Ind. Eng. Chem. Res. 1992,31, 1659-1664 metal chelates by a colloidal clay. J. Phys. Chem. 1990,94, 5896. Taniguchi, M.;Yamagishi, A.; Iwamoto, T. X-ray diffraction and electric dichroism studies on the adsorption of metal complexes by a clay. Inorg. Chem. 1991, 30, 2462. van Olphen, H., Fripiat, J. J., E&. Data Handbook for Clay Materials and other Non-metallic Minerals; Pergamon Press: Elmsford, NY, 1979; p 198.
1659
Yariv, S.; Ghosh, D. K.; Hepler, L. G. Metachromasy in clay-mineral systems: adsorption of cationic dyes crystal violet and ethyl violet by kaolinite from aqueous and organic solutions. J. Chem. SOC., Faraday Trans. 1991,87, 1201. Received f o r review January 8, 1992 Accepted March 24, 1992
Effects of Drying and Aging Treatments on the Migration of a Nonionic Surfactant in Gelatin-Based Coatings Leslie J. Fina* and Gangchi Chen Department of Materials Science and Engineering, Rutgers University, P.O. Box 909, Piscataway, New Jersey 08855-0909
Jose E. Valentini Imaging S y s t e m Department, E. I . du Pont de Nemours, Imaging Systems Department, Brevard, North Carolina 28712
Formulations composed of the nonionic surfactant Triton X-100, gelatin, and water were coated onto poly(ethy1ene terephthalate) (PET) substrates and subjected to various drying and aging treatments. The migration of the surfactant is studied with a combination of single-angle and variable-angle Fourier transform infrared and attenuated total reflection (FTIR-ATR) spectroscopies. The single-angle data provide a qualitative description of the surfactant migration whereas the variable-angle data offer a quantitative assessment of the concentration gradient of surfactant with depth from the air-coating interface.
Introduction The migration of low molecular weight surfactant molecules in aqueous polymer matrices in response to drying and aging treatments is of considerable importance to the photographic film, paint, adhesion, and paper industries. Surfactant migration depends on a number of variables, e.g., relative strengths of interaction between the components, competing surface activities, surface and interfacial free energies, water migration, and drying conditions. If a surfactant migrates during drying, the sites of concentration are the air-matrix interface and/or the matrix-substrate interface. On the other hand, the surfactant can simply be excluded from an interface due to a small interfacial free energy. Lastly, with strong intermolecular interactions between the surfactant molecules and the matrix, it is possible that no surfactant migration occurs. The migration of surfactants in copolymer latices has been studied recently since surfactants are used in emulsion polymerizations and they influence the surface properties of the prepared films or coatings. Zhao et al. (1987) have examined the behavior of a sulfate and a disulfonate (anionics) which were used to stabilize latex particles during polymerization. A combination of attenuated total reflection and Fourier transform infrared (ATR-FTIR) spectroscopies was used to characterize the time and concentration dependence of surfactant migration in the prepared films. Surfactant was found to accumulate at both the air-film and film-substrate (glass) interfaces resulting in a parabolic shaped distribution. It was also found that the distribution is nearly completely developed in 3 h of slow water removal. Extensive studies have recently been conducted by Evanson et al. (Evanson and Urban, 1991a,b;Evanson et al., 1991) with FTIR-ATR. They have attempted to understand the relative importance of the forces which control surfactant exudation in copolymer latices. The surfactant-latex interaction was altered by an acid to ester conversion of latex moieties. The increased polarity of the
ester group promoted binding and inhibited anionic surfactant migration. The surface tension of the film-substrate interface was varied by the use of a variety of substrate materials. Substantial differences in anionic surfactant migration toward this interface were observed in response to the drive toward equilibrium. In the case of a nonionic surfactant, no migration was observed, and this behavior was attributed to surfactant-latex interactions. In this work we examine the migration of Triton X-100, which is a polydispersed poly(ethy1ene oxide)-based nonionic surfactant, in the natural product gelatin. Aqueous solutions of Triton X-100 and gelatin are commonly used in the photographic industry as the basis for imaging films. The surfactant serves to decrease the inherent brittle mechanical behavior of pure gelatin and to provide a means for removing static charge buildup. The following two paragraphs provide background information on the structure of gelatin and its interaction with water and surfactants. The structure of both native collagen and extracted gelatin is generally accepted to be a right-handed triple helix composed of three left-handed single helicies (Wetzel et al., 1987; Seeboth et al., 1990). The