Interaction of Hydrophobic Ions with a Langmuir Monolayer of

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Langmuir 1997, 13, 4870-4876

Interaction of Hydrophobic Ions with a Langmuir Monolayer of Dioctadecyldimethylammonium Bromide V. Shapovalov*,† and A. Tronin‡ Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygina 4, 117334 Moscow, Russia, and Institute of Crystallography, Russian Academy of Sciences, Leninsky 59, 117333 Moscow, Russia Received December 2, 1996. In Final Form: June 6, 1997X Interaction of a positively charged dioctadecyldimethylammonium bromide monolayer with three different single-charged hydrophobic anions, namely, hexafluorophosphate (HFP), picrate, and tetraphenylborate (TPB), was studied by surface pressure, surface potential isotherm, and ellipsometric techniques. Two models of monolayer-ion interaction were considered: adsorption to the head groups and incorporation into the monolayer. It was shown that the nature of ion-monolayer interaction depends on the hydrophobic properties of ions. The least hydrophobic HFP anion adsorbs to the head groups of the monolayer. This leads to a moderate decrease of the surface potential and more close packing of the monolayer. Ellipsometric data as well agree with the adsorption model. More hydrophobic picrate and strongly hydrophobic TPB anions incorporate into the monolayer. It reveals a drastic drop of the surface potential, which is due to the formation of an additional double electric layer inside the monolayer. Variations of φ-A, π-A, and ellipsometric isotherms support the incorporation model in this case. The experimental data show that hydrophobic ions cause much more pronounced changes in Langmuir monolayers than those produced by the usual (hydrophilic) ions. The mixed monolayer with a high content of large hydrophobic particles has its own particular mechanical and electrical properties, phase behavior, etc., which are quite different from those of the precursor monolayer.

1. Introduction It is well-known that the state of charged Langmuir monolayers depends drastically on the ion composition of the subphase solution. By now the influence of various inorganic (hydrophilic) cations1 and anions2 on the properties of some Langmuir monolayers was closely investigated. Interesting results were published recently on the interaction of ionic organic dyes3,4 and polyelectrolytes5 with monolayers of different amphiphiles. According to the chemical nature of an amphiphile head group and subphase ions, their interaction may combine various processes, such as double electric layer formation, dissociation or protonation of the head group, formation of coordination bonds, etc. Besides the above-mentioned types of ions, there is one more particular class, which may be called by convention “hydrophobic ions”. This class comprises strongly hydrophobic ions, which due to high symmetry possess a very low surface activity. Illustrative examples of hydrophobic ions are tetraphenylphosphonium (TPP) cation and tetraphenylborate (TPB) anion. Owing to their unique properties, hydrophobic ions are widely used in various fields of chemistry and biology. One of us successfully used TPB and TPP for charging the oil droplets in microemulsion to arbitrarily given electric potential.6 Recently we observed strong interaction of TPB anions † Semenov Institute of Chemical Physics. E-mail: [email protected]. ‡ Institute of Crystallography. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, August 1, 1997.

(1) Bettarini, S.; Bonosi, F.; Gabrielli, G.; Martini, G.; Pugelli, M. Thin Solid Films 1992, 210/211, 42. (2) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D. Thin Solid Films 1994, 242, 195. (3) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D.; Wildburg, G.; Ringsdorf, H.; Philp, D.; Preece, J. A.; Stoddart, J. F. Langmuir 1993, 9, 1534. (4) Gregory, B. W.; Vaknin, D.; Cotton, T. M.; Struve, W. S. 7th Int. Conf. Organized Mol. FilmssSuppl. Abstr. Book, Ancona 1995, 42. (5) Stroeve, P.; Hva, M. 7th Int. Conf. Organized Mol. FilmssAbstr. Book, Ancona 1995, 87. (6) Shapovalov, V. L. Izv. Akad. Nauk, Ser. Khim. 1992, 2245 [Bull. Russ. Acad. Sci., Div. Chem. Sci. 1992, 41, 1756].

