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FT-IR spectroscopic studies on interactions of fatty acids in solution and on a salt surface
Colloids and Surfaces, 30 (1988) 287-294 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
287
F T - I R S p e c t r o s c o p i c S t u d i e s on I n t e r a c t i o n s of F a t t y A c i d s in S o l u t i o n and on a Salt S u r f a c e SOMNATH GANGULY and V. KRISHNA MOHAN
IDL-NitroNobel Basic Research Institute, P.O. Box 397, Sankey Road, Malleswaram, Bangalore 560 003 (India) (Received 18 May 1987; accepted in final form 18 September 1987)
ABSTRACT
Hydrogen bonding interactions of heptanoic acid and cis-9-octadecenoic acid have been studied in carbon tetrachloride. Hydrogen bonding and association increases till 10-2 M concentration of the acids. Sodium nitrite particles were coated with these acids in a rotary vacuum evaporator. Hydrogen bonding and other interactions of the acids on the salt surface were studied by diffuse reflectance IR spectroscopy. The acid molecules are bound to water molecules on the salt surface. Intermolecular hydrogen bonding increases with increasing acid concentration. The maximum amount of acid required to coat the particles decreases with increasing chain length of the acid.
INTRODUCTION
The study of hydrogen bonded vibrational modes provides much information on intermolecular interactions [ 1,2]. In recent years FT-IR spectroscopy has been used to study molecular interactions on the surfaces of metals and catalysts [ 3-5 ], biological systems [ 6 ] and to look at surfactant interactions, mainly in aqueous media [ 7 ]. The first part of this paper deals with hydrogen bonding interactions and association behaviour of two long-chain fatty acids, cis-9-octadecenoic acid and heptanoic acid in carbon tetrachloride. Previous workers have been mainly concerned with the energy of association of fatty acids in organic solvents [8 ]. We have been interested in hydrogen bonding and micellisation of these two fatty acids in a non-aqueous environment since there is relatively little work in this direction. We also thought that the study of hydrogen bonding behaviour of fatty acids with differing chain lengths would lead to an understanding of the effect of hydrocarbon chain length on the formation of micelles. The latter part of this paper concerns the interactions of these two long-chain acids on the surface of sodium nitrite particles. This salt has been coated with the acids at different levels of concentration and the diffuse reflectance infrared spectra recorded. An attempt has been made to determine the optimum level of coating material necessary to cover these salt 0166-6622/88/$03.50
© 1988 Elsevier Science Publishers B.V.
288 particles. Sodium nitrite is used in the explosives industry to gas up water gels. Also food, drugs and fertilisers are coated to protect them from external influences. This technique might be applied in these industries so that only an optimum quantity of coating material is used. EXPERIMENTAL Heptanoic and cis-9-octadecenoic acid used for the study were obtained from Fluka. For the solution studies, the acids were dissolved in analar grade carbon tetrachloride and the infrared spectra recorded as 100 micron thick films between NaC1 windows. Analar grade sodium nitrite was accurately weighed, finely powdered and then passed through a 25 mesh sieve. The salt was then heated in an oven at 120°C for an hour. The material was weighed again to calculate the water loss. The sample was then left at room temperature for an hour after which it was weighed again and the adsorbed water estimated. It was always found that the water adsorbed by the salt was exactly the same when this was repeated for the different batches before vacuum coating the samples with the fatty acids. The samples always adsorbed 0.51% by weight of water. Before vacuum coating the samples they were studied under the scanning electron microscope. The electron microscope pictures show that the average size of the salt particles is around 100 microns. Thus the area of salt surface per gram is 0.03 m 2 g-1 which is a very low surface area. From the weight of the water adsorbed it can be shown that 8 × 10-7 mol c m - 2 of water are present on the salt surface. A calculation based on the dimensions of the water molecule shows that only 2 × 10 - s mol cm -2 of water are required to cover the salt surface. Hence the surface can be said to be completely covered with water. Accordingly, the F T - I R spectrum of sodium nitrite particles which had adsorbed water showed bands at 3430 and 3390 c m - 1 which indicates hydrogen bonded water (see Fig. 3a). After vacuum coating the salt with the acids, the increase in weight of the salt was estimated which indicated the amount of acid adsorbed on the salt. Weights were recorded on a Mettler H K 60 balance. Scanning electron microscope photographs were recorded on a J E O L J S M - 3 5 C F scanning electron microscope. IR spectra were recorded on a Bruker IFS-85 F T - I R spectrometer. Diffuse reflectance infrared spectra of the coated and uncoated salt particles were recorded using a Harrick diffuse reflectance unit with K B r as the reflectance standard; 2 mg sample was used with 300 mg KBr. The spectra were not treated with the K u b e l k a - M u n k function. CH stretching band intensities of coated particles were integrated using an integration program. ( The arrows in Fig. 3b show the limits of integration. ) The integrated intensities were normalised using the area under the NO2 bending mode of sodium nitrite at 825 c m - 1. All infrared spectra were recorded at a resolution of 2 c m - 1; 500 scans were used for each spectrum. Dry air was used to purge the instrument.
