Langmuir 1988,4, 732-735
732
6 Au
Figure 5. Schematic diagram of the structure of the underpotentially deposited copper adlayer on gold(ll1)at full monolayer coverage.
Conclusion With grazing incidence geometry and fluorescence-detected S E W S , unambiguous structural determinations for monoatomic adlayers a t a metal/liquid interface can be made in situ. For copper underpotentially deposited on gold(ll1) at full coverage, copper-copper and copperold distances were found to be 2.92 f 0.03 and 2.58 f 0.03 ,respectively. This, coupled with the EXAFS intensity, shows that the adlayer has a (1x1) structure with the
1
copper atoms occupying 3-fold hollow sites. Both the position and shape of the absorption edge show that the adsorbed copper atoms are fully reduced. Unlike measurements where the polarization of the incident beam was perpendicular to the electrode surface, no scattering from adsorbed oxygen was observed. This shows that the sulfate ions or water molecules are adsorbed on "atop" sites. The ability to make structural determinations in situ should significantly advance our understanding of the metal/ electrolyte interface.
Acknowledgment. This work was partially supported by the Office of Naval Research, the Materials Science Center at Cornell University, and the Army Research Office and was carried out at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the NSF. H.D.A. is the recipient of a Presidental Young Investigator Award and a Sloan Fellowship. The CuAu, and Cu3Au alloys were graciously supplied by Prof. B. W. Batterman. The help of W. Ausserer in performing SIMS is gratefully acknowledged. Registry No. Au, 7440-57-5;Cu, 7440-50-8.
Design and Synthesis of Novel Fluorinated Surfactants for Hydrocarbon Subphases Alan R. Katritzky,* Terry L. Davis, and Gordon W. Rewcastle Department of Chemistry, University of Florida, Gainesville, Florida 3261 1
Glenn 0. Rubel US. Army Chemical Research, Development and Engineering Center, Aberdeen Proving Ground, Maryland 21 010-5423
Mike T. Pike Commercial Chemical Division, 3M Company, S t . Paul, Minnesota 55144-1000 Received April 1 , 1987. In Final Form: January 15,1988 A variety of perfluorooctanesulfonamido derivatives of piperazine have been synthesized and investigated as potential surfactants for the limitation of evaporation of hydrocarbon subphases. Measurements of surface tension on diesel fuel showed significant reductions in surface tension for several of these compounds. The postulated structural requirements for the optimum effect include a fluoroalkyl group, a rigid polar central section, and a lipophilic alkyl or aryl-alkyl substituent.
Introduction It has long been known'-, that certain surfactants can effectively limit evaporation of aqueous subphases. (This subject was addressed in detail at the Symposium on the Retardation of Evaporation by Monolayers, New York City, September, 1960.4) Such behavior implies very highly oriented, closely packed surfactant films. Our goal in this research has been to demonstrate the same phenomenon on hydrocarbon subphases and in particular to achieve the retardation of evaporation (stabilization) of hydrocarbon aerosols. We have been able to show that conventional Surfactants are also capable of inhibiting the evaporation (1) Rideal, E. K.J. Phys. Chem. 1926,29, 1585. (2) Langmuir, I.; Langmuir, D. B. J . Phys. Chem. 1927, 31, 1719. (3) Sebba, F.; Briscoe, H. V. A. J. Chem. SOC.1940, 106. (4) Retardation of Evaporation by Monolayers: Transport Processes; La Mer, V. K.,Ed.; Academic: New York, 1962. Proceedings of the Symposium on the Retardation of Evaporation by Monolayers, New York City, September, 1960.
