions and a contribution to the surface pressure, related to the ion-ion interaction. The experimental Π-A isotherms can be well described over the entire region of the gaseous and condensed states by the equation of state. The results allow the conclusion that the dissociation of arachidic acid in the monolayer increases with increasing pH, does not exceed 15%, and can be neglected for pH e 12. The thermodynamic characteristics of the aggregation process of arachidic acid in the monolayer, particularly the negative value of standard enthalpy of aggregate formation and that of standard entropy of aggregation, indicate the formation of a highly ordered condensed phase at the expense of the nonordered fluid phase.
Introduction Fatty acids are the mostly studied amphiphilic substances used as models for obtaining general information on the properties of Langmuir monolayers,1-4 such as thermodynamics, phase behavior, structure, and texture of the condensed phases. It has been found that the medium-chain fatty acids show an impressing variety of morphological structures.5,6 Simultaneous grazing incidence X-ray diffraction (GIXD) studies provided detailed knowledge on the lattice structures of the homologous fatty acids whereby numerous monolayer phases were assigned.7-10 However, these systematic investigations were mostly restricted to fatty acid monolayers in the nondissociated state. Much less information exists on the effect of an increasing pH value of the aqueous subphase on 2D phase behavior of fatty acid monolayers. In aqueous solutions of alkali salts of long-chain fatty acids, above pH 10 the headgroups are fully dissociated, between pH values of 10 and 7 they are partially dissociated, and below pH 7 the fatty acid exists increasingly only in the † ‡
Donetsk Medical University. Max-Planck-Institut.
(1) Harkins, W. D. Physical Chemistry of Surface Films; Reinhold: New York, 1952. (2) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers, John Wiley & Sons: New York, 1966. (3) Lundquist, M. Chem. Scr. 1971, 1, 197. (4) Pallas, N. R.; Pethica, B. A. Langmuir 1985, 1, 509. (5) Moore, B. G.; Knobler, C. M.; Akamatsu, S.; Rondelez, F. J. Phys. Chem. 1990, 94, 4588. (6) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591. (7) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Laxhuber, L. A.; Mo¨hwald, H. Phys. Rev. Lett. 1987, 58, 2224. (8) Barton, S. W.; Thomas, B. N.; Rice, S. A.; Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. J. Chem. Phys. 1988, 89, 2257. (9) Shi, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. Phys. Rev. A 1992, 45, 5734. (10) Kaganer, V. M.; Peterson, I. R.; Kenn, R. M.; Shish, M. C.; Durbin, M.; Dutta, P. J. Chem. Phys. 1995, 102, 9412.
nondissociated form.11 The protons of the carboxyl group can increasingly dissociate depending on the individual acid pK value. Within monolayers of fatty acids, increasing electrostatic repulsion should be expected with increasing pH of the aqueous subsolution so that their properties should also be changed. The experimental effect of subphase counterions is well-known, especially the stabilizing effect of many divalent counterions on the monolayer transfer onto solid substrates. Some theoretical efforts have been made to explain the effect of counterions on stability and structure of monolayers and LB films. In particular, recent theoretical and experimental studies of adsorbed monolayers have shown that considerable amounts of counterions can be bonded in the Stern layer, so that a small value of the dissociation degree should be expected.12 A recent paper of Oichi et al.13 drew our attention on the monolayer properties of fatty acids at high pH values. They recorded surface pressure-area (Π-A) isotherms of arachidic acid monolayers at about pH 12 and found plateau regions for the main phase transition in the accessible temperature region. Accordingly our following systematic BAM studies have revealed that the dissociated long-chain fatty acids (C18, C20, C22) at pH g 12 evolve also regularly shaped condensed phase domains with a defined inner texture and a corresponding long-range orientational order.14,15 The objective of the present paper is to consider the dissociation or at least a partial dissociation in the (11) Small, D. M. The Physical Chemistry of Lipids, Handbook of Lipid Research; Plenum Press: New York, 1986; Vol. 4, p 300. (12) Kralchevsky, P. A.; Danov, K. D.; Broze, G.; Mehreteab, A. Langmuir 1999, 15, 2351. (13) Oishi, Y.; Takashima, Y.; Suehiro, K.; Kajiyama, T. Langmuir 1997, 13, 2525. (14) Johann, R.; Vollhardt, D. Mater. Sci. Eng. C 1999, 8-9, 35. (15) Johann, R.; Vollhardt, D.; Mo¨hwald, H. Colloid Polym. Sci. 2000, 278, 104.
10.1021/la0003903 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/26/2000
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thermodynamic description of ionized monolayers. The equation of state developed for bimodal distribution between large aggregates and monomers of nondissociated molecules is extended to the case of dissociation of the monolayer material. The theoretical model is confronted with experimental data.
complete or partial) can result in an increase of the area per 1 molecule in the gaseous monolayer. If the amphiphilic substance is the 1:1 ionic surfactant RX, with the dissociation degree R in the monolayer, then the value of ω in the above equations should be calculated as the average area per one partially dissociated molecule
Theory
ω ) ω0(1 - R) + RωR + RωX
An approach based on the combination of the generalized Volmer’s equation16 and the quasichemical equilibrium model of 2D aggregation in monolayer17 was developed in ref 18. It has been assumed that the area per 1 molecule in the aggregate can differ from the area per free monomer molecule. The condition which describes the equilibrium between the monomers and aggregates was formulated with due account for the free surface existing in the monolayer. For bimodal distribution (large clusters and monomers), the following equation of state derived in ref 18 holds
Π)
RT(A/AcΠ)β A - ω[1 + ((A/AcΠ)β - 1)]
-B
β)1+
ω(1 - ) ω(1 - ) AcΠ A
(
)
(2)
(Π - Πc)ω AcΠ) Ac exp RT
(3)
) 1 - ω(n)/ω
(4)
where Π is the surface pressure, A the current area per 1 mol, R the gas constant, T the temperature, ω the area per 1 mol of surfactant in the extremely compressed gaseous monolayer, ω(n) is the area per 1 mol of monomers in a cluster, B is the constant, Ac is the molar area which corresponds to the point of the commencement of the 2D transition, i.e., at Π ) Πc. Equation 1 is valid within the region A < Ac. For the region A g Ac, the obvious relations A/Ac ) 1 and ) 0 can be used to derive the ordinary Volmer’s equation:19
Π)
RT -B A-ω
where the ω values with subscripts 0, R, and X refer to the areas of the nondissociated molecule, surface-active ion, and counterion, respectively. Another possible phenomenon is the increase in the surface pressure caused by the increase in the number of kinetic entities. In the present case, the dissociation of 1 molecule results in the formation of a pair of ions. This result follows from the generalized Volmer’s equation16,18
∑i Γi
Π ) RT 1-
(1)
with
(5)
Consequently, the equation of state (eq 1) involves four constants (B, Ac, ω, and ) and describes the entire region of surface pressure, from gaseous or liquid-expanded monolayer to the condensed state. The two constants B and ω are applicable to the gaseous monolayer region. The value Ac is the coordinate of the kink point of the Π-A isotherm located at the beginning of the phase transition region. Only the parameter refers explicitly to the 2D phase transition region and affects additionally the shape of the isotherm at A < Ac. The model defined by eqs 1-4 does not provide for the dissociation of amphiphilic molecules in the monolayer. This dissociation can lead to phenomena which affect the behavior of the Π-A isotherms in the region of the gaseous and 2D aggregating monolayers. The dissociation (either (16) Fainerman, V. B.; Lucassen-Reynders, E. H.; Miller, R. Colloids Surf. A 1998, 143, 141. (17) Fainerman, V. B.; Vollhardt, D.; Melzer, V. J. Phys. Chem. 1996, 100, 15478. (18) Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. B 1999, 103, 145. (19) Volmer, M. Z. Phys. Chem. (Leipzig) 1925, 115, 253.
(6)
∑i Γiωi
-B
(7)
where Γi is the surface concentration (adsorption) value of the ith component or state. Taken into account eq 7, one can rewrite eqs 1 and 5 as
Π)
RT(1 + R)(A/AcΠ)β A - ω[1 + ((A/AcΠ)β - 1)] Π)
-B
(1 + R)RT -B A-ω
(8) (9)
Finally, there exists a contribution to the surface pressure from the dissociation of ions, related to the ionion interaction. Assuming that the counterions form a diffuse layer, Davies20,21 calculated this contribution. This result was reproduced subsequently by other authors.22-26 The interion interaction results in an additional surface pressure jump
∆Π )
4RT (2RTcΣ)1/2[chφ - 1] F
(10)
where F is the Faraday constant, the dielectric permittivity, zi is the charge of the ion, cΣ is the total concentration of ions within the solution, and φ ) ziFψ0/2RT, where ψ0 is the electric potential of the surface. The electric potential is determined by the surface charge density.
shφ )
zRΓRF (8RTcΣ)1/2
(11)
In the theory developed by Kalinin and Radke,27 it was taken into account that some portion of the counterions is bonded to surface-active ions within the SternHelmholtz (S-H) layer, while another (nonbounded) portion is located within the diffuse region of the electric double layer. Similar concepts concerning the structure (20) Davies, J. T. Proc. R. Soc., Ser. A 1951, 208, 224. (21) Davies, J. T. Proc. R. Soc., Ser. A 1958, 245, 417, 419. (22) Borwankar, R. P.; Wasan, D. T. Chem. Eng. Sci. 1988, 43, 1323. (23) MacLeod, C. A.; Radke, C. J. Langmuir 1994, 10, 3555. (24) Diamant, H.; Andelman, D. J. Phys. Chem. 1996, 13732. (25) Poberezhnyi, V. Ya.; Kul’skiy, L. A. Kolloidn. Zh. 1984, 46, 735. (26) Poberezhnyi, V. Ya.; Sotskova, T. Z.; Kul’skiy, L. A. Khim. Tekhnol. Vody. 1996, 18, 570. (27) Kalinin, V. V.; Radke, C. J. Colloids Surf. A 1996, 114, 337.
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Langmuir, Vol. 16, No. 20, 2000 7733
of the adsorption layer for ionic surfactants were used in other publications.26,28,29 It can be supposed that bonded counterions are located within the same plane where the surface-active ions are situated and, therefore, they do not affect the surface pressure. An expression for an additional surface pressure jump which arises when the ions possessing opposite signs are located in different planes was derived in ref 26. In our notation, for the case when partial bounding of counterions takes place, eq 11 can be transformed into
shφ )
RzRΓRF (8RTcΣ)1/2
(12)
It is seen that for maximum ionization of the monolayer (R ) 1), this last expression is reduced to eq 11, while for nondissociating molecules (R ) 0) the additional jump of surface pressure becomes zero, cf. eq 10. According to the definition of the dissociation degree R ) (ΓR - ΓXs)/ΓR, eq 12 can be replaced by an expression, which contains the difference ΓR - ΓXs instead of ΓR, where ΓXs is the adsorption of counterions localized within the monolayer.26
shφ )
zR(ΓR - ΓXs)F (8RTcΣ)1/2
(13)
It follows from the model described by eqs 10 and 12 that if all counterions are localized within the monolayer (within the S-H layer), then ∆Π ) 0. However, this is not exactly the case. If no bulk charge separation takes place, and opposite charged ions are located at the interface (twodimensional electrolyte solution model), the Coulombic interaction leads to a certain arrangement of the ions. In this case, also an additional contribution to the surface pressure ∆Π exists, as was shown by Muller and Derjaguin.29 This contribution, however, is negative, and its value is significantly lower than that given by eq 10 for the case of a formation of a diffuse region of the electric double layer. The analysis of eq 10 has shown that for 1:1 ionic surfactants, the approximate relation φ . 1 is valid at low bulk concentration.30 This approximation leads to a linear dependency of ∆Π on Γ in eq 1024,31
∆Π ) 2RTΓ
(14)
For the opposite case of large ion bulk concentrations and low φ values, ∆Π is proportional to the squared adsorbed amount.24,31 Moreover, in the case of 1:1 ionic surfactant the value of ∆Π appears to be small enough for the effect of the diffuse part of the electric double layer to be neglected. Noting that the counterions located within S-H layer are partly bonded, and also taking into account that Γ ) 1/A, one can transform eq 14 into the form
∆Π ) 2RTR/A
(15)
This value should be added to the right-hand side of eq 9. In the 2D phase transition region, described by eq 8, (28) Vlahovska, P. M.; Danov, K. D.; Mehreteab, A.; Broze, G. J. Colloid Interface Sci. 1997, 192, 194. (29) Muller, V. M.; Derjaguin, B. V. J. Colloid Interface Sci. 1977, 61, 361. (30) Fainerman, V. B. Colloids Surf. 1991, 57, 249. (31) Fainerman, V. B. Zh. Fiz. Khim. 1982, 56, 2506.