289 RESULTS
Solution studies Figure I shows the OH stretching region of c/s-9-octadecenoic acid in carbon tetrachloride at different concentrations. At 10 -4 M concentration of the acid, a strong OH stretching band is seen at 3450 c m - 1. W h e n the concentration is increased to 9 × 10- 4 M a second broad band appears around 3200 c m - 1. With increasing concentration this band moves to low frequencies while gaining in intensity. The band at 3450 cm -1 becomes weaker. Similar effects are seen in the case of the shorter chain heptanoic acid. Figure 2 gives the variation of the hydrogen bonded OH stretch frequency of heptanoic acid against its concentration in carbon tetrachloride. For both acids the second band stops shifting
"4 M '4 M
S3M
03M
E
t~3M
c
e
~3M ,103M
~-2 M
3600
3400
3200
Frequency
3000
(cm-1)
Fig. 1. The OH stretching region in the infrared spectrum of c/s-9-octadecenoic acid at different
concentrations in carbon tetrachloride.
290
3200
m£ U v
-r 0 3150
\
--
\
I
lo-3
1
i
lo-2 acid concentration
10-'
(reals)
Fig. 2. The hydrogen bonded OH stretching frequency of heptanoic acid against its concentration in carbon tetrachloride.
when the frequency reaches about 3120 cm -1. For c/s-9-octadecenoic acid this frequency is reached at 1.8X 10 -2 M concentration while for heptanoic acid the shifts stop at 3.3 × 10 -2 M.
Surface studies Figure 3a shows the 3600-2800 cm-1 region in the IR spectrum of sodium nitrite particles which had been heated to 120 °C and cooled to room temperature. The spectrum shows an O H stretching band with a doublet-like structure. One of the peaks of this doublet is at 3430 cm-1 and has been assigned to bridged hydroxyl groups while the other peak at 3390 cm-1 is due to surface water molecules [ 10]. Figure 3b shows the same region with cis-9-octadecenoic acid on the surface of the salt at a concentration of 2 × 10 -8 mol cm -2. The OH stretch band at 3390 c m - 1 (of uncoated sodium nitrite) has now shifted to 3366 c m - 1. Similar shifts were seen when heptanoic acid was coated on the salt. Figure 4 is a plot of the variation of the 3390 cm-1 band against acid concentration for both acids. It can be seen that as the concentration of the acids is increased on the surface of the salt the O H stretching band at 3390 cm-1 moves to lower frequencies. The frequency decrease, however, stops at 2 ><10 - s mol cm -2 cis-9-octadecenoic acid and 7X 10 - s mol cm -2 heptanoic acid. The band at 3430 c m - 1 does not shift. A band was seen at 3720 c m - 1 due to surface water molecules. However, since the band does not shift with acid concentration, it will not be discussed. Figure 5 is a plot of the normalised integrated CH stretch intensity against acid concentration on the surface for both acids. The intensity increases with increasing acid at first and then tends
291
(Q) (a)
-
8
~
I
~
r
3600
3400
(b)
I
3200
I
3000
2800
Frequency (cn~ t )
Fig. 3. The 3600-2800 cm -1 region in the infrared spectrum of sodium nitrite particles with (a) adsorbed water on the surface (8 X 10- 7 tool cm - 2 water); (b) 2 X 10- 8 tool c m - 2 cis-9-octadecenoic acid adsorbed on the surface water molecules. 3400 C" tE u T
338o-
o >.
3360
--
i l o -9
I
lo-8
lo- ~
ocid concentro~'ion ( mols cr~ 2)
Fig. 4. Variation of the OH stretching frequency of surface water molecules against concentration ofcis-9-octadecenoic acid ( • ) and heptanoic acid ( • ) on the surface of the salt. Arrows indicate
points where saturation of the frequency begins.
to taper off. The tapering off being at 2 × 10 - s mol acid and 9X 10 - s mol cm -2 for heptanoic acid.
cm -2
for c/s-9-octadecenoic
292 DISCUSSION
Solution studies At 10 -4 M concentration of the acids in carbon tetrachloride, the OH stretching band is at 3450 c m - 1and is probably due to monomeric species ( Fig. 1 ). Norkov et al. [ 9 ] have, in their IR studies of fatty acids, shown that an intermolecular H-bond exists between the carbonyl and methylene groups. This could be the reason for the monomeric OH frequency being so low ( 3450 c m - 1). At 9 × 10 -4 M concentration, the broad band which appears at 3200 cm -~ is due to hydrogen bonded acid molecules. The intensity of the monomeric band at 3450 c m - 1 decreases with increasing concentration and is due to a decrease in the number of unbonded molecules. The hydrogen bonded OH stretch at about 3200 c m - ~shifts to low frequencies due to increasing hydrogen bonding. The shift to low frequencies occurs till 1.8X 10 -2 M c/s-9-octadecenoic acid and till 3.3 × 10 -2 M heptanoic acid. This agrees with literature reports that hydrogen bond formation is easier for the longer chain acid [ 8 ]. At these concentrations the OH stretching frequency of the acids is about 3120 cm -~, a value which is close to the OH stretch frequency of the acids in the bulk liquid state. It is known from literature t h a t fatty acids in solution hydrogen bond to form dimers. It is difficult to understand how a continuous change in frequency occurs from 3200 cm-~ to 3120 cm-~ with increasing concentration if dimers
to c
~I.0 o
.c=
0.5--
.J 10-9
I
I
10 -8
10-7
acid concenfrotion (toolscm-2 ) Fig. 5. Plot of the normalised integrated C - H stretching intensity against concentration of c/s-9octadecenoic acid (• ) and heptanoic acid ( • ) on the salt surface. Arrows indicate points at which tapering of the intensitiesbegin.
293 are the only structures possible in solution. Probably at high concentrations fatty acid molecules hydrogen bond to form trimers, tetramers and oligomers.
Surface studies The 3600-2800 c m - 1 region in the infrared spectrum of sodium nitrite particles which had been heated to 120 ° C and cooled to room temperature showed an OH stretching band with a doublet-like structure (Fig. 3a). These bands have been observed on Ti02 surfaces. The higher frequency peak (at 3430 cm-1) has been assigned to bridged hydroxyl groups while the peak at 3390 c m - 1 is due to the vibrations of surface water molecules [ 10 ]. The drop in the frequency of the band at 3390 c m - 1when the acid is coated on the salt indicates that the acid molecules hydrogen bond to the surface water molecules (Fig. 3b). Thus the polar OH group of each acid molecule bonds with the water molecules on the surface. As the concentration of the acids is increased, more OH groups hydrogen bond to the molecules already there, causing a reduction in the OH stretching frequency (Fig. 4). The shift of the band to low frequency stops after a certain acid concentration because no more acid molecules can be adsorbed onto the hydroxyl groups already present. The OH stretch frequency is now 3365 cm-1. Thus, hydrogen bonding stops at 2 × 10 -s mol cm -2 cis-9octadecenoic acid and at about 7×10 -s mol cm -2 heptanoic acid (Fig. 4). However, the bridged hydroxyl groups do not enter into hydrogen bonding since the band at 3430 cm -1 does not shift with increasing acid (Fig. 3 ). This was noticed by previous workers [ 10,11 ]. However, Graham et al. [ 11 ] found that the band at 3655 cm-1 due to terminal hydroxyl groups on Ti02, shifted to 3640 cm -~ when acetic acid was adsorbed on the surface. As the acids are adsorbed on the surface, the hydrocarbon chains cover the surface and the CH stretching intensity increases (Fig. 5). However, as the surface is covered up, the acid molecules have fewer water molecules to adsorb onto and hence adsorption tapers off. Thus, the CH stretching intensities of the acids also tend to level off. Since the longer chain c/s-9-octadecenoic acid covers the sites and the surface more rapidly the tapering off of the intensities occurs at a lower concentration of the acid. Figure 5 shows that this occurs at about 2 × 10 -s mol cm -2 for c/s-9-octadecenoic acid and at about 9 × 10 -s tool cm-2 for heptanoic acid. These values are close to the concentrations of the acids where hydrogen bonding stops. Thus, these values may be taken as the amount of acid necessary to coat the sodium nitrite particles. Figure 5 also shows that the integrated intensities do not really reach a constant value but continue to show a very small increasing trend. This intensity increase is caused by molecules adsorbing due to van der Waals forces. A rough calculation based on the dimensions of the fatty acid molecules shows that about 0.16 × 10- 8 mol c m - 2 of cis-9-octadecenoic acid are required to cover the sodium nitrite particles. However, FT-IR data indicate that at the
294 point where the O H frequency shift saturates, about 2 . 0 × 10 - s mol cm -2 of cis-9-octadecenoic acid had been used. Similarly, for heptanoic acid, a calculation shows t h a t 0.4 × 10 -8 mol cm -2 of the acid are required to cover the salt surfaces. F T - I R data here show t h a t saturation occurs at 7 × 10 - s mol cm -2. T h e higher values of acid used in the e x p e r i m e n t could be due to various reasons. It is possible t h a t the molecules do not lie flat on the surface. T a n e k a et al. [ 12 ] used A T R to obtain polarised IR spectra of 33 layers of stearic acid on a germanium A T R plate a nd t h e y estimated a tilt angle of 25-30 ° between the stearic acid tail and the surface normal. Chollet's studies [ 13 ] of behenic acid monolayers indicated t h a t the acid was inclined at an angle of 25 + 4 ° from the normal. It is possible t h a t in our work the molecules behave in a similar way and hence do n o t cover the surface by lying flat. T h e y are probably hydrogen bonded to the water molecules at an angle a n d t h e n curve down to surface. If this is t a k e n into account the discrepancy between observed and calculated values of f atty acids is almost the negligible. ACKNOWLEDGEMENTS T h e authors wish to t h a n k t he M a n a g e m e n t of I D L Chemicals L t d for granting permission to publish the paper and Mr. R. Vedam for useful discussions.
