Langmuir 1988, 4, 1277-1283 aqueous solution/low-energy solid interfaces will increase with increase in the length of the hydrophobic group. However, the AGOadvalues at the aqueous solution/Teflon and Parafilm interfaces are each based on only a single data pair, and consequently this conclusion should be considered tentative. The addition of electrolyte (NaC1,O.l M) causes almost no change in the properties at either the Parafilm or Teflon interfaces. Properties at the Aqueous Solution/Hexadecane Interface. When the solubility of the surfactant in the nonaqueous phase is small, then the replacement of air by a low surface energy liquid (e.g., alkane) results in little change in the interfacial properties of the surfactant. As the solubility of the surfactant in the nonaqueous phase increases, differences between interfacial properties at the two interfaces increase. Especially apparent is the decrease in the surface excess concentration (increase in interfacial area/surfactant molecule) a t the aqueous/nonaqueous interface,"J2 presumably because of desorption of the surfactant from the interface into the nonaqueous phase. Even when solubility of the surfactant in the nonaqueous (11)Rehfeld, S. J. J. Phys. Chem. 1967,71,738. (12)Murphy, D.S.; Rosen, M. J. J. Phys. Chem. 1988,92,2870.
1277
phase is insignificant, the AGo,d becomes more negative by a few kilojoules per mole (compared to the value for the aqueous solution/air interface), as a result of the intercalation of molecules of the nonaqueous phase between the hydrophobic groups of the surfactants at the interface and their interaction with them.12 For the compounds investigated here, partition coefficients (C,/Cw) show the expected sharp solubility increase in hexadecane with increase in the length of the alkyl chain from c8 to Clo and with addition of NaCl to the aqueous phase. This is accompanied by increases in the area per molecule a t the interface and in the negative value of AGOad,compared to the values for the aqueous solution/air interface. The increases are particularly pronounced upon addition of NaCl. The increase in the value of nmax for CsP is believed to be due to the larger p(C,) values in the presence of hexadecane, compared to those in its absence.12
Acknowledgment. This material is based upon work supported by a grant from GAF Corp. The assistance of David Yang in obtaining the ultraviolet molar absorptivity values of the N-alkyl-2-pyrrolidones is appreciated. Registry No. C2,6P, 66397-78-2; CSP, 2687-94-7; ClOP, 55257-88-0; C12P, 2687-96-9; Teflon, 9002-84-0; hexadecane, 544-76-3.
Detergency of Nonionic Surfactants toward a Solid Hydrocarbon Soil Studied by FT-IR David R. Scheuing* and Jeff C. L. Hsieh Clorox Technical Center, 7200 Johnson Drive, Pleasanton, California 94566 Received February 12, 1988. I n Final Form: May 23, 1988 The interactions of two different aqueous nonionic surfactant solutions with a model solid soil, the hydrocarbon eicosane, have been studied by Fourier transform infrared (FT-IR)spectroscopy. Time-resolved spectra of the eicosane-surfactant solution interface indicate that disordering of eicosane molecules is necessary for removal from a surface, Le., for successful detergency, using ethoxylated alcohols as detergents. For disordering of eicosane to occur, adsorption of surfactant onto the eicosane surface must be followed by penetration of surfactant and water molecules into the eicosane layer. Neodol 25-3, a relatively hydrophobic ethoxylated alcohol, adsorbs to a significantly greater extent than the more hydrophilic Neodol 23-6.5. Neodol25-3 is, however, inefficient in promoting the disordering of eicosane. Hence, the removal of eicosane from a surface by Neodol25-3 is much slower compared to that achieved by Neodol 23-6.5.
Introduction Removal of a solid organic soil from a surface by an aqueous surfactant solution involves the operation of several processes at the soil-water interface. Improvement of our understanding of the removal mechanism for solid soils will aid development of a variety of products such as low wash temperature laundry detergents and hard surface cleaners. The detergency of liquid soils, such as decanol or hexadecane, is understood in terms of rollup, direct emulsification, and solubilization of the soil into micelles or lamellar phases formed a t the soil surfa~e.l-~For liquid (1) Schwartz, A. M. In Surface and Colloid Science; Matiijevic, E., Ed.; Wiley: New York, 1972;Vol. 5. (2) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York, 1978. (3) Kielman, H. S.; van Steen, P. J. F. In Surface Actiue Agents; The Society of Chemical Industry: London, 1979;pp 191-198. (4) Pierce, R. C. presented a t AICHE Spring National Meeting, Houston, TX, March, 1983.
0743-7463/88/2404-1277$01.50/0
hydrocarbon soils and nonionic surfactants, some progress has been made in relating optimum detergency conditions to the composition of ternary hydrocarbon-water-nonionic-surfactant mixtures. Solubilization of liquid hydrocarbon into a variety of surfactant-rich phases has been observed with a videomicroscopy technique.6 Optimal detergency was correlated with the phase inversion temperature of the ternary mixtures and the presence of a dispersion of a lamellar phase in the dilute surfactantwater s~lution.',~ The detergency of solid oily soils, such as lanolin and tristearin, is more complex. An initial soil "liquefaction" or softening and swelling step, caused by the penetration ( 5 ) Hsieh, J. C. L.; Turley, W. D.; Briones, J. G.; Weers, J. G.; Scheuing, D. R. presented at JAOCS National Meeting, New Orleans, LA, May, 1987. (6) Raney, K. H.; Benton, W. J.; Miller, C. A. J. Colloid Interface Sci. 1987,117,282. (7) Raney, K. H.; Miller, C. A. J. Colloid Interface Sci. 1987,119,539. (8) Schambil, F.; Schwuger, M. J. Colloid Polym. Sci. 1987,265,1009.
0 1988 American Chemical Society
1278 Langmuir, Vol. 4,No. 6, 1988 of surfactant and water molecules, was proposed as a necessary precursor to soil removal by mechanical or emulsification proces~es.~The gravimetric data used to develop this model did not, however, reveal much about the nature of the soil-watemurfactant interactions at the soil-solution interface. Results for solid hydrocarbon removal (paraffin wax) were reported as nonreproducible and not extensively discus~ed.~ A recent study employing ellipsometry to quantitate the removal of triglycerides was sensitive enough to detect surfactant adsorption preceding triglyceride removal but did not address the liquefaction process directly.1° Fourier transform infrared spectroscopy (FT-IR) is suited for the study of changes in the packing of long methylene chains, a common structural unit of solid oily soils, in a wide variety of systems. FT-IR has been applied to studies of thermally induced changes in fluidity of methylene chains in hydrocarbons” and phospholipid bilayer^,'^*'^ upon micellization of alkan~ates,’~ and in the gel to liquid crystal transition of cationic surfactants.15 In this paper, we report the application of a FT-IR method, developed in our laboratory, to the study of the removal of a solid hydrocarbon soil by nonionic surfactants. Of special interest is the spectroscopic observation of the different hydrocarbon-water-surfactant structures formed at the interface during exposure of a solid hydrocarbon layer to different aqueous nonionic surfactants.
Scheuing and Hsieh
a
a
a
A
A
a e
.
NEOWL NEOWL NEODOL NEODOL
23-8.5 I . W X ) 23-8.5 1 1 1 ) 25-3 I 0311 25-3 11x1
TIME ( M I N ) Figure 1. Removal of eicosane from IRE surface by flowing
surfactant solutions. The absorbance of the eicosane CH2scissoring band, relative to the local base line at 1500 and 1430 cm-’, is plotted vs time of exposure. Neodol 23-6.5 is much more efficient, at both 1% and 0.03% concentration, in removing eicosane from the IRE surface than Neodol 25-3.
* .
NEODOL 23-8.5 I . 0 3 X i NEODOL 25-3 I . 0 3 X I
Experimental Section The use of a cylindrical internal reflectance cell (CIRCLE) in FT-IR studies of detergency has been described in detail elsewhere.16 Briefly, the experiment consists of coating the internal reflection element (IRE) of the CIRCLE with a layer of a solid “model soil” (the hydrocarbon eicosane mp = 37 “C) and then recording time-resolved infrared spectra, at ambient temperature, of the eicosane layer as it is exposed to an aqueous surfactant solution. The solution can be flowing over the IRE or static. If the surfactant solution exhibits effective detergency toward the eicosane, removal results in a decrease in intensity of the infrared bands due to eicosane in a series of spectra. Adsorption of surfactant onto the eicosane is also readily detectable. All spectra shown were obtained with the CIRCLE, equipped with a ZnSe IRE. It is the intention of this work to model a ”real world” surface and not a molecular monolayer. The eicosane layers were deposited on the IRE by slowly withdrawing the IRE from a hexane solution of eicosane. A syringe drive was used to slowly pull a thread from which the IRE was suspended. As the IRE leaves the solution,the hexane flashes off, leaving a reproducible eicosane layer on the IRE. Eicosane layer thicknesses could be varied by changing the concentration of eicosane in the hexane and the withdrawal rates. The sampling depth of the infrared radiation from the surface of the IRE outward is often calculated from the following expression:” dp =
A1
%(sin2 e - nz12)1/2
(9) Cox, M. F.J. Am. Oil. Chem. SOC.1986, 63, 559. (10) Engstrom, S.;Backstrom, K. Langmuir 1987, 3,568. (11) Casal, H.L.;Mantsch, H. H.; Camerson, D. G.; Snyder, R. G. J. Chem. Phys. 1982, 77, 2825. (12) Cameron, D.G.;Casal, H. L.; Mantsch, H. H. Biochemistry 1980, 19, 3665. (13) Casal, H.L.;Mantsch, H. H. Biochim. Biophys. Acta 1984, 779, 381. (14) Umemura, J.; Mantsch, H. H.; Cameron, D. G. J. Colloid Interface Sci. 1981, 83, 558. (15) Umemura, J.; Kawai, T.; Takenaka, T.; Kodama, M.; Ogawa, Y.; Seki, S.Mol. Cryst. Li9. Cryst. 1984, 112, 293. (16) Scheuing, D. R. Appl. Spectrosc. 1987, 44, 1343.
TIME ( M I N I Figure 2. Removal of eicosane from IRE surface by static 0.03% surfactant solutions, as indicated by the absorbanceof the eicosane
CHzscissoring band. The removal is much slower, compared to flowing solutions. Ned01 23-6.5 is again found to be more efficient than Neodol 25-3.
where AI is the wavelength of the radiation in the IRE (given by A/nl), 0 is the angle of incidence at the IRE-sample interface, and nZ1is the ratio of the refractive index of the sample to that of the IRE. With an average angle of incidence of 45O for the CIRCLE, n2 of eicosane = 1.5, and nl of a ZnSe IRE = 2.4, a sampling depth of 1.2 pm is calculated at the frequency of the water bending band (1640cm-’). Since the water band was always observed immediately upon filling the CIRCLE with either static or flowing surfactant solutions,the thickness of the eicosane layers prepared must have been, on the average, somewhat less than the sampling depth calculated above. The two nonionic surfactants investigated, Neodol 25-3 and Neodol23-6.5 (Shell Chemical Co.), were commercial materials used without further purification, dissolved in HPLC grade water (J.T. Baker). The structure of the Neodol materials is RO(CH2CH20),H. R is 12 to 15 carbons long, and x (the number of ethylene oxide groups) averages 3 for Necdol25-3, the relatively hydrophobic material. In the case of Neodol 23-6.5, R is 12 or 13 carbons long and the ethylene oxide chains are considerably longer; Le., x averages 6.5. Eicosane (99%) was obtained from Aldrich Chemical Co.
Results and Discussion A. Eicosane Removal. Comparison of Surfactants. The removal of eicosane from the surface of the IRE is monitored by the decrease in the absorbance of the eicosane CH2 “scissoring” band at 1471 cm-l, relative to local base line, in a series of time-resolved spectra. Eicosane (17) Harrick, N. J. Internal Reflection Spectroscopy; Interscience: New York, 1967.
Detergency of Nonionic Surfactants
Langmuir, Vol. 4, No. 6,1988 1279
was not removed by surfactant-free water, even at fast flow rates (60 mL/min). Neodol 23-6.5 always caused more extensive and more rapid removal of eicosane than did Neodol 25-3. Figure 1 compares the behavior of the flowing surfactant solution at both high concentration (1.0 w t %) and a lower concentration (0.03%). Qualitatively, increases in flow rate decreased the time required for removal. Even with static solutions, faster removal of eicosane by Neodol23-6.5 could still be detected (Figure 2). In both flowing and static experiments, removal was initially rapid and slowed significantly at longer times, a trend which agrees with that found by ellipsometric studies of triglyceride removal.1° A quantitative comparison of the kinetics of eicosane removal was not attempted for several reasons. Even though the absorbance of the CH2band was approximately equal in the spectra of the various layers prepared for each WAVENUMBERS IS00 I250 removal experiment, details about the surface roughness Figure 3. Spectra of hexadecane (a), a liquid hydrocarbon, and were lacking. More importantly, the calculation of layer of an eicosane (b) layer prepared for a detergency experiment. thicknesses from the absorbance of eicosane bands is The CH,scissoring band of hexadecane is found at lower frecomplicated by the fact that at least two optically distinct quency (1466.6 cm-’ vs 1470.3 cm-’) and is considerably broader layers are present: eicosane and aqueous surfactant sothan that of eicosane. The asymmetric (1458.4 cm-’) and symmetric (1378.3 cm-’) CH3deformation bands are also relatively lution. The sampling depth (dp) of the infrared radiation more intense in the hexadecane spectrum. from the IRE surface outward is a function of, among other things, the ratio of the refractive indices of the IRE and 4 4 1410 2 the optically rarer external medium (eicosane layer and aqueous sol~tion).’~ The relative thicknesses of eicosane . . e ~ * o e 1470 P F B ~PP( m and aqueous solution vary continuously during the removal A R a N40 e 1488 8 E process, which will affect dp continuously as well. The e D quantitative approach of Iwamoto and Ohta to multipleQ . YlDTH 23-8 5 14~8 WIDTH 25-3 layer internal reflection spectra may not be applicable W 3 8 t e FREO 23-8 5 rRE0 25-3 1 1 . E because of uncertainty in the refractive indices.l8 The A ’ 1480 4 N D 38 simplifying assumptions used in studies of protein adA A C T a sorption onto an IRE surface cannot be used.lg Clearly, 1489 2 y H 34? A 4 further work, perhaps with flat plate IRES and polarized , &AsA4 infrared radiation, will be necessary to evaluate the pos1488 0 3 2AA 3 10 13 20 25 30 sibility of extracting quantitative kinetic information from FT-IR spectra. TIME ( M I N I B. Eicosane CH2 Band Analysis. Solid methylene Figure 4. Frequency and bandwidth changes of the CH,scischains in an all-trans conformation within a triclinic unit soring band during detergency runs of Figure 2. The center of gravity frequency is calculated from the topmost five data points cell exhibit the CH2 deformation (“scissoring”)band near on either side of the peak maximum. Bandwidth is given at 1470 cm-1.20,21The frequency and width of this band will nine-tenths peak height, relative to the local base line.14 respond to any changes in the packing of the chains. For example, melting of a hydrocarbon chain introduces an layer to layer, which results in slightly different amounts increased population of gauche conformers, which causes of “amorphous” or disordered chains and which is detected a decrease in frequency of the band toward that of chains by slight variations in the CH2 scissoring bandwidths. in the completely disordered state, approximately 1467 Penetration of surfactant and water into the eicosane cm-’. Bandwidth increases are also noted with increases layer in a disordering (“liquefaction”) step would be exin chain disorder. Even more subtle changes in methylene pected to cause a downshift in frequency and increase in chain packing can be detected by changes in the scissoring bandwidth of the CH2 scissoring band of eicosane. The band. Factor group splitting of the band occurs when magnitude of the changes in the band will depend on the methylene chains are packed in an orthorhombic subcell relative fraction of eicosane which is disordered. in an orthorhombic or monoclinic crystal l a t t i ~ e . ’ ~ ? ~ ~ Under conditions of rapid removal (Neodol 23-6.5 a t Distortions of the subcell dimensions caused by temperhigh flow rates) most of the disordered eicosane is swept ature changes are detected by changes in the splitting of away from the surface of the IRE, and small changes in the two CH, bands in phospholipid bilayers.” frequency and width of the CH2band were observed. The Figure 3 compares the spectra of liquid hexadecane with use of a static surfactant solution, however, enhances dethose of two eicosane layers used in detergency experitection of an interfacial layer of disordered eicosane formed ments. The differences in the CH, scissoring bands (1466.6 (Figure 4) because the overall removal process proceeds cm-I vs 1470.3 cm-l) are readily observable, as are the much more slowly. In either case, removal of eicosane from differences in the symmetric methyl deformation band the IRE is correlated directly with changes in the CH, (1379 cm-’ in the liquid vs ~ 1 3 7 in 2 the solid layers). The band, which indicate a perturbation of the packing of the perfection of the eicosane crystals varies somewhat from molecules. Neodol23-6.5 is more effective at “liquefaction” and hence removal of eicosane than is Neodol25-3, even though, as discussed below, more of the latter adsorbs onto (18) Ohta, K.;Iwamoto, R. Appl. Spectrosc. 1985,39,418. the eicosane layer. (19) Fink, D. J.; Gendreau, R. M. Anal. Biochem. 1984, 139, 140. (20) Snyder, R.G.; Schactschneider, J. H. Spectrochim. Acta 1963,19, I
~
”
8.5.
(21) Holland, R. F.; Nielsen, J. R. J. Mol. Spectrosc. 1962,8, 383.
(22) Snyder, R. G.J. Chem. Phys. 1979,71,3229.
Langmuir, Vol. 4, No. 6, 1988
Scheuing and Hsieh r
J
I V!O
U
1sw
WAVENUMBERS
1m(!
Figure 5. Time-resolved difference spectra obtained from a detergency experiment with 1%Neodol25-3. The spectra of water and e i c m e have been subtracted. Times of exposure of eicosane
layer to static surfactant solution are, from the bottom spectrum, 1, 3, 10, and 30, min. Topmost spectrum is a reference of 1% Neodol 25-3 obtained in CIRCLE without a layer of eicosane present. Bands due to eicosane have been nulled out successfully, indicating that little disordering of the eicosane layer occurs, even though significant surfactant adsorption is indicated by increases in CH2 wagging bands due to ethoxylate groups (1350,1300,1250 cm-’, *) and C-0 stretching bands (1150-1000 cm-’). The spectra of the surfactants contain a broad, weak CH2 scissoring band near 1468 cm-’, which partially overlaps that of eicosane. The contribution of surfactant CH2absorbance to that of eicosane is practically negligible, however, even a t 1% surfactant concentrations. The largest eicosane CH2 band shifts are observed in the spectra obtained early in the Neodol23-6.5 runs, i.e., where the amount of adsorbed surfactant is a minimum. As will be discussed further below, Neodol25-3 adsorbs onto the eicosane layer to a greater extent, yet causes less shift in the CH2 scissoring band and less removal than Neodol 23-6.5. C. Time-Resolved Spectra. The spectrum of the surfactants can be obtained by sequential subtractions of the spectra of liquid water and the original, dry eicosane layer. Figures 5 and 6 show a series of such difference spectra for both surfactants, obtained from detergency runs with static solutions a t 1% surfactant concentration. The intense eicosane bands are nulled out fairly well in the difference spectra from the 1% Neodol25-3 run. The slight oversubtraction evident as a negative-going peak at 1470 cm-l indicates a slight downward frequency shift of the CH2 band, as discussed above. The increase in the intensity of the intense C-0 stretching bands between 1150 and 1040 cm-’ indicates adsorption of the surfactant onto the eicosane layer. The C-0 band maximum is shifted slightly, toward 1110 em-’, early in the run. With increasing exposure time, this complex band changes until it becomes similar to, but not identical with, the reference spectrum. The series of spectra from the 1% Neodol23-6.5 run are quite different. The shift in the eicosane CH2 band is largest in the 1.0-min spectrum, decreasing with exposure time. The substantial perturbation of the eicosane structure is also indicated by the disordered or ”liquid-like” CH3 band a t 1378 cm-’. The C-0 band is composed of several components, at 1130,1110, and 1088 cm-l. During the first 10 min, the period of most rapid eicosane removal,
\\ A I F N L M BERS
IO00
Figure 6. Time-resolveddifference spectra from a detergency
experiment with 1%Neodol23-6.5. Exposure times (from bottom) are 1,3,10,and 30 min. Topmost spectrum is a 1%Neodol23-6.5 reference. Significant disordering of eicosane is indicated by the CH2scissoring band which, because of a frequency and bandwidth change, cannot be completely nulled out. The intensity of this shifted CH2 band (as well as that of the “disordered”CH3 group band at 1379 cm-’) is largest at 1min, indicating rapid disordermg of the eicosane layer. The surfactant C-0 bands are marked with an arrow.
1 L l 1.500
WAVENUMBERS
1om
Figure 7. Time-resolved difference spectra from a detergency experiment with 0.03% Neodol25-3. Times of exposure are,from the bottom spectrum, 1,3,15,25, and 30 min. At 1and 3 min, loss of some disordered eicosane is indicated by negative-going bands at 1469 and 1379 cm-l. Only a slight increase in eicosane disordering is noted at longer times. Surfactant C-0 bands indicate a much lower interfacial concentration and EO group
conformations quite different from those of an aqueous envi-
ronment.
