9114
J. Phys. Chem. 1992, 96, 9114-9120
Raman Spectra of Aqueous Sodium Octanoate Solutions: Solute and Solvent Study M. Picquart* and G.Lacrampe Laboratoire de Physique et Biophysique des Milieux Mal Ordonnb, UniversitP R e d Descartes-Paris V, UFR BiomPdicale des Saints-PPres, 45 rue des Saints-PPres, 75270 Paris Cedex 06, France (Received: February 19, 1992: In Final Form: August 3, 1992)
Aqueous solutions of sodium octanoate between 3 and 40% of NaCBhave been studied by Raman scattering. The spectral behavior is followed as a function of concentration in the low-temperature solid phase and in the liquid phase for all samples. We have focused our study on the modifications that appear in the spectrum of the solutions when the concentration is lower or higher than the critical micellar concentration, which is quite high for this surfactant (6% in weight). We observed the changes of the spectrum when the micellization occurs. It is to be noted that we present some results about the water structure in these solutions studying the stretching modes of the 0-H bond of the water. In particular the effect of concentration on water structure is quite identical to the effect of temperature on water structure.
Introduction Ionic surfactants are important as cleansing agents, as models for biological membranes, etc. They belong to the amphiphile family and poaseas a long nonpolar parafim chain and a hydrophilic polar headgroup. In a polar solvent and at a certain concentration (critical micellar concentration (cmc)) these molecules associate to form micelles. When the concentration increases further they can form liquid-crystal phases. Sodium octanoate is an octanoic (caprylic) acid salt with a formula CH3(CH2)&OO-Na+, It is composed of an hydrocarbon chain of eight carbon atoms with a carboxylic group at one end. Aqueous micelles of sodium octanoate have been extensively because this compound has a large cmc (-0.4 M) and a low aggregation number. Different experimental methods were used to determine aggregation numbers3" Other studies7-'* of solubilization have been made on ternary systems water-sodium octanoate-pentanol/deumol,and we found only one paperI5 using Raman scattering technique. We present here results using this technique on aqueous solutions with concentrations lower or higher than the cmc. Many investigations both experimental and theoretical'bm have been performed on long-chain molecules. These results have been used to interpret the Raman spectra of phospholipids and biomembranes;21s5.2 we shall used them to interpret our experimental work. The Raman spectrum of a surfactant is dominated by the vibration modes of the hydrocarbon chain as described hereafter. In the low-energy part (150-200 cm-') we find a longitudinal acoustical mode (LAM) whose frequency depends on the length of the chain. At higher frequency we have the skeletal vibration modes ( C 4 stretching modes between 1050 and 1150 cm-I), near 1160cm-I we find the CH2rocking mode, the CH2twisting mode is around 1300 cm-', and the CH2bending modes between 1430 and 1460 cm-I. The high-energy part between 2800 and 3000 cm-I contains the C-H stretching vibration modes. We can find also some features due to the polar headgroup with generally low intensity. The high-energy part is the most complex of the spectrum. It is interpreted as the combination of the C-H stretching fundamental modes of the molecule and the Fermi resonance interactions of these modes with the overtones of the CH2bending modes. This Fermi interaction in the crystal is also affected by the coupling between the CH2bending modes of adjacent chains. This produces a greater dispersion of these modes and results in a broadening of the bands. The main featum of the high-energy part are the peaks at 2850 and 2885 cm-'respectively attributed to the symmetrical and the antisymmetrical CH2 stretching modes. In the solid phase (low temperature) the antisymmetrical mode is more intense (if we compare the peak heights) than the symmetrical one. In the liquid phase, the heights of these two peaks are inverted. is generally used to measure the The peak height ratio 128s5/128w 0022-365419212096-9114$03.00/0
lateral order and gives information on the chain packing and the chain mobility.2'*22This ratio decreases if there is an increasing chain disorder. The C-C stretching vibration modes region between 1050 and 1150 cm-I is characterized in the solid phase by two intense peak9 near 1065 and 1120 cm-I due to the C-C vibration modes in all-trans configuration (respectively antisymmetrical and symmetrical modes). Another band near 1080 cm-I appears in the high-temperature phases (crystal-liquid phases or isotropic liquid phases) and is due to the C-C vibration modes in the presence of gauche conformations. Meanwhile the 11U)cm' band reducea its intensity and the peak at 1065 om-'becomes a shoulder in the band due to gauche conformations.22-26 The C-C stretching vibration modes are affected by the intrachain disorder and give information about the trans-gauche isomerization. The aqueous solution is composed of two parts: the solute, described above, and the solvent, Le., the water. The study of the water in these systems, near ions, interfaces, is interesting to understand the nature of the interaction for micellbation, interface curvatures, etc. The ability of water molecules to establish hydrogen bonds either with other water molecules or with solute molecules can give us information on the structure of micellar solutions. The Raman spectrum of water is known since the fmt studies of W e s t ~ nand ~ ~Walrafen.28 It has been extensively studied, and the interpretation is a still debated question. Nevertheless it seems that the strong 0-H stretchingband (300&3800 cm-l) is mainly composed of four bands, each corresponding to different degrees of the hydrogen bond strengthB between water molecules and the surrounding molecules. The lower the frequency, the greater the hydrogen bond strength. We can use this property to analyze our experimental results and to reach some conclusions about molecular environment.