REFERENCES 1 P. Schuster, G. Zundel and C. Sandorfy (Eds), The Hydrogen Bond: Advances in Theory and Experiment, North Holland, Amsterdam, 1976. 2 S. Bratos, J. Lascombeand A. Novak, in H. Ratajczak, W.J. OrvilleThomas and M. Redshaw (Eds), Molecular Interactions, Vol. 1, Wiley, New York, 1981. 3 F.M.Hoffman, Surf. Sci. Rep. 3 (1983) 1. 4 Tinh Nguyen, Prog. Org. Coat., 13 (1985) 1. 5 P.R. Griffiths, K.W. Van Every, I.M. Hamadeh and N.A. Wright, in J.R. Durig (Ed.), Chemical, Biologicaland Industrial Applications of FT-IR Spectroscopy,Wiley, New York, 1985. 6 I.W. Levin, in R.J.H. Clark and R.E. Hester (Eds), Advances in IR and Raman Spectroscopy, Vol. 11, Wiley, New York 1984. 7 H.H. Mantsch, in K.L. Mittal and B. Lindman (Eds), Surfactants in Solution,Vol. 1, Plenum Press, New York, 1982. 8 M.A. Taylor and L.H. Princen, in E.H. Pryde (Ed.), Fatty Acids, Amercian Oil Chemists Society, Champaign, IL, 1979. 9 A.M.Norkov, A.M. Zomlerand T.A.V. Urshinin, Zh. Prikl. Spectrosk., 31 (1972) 1042. 10 C.H.Rochester, Colloids Surfaces, 21 (1986) 205. 11 J. Graham, C.H. Rochester and R. Rudham, J. Chem. Soc.Faraday Trans. l,77 (1981) 1973. 12 T. Taneka, K. Nogami, H. Gotoh and R. Gotoh, J. Colloid Interface Sci., 35 (1971) 395; 40 (1971) 409. 13 P.A.Chollet, Thin Solid Films, 52 (1978) 343.
287
F T - I R S p e c t r o s c o p i c S t u d i e s on I n t e r a c t i o n s of F a t t y A c i d s in S o l u t i o n and on a Salt S u r f a c e SOMNATH GANGULY and V. KRISHNA MOHAN
IDL-NitroNobel Basic Research Institute, P.O. Box 397, Sankey Road, Malleswaram, Bangalore 560 003 (India) (Received 18 May 1987; accepted in final form 18 September 1987)
ABSTRACT
Hydrogen bonding interactions of heptanoic acid and cis-9-octadecenoic acid have been studied in carbon tetrachloride. Hydrogen bonding and association increases till 10-2 M concentration of the acids. Sodium nitrite particles were coated with these acids in a rotary vacuum evaporator. Hydrogen bonding and other interactions of the acids on the salt surface were studied by diffuse reflectance IR spectroscopy. The acid molecules are bound to water molecules on the salt surface. Intermolecular hydrogen bonding increases with increasing acid concentration. The maximum amount of acid required to coat the particles decreases with increasing chain length of the acid.
INTRODUCTION
The study of hydrogen bonded vibrational modes provides much information on intermolecular interactions [ 1,2]. In recent years FT-IR spectroscopy has been used to study molecular interactions on the surfaces of metals and catalysts [ 3-5 ], biological systems [ 6 ] and to look at surfactant interactions, mainly in aqueous media [ 7 ]. The first part of this paper deals with hydrogen bonding interactions and association behaviour of two long-chain fatty acids, cis-9-octadecenoic acid and heptanoic acid in carbon tetrachloride. Previous workers have been mainly concerned with the energy of association of fatty acids in organic solvents [8 ]. We have been interested in hydrogen bonding and micellisation of these two fatty acids in a non-aqueous environment since there is relatively little work in this direction. We also thought that the study of hydrogen bonding behaviour of fatty acids with differing chain lengths would lead to an understanding of the effect of hydrocarbon chain length on the formation of micelles. The latter part of this paper concerns the interactions of these two long-chain acids on the surface of sodium nitrite particles. This salt has been coated with the acids at different levels of concentration and the diffuse reflectance infrared spectra recorded. An attempt has been made to determine the optimum level of coating material necessary to cover these salt 0166-6622/88/$03.50
© 1988 Elsevier Science Publishers B.V.
