18458
J. Phys. Chem. 1996, 100, 18458-18463
Langmuir Monolayers with a CF3 Group in the Hydrophilic Head. Monolayers of Trifluoroethyl Ester of Behenic Acid Jordan G. Petrov*,† and Helmuth Mo1 hwald Max-Planck Institute of Colloids and Interfaces, Rudower Chaussee 5, 12489 Berlin, Germany ReceiVed: May 10, 1996; In Final Form: August 22, 1996X
Insoluble monolayers of the trifluoroethyl ester of behenic acid possessing a terminal CF3 group of the ester radical of the hydrophilic head are studied at the air-water interface. The surface pressure-area isotherms show a formation of condensed monolayer. The relatively low collapse pressure and significant reduction with time of the molecular area at constant surface pressure illustrate considerable decrease of the monolayer stability when compared to the one of the ethyl behenate monolayers. The CF3 terminal of the ester radical reverses the sign of the surface potential and the vertical dipole moment component; negative values of ∆V and µ⊥ are obtained for the trifluoroethyl behenate monolayer, while positive values of ∆V and µ⊥ are characteristic for other neutral behenyl chain derivatives such as methyl and ethyl behenate, behenyl alcohol, undissociated behenic acid, and behenyl amine. This fact demonstrates the determining role of the hydrophilic head for these electrostatic properties. The new result is important for the most widely used interpretation of the surface potential by means of the three-capacitor model, which distinguishes among the contributions of the hydrocarbon chains, hydrophilic heads, and hydration water of the condensed monolayer. When compressed beyond the collapse point, the monolayer of the trifluoroethyl behenate forms a bilayer. The observed increase of ∆V after collapse implies a “heads-to-tails” structure leading to a summation of the vertical dipole moments of the two monolayers. Such a structure is probably due to the opposite orientation of the dipole moments of the head groups and the ω-methyl groups of the trifluoroethyl behenate. This collapse mechanism basically differs from the one of the fatty acids, alcohols and other long chain amphiphiles where “heads-to-heads” bilayers are formed atop of the first monolayer.
Introduction Insoluble monolayers of ω-halogenated fatty acids, amines, and alcohols spread at the air-water interface or adsorbed on solid surfaces have been studied since the middle of the 30’s.1-10 They attract scientific interest because the substitution of one or more hydrogen atoms of the terminal CH3 group by halogens significantly changes their mechanical and electrostatic properties. The nonsubstituted substances form stable condensed monolayers on water that show positive surface potentials, while most of the monolayers of their ω-halogenated derivatives are considerably less stable and exhibit negative ∆V values. This difference was demonstrated first by Frumkin and co-workers1,2 who investigated spread monolayers of 16-bromohexadecanoic acid and found large negative values of the ∆V potential. Davies3 and Davies and Rideal4 confirmed this result and showed that the same substance gives a negative surface potential also at the oil-water interface. Fox5 has found that surface potentials of monolayers of ω-trifluorooctadecanoic acid and ω-trifluorooctadecylamine at the air-water interface were also strongly negative. Barnet, Jarvis, and Zisman6 studied spread monolayers of ω-monohalogenated acids and alcohols and determined the specific contribution of different X-CH2 groups (X ≡ F, Cl, Br, or I) to the negative dipole moment of the monolayer. In a series of investigations Zisman and co-workers studied the wettability of solid substrates covered by monolayers of fatty acids and amines with ω-trifluoro groups11 or perfluorinated hydrocarbon chains12,13 and showed that they are strongly * To whom correspondence should be addressed. † On leave of absence from Institute of Biophysics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 21, 1113 Sofia, Bulgaria. X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)01359-7 CCC: $12.00
