10166
J . Phys. Chem. 1993,97, 10166-10174
Insoluble Perfluorinated Monolayers and Thin Films on the Surfaces of Corrosive Acids Jane K. Klapsen, Melissa B. Mitchell, Steven T. Covoni, and Gilbert M. Nathanson' Department of Chemistry, University of Wisconsin, I IO1 University Avenue, Madison, Wisconsin 53706 Received: May 27, 1993; In Final Form: July 21, 19936
Long-chain perfluoropolyethers spread spontaneously on concentrated sulfuric, nitric, and phosphoric acids, forming stable and insoluble films on these acid surfaces. On HzS04, the fluorinated ethers begin to spread at a concentration of 3.5 M, characterized by steady-state spreading pressures that rise continuously with acid concentration up to 9 mN/m at 17.4 M. Measurements of surface pressure versus area per monomer indicate that the long-chain perfluoroethers preferentially pack lying down on sulfuric acid at monolayer coverage. We find that the spreading pressures correlate most directly with the Hammett acidity of the acid subphase, suggesting that spreading is driven by hydrogen bonding between the acid and the ether oxygen or fluorine atoms of the perfluorinated ether molecules. Proton transfer may also occur at the highest acid concentrations but should play a minor role for the weaker or more dilute acids because of the extremely weak basicity of the fully fluorinated ethers.
Introduction Many simple molecules and polymers containing polar and nonpolar groups spread spontaneously on water, forming insoluble monolayers that adopt a wide variety of structures.I4 These compounds are surface active because the insolubility of the nonpolar segments opposes the solubility of the polar functional groups.' The resulting monolayers are often so compact that they can impede mass transport and reduce evaporati~n.~.~ The surfaces of corrosive acids provide a very different environment from those of dilute aqueous solutions. Reactive liquids like sulfuric and nitric acids expose surfacesthat protonateand oxidize a wide variety of substances, such that many compounds which spread also react and dissolve. However, there exists at least one type of surfactant, the perfluorinated polyethers (PFPE), which forms stable, insoluble films on the surfaces of concentrated and corrosive acids. In this paper, we investigate how the perfluorinated ethers spread on and bind to acid surfaces and show how this spreading varies with the identity and strength of the acid subphase. In particular, we explore evidencefor hydrogen bonding and proton transfer between the acids and PFPE and compare the contributions of the CF groups and the ether oxygen atoms to the spreading process. In a second study, we will report measurements of water penetration through PFPE films on concentrated sulfuric acid. Ongoing experiments indicate that micrometer thick films of PFPE on HzSO4 possess water permeation rates comparable to compact, long-chain alcohol monolayers on water.' These perfluorinated ether thin films may be useful for regulating interfacial transport, impeding water condensation, and extending the lifetimes of concentrated acids. Two groups have previously reported studies of monolayers on strong acids. Arnett and co-workers have shown that long-chain amides spread on sulfuric acid above 6 M and that the spreading pressure increaseswith the molarity of the acid, at least up to 14 Ma8s9The correlationbetween spreading pressure and the acidity of HzSO4 was interpreted in terms of protonation of the carbonyl oxygen of the amide group. They further demonstrated that the racemic amide spreads more extensively than either of the enantiomers, implying that the aggregation of molecules on acid surfaces can be very sensitive to their stereochemistry. More recently, Ahmad and Astin have explored olefin cyclization, alcohol dehydration, and ester hydrolysis on sulfuric acid at concentrations up to 8.5 M.10 Their studies show that reaction .Abstract published in Aduunce ACS Absrrucrs, September 1, 1993.
