Langmuir 1994,10, 218-224
218
A Study of the Competitive Adsorption of a Fluorosurfactant at the Gelatin-Air Interface Using Time-of-Flight Secondary Ion Mass Spectrometry G . N. Battst and A. J. Paul'** Kodak Limited, Headstone Drive, Harrow, Middlesex HA1 4TY, U.K., and Centre for Surface and Materials Analysis, Armstrong House, Oxford Road, Manchester MI 7ED, U.K. Received November 30, 1992. In Final Form: October 18, 199P Time-of-flightsecondary ion mass spectrometry (ToF-SIMS)has been used to follow qualitatively and quantitatively the displacement of gelatin by a fluorosurfactantfrom an interfaceover a wide concentration range from zero to beyond monolayer coverage of the fluorosurfactant. The ToF-SIMS results are in good agreement with published wettability data and also show a good correlation with previously reported measurements using X-ray photoelectron spectroscopy (XPS). The ToF-SIMS results indicate no change in the outermost surface chemistry above the point of monolayer coverage as defined according to the Gibbs adsorption model and that the surface displacement of gelatin by the fluorosurfactant is effectively complete. 0.5 r
1. Introduction
The surface chemistry of dried gelatin layers containing different amounts of fluorosurfactant FC134 has been investigated previously using X-ray photoelectron spectroscopy (XPS or ESCA) and wettability measurementa.'v2 The study showed that there is a close correlation between the two techniques and how fluorosurfactant adsorption at the aqueous gelatidair interface can be monitored from the surface analyses of the corresponding dried gelatin layers. Since gelatin itself is mildly surface active and does adsorb at the air/water interface (reducing the surface tension to -60 mN m-l), addition of the significantlymore surface active FC134 leads to competitive adsorption for the airlwater interface. In other words, the two species independently try to reduce the surface energy of the system by having a surface excess. As expected, the FC134 displaces the gelatin from the interface since it is far more surface active. that monolayer I t was shown in the previous coverage of the fluorosurfactant is reached at a predried FC134 bulk concentration close to that predicted from the surface tension data obtained for aqueous gelatinFC134 solutions. As regards the definition of monolayer coverage, according to the classical adsorption model proposed by Gibbs? monolayer coverage is said to be reached when the plot of interfacial tension versus loglo[substance] becomes linear. In this previous work,1,2however, there was uncertainty over the exact chemical composition of the outermost atomic layers of the gelatin-FC134 system after FC134 monolayer coverage had been attained and it was not possible to ascertain whether the gelatin was ever completely displaced from the interface. This is due to the inability of XPS to distinguish between some of the carbon, oxygen, and nitrogen species present in the gelatin and FC134 components coupled with the fact that the sampling depth of the XPS technique is such that information + Kodak Limited. Centre for Surface and Materials Analysis. .s Abstract published in Advance ACS Abstracts, December 1. 1993. (1) Batta, G. N. Colloids Surf. 1987,22,133. (2)Batta, G.N. Ph.D. Thesis, Imperial College, London, 1984. (3)Gibbs, J. W.Sci. Pap. 1987,1, 219.
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Figure 1. Fls/(Fls + Cls) XPS photoelectron intensity ratio
as a function of loglo[predried FC134 bulk concentration (mol/ L)] (from ref 2, photoelectron take-off angle = 3 5 O ) .