288 particles. Sodium nitrite is used in the explosives industry to gas up water gels. Also food, drugs and fertilisers are coated to protect them from external influences. This technique might be applied in these industries so that only an optimum quantity of coating material is used. EXPERIMENTAL Heptanoic and cis-9-octadecenoic acid used for the study were obtained from Fluka. For the solution studies, the acids were dissolved in analar grade carbon tetrachloride and the infrared spectra recorded as 100 micron thick films between NaC1 windows. Analar grade sodium nitrite was accurately weighed, finely powdered and then passed through a 25 mesh sieve. The salt was then heated in an oven at 120°C for an hour. The material was weighed again to calculate the water loss. The sample was then left at room temperature for an hour after which it was weighed again and the adsorbed water estimated. It was always found that the water adsorbed by the salt was exactly the same when this was repeated for the different batches before vacuum coating the samples with the fatty acids. The samples always adsorbed 0.51% by weight of water. Before vacuum coating the samples they were studied under the scanning electron microscope. The electron microscope pictures show that the average size of the salt particles is around 100 microns. Thus the area of salt surface per gram is 0.03 m 2 g-1 which is a very low surface area. From the weight of the water adsorbed it can be shown that 8 × 10-7 mol c m - 2 of water are present on the salt surface. A calculation based on the dimensions of the water molecule shows that only 2 × 10 - s mol cm -2 of water are required to cover the salt surface. Hence the surface can be said to be completely covered with water. Accordingly, the F T - I R spectrum of sodium nitrite particles which had adsorbed water showed bands at 3430 and 3390 c m - 1 which indicates hydrogen bonded water (see Fig. 3a). After vacuum coating the salt with the acids, the increase in weight of the salt was estimated which indicated the amount of acid adsorbed on the salt. Weights were recorded on a Mettler H K 60 balance. Scanning electron microscope photographs were recorded on a J E O L J S M - 3 5 C F scanning electron microscope. IR spectra were recorded on a Bruker IFS-85 F T - I R spectrometer. Diffuse reflectance infrared spectra of the coated and uncoated salt particles were recorded using a Harrick diffuse reflectance unit with K B r as the reflectance standard; 2 mg sample was used with 300 mg KBr. The spectra were not treated with the K u b e l k a - M u n k function. CH stretching band intensities of coated particles were integrated using an integration program. ( The arrows in Fig. 3b show the limits of integration. ) The integrated intensities were normalised using the area under the NO2 bending mode of sodium nitrite at 825 c m - 1. All infrared spectra were recorded at a resolution of 2 c m - 1; 500 scans were used for each spectrum. Dry air was used to purge the instrument.
289 RESULTS
Solution studies Figure I shows the OH stretching region of c/s-9-octadecenoic acid in carbon tetrachloride at different concentrations. At 10 -4 M concentration of the acid, a strong OH stretching band is seen at 3450 c m - 1. W h e n the concentration is increased to 9 × 10- 4 M a second broad band appears around 3200 c m - 1. With increasing concentration this band moves to low frequencies while gaining in intensity. The band at 3450 cm -1 becomes weaker. Similar effects are seen in the case of the shorter chain heptanoic acid. Figure 2 gives the variation of the hydrogen bonded OH stretch frequency of heptanoic acid against its concentration in carbon tetrachloride. For both acids the second band stops shifting
"4 M '4 M
S3M
03M
E
t~3M
c
e
~3M ,103M
~-2 M
3600
3400
3200
Frequency
3000
(cm-1)
Fig. 1. The OH stretching region in the infrared spectrum of c/s-9-octadecenoic acid at different
concentrations in carbon tetrachloride.
290
3200
m£ U v
-r 0 3150
\
--
\
I
lo-3
1
i
lo-2 acid concentration
10-'
(reals)
Fig. 2. The hydrogen bonded OH stretching frequency of heptanoic acid against its concentration in carbon tetrachloride.
when the frequency reaches about 3120 cm -1. For c/s-9-octadecenoic acid this frequency is reached at 1.8X 10 -2 M concentration while for heptanoic acid the shifts stop at 3.3 × 10 -2 M.