rates are often controlled by the extent of contact between the C = C , OH, or COOR groups and the acid surface. In particular, compression of the monolayer can severely reduce the reaction rate if the functional group is squeezed away from the surface or if there is insufficient room for the molecules to adopt reactive conformations. This paper explores inert, fluorinated ether monolayers and thin films on sulfuric acid, supplemented by studies of the ether spread on concentrated acetic, nitric, and phosphoric acids. Selected propertiesof the acids are listed in Table I at the highest concentrationsused in the experiments,along with data for H2O and 8.4M HzSO4. The concentrated acids are characterized by high molaritiesand acid mole fractions, although the formal mole fractions of water in nitric and phosphoric acids are substantial. The activities of unprotonated water are much smaller, and in 17.4M H2S04 the HzO activity is only 7 X 10-5.11Perhaps the most useful estimate of the strength of an acid is the Hammett acidity function Ho, which yields the fraction of protonated base through the equation log([BH+]/[B]) = PKBH+- H0.12J3The Hammett acidity scale is a logarithmic measure of the acid's ability to protonate weak bases and provides an extension of the pH scale to strong acids.14 The HOvalues in Table I show that sulfuric acid is much stronger (Ho more negative) than nitric, phosphoric, or acetic acids. In particular, sulfuric acid must be diluted to 8.4 M before it has the same Hammett acidity as the concentrated nitric and phosphoric acids used here.l3.*5 One goal of this paper is to explore correlations between film formation, the bulk acidities, and the surface properties of the acid^.^^^ Table I shows that there is no simple relationship between the reactivity of the bulk acid, as measured by Ho,and the attractive forces acting at the acid's surface, as gauged by the surface tension. Water is the least acidic of the liquids in Table I, but its surface tension is greater than that of concentrated acetic, sulfuric, and nitric acids. Table I also shows that at similar Hammett acidities of -3.8 to -4.0,the surface tensions vary from 77 mN/m for phosphoric acid to 56 mN/m for nitric acid. Our studies further demonstrate that, despite the tendency for high surface tension liquids to promote spreading of lower surface tension surfactants, there is no simple correlation between PFPE film formation and the surface tensions of the bare acids. Among the corrosive acids studied here, sulfuric acid most extensively promotes the spreading of organic molecules, including long chain hydrocarbons that do not spread on water. Many substances, however, react and dissolve within seconds upon contact with HzS04. The perfluorinated polyethers are an
0022-3654/93/209710166$04.00/0 Q 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 10167
Insoluble Films on Concentrated Acids
TABLE I Selected Properties of Concentrated Acids and Water acid HzsO4 HNO3 H3W4 CH3COOH H20
formal acid concentration wt% molarity 93.3 56.4 69 85.7 99.7 100
formal mole fractions acid H20
17.4 8.4 15.4 14.8 17.5 55.5
71.9 19.2 38.9 52.4 99.0 0
molecular
formula F-(CF(CF3)CF20)27(*w)-CFKFs
28.1 80.8 61.1 47.6 1.0 100
-9.6 -3.8
4.0 -3.9 -1 7
viscosity (CP)
56.9 75.1 56 77 27 72.4
20 4 2 33 1.3 1.0
Selected Prowrties of Krvtox 1625 surface tension viscosity (mN/m) at 25 ‘C (cP) at 20 OC
weight
density (g/mL) at 20 O C
4600
1.9
-
HQ
surface tension# (mN/m)
19
486
electronic polarizability
HOat
wb(A3) 3.8 (so42-) 4.5 (NO,-) 5.6 5.1 1.5
Mmip
3.5
-1.6
6.2 6.7 17.5
-2.0 -1.0 -1
compressibility (m2/N)
vaporization enthalpy (Id/mol)
10-9
116
As measured in the experiments. and NO3- polarizabilities from Hastel, J. B. Aqueous Dielectrics; Chapman and Hall: London, 1973; DD 171-172. H20, CH3COOH. and H3PO4 values calculated from refractive index data using the Lorenz-Lorentz quation. See ref 25. Molarity bi acid at minimum concentration rquired for spreading to occur. a
Figure 1. Space-filling model of a three-monomer fragment of the perfluoropolyether (PFPE) surfactant used in the experiments. The full chemical structure is CF3(CF(CFs)CF20)27(lvc)CF~CF3.
exception, forming stable monolayers and thin films that do not react even with boiling sulfuric acid. These fluorinated liquids are widely used as lubricants, hydraulic fluids, and vacuum pump oils in corrosive environments.16 Figure 1 shows a space-filling model of a fragment of Krytox 1625, the fluorinated ether used in the experiments. The full ether has the structure F[CF(CFj)CF20]27(aw)CF~CF3.The molecules do not possess separate polar and nonpolar “head“ and “tail” groups but are instead constructed from repeating perfluoroisopropylsegments bridged by oxygen atoms. These ether oxygens are in turn shielded by surroundingfluorineatoms which render the C-0-C bonds highly resistant to attack. The alternating CF3 groups and oxygen atoms disrupt thenormally tightly packed, helical structureof a straightchain perfluoroalkane: the CF3 groups inhibit close packing of the chains while the ether oxygens enhance the flexibility of the carbon ske1et0n.l~Together, thesedisruptions make the polyether a fluid at room temperature even when the number of monomer units exceeds 65.16 The properties listed in Table I show that PFPE has a surface tension of only 19 mN/m, in accord with its low dielectric constant and small enthalpy of vaporization of only =5 kJ per mole of monomer. Its vapor pressure of less than 3 X 10-10 Torr arises from the large number of monomer units in the chain, and it imposes evaporation rates of only 1 monolayer per hour in vacuum. The studies reported here indicate that PFPE monolayers on acids survive for at least several days in air without detectable evaporation. Two sets of surface tension measurements are reported for Krytox 1625 on sulfuric acid. In each case, we measure the surface pressure T = Yadd - YPWE film, where Yadd is the surface tension of the bare acid and YPWE film is the surface tension of the acid with added PFPE. In one set of experiments, we record u as a function of PFPE concentration on 10 and 16 M H2S04 at values below and above the density of 1 monolayer. These u values aid in determining the orientation and adsorbed area of the fluorinated ether chains at close-packing densities on the ~urface.1~2.18~19