Experimental Procedure The Raman spectroscopy experiments were performed on a Jobin-Yvon Ramanor HG2S double monochromator, using a Spectra Physics Ar+ laser. We used the 514.5-nm laser line with a power of 400 mW. The spectral resolution was 4 cm-I. A small cryostat with nitrogen circulation and temperature regulation was used for the temperature experiments. For technical reasons, the thermocouple is quite far from the focusing point of the laser beam, and the error in temperature measurements is estimated at 2 K. The sample was allowed to equilibrate for 15 min to stabilize the temperature before recording each spectrum. The scattering light, detected at a right angle from the incident light, is collected on the photocathode of a cooled photomultiplier and amplified by a dc operational amplifier. All the samples are enclosed into sealed 1-mm-diametercap illaries. The mixture with deionized water was made by sonication and centrifugation, and we checked the sample homogeneity by successive heating and cooling proceaoes between crossed polarizers. Q 1992 American Chemical Society
7'he Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9115
Aqueous Sodium Octanoate Solutions ZD1
I 10
Octane
lZ6
i
h
268
I
"'l
300
I
I
2 00
I
1
100
1
Figure 1. Raman spectrum of the low-frquency region of octane ( u p per), anhydrous sodium octanoate (middle), and 40% solution of sodium octanoate (lower) at 79 K between 50 and 350 cm-I. Values on the spectra indicate the peak frequency.
We first compare the different parts of the octane spectrum in the solid phase at 79 K, of the anhydrous sodium octanoate, and of an aqueous solution of octanoate (4075,w/w) and then the liquid phase, which has not been extensively studied. In particular, we do not study parts of the spectrum that do not show any differences with other c o m p o ~ n d s . ~We * ~insist ~ mainly on some differences noted in the transition between the monomers solution (concentration lower than the cmc) and the micellar solution (concentrationhigher than the cmc) using different solutions with concentrations between 3 and 40%. Finally, we present some results about water in these solutions using a decomposition of the 0-H stretching vibration band of water in four Gaussians by means of a least-squares fitting program. Three adjusting parameters (frequency, intensity, bandwidth at half-height) were considered for each Gaussian. An initial bandwidth of 150 cm-I was attributed to each Gaussian. The frequency and intensity of the evident maximum of the original spectrum were considered as the initial frequency and intensity of the main component (band 11; see Table I), The initial intensities and frequencies of the other components were chosen in order to give the best adjustment, and each essay was visualized before calculation was initialized. The fitting adjustment is performed until the synthetic curve matches the experimental one with a precision factor 11%. Such a precision factor was reached when the number of Gaussians was at least equal to four. Decreasing the number of Gaussians (three for example) leads to a lower precision factor whereas increasing this numbor (five or more) makes difficult the attribution of the different bands using the actual models.32 Consequently the number of Gaussians was limited to four. The percentages corresponding to the relative surface of each Gaussian were determined in order to compare the different decomposed spectra. Sodium octanoate is purchased from Sigma with a 99-10096 purity and used without further purification.