of aqueous aerosol droplets: one of us used a single particle suspension technique to measure the evaporation rate of aqueous droplets with adsorbed monolayers of hexadecan01.~ It was shown that the droplet evaporation rate could be significantly reduced by adsorbing a solid monolayer of hexadecanol onto the droplet surface.6 Although hydrocarbon-active surfactants have apparently long been sought by the petroleum industry, to date no agent effective as an evaporation inhibitor has been forthcoming.' Such surfactants have obvious applications in fuel economy, fire control, and suppression of flammability? it is furthermore our hope that such agents will be efficient surfactants on nonpolar substrates in general. (5) Rubel, G. 0.; Gentry, J. W. J. Phys. Chem. 1984,88, 3142. (6) Rubel, G. O.,unpublished results. (7) Cross, J., Exxon Oil Co., personal communication, 1985. (8) Blake, G. B.;Albrecht, A. H. 'Perfluoroalkyl Surface Agents For Hydrocarbon Systems", a special report to the ACS from 3M Central Research.
0743-7463/8S/2404-0732$01.50/00 1988 American Chemical Society
Langmuir, Vol. 4, No. 3, 1988 733
Design and Synthesis of Fluorinated Surfactants Crude experiments with certain fluorinated surfactants have been claimed to suppress the evaporation of beakers of gas0line.8*~ However, the failure of these agents to suppress the evaporation of diesel fuel aerosols3is taken to indicate that this behavior is not due to a surfactant monolayer. These droplet evaporation experiments were carried out with an electrodynamic particle suspension device recently developed by one of US.^ The molecular design of surfactants for hydrocarbon systems is nontrivial. Lyophobic groups are transported to the surface of a bulk phase: this results in a decrease in the free energy of the system.1° In an aqueous bulk phase, such an increase in free energy results because the large-scale restructuring of the bulk phase caused by the presence of the lyophobe is thus alleviated (the "hydrophobic bond")." In h y d " bulk phases, there is little or no hydrogen bonding, and polar lyophobic groups do not necessarily affect restructuring; hence they do not necessarily lead to surface activity. Furthermore, since the surface tension of hydrocarbon liquids is quite low (ea.27 dyn/cm),12stringent demands are made upon the design of the surface-active agent. Surfactanta are broadly of three types,'3 depending upon their organization with respect to the interface: (i) parallel to the surface, (ii) orthogonal to the surface, and (iii) at some angle to it. Agents that successfully inhibit evaporation of aqueous phases are typically l-alkan~ls'~ or 1alkanoic acids2s3 with chain lengths of 14 carbons or longer;1SJ6all of these are of type ii and form phases characterized as "solid phase" monolayers."J8 Surfactants forming films of type ii ("vertical films") give an area per molecule on the order of the cross sectional area of the molecule. Any deviation from orthogonality will increase this number somewhat. Parallel film formers are, by comparison to solid-phase films, "gaseous film" formers, giving phases which are nonresistant to compression. Parallel films are recognizable by their ready compressibility with limiting area per molecule of very low dimensions and often by their characteristic birefringence. Poly(dimethylsiloxanes)form parallel polymolecular films on hydrocarbons, but they do not suppress evaporation to any great degree until the film attains niacromolecular proportions, and even then are not very effective.lg A problem that is encountered in the design of surfactants for hydrocarbon subphases is that of the adhesional/cohesional balance: in short, in order to spread efficiently on a given subphase, the work of adhesion (to the interface) must exceed the work of cohesion (between surfactant molecules).20 Since the maximum change in surface tension, and therefore adhesion, that can be sustained is relatively small in hydrocarbons (ca. 27-11 = 18 dyn/cm) relative to aqueous subphases, the maximum cohesional forces that can be sustained are also of greatly (9) Bryce, H. G. In Fluorine Chemietry; S i o n s , J. H., Ed.;Academic: New York, 1961, Vol. 2, pp 297-498. (10) h e n , M. L. Surfactants and Interfacial Phenomena; Wiley: New York, 1978. (11) Tanford, C. L. The Hydrophobic Effect; Wdey New York, 1973. (12) Holzleiter, R. Report to First Diesel Fuel Chemical Conference, Aberdeen Proving Grounds, MD, January, 1986. (13) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic: New York, 1963; p 226. (14) Gaines, G. L., Jr. J. Colloid Sci. 1964,19,679. (15) Gaines, G. L., Jr. Imoluble Monoluyers at Liquid-Gas Interfaces; Interscience: New York, 1966; p 212. (16) Archer, R.; La Mer, V. K., J. Phys. Chem. 1965,60, 348. (17) Harkme, W. D. Physical Chemistry of Surface Films; Reinhold New York, 1952; pp 106-107. (18) Dervichian, D. G. J. Chem. Phys. 1939,7,931. (19) Bernett. M. K.: HelDer, L. A.: Jarvie, N. L.: Thomas, T. M. Znd. E G . Chem. Fundam. 1970: 9,.150. (20) Reference 17; p 97.
Table I. Surface Tension Depression by Zonyl Surfactants in Diesel Fuel"
Zonvl FSA FSC FSE FSJ
FSK FSN FSP
Ayldiesel fuel, structure (R, d-d .c m - = C.F*+,; x = 3-61 RfCH2CH2SCH2CH2COz-Li+ 0.5 [R~CH~CH~SCHZCHZN+(CH~)~~~SO~'-6.4 (R&H2CH20)2P(O)O-NH4' 4.9 FSP +-nonfluorinated 9.6 R&H~CH~CH(OCOCH&N+(CH~)ZCH~COZ4.6 RfCH2CHZO(CH&H20),H 0.4 (RfCH,CH,O)P(O)(0-)2(NH1+)2 0.7
reduced magnitude. Since significant cohesional force must be sustained in order to organize and maintain a closest-packed, solid-state monolayer, it is our suspicion that this point is the actual crux of the problem. The film orientation may often be predicted for a given subphase on the basis of structural features. General structures for a parallel or a vertical film of these two types might be represented as vertical: (lyophile)-(lyophobe) 1
parallel: (lyophobe)-(lyophile)-(lyophobe) 2 Previous research along these lines12has employed Zonyl surfactants manufactured by E. I. du Pont de Nemours, Inc., of structures shown in Table I. Although these agents did show surface activity on diesel fuel, they have failed to inhibit evaporation of aerosols.6 From an aqueous to a hydrocarbon bulk phase, the identities of the lyophilic and lyophilic groups are usually inverted: hence the inapplicability of aqueous system structural paradigms to hydrocarbon systems. Inspection of the structures of the Zonyls (Table I) shows that these compounds, primarily intended for use on aqueous subphases, would not be expected to form vertical films on hydrocarbons. The results shown for Zonyl FSJ indicate that the nonfluorinated surfactant component in this system probably increases the interaction (effective solubility) of Zonyl FSP at the interface. One design goal was to incorporate structural features with limited rotational freedom. On the basis of the resemblance of solid-phase films to smectic mesophases, rodlike conformations are more likely to form such a highly ordered, condensed phase. This is supported by previous descriptions of certain condensed-phase surfactant behavior as meso-phase systems. Certain interesting structural guidelines arise from this approach, such as the inclusion of several linked aryl functionalitiesand axial ratios which may be either 4-8 or up to 15.21
Experimental Section Gas chromatography (GC) was performed using a Varian 1600 flame-ionization chromatograph, with 2 m X 2 mm i.d. glass columns packed with 3% OV-1 on Chromasorb WAWIDMCS (Alltech Co.). Retention indices [KI] were obtained by the method of Kovata" using hydrocarbon standards. Thin-layer chromatography was carried out on silica gel (Kodak 13181) visualized by spraying with a 1% aqueous solution of New Fuchsin (Aldrich). 'H N M R spedra were obtained with a Varian EM-360 or XL-200 spectrometer; NMR spectra were normally obtained on the XL-200 spectrometer. Carbon and fluorine spectra (300-MHz) were obtained on a Nicolet NT-300 spectrometer. Compounds prepared are listed in Table 11. Synthetic details are available (see supplementary material). (21) Priestley, E. B. In Introduction to Liquid Crystak; Priestley, E. B., Wojbwicz, P. J., Sheng, P., Eds.; Plenum: New York, 1975. (22) Kovata, E. Adu. Chromatogr. 1965,1, 229.
734 Langmuir, Vol. 4, No. 3, 1988
Katritzky et al.
Table 11. New Compounds Preparedo compd mp, O C 4-tetradecylbromobenzene(5) ca. 25 diethyl 4-tetradecylbenzenephosphonate(6) ca. 25 4-tetradecylbenzenephosphonicacid (7) 104-105 1-(4-bromophenyl)-l-tetradecanone(8) 69.5-71.5 l-(4-tetradecylphenyl)-2,4,6-trimethylpyidinium 110.5-112 triflate (12) 107-108 N-(uerfluorooctanesulfony1)uiuerazine (16) - - N’-bnzoyl-N-(perfluorooctanesulfony1)piperazine(17) 136-127 N’-propanoyl-N-(perfluorooctanesulfonyl)piperazine 154-155 (18) N‘-butanoyl-N(perfluorooctanesulfony1)piperazine 138-140 (19) N’-hexanoyl-N-(perfluorooctanesulfonyl)piperazine 147-149 (20) 147.5-148 N‘-nonanoyl-N(perfluorooctanesulfony1)piperazine (21) N’-dodecanoyl-N-(perfluorooctanesulfonyl)piperazine 134-136 (22) N’-methanesulfonyl-N-(perfluorooctanesulfony1)185 (dec) piperazine (23) N’-hexanesulfonyl-N-(perfluorooctanesulfony1)200 (dec) piperazine (24) 245-248 N’- (4-toluenesulfonyl) -N-(perfluorooctanesulfony 1)piperazine (25) N’- (4-octylbenzenesulfonyl)-N-(perfluorooctane210 (dec) sulfony1)piperazine (26) N’-methyl-&(perfluorooctanesulfonyl)piperazine (27) 69-70 N’-ethyl-N-(perfluorooctanesulfony1)piperazine (28) 60-61 N’-butyl-N-(perfluorooctanesulfony1)piperazine (29) 77-78 N’-hexyl-N-(perfluorooctanesulfony1)piperazine (30) 84-85.5 N’-octyl-N-(perfluorooctaneaulfony1)piperazine(31) 79-81 N,N-dimethyl-N’-(perfluormtanesulfony1)260 (dec) piperazinium iodide (32) N,N-diethyl-N’-(perfluorooctanesulfonyl)piperazinium 283-286 iodide (33) N,N-dibutyl-N’-(perfluorooctanesulfony1)262-265 piperazinium iodide (34) NJ-dihexyl-N’-(perfluorooctanesulfony1)252-255 piperazinium iodide (35) N,N-dioctyl-N’-(perfluorooctanesulfonyl)piperazinium 242-244 iodide (36) a Satisfactory microanalyses (C, H,N)obtained for all new compounds.
Results and Discussion Initial Synthetic Targets. Nonfluorinated Compounds. Nonfluorinated agents were prepared and evaluated, including 4-tetradecylbenzenephosphonicacid (7) and 1-(4-tetradecylpheny1)-2,4,6-trimethylpyridinium triflate (12).
5 acylation of bromobenzene by the entrainment procedure of Smeets and Verhulst,= followed by reduction with sodium borohydride, gave the benzylic alcohol 9. This was dehydrated and catalytically reduced to give 5. Several literature were applied in attempts at direct reductions of 8: all resulted in extensive hydrogenolysis of the bromime atom.24~26
10
Pyridinium salt 12 was prepared from 4-tetradecylaniline via the pyrylium salt 11. Both 7 and 12 failed to spread on diesel fuel, although 7 did spread, forming a stable layer, on water.
..
11
12
Fluorinated Surfactants. Perfluorinated lyophobic functionalities were next designed. The nature of the linking function between the lyophobic and lyophilic groups was critical: linkage via single, simple functionalities, such as esters, amides, and sulfones, shows only minimal surface activity on the hydrocarbon subphase (AY < 1d y n / ~ m ) . ~ ,Bipolar ~’ functionality, however, results in good activity, and the length is important. The structural paradigm for a vertical, solid-phase agent on hydrocarbons was revised to 13: where X and Y are polar, linking functionalities, separated by an ethylene, propylene, or other bridge. RFX- - - -YRH
3
13
4
6
5
6N HC1, h ‘1 4 ‘ 2 9 0
OH 2
7
Several routes were investigated for preparation of the bromide 5. The Sandemeyer reaction carried out with 4-tetradecylbenzenediazoniumbromide gave only tetradecylbenzene. The same procedure, performed upon the diazonium fluoroborate 4 (which, by the modified procedure, is much more easily prepared than the bromide),gave 55% of the desired bromide 5, in addition to the alkylbenzene. An alternate procedure, via l-(l-bromophenyl)-1-tetradecanone (8), gave m w h better yields of
The associative energy that maintains the stability on water of the monolayer of a vertical, solid-phase agent, such as tetradecanol, arises from van der Waals forces among the lyophobic groups: it is not to be expected that perfluoroalkyl chains can sustain sufficient interaction to provide sufficient associative force, since perfluoroalkanes interact only weakly. Hence we designed agents in which the necessary interaction energy comes from the linking functions in 13. Derivatives of the Perfluorooctanesulfonamideof Piperazine. Our starting materials stem ultimately from perfluorooctanesulfonyl fluoride (14; 3M FX-8), which is (23) Smeeta, F.;Verhulst, J. Bull. SOC.Chim. Belg. 1952, 61, 694. (24) Hug-Minlon J. Am. Chem. SOC.1949, 71, 3301. (26) Yamamura, S. J . Chem. SOC.C 1968,2888. (26) Jarvis, N. L.; Zisman,W.A. J. Am. Chem. SOC.1969, 63,727.
(27)Pike,M.T.,unpublished results.
Langmuir, Vol. 4, No. 3, 1988 735
Design and Synthesis of Fluorinated Surfactants typically only ca.7+75% n-perfluorooctyl? it was desired that any synthetic intermediates employed overcome this essential structural inhomogeneity. N-(Perfluorooctanesulfony1)piperazine (16) was found to be suitable for an intermediate of this type. It was prepared by reaction of 14 with piperazine (15). Chromatography at high loading on basic alumina, followed by recrystallization from hexane, enabled us to eliminate all homologues to obtain a material which is homogeneous in terms of its physical properties (i.e., melting point and chromatographic behavior). High-resolution 'gF NMR of this intermediate showed no straight-chain homologues and only some 10% of branched homologues.
Table 111. Equilibrium Surface Tensions of 1% w/v Solutions in Diesel Fuel" at 24 'C compd y, dyn/cm compd y, dyn/cm blank 27.2 25 23.0 7 26.9 26 22.8 12 26.9 27 26.7 16 26.2 28 25.8 17 27.15 29 26.0 18 24.6 30 26.2 19 27.2 31 26.2 32 26.9 20 25.1 26.5 33 24.3 21 22 26.6 34 23.9 23 27.0 35 27.0 24 21.7 36 26.8 Corrected for buoyancy as specified by Zuidema and Waters.32
14
15
16
In this paper we present a number of potential surfactants based upon this intermediate. These derivatives include a number of carboxamides, 17-22; sulfonamides, 23-26; tertiary amines, 27-31; and quaternary ammonium compounds, 32-36. They incorporate the design features previously detailed, namely, the linking functionality is bipolar and the cyclic amine, having fewer vibrational/ rotational degrees of freedom than acyclic analogues published to date,mapproaches the mesogen-like character of the paradigm 13. /7 17: Y
18: Y 19: Y
-
c8 F1 7 S o 2 N u N - Y COC6H5
25: Y
COC2H5
26: Y
COC3H7
27: Y
20: Y = COC5H11
28: Y
21: Y 22: Y 23: Y 24: Y
=
-
COC8H17
29: Y
COC11H23
30: Y
S02CH3
31: Y
-
=
SO2-4-C6HqCH3 S02-4-C6H4C8H17 CH3 C2H5 C4Hg C6H13 C8H17
S02C6H13 Y C 8 F l 7 S O 2 Nn u y l I-
32: Y
-
Y
CH3
33: Y = C2H5
34: Y 35: Y 36: Y
-
C4Hg C6H13 C8H17
Application of standard reactions (see supplementary material) to the key intermediate 16 led to a broad array of surfactants: carboxamides 17-22 and sulfonamides 23-26 were all prepared in the usual way, and many were surface active on diesel fuel. The dimethylammoniun iodide 32 was also surface active, although it was not very soluble in diesel fuel, and seemed to form an insoluble film on the surface. There(28)3 M Product Information Bulletin Fluorad Fluorochemical Solfonyl Fluoride; Technical Report No. 98-0211-1909-8; 3 M Company, St. Paul, MN,1984. (29)Guenthner, R.A.;Vietor, M. L. Ind. Eng. Chem., Prod. Res. Deu. 1962,1, 165.
fore, analogues with longer lyophilic alkyl chains were prepared, specifically the diethyl to dioctyl compounds 33-36. All these quaternary compounds were obtained by alkylation of the analogous tertiary amines 27-31, which in turn were produced by reductive alkylation of 16. The methyl derivative 27 was prepared by the EschweilerClarke procedure with formaldehyde and formic acid,30 while the longer alkyl chain analogues 28-31 were obtained from 16 by reductive alkylation with sodium borohydride and the appropriate carboxylic acid.31 The results obtained from the surface tension measurements are shown in Table 111. Evaporation studies will be reported at a later date.
Summary The larger sulfonamide derivatives 24,25, and 26 show excellent surface activity on diesel fuel, while moderate results are seen with the dibutyl and dihexyl quaternary iodides 34 and 35. However, none of the other compounds shows any significant effect. In some cases this lack of surface activity can be explained by the fact that the surfactant molecule is probably too soluble in the organic subphase, while in other cases the shorter chain compounds may not possess sufficient affmity for the subphase to achieve a vertical packing. Acknowledgment. We thank Dr. W. S. Brey for running the 300-MHz NMR spectra and the Instrument Program, Chemistry Division, NSF, for assistance in the purchase of the Nicolet NT-300 spectrometer. We also thank D. S. Liang for some synthetic assistance and Drs. A. Manzarro and D. Newman of the 3M Company, St. Paul, MN, for their generous gift of materials and technical information. Registry No. 3, 91323-12-5; 4, 113584-24-0; 5, 113584-25-1; 6, 113584-26-2; 7, 17166-60-8; 8,113584-27-3; 9,113584-28-4; 10, 113584-29-5; 11, 40927-60-4; 12, 113584-31-9; 14, 307-35-7; 15, 110-85-0; 16, 113584-32-0; 17, 113584-33-1; 18, 113584-34-2; 19, 113584-35-3; 20, 113584-36-4; 21, 113584-37-5; 22, 113584-38-6; 23,113584-39-7; 24,113584-40-0; 25,113584-41-1; 26,113584-42-2; 27,113584-43-3; 28,113584-44-4; 29,113584-45-5; 30,113584-46-6; 31,113584-47-7;32,113584-48-8; 33,113584-49-9;34,113584-50-2; 35,113584-51-3;36, 113584-52-4; P(OC2H,),, 122-52-1; C6H,Br, 108-86-1. Supplementary Material Available: Synthesis of compounds in Table I1 (15 pages). Ordering information is given on any current masthead page. (30) Clarke, H. T.;Gilleapie, H. B.; Weisshaus, S. Z. J.Am. Chem. Soc. 1933,55,4571.
(31) Gribble, G. W.; Jaeinski, J. M.; Pellicone, J. T.; Panetta, J. A. Synthesis 1978,766. (32)Zuidema, H.H.; Waters, G. W. Ind. Eng. Chem. Anal. Ed. 1941, 31, 312.