hydrophobic. On the other hand, surface force measurements14,15 and other experimental data16-18 lead to the conclusion that interaction between methylated or perfluorinated surfaces in water possesses a strong and long-range non-van der Waals attractive component. Although the origin of this “hydrophobic force” is still a matter of debate, some authors relate it to the water structure at the interface.15 Compiling the results of the above investigations, one comes to the curious question of how an incorporation of a CF3 group in the hydrophilic head would affect the mechanical and electrostatic properties of a monolayer at the air-water interface. Would it also decrease its stability, and if so, why? Is it because of the stronger lateral repulsion, as usually assumed for the ω-halogenated monolayers,6 or due to the weaker adhesion toward water? Would the surface potential of such monolayers be significantly altered or even reversed in sign (when compared with the corresponding nonfluorinated amphiphilic compound), or would this change be minor as one might expect from the close ∆V values of condensed monolayers with different neutral hydrophilic heads and the same hydrocarbon chains?19 Would the structure of hydration water in the vicinity of the hydrophilic groups differ for fluorinated and nonfluorinated heads, and would this difference play an important role for the surface potential and dipole moment in contrast to the widely held opinion, based on the three-capacitor model of Demchak and Fort, that the contribution of hydration water is almost negligible?8,19 In this paper we report an investigation of Langmuir monolayers of trifluoroethyl ester of behenic acid as a first attempt to answer some of these questions. The long C21H43 hydrocarbon chain was chosen to stabilize the fluorinated monolayer opposing the van der Waals chain-to-chain attraction to the strong repulsion between the equally oriented dipoles of © 1996 American Chemical Society
Langmuir Monolayers the head groups. The neutral hydrophilic head avoids double layer effects in the aqueous phase, thus simplifying the interpretation of the surface potential and dipole moment.19 The classical liquid substrate, pure water, was chosen for this study, and the effects of pH and indifferent and chaotropic ions (breaking the hydration water structure) on surface pressure and surface potential isotherms have been further examined. Analysis of monolayer dipole moments based on semiempirical quantum mechanical molecular models of the fluorinated and nonfluorinated molecules in vacuo is also in progress. The effective dielectric constant in the region of the head groups will be estimated from the pKi of the interfacial polarity probe embedded in the trifluoroethyl behenate monolayers as done in some previous investigations for other neutral monolayers at the air-water9,10,21 or solid-water interface.20,22,23 This series of studies should help to distinguish among the relative contributions of the hydrocarbon chains, hydrophilic heads, and hydration water to the electrostatic properties of condensed monolayers at the air-water interface.7-10,19 More generally, it could serve to construct a realistic model of the electrostatic field profile of neutral biointerfaces and biomembranes.24
J. Phys. Chem., Vol. 100, No. 47, 1996 18459
Figure 1. Isotherms of surface pressure versus area per molecule. The overlap of the individual curves illustrates the reproducibility of the mechanical properties of trifluoroethyl behenate monolayer on water.
10 mV in the condensed part of the isotherm were considered as representative.