Experimental R d t a and Discussion soyd phrae at L a w Tempatwe!. We show in Figure 1 Raman spectra between 50 and 350 cm-'of octane, NaC8, and an aqueous
solution of 40% of NaC8 at 79 K. The octane spectrum is characterized by the LAMl band at 287 cm-l and a smaller band at 70 cm-'.The NaC8 spectrum is characterized by a strong band at 83 cm-' with a shoulder near 80 cm-', by a stronger band at 125 cm-' with a shoulder near 109 cm-', and a weaker band at 258 cm-'. A higher frequency band is situated near 298 cm-l. We find also these bands in the spectrum of the 40% solution at nearly the same frequency but with two new bands situated at 229 and 3 10 cm-l. The shift of the LAMl band from 287 cm-' in octane to 258 cm-I in sodium octanoate is due to the modification of one end of the chain by a heavier polar head.33*34Bands at frequency smaller than 150 cm-I are transversal acoustical modes. We can observe that all the bands of octane are split in sodium octanoate, this could be produced by an adjacent chain coupling. F w e 2 shows the evolution of the 5&35o-Cm' spectral region with the concentration of NaC8 and explains the origin of these two bands at 229 and 310 cm-l. It is quite evident that they are due to the solvent because their intensities increase as the NaC8 concentration diminishes. Wong and Whall~y'~ observed these two bands respectively at 228 and 305 cm-'at 100 K, and are attributed to the translational vibrations of the Zh hexagonal ice. In our experimental conditions we observed them in pure water at 228 and 31 1 cm-' at 79 K (spectrum not shown). The bandwidth of the vibration modes of the ice is nearly 23 cm-I in pure ice and only 10 cm-' in the solutions. In an anhydrous state all long-chain molecules have structures more or less identical: chains are parallel, and polar heads are face to face, and at the other end methyl groups are facing each other, forming a bilayer. Because of the hydrophobicity of the chain when water molecules are added to these structures, they place themselves between the polar headgroups of two successive bilayers. In the lamellar phase or liquid-crystal phase we have alternative layers of water and surfactant. At higher concentration of water and at high temperature we have a micellar liquid phase. When the sample is cooled from the micellar phase (or from another phase), we obtain a spectrum of the surfactant characteristic of a solid phase at low temperature. What is the structure of this phase? In particular, have we alternative layers of ice and surfactant? As it can be seen in Figure 2, the band due to ice does not change in frequency and increases its intensity when the quantity of water increases. The LAM frequency does not change even for the lowest concentrations ( 5 , 4,and 3%) which are smaller than the cmc (6.2% in weight). With surfactant, whatever the concentration, higher or lower than the critical micellar concentration, the ice seems highly structurated as a consequence of interactions between water and surfactant molecules as can be seen with the decreasing of the bandwidth of the vibrational modes of the ice in the octanoate solutions. The LAM frequency is not modified (258 cm-l) as the concentration changes. This frequency is identical to that of the anhydrous sample. This indicates that the molecules are isolated. Neverthelms, using the LAM model of Minoni and Z ~ r b ithe ,~~ LAMl frequency for a molecule with eight carbon atoms and a ~ value terminal carboxylate group should be of 232 ~ m - I . 3This is quite far from that observed (258 cm-I) to confirm this hypotheais. We must conclude that they stay as dimers. In this case, in the low-temperature phase we have microcrystalsof surfactant inside the ice. This explains why the ice spectrum and the surfactant spectrum are not modified with concentration. In Figure 3 we see the 85&1200-cm-l region for the three samples: we observe that in the NaC8 spectrum the vibrational modes observed in the octane; the CH3rocking deformation mode the antisymmetricaland symmetrical C< stretching at 893 cm-'; modes at 1066 and 1125 cm-l. These bands characterize conformers in the all-trans structure. The shift observed on the CH3 rocking deformation mode of the NaC, is due to the modification of one end of the chain and proves that this mode is also coupled to a vibration mode of the last bonds of the chain.
9116 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
Piquart and Lacrampe
h
crn-'
300
2 00
loo
Finun 2. Evolution of the low freauencv - - Raman suectrum of sodium octanoate at 79 K with different concentration of sodium octanoate from 3 to 10%.
We also see a mode at 927 cm-I and a less intense one at 1030 These modes are certainly due to the C-O stretching modes of the carboxylate group. They have been observed in sodium laurate and in fatty acids that also possess a terminal carboxylate We can observe that the NaCs solution bandwidths are broader than the octane and anhydrous octanoate bandwidths. There are few differences between the NaCs and octane spectra in the 1250--1500-~m-~region (spectra not shown). They look like Raman spectra of others c o m p o ~ n d s . ~An * ~intense l ~ ~ ~ mode situated near 1293 cm-I is the CH2torsional made. Bands between 1420 and 1480 cm-l are due to CH2scissor vibrations. We observe vibrational modes with bandwidths broader in the case of octanoate than in the case of octane. Also the broader bandwidths for the aqueous solution modes compared to those of the anhydrous octanoate indicates a slightly different chain environment. The band at 1436 cm-I is more intense in octanoate than in octane, and frequencies of deformation modes are identical in the anhydrous octanoate and in the solution. In the C-H stretching modes region there is little difference between the spectra of the three samples (spectra not shown). The CH3 modes are less intense in sodium octanoate than in octane because there is only one CH3 in sodium octanoate chains. The antisymmetrical stretching mode is at 2887 cm-I in octanoate instead of 2883 cm-I in octane. We also observe that the NaC8 band at 2960 cm-'is split in octane at 2955 and 2967 cm-l. The symmetrical stretching band at 2850 cm-I is much more intense in sodium octanoate than in octane. riplia Phse. The spectrum of sodium octanoate in the micellar liquid phase shows few differences with those of others surfact a n t ~ , ~and ~ *we~consider ~ , ~ ~ only several specific points. The first one concerns the CHI deformation modes near 1450 an-'.These arc shown in Figure 4 in three samples the aqueous solution at 40% at low temperature, the liquid octane, and the 40% solution in the micellar phase. Abbate et a1.I" have shown that the frequency of an isolated CHI group is nearly 1450 cm-l. The broad shoulder that appears on the high-frequency side results from Fermi resonances in the trans chain. For all-gauche conformations, the 146O-Cm-' band should not exist because it is the
1141
cm-l.