In the second set of experiments, PFPE is placed on the acid in substantial excess of 1 monolayer. The surface pressure attains a constant value after the spread film reaches equilibrium with the excess bulk surfactant. This value of u is defined as the equilibrium spreading pressure (ESP), and it is equal to the reduction in work per unit area required to expand the film covered surface reversibly at constant temperature, pressure, and surface ~oncentration.~8~1~ For insoluble monolayers, this free energy reduction occurs when molecules escape from the bulk surfactant and bind to the surface of the liquid. This spreading can also be viewed as a hypothetical, two-step process in which molecules first vaporize from the bulk surfactant and then adsorb from the gas phase onto the liquid at their equilibrium configurations. The ESP is thus equal to the difference in stabilities, or free energies, between the bulk surfactant and the equilibrium spread film with respect to gas-phase PFPE. By applying the same surfactant to different acid solutions, we vary only the monolayer-subphase interactions and focus on the second adsorption step in the spreading process. This procedure allows us to explore the geometry and bonding of fluorinated ether chains to acids of differing concentration and composition. For viscous liquids composed of long-chainmolecules like PFPE, the approach to equilibrium may be very slow since entangled or partially buried chains will encounter large barriers in moving between the bulk and surface The surface tension reductions reported here are therefore more properly labeled as steady-state spreading pressures (SSP), which refer to pressures generated by films in contact with excess PFPE for long times (10 or more hours in our experiments). All spreading pressures measured in the presence of excess perfluorinated ether are referred to as SSP values, although we believe that they should be very close to the equilibrium values. We find below that the fluorinated ether chains preferentially pack lying down on the acid surfaces at monolayer coveragesand that the SSP values for PFPE films on strong acids vary almost linearly with the Hammett acidities of the acids. The fluorinated ethers, however, are extremely weak bases and therefore are unlikely to be protonated. The correlation between spreading pressure and Hammett acidity may thus suggest that hydrogen bonding between the underlying acid and the ether oxygen and fluorine atoms is responsible for the adhesion between the perfluorinated ether and the acid surface. The surface pressures we measure indicate that the PFPE-acid interactions generate free energy reductions similar, for example, to hydrocarbon ethers spread on pure water.21.22 Although the SSP values are low, they are sufficient to promote formation of stable, micrometer thick PFPE films which can impede condensation of water vapor even in strongly desiccating acids like H2S04.
Klassen et al.
10168 The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
Experimental Section The experiments comprise surface pressure measurements of sulfuric acid solutions of different molarities covered by PFPE films. For comparison, surface pressures are also reported for PFPE films on concentrated nitric, phosphoric, and acetic acids. The sulfuric acid solutions were mixed from 95-98 wt % H2S04 (A.C.S. reagent grade, Fisher Scientific) and from purified water (Milli-Q filtering system, Millipore, resistivity >17 Mil cm). The sulfuric acid, nitric acid (69-71 wt %, A.C.S. reagent grade, Fisher Scientific), and phosphoric acid (85.7 wt %, analytical reagent, Mallinckrodt) were not purified further except to aspirate the acid surface before spreading the surfactant. Perfluorohexane (PCR Inc.) was used as the spreadingsolvent for the PFPE (Krytox 1625, Du Pont). Both chemicals were used as received. The solid and liquid perfluoroalkanes described in the final section of the Discussion were also purchased from PCR Inc. They are listed at a minimum purity of 97% and were used without further purification. Surface tensions were measured using the Wilhelmy plate methode2 Three 0.1 mm thick by 8 mm high platinum plates were employed with perimeters of 35.7 f 0.2,35.40 f 0.12, and 35.32 f 0.12 mm. The tensiometer (Model ST900 modified with a 250-mg counterweight, NIMA Technologies, Coventry, England) was calibrated to produce a voltage in the range 80104 mg/V over the course of the experiments. A Fluke 45 voltmeter recorded measurements in 30 and 60-s intervals for P A and SSP experiments, respectively. All experiments were performed inside a closed lucite box maintained at 2 1.1 f 0.4 C. The box was often flushed with dry N2 to adjust the humidity before the experiments commenced. Acid samples were then placed in the box and allowed to equilibrate for 20 min before aspirating the surface to remove impurities. The Wilhelmy plate was lowered to the surface and allowed to equilibrate with the acid and the surrounding air for up to 14 h before the addition of the surfactant. The SSP measurements were performed by adding PFPE to the acid surface in substantial excess of 1 monolayer. A 10-pL aliquot of a PFPE solution (48 mg PFPE/mL perfluorohexane) was placed in 2-4 equally spaced additions around the perimeter of a 62.9-63.6 cm2 pyrex dish containing 50-75 mL of acid. These additions resulted in an average PFPE area of 0.2 A2/ monomer. This area is -150 times smaller than the 30 Az/ monomer value predicted for PFPE chains lying parallel to the surface and 4 times smaller than the 1.1 Az/monomer value for vertically oriented chains. Thus, PFPE is present in excess of one monolayer for both geometries. Separate tests performed with a single addition of =0.1 mL of PFPE yielded identical SSP values. The spreading pressures were monitored over periods of 2 4 8 h. At the end of each run, the acid was titrated to determine its concentration. Similar procedures were followed in the *-A experiments, except that the 18 surfactant additions were added in 1-h intervals to a 170.0 cm2 dish using aliquots ranging from 3 to 30 pL of a PFPE solution (0.6 mg PFPE/mL perfluorohexane). All glassware used in the experiments was soaked in a Nochromix bath (Godax Laboratories) and thoroughly rinsed with Millipore water. The platinum plate was stored in a 50% nitric acid-50% sulfuric acid bath between experiments and was flamed and periodically sandblasted before storage. In order to determine if any impurities exist in the spreading solvent that would have altered the surface pressure, we spread 0.5 mL of perfluorohexane on 4 M and undiluted H2SO4. After solvent evaporation, y changed by less than 0.1 mN/m on both solutions.