Surface studies Figure 3a shows the 3600-2800 cm-1 region in the IR spectrum of sodium nitrite particles which had been heated to 120 °C and cooled to room temperature. The spectrum shows an O H stretching band with a doublet-like structure. One of the peaks of this doublet is at 3430 cm-1 and has been assigned to bridged hydroxyl groups while the other peak at 3390 cm-1 is due to surface water molecules [ 10]. Figure 3b shows the same region with cis-9-octadecenoic acid on the surface of the salt at a concentration of 2 × 10 -8 mol cm -2. The OH stretch band at 3390 c m - 1 (of uncoated sodium nitrite) has now shifted to 3366 c m - 1. Similar shifts were seen when heptanoic acid was coated on the salt. Figure 4 is a plot of the variation of the 3390 cm-1 band against acid concentration for both acids. It can be seen that as the concentration of the acids is increased on the surface of the salt the O H stretching band at 3390 cm-1 moves to lower frequencies. The frequency decrease, however, stops at 2 ><10 - s mol cm -2 cis-9-octadecenoic acid and 7X 10 - s mol cm -2 heptanoic acid. The band at 3430 c m - 1 does not shift. A band was seen at 3720 c m - 1 due to surface water molecules. However, since the band does not shift with acid concentration, it will not be discussed. Figure 5 is a plot of the normalised integrated CH stretch intensity against acid concentration on the surface for both acids. The intensity increases with increasing acid at first and then tends
291
(Q) (a)
-
8
~
I
~
r
3600
3400
(b)
I
3200
I
3000
2800
Frequency (cn~ t )
Fig. 3. The 3600-2800 cm -1 region in the infrared spectrum of sodium nitrite particles with (a) adsorbed water on the surface (8 X 10- 7 tool cm - 2 water); (b) 2 X 10- 8 tool c m - 2 cis-9-octadecenoic acid adsorbed on the surface water molecules. 3400 C" tE u T
338o-
o >.
3360
--
i l o -9
I
lo-8
lo- ~
ocid concentro~'ion ( mols cr~ 2)
Fig. 4. Variation of the OH stretching frequency of surface water molecules against concentration ofcis-9-octadecenoic acid ( • ) and heptanoic acid ( • ) on the surface of the salt. Arrows indicate
points where saturation of the frequency begins.
to taper off. The tapering off being at 2 × 10 - s mol acid and 9X 10 - s mol cm -2 for heptanoic acid.
cm -2
for c/s-9-octadecenoic
292 DISCUSSION
Solution studies At 10 -4 M concentration of the acids in carbon tetrachloride, the OH stretching band is at 3450 c m - 1and is probably due to monomeric species ( Fig. 1 ). Norkov et al. [ 9 ] have, in their IR studies of fatty acids, shown that an intermolecular H-bond exists between the carbonyl and methylene groups. This could be the reason for the monomeric OH frequency being so low ( 3450 c m - 1). At 9 × 10 -4 M concentration, the broad band which appears at 3200 cm -~ is due to hydrogen bonded acid molecules. The intensity of the monomeric band at 3450 c m - 1 decreases with increasing concentration and is due to a decrease in the number of unbonded molecules. The hydrogen bonded OH stretch at about 3200 c m - ~shifts to low frequencies due to increasing hydrogen bonding. The shift to low frequencies occurs till 1.8X 10 -2 M c/s-9-octadecenoic acid and till 3.3 × 10 -2 M heptanoic acid. This agrees with literature reports that hydrogen bond formation is easier for the longer chain acid [ 8 ]. At these concentrations the OH stretching frequency of the acids is about 3120 cm -~, a value which is close to the OH stretch frequency of the acids in the bulk liquid state. It is known from literature t h a t fatty acids in solution hydrogen bond to form dimers. It is difficult to understand how a continuous change in frequency occurs from 3200 cm-~ to 3120 cm-~ with increasing concentration if dimers
to c
~I.0 o
.c=
0.5--
.J 10-9
I
I
10 -8
10-7
acid concenfrotion (toolscm-2 ) Fig. 5. Plot of the normalised integrated C - H stretching intensity against concentration of c/s-9octadecenoic acid (• ) and heptanoic acid ( • ) on the salt surface. Arrows indicate points at which tapering of the intensitiesbegin.
293 are the only structures possible in solution. Probably at high concentrations fatty acid molecules hydrogen bond to form trimers, tetramers and oligomers.