DSI 0
1
1200
I
I
1100
I
I
1000
I
I
7
1
900
I
cm-'
Figure 3. Raman spectrum of the 850-1200-~m-~region of octane (up per), anhydrous sodium OCtanoatc (middle), and 40% solution of sodium octanoate (lower) at 79 K. Values on the spectra indicate the peak frequency.
evidence of intramolecular coupling of trans structures. Its intensity is generally considered as proportional to the number of these trans structures.36 Looking at the experimental spectra, we note that in octanoate there are more gauche conformers than in octane. The shoulder
The Journal of Physical Chemistry, Vol. 96, NO. 23, 1992 9117
Aqueous Sodium Octanoate Solutions
cm"
1500
1400
Figure 4. Deformation modes spectrum of the solid phase of the 40% solution at low temperature (upper), liquid octane (middle), and liquid phase of the 40%solution of sodium octanoate (lower).
on the high-frequency side is less important in the samples containing 3 and 5% of octanoate, Le., with a concentration smaller than the cmc. This seems to indicate that there are more gauche conformers in monomers than in micelles. An identical result was observed by O k a b a y a ~ h and i ~ ~ Rmenholm3*in a Raman study ~ hexof short chain carboxylates and by U m e m ~ r aon~ sodium anoate in an infrared study on micelles formation. In the solution spectrum an intense band appears near 141 1 cm-l. It exists in melted octanoate only as a small shoulder (spectrum not shown). This band exists on all the solutions spectra whatever the concentration. This band is attributed37 to the stretching symmetrical mode of COO- carboxylate. I@frequency changes from 141 1 cm-'in the 40% solution to 1406 cm-'in the 3% solution. Its intensity also slightly changes with the concentration 88 the Z,411/Zlw ratio passes from 0.71 for the 40% solution, to 0.86 for the 3% solution. This ratio changes more rapidly for concentrations smaller than the cmc and less rapidly when the concentration is higher. When the sodium octanoate solution passes from solid to liquid phase, the adsymmetrical (1063 cm-l) and the symmetrical (1118 cm-')vibration modes, characterizing the trans configuration, still
exist, but a new mode appears near 1078 cm-l due to gauche conformers (Figure 5 ) . We can observe that the bands are shift toward lower frequency in the aqueous solution as compared with melted octanoate. The 1078nn-'band can be used to follow the evolution of gauche conformers in the chains. In Figure 6 we present the evolution of the intensity ratios of skeletal vibration modes ZlMs/ZIo78 and ZI138/Z1078versus concentration. We see that for a concentration of Co= 6-7% a change appears in these ratios. They are smaller than 1 under Coand higher than 1 for concentrations greater than C,. This concentration is a p proximately the cmc found in the literature.@ Iq the lower frequency part of the spectrum (Figure 5), between 750 and 950 an-',we observe vibration modes due to the rocking CH3mode with different environments. The change in frequency is due to different bonds at the end of the chain. Bands at 897, 892,865,and 847 cm-l in octane are due to tt, gt, gg,and tg bonds, respectively,4I at the chain termini. The same phenomena is observed for the sodium octanoate chain but with frequencies at 898,889,873,and 847 cm-'. We observe in Figure 7 the evolution of the band centered nearly 2900 cm-'.The less concentrated the solution, the more this band shifts toward higher frequencies. The band situated near 2900 cm-' is extremely complicated. It is known from the works of Zerbi et al.42that the antisymmetrical methylene stretching mode causes an intense band near 2900 cm-'. If gauche conformers are introduced in the chain, this mode is shift toward higher frequency, its intensity decreases, and it broadens. Simulations have shown%that this region would be composed of a band having a frequency smaller than 2900 cm-', corresponding to trans conformers still existing in the liquid state, and of a band having a frequency higher than 2900 cm-I corresponding to gauche conformers in the chain. The shift of the maximum is then due to the relative proportion of each component. The shift observed in Figure 7 indicates that there are more gauche conformers in the less concentrated compounds. This result was also observed in the deformation and in the skeletal mode regions. We can also observe the evolution of the intensity ratios 12$,38/12910 and Z2910/Zz8azusually used to estimate the chain ordering. These ratios give informations about interactions between contiguous chains. In Figure 8, these ratios are reported as a function of concentration. At a concentration of approximately 5.5-696, a change occurs in these ratios. The Z2910/Z2862 ratio decreases below this concentration, showing a higher disorder of the chains. At the
A
a75
860
NaC8 40'1.
