Adsorption of sodium laurate from its aqueous solution onto an

Langmuir 1993, 9, 3414-3421

3414

Adsorption of Sodium Laurate from Its Aqueous Solution onto an Alumina Surface. A Dynamic Study of the Surface-Surfactant Interaction Using Attenuated Total Reflection Fourier Transform Infrared Spectroscopy Alexander Couzist and Erdogan Gulari* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109 Received July 15, 1993. I n Final Form: September 30, 199P The interaction dynamics and the structure of the adsorbing sodium laurate from its aqueous solution onto an alumina surfacewere studied in situ using infrared attenuated totalinternal reflection spectroscopy. For pH values under the isosteric point of alumina, electrostaticallybound laurate is irreversibly adsorbed on the surface. With time, the laurate ion reacts with the alumina, through a water abstraction reaction, forming a unidentate aluminum salt. Desorption experiments show that the latter structure is reversibly bound to the surface. Spectroscopic evidence indicates that the electrostatically bound layer has Czu symmetry. The aluminum laurate does not exhibit such symmetry, and no organization on the surface is observed upon formation of the salt. At higher pH values, when the alumina surface is not positively charged, surface aluminum atoms and the laurate ions form a chelating structure, through a hydroxyl abstraction reaction.

Introduction Adsorption of surfactants from solution on the solidliquid interface attracts great technological and scientific interest. The large number of applications in areas such as detergency, optics, microelectronics,ore floatation, toxic waste management, and enhanced oil recovery motivate this interest. The study of the adsorption of surfactants can be classified into two categories: first, determining the amount adsorbed on the solid-liquid interface; second, determining the structure and properties of the adsorbed layer. Traditionally, the studies reported in the literature involve the determination of “equilibrium” amounts adsorbedla and the structure and properties of the adsorbing layer at “ e q ~ i l i b r i u m ”Unfortunately, .~~~ these types of studies yield no important information about the

(8)Somasundaran, P. Proceedings of the ACS Sympcaium on Surfactant Adsorption on Surfaces; ACS annual meeting, New York, 1986. (9) Timmons, C. 0.; Zisman, W. A. J. Phys. Chem. 1966,69,984. Patterson, R. L.; Lockhart, L. B., Jr. J. Colloid (10) Timmons, C. 0.; Interface Sci. 1968,26, 120. (11) Tompkins, H. G.; Allara, D. L. J. Colloid Interface Sci. 1974,49,

kinetic mechanisms and surface reactions involved. By linking results from kinetic studies and studies on the dynamics and development of the surface-surfactant interaction, important information on the mechanisms involved can be deduced. The traditional bulk methods for studying adsorption generally involve bringing the solid, in powder form, and the solution in contact, letting the system equilibrate, and then separating the slurry and analyzing the residual s~lution.”~These techniques lack the speed to monitor the kinetics and, obviously, cannot be conducted in situ. Furthermore, they do not provide any information about the surface-surfactant interaction. The problem of studying adsorption is further complicated by the fact that most surface science techniques are not suitable for looking at the liquid-solid interface. This forces studying the adsorption process ex situ, limiting the ability to study the nature of the surface-adsorbate interaction and the structure of the adsorbate in its natural solution environment. Nevertheless, these techniques have yielded important information concerning the structure of the adsorbate after long contact times of the solution and the solid surface. Reflection absorption infrared spectroscopy has been used to study the equilibrium structures of n-alkanoic acids on aluminum,13oxidized aluminum,14J6 iron and copper surfaces,11J2and fluorite surfaces.22 The reported data have established that the structure of the film depends on the solution concentration of the surfactant and the aging of the adsorbed surfactant layer.14J6 In total contrast with the bulk techniques, employing spectroscopic techniques provides solutions to most of the above problems. Two such techniques have appeared in the recent years in the literature, electron spin resonance (ESR)17and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR).lSz7 The latter is the technique employed in this study. Attenuated total

410. (12) Boerio, F. J.; Chen, S. L. J. Colloid Interface Sci. 1980, 73, 176. (13) Golden, W. G.; Snyder, C. L.; Smith, B. J.Phys. Chem. 1982,86, 4675. (14) Allara, D. L.; Nuzzo, R. G. Langmuir 1986, 1, 45. (15) Allara, D. L.; Nuzzo, R. G. Langmuir 1986,1, 52. (16) Brown, N. M. D.; Floyd, R. B.; Walmsley, D. G. J. Chem. SOC., Faraday Trans. 2 1979, 75, 17.

(17) Malbre1,C. A.;Somasundaran,P.;Turro, N. J. J.Colloidlnterface Sci. 1990, 137, 600. (18) Higashiyama, T.; Takenaka, T. J.Phys. Chem. 1974, 78, 941. (19)Kellar, J. J.;Cross, W. M.; Miller, J. D. Appl. Spectrosc. 1989,43, 1456. (20) Kellar, J. J.; Cross, W. M.; Miller, J. D. Appl. Spectrosc. 1990,44, 1508.

* To whom correspondence should be addressed. Present address: -International Paper Company, Long Meadow Road, Tuxedo, NY 10924. Abstract published in Advance ACS Abstracts, November 15, t

1993.

(1) Hanna, H. S.; Somasunadaran, P. J. Colloid Interface Sci. 1979, 70, 181. (2) Partyka,S.;Lindheimer,M.; Bottero, J. Y.; Brun, B.; Somasundaran, P.; Viswanathan, K. V. Calorim. Anal. Therm. 1986,16,76. (3) Rathman, J. F.; Scamehom, J. F. J.Phys. Chem. 1984,88,5807. (4) Scamehorn, J. F.; