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Langmuir 1987, 3, 568-574
Ellipsometry as a Tool To Study Detergency at Hard Surfaces Sven Engstrom" Physical Chemistry, University of UmeA, S-901 87 UmeA, Sweden
Kjell Backstrom Physical Chemistry 1, Chemical Center, S-221 00 Lund, Sweden Received October 16, 1986. I n Final Form: February 18, 1987 We have investigated the possibility of using ellipsometry for the study of triglyceride removal from poly(viny1chloride) (PVC) and chromium surfaces by means of detergents. The reason for our interest in ellipsometry is twofold: (i) the technique may be used for determining the efficiency of a detergent solution in its ability to remove soily dirt from a hard surface; (ii) the detailed information about the optical properties of the adsorbed f i i obtained with ellipsometry may be helpful in the interpretationof the removal process at a molecular level. In this work we focus our interest on various aspects of the triglyceride/substrate interface. We have found that the need for highly reflecting surfaces in ellipsometry does not rule out the use of PVC as a substrate. The spinning technique used for depositing the triglyceride layers on the substrates was shown to give fairly reproducible values of the ellipsometer angles. In order to determine the amount of deposited triglyceride on the surfaces, we have used a formula proposed by McCrackin and Cuypers et al. The masses on PVC obtained with this formula were shown to be lower for triolein and higher for tripalmitin than the corresponding masses determined with a radiotracer technique. The reasons for these discrepancies are probably many, one being the insufficient optical contrast between the PVC surface and the triglyceride film. Since only the difference in the amount of triglyceride on the surface before and after application of a detergent solution is needed in order to study the removal efficiency, we conclude that ellipsometrymay be used for this purpose. Two examples of triglyceride removal by means of detergent solutions are given.
Introduction Ellipsometry is an optical technique for measuring properties such as the thickness and refractive index of thin films at interfaces. The method is based on the fact that the film will change the state of polarization of an incident light beam upon reflection a t the interface, and the size of this change is dependent on the film properties mentioned above. If some kind of process (adsorption, desorption, etc.) occurs at the interface, ellipsometry makes it possible to follow that process continuously. Comprehensive reviews of ellipsometry are given by Azzam and Bashara' and Rzhanov and Svitashev.* Ellipsometry has been applied to the study of interfaces of various types, e.g., solid/gas, solid/liquid, liquid/liquid, and liquid/gas. Semiconductor surfaces, which are examples of the first kind, have been extensively studied, and the resolution of measured film thicknesses are in some cases as good as one or a few tenths of a nanometer. For interfaces involving a liquid as one of the interfacial components, the resolution is less striking, except for the case where well-defined layers of amphiphilic molecules are applied onto a surface with the so-called LangmuirBlodgett t e ~ h n i q u e . ~ Representative examples of applications of ellipsometry with various types of solid/liquid interfaces are polymer/polyelectrolyte adsorption* and interaction between proteins and lipids6and between antibodies and antigens.6 The competitive adsorption of milk proteins has been investigated by Arnebrant and N ~ l a n d e r . ~ These studies (1) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light;North-Holland: Amsterdam, 1977. (2) Rzhanov, A. V.; Svitashev, K. K. Adu. Electron. Electron Phys.
1979, 49, 1.
(3) Cuypers, P. A. Thesis, Rijkuniversiteit Limburg, The Netherlands,
all have in common that the substrate is a solid surface. An expample where both phases forming the interface are liquids is the study of microemulsion structure by Beaglehole et a1.8 We have previously reported some preliminary results obtained with ellipsometry applied to the study of triglyceride removal from poly(viny1 chloride) (PVC) and chromium surfaces by means of detergent^.^ The reason for our interest in ellipsometry in this context is twofold: (i) the technique may be used for determining the efficiency of a detergent solution in its ability to remove dirt froma soily surface; (ii) the detailed information about the optical properties of the adsorbed film obtained with ellipsometry may be helpful in the interpretation of the removal process at a molecular level. In this work we focus on various aspects of the triglyceride/substrate interface, while the efficiency of removing triglycerides of various detergents is the subject of a separate paper.1° Chromium surfaces have been extensively used as substrates in ellipsometry studies, but PVC has, as far as we know, not been used to any greater extent. The problem with PVC in this case, in comparison with chrominm, is its low reflectivity and its low refractive index (comparable to that of triglycerides). We investigate which consequences these properteis of PVC have on the sensitivity in the ellipsometer readings for surfaces immersed in water, without and with deposited triglyceride films. The lower resolution in measured thicknesses of liquid systems is a consequence of a less well-defined boundary between the film and the liquid phase. In fact, in many cases it is inadequate to discuss the film properties in terms of thickness due to inhomogeneity of the film, and a better measure is the amount of material (protein, lipid, etc.) in
1976. (4)
Takahashi, A.; Kawaguchi, M. Adu. Polym. Sei. 1982, 46, 1. (5) Cuypers, P. A.; Corsel, J. W.; Janssen, M. P.; Kop, J. M. M.; Hermens, W. T.; Hemker, H. C. J. Biol. Chem. 1983, 258, 2456. (6) Jonsson, U.; Malmqvist, M.; Ronnberg, I. J. Colloid Interface Sci. 1985,103, 360. (7) Amebrant, T.; Nylmder, T. J . Coloid Interface Sei. 1986,I 11,529.
(8) Beaglehole, D.; Clarkson, M. T.; Upton, A. J. Colloid Interface Sei.
1984,101,330. (9) Backstrom, K.; Engstrom, S.; Lindman, B.; Arnebrant, T.; Nylander, T.; Larsson, K. J. Colloid Interface Sci. 1984,99,549. (10) Backstrom, K.; Engstrom, S. J . Am. Oil Chem. SOC.,in press.
0743-7463187/2403-0568$0l.50/0 0 1987 American Chemical Society
Langmuir, Vol. 3, No. 4 , 1987 569
Detergency a t Hard Surfaces
Personal computer
F i g u r e 1. Schematic picture of a n automated thin-film ellipsometer.
the film. This has led to the development of some equations in an attempt to relate the ellipsometer readings to adsorbed mass. Some relevant equations of this kind, McCrackin’s,’l de Feijter’s,12 and C ~ y p e r s ’ , ~will J ~ be discussed below, and the results obtained with one of them will be compared to masses obtained with a radiotracer technique. The rest of the paper deals with experimental details and theory, the latter of which includes a brief description of the fundamentals of so-called null ellipsorhetry and a discussion of the procedure for mass determination presented by the authors mentioned in the previous paragraph. Under Results and Discussion we present data from various ellipsometer measurements and discuss them with respect to substrate quality, the reproducibility in film deposition, and the validity of the ellipsometer mass equations. Two examples of the effect of detergent on the triglyceride film are given as well and discussed briefly. Finally, we summarize our findings under Conclusions. Experimental Details The Ellipsometer. The measuring instrument was a modified automated Rudolph thin-film ellipsometer, Type 43603-2003, s c h e m a t i d y depicted in Figure 1. The instrument was equipped with computer-steered stepping motors on the polarizer and the analyzer, making it possible to measure one position every 4 s and thus to follow the process occurring a t the interface continuously. Moreover, besides steering the motors, the computer has also been equipped with a n analysis program which solves the relevant equations (see below) for optical properties of the system under study during the course of the experiment. This calculation somewhat lengthens the interval between two successive measurements (by about l s). Test-Slide P r e p a r a t i o n . The slides used were of poly(viny1 chloride) (PVC) and vacuum-deposited chromium on glass, with a film thickness of a few hundred nanometers. Precleaning of the slides was very important in order to obtain reproducible results. Both kinds of slides were subjected to the following procedure: (i) cleaning in detergent solution and (ii) rinsing in tap water and doubly distilled water. Furthermore, the PVC slides were finally cleaned in ethanol, while the chromium slides were treated with hot acetone and finally plasma cleaned with a Harrick plasma cleaner, Type PDC-BXG, connected to a vacuum pump. (11)McCrackin, F. L. A Fortran Program for Analysis of Ellipsometer Measurements; National Bureau of Standards Technical Note 479;National Bureau of Standards: Washington, DC, 1969. (12)de Feijter, J. A.; Benjamina, J.; Veer, F. A. Biopolymers 1978,17, 1759. (13)Kop, J. M. M.; Cuypers, P. A.; Lindhout, T.; Hemker, H. C,; Hermens, W. T. J. Biol. Chem. 1984,259,13993.
The plasma cleaning removed all organic contamination and gave a hydrophilic character to the chromium surface, as was seen from water wettability. Film Deposition. Prehaps the simplest way to obtain a layer of material on a slide is to dip the slide manually in a solution of the material in question. However, in order to improve reproducibility, we have found it more convenient to use the following method: a drop of solution, in our case typically a 10 mg/mL solution of triglyceride in toluene, is put on the slide, which is placed horizontally in a spinner, and then rotated a t a speed of 4000 rpm for 5 min. During that time the solution spreads on the surface, the solvent evaporates, and a film of the solute remains. Measurements. After it was cleaned, the slide was placed in a cuvette containing 4.5 mL of doubly distilled water, and the optical properties of the slide were recorded. The same procedure was repeated with the slide after film deposition, and with the new optical parameters the amount of material in the film could be determined. Surfactant solution (0.5 mL) was then pipetted into the cuvette and the change in polarizer and analyzer angles were recorded continuously. All measurements were carried out a t 25 “C. R a d i o t r a c e r Counting. For the radiotracer counting, 14Clabeled tripalmitin (=TP) and triolein (=TO), with known specific activity, were used. The deposition of the labeled substances on the surface was performed as described above. The radioactivity of a film was measured by a Geiger-Muller counting apparatus. In order to obtain absolute masses on the substrate, the measured film activity was compared to the activity of a polymer sheet, the latter with known specific activity. The film thicknesses were sufficiently low to prevent self-absorption of the radiation in the film. The 14C-labeled triglycerides and the reference sheet were manufactured by Amersham, England.
Theory Null Ellipsometry. The basic equations of ellipsometry were derived by Drude about 100 years a g ~ . ~ ~InJ p ~ the variant called null ellipsometry, used in this work, Drude’s equations relate two angles, A and J/,which in turn depend on the angles of the polarizer and the analyzer of the ellipsometer (see Figure l),to the ratio of the overall reflection coefficients of light parallel (R,) to the plane of incidence and of light normal (R,) to this plane. The key equation is R,/R, = tan 1c. exp(iA) (1) where the definitions of R, and R,, relevant for the present ( 1 4 ) Drude, P. Ann. Phys. 1889,272, 532;1889,275, 481;1890,275,
481.
Engstrom and Backstrom
570 Langmuir, Vol. 3, No. 4, 1987
Figure 2. Schematic picture of a substrate/film/bulk system. The definitions of the symbols are given in the text.
work, are given in the Appendix. In null ellipsometry one adjusts the polarizer (P) and the analyzer (A) angles in order to minimize the light intensity to the detector (see Figure 1). There are four different combinations of (P,A) angles, giving rise to a minimum detector signal, and if all of them are determined, a so-called four-zone measurement is obtained. An advantage with such a procedure is that almost all optical deficiencies of the equipment are compensated for, but at the expense of time. For absolute determinations of film optical properties one should make four-zone measurements. However, for relative measurements, as in our case (amount of triglyceride on the surface before and after treatment with detergent solution), one-zone determinations should, in principle, be sufficient. Examples of measurements in different zones are given below. Evaluation of Film Refractive Index a n d Thickness. For the simplest case of one isotropic, homogeneous, and perfectly planar film, visualized in Figure 2, we may summarize eq 1 in the following form
f(No,N,,N2,X,90,d) = tan $ exp (;A)
(2)
indicating the many parameters involved. In eq 2, No,N1, and N2 are complex indices of refraction. In all six parameters, N = n - i k , which in our case are reduced to four. These are No = nb (b = bulk), N,= nf (f = film), and N 2 = n, - i k , (s = surface). X is the laser light wavelength (632.8 nm), a0is the angle of incidence of the light beam (6S0), and d is the thickness of the film. From eq 2 we conclude that from the two measured angles one may determine a t most two of the remaining parameters in f a The wavelength and angle of incidence are given beforehand and the refractive indices of both the bulk solution and the bare surface in this bulk solution may be determined separately, the latter from another of Drude’s equations.’ We are thus left with nf and d. Even for this simple case, eq 2 has to be solved iteratively in order to determine nf and d. However, the films occurring in most liquid systems do not meet the “ideal” conditions on which eq 2 is based. The films, whether they are polymer, protein, or lipid, will be more or less anisotropic and inhomogeneous and have rough surfaces. These facts imply that nf and d , determined from eq 2 for a real film, should be regarded as averages since these numbers represent the ideal film with optical properties of the real film ( n fand d determined from eq 2 for real films are from now on denoted by iif and d, respectively). In fact, we always determine film refractive indices which are lower than one would have expected for a pure triglyceride film, and, furthermore, iif decreases while d increases when the triglyceride film is removed with a detergent. These observations strongly indicate that the tryglyceride films are inhomogeneous and/or have rough surfaces. At first thought one would perhaps have expected d to decrease as triglyceride is removed. However, during the
detergency process an inhomogeneous transport layer forms consisting of solubilized and/or emulsified triglyceride in the water phase outside the triglyceride film. Thus, d is not a suitable parameter to study if one wants to determine the capability of a detergent to remove triglyceride from a film, since d also depends on the thickness of the inhomogeneous layer. In the next subsection we present two equations from which we are able to estimate the amount of triglyceride in the film. Mass Calculations from Ellipsometer Data. The amount of adsorbed substance (triglyceride, protein, etc.) per unit surface may formally be written
where c(z) is the concentration profile of the substance (the z-axis is directed normal to the substrate surface located at z = 0). We are thus faced with the problem of relating the measured film properties, iif and d , to the integral of c ( z ) . For the case of protein films, two equations are in use, one due to McCrackinl’ and Cuypers et aL5 and the other due to de Feijter et a1.I2 Both these equations consider the effect of homogeneously mixed films with two components (bulk solution and protein) on the film refractive index and thickness. To obtain McCrackin’s mass formula, eq 3 is rewritten as 2‘
rMcC
=
C(z)
dz = p o p p p ( f i f ) ~
(4)
where pop is the specific density of the pure protein (thus, change of the protein partial specific density with concentration is not considered), pp(ftf)is the volume fraction of the protein in the film, and d is the average film thickness as obtained from eq 2. pp(ftf)is assumed to be dependent on the average refractive index of the film through the fundamental Lorenz-Lorentz equation of dielectric continuum theory15 pP(fif)= k ( 4 - g ( n b ) ) / k ( n , )- g(nb)) (5a) where g ( n J = (n? - U / ( n L 2+ 2)
(5b)
de Feijter et al. begin by assuming that the refractive index profile of the film is a linear function of the protein concentration, Le., nf(z)= nb (dnf/dc)c(z) (6)
+
This assumption was shown to be valid up to high concentrations (-0.4 g/cm3) of the bovine serum albumin and lysozyme solutions. The equation for nf(z)is then inserted into two equations proposed by McCrackin and Colson,16 which relate the average itf and d to the real refractive index profile. The McCrackin-Colson formulas read fif =
L‘nf(z)(nf(r)- nb) dz/Lz’(nf(z)- nb) dz ( 7 )
and
d = l z0’ ( n f ( z -) nb) dz/(fif - nb)
(8)
and were shown to be valid for linear, exponential, and Gaussian refractive index profiles.16 Combining eq 3 and (15) Bottcher, C. J. F. Theory of Electric Polarization; Elsevier: Amsterdam, 1973; Vol. I. (16) McCrackin, F. L.; Colson, J. P. Natl. Bur. Stand. (US’.),Publ. 1964, 266, 61.
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Langmuir, Val. 3, No. 4,1987 571
Figure 3. Electron micrographs of a PVC (A, C, E) and a chromium (B, D, F)surface without (A, B) and with (C-F)a deposited layer of tripalmitin. The white line in each picture represents 1wrn on the surface, except for (F),where the line represents 10 pm. The magnifications are given in the lower right corner of each picture. eq 6-8 gives the following equation for the adsorbed masn according to de Feijter et al:
refractive index of the film is lowered when roughness is taken into account (if Re N , is >l). The anisotropy of triglyceride films may also play a nonnegligible role, esrdeF = a(%- nd/(dnr/dc) (9) pecially for the tripalmitin films, since this substance is Both eq 4 and 9 have been tested for different protein solid at the temperature used. systems, and the estimated masses have been compared In summary, we use eq 4 to determine the amount of to the masses obtained with the radiotracer t e c h n i q ~ e . ~ . ~ adsorbed triglyceride, having in mind its applicability to These comparisons show that the mases given by eq 4 and inhomogeneousfilms only, not forgetting the influence of 9 deviate from the radiotracer results by at most about surface roughness and anisotropy which are difficult to +/-lo%. Furthermore, rd, is generally lower than rMec, take into account. In the next section we compare masses but the difference is small. One difficulty with eq 4 is to estimated with eq 4 with the corresponding radiotracer estimate a reasonable value for the refractive index of the values. pure protein, n,,,and this problem has been discussed by Kop et al.I3 With eq 9 the