I
I
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I
DOO
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703 Figure 5. Raman spectrum of the 700-1 15O-cm-I region of melted sodium octanoate (upper), liquid octane (middle), and liquid phase of the 40% solution of sodium octanoate (lower). Valua on the spectra indicate the peak frquency. cm-'
1100
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9118 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
b
b
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b h
0
10
Picquart and Lacrampe
20
30
40
o
.I
Figure 6. 1 1 ~ ~ / 1 (opcn 1 ~ 8 triangle) and I,l,8/Ilo7s (black circle) intensity ratios at 30 O C versus concentration of sodium octanoate solutions between 3 and 40%. The arrow indicates the
0
Figure 8.
10
A
P
20
30
40
.I.
(open square) and 12910/12862 (black triangle) inC versus concentration of sodium octanoate solutions tensity ratios at 30 ' between 3 and 40%. The arrow indicates the C ~ C . ~ ~ 12938/12910
cm-4 3000 3400 3800 Figure 9. Raman spectrum of 0-H stretching region of water at 30 OC for 3% (black line) and 40% (dashed line) sodium octanoate solutions.
cm*'
I
I
I
3000
2900
2 BOO
Figure 7. Raman spectrum of the C-H stretching region at 30 'C for different concentrations from 3% (lower) to 40% (upper). The dashed line indicates 2900-~m-~ frequency. Values on the spectra indicates the peak frequency.
same time the 12938/12910 ratio increases below this concentration, showing an increasing motion of the CH3 groups. Both ratios are nearly constant above this concentration. A concentration of 3% of octanoate corresponds to nearly 298 water molecules for a molecule of surfactant, while a concentration of 40% corresponds to 14 water molecules for a molecule of surfactant. Roscnholm et al. indicate in a paper that 5-6 molecules of water/molecule of sodium octanoate is the minimum amount of water required for hydration of the sodium carboxylic group.l0 The various concentration we have studied show that all the , ~ ~observed effect in molecules of NaC8 can be h ~ d r a t e d . ' ~The Figures 6 and 8 is then not due directly to a modification of hydration but must be certainly attributed to the transition of a monomeric solution to a micellar solution. In the monomeric solution each ion is hydrated and the molecules of octanoate are surrounded by a network of water. In the micelles, ions are surrounded first by water but also by ions of neighboring molecules. This can modified the orientation of the polar heads, the symmetrical stretching mode of carboxylate being prohibited in some configurations. This could explaii the variation of relative intensity of the symmetrical stretching mode of carboxylate COO- ion. This effect appears mainly at concentrations
below the cmc and can be interpreted as a progressive aggregation and counterion binding process as observed by Umemura et al. on sodium h e ~ a n o a t e . ~ ~ We can conclude that the monomer chains in solution have more gauche conformers than when the molecules form micelles. A similar effect was observed by Kamogawa et al.# studying submillimolar surfactant solutions of sodium decyl sulfate, sodium dodecyl sulfate, and lithium tetradecyl sulfate. Water Structure. It seems interesting to look at 0-H stretching modes of water, situated in the 3000-38oo-Cm-' region in all these solutions. We analyzed this region at 30 OC for various concentrations. All the solutions are in the liquid state (micellar or monomeric) at this temperature. On Figure 9 we reproduce spectra of this region for two concentrations of 3 and 40%. We can see that the intensity of the band situated nearly 3250 cm-' decreases and is shift toward higher frequency. The interpretation of this region is still under discussion because the intramolecular vibration region of the Raman spectrum of liquid water is comp e d of several overlapping bands. BrookeP analyzes this region as the superposition of different 0-H stretching modes, corresponding respectively to a different environment of the water molecules. He makes an arbitrary classification: when the 0-H bond stretching frequency is above 3600 cm-I, a weak hydrogen bond is implicated, and when this frequency is below 3600 cm-*, stronger hydrogen bonds are implicated. This can be complicated by coupling effect and Fermi resonances. To analyze the evolution of this broad band, we used a decomposition program in four Gaussian bands as in the Walrafen ~ o r k . Z * J zWe ~ needed at least four bands to have the best fitting spectrum. In pure water at 30 OC these bands are situated at 3226, 3410,3522,and 3617 cm-' after the decomposition procedure. Walrafen32 found them at 3245, 3420, 3520, and 3620 cm-I, respectively. We can see in Table I, the evolution of the intensity (band area) of these four bands with surfactant concentration. We can observe that bands I and 111decrease their intensities as the sodium octanoate concentration increases; meanwhile band I1 increases its intensity, and band IV stays more or less constant. It is known43that in solution the integrated intensity of a Raman
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9119
Aqueous Sodium Octanoate Solutions TABLE I: Relative Intensity, after Decomposition, of the 0-H Stretching B.nds for Different Concentmtim of Sodium Octanorte It MY? mnc band I band I1 band I11 band IV
water 3% 5% 9.6% 20% 30% 40%
40.8% 40.2% 39.9% 39.1% 38.2% 36.7% 32.8%
44.6% 44.9% 45.2% 47.0% 49.3% 51.0% 53.3%
9.4% 8.2% 8.5% 8.0% 7.2% 7.6% 7.9%
5.4% 7.0% 6.4% 5.9% 5.3% 4.7% 6.0%
Bands are numbered in the way of increasing frequencies and percents are calculated in relation to the total surface between 3000 and 3800
cm-I.