single chains are composed of peptides and assume the polyproline I1 helix structure, and the chain conformation is stabilized by the formation of the triple helix (von Hippel, 1967). Investigations using circular dichroism of various gelatins in the form of films have established that thermal denaturation and isothermal dehydration/rehydration promote helix to random coil transitions (Wetzel et al., 1987; Gardi et al., 1973). Extensive dehydration studies have shown that a trans to cis conversion occurs in large part between 20 and 0% relative humidity. It has been suggested that the binding of water molecules to the peptide groups takes place after the cis to trans conversion upon rehydration and is essential for triple helix formation (Ramachandran and Chandrasekharan, 1968). In general the interaction of surfactants with peptides depends on the pH of the solution (i.e., net charge on the
0888-5885 92/ 2631-1659$03.00/0 0 1992 American Chemical Society
1660 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
peptides), nature of the surfactant, whether ionic or nonionic, and the availability of binding sites on the peptides. Additionally, gelatin-gelatin mociation and the formation of surfactant micelles above the critical micelle concentration (cmc) influence peptidesurfactant interactions and peptide conformation (Seeboth et al., 1990;Makino et al., 1973). The conformation of gelatin changes with the addition of ionic surfactants (Wustneck et d., l988,1989a), where the triple helix unfolds in response to surfactant binding. On the other hand, nonionic surfactants are thought to interact weakly with peptides. Studies of the association between bovine serum albumin and Triton X-100 have revealed a lack of protein conformational changes, although some binding occurred (Makino et al., 1973). The conclusion of weakly interacting components was based on the preferential formation of surfactant micelles. Aqueous solutions of gelatin and several nonionic surfactants similar in chemical composition to Triton X-100were studied by Wustneck et al. (1989b). An interesting and unexpected observation of the work was that at low surfactant concentrations (i.e., below the cmc) the triple helix content of gelatin increased, whereas above the cmc the helix content decreased. The primary purpose of this study is to understand how the drying and aging of coated aqueous gelatin-surfactant solutions affect the distribution of Surfactants in the dried films. It is not the intention of the authors to elucidate the complexities of the forces which contribute to the migration but to explore accessible experimental variables which control the distribution. Surfactant migration in gelatin films after drying is an area that has received no attention in the literature but is of considerable importance to the long term stability of postprocessed imaging f i i s . This work examines surfactant distributions with singleand variable-angle FTIR-ATR spectroscopies.
Experimental Section Sample Preparation. Sample films were prepared by E. I. du Pont de Nemours, Imaging System Department in North Carolina. Thick f i i s of commercially prepared poly(ethy1ene terephthalate) (PET) (Du Pont Mylar) were coated with aqueous solutions of gelatin and the nonionic surfactant Triton X-100. Different concentrations of aqueous coating formulations were prepared which consisted of 7% gelatin and T% Triton X-100(where T = 0.5, 1.11, 1.47,1.82,and 2.22). The coating temperature was 40 "C. Following the coating process, a drying procedure was conducted to remove the water within the system. Two different drying conditions resulted in relatively wet and relatively dry films. The relative humidities of the wet and dry films were calculated to be 55 and 2570,respectively. The temperature of drying was 24 or 30 "C for different film preparations. Aging treatments were done at room temperature or 35 "C. Spectroscopy. FTIR-ATR spectra in the mid-infrared frequency range were collected with a Perkin Elmer 1750 spectrometer bench, a dry air purge, and an MCT detector. A commercially manufactsued reflection accessory (Specac 19653)in conjunctionwith an hemispherical ZnSe internal reflection element was used to collect variableangle single reflection ATR spectra. A multiple internal reflection accessory (Foxboro 099-5090-2)was used in conjunction with a 60° germanium reflection element for collection of single-angle spectra. In all experiments a constant torque of 5 in-lbs. was used for optical contact reproducibility. The final spectra used in data analysis are a result of 200 coadded scans at a resolution of 2 cm-l, which was postcollection interpolated to an increment of 1 cm-'. All the variable-angle ATR spectra were collected with light po-
1 , O n
E
0 . 6 h
,
0
U
0.2
III
m
0.4i
nn
2
4
6
D
m 01)
8
10
Figure 1. Model surfactants distributions each based on the same amount of surfactant. The series represents the migration of species in reference to an interface (at zero depth).
larized perpendicular to the plane of incidence, where the plane is defined by the surface normal and direction of incident light. The infrared beam was apertured down to 1 mm in all variable-angle ATR experiments to reduce polarization mixing caused by the presence of beam divergence. All the single-angle ATR spectra were collected with unpolarized light. All data files were transferred to a DEC VAX cluster for processing and analysis.
Optical Background ATR spectroscopy as a probe for concentration gradient or depth profiling analysis can be used in either a qualitative or quantitative way. The basic principle involved in any ATR depth analysis is a dependence of the depth of penetration of the exponentially decaying evanescent wave on the refractive index ratio of the sample and the reflection element, the wavelength of incidence light, and the incident light angle. A variation in either the refractive index or the wavelength produces intensity changes which are descriptive of the depth dependence of species located close to the probed interface. Hirschfeld has explored information available from both a variation in incident angle (Hirschfeld, 1977) and the composition of the reflection element (Hirschfeld, 1970,1978). Various other studies have been conducted which treat depth profiling semiquantitatively (Hobbs et al., 1983;Popli and Dwivedi, 1989;Zerbi et al., 1989),and the area has been reviewed by Mirabella and Harrick (1985). Semiquantitative information can be readily obtained from systems in which the gradient of a species changes but the total concentration in the field of view remains constant. This concept is modeled in Figure 1, where various distributions of an absorbing species are shown with equal total concentrations in each distribution. The surface of the model film is located a t 0 pm. The relationship between the distribution type and the absorption intensity observed in an ATR experiment is (Fina and Chen, 1991;Fina, 1992)
where R is the experimentally observed reflected intensity, B is the incident angle, (Em2)is the time averaged electric
Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 1661 0.12
€
s
E E
e
0.06
2 r
8U
' \
A
0.0 I
DISTRIBUTION RANGE Figure 2. Calculated infrared absorption intensities based on eq 1 and the distributions shown in Figure 1. The x-axis is the distance at which each distribution in Figure 1 falls to zero absorption coefficient.
field intensity, n2' is the refractive index ratio, d is the depth of penetration, z is the depth from the inteAce, Az is the interval over which a is assumed to be constant, and a(z) are the absorption coefficients. The distributions modeled in Figure 1are used in eq 1to calculate the reflected intensities at one incident angle, 60°, an a value of lo00 cm-', a refractive index ratio of 0.651, and a wavenumber of 1100 cm-', using perpendicular polarized light. The reflected absorption intensities are shown in Figure 2, where the x-axis refers to the maximum depth at which the analyte species is present. The decrease in intensity as the analyte migrates away from the interface is readily apparent and can be attributed to the exponential decrease in intensity of the evanescent electric field with distance.
Results and Discussion An aqueous coating formulation consisting of 7% gelatin and 0.5% Triton X-100 was prepared, coated on a PET thick film substrate, and dried. The thickness of the dried coating material was 5.0 pm, as measured with a micrometer. In order to demonstrate the migration of Triton X-100 during the drying process, the dried coating material was separated from the substrate and both the coating-air and coating-PET interfaces were examined. Each of the interfaces was scanned separately with ATR spectroscopy using a 60° germanium crystal. The spectra shown in Figure 3 are the result of a subtraction of the gelatinTriton X-100 mixture spectrum minus a pure gelatin spectrum. The amide I and I1 bands located in the 1500-1700-crn-' region (not shown) were used to find the scale factor for gelatin subtraction. The top two spectra in Figure 3 are from the coating interfaces while the bottom spectrum is the pure surfactant scanned in the transmission spectral mode shown for the purpose of comparison. The AA value is about 30% larger in spectrum A as compared to that in B. Since the only difference between spectra A and B is the interface that is probed (i.e., the total concentration is the same), the AA difference establishes that Triton X-100 has migrated to the aircoating interface and away from the coating-PET interface. A common source of error in ATR spectroscopy is the lack of optical contact between the reflection element and the probed samples. In this work all samples require a subtraction to remove the gelatin contributions to the spectra, so optical contact of both the pure gelatin and
AA
= 0.732
\J
L
1325
1200
1100
1000
WAVENUMBERS (cm-1)
Figure 3. (A and B) ATR subtraction spectra of gelatin-Triton X-100 mixture minus pure gelatin. (A) Air-coating interface in contact with the reflection element. (B)Coating-PET interface in contact with the reflection element. (C) Transmission spectrum of pure Triton X-100. 6-
5-
44
I
1-
04 0
1
10
20
30
40
TRIALS
Figure 4. Peak integrations of the 1100-cm-' peak from Triton X-100-isolated subtraction spectra. The original coating material before drying was composed of 1.47% Triton X-100 and 7.0% gelatin in water: (0)integration values after 5 days of room temperature aging; ( 0 )values after 20 days of room temperature aging.
