Effect of the Partial Hydrogenation of Hydrophobic Chains on the

Mar 1, 2008 - Effect of the Partial Hydrogenation of Hydrophobic Chains on the Mixing of Fluoroalkanols in an Adsorbed Film at the Hexane/Water Interf...
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J. Phys. Chem. C 2008, 112, 4564-4568

Effect of the Partial Hydrogenation of Hydrophobic Chains on the Mixing of Fluoroalkanols in an Adsorbed Film at the Hexane/Water Interface Daiki Murakami,* Takenori Fukuta, Hiroki Matsubara, Makoto Aratono, and Takanori Takiue Department of Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu UniVersity, Fukuoka 812-8581, Japan ReceiVed: July 31, 2007; In Final Form: January 15, 2008

The mixed adsorbed film of 1H,1H-perfluorooctanol (DFC8OH) and 1H,1H,2H,2H-perfluorodecanol (TFC10OH) at the hexane/water interface was studied on the basis of interfacial tension measurement and its thermodynamic data analysis. An adsorbed film at any composition of the mixed system as well as those of pure DFC8OH and TFC10OH systems exhibits three states: the gaseous, expanded, and condensed states. Construction of the phase diagram of adsorption clarified that DFC8OH and TFC10OH mix almost ideally in the gaseous and expanded states. On the contrary, the excess Gibbs energy of adsorption gH,E value evaluated in the condensed state was positive. These results are explained by considering the following two factors: (1) The mixing of binary alcohols is accompanied by the loss of dispersion interaction energy due to the difference in extent of fluorination of hydrophobic chains and in their chain length and increases the gH,E value. (2) Since the interchange energy concerning the interaction between dipoles with different dipole moments is negative, the mixing of these alcohols reduces the repulsive force between hydrophilic groups and thus leads to a decrease in the gH,E value. In the gaseous and expanded states, both of above two factors are not effective. On the other hand, the positive gH,E value in the condensed state is attributable to more effective dispersion interaction than the dipole-dipole interaction in short molecular distance, and so factor 1 becomes dominant. Comparison of the gH,E value of the present system with that of the homologous TFC10OH-TFC12OH mixture leads us to a conclusion that the hydrogenation on β-carbons in hydrophobic chains affects appreciably the balance of interactions between hydrophilic and hydrophobic groups which governs the mixing of molecules in adsorbed films.

Introduction Studies on adsorbed films at oil/water interfaces are of great importance because they are regarded as a simple model of more complicated molecular organized systems such as microemulsions and biomembranes, and thus many groups have investigated the structure and property of adsorbed films from macroscopic and microscopic viewpoints. We have studied the adsorption of various surface-active substances at oil/water interfaces mainly on the basis of interfacial tension measurements and their thermodynamic data analysis. Among others, the state of adsorbed films of alcohols with fluorocarbon (FC) chains as hydrophobic groups at the hexane/water interface was examined systematically1-5 because of unique properties of FC such as greater hydrophobicity, higher melting point than the corresponding hydrocarbon (HC), very weak mutual interaction between FC and HC, and so on.6 Some FC alcohols form condensed films in which molecules are closely packed like two-dimensional solids, which were confirmed from the X-ray reflection (XR) measurement of their films.7-9 One of the characteristic properties of the fluorine atom is the most mighty electronegative nature among all atoms. Therefore, the substitution of a fluorine atom at the ω-position of a FC chain into a hydrogen atom generates a ω-dipole and is expected to change the structure of the adsorbed film. In our previous studies on adsorbed films of individual and mixtures * To whom correspondence should be addressed. Phone: +81 92 642 2578. Fax: +81 92 642 2607. E-mail: [email protected].