the additional term is:
∆Π ) 2RTRβ/AcΠ
(16)
When comparing the contributions to the surface pressure determined by eqs 15 and 16 with the first terms in eqs 8 and 9, one can easily see that for relatively high extent of counterion bonding in the surface layer (say, for the dissociation degree R < 0.2), it cannot be expected that neglect of an additional electrostatic term introduces any appreciable error. Experimental Section The experimental setup consisted of a computer-interfaced film balance coupled with a Brewster angle microscope (BAM 2, NFT, Go¨ttingen). Using a Wilhelmy-type pressure measuring system the surface pressure was measured to within 0.1 mN/m. The lateral resolution of the BAM was about 4 µm. An image processing software were used for cutting-out selected image parts and for improving the contrast. Arachidic acid (eicosanoic acid) (g99% purity) obtained from Merck (Darmstadt, Germany) was used as received. The substance was dissolved in a 20:1 (v:v) mixture of n-heptane (for spectroscopy, Merck) and ethanol (p.a., Merck) to yield a 10-3 M spreading solution. Ultrapure MilliQ-filtered water (Millipore Co.) with a specific resistance of 18.2 MΩcm was used for the subphase. The high subphase pH was adjusted with NaOH (Titrisol, Merck) without using a buffer. To avoid lowest traces of polyvalent cations in the subphase, 10-5 M polyethylenediamine tetraacetic acid (EDTA, 99.9995%, Aldrich, Steinheim, Germany) was added to the alkaline subsolutions.
Results and Discussion With the increase of pH value of the aqueous solution, the degree of dissociation of fatty acids into ions RCOOand H+ also becomes higher. However, arguments have been discussed that the dissociation degree in the surface layer should be lower than in the aqueous subphase.32 Our attention has been drawn to the surface properties of long chain fatty acids at high pH values where the Π-A isotherms reveal two-phase transition regions similar to the medium chain fatty acids at low pH values. In the present work, the pH and temperature dependence of arachidic acid monolayers has been systematically studied. The solid lines of Figures 1 and 2 represents the theoretical results. As is clearly seen, the two-phase transition region is continuously shifted between pH 11 and 13 (Figure 1) at the accessible temperature range between 10 and 35 °C (Figure 2). Therefore these systems are good candidates to study not only the morphology of the condensed phase domains which are formed in the two-phase transition region but also the phenomena discussed above within these conditions. It is interesting to note that at constant temperature a continuous pH change of the aqueous subphase affects the Π-A isotherm in a similar way as temperature changes at a constant pH. The results of the BAM studies support the conclusions on the two-dimensional phase behavior of arachidic acid monolayers at high pH values which can be drawn from the Π-A isotherms. At compression, regularly shaped condensed phase domains are formed after the break point in the “plateau” region in the isotherm, i.e., in the twophase transition region after the main phase transition point Ac. At A > Ac, the monolayer is in the fluid (gaseous) state so that condensed phase domains cannot be observed. Figure 3 shows a sequence of typical BAM images for selected pH values at 25 °C. At pH 11, the two-phase coexistence region is already at zero pressure (Figure 1). (32) Ahn, D. J.; Franses, E. I. J. Chem. Phys. 1991, 95, 8486.
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Figure 1. Experimental and theoretical Π-A isotherms for arachidic acid monolayers at T ) 25 °C and various pH values. The values of the parameters in eqs 1-4, 7, and 8 are shown in Table 1. Key: ×, pH 11.0; 0, pH 12.0; ∆, pH 12.3; [, pH 12.5; +, pH 12.7; O, pH 13.0. The solid lines are the adequate calculated curves.
Figure 2. Experimental and theoretical Π-A isotherms for arachidic acid monolayers at pH 12 and various temperatures. The values of the parameters in eq 1-4 are shown in Table 2. Key: [, 10 °C; 0, 15 °C; ∆, 20 °C; × , 25 °C; +, 30 °C; O, 35 °C. The solid lines are the adequate calculated curves.