derives from several atomic layers. Above the point of FC134 monolayer coverage, the XPS data indicate that the surface concentration of FC134 is still increasing (see Figure 1, reproduced from ref 2), whereas the surface tension data suggest no further change in the outermost surface chemistry (see Figure 2, reproduced from ref 2). The work has now been extended to exploit the advances made in secondary ion mass spectrometry (SIMS) with the development of time-of-flight SIMS (ToF-SIMS). As a surface mass spectrometry,Pll ToF-SIMS provides definitive chemical and molecular structure information which is unattainable by other surface analysis techniques (4)Standing, K.G.; Chait, B. T.; Em,W.; McIntosh, G.; Beavie, R. Nucl. Imtrum. Methods 1982,198,33. (5)Bletaos, I. V.; Hercules, D. M.; Greifendorf, D.; Benninghoven, A. Anal. Chem. 1985,57,2384. (6) Lub, J.; Benninghoven, A. Org. Mass Spectrom. lSSS,24,164. (7)Bletaos, I. V.; Hercules, D. M.; van Leyen, D.; Benninghoven, A. Macromolecules 1990,20,407. (8)Eccles, A. J.; Vickerman, J. C. J. Vac. Sci. Technol. 1989,A7,234. (9) Paul, A. J.: Vickerman, J. C. Philos. Tram.R. SOC. London, A 1990. 333,147. (IO) Briggs, D., Br. Polym. J. 1989,21,3. (11) Davies,M. C.;Lynn,R.A. P.; Watts, J. F.;Paul,A. J.;Vickerman, J. C.; Heller, J. Macromolecules 1991,24,5508.
0743-7463/94/2410-0218$04.50/00 1994 American Chemical Society
Displacement of Gelatin by a Fluorosurfactant
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Langmuir, Vol. 10, No. 1,1994 219 qualitativelyand quantitatively the displacementof gelatin by a fluorosurfactant (FC134) from an interface. The results are compared with XPS and wettability data reported previously for this gelatin-fluorosurfactant system.
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including XPS. As such, SIMS and ToF-SIMS are particularly well suited for the analyses of organic and polymer surfaces.611 Furthermore, such analyses can be carried out under static conditions12which are effectively nondamaging to the organic and polymeric materials of interest.
Inthestaticmode,SIMSandToF-SIMSareverysurface sensitive with a sampling depth of only 1-2 monolayers, which is somewhat less than XPS under routine conditions. Indeed, as shown in previous studies of polymeric systems,loJ3J4it is only when XPS analysis is carried out under very surface sensitive conditions (i.e., photoelectron take-off angle 10" with respect to the surface) that the XPS technique has a similar sampling depth to that of static SIMS. In contrast to XPS, SIMS and ToF-SIMS data are very difficult to quantify in absolute terms16 without, for example, the use of internal standards.I6 A number of studies, however, have illustrated that the molecular specificity of SIMS and ToF-SIMS may be effectively exploited in a quantitative manner where the use of secondary ion ratios provides good and reliable relative quantification of surface composition^."^^^^^^^^^^'-^^ This has been particularly valuable in the analyses of complex copolymer surface structures, including segmented polyu r e t h a n e ~p~lymethacrylates,~' ,~~ and p01yorthoesters.l~ ToF-SIMS has also been used to study quantitatively the surface compositions of miscible and immiscible polymer blendsz0 and the surface versus bulk relationships of biomolecules at the surfaces of model membranesg and colloidal latex particles.21 In this study, the molecularly specific and surfacesensitive capabilitiesof ToF-SIMS have been used to follow N
(12) Briggs, D.; H e m , M. J. Vacuum 1986,36,1006. (13) Hearn,M. J.;Briggs,D.;Yoon,S. C.;Ratner,B. D.Surf. Interface Anal. 1987,10,384. (14) Hearn, M. J.; Ratner, B. D.; Briggs, D. Macromolecules 1988,21, 2960. (16) Vickerman, J. C.; Brown, A.; Reed, N. M. Secondary Ion Mass Spectrometry; Oxford Science, 1989; p 34. (16) Hagenhoff,B.; Kock,R.;Deimel, M.; Benninghoven, A. Secondary Ion Mass Spectrometry SIMS VIII; John Wiley and Sone: New York, 1992; p 831. (17) Briggs, D.; Ratner, B. D. Polym. Commun. 1988,29, 6. (18) Briggs, D. Org. Mass Spectrom. 1987,22,91. (19) Davies,M. C.; hadley, S. R.; Paul, A. J.; Vickerman, J. C.;Heller, J.; Franson, N. M. Polym. Adu. Technol. 1992,3, 293. (20)Thompson, P. M. Anal. Chem. 1991,63, 2447. (21) Daviee, M. C.; Lynn, R. A. P.; Davie, S. S.;Hearn, J.; Watts, J. F.; Vickerman, J. C.; Paul, A. J. Langmuir 1993,9,1637.