of 1H,1H-perfluorononanol (FDFC9OH) and its ω-hydrogenated analogue 1H,1H,9H-perfluorononanol (HDFC9OH) at the hexane/water interface,10-12 we clarified that HDFC9OH has smaller partial molar entropy and volume at the interface compared to FDFC9OH because HDFC9OH molecules take an oblique orientation from the interface normal in order to make interactions between ω-dipoles and between ω-dipole and water in the interfacial region more effective. Furthermore, it was concluded that the mixing of FDFC9OH and HDFC9OH in the adsorbed film is dominated by the balance of two effects: (i) the reduction of the repulsive interaction between ω-dipoles aligning parallel in the film by mixing different species and (ii) the loss of dispersion interaction between hydrophobic chains due to the mixing of HDFC9OH with oblique orientation and FDFC9OH with perpendicular orientation. The electronegative properties of fluorine atoms in the hydrophobic chain affect appreciably the polarization of hydrophilic group. In order to know the effect of fluorination on β-carbons in the hydrophobic chain of fluoroalkanol on the adsorption behavior, we studied the adsorption of 1H,1H,2H,2H-perfluorooctanol (TFC8OH) and 1H,1H-perfluorooctanol (DFC8OH) at the hexane/water interface.5 The comparison of interfacial pressure π versus mean area per adsorbed molecule A curves shows that DFC8OH molecules form the condensed film in addition to the gaseous and expanded ones, whereas TFC8OH molecules do not. This was explained by the following factors: (1) Fluorination accompanies an increase in molecular weight, and thus, the dispersion interaction between hydrophobic

10.1021/jp076108j CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008

Mixing of Fluoroalkanols in an Adsorbed Film

J. Phys. Chem. C, Vol. 112, No. 12, 2008 4565

Figure 1. Chemical structures of surface-active substances: (1) DFC8OH, (2) TFC10OH, and (3) TFC12OH.

chains is strengthened. (2) Since the electronegative effect of the FC chain is larger for DFC8OH than for TFC8OH, DFC8OH molecules have a larger dipole moment in the hydroxyl group and interact more strongly with each other than TFC8OH molecules. Our aim in this work is to shed light on the effect of the difference in polarizations of hydroxyl groups on the mixing of molecules in the adsorbed film by employing a mixed system of DFC8OH and 1H,1H,2H,2H-perfluorodecanol (TFC10OH), whose chemical structures are shown in Figure 1. The reasons why this mixture was chosen are as follows: (1) Polarization of the hydroxyl group is larger for DFC8OH than for TFC10OH. (2) The adsorbed films of both alcohols exhibit three kinds of states, that is, gaseous, expanded, and condensed states, and thus the mixing of molecules can be discussed at all film states. Interfacial tension measurement and thermodynamic data analysis were performed through the construction of the phase diagram of adsorption and the evaluation of excess Gibbs energy of adsorption. Results obtained are compared to those of the TFC10OH-1H,1H,2H,2H-perfluorododecanol (TFC12OH) system13 in order to elucidate the effect of interaction between hydrophilic groups as well as that between hydrophobic chains on the mixing of molecules in the adsorbed film.

Figure 2. Interfacial tension vs total molality curves at constant composition: X2 ) (1) 0 (DFC8OH), (2) 0.100, (3) 0.200, (4) 0.300, (5) 0.400, (6) 0.500, (7) 0.600, (8) 0.700, (9) 0.800, (10) 0.900, and (11) 1 (TFC10OH). Data for different compositions are progressively offset by 2 mN m-1 (γ ) 50.3 mN m-1 at m ) 0 mmol kg-1).

Experimental Section 1H,1H-Perfluorooctanol (DFC8OH) and 1H,1H,2H,2H-perfluorodecanol (TFC10OH), purchased from PCR Inc., were purified by recrystallization from hexane solution once and three times, respectively. Their purities were checked by observing no time dependence of interfacial tension between the hexane solution of each alcohol and water and by liquid-gas chromatography. Hexane (Aldrich Chemical Co. Inc.) was distilled once and water three times; the second and third stages were done from alkaline permanganate solution. Interfacial tension γ was measured as a function of total molality m and composition of TFC10OH X2 at 298.15 K under atmospheric pressure by the pendant drop method.14,15 Here m and X2 are defined by

m ) m 1 + m2

(1)