the llOO-cm-’ component increases in relative intensity. The overall intensity of this band is much weaker than in the case of Neodol 25-3. The difference spectra from the 1% surfactant runs support the conclusions concerning eicosane disordering. The C-0 bands, however, contain contributions from both interfacially bound surfactant and surfactant in the bulk aqueous phase. In the case of 0.03% surfactant, the C-0 bands observed are due almost entirely to interfacially bound molecules, since the bands are not observed in an aqueous reference solution spectrum. The difference spectra in Figures 7 and 8 indicate that eicosane disordering is slower at the lower surfactant
Langmuir, Vol. 4, No. 6, 1988 1281
Detergency of Nonionic Surfactants
L
1200 ~
1500
WAVENUMBERS
~~~
I 000
Figure 8. Time-resolved difference spectra from a detergency experiment with 0.03% Neodol23-6.5. Times of exposure are, from the bottom spectrum, 1,3, 10, 20, and 30 min. Significant disordering of eicosane, indicated by increasing CH2 and CH,
bands, occurs but more slowly than at higher surfactant concentration. Increases in the lower frequency surfactant C-O bands (1100-1080 cm-l), relative to the bands at 1150-1120 cm-', are correlated with the disordering of eicosane caused by water uptake and surfactant penetration. concentrations corresponding to slower adsorption and penetration of surfactant into the eicosane layer. The C-O band pattern in the case of Neodol 23-6.5 consists of multiple components, as are observed early in the high concentration run (Figure 6). The overall extent of eicosane disordering is much smaller in the 0.03% Neodol 25-3 run. The difference spectra suggest a slight increase in ordering of the eicosane layer, or a preferential loss of a small amount of disordered eicosane, during the first few minutes. The trend is reversed at longer times, with a downward frequency shift of the CH2 band observed. The amount of adsorption of the surfactant is much lower at this concentration. Several components comprise the C-0 band, but the intensity ratios are quite different from those in the spectra from the Neodol 23-6.5 run. D. Interpretation of Surfactant Spectra. The major band of the ethylene oxide (EO) groups is broad and complex for several reasons. The band is due primarily to C-0 stretching but contains contributions from C-C stretching and CH2 rocking.23 The variety of conformers of the EO chains in the melt or in aqueous solutions broadens the band so that assignment of specific conformations to the bands between 1160 and 1050 cm-' is apparently not possible. The extent of hydrogen bonding of EO groups to water molecules was shown to affect the Raman spectra of nonionic surfactants, but exact conformational assignments of certain bands were again somewhat s p e ~ u l a t i v e . ~ ~ We do not seek to establish exact EO group conformational assignments to C-0 bands observed in this study but only to relate band shape changes to the extent of interaction of EO groups with water and hydrocarbon molecules. A study of a model system (CI2EO5in hexadecane and water, Figure 9) clearly established that the (23)Matsuuro, H.; Fukuhara, K. J. Polym. Sci., Polym. Phys. E d . 1986,24, 1383.
(24) Bortlett, J. R.; Cooney, R. P.J. Chem. SOC.,Faraday Trans. 1 1986,82, 597.
WAVENUMBERS
I000
Figure 9. Changes in C-0 stretching bands of C12E05caused by addition of water. The band maximum is shifted toward lower frequency (1098 cm-') at high water levels (98% H20,spectrum a) relative to neat surfactant (1108 cm-l, b). Reduction of sur-
factant-water and surfactant-surfactanthydrogen bonding in the L2 phase (98% hexadecane) causes a shift in band maximum to higher frequency (1119 cm-', c). Spectra of water and hexadecane have been subtracted. All spectra obtained in CIRCLE.
I
1200
WAVENUMBERS
Io00
Figure 10. Comparison of the spectra of solid and molten Neodol 23-6.5. Increased ordering of EO groups in the solid state yields C-0 stretching bands at 1149, 1106, and 1064 cm-'. Spectra obtained from layer of surfactant clinging to IRE of CIRCLE.
C-0 band maximum shifts toward lower frequency (1108 to 1098 cm-l) as the degree of surfactant hydrogen bonding to water increases. Dilution of surfactant in a liquid hydrocarbon, such as hexadecane, shifts the maximum to about 1119 cm-l, as hydrogen bonds between terminal hydroxyl and EO groups are eliminated. It should also be noted that in the solid state the preferred helical conformation adopted by the surfactant molecules gives rise to several narrow C-0 bands at 1149, 1114, and 1064 cm-', as illustrated in Figure 10. The high-frequency component (1150 cm-') of the C-0 bands in the spectra obtained early in the 0.03% surfactant runs (Figure 8) could be due to a population of adsorbed EO groups with a specific, solid-like conformation. The disordering of eicosane already discussed in terms of eicosane band shifts, caused by penetration of Neodol23-6.5, results in a transfer of EO groups from a micellar environment to a hydrocarbon environment. The C-0 band shoulders between 1120 and 1100 cm-' may reflect this transfer. As eicosane disordering proceeds, swelling of the hydrocarbon-surfactant mixture with water molecules
1282 Langmuir, Vol. 4, No. 6, 1988
Scheuing and Hsieh
Eicosane
N25-3
H20
Eicosane
1200
WAVENUMBERS
1000
Figure 12. Changes in the C-O stretching bands of commercial surfactanb with water content changes. Bottom spectra are neat Neodol25-3 (C-0 band maximum 1111cm-l, a), L2 phase (1104 cm-', b), and lamellar el (1096 cm-l, c). Top spectra are neat Ned01 23-6.5 (1106 cm-K,d), L2 phase (1096 cm-l, e), and L1 phase (1094 cm-I, f). HD