gelathawfactant mixtures are of concern. Figure 4 shows the typical variation in the integrated area of the llOo-cm-' Triton X-100 peak in the subtraction spectra of pure gelatin and gelatinaurfactant mixture samples. Also shown in Figure 4 are two sets of room temperature aging data, where scanning was conducted at 5 and 20 days after the coating and initial drying procedures. For 5 days of aging the average integrated area is 2.290 with a standard error of *0.0837, and for 20 days the area is 2.563 f 0.115. Since there is a statistically significant difference between the data at different aging times and the total surfactant concentration remains constant, the data establish that the surfactant is moving toward the ail-coating interface with aging time after the initial drying treatment. A series of bulk Triton X-100 concentrations and aging data equivalent to Figure 4 are shown in Figure 5. The increasing areas with concentration can be accounted for by the
1662 Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 41
k1'
3
z 2
kEs!
am 4
z
Pc.
Y
i
OJ 1.11
1.47
1-82
2000
2.22
TRITON CONCENTRATION (%)
Figure 5. Peak integrations of the llOO-cm-' peak in the Triton X-100-isolated subtraction spectra. The points above each Triton X-100concentration level correspond to different room temperature aging treatments. For each concentration level the number of aging days after coating from left to right are as follows: 1.11%, 155 and 202; 1.47%, 5, 8, and 20; 1.82%, 4, 7, and 13; 2.22%, 9 and 14.
presence of more surfactant in the bulk. All of the aging data of Figure 5 support the fact that the Triton X-100 continually moves toward the surface with time. The 1.11% Triton X-100data have been aged to much longer times than the remaining three concentrations. The fact that the Triton X-100moves toward the air-coating interface after several months of aging suggests that driving forces are almost in balance. The tendency toward phase separation and/or surface tension reduction is slightly stronger than the molecular interactions between the Triton X-100and the peptides. An additional feature in Figure 5 is the considerably larger error bar at 2.22% Triton X-100concentration for the smaller aging time. The higher variation in areas in this preparation is most likely due to the occurrence of phase separation of the Triton from the gelatin, as evidenced by substantially increased cloudiness in these films. This contention is supported by the fact that at 14 days of aging both the magnitude of the error bars and the film cloudiness decrease. In the present application it is not only of interest to predict the migration direction of the surfactant, as in single-angle ATR studies, but also the form of the surfactant distribution using variable-angle FTIR-ATR spectroscopy. The use of the latter technique to probe unknown distribution types in concentration and orientation has been described in detail elsewhere (Fina and Chen, 1991;Fina, 1992). It will be briefly presented here in order to support the data analysis. The relationship between the reflected intensity in ATR and the absorption coefficients is as follows: lm 0 a ( z )exp( 1 - R(6) = nzl(E,,2) cos 6
&)
dz (2)
where R(6) is the measured reflected intensity for a single reflection and a(z) are the absorption coefficients as a function of depth z. The remainder of the variables are described elsewhere (Fina and Chen, 1991;Fina, 1992). Equation 2 is based on the assumption that cy is small in order to preserve the linearity in Beer's law. In practice the integral in eq 2 can be reduced to a summation with the assumption of discrete depth intervals in a(z) with constant values. A set of simultaneous equations can then be used to find cy(z).
1800
1600
1400
1200
1000
800
WAVENUMBERS (cm-I) Figure 6. Variable-angle ATR spectra of the dried coating. The coating is composed of 2.22% Triton X-100 and 7.0% gelatin in water before drying. The angles of incidence of the s-polarized light are from top to bottom as follows: 60°, 5 8 O , 56O, 54', 52', 50°, 48O, 46O, 45O, 44', 43', and 42'. Z
E %m 4
z
9 I-
= 0.28
8 ARA
Y
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8Z
E %m 4
2000
1800
1600
1400
1200
1000
800
WAVENUMBERS (cm-1) Figure 7. (top seven spectra) Variable-angle ATR subtraction spectra of the same sample as shown in Figure 6. From the top down the angles of incidence are as follows: 60°, 58', 54O, 50°, 46', 44O, and 42'. The bottom spectrum is a transmission scan of pure Triton x-100.
Figure 6 shows the variable-angle spectra for a sample coating composed of 2.22% Triton X-100by volume and 7.0% gelatin by weight in water. The formulation was coated on a PET substrate and dried at 30 "C in an atmosphere of 25% humidity. The incident light was polarized perpendicular to the plane of incidence (s-polarized). Using a zinc selenide hemispherical crystal and the refractive index of gelatin calculated from specular reflectance on the metal surface (i.e., 1.614), the critical angle (e,) of this interface is 41.83'. According to modeling calculations the first angle above 6, where the refractive index dispersion does not significantly distort the spectral intensities is 4 4 O . The Triton X-100has absorption coefficients significantlysmaller than gelatin, so the Triton X-100is more or less invisible in Figure 6. Subtraction spectra of the Triton X-100-gelatin mixture minus the pure gelatin are shown in Figure 7, along with a transmission spectrum of pure Triton X-100. The wavelength dependence of the depth of penetration which is expressed in
(3)
Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992 1663 0.06
0.0
0.0
1.o
2.0
3.0
4.0
5.0
DEPTH FROM INTERFACE (p) Figure 8. Calculated Triton X-100 concentration profiles of a coating material composed of 1.82%Triton X-100 and 7.0% gelatin. Drying conditions produced a 'wet" coating (see text): (A) 3 days of aging at room temperature (after coating and drying) and 0 days of aging at 35 OC; (B) 3 days of aging at room temperature and 14 days of aging at 35 "C; (C) 3 days of aging at room temperature and 29 days of aging at 35 "C.
where E2 = fi2 cos e2, is clearly visible in the subtraction spectra where the intensity decreases with increasing frequency. A decrease in intensity with increasing angle is also apparent, and resulta from the concurrent decrease in the surface electric field intensity. Another notable feature in Figure 7 is the dispersion-type peak shape of the amide I and I1 peaks (ca. 1656 and 1542 cm-', respectively). As referred to earlier, the presence of water in the gelatin tends to stabilize the triple helix structure. The pure gelatin used in the subtractions of Figure 7 was thoroughly dried. This treatment is expected to promote the trans to cis conversion around the peptide bond and degrade the helix (Wetzel et al., 1987). The derivative shapes of the amide I and I1 peaks in Figure 7 can be attributed to conformational changes in the gelatin. It has been shown previously that eq 2 requires the use of peaks of small intensity (Fina and Chen, 1991). This can also be stated as the absorption coefficient k must be less than 0.10. The present analysis uses the 1100-cm-' peak of Triton X-100. An absorbance versus concentration plot for this peak yields a k value of 0.20. However, since the concentration of Triton X-100 in the final dried films is about 25%, the k value is 0.05. This value is in accordance with the calculations shown later (Figures 8 and 9), which is an indication of the quantitative nature of the results. With the variable-angle subtracted intensities measured, solutions to eq 2 with no assumptions about the form of the gradient can be found. The refinement procedure uses a nonlinear finite differences Levenberg-Marquardt least squares optimization algorithm (Brown and Dennis, 1972) to minimize the difference between calculated and observed data. In order to solve the gradient problem, initial guesses are required. If the guesses are close to the true solution, the refinement will globally converge. The refinement seeks an exact solution to a set of intensities that contain small errors due to inexact base lines, finite signal-to-noise ratio, angular setting of reflection apparatus, and scaling factors in the subtraction procedure. These errors will translate to the solution in the refinement procedure. Therefore, an alternate fitting procedure was also used based on solving a set of simultaneous equations. Solutions are considered to be physically meaningful if
0.0
1
I
0.0
1.o
I
2.0
I
3.0
I
4.0
5.0
DEPTH FROM INTERFACE (p) Figure 9. Same as in Figure 8 for drying conditions which produced a 'dry" coating (see text): (A) 2 days of aging at room temperature and 0 days of aging at 35 OC; (B) 2 days of aging at room temperature and 16 days of aging at 35 "C; (C) 2 days of aging at room temperature and 31 days of aging at 35 OC.
both refinement procedures produce similar results. Figures 8 and 9 show the result of the fitting procedure for two seta of coating samples composed of 1.82% Triton X-100 and 7% gelatin. The y-axis is expressed as the optical constant k and is related to the linear absorption coefficient Q! by k = aA/41r. Figure 8 contains distributions from coated films initially dried at 24 "C in ca. 55% humidity, which resulted in relatively wet films. Figure 9 shows distributions from coated films initially dried at 30 "C in ca. 25% humidity, which resulted in relatively dry films. Both sets of samples have been aged at 35 "C, somewhat accelerated over the room temperature aging studies shown in Figure 5. The solid lines in Figures 8 and 9 represent the depth to which the calculation procedure (eq 2) is the most accurate. Beyond about 2 pm, a distance identified by independent calculations, the refinement is increasingly inaccurate and is shown as dashed lines in Figures 8 and 9. The distribution curves show that Triton X-100 migrates toward the surface with aging, and this is verification of the trends shown in the single-angle studies. Both "wet" and "dry" samples of Figures 8 and 9 show substantial distribution changes between the first and intermediate aging trials and minor changes between the intermediate and final trials. This is indicative of pseudoequilibrium distribution. The distribution curves also establish that at short aging times the surfactant is concentrated a t about 1 pm below the air-gelatin interface. Figure 9 shows an increased tendency of the Triton X-100 to migrate toward the surface in dry films. This observation suggests that the preeence of water promotes molecular-level interactions between the surfactant and the gelatin and increases miscibility. An alternate explanation which can account for the observed behavior is the influence of water on the relative abilities of the surfactant, gelatin, and molecular complex to reduce the surface tension a t the air-coating interface. Conclusions Single-angle FTIR-ATR spectroscopic studies of dried and aged coating materials composed of Triton X-100 and gelatin provide a qualitative picture of the migration of surfactant. Triton X-100 is shown to migrate toward the air-coating interface and away from the coating-substrate interface during both initial drying and aging treatments. This migration occurs independent of the concentration
1664 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
of Triton X-100in the aqueous coating formulation. Aging trials establish that Triton X-100continues to migrate toward the air-coating interface at room temperature for several months after the initial drying treatment. Variable-angle ATR studies provide a quantitative prediction of the concentration profile of Triton X-100as a function of depth from the air-coating interface. These studies demonstrate that the presence of increased amounts of water in the dried coating inhibit the movement of surfactant toward the interface.
Acknowledgment Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Literature Cited Brown, K. M.; Dennis, J. E. Derivative Free Analogues of the Levenberg-Marquardt and Gums Algorithms for Nonlinear Least Squares Approximation. Numer. Math. 1972,18,289-297. Evanson, K. W.; Urban, M. W. Surface and Interfacial FTIR Spectroscopic Studies of Latexes. I. Surfactant-Copolymer Interactions. J. Appl. Polym. Sci. 1991a,42,2287-2296. Evanson, K. W.; Urban, M. W. Surface and Interfacial FTIR Spectroscopic Studies of Latexes. 111. The Effects of Substrate Surface Tension and Elongation on Exudation of Surfactants. J. Appl. Polym. Sci. 1991b,42, 2309-2320. Evanson, K. W.; Thoretenson, T. A.; Urban, M. W. Surface and Interfacial FTIR Spectroscopic Studies of Latexes. 11. Surfactant-Copolymer Compatibility and Mobility of Surfactants. J. Appl. Polym. Sci. 1991,42,2297-2307. Fina, L. J. In Structure-Property Relatiom in Polymers: Spectroscopy and Performance; Claver, C., Urban, M., Eds.: 1992; submitted for publication. Fina, L. J.; Chen, G. C. Quantitative Depth Profiling with Fourier Transform Infrared Spectroscopy. Vib. Spectrosc. 1991, I , 353-361. Gardi, A.; Nitschmann, H S.; Rieder, K. Vergleichende Optische Untersuchungen An Modifizierten Gelatinen Und An Kalbshautkollagen Einfluss Der Modifizierung Auf Daa Konformative Verhalten. Chimia 1973,27,116-121. Hirschfeld, T.Accuracy and Optimization of the Two Prism Technique for Calculating the Optical Constants from ATR Data. Appl. Spectrosc. 1970,24 (2),277-282. Hirschfeld, T. Subsurface Layer Studies by Attenuated Total Reflection Fourier Transform Spectroscopy. Appl. Spectrosc. 1977, 31 (4),289-292.