X2 ) m2/m

(2)

and

where m1 and m2 are the molalities of DFC8OH and TFC10OH in hexane solution, respectively. The experimental error of γ value was within (0.05 mN m-1. For the calculation of interfacial tension, the densities of pure hexane and water were used instead of those of hexane solution and water in equilibrium

Figure 3. Total molality vs composition curves at constant interfacial tension: γ ) (1) 25, (2) 27, (3) 29, (4) 30, (5) 32, (6) 35, (7) 37, (8) 40, (9) 42, (10) 45, (11) 47, and (12) 49 mN m-1; (- - -) meq vs X2 curve. (Filled diamonds in the figure represent the data read from Figure 2.)

with each other,16,17 because the concentrations of hexane solutions were sufficiently low and mutual solubility between them was negligibly small. Results and Discussion Results of interfacial tension measurement are shown as the γ versus m curves at constant X2 in Figure 2. Data for different composition are progressively shifted downward by 2 mN m-1 (γ ) 50.3 mN m-1 at m ) 0 mmol kg-1). The γ value decreases gradually with increasing m, and slopes of the γ versus m curves change abruptly at two concentrations corresponding to phase transitions in the adsorbed film. In the low-concentration region, the curves of the mixed systems are very close to those of pure DFC8OH and TFC10OH systems. The variations of m value at given γ read from Figure 2 are clearly seen in Figure 3. The broken lines show the dependence of m values at the break

4566 J. Phys. Chem. C, Vol. 112, No. 12, 2008

Murakami et al.

Figure 4. Interfacial density vs total molality curves at constant composition: X2 ) (1) 0 (DFC8OH), (2) 0.200, (3) 0.400, (4) 0.600, (5) 0.800, and (6) 1 (TFC10OH).

points, meq, determined in the γ versus m curves on X2. It is realized that three states appear in the adsorbed film, which are divided by the broken lines, in the present mixture. Although the kinks on the m versus X2 curves (curves 6 and 7) at the phase transition points are hardly recognizable in contrast to those on the γ versus m curves, as shown later, this indicates that the interfacial density of adsorbed molecules changes largely and discontinuously at transition points but the change in film composition is not so noticeable. In order to assign the state of the adsorbed film, first, the total interfacial density, ΓΗ, was evaluated by applying the following equation

Γ ) -(m/RT)(∂γ/∂m)T,p,X2 H

(3)

to the γ versus m curves. Here ΓΗ is the sum of an individual interfacial density, ΓHi (i ) 1, 2), defined with reference to the two dividing planes making the excess numbers of moles of water and hexane zero concurrently, as

ΓH ) ΓH1 + ΓH2

(4)

Then, interfacial pressure π and mean area per adsorbed molecules A values were calculated by

π ) γ0 - γ

(5)

A ) 1/NAΓH

(6)

and

respectively. Results are illustrated as the ΓΗ versus m and π versus A curves in Figures 4 and 5. The ΓΗ value increases with increasing m and changes discontinuously at concentrations corresponding to the breaks on the γ versus m curves. The π versus A curves consist of three parts connected by two discontinuous changes. The A value in the almost vertical region converges into 0.3 nm2 which is almost equal to the crosssectional area of the FC chain, and therefore the adsorbed molecules are regarded to be very closely packed in this state like a two-dimensional solid. So, it is concluded that three states are assigned as the gaseous, expanded, and condensed states in decreasing order of the A value; adsorbed films at any composition of the mixed systems as well as those of pure DFC8OH and TFC10OH systems exhibit three states.

Figure 5. Interfacial pressure vs mean area per molecule curves at constant composition: X2 ) (1) 0 (DFC8OH), (2) 0.200, (3) 0.400, (4) 0.600, (5) 0.800, and (6) 1 (TFC10OH).