Experimental Section
Results
Materials and Methods. The trifluoroethyl ester of behenic acid was synthesized by Dr. R. Wagner and Mrs. Y. Wu at our institute from behenyl chloride and trifluoroethanol.25 The reaction was performed in dioxane with an equimolar amount of N(C2H5)3 added to bind HCl:
Surface Pressure-Area Isotherms and Stability of the Monolayer. Figure 1 shows three isotherms of surface pressure, π, versus area per molecule, F, recorded at a compression speed of 0.033 nm2 molecule-1 min-1. They are smooth without kinks, which would indicate phase transitions, and are located at molecular areas characteristic of a condensed monolayer. The individual curves overlap within 0.001 nm2 until the point corresponding to 0.191 nm2 and 32.5 mN/m, where the second derivative, d2π/dF2, reverses sign. The divergence beyond that point is probably due to a significant collapse. The characteristic maximum of the surface pressure appearing at 0.186 nm2 and 36-38 mN/m indicates a plastic failure or “fracture” of the monolayer.26 Its value, πfr, depends on the compression speed26 and is somewhat irreproducible even under the same dynamic conditions (see Figure 1). Enlarged representations of the high- and the low-pressure parts of the isotherm show that the high-pressure section between 0.191 and 0.198 nm2 is linear. At areas larger than 0.198 nm2 a gradual transition from the solid to the liquid condensed state occurs. The surface pressure starts to rise at about 0.52 nm2, and its smooth increase becomes less steep at ∼0.42 nm2. At this monolayer density the transition between the vaporexpanded state (described by Adam for monolayers of long chain esters25) and the liquid-condensed state seems to begin. The transition ends at about 0.22 nm2 with a strong increase of the surface pressure due to the much lower compressibility of the LC phase. A significant difference exists between the isotherms at compression and expansion (Figure 2). The hysteresis cycle was recorded up to 30 mN/m at the same speed of 0.033 nm2 molecule-1 min-1. The compression isotherm overlaps with the one given in Figure 1, but the expansion isotherm is considerably different. It has a very steep initial part followed by two other almost linear sections with lower compressibility. Two transition points, one at 17.5 mN/m and 0.193 nm2 and the other at 4.5 mN/m and 0.197 nm2, can be clearly distinguished in this mode. The second hysteresis cycle of the same monolayer entirely superimposes the first one. Figure 3 illustrates the stability of the trifluoroethyl behenate monolayer at 15 mN/m (curve 1) and 30 mN/m (curve 2). The compression at 0.033 nm2 molecule-1 min-1 was continued up to the given surface pressure, and then the decrease of the molecular area with time was followed, keeping π constant.
C21H43COCl + HOCH2CF3 + N(C2H5)3 f C21H43COOCH2CF3 + HCL‚N(C2H5)3 The product was recrystallized 5 times in dry ice from a diethyl ether-benzene mixture (bp 30-60 °C) and finally had a melting point of 55-56 °C. It was characterized by gas chromatography that showed 98% purity and by elemental analysis confirming the expected amounts of C and H within 0.2%. The substance was dissolved in chloroform with addition of several drops of ethanol to a concentration of 1 × 10-3 M. Typically, 100 µL of that solution was spread on the aqueous substrate in a Langmuir trough and left for 2 min before compression for evaporation of the solvent. The computerized Lauda film balance FW-2 was used. Its dynamometric system with a floating barrier registered the surface pressure with an accuracy of 0.1 mN/m. The Teflon trough had a large analytical area of 927 cm2, enabling a very accurate determination of the area per monolayer molecule. A thermostating plate underneath the trough maintained a constant subsolution temperature of 20.0 ( 0.1 °C, and a metal serpentine with circulating water kept the air temperature in the closed cabinet also constant and close to that value. Surface pressure-area isotherms, variation of the molecular area with time at constant surface pressure, and compression-expansion hysteresis curves of surface pressure versus molecular area were recorded with this equipment. The π/F isotherms were triplicated and the hysteresis curves were duplicated in order to check the reproducibility of the properties studied. The measurements of the surface potential, ∆V, were performed with the vibrating capacitor method also at 20 °C . Another Langmuir balance equipped with a smaller Teflon trough and a Wilhelmy plate dynamometric system registering the surface pressure within (0.2 mN/m was used in this case. An accuracy of 5 mV was typical for surface potential measurements. Three to five surface potential-area dependences were usually recorded, and only those overlapping within
18460 J. Phys. Chem., Vol. 100, No. 47, 1996
Petrov and Mo¨hwald
Figure 4. Surface potential (curve 1) and vertical dipole moment (curve 2) versus area per molecule of the trifluoroethyl behenate monolayer. Figure 2. Compression-expansion hysteresis of the surface pressurearea isotherms.
Figure 3. Variation with time of the molecular area at constant surface pressure of 15 mN/m (curve 1) and 30 mN/m (curve 2), illustrating the stability of the trifluoroethyl behenate monolayer at the air-water interface.