2.1. Materials. A stock solution ( W M ) of the cationic fluorosurfactant FC134 (3M,Ltd.)in aqueous gelatin (5 9% (w/w))waspreparedandused tomakeother gelatin-FC13.4 solutionsto give a wide range of fluorosurfactant coverages. A thin layer was made from each solution by spreading onto a gelatin-subbed plastic support and allowing to dry. This is analogous to the production of a photographic product where the plastic support has a precoated gelatin layer applied before the sensitized material is coated. This precoated layer provides good adhesion of the photographic material without itself influencing the properties of the layers above. Thus, in this study, the support played no part in the surface characterization of the dried gelatinFC134 layers. The full range of gelatin-FC134 solutions prepared and studied were as follows: (1)0 M; (2) 1 X 10-7 M; (3) 1X 10-8 M; (4) 5 X 10-8 M (5) 1X 106 M; (6) 2 X 106 M (7) 5 X 106M; (8) 5 X 1 V M (9) 5 X 1 W M (10) 5 X 10-2 M. The gelatin used in this study was a type IV (Rousselot, Ltd., batch no. RG-623) with 100% deionization to completely remove Ca2+ ions (but insufficient to fully remove Na+). Water was obtained from a MilliporeMilli-Q water purification system comprising three ion-exchange columns with a final column of activated carbon to remove organics. The water was dispensed through a 0.22-pm filter and had a conductivity better than 0.1 pS.cm-l at 25 "C. A reference ToF-SIMS spectrum of FC134 itself was facilitated by dissolving some of the fluorosurfactant in methanol (Aldrich,HPLC grade, 99.9+ % ) and depositing a few drops of the solution onto aluminum foil. The resultant film of FC134 was thin enough not to cause problems with sample charging but was sufficiently thick such that signals associated with the foil substrate were not observed. 2.2. Instrumentation. Time-of-flight SIMS spectra were obtained using a VG IX23S instrument8 comprising a Poschenrieder energy-compensating ToF analyzer and a pulsed liquid metal ion source (Ga+, ion energy = 30 keV, pulse length = 20 ns, pulse frequency = 10 kHz, ion flux density = 5 X 10-l' A cm-2). For each sample, the Ga+ primary ion beam was focused into an &rea of 0.6 mm X 0.6 mm and the resultant secondary ions were accelerated to 5 keV for the ToF analysis by applying a bias of +5 kV to the sample. Charge compensation of the insulating gelatin-FC134 specimens was facilitated using a pulsed electron flood source (electron energy = 14 eV, pulsed at '/loth ion gun pulse frequency, electron flux density = 5 X 1WAcm-2) which was synchronized (out-of-phase)with pulsing of the applied bias to the sample stage, as described elsewhere.22 Secondary ion spectra were collected using a primary ion dose which did not exceed 2 X 10" ions cm-2. Such a dose lies well below the damage threshold of 1 X 10'3 ions cm-2 for static SIMS.12 A DEC PDP 11 computer system was used for spectral acquisition, storage, and processing. As described in this paper, one of the major aims of this work was to try and exploit the molecular specificity and surface sensitivity of the ToF-SIMS technique in a (22) Briggs, D.; Hearn, M. J.; Fletcher,I. W.; Waugh, A. R.; McIntosh, B. J. Surf. Interface Anal. 1990,15, 62.