Surface studies The 3600-2800 c m - 1 region in the infrared spectrum of sodium nitrite particles which had been heated to 120 ° C and cooled to room temperature showed an OH stretching band with a doublet-like structure (Fig. 3a). These bands have been observed on Ti02 surfaces. The higher frequency peak (at 3430 cm-1) has been assigned to bridged hydroxyl groups while the peak at 3390 c m - 1 is due to the vibrations of surface water molecules [ 10 ]. The drop in the frequency of the band at 3390 c m - 1when the acid is coated on the salt indicates that the acid molecules hydrogen bond to the surface water molecules (Fig. 3b). Thus the polar OH group of each acid molecule bonds with the water molecules on the surface. As the concentration of the acids is increased, more OH groups hydrogen bond to the molecules already there, causing a reduction in the OH stretching frequency (Fig. 4). The shift of the band to low frequency stops after a certain acid concentration because no more acid molecules can be adsorbed onto the hydroxyl groups already present. The OH stretch frequency is now 3365 cm-1. Thus, hydrogen bonding stops at 2 × 10 -s mol cm -2 cis-9octadecenoic acid and at about 7×10 -s mol cm -2 heptanoic acid (Fig. 4). However, the bridged hydroxyl groups do not enter into hydrogen bonding since the band at 3430 cm -1 does not shift with increasing acid (Fig. 3 ). This was noticed by previous workers [ 10,11 ]. However, Graham et al. [ 11 ] found that the band at 3655 cm-1 due to terminal hydroxyl groups on Ti02, shifted to 3640 cm -~ when acetic acid was adsorbed on the surface. As the acids are adsorbed on the surface, the hydrocarbon chains cover the surface and the CH stretching intensity increases (Fig. 5). However, as the surface is covered up, the acid molecules have fewer water molecules to adsorb onto and hence adsorption tapers off. Thus, the CH stretching intensities of the acids also tend to level off. Since the longer chain c/s-9-octadecenoic acid covers the sites and the surface more rapidly the tapering off of the intensities occurs at a lower concentration of the acid. Figure 5 shows that this occurs at about 2 × 10 -s mol cm -2 for c/s-9-octadecenoic acid and at about 9 × 10 -s tool cm-2 for heptanoic acid. These values are close to the concentrations of the acids where hydrogen bonding stops. Thus, these values may be taken as the amount of acid necessary to coat the sodium nitrite particles. Figure 5 also shows that the integrated intensities do not really reach a constant value but continue to show a very small increasing trend. This intensity increase is caused by molecules adsorbing due to van der Waals forces. A rough calculation based on the dimensions of the fatty acid molecules shows that about 0.16 × 10- 8 mol c m - 2 of cis-9-octadecenoic acid are required to cover the sodium nitrite particles. However, FT-IR data indicate that at the
294 point where the O H frequency shift saturates, about 2 . 0 × 10 - s mol cm -2 of cis-9-octadecenoic acid had been used. Similarly, for heptanoic acid, a calculation shows t h a t 0.4 × 10 -8 mol cm -2 of the acid are required to cover the salt surfaces. F T - I R data here show t h a t saturation occurs at 7 × 10 - s mol cm -2. T h e higher values of acid used in the e x p e r i m e n t could be due to various reasons. It is possible t h a t the molecules do not lie flat on the surface. T a n e k a et al. [ 12 ] used A T R to obtain polarised IR spectra of 33 layers of stearic acid on a germanium A T R plate a nd t h e y estimated a tilt angle of 25-30 ° between the stearic acid tail and the surface normal. Chollet's studies [ 13 ] of behenic acid monolayers indicated t h a t the acid was inclined at an angle of 25 + 4 ° from the normal. It is possible t h a t in our work the molecules behave in a similar way and hence do n o t cover the surface by lying flat. T h e y are probably hydrogen bonded to the water molecules at an angle a n d t h e n curve down to surface. If this is t a k e n into account the discrepancy between observed and calculated values of f atty acids is almost the negligible. ACKNOWLEDGEMENTS T h e authors wish to t h a n k t he M a n a g e m e n t of I D L Chemicals L t d for granting permission to publish the paper and Mr. R. Vedam for useful discussions.
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