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with a stearylamine monolayer, which results in the formation of a significantly expanded monolayer of “surface salt”.7 According to the suggested model, this monolayer consists of an approximately equal amount of fully protonated stearylamine and TPB ions. Strong interaction of hydrophobic ions with Langmuir monolayers7 seems to be very interesting for fundamental research and rather prospective for nanomanufacturing applications. In the present paper we studied the interaction of a positively charged monolayer of dioctadecyldimethylammonium bromide (DODA) with three different singlecharged anions, namely, hexafluorophosphate (HFP), picrate, and TPB. These ions represent somehow a series of ions with different hydrophobic properties. The mass and the size of ions increase from HFP to TPB. All ions are compact in shape; moreover, HFP and TPB are highly symmetric. As first approximation they can be treated as spherical particles. In this case the hydrophilic (or hydrophobic) properties of ions are directly controlled by their size. The larger ion interacts with water more weakly; i.e., it is less hydrophilic (or more hydrophobic). The smallest of the ions used, HFP, is close in any sense to common inorganic single-charged anions, such as NO3and ClO4-. The largest ion, TPB, has almost nothing to compare with inorganic ions, except the charge. The picrate ion has an intermediate size and possesses intermediate properties. In this study we used complementary data obtained from φ-A,π-A isotherms and ellipsometric measurements to get insight on the structure of a pure amphiphile monolayer and its modification caused by amphiphileion interaction. 2. Materials and Methods Monolayers of dioctadecyldimethylammonium bromide (DODA) were formed by spreading of a 0.5 mM solution in freshly distilled chloroform. The sample of DODA, kindly supplied by Prof. F. (7) Shapovalov, V. L. Izv. Akad. Nauk, Ser. Khim. 1996, 1701 [Russ. Chem. Bull. 1996, 45, 1611].

© 1997 American Chemical Society

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Figure 2. Compression modulus of the DODA monolayer spread on the subphase without hydrophobic ions and containing TPB, picrate, and HFP ions. Concentrations used are as follows: for TPB and picrate, 10-5 M; for HFP, 10-4 M.

Figure 1. Electric potential (a) and surface pressure (b) versus area per molecule for the DODA monolayer spread on the subphase without hydrophobic ions and with different concentrations of TPB ions. T ) 297 K. M. Menger (Emory University, Atlanta, GA), was used as received. Surface pressure and surface potential isotherms were measured with a homemade Langmuir trough equipped with a Wilhelmy balance and surface potential meter (Calvin probe) described elsewhere.7 The accuracy of the measurements was 0.05 mN/m and 5 mV, respectively. Subphase solutions were prepared using double-distilled water additionally purified by passing through a column with activated carbon. The ionic strength in all experiments was fixed at the level of 0.01 M by addition of NaCl. Since the DODA monolayer is not sensitive to pH, no buffer was added. Hydrophobic ions tetraphenylborate (TPB), picrate, and hexafluorophosphate (HFP) were supplied to the subphase by addition of sodium tetraphenylborate (STPB), sodium picrate (SP), and ammonium hexafluorophosphate (AHFP). These salts were purified by recrystallization. Their stock solutions (0.1 M) were additionally purified from uncontrolled amphiphilic impurities by passing through a column with activated carbon. The absence of such impurities in the subphase solution was controlled in each experiment by registration of the surface pressure and surface potential during compression of the subphase surface before DODA spreading. Normally, variations were less than 0.1 mN/m and 10 mV, respectively, upon 6-fold compression. To check whether the distribution of hydrophobic ions between the monolayer and bulk subphase reaches the equilibrium state, both compression and expansion isotherms were recorded. Normally, the hysteresis was less than several percent, except for the lowest concentrations of hydrophobic ions, when the diffusion restriction3 evidently appears. Ellipsometric measurements were performed using a PCSA null ellipsometer LEPh-2 (Special Design and Production Bureau for Scientific Devices of the Siberian branch of the Russian Academy of Sciences, Novosibirsk, Russia) with a He-Ne laser (wavelength 632.8 nm) as a light source. The angle of incidence was 56°. The accuracy of the device is 0.02° with respect to Ψ and ∆. Ellipsometric parameters were measured according to a two-zone technique. The accuracy of the measurements on the water surface was lower than that of the solid samples due to residual capillary waves at the surface. The errors were estimated to be 0.1° for Ψ and 0.2° for ∆. A home-built Langmuir trough, installed at the ellipsometer, was described elsewhere.8

3. Results and Discussion 3.1. Pure DODA Monolayer. Surface pressure and surface potential isotherms for the DODA monolayer on the subphase without hydrophobic ions are presented in Figure 1 by solid lines. Both curves are close to that (8) Tronin, A.; Dubrovsky, T.; Nicolini, C. Langmuir 1995, 11, 385389.

reported for the DODA monolayer on a 10-3 M NaCl solution.2 The isotherms reflect different phase transitions taking place in the monolayer upon compression. There is no detectable pressure at an area per molecule larger that 1.2 nm2. This region corresponds to the coexistence of a rarefied two-dimensional gas and liquid-expanded (LE) phase. Surface potential cannot be interpreted in this region because it depends on the surface fraction of the LE phase under the vibrating electrode. Onset of the pressure at ca. 1.15 nm2 accompanied by a sharp jump of the potential is associated with continuous LE phase formation. Further smooth growth of the potential and surface pressure is typical for the monophase film. The kink at ca. 0.8 nm2 is associated with the beginning of LE-LC (liquid condensed) phase transition. This transition is of first-order, and there is a LE-LC phase coexistence region. Accomplishing LC phase formation gives rise to a sharp increase of surface pressure at ca. 0.6 nm2. The phase behavior of the monolayer is clearly seen in the dependence of the compression modulus on the area per molecule (Figure 2). The regions with low and high values at large and small area per molecule, respectively, are separated by a pronounced drop. The regions with low and high elasticity correspond to pure LE and LC phases, and the drop corresponds to LE-LC coexistence. 3.2. DODA/TPB Interaction. Addition of STPB into the subphase solution results in significant changes of the φ-A and π-A isotherms (Figure 1). One can see that with an increase of the TPB concentration the features of the π-A isotherm, related to phase transition, vanish. However, the monolayer behavior in this case cannot be treated in terms of 2D phase transitions since it is an open system. As will be shown later, the monolayer contains some amount of hydrophobic ions. The ratio of hydrophobic ions/amphiphile in the monolayer (i.e., its composition) most likely changes during compression. Both surface pressure and surface potential decrease drastically with the TPB concentration. The evolution of the curves shows saturation behavior: the changes are strong for small concentrations, while the curves for 10-5 and 10-4 M TPB almost coincide with each other. The pressure of collapse decreases from ca. 50 mN/m for a pure DODA monolayer down to 8 mN/m for the maximum concentration of TPB used. This means that the monolayer-subphase interaction becomes weak, which is reasonable since the TPB ions are strongly hydrophobic and neutralize the surface charge of the DODA monolayer. Detailed analysis of π-A isotherms shows that the onset of the pressure shifts in the presence of TPB ions toward smaller area. Its position depends on the TPB concentration not monotonically (see Figure 3 where area per molecule, corresponding to the pressure onset (A0) is

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Figure 3. Concentration dependence of the area per molecule corresponding to the surface pressure onset for the DODA monolayer on the subphase containing different hydrophobic ions.

plotted versus ion concentration). A0 decreases for the concentration lower than 10-6 M and then increases; that is, the monolayer compresses at small ion concentration and expands with further increase of concentration. Obviously such behavior reveals the competition of two effects: decrease of Coulomb repulsion caused by the decrease of surface charge density and increase of film volume due to incorporation of large ions. The supposition that the ions incorporate into rather than adsorb to the monolayer is supported by surface potential data (Figure 1a). Without hydrophobic ions at A ) 1 nm2 the surface potential is 480 mV, while for [TPB] ) 10-4 M it drops down to 90 mV. In the case of adsorption the ions form a counterion layer adjacent to the head groups, which changes the double electric layer potential. In the absence of these ions the latter is 170 mV according to Gouy-Chapman model estimation. If the surface density of adsorbed ions is not more than that of the head groups, the drop of surface potential will not exceed 170 mV. The observed change in surface potential is 390 mV, which could be accounted for by a large excess of adsorbed ions resulting in a change of the surface charge sign. However, changing the charge sign is improbable since it implies the adsorption of the ions to a likewise charged monolayer. In a previous study7 we did not observe such an interaction in systems of this type. Moreover, there is not enough space in the vicinity of the head groups to accommodate the necessary amount of large TPB anions. Contrary to adsorption, which changes the double electric layer below the head groups, incorporation of hydrophobic ions results in the formation of one more double electric layers inside the monolayer. This additional layer consists of positively charged head groups (bottom plate) and TPB anions (top plate). Its contribution to the total potential is negative and may be very high due to the low dielectric constant inside the monolayer. Incorporation of TPB ions into the DODA monolayer changes not only its electric potential but its mechanical properties as well. Besides a drastic decrease of the collapse pressure, a pronounced increase of the compression modulus takes place (Figure 2). At molecular area 1 nm2 it is approximately 2 times greater than that of the pure DODA monolayer. Apparently, the incorporation of a great amount of large rigid anions results in the formation of a more rigid film. As one of the possible explanations of this effect, we can suggest that hydrocarbon chains in the mixed monolayer take almost a vertical position even at zero surface pressure, so that the monolayer cannot be compressed at the cost of an increase of thickness. After the collapse both π-A and φ-A curves have plateaus. The fact that the surface potential does not change with further compression suggests two models of collapsed film:

Figure 4. Electric potential (a) and surface pressure (b) versus area per molecule for the DODA monolayer spread on the subphase with different concentrations of picrate ions. T ) 295 K.

1. After the collapse the film remains in the form of a maximum compressed monolayer. Its density, structure, etc., remain constant, and decrease of the film area is accounted for by an exclusion of the substance in a compact strictly localized 3D formation, which does not enter into the acquisition area (roller just before the barrier, for example). 2. The collapsed monolayer folds in small 3D structures with an odd number of monolayers. In adjacent monolayers the molecules are packed head-to-head and tailto-tail, and the surface potential of such structures is equal to that of the initial monolayer. These collapsed domains are rather uniformly distributed over the monolayer. Crucial for distinguishing between these two models is the film thickness behavior. It would remain constant in the first case and grow in the second one. Our ellipsometric measurements (see below) show that the thickness monotonically increases after the collapse, which supports the second model. 3.3. DODA/Picrate Interaction. The effect of picrate ions on the isotherms of the DODA monolayer is qualitatively the same as that of the TPB ones. Corresponding curves are shown in Figure 4. The collapse pressure decreases and the surface potential drops with picrate concentration. The drop in the potential is very large. According to the reasons discussed above, the picrate ions as well as TPB ions are likely to incorporate into the monolayer rather than adsorb to it. The onset of surface pressure shifts with picrate concentration in the same manner as in the case of TPB (Figure 3). Absence of the kink in the surface pressure isotherm at high picrate concentration indicates the disappearance of the LE-LC phase transition. This effect shows that the phase behavior of the monolayer is influenced to a great extent by the amphiphile-ion interaction. The compression modulus curve (Figure 2) is featureless and resembles the LE part of the curve for the pure DODA monolayer. It seems likely that relatively small picrate ions do not impede the change of hydrocarbon chain tilt during compression of the expanded monolayer. However, the ions prevent such close packing of the compressed monolayer as it is in the LC phase of pure DODA. The pressure of collapse decreases down to 32 mN/m for the maximum concentration of SP used. This is

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by den Engelsen early in 1970s9 and exploited since then in several studies. According to this model, the long-chain molecules at the water surface are represented by solid rods, which are tilted from the normal by an angle θ. This angle varies during monolayer compression. As was shown,9-11 such a monolayer can be treated as a uniaxial layer with optical axis perpendicular to the interface. This anisotropy is due to the fact that the monolayer is a wellordered structure of rodlike molecules, which have different polarizabilities along the hydrocarbon chain and perpendicular to it. The polarizabilities are related to the components of molecular dielectric tensor mol through the generalized Lorentz-Lorenz expression9

moli - 1 ) 4πRi/(Vmol - 4πRiLi)

Figure 5. Electric potential (a) and surface pressure (b) versus area per molecule for the DODA monolayer spread on the subphase with different concentrations of HFP ions. T ) 293 K.

reasonable since the picrate ions compensate the head group charge and weaken the monolayer-water interaction. Evolution of the surface potential is rather complicated (Figure 4a). Normally it has a general trend to decrease in absolute value with molecular area owing to decrease of dipole and charge density. In the DODA-picrate system inverse behavior is demonstrated in some regions. For example, at molecular areas from 0.5 to 1 nm2 for 10-6 M picrate concentration the potential increases with area per molecule. Such behavior may be due to an increase of the ion concentration in the monolayer during compression or to lifting of the already incorporated ions toward the monolayer-air interface. It is impossible to judge from our data which process takes place; however, we can state that squeezing out picrate ions during monolayer compression is unlikely. 3.4. DODA/HFP Interaction. The effect of HFP ions on the DODA monolayer is different from that of the picrate and TPB ones (see Figure 5). The shift of the pressure onset is very large, and it increases monotonically with HFP concentration (Figure 3). The drop of the surface potential is not so pronounced as in the previous cases. For maximum concentration of AHFP, at molecular area 0.5-0.75 nm2 it is about 100 mV, i.e., less than the potential of the double electric layer for a pure DODA monolayer. Thus, in the case of HFP we get no evidence for the ion incorporation into the monolayer. One can estimate the ion/amphiphile ratio Ri from the drop of the surface potential. According to the Gouy-Chapman model, the decrease in potential of 100 mV corresponds to a decrease of the surface charge density by a factor of ca. 7, i.e., Ri ) 0.8-0.9. The kink of LE-LC transition vanishes; however, the state of the monolayer in this case resembles in some sense the LC phase of the initial monolayer. One can see that the monolayer formed on the HFP-containing subphase is much more compact than those in the previous cases. The compression modulus curve (Figure 2), in turn, looks like one for the LC phase of the pure DODA monolayer. Obviously, HFP anions adsorbed to cationic DODA head groups decrease their electrostatic repulsion and assist compact packing of the monolayer. 3.5. Ellipsometry. 3.5.1. Monolayer Model. Our ellipsometric model is based on the approach suggested

(1)

where i ) z, x (z ) direction along the hydrocarbon chain, x ) perpendicular to it), Vmol is the volume occupied by one molecule in the monolayer, Ri are molecular polarizabilities, and Li are Lorentz factors. Vmol is equal to van der Waals molecular volume plus free space corresponding to one molecule. The direction in which the molecules are tilted, characterized by the azimuth angle, is random for the domains, which are usually much smaller than the acquisition area, so the monolayer is laterally isotropic and its optical axis is perpendicular to the interface. Passing to the laboratory coordinates and averaging over the azimuth angle, one obtains for the ordinary and extraordinary components of the monolayer dielectric tensor9

o ) 1/2{(1 + cos2 θ)molx + molz sin2 θ} e ) molx sin2 θ + molz cos2 θ

(2)

Tilt angle θ is given by:

cos θ ) dm/lmol

(3)

where dm is monolayer thickness and lmol is the length of the chain. The former is related to the area per molecule A and Vmol by

dm ) Vmol/A

(4)

This relation reflects the fact that the layer thickness is equal to the total volume of the substance divided by an area this substance is spread on. The polarizabilities of fatty acids with different hydrocarbon chains lengths are reported in refs 9 and 12. For stearic acid their values are Rz ) 51.3 Å3 and Ry ) Rx ) 28 Å3. Hydrocarbon chains of DODA are similar to the stearic acid molecule; thus, we suppose that their polarizabilities are the same. Since polarizability is an additive property, the corresponding values for DODA are 2 times greater. Strictly speaking, the polarizabities along the x and y axes are different. However, we can consider them to be equal. To justify this assumption, one can take into account that the monolayer can be regarded as an assembling of the chains themselves. The whole consideration will be the same but for the use of chain volume, polarizabilities, and area per one chain instead of corresponding molecular values. The Lorentz factors which appear in eq 1 are complicated functions of molecular (9) den Engelsen, D.; de Konig, B. J. Chem. Soc., Faraday Trans. 1974, 9, 1603. (10) Ducharme, D.; Max, J.-J.; Salesse, C.; Leblanc, R. J. Phys. Chem. 1990, 94, 1925. (11) Paudler, M.; Ruths, J.; Riegler, H. Langmuir 1992, 8, 184. (12) den Engelsen, D. Surf. Sci. 1976, 56, 272.