Batte and Paul
220 Langmuir, Vol. 10, No. 1, 1994
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3. Results a n d Discussion As regards the specific aims of this study, only positive secondary ion spectra were recorded for the full series of gelatin-FC134 samples. On the basis of published24v2Sand unpublished reference spectra, it was clear that the FC134 species would yield a rich array of positive ion fragments characteristic of the fluorocarbon and aminelammonium parts of the molecules, in addition to the fully intact preformed molecular cations. 3.1. Cations Characteristic of Gelatin. From inspection of the data recorded for the pure gelatin sample (Le. where the predried FC134 bulk concentration was 0 M), the following species were identified as being principally diagnostic of gelatin. At low mlz, an intense peak corresponding to sodium (23+) was evident (see Figure 3A). In addition, prominent signals characteristic of hydrocarbon fragments of the form C,Hy+ were observed, Le. CH3+ (mlz = 159, CzH3+ (mlz = 27+), C2H5+ (mlz = 29+), C3H3+ (mlz = 399, C3H5+ (mlz = 41+), C3H7+ (mlz = 439, C4H7+ (mlz = 559, C4H9+ (mlz = 579, etc. The detection of such C,H,+ species is expected in the case of a carbonaceous (hydrocarbon-based) material such as gelatin. As has been well demonstrated in other SIMS st~dies,~4,26127 positive ion SIMS spectra of hydrocarboncontaining materials normally exhibit rich arrays of C,Hy+ signals. Signals detected at mlz values of 70+ (see Figure 3A) and 86+ (see Figure 4A) are attributed to the presence of proline and hydroxyproline residues within the gelatin structure i.e.
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Figure 3. Positive ion ToF-SIMSspectra (m/z 0-80) recorded for dried FC134-gelatin layers with predried FC134 bulk concentrationsof (A) 0 M,(B)1 X 10-8M, (C) 5 X 1W M, (D) 1 X 1od M, (E) 5 X 1od M, and (F) 5 X 1o-L M.
quantitative manner as far as possible. As has been defined s ~ necessary ~ ~ ~ ~ selection in this and other l a b ~ r a t o r i e the of a well-defined and reproducible instrument protocol for all of the gelatin-FC134 sample analyses was rigidly adhered to. This included fixing all of the important instrumental variables such as the primary ion and electron gun parameters (e.g. energy, flux density, pulse length, and frequency), ToF analyzer voltages, ToF analyzer acceptance angle, sample bias, and sample position.8 The instrumental conditions used for the acquisition of the FC134 reference spectrum differed from that employed for the gelatin-FC134 samples, as a consequence of the fact that FC134 itself was analysed in the form of a thin film deposited onto aluminum foil such that charge compensation was not necessary. As such, the FC134 reference spectrum was of more value on a qualitative rather than quantitative basis. As regards the latter, some differences in the relative FC134 peak intensities for the FC134 reference compared to the gelatin-FC134 samples are attributed, in part, to the use of different instrument conditions. This is not unexpected in view of the fact that relative secondary ion peak intensities are dependent upon, and sensitive to (albeit to varying degrees), some of the instrument variables outlined previously. (23) Briggs, D. Surf. Interface Anal. 1990,15, 734.
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Signals indicative of aromatic structures were also observed (Figure 4A), the most intense being (note, the likelihood of structural rearrangements) 0 II
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m/z = 1 w
While these species possibly correspond to gelatin contaminants, they are, nevertheless, characteristic of the gelatin component. Finally, at higher mlz, unidentified gelatin characteristics were observed a t mlz values such as 419+,433+,441+, and 455+ (see Figure 5A). These species are thought tentatively to correspond to the protonated and sodiumcationized quasi-molecular ions of discrete organic molecules of molecular weights 418 and 432. It is not possible ~~
(24) Briggs, D.; Brown, A.; Vickerman, J. C. Handbook of Static Secondary Ion Mms Spectrometry;John Wiley & Sons: New York, 1989. (25) Swift, A. J.; Paul, A. J.; Vickerman, J. C. Surf. Interface Anal. 1993,20, 27. (26) Newman, J. G.; Carhon, B. A.; Michael, R. S.;Moulder, J. F.; Hohlt, T. A. Static SIMS Handbook of Polymer Anulysis; Perkin-Elmer Corporation Physical Electronics Division: Eden Prairie, MN, 1991. (27) Briggs, D. Surf. Interface Anal. 1982,4, 151.