Resulte: Spontaneous Spreading, SSP Values, and
Pressure-Area Measuremenb Visual Observations of Spontaneous Spreading. Initially, we performed visual tests to determine if the perfluorinated ethers
spread on acids, bases, salt solutions, or organic liquids.18 These tests were carried out by dusting the surface of the subphase with teflon powder and then placing a drop of PFPE on the surface. The teflon flakes remain stationary in the absence of a spread film but move away from the droplet if the film spreads. These spreading tests determine if a thick surfactant film will spread initially on the surface. Thermodynamically, film formation occurs when the initial spreading coefficient, S = - (YPPPE + Yacid-PwE), is greater than zero, where yadd-pF-pE is the liquidliquid interfacial tension.18 Negative initial spreadingcoefficients do not imply that the surface ultimately remains free of surfactant, but typically only very dilute monolayers will form when S is less than zero.18 The spreading tests show that PFPE films do not form spontaneously on pure water, on solutions of NaOH or NH,OH, or on solutions of NaCl or Na2S04. PFPE also does not spread on the organic solvents diethyl ether, butanol, acetone, or acetonitrile, but it does spread sluggishly on carbon tetrachloride and chloroform. PFPE also spreads sluggishly on concentrated acetic and formic acids but rapidly on nitric, hydrochloric, phosphoric, and sulfuric acids. The minimum acid molarities M m i n for spontaneous spreading are listed in Table I. Sulfuric acid displays the lowest threshold concentration of 3.5 M, while even a small dilution of acetic acid inhibits spreading. The final column in Table I displays the Hammett acidities corresponding to the threshold concentrations, showing that the values cluster between HO= -1 and -2. We note that PFPE's lack of surface activity on water has recently been explored by Frank and coworkers, who showed that fluorinated ethers can be tethered at the air-water interface by functionalizing them with CHzOH or COOH end groups.23 The spreading tests also show that PFPE in excess of the spread film collects into patches on the surface which oscillate in shape and color as their thickness changes. This patch formation is in contrast to the spreading of many surfactants on water, where the excess surfactant often contracts into discrete droplets.2J8 When excess PFPE is placed on a sample of concentrated sulfuric acid and the surface is then dusted with teflon, a second drop of PFPE applied to the film covered acid appears to spread slowly across the ~urface.~J* This implies that bulk PFPE may spread on the PFPE-acid interface, forming a thick or 'duplex" film. If the SSP values are close to the equilibrium spreading pressures, then the SSP is equal to the initial spreading coefficient S,making possible a determination of the acid-PFPE interfacial tension, Yacid-PFPE = Yacid Y ~ F ~ E SSP.4J* Because of our uncertainty in interpreting this unusual result,4J8 we postpone further analysis until the interfacial tension can be measured independently. Visual tests and NMR analysis indicate that the perfluoropolyethersare insolublein concentrated sulfuricacid. A solution of 17 M H2SO4 shaken with Krytox 1625 showed no evidence of fluorine contamination as determined by 19F NMR. By comparison with the NMR spectrum of a 0.1 M KF solution in H2SOC we place a lower limit on the solubility of Krytox 1625 of [monomer]/[HzS04] < 2 X 1 V . This insolubility is not a characteristic of the oligomer alone. A dimer fragment of PFPE which differs by one hydrogen, F[CF(CF~)CF~O]~CHFCFS, also does not dissolvevisibly in the concentrated acid. NMR analysis places a lower limit for the dimer solubility of [dimer]/[H2SO,]