Now, let us discuss the mixing of DFC8OH and TFC10OH molecules in the adsorbed film. For doing this, first, we evaluated the composition of TFC10OH in the adsorbed film X H2 by applying the following equation18,19

X H2 ) X2 - (X1X2/m)(∂m/∂X2)T,p,γ

(7)

to the m versus X2 curves in Figure 3. Here X H2 is defined by

X H2 ) ΓH2 /ΓH

(8)

Then, the m values are plotted against X H2 together with the m versus X2 curves at given γ. Results are shown in Figure 6a for the gaseous and expanded states and in Figure 6b for the condensed one. These figures give the quantitative relationship of composition in between the bulk solution and the adsorbed film at given γ and are called the phase diagram of adsorption (PDA). Furthermore, in the case of the ideal mixing in an adsorbed film for a binary mixture of nonionic surface-active substances, the m versus X H2 curve in the PDA is a straight line connecting the molalities of pure components at the same γ expressed as18,20

m ) m01 + (m02 - m01)X H2

(9)

where m0i is a molality of pure component i. Concerning the PDA of the gaseous and expanded states shown in Figure 6a, it is noted that the m versus X H2 curves are almost linear, and therefore the mixing of DFC8OH and TFC10OH molecules is ideal both in the gaseous and expanded states. On the other hand, in the condensed state (Figure 6b), the m versus X H2 curve is convex upward; a deviation from the criterion expressed by eq 9 indicates that the mixing of DFC8OH and TFC10OH molecules is nonideal. This nonideal mixing in the adsorbed film is examined more quantitatively by evaluating the excess Gibbs energy of adsorption gH,E given by18,19

gH,E ) RT(X H1 ln f H1 + X H2 ln f H2 )

(10)

Here f Hi is an activity coefficient of component i, defined symmetrically as f Hi f 1 when X Hi f 1, and calculated by using the PDA and the following equation

f Hi ) mXi/m0i X Hi

(11)

Mixing of Fluoroalkanols in an Adsorbed Film

J. Phys. Chem. C, Vol. 112, No. 12, 2008 4567 and therefore the mixing of DFC8OH and TFC10OH molecules is energetically unfavorable in the condensed state. In order to understand the difference in the mixing states of DFC8OH and TFC10OH molecules depending on the interfacial density, we should consider the following two factors. (1) Dispersion interaction between hydrophobic chains of TFC10OH molecules is different from that of DFC8OH molecules due to the difference in extent of the fluorination of hydrophobic chains and their chain length. The mixing of these alcohols is accompanied by the loss of dispersion interaction energy and induces an increase in gH,E value.13 (2) Dipole-dipole interaction between hydroxyl groups with parallel orientation is repulsive and expected to be larger for DFC8OH than for TFC10OH because of larger dipole moments in hydroxyl groups of DFC8OH molecules compared to those of TFC10OH molecules due to the fluorination of β-carbons of DFC8OH molecules. The interchange energy concerning dipole-dipole interaction given by21

w ) ku1u2 - k(u12 + u22)/2 ) -k(u1 - u2)2/2

Figure 6. Phase diagram of adsorption (PDA) at constant interfacial tension in gaseous and expanded states: (s) m vs X2, (- - -) m vs X H2 curve; γ ) (1) 49, (2) 42, (3) 32, (4) 30, and (5) 25 mN m-1; (a) gaseous and expanded state, (b) condensed state. (Filled diamonds in the figures represent the data read from Figure 2.)

Figure 7. Excess Gibbs energy vs composition curves at γ ) 30 mN m-1: (1) DFC8OH-TFC10OH system; (2) TFC10OH-TFC12OH system.