Langmuir, Vol. 10, No. 1, 1994 221
Displacement of Gelatin by a Fluorosurfactant *"
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Table 1. FAB-MS Analysis of 3M Fluorosurfactant FC1342 cation structure mol w t % CsFi7SOzNH(CHz)sN+(CHa)3 599 -80 C~F~~SO~NH(CHZ)~N+(CH~)~ 613 15 CsFi7SOzNH(CHz)zN+(CHs)s
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to say if these species derive from the gelatin structure itself or correspond simply to gelatin contaminants. 3.2. Cations Characteristic of FC134. Previous analysis of this cationic fluorosurfactant by fast atom bombardment mass spectrometry (FAB-MS) (ref 2) gives the structural information presented in Table 1. As expected, the fully intact and highly distinctive molecular ions corresponding to the two major FC134 components were observable easily in the ToF-SIMS spectra recorded in this study (see Figures 5 and 61,i.e.
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A t lower mlz (see Figures 3, 4, and 6), a number of diagnostic FC134 fragments were observable. As expected,
the fluorocarbon part of the FC134 molecular structures gave rise to a number of intense C,F,+ species such as C+ (m/z = 129, CF+ (mlz = 31+),CF2+ (mlz = 5 0 9 , CFs+
Batts and Paul
222 Langmuir, Vol. 10, No. 1, 1994
(mlz = 699, C3F3+ (mlz = 93+), mlz = 100+ (C2F4+),and (mlz = 1319. Peaks were also observed corresponding to C,Hy+ fragments. Other prominent signals detected are assigned to the following structures ICH~=N=CHZI+
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At higher mlz, a rich array of signals were detected, corresponding to fragmentation within the fluorocarbon chain component of the FC134 molecules, e.g. mlz = 261+ = C2F&302NH[CH213N[CH313+,mlz = 361+ = C4F7S02NH[CH213N[CH&+, mlz = 399+= C4F&02NH[CH213Nmlz [CHaIs+,mlz = 449+ = CJ?llSO2NH[CH2lgN[CH313+, = 461+ = CsF11S02NH[CH2]3N[CH333+, mlz = 499+ = C@I~SO~NH[CH~I~NCCH~I~+, mlz = 549+ = C~FlsS02NH[CH2lsN[CH&+, mlz = 561+= C~ISSO~NH[CH&,N[CH&+. A low intensity signal at mlz = 581+ is assigned to the hydrogen-capped species HC8&02NH[CH213N[CHda+. 3.3. Variation in Cation Intensities. From visual comparison of the ToF-SIMS data, it is immediately apparent that the spectra reflect directly the qualitative and quantitative changes in the surface chemical composition of the gelatin-FC134 system as a function of the predried FC134 bulk concentration. This is most clearly illustrated by the spectra presented in Figures 3, 4, and 5. In Figure 3 (mlz = 0-80 region), signals characteristic of the C,Hy+ and Na+ species, principally associated with the gelatin component, diminish in intensity (Figure 3) with increasing predried FC134 bulk concentration in contrast to the progressive enhancement of the C+/C,Fy+/ C,HyN+ signals which derive from the FC134 component. This behavior is mirrored in the mlz region 80-190 (see Figure 4) where gelatin characteristics are gradually replaced by those specieswhich are diagnostic of the FC134 component as the predried FC134 bulk concentration increases. At higher mlz (380-6501, the changes in surface composition are again reflected by the relative intensity variations of the secondary ion signals which characterize the gelatin and FC134 components. In this case, however, the relative surface level of gelatin cannot be followed over the full range of predried FC134 bulk concentrations as a consequence of the low signal intensities associated with the gelatin peak series observed in the mlz range 400-470. This observation contrasts with the behavior of the fully intact FC134 molecular cations which are detected with very high intensity and are discernible easily at concentrations below the detection limits of XPS.ls2 Finally, it should be noted that the ToF-SIMS spectra recorded at the three highest predried FC134 bulk concentrations (i.e. 5 X 1W M, 5 X 1V M, and 5 X le2 M) were all identical, indicating no significant change in the outermost surface chemistry.
Log [ FC134 concentration (moles/litre)1 Figure 7. Plot of the positive ion peak area ratios of the signals at m/z 31 and 29 (principallydiagnostic of FC134 and gelatin, respectively) as a function of loglo[predried FC134 bulk concentration (mol/L)l.