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shape. Fortunately, their sum equals unity. According to the reasons discussed above, we assume that Lx ) Ly and relate Lz with Lx:

Lz ) 1 - 2Lx

(5)

In order to consider LE-LC phase transition, we take into account that volume packing densities in LE and LC phases are different. This means that the effective molecular cross section Smol changes during monolayer compression. We assume that Smol takes two different constant values in LE and LC phases, SLE and SLC, and in the coexistence region it decreases linearly from SLE to SLC, that is:

{

SLC for A < At Smol ) SLC + ∆S(A - Ai)/(Ai - At) for At < A < Ai SLE ) SLC + ∆S for A > Ai (6) where Ai and At are initial and terminal points of the LE-LC transition, and ∆S is the difference of cross sections in LC and LE phases. Variation of Smol results in variation of Vmol, which is used in eqs 1 and 4. Linear decrease of the molecular cross section in the transition region should be interpreted as a linear change of the ratio of areas occupied by LE and LC phase domains. Linear change of this ratio in the coexistence region was observed earlier13 via fluctuations of ellipsometric parameters. We did not notice such fluctuations, probably due to the much smaller domain dimension in our systems. The molecular cross section in the LC phase can be determined from following consideration. The area per molecule just before the monolayer collapse at ca. 50 mN/m is 0.46 nm2. As was shown by X-ray reflectivity studies,14 the molecules of some amphiphiles with two hydrocarbon chains just before the collapse are tilted by 30°. We assume that the same is valid for the DODA monolayer. Substituting eq 4 into eq 3 and assuming 0.46 nm2 for A and 2.42 nm for lmol, we get the value of 0.4 nm2 for SLC. This value is typical for two-chain molecules. It is approximately double the value that is commonly ascribed to the cross section of one hydrocarbon chain. Ellipsometric parameters Ψ and ∆, which describe the ratio of amplitude reflection coefficients for the light polarized parallel and perpendicular to the incidence plane, can be calculated as functions of dm and  by the well-known formulas for the uniaxial layer with optical axis perpendicular to the surface (see, for example, ref 9). Equations 1-6 relate thickness and the effective dielectric tensor of the monolayer to four fitting parameters (Lx, Ai, At, ∆S) and five fixed parameters (Rx, Rz, lmol, SLC, and A). Ai and At may be determined from the π-A isotherm; however, we shall try to determine these parameters from ellipsometric measurements alone; agreement (or disagreement) of the results will indicate somehow the adequacy of the proposed model. 3.5.2. DODA Monolayer. Dependence of the ∆ parameter on area per molecule for a pure DODA monolayer is shown in Figure 6 by circles. The solid line corresponds to the best-fitted model curve, calculated according to the above-described procedure. One can see very good agreement with the experiment. Criterion χ2 gives an estimation of the model probability of about 80%. The values of Ai and At, corresponding to the best fit, are 0.95 and 0.60 nm2, respectively, which agree rather well with the (13) Rasing, Th.; Hsiung, H.; Shen, Y. R.; Kim, M. W. Phys. Rev. A 1988, 7, 2732. (14) Helm, C. A.; Mo¨hwald, H.; Kjaer, K.; Als-Nielsen, J. Europhys. Lett. 1987, 4, 697.

Figure 6. Ellipsometric parameter ∆ of the DODA monolayer spread on the subphase without hydrophobic ions versus area per molecule: circles, experiment; solid line, calculation according to the model accounting for the LE-LC phase transition (best-fitted curve); dotted lines, calculation without accounting for the LE-LC phase transition. Different curves correspond to different indicated molecular Lorentz factors; the best-fitted curve from this family is shown by a dashed line.