line is, to a very good approximation, linear in the molar concentration of the species which gives rise to the line. When studying the evolution of the intensity of the bands of the 0-H vibration of water, we observed a result identical to the temperature effect. It is that when temperature increases, the intensity of the lower frequency band decreases. This result is explained quite simply if we suppose that there are mainly two types of water molecules, those with a strong hydrogen bond on each 0-H bond (symmetrical structures), and those with one strong hydrogen bond on one 0-H bond and a weaker hydrogen bond on the other one (asymmetrical structures). When the temperature decreases, the strength of hydrogen bonds increases and the intensity of the lower frequency band increases. On the contrary, when the temperature increases, the strength of hydrogen bonds decreases and the intensity of the lower frequency band decreases. Here we observe that when concentrationof surfactant increases, the intensity of the lower frequency band decreases (Table I). The higher frequency band (band IV at 3617 cm-l) could be due to free 0-H bonds4sand the band I11 situated at 3522 cm-', to 0-H bonds with two weak hydrogen bonds.& As it can be seen in Table I, band IV is approximately constant, while the intensity of band I11 decreases as concentration increases. The main changes concern the bands corresponding to symmetrical and asymmetrical structures. But there is not only a change in intensity because, for the two lower concentrations bands I-IV are situated at 3230,3413,3520, and 3619 cm-l, respectively, and near 3240-3250, 3426, 3530, and 3620 cm-l for the higher concentrations. We observe also a shift of the bands I and I1 toward higher frequencies. It is known that in the hexagonal structure the ice is also showing (spectrum not shown) a very strong band at higher frequency (3085 cm-')and a smaller one at 3212 cm-1.28 These two bands are the stretching modes of the 0-H bond. Reciprocally when temperature increases the bands are shifted to higher frequency because of the lowering of the strength of hydrogen bonds. These observed shifts are due29 to the anion nature and are proportional to the concentration. They come from the decoupling of intermolecular interactions between neighboring water molecules. The vibrational frequency corresponding to the 0-H stretching region of a given oscillator is perturbed by the environment in which it find itself!' As we observe, when the concentration increases, there are more water molecules in contact with the COO- anions of the polar head of the surfactant. The C-0 bond is less polarized than the 0-H bond and "behind" the carbon atom is a nonpolar hydrocarbon chain. This fact increases the number of weak hydrogen bonds between hydrogen of water molecules and oxygen of carboxylate group and increases the number of asymmetrical structures. But this process is really more complicated because the introduction of octanoate molecules in water breaks or causes distortion of hydrogen bonds and can produce a more structured reorganization of the water neighboring the molecule of surfactant. This well-known phenomena, called "hydrophobic hydration",4' reduces the distortion of hydrogen bonds. As an example, a concentration of 3% (below the cmc) of sodium octanoate corresponds to an
octanoate molecule for nearly 300 water molecules. It means that an octanoate molecule is more or less surrounded by 20 water molecules. The distortion effect should concern 6-7% of water molecules (if we consider only the first neighbor molecules), but the variation of band I intensity is in fact smaller than 2%. On a more concentrated sample (20%), the variation of band I intensity is approximately 6%. This concentration corresponds to 1 molecule of surfactant/l8 water molecules. Zemb et aL6have shown that at this concentration the micellar aggregation number is 26. If we consider that each carboxylic group of a micelle is H-bonded to a water molecule, the proportion of water molecules in interaction would be approximately of 5.6%. It means that only the first neighbor water molecules are perturbed in this case.