problems are the validity of the Results a n d Discussion linear aemmption, i.e., eq 6, at high protein concentrations, PVC and Chromium Slides. We have used grey PVC and the difficulty of determining (dnr/dc) for proteins of slides and chromium slides. The chromium slides were low solubility. prepared especially for ellipsometry and were thus exIn our case the problem of obtaining (dnl/dc) is certainly tremely smooth, which was also seen by electron microsdue, as well, to the insolubility of triglyceride in water. copy (see Figure 3B). The PVC slides were found to he Refractive indices of triglycerides in the liquid state are reasonably smooth as well, but some dark spots appeared available, which therefore motivates the use of McCrackin’s on the micrographs, as is seen in Figure 3A! However, formula, eq 4. However, one should note that eq 4 takes these spots were few (less than five percent of the area) only inhomogeneityof the film into account and does not and their size was at most 500 nm in one direction, more consider surface roughness. Fenstennaker and McCrackin often about 150 nm. Since the light wavelength used is have performed model calculations of the influence of 632.8 nm and the light beam area on the slide is about 2 surface roughnegs on elliwometer parameters.” They find mm2, we conclude that the PVC slides ought to be suffithat the effect may be substantial and that the apparent ciently smooth to serve as substrates in ellipsometry. The optical properties of the slides are given in Table r (17) Fenstermaker, C. A.; McCrackin, F. L. Surf. Sci. 1969, 16, 85. I. The complex refractive index N = n - ik is an average
572 Langmuir, Vol. 3, No. 4,1987
Engstrom and Backstrom Table I. Average Complex Refractive Index, N, = n I - ik,, for PVC and Chromium Slides Immersed in Watera slide PVC (grey) chromium
n '
5
13
9
I
1
-10
0
20
10
30
30
20
'
d-0
nrn I
100
k8
0.006 i 0.003 2.696 i 0.028
*
"At 25 "C; obtained by ellipsometry in zone 1. The standard deviation is based on 25 values, and the refractive index of water, nb, is 1.332.
I
26l
n8
1.530 0.001 3.314 i 0.027
110
120
130
3 / degrees
Figure 4. Simulated (+) values of A and $ due to ideal films with different optical properties on a PVC slide (A), a n absorbing PVC-analogue slide (B), and a chromium slide (C). The simulated points with constant refractive index and varying thickness are connected. nr= 1.350, 1.375,..., 1.525 and d = 0, 20, ..., 140 nm. Typical experimental values of A and IC. are dso given in (A) and the arrow indicates the direction of the changes with time.
for each type of slide immersed in water, determined from the ellipsometer readings (one-zone measurements) using Drude's equation for a film-free surface. The calculated numbers in Table I are based on 25 different measure-
ments. Since we are mainly interested in films containing triglycerides, which are nonabsorbing at the wavelength used and have real refractive indices in the approximate range of 1.46 (triolein) to 1.52 (tripalmitin), we find from Table I that the optical contrast is large between chromium and triglyceride, but small between PVC and triglyceride. This has a consequence that one might expect the sensitivity in the measurements to be less for the PVC slides than for the chromium slides. It is possible to check whether or not the optical contrast between the adsorbed film and the substrate is sufficient for reliable determinations of optical properties of the film by performing simulations. One then calculates the angles A and # for relevant values of the different parameters of eq 2, especially nf and d, since the others are fixed or vary little (n,, k,, nb). From the simulation one is able to determine the expected variation of A and when nf and d are changed and compare these changes with the experimental accuracy of the angle determinations (0.01O). Figure 4A,C shows the results of such simulations under conditions relevant to the present PVC and chromium systems. In Figure 4A the ellipsometer readings from a typical experiment are given as well (the calculated mass vs. time is shown in Figure 5). Since all curves in Figure 4A,C show that small variations of nf and d give sufficient changes in A and # (compared to O.0lo), we conclude that ellipsometry studies of the systems should be feasible. One may also note that the sensitivity is largest for the chromium system, which is to be expected due to the better optical contrast. Figure 4B shows simulated ellipsometer angles for a PVC analogue slide which is able to absorb light (i.e., k = 0.1). It is clearly seen that the sensitivity increases and
time / min
Figure 5. Removal of tripalmitin from a PVC surface by means of a nonionic surfactant solution (0.04% (w/w) of pentakis(oxyethy1ene) dodecyl ether vs. time.
Langmuir, Vol. 3, No. 4, 1987 573
Detergency at Hard Surfaces
Table 11. Average Ellipsometer Angle Changes, Film Refractive Index, Film Thickness, and Triglyceride Amount according to Eq 4' slide 1% TP (ll)* 1% TO (6) 2% TO (8) 1% TP (22) 2% TO (2)
An+