Figure 7. Ellipsometric parameter ∆ of the DODA monolayer spread on the subphase without hydrophobic ions and containing TBP, picrate, and HFP ions. Concentrations used are as follows: for TBP and picrate, 10-5 M; for HFP, 10-4 M. Area per molecule indicated as Ac corresponds to the collapse of the monolayer formed on a TBP-containing subphase.

estimation made from the π-A isotherm. The value obtained for ∆S is 0.05 nm2. For Lx we got the value of 0.52, which gives reasonable values for the ordinary and extraordinary refraction indices. For the monolayer in the LC state they are 1.471 and 1.506, respectively. To demonstrate the importance of accounting for the LE-LC phase transition suggested here, we tried to fit the data by varying only one parameter Lx assuming that Smol is equal to SLC in the whole compression region. The curves with different Lx are shown by dotted lines; the best-fitted curve is shown by a dashed line. It is clearly seen that the curves cannot be fitted well to the entire experimental data set, and the χ2 criterion gives a low estimation of the model probability (less than 1%). 3.5.3. DODA/Hydrophobic Ions. Changes occurring in the monolayer structure due to DODA-ions interaction are reflected in ellipsometric isotherms as well. The results of ellipsometric measurements of the pure DODA monolayer and DODA + ions systems are shown in Figure 7. The data for monolayers formed on subphases containing different ions deviate noticeably from each other and from that of the pure DODA monolayer. To treat ellipsometric data for DODA + hydrophobic ions systems, we used two models, which are modifications of that for pure DODA. In the first model the ions are supposed to be adsorbed to the DODA monolayer, and in the second one they are incorporated into the DODA matrix. We denote these models as adsorption and incorporation, respectively. For both of them we specify ion “refractivity” ni and the ions/DODA ratio in monolayer Ri as unknown parameters. We assume that Lorentz factors of the DODA molecule do not change in the presence of the ions and use Lx and Lz obtained from the fitting for

Hydrophobic Ions/DODA Interaction

Figure 8. Ellipsometric parameter ∆ of the DODA monolayer spread on the subphase containing 10-5 M picrate ions. Shown is the interval of an uncollapsed continuous monolayer: circles, experiment; solid line, best-fitted curve for the model of ions incorporated into the monolayer; dashed line, best-fitted curve for the model of adsorbed ions.

a pure monolayer. Cross section of the DODA molecule in the modified monolayer is unknown, and thus we have three fitting parameters: ni, Ri, and ∆S. The adsorption model simply adds one effective layer below that of the DODA. The thickness of this layer is equal to the ion diameter, and its refractive index is calculated according to the Maxwell-Garnett expression, which is valid for spherical inclusions in a continuous medium:

(eff - med)/(eff - 2med) ) f(i - med)/(i - 2med) (7) where eff, i, and med are dielectric constants of the effective layer, ions, and matrix medium (subphase), respectively, f-filling factor, which in this case is given by ion volume Vi, ion diameter di, area per molecule A, and the Ri ratio:

f ) ViRi/Adi In the incorporation model the monolayer thickness increases since the total volume increases:

dm ) (Vmol + RiVi)/A Dielectric constants o and e of the film are changed as well. We calculated them through eq 7 by substituting in place of med corresponding parameters of the matrix DODA monolayer calculated according to eq 4. The filling factor in this case is given by

f ) RiVi/(Vmol + RiVi) 3.5.4. DODA/Picrate. Results of the fitting show that for the adsorption model as well as for the incorporation one the molecular cross section does not change in the whole range of the monolayer compression and is equal to SLE of the pure DODA monolayer. This result agrees with the absence of phase transition in the mixed DODA + ions monolayer as seen in π-A isotherms. The calculated curves obtained for the adsorption and incorporation models are shown in Figure 8 by dashed and solid lines, respectively. One can see that the incorporation model fits the experiment better. The χ2 criterion gives 20% and 70% estimation for the plausibility of the adsorption and incorporation models, respectively. Parameters of the ions, Ri and ni, appear to be correlated. It is impossible to determine them independently; however, there are sets of reasonable values, for example, 1.61 and 0.7 or 1.56 and 0.9 for ni and Ri, respectively. 3.5.5. DODA/TPB. Unfortunately, in this case the interval of area per molecule for the uncollapsed monolayer is very narrow, and ellipsometric data do not allow one to distinguish between the models. Corresponding calculated curves almost coincide with each other. However,