Curve 1 and the initial stages of curve 2 show an accelerated area loss that, according to Brooks and Alexander27 and Smith and Berg,26 indicates a monolayer collapse due to nucleation and growth of a bulk solid phase. However, the latter stages of curve 2 exhibit an opposite curvature and a decreasing rate of the area decay, which implies a transition to a more stabile formation. When the relaxation time is made long enough, the final area per molecule asymptotically approaches half the initial value. This fact infers that a bilayer of trifluoroethyl behenate is formed at the air-water interface that is more stable then the condensed monolayer. Surface Potential-Area and Dipole Moment-Area Isotherms. Figure 4 presents an isotherm of surface potential, ∆V, versus molecular area, F, recorded at a compression speed of 0.033 nm2 molecule-1 min-1 (curve 1). Curve 2 shows the vertical dipole moment of the monolayer, µ⊥, evaluated from the Helmholtz formula
µ⊥ ) 0F∆V
(1)
where 0 is the permittivity of vacuum and is the mean permittivity of the monolayer assumed to be 1. The π/F isotherm registered simultaneously is also included for comparison (curve 3). The negatiVe sign of the surface potential at all molecular areas considered is the most interesting result of this study. Since ∆V for a monolayer of an ethyl ester of a long chain fatty acid is positiVe (see the next section), the negative ∆V value of the
Figure 5. Surface potential-area (curve 1) and surface pressurearea (curve 2) isotherms recorded beyond the collapse point. The continuing increase of ∆V and π after collapse implies a formation of a bilayer under compression.
trifluoroethyl behenate shows that the substitution of the CH3 group of the ethyl radical by a CF3 group reverses the sign of ∆V. The absolute value of ∆V increases monotonically and smoothly during compression, in contrast to the ∆V/F dependences of the nonfluorinated monolayers exhibiting irregularities at areas larger then 0.22 nm2/molecule. The corresponding variation of the vertical dipole moment, µ⊥, at compression is illustrated by curve 2. Its absolute value continuously increases at areas below ∼0.50 nm2 with a linear part between 0.425 and 0.203 nm2. At 0.203 nm2 the slope of the µ⊥/F dependence abruptly increases. The following steep linear part ends at 0.198 nm2 with the transition to the extremum characteristic for the “fracture” of the monolayer. Formation of a Bilayer at the Air-Water Interface. Figure 5 presents a section (between 0.350 and 0.125 nm2) of a couple of π/F and ∆V/F dependences for which compression was continued beyond the collapse point. The monolayer was spread at a small initial density, and the compression was performed at the lowest available speed of 0.016 nm2 molecule-1 min-1. These conditions were chosen in a hope to decrease the initial monolayer heterogeneity and to diminish the kinetic effects during the monolayer-to-multilayer transition at compression. Unfortunately, because of the small Langmuir trough used for the ∆V/F measurements, it was not possible to simultaneously achieve a very low initial density and considerable degree of compression beyond the collapse point. Figure 5 shows that a second increase of the surface pressure at 0.150 nm2 follows after the characteristic collapse maximum in the π/F dependence. The negative surface potential continues to increase beyond the collapse point, in contrast to the usual
Langmuir Monolayers
J. Phys. Chem., Vol. 100, No. 47, 1996 18461
TABLE 1: Comparison of Some Mechanical and Electrostatic Characteristics of the Trifluoroethyl Behenate Monolayer with the Corresponding Values for Other Neutral Monolayers of Behenyl Derivatives Obtained under the Same Experimental Conditionsa
a
monolayer substance
πfr (mN/m)
(∆F30mN/m/F30mN/m)15 min (%)
F30mN/m (nm2)
∆V30mN/m (V)
µ⊥,30mN/m (D)
trifluoroethylbehenate behenyl alcohol methyl behenate ethyl behenate
∼36 ∼65 ∼50 ∼64
∼50