Conclusion Raman scattering gives us information about chain ordering in sodium octanoate aqueous solutions. We have observed the modifications that appear in the Raman spectrum of octane when one end (CH,) is changed by a carboxylate group (COO-). In particular the shift observed in some bands of deformation modes is directly due to the effect of the weight of the polar head. We have also observed the spectral modifications that appear for concentration smaller than the critical micellar concentration, pecularly in the carboxylate vibration modes, and in the relative intensities of the skeletal vibration modes and the C-H stretching modes. The fact that there are more gauche conformers in monomers than in the micellar state suggests that the each monomer is forming a small ball. We have observed the changes in the 0-H vibration modes of water in these aqueous solutions due to the reorganization of the water molecules around surfactant molecules. We have seen that the polar head forms hydrogen bonds with water molecules that compensate the initial distortion in the monomeric solution. In micellar solutions, it seems that each micelle is surrounded by a shell of water molecules H-bonded to each carboxylic group. It means that the symmetric structure of the water is destroyed by the polar head of the surfactant. We confm on sodium octanoate, with another method, the results observed by D'Aprano et al. on AOT reversed micelles.48 References and Notes (1) Wennerstrom, H.; Lindman, B. Phys. Rep. 1979, 52, 1. (2) Eriksson, F.; Eriksson, J. C.; Stenius, P. In Solution Chemistry of Surfactants; Mittal, K., Ed.; Plenum: New York, 1979; Vol. I1 p 297. (3) Lindman, B.; Brun, B. J. Colloid Interface Sci. 1973, 42, 388. (4) Persson, P. 0.;Drakenberg, T.; Lindman, B. J. Phys. Chem. 1979,83, 3011. (5) Hayter, J. B.; Zemb, T. Chem. Phys. Lett. 1982, 93, 91. (6) Zemb, T.; Drifford, M.; Hayoun, M.; Jehanno, A. J . Phys. Chem. 1983, 87, 4524. (7) James, C. J.; Heathcock, J. F. J . Chem. SOC.,Faraday Trans. 1981, 77, 2857. (8) Boussaha, A.; Ache, H. J. J . Phys. Chem. 1981,85, 2444. (9) SjBblom, J.; Nylander, C.; Lundstrom, I. Colloid Polym. Sci. 1982, 260, 89. (10) Rosenholm, J. B.; Sjoblom, J. and Osterholm, J. E. Chem. Phys. Lipids 1982, 31, 117. (11) Hayter, B.;Hayoun, M.; Zemb, T. Colloid Polym. Sci. 1984, 262, 798. (12) Kertes, A. S.; Tsimering. L.; Garti, N. Colloid Polym. Sei. 1985,163, 67. (13) WPrnheim, T.; Henriksson, U.; Klason, T. Mol. Cryst. Liq. Cryst. 1985, 126, 247. (14) Warnheim, T.; Henriksson, U. J . Colloid Interface Sci. 1987. 115, 583. (15) Rosenholm, J. B.;Larsson, K.; Dinh-Nguyen, N. Colloid Polym. Sci.
1977, 255, 1098. (16) Snyder, R. G.; Hsu, S.L.; Krimm, S . Spectrochim. Acta 1978, 34A, 395.
(17) Snyder, R. G.; Scherer, J. R. J . Chem. Phys. 1979, 71, 3221. (18) Abbate, S.;Zerbi, G.; Wunder, S.L. J. Phys. Chem. 1982,86,3140. (19) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J . Phys. Chem. 1982, 86, 5145. (20) Lippert, S.L.; Petitcolas, W. L. Biochim. Biophys. Acta 1972, 282, 8. (21) Snyder, R. G.; Scherer, J. R.; Gaber, B. P. Biochim. Biophys. Acta 1980, 601, 47. (22) Gaber, B. P.; Petitcolas, W. L. Biochim. Biophys. Acta 1977, 465, 260. (23) Yellin, N.; Levin, I. W. Biochim. Biophys. Acta 1977, 489, 177.
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(38) Rosenholm, J. 9.;Stenius, P.; Danielson, I. J . Colloid InterfaceSci. 1976,57, 551. (39)Umemura, J.; Cameron, D. G.; Mantsch, H.H.J. Phys. Chem. 1980, 84,2272. (40)Lindman, B. In Surfacranrs; Tadros, Th. F., Ed.; Academic Press: London, 1984; p 83. (41) Kim, Y.;Straws, H. L.; Snyder, R. G. J . Phys. Chem. 1988,92,5080. (42) Zerbi, G.; Magni, R.; Gussoni, M.; Moritz, K. H.;Bigotto, A,; Dirlikov, S.J . Chem. Phys. 1981, 75, 3175. (43) Lilley, T. H.In Water. A Comprehensive Treatise; Franks, F., Ed.; Plenum Prcss: New York, 1975;Vol. 3, p 265. (44) Kamogawa, K.;Tajima, K.; Hayakawa, K.; Kitigawa, T. J . Phys. Chem. 1984,88,2494. (45)Scherer, J. R.; Go, M.K.; Kint, S.J . Phys. Chem. 1974,78, 1304. (46) Walrafen, G. E. J . Chem. Phys. 1970,52,4176. (47) Franks, F.; Ravenhill, J.; Egelstaff, P. A.; Page, D. I. Proc. R. Soc. London 1970,A319, 189. (48) DAprano, A.; Lizzio, A.; Turco Liveri, V.;Aliotta, F.; Vasi, C.; Migliardo, P.J . Phys. Chem. 1988,92,4436.