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Log [ FC134 Concentration (moles/litre)1 Figure 8. Plot of the positive ion peak area ratios of the signals at mlz 58 and 29 (principallydiagnostic of FC134 and gelatin, respectively) as a function of loglo[predried FC134 bulk concentration (mol/L)]. 3.4. Relative Quantification Using Cation Intensities. As discussed in the preceding sections, diagnostic ions specific to the gelatin and FC134 components of the gelatin-FC134 system are identifiable in the ToF-SIMS spectra. As has been carried out in previous semiquantitative ToF-SIMS s t u d i e ~ ~ Jmeasurement lJ~ and comparison of secondary ion peak areas was used to assess the relative surface level of FC134 as a function of the predried FC134 bulk concentration. Note, that peak areas (A) rather than peak heights were chosen for the quantitative evaluation because each peak in a secondary ion spectrum is defined over a number of spectral channels rather than one. A number of peak area ratios (A,/[A, + A,]) were measured and calculated. These were (a) A31/[A31+ Am], (b) Ad[Asa + Am], (c) Ad[Aai + A231, (4 A93I[A93 + &ll, (e) Ald[A115 + A1491, ( f ) A163/[&63 + A1491, and (g) AS99/[A599 + &,I, where A319 AM,-493, AIIS, A163, and A699 are principally diagnostic of FC134 and A239 Am, Agl, and A149 are principally diagnostic of gelatin. Some of the graphs of these peak area ratios as a function of logdpredried FC134 bulk concentration] are presented in Figures 7-10. All graphical relationships investigated were similar over the full predried FC134 bulk concen-
Displacement of Gelatin by a Fluorosurfactant
Langmuir, Vol. 10, No. 1, 1994 223
+
Log [ FC134 concentration (moles/Jitre) 1 Figure 9. Plot of the positive ion peak area ratios of the signals at mlz 163and 149 (principallydiagnostic of FC134 and gelatin, respectively) as a function of loglo[predried FC134 bulk concentration (mol/L)I.
0.0
-7
-6
-5
-4
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-a
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o
Log 1 FC134concentration (moles/litm)1 Figure 10. Plot of the positive ion peak ratios of the signals at mlz 599 and 23 (diagnosticof FC134 and gelatin, respectively) as a function of logdpredried FC134 bulk concentration (mol/ L)1* tration range studied. These ToF-SIMS graphs bear a close resemblance to similar plots of XPS intensity ratios against predried FC134 bulk concentration (refs 1and 2, see also Figure 1). There appears, however, to be some difference in behavior at high predried FC134 bulk concentrations. The XPS data obtained in the earlier studylJ suggest that the surface level of FC134 is still increasing beyond monolayer coverage as defined according to the Gibbs adsorption model. In this work, the data provided by ToF-SIMS suggest no change in the outermost surface chemistry above the classical point of monolayer coverage. The ToF-SIMS data show a better correlation with the wettability datal where, in both cases, the corresponding plots reach a plateau at a similar value of predried FC134 bulk concentration. In the previous study,lJ it was proposed that the different sampling depths of the XPS and wettability determinations account for the deviation in their behavior. The wetting properties of a solid surface are determined predominantly by the chemical and physical properties of the outermost surface layer. Changes, therefore, in chemical composition below a complete monolayer (which are detected by XPS)are unlikely to significantly influence the wetting properties of the gelatin-FC134 surface. The
close agreement of the ToF-SIMS and wettability data is reasonably attributed to the sampling depth of the ToFSIMS technique which is more comparable with that of wettability determinations than is XPS. From visual assessment of the ToF-SIMS spectra and their comparison with the reference spectrum obtained for the FC134 fluorosurfactant itself, the results suggest that the surface displacement of gelatin by FC134 is effectively complete above monolayer coverage. It is not possible to conclude definitively that the concentration of gelatin in the outermost molecular layer is absolutely zero. Unfortunately, an inherent feature of the gelatin type studied in this work is that all of the secondary ion signals which are considered to be diagnostic of gelatin are not completely unique (unlike many of the FC134 peaks) and that there are small contributions associated with FC134 (e.g. CxHy+: even a trace level of sodium). 