Accordingly the condensed phase domains are formed already after the spreading, so that only irregularly shaped condensed phase domains surrounded by the fluid phase are formed (Figure 3a). At higher pH values the Π-A isotherms have the phase transition region at Π > 0. Consequently regularly shaped domains are developed which show various inner textures. At pH 12, circular domains are formed (Figure 3b). With further pH increase, interesting shape changes occur. Already a small change to pH 12.3 causes faceting of domains (Figure 3c). Finally, the faceting effect dominates so strong that at pH 13 the appearance of the domains seems to be irregular (Figure 3d).
A single experimental Π-A isotherm of arachidic acid monolayers at pH 13 and at 25 °C is presented in Figure 4. Two theoretical models were used to process these results. In the first model, hereinafter referred to as model I, the ordinary eqs 1-5 were used, that is, the ionization was assumed to affect the value of the molar area ω only. In the second model (model II), eqs 8 and 9 were employed, where the additional parameter R was introduced to take into account explicitly the ionization degree of the arachidic acid monolayer. The values of model parameters which correspond to the best fit of the experimental data by the theoretical curve are presented in Table 1; here the areas are calculated per 1 arachidic acid molecule. It is seen
Arachidic Acid Monolayers at High pH
Langmuir, Vol. 16, No. 20, 2000 7735 Table 1 pH parameter
model
11.0
12.0
12.3
12.5
12.7
13.0
Ac, nm2
I II I II I II I II I II
0.98
0.568 0.57 0.204 0.202 0.188 0.186 6.0 6.5 0.0 0.05
0.514 0.514 0.244 0.218 0.171 0.179 6.5 6.8 0.0 0.11
0.484 0.484 0.246 0.216 0.172 0.179 6.4 6.5 0.0 0.11
0.463 0.463 0.250 0.222 0.175 0.178 6.8 7.6 0.0 0.15
0.453 0.452 0.250 0.225 0.175 0.176 6.9 7.9 0.0 0.15
ω, nm2 ω(n),
nm2
B, mN/m R
Figure 3. BAM micrographs of the condensed phase domains of arachidic acid monolayers obtained in the two-phase coexistence region of the Π-A isotherm at different high pH values. T ) 25 °C. Size: 370 × 290 µm2.
from Figure 4, where the curves which correspond to the two models are shown, that both models agree well with the experiment and neither of them can be reasonably preferred. Comparing the parameters of the models, one can see that the values ω(n) (area per 1 monomer molecule
0.224 0.208 6.5 0.0
in the aggregate) for the two models are the same, while the value ω for model I is significantly larger than for model II. This difference is quite understandable because in the framework of model I the value of ω is assumed to be responsible for all effects related to the ionization, while for model II, due to taking into consideration the possibility of dissociation, a more definite physical meaning is ascribed to this parameter. When, however the value of ω for model I was adjusted with respect to the ionization degree (see above) which follows from model II (15%). This ω value was found to be close to that calculated for model II. The experimental and theoretical Π-A isotherms for 25 °C and various pH values are shown in Figure 2. Table 1 summarizes the parameter values which enter the equations for the two models. As both models lead to quite similar ω(n) values, only the results followed from model II which considers the dissociation within the monolayer are presented in Figure 2. With regards to the parameters of the models, all conclusions drawn above with respect to the particular case of pH ) 13 are valid. Also, ω and R decrease quite expectably when pH value is decreased from 13 to 12. For pH values 12 and probably lower, no dissociation of the arachidic acid monolayer was found. In Figure 2 the Π-A isotherms of arachidic acid monolayer obtained at the subphase pH 12 and at different temperatures are shown. It should be mentioned that a small difference exists between the corresponding curves
Figure 4. Experimental and theoretical Π-A isotherms for arachidic acid monolayers at temperature 25° and pH 13. Experimental data is represented by open circles (O). Theoretical curves: dotted line, model I; solid line, model II. The values of the parameters in eqs 1-4, 7, and 8 are shown in Table 1.
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Table 2 temp, °C parameters
10
15
20
25
30
35
B, mN/m 6.3 6.3 6.3 6.1 5.8 6.0 ω, nm2 0.205 0.204 0.202 0.196 0.188 0.185 2 ω(n), nm 0.198 0.192 0.182 0.176 0.167 0.160 Ac, nm2 0.82 0.709 0.616 0.558 0.505 0.449 ∆G°, kJ/mol -3.34 -3.13 -2.97 -2.86 -2.76 -2.64 ∆H°, kJ/mol -10.2 -10.2 -10.2 -10.2 -10.2 -10.2 T∆S°, kJ/mol -6.9 -7.1 -7.2 -7.3 -7.4 -7.6
and the parameters (for pH 12 and 25 °C) in Figures 1 and 2 and in Tables 1 and 2, because the data obtained from two different experimental series were used. The parameters of eqs 1-5 which correspond to the theoretical curves shown in Figure 2 are summarized in Table 2. Here again a satisfactory agreement between theory and experiment exists. The trends of the parameters are quite expectable: an increase of the temperature leads to a significant decrease in Ac and a slow decrease of B, ω, and ω(n). In addition, the thermodynamic characteristics of the aggregation process of arachidic acid in the monolayer are also listed in Table 2. The value of the standard free energy of aggregation ∆G° was calculated from the equation presented in ref 33:33
∆G° ) RT ln(ω(n)/Ac)
(17)
The results obtained for ∆G° at various temperatures were used to estimate the values of standard enthalpy (∆H°) and standard entropy of aggregation (∆S°):
∆G° ) ∆H° - T∆S° -
∂ ∆G° ∆H° ) ∂T T T2
( )
(18) (19)
The value of mean standard enthalpy of aggregate formation ∆H° ) -10.2 ( 2.5 kJ/mol, calculated for temperatures from 15 to 35 °C is shown in Table 2; here the range of the variation of ∆H° indicated refers to any pair of the temperature values within this interval. For narrow temperature intervals (∆T ) 5°), it holds that the lower the mean temperature is, the lower is the ∆H° value. Similar to some phospholipid monolayers,33 the calculated values of T∆S° are negative. It was stressed that the ordered aggregation of insoluble molecules within the monolayer results in the decrease of the entropy of the (33) Vollhardt, D.; Fainerman, V. B.; Siegel, S. J. Phys. Chem. B 2000, 104, 4115.
system.33 According to the conclusions drawn for the monolayers of the phospholipid DPPG,33 also in the present system the T∆S° value represents a quantity for characterizing the degree of ordering, i.e., the two-dimensional crystallinity, after aggregation to the condensed phase. The negative entropy values are due to aggregation of the arachidic acid molecules to highly ordered condensed phase, whereas simultaneously the nonordered fluid phase decreases. The fact that the T∆S° value is negative but its absolute value is comparably low agrees reasonably with the liquid crystalline character of the condensed phase formed by arachidic acid monolayers at high pH values, as visualized by BAM (Figure 3). Conclusions Arachidic acid monolayers at high pH values (pH 1113) and accessible temperatures (10 °C e T e 35 °C) have a two-phase coexistence region of the surface pressure (Π)-area (A) isotherms which is shifted to higher surface pressures with increasing pH value, just as with increasing temperature. The formation of regularly shaped condensed phase domains supports the conclusions on the twodimensional phase behavior of arachidic acid monolayers at high pH values which can be drawn from the Π-A isotherms. The equation of state derived recently using the generalized Volmer’s equation and the quasichemical equilibrium model of 2D aggregation was developed for the case of dissociating monolayer material. The effects caused by the increase in the number of kinetic entities from the dissociation of ions and contribution to the surface pressure, related to the ion-ion interaction was taken into account. The experimental Π-A isotherms for arachidic acid at different temperature and pH value in the range of 11-13 can be well described over the entire region of the gaseous and condensed state by an equation of state valid for a bimodal distribution of large aggregates and monomers. The dissociation degree of the arachidic acid in monolayer increases with increasing pH but does not exceed 15%. For pH e 12 dissociation of arachidic acid in the monolayer can be neglected. The thermodynamic characteristics of the aggregation process of arachidic acid in the monolayer was calculated. The negative value of standard enthalpy of aggregate formation and that of standard entropy of aggregation are due to the formation of a highly ordered condensed phase, whereas simultaneously the nonordered fluid phase decreases. LA0003903