Time-Resolved Resonance Raman Studies of the Structures of the Lowest Triplet State and the Radlcai Anion of Benzii Ken Ebihara, Hidefumi Hiura, and Hiroaki Takabashi* Department of Chemistry, School of Science and Engineering, Waseda University, Tokyo 169, Japan (Received: April 23, 1992; In Final Form: July 28, 1992)
Timeresolved resonance Raman spectra of the TI state and the radical anion of benzil and its isotopically substituted analogues have revealed that the c-0symmetric stretch exhibits dramatically large downshifts in the TI state and in the radical anion, while the downshifts of the phenyl vibrations are small. This indicates that the spin density is predominantly localized on the C - 0 groups, which drastically weakens the C = O bonds of the two transients. The much larger weakening of the C 4 bond in the TI state than in the radical anion is interpreted in terms of the nature of HOMO and LUMO. On the other hand, the central C-C stretch is markedly upshifted in the radical anion and in the TI state. In particular, the remarkably high frequency of the C-C stretch of the radical anion may indicate that the radical anion takes a more planar structure than the So state. The C-C stretch of the radical anion exhibits unusually large high-frequency shifts in strong hydrogen-bond-donor solvents. This may imply that the planarity of the molecule increases as the hydrogen bond between the radical anion and the solvent becomes stronger.
Introduction The photochemistry of benzil (diphenylethanedione) has been the subject of many investigations with regard to the conformations about the central C-C bond in the ground state, So,the excited states, SIand TI, and the radical anion. The structure of the So state is reported to be twisted about the central C-C bond with the two benzoyl planes making an angle of 111'36' in the crystalline state' and 98O in solutions.2 Under matrix-imposed geometric restraints, such as in benzil-doped stilbene crystals3and in methylcyclohexane glass? a trans-planar configuration was suggested for the So state structure. Both skewed and relaxed trans-planar structures have been suggested for the excited states, SIand TI. Dual fluorescence with the peaks at 440 and 500 nm5 and dual phosphorescence at 525 and 565 nmS7 were observed from benzil, which were assigned to skewed (440 nm for SIand 525 nm for TI)and trans-planar (500 nm for SIand 565 nm for TI) configurations of the SIand TIstates. Microwave dielectric absorption measurements showed that the TIstate has zero or near-zero dipole moment, which implies that the TIstate is of the trans structure.* An ENDOR study suggested that the structure of the T I state has the configuration with the dihedral angle of 157O and the ring twist angle of 24O? A transient absorption peak at 490 nm of b e d in ethanol glass observed in a photolytic studylo was assigned to the skewed conformer of the TI state and the peak at 470 nm in benzene to the trans-planar conformer. picosecond time-resolved absorption study of benzil showed that the absorption peaks of the SIand TI states are located at 525 and 490 nm, respectively, in cyclo0022-3654f 92f 2096-9120$03.00/0
hexane, and at 560 and 500-510 nm, respectively, in the microcrystalline state; the geometries in cyclohexane were considered to be different from those in microcrystalline state.' The photochemistry of the production of benzil radical anion is rather complicated. McGimpsey and Scaiano" have proposed that a homolytic cleavage of the central C-C bond occurs from a higher excited triplet state and generates benzoyl radical. Time-resolved ESR study by Mukai et a1.I2 showed that in the presence of triethylamine the radical anion is produced from the lowest triplet state TI through a one-photon prows, while the benzil ketyl radical and the benzoyl radical are produced from the higher triplet states T, through a two-photon process. The structure of the radical anion has not been studied in detail. Although the transient absorption peak at 620 nm in acetonitrile-triethylamine has been assigned to the skewed conformer of the radical anion and the peak at 540 nm in ethanol-triethylamine to the trans-planar conformer,1° the evidence for this assignment appears to be insufficient. In the present investigation we have measured time-resolved resonance Raman spectra of the TI state and the radical anion of benzil and its isotopically substituted analogues in order to obtain information on the structures of these transients based on vibrational assignments. Experimental Seetion
Benzil (BZhlo) was purchased from U n t o Chemical CQ.,Inc., and was recyrstallized from benzene. C6H5'3C013COCpHS (BZ-13C2)was obtained by the coupling of two benzoylchlonde0 1992 American Chemical Society