N23-6.S
Figure 11. Phase diagrams of Neodol25-3 and Neodol23-6.5eicosane-water mixtures. causes the increase of C-0 bands characteristic of more highly hydrated EO groups, between 1100 and 1080 cm-l. Neodol25-3 adsorbs onto the eicosane layer, but a rapid increase in the relative amount of hydrogen-bonded EO groups is not observed. Hence, swelling of the eicosane layer with surfactant and water molecules does not occur nearly as rapidly, and the removal process stalls. The time-resolved spectra suggest that several surfactant-water-hydrocarbon phases are present at the interface during the removal process. The relative amounts of various surfactant-eicosane structures change continuously a t the interface, depending on surfactant type, concentration, and the amount of mechanical action accomplished by flow of the solution. E. Interfacial Structure vs Phase Diagrams. The spectra of eicme-surfactant-water mixtures from various areas of the phase diagram and of the surfactants themselves can be used to interpret the time-resolved interfacial spectra from the detergency experiments. Various one-phase regions are identified from the phase diagrams (Figure 11). They only exist for eicosane levels less than 10%. Mesophases cover a broad range of composition along the surfactant-water line. The relationship of the detergency phenomenon to the formation of mic r ~ e m u l s i o n sand ~ * ~lamellar4~6~7*8 phases in liquid soil removal has been discussed and a possible diffusion pathway proposed.+8 Selected samples from each sample phase region were subjected to IR analyses. The resulting spectra (Figure 12) show that the C-0 band undergoes the same trend as observed in the ClzEO5/hexadecane/H2O system (Figure 9). The band maximum decreases in frequency as the amount of water increases in the composition. Notice, however, that both the CH2 scissoring (- 1467 cm-') and the methyl deformation (- 1379 cm-l) remain much the same during the L2 liquid crystal L, transitions, which is indicative of a liquid-like environment for the alkyl chain of the surfactant. The spectra of the single-phase samples prepared from both surfactants show similar changes in the C-0
-
-
stretching bands with degree of hydration of the ethylene oxide groups. The time-resolved spectra, however, indicate substantial differences between the two surfactants in the interfacial structures and compositions formed. In the Neodol 23-6.5 runs (Figures 5 and 7) the adsorption of surfactant onto the eicosane layer promotes the penetration of both surfactant and water into the solid soil, inducing the disordering of eicosane very effectively. The intake of water and resulting disordering of eicosane appear to follow the initial enrichment of the interface with Neodol23-6.5. The interfacial kinetics may well follow the phase transitions as illustrated in the phase diagram (Figure l l ) , which eventually leads to the release of soil to the wash liquid by the mechanisms discussed in the introduction. In the Neodol25-3 runs (Figures 6 and 8), however, the hydration of the ethylene oxide groups of adsorbed molecules is much smaller. At low bulk surfactant concentration, the adsorbed surfactant may adopt a solid-like character, while a t higher bulk concentration many of the ethylene oxide chains appear to be in an environment similar to that of neat surfactant. The phase transitions at the interface are hampered, and the disordering of the eicosane layer becomes very inefficient. The significance of phase transformations for both polar3,5and oil in detergency action has been addressed. Generally, the involvement of a lamellar phase was found essential. Thus the formation of a lamellar layer at oillwater interface has been r e p ~ r t e d . ~In, ~this . ~ study with a nonpolar solid hydrocarbon, however, we find that the interface is comprised of a variety of compositions from different phases. The key element for the removal of solid hydrocarbon is the disordering process resulting from the surfactant and water penetration into the soil. A definite correlation between the detergency and phase-inversion temperature (PIT) of hydrocarbon oil/ nonionic surfactant/water has been The PIT concept for solid hydrocarbons has yet to be investigated. A temperature-dependent phase-performance profile is needed to define the existence and the role of a PIT for a solid hydrocarbon. At the temperature of our study (25 "C), defining PIT as the detachment of the L1 phase from the water corner, the Neodol 23-6.5/eicosane/H20 system is below its PIT and the Neodol25-3/
Langmuir 1988,4, 1283-1288 eicosane/H20 system is above its PIT.
Conclusions Disordering of solid eicosane “soil” is a necessity for removal from a surface using an aqueous nonionic surfactant solution as a “detergent”. Neodol23-6.5 provides a greater disordering of solid eicosane, with the assistance of water, than does Neodol25-3. The penetration of water into the eicosane layer occurs after the adsorption of surfactant onto the eicosane surface. Neodol 25-3, although it adsorbs to a much greater extent onto the eicosane surface, is relatively inefficient at disordering the eicosane layer because of a lack of significant participation of water in the process.
1283
FT-IR has been shown to reveal structural and compositional changes in the region of interest in studies of detergency, i.e., the soil-surfactant solution interface. While the spectroscopic data cannot specify the exact detergency pathway due to the complexity of the interfacial phases involved, the key elements of surfactant and water penetration and the disordering of solid eicosane are clearly established.