When f Hi ) 1, eq 11 yields an ideal mixing line given by eq 9. The gH,E values calculated at γ ) 30 mN m-1 are shown as the gH,E versus X H2 curve in Figure 7 together with the corresponding curve obtained for the TFC10OH-TFC12OH system13 for comparison. The gH,E values are positive at any composition,

(12)

is negative, where k ) 1/(4π0rr3) is constant at given intermolecular distance r, 0 is the permittivity of vacuum, r is the dielectric constant of the medium, u1 () 3.56 D) and u2 () 2.69 D) are the dipole moments of hydrophilic groups of DFC8OH and TFC10OH, respectively, and the directions of the dipoles are assumed to be perpendicular to the interface. So the mixing of these alcohols reduces the repulsive force between dipoles and thus leads to a decrease in the gH,E value. An essential difference between the two factors is that the dispersion interaction is short-range interaction which is proportional to r-6, but the dipole-dipole interaction is long-range one as shown above. Since the mean distances between molecules are relatively large in the gaseous and expanded states, factor 1 is not effective to determine the state of the adsorbed film. Furthermore, factor 2 is also expected not to be effective in these states because of the large dielectric constant (r ≈ 78) of water. This is a striking contrast to the dipole-dipole interaction in our previous 1H,1H-perfluorononanol (FDFC9OH)-1H,1H,9H-perfluorononanol (HDFC9OH) system;12 the interaction between dipoles at the ω-position of hydrophobic chains acts via hexane (r ≈ 2) and is the predominant factor for the negative gH,E value. Consequently, the mixing of DFC8OH and TFC10OH in the gaseous and expanded states is almost ideal as we can see from the PDA in Figure 6a. In addition, it is possible that the expanded film is in a heterogeneous state which consists of condensed domains and a gaseous region as observed for the TFC10OH-1-icosanol system by X-ray reflectivity measurement.9 Spectroscopic techniques such as X-ray or neutron reflectivity measurements are useful to clarify the mixing state of molecules from the microscopic viewpoint. In the condensed film in which adsorbed molecules are arranged very densely with each other, the intermolecular dispersion interaction in addition to the dipole-dipole interaction becomes dominant for the mixing of molecules. So we can understand the positive gH,E values as a consequence of the dispersion interaction energy loss by factor 1 exceeding the energy gain by factor 2; DFC8OH and TFC10OH molecules prefer to gather with the same species rather than to mix with each other. Here we note that the gH,E value of the DFC8OHTFC10OH system is lower than that of the homologous TFC10OH-TFC12OH system. Since the polarization of hydroxyl groups of TFC10OH molecules are reasonably assumed to be equal to those of TFC12OH, a large gap between the gH,E values

4568 J. Phys. Chem. C, Vol. 112, No. 12, 2008 of the two systems is mainly due to the mixing of dipoles with different moments in the adsorbed films. The interchange energy concerning the dipole-dipole interaction estimated for the condensed state of DFC8OH-TFC10OH system is around -0.10 kJ mol-1 when u1 - u2 ) 0.87 D and r ) 0.6 nm. This is comparable in order with the magnitude of the gap between the gH,E values. The residual gap between two systems may be attributable to the difference in the interaction between hydrophobic chains. The energy loss of the dispersion interaction which causes the positive gH,E value is expected to be smaller for the DFC8OH-TFC10OH system than for the TFC10OHTFC12OH system, because difference in chain length of the fluorinated parts is smaller for the former than for the latter. Therefore, we can say that the hydrogenation on β-carbons in hydrophobic chains influences the balance of interaction between hydrophilic groups and that between hydrophobic groups of surface-active substances which governs the mixing of molecules in adsorbed films. Acknowledgment. This work was supported in part by the Grant-in-Aid for Scientific Research (C) of the Japan Society for the Promotion of Science (No. 19550021) and The Mitsubishi Foundation. References and Notes (1) Hayami, Y.; Uemura, A.; Ikeda, N.; Aratono, M.; Motomura, K. J. Colloid Interface Sci. 1995, 172, 142. (2) Takiue, T.; Yanata, A.; Ikeda, N.; Motomura, K.; Aratono, M. J. Phys. Chem. 1996, 100, 13743. (3) Takiue, T.; Yanata, A.; Ikeda, N.; Hayami, Y.; Motomura, K.; Aratono, M. J. Phys. Chem. 1996, 100, 20122.

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