Hirschfeld, T. Accuracy and Optimization of the Two Polarization Technique for Obtaining the Optical Constants from Attenuated Total Reflection Measurements. Appl. Spectrosc. 1978,32(2), 160-164. Hobbs, J. P.; Sung, C. S. P.; Krishnan, K.; Hill, S. Characterization of Surface Structure and Orientation in Polypropylene and Poly(ethy1eneterephthalate) Films by Modified Attenuated Total Reflection IR Dichroism Studies. Macromolecules 1983, 16, 193-199. Makino, S.;Reynolds, J. A.; Tanford, C. The Binding of Deoxycholate and Triton X-100to Proteins. J. Biol. Chem. 1973,248, 4926-4932. Mirabella, F. M., Jr.; Harrick, N. J. Internal Reflection Spectroscopy: Review and Supplement; Harrick Scientific: Ossining, NY, 1985. Popli, R.; Dwivedi, A. M. Attenuated Total Reflectance (ATR) Study of Polymer Blend Films. J. Appl. Polym. Sci. 1989, 37, 2469-2484. Ramachandran, G. N.;Chandraaekharan, R. Interchain Hydrogen Bonds via Bound Water Molecules in the Collagen Triple Helix. Biopolymers 1968,6,1649-1658. Seeboth, A.; Wiistneck, R.; Otto, K. The Influence of Surfactants on the Structure of Transparent Gelatin Films. Colloid Polym. Sci. 1990,268,286-289. von Hippel, P.H. In Treatise on Collagen; Ramachandran, G. N. Ed.; Academic Press: New York, 1967;Vol. 1. Wetzel, R.; Buder, E.; Hermel, H.; Htittner, A. Conformations of Different Gelatins in Solutions and in Films; An Analysis of Circular Dichroism (CD) Measurements. Colloid Polym. Sci. 1987, 265,1036-1045. Wiistneck, R.; Wetzel, R.; Buder, E.; Hermel, H. The Modification of the Triple Helical Structure of Gelatin in Aqueous Solution; The Influence of Anionic Surfactants, pH-Value and Temperature. Colloid Polym. Sci. 1988,266,1061-1067. Wiistneck, R.; Buder, E.; Wetzel, R.; Hermel, H. The Modification of the Triple Helical Structure of Gelatin in Aqueous Solution; The Influence of Cationic Surfactants. Colloid Polym. Sci. 1989a, 267,429-433. Wiistneck, R.; Buder, E.; Wetzel, R.; Hermel, H. The Modification of the Triple Helical Structure of Gelatin in Aqueous Solution; The Influence of Nonionic Surfactants. Colloid Polym. Sci. 1989b,267,516-519. Zerbi, G.; Gallino, G.; Del Fanti, N.; Baini, L. Structural Depth Profiling in Polyethylene Films by Multiple Internal Reflection Infrared Spectroscopy. Polymer 1989,30,2324-2327. Zhao,C. L.; Holl, Y.; Pith, T.; Lambla, M. FTIR-ATR Spectroscopic Determination of the Distribution of Surfactants in Latex Films. Colloid Polym. Sci. 1987,265,823-829. Receiued for review December 26, 1991 Revised manuscript receiued April 13,1992 Accepted April 24, 1992