Acknowledgment. We thank Daniel Webster for his operation of the FT-IR and Janice Briones for her determination of the phase diagrams. The support and encouragement by the management of the Clorox Technical Center are also gratefully acknowledged. Registry No. Eicosane, 112-95-8.
Adsorption Sites for Water on Graphite. 4. Chemisorption of Water on Graphite at Room Temperature Kazuhisa Miurat and Tetsuo Morimoto*s$ Department of General Education, Tsuyama National College of Technology, Tsuyama, 708, Japan, Department of Chemistry, Faculty of Science, Okayama University, Okayama, 700, Japan Received December 30, 1987. I n Final Form: April 20, 1988 The chemisorption of H2O on graphite is investigated by repeating the measurement of the adsorption isotherm of H20 at 25 “C and the pyrolytic analysis of surface oxides formed therefrom. It is found that five kinds of gases, H20, C02,CO, H2, and CH,, are evolved by the pyrolysis of the sample after the adsorption of H20. The total amount of the gases evolved increases with increasing final H20vapor pressure at which the adsorption isotherm is measured. Three distinct steps appear in the adsorption isotherm of H20 on the sample treated at 1000 “C in vacuo, i.e., at relative pressures of 0.005, 0.025 and 0.444,respectively, while they disappear when the sample is preexposed to saturated H 2 0 vapor at 25 “C. The first and second steps are clcsely related to the chemisorption of H20,and the third step is due to physisorption. The second and third steps are associated with pores in graphite. Especially, the second step is mainly due to the pore filling of H20 into the slit-shaped pores that are formed by removal of a sheet of the basal plane of graphite, and it takes a long time for the attainment of the adsorption equilibrium. The mechanism of the chemisorption of H20 on graphite, which takes place at the terminal edge carbon atoms around basal planes, is discussed.
Introduction In order to identify the physisorption sites for H 2 0 on graphite, the pyrolysis of surface oxides present on the surface after the adsorption of H 2 0 has been Through these investigations an unexpected phenomenon has been discovered; on the sample treated a t 1000 OC in vacuo, two distinct steps appear around relative pressures of 0.1 and 0.5 in the first adsorption isotherm of H20,while they disappear from the sample exposed to saturated H 2 0 vapor. Such steps have never been observed when the sample is pretreated a t temperatures lower than 700 “C. The appearance of the steps was accompanied by a long equilibration time for adsorption, which suggests the occurrence of the chemisorption of H 2 0 on the surface of graphite and/or the pore filling of the adsorbate. Some papers report the chemisorption of H 2 0 on carbonaceous materials. Pierce et al.4 contacted a Graphon sample with liquid H 2 0 in a sealed tube a t temperatures between 25 and 150 “C and detected C02 and H2 in the gas phase. Smith et aL5 also reacted Spheron 6 and Graphon with H 2 0 in a sealed vessel a t temperatures from 25 t Tsuyama
National College of Technology.
* Okayama University.
to 200 “C. As the result, H2, CO, and C 0 2were detected in the gas phase, and two kinds of surface oxygen complexes, which generate C 0 2 and CO on ignition, were postulated. For the investigation of the kinetics and mechanism of the reaction between carbon and steam, Yang and Dum6 examined the growth rate of etch pits and the change in their conformations with the aid of transmission electron microscopy around 700 OC; they concluded that H 2 0 molecules are dissociatively adsorbed on the (1010) surface of the graphite structure. These experiments are concerned with the chemisorption of H 2 0 on the surface oxygen complexes at relatively low temperature8 or on the bare surface of graphite at high temperatures.6 However, the chemisorption of H20 on graphite at room temperature remains to be studied. In the present investigation, we have attempted to clarify the nature of the step in the H20adsorption isotherm and the details of the H 2 0 chemisorption on graphite by measuring (1)Morimoto, T.; Miura, K. Langmuir 1985, 1 , 658. (2) Morimoto, T.; Miura, K. Langmuir 1986, 2, 43. (3) Miura, K.; Morimoto, T. Langmuir 1986, 2, 824. (4) Pierce, C.; Smith, R. N.; Wiley, J. W.; Cordes, H. J. Am. Chem. SOC.1951, 73, 4551. (5) Smith, R. N.; Pierce, C.; Joel, C. D. J.Phys. Chem. 1954,58,298. (6) Yang,R. T.; Duan, R. Z. Carbon 1985,23, 325.
0743-7463/88/2404-1283$01.50/0 0 1988 American Chemical Society