3.5. General Discussion. The results show the suitability of ToF-SIMS for the study of the gelatinFC134/air interface. The technique provides definitive qualitative information in addition to data which is quantifiable in a relative sense. The good agreement of the ToF-SIMS data with that of XPS, itselfa quantitative technique in absolute terms, indicates the validity of the ToF-SIMS relative quantification method. It should be emphasized that the similar behavior of all of the graphical relationships studied (Le. relative signal intensities vs predried FC134 bulk concentration) indicates a consistent quantitative relationship between the ToFSIMS spectra and the compositions of the gelatin-FC134 surfaces. These data lend support to the literature work indicating that SIMS and ToF-SIMS analysis of organic and polymer systems may be exploited in a quantitative manner, albeit under carefully controlled experimental conditions, and always taking into account the possible absolute and relative variations in secondary ion yields as a function of any matrix effect(s) associated with changes in surface ~omposition.'~~20 The results obtained by ToF-SIMS appear to be in closer agreement with published wettability data than those obtained by XPS.1r2 Unlike the previous XPS study, the ToF-SIMS results obtained in this work suggest no change in the outermost surface chemistry above the classical point of monolayer coverage and that the surface displacement of gelatin by FC134 is effectively complete. This observation is ascribed to the inherent sampling depth of the ToF-SIMS technique which is more comparable with that of wettability determinations than is XPS. As such, the ToF-SIMS results infer that the proposal put forward in the earlier work,l as regards differing XPS and wettability sampling depths, was correct. This work also shows that ToF-SIMS is able to follow the surface displacement of gelatin by the FC134 fluorosurfactant over a wider predried FC134 bulk concentration range than either of the previously reported wettability or XPS studies. As regards the latter, the FC134 fluorosurfactant is easily observable at concentrations below the detection limits of xPS.1~2 In this study, ToF-SIMS analyses of the gelatin-FC134 samples were undertaken using positive secondary ion spectra only. Thus, relative quantification was only made on the basis of positive ion signal comparisons. There is no reason why similar data could not have been obtained using negative ion spectra (subject, again, to a well-defined and reproducible instrument protocol). The consistency of both positive and negative ion ToF-SIMS data has already been shown in a study of a polymer system where variations in relative secondary ion intensities were correlated with the changes in a polymer molecular weight.28
224 Langnuir, Vol. 10, No. 1, 1994
This work further illustrates the realizable and widespread capability of ToF-SIMS for the combined qualitative and semiquantitative evaluation of organic and polymer surface compositions. This capability largely derives from the fact that ToF-SIMS is a mass spectrometry technique and therefore is expected to, and does, provide detailed chemical and molecular structure information from most types of organic and polymer material. ToF-SIMS analysis can be carried out directly without prior surface modification under experimental conditions which are effectively nondamaging (static)to the surface of interest. The high surface sensitivity (low sampling depth) of the ToF-SIMS is clearly important for many areas of study where the composition of the outermost surface layer(@is the determining factor. In principle, the molecular selectivity of ToF-SIMS coupled with the other features of the technique not only can be exploited to study the competitive behavior of two components at an interface but could be extended to investigate the surface compositions of multicomponent (28) Treverton, J. A.; Paul, A. J.; Vickerman, J. C . Surf. Interface A d . 1993,20,449.
Batts and Paul systems. This could be particularly fruitful in cases of complex organic and polymer surfaces involving components of slightlydifferent chemicalstructures which would be difficult, perhaps even impossible, to study by other techniques. The success of this will clearly depend on the inherent chemical nature of the surface under study, relying upon the production of suitably diagnostic secondary ions per component. Recent advances in ToFSIMS instrumentation and the development of high mass resolution capabilities may help in this respect where components characterized by ions of similar mass can be resolved. 4. Conclusions
The surface displacement of gelatin by the fluorosurfactant FC134 has been investigated using time-of-flight SIMS. The results are in good agreement with previously reported wettability and X P S measurements. The ToFSIMS data suggest no change in the outermost surface chemistry above monolayer coverage as defined according to the adsorption model proposed by Gibbs and that the surface displacement of gelatin by FC134 is effectively complete.