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Figure 9. Ellipsometric parameter ∆ of the DODA monolayer spread on the subphase containing 10-4 M HFP ions. Shown is the interval of an uncollapsed continuous monolayer: circles, experiment; solid line, best-fitted curve for the model of ions adsorbed to the monolayer ions; dashed line, best-fitted curve for the model of incorporated ions.

the incorporation model is favored by the following consideration. Monolayer thickness, calculated according to this model at the moment of collapse, coincides with tail length. Within the framework of the adsorption model, it is 0.7 of the tail length; that is, the tilt angle is 45°. The last value seems to be too large for tilt just before collapse. As we stated above, the behavior of the ∆ parameter for TPB in the region of constant surface pressure and potential (see Figure 1) allows one to distinguish between the models of monolayer collapse. A pronounced increase of ∆ (in absolute value) after the collapse (Figure 7; the area per molecule corresponding to the collapse is indicated as Ac) gives unambiguous evidence for film thickness growth, which agrees with the multilayer structure of the collapsed film. 3.5.6. DODA/HFP. This system is different from the previous ones. The molecular cross section appeared to be constant during compression; however, its value is equal to that of the LC but not LE phase of pure DODA. This result is in accordance with π-A isotherm and compression modulus data. The results of model fitting are shown in Figure 9. One can see that the adsorption model agrees with the experiment a little better. Unfortunately, the considered interval of compression is too narrow to get a reliable χ2 criterion estimation. As in the previous cases, Ri and ni are correlated. There are reasonable pairs, for example, 1.66 and 0.7 or 1.58 and 0.9 for ni and Ri, respectively. 4. Conclusions The observed picture of ion-monolayer interaction varies regularly with hydrophobic properties of ions. The smallest and least hydrophobic HFP anion adsorbs to the head groups of the monolayer. This leads to a moderate decrease of the surface potential of the monolayer and its more close packing due to a decrease of the surface charge density. Ellipsometric data as well agree with the adsorption model of HFP-DODA interaction. Similar behavior was recently reported for the iodide (I-) anionDODA system.2 Surface potential and ellipsometry data show that at a high concentration of HFP anions in the subphase the monolayer consists of approximately equivalent amounts of charged amphiphile and HFP counterions. There is no evidence for the composition of such a monolayer to change markedly during its compression. More hydrophobic picrate anions do not adsorb to but incorporate into the DODA monolayer. Incorporation of a large amount of picrate anions leads to some contraction

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of the monolayer due to Coulomb attraction, but at the same time it prevents close packing of the monolayer upon compression. Ellipsometric study confirms the incorporation of an approximately equivalent amount of picrate ions into the monolayer when their concentration in the subphase is sufficiently high. The most hydrophobic TPB anions cause the most pronounced changes of the DODA monolayer. When the concentration of TPB in the subphase is high enough, the observed π-A isotherm has nothing in common with that of the parent DODA monolayer. TPB ions, as picrate ions, incorporate into the DODA monolayer. This incorporation prevents the compression of monolayer at the cost of a decrease of tilt angle, changes the surface potential, and decreases the collapse pressure, which depends on the energy of monolayer-subphase interaction. At the initial stage of compression the mixed monolayer is more rigid and further compression leads to collapse at the surface pressure of only 8-10 mN/m. The behavior of the

Shapovalov and Tronin

collapsing monolayer can be rationalized in terms of formation and growth of flat multilayer domains consisting of an odd number of monolayers, packed head-to-head. Summarizing, hydrophobic ions may cause much more pronounced changes in Langmuir monolayers than those produced by the usual inorganic (hydrophilic) ions. Hydrophobic ions most likely incorporate into the amphiphile matrix, yielding a mixed monolayer. The mixed monolayer with a high content of large hydrophobic particles has its own particular mechanical and electrical properties, phase behavior, etc., which may be quite different from those of the precursor monolayer. Acknowledgment. This work was partially supported by the Russian Foundation for Basic Research (Project No. 96-03-34122a). LA962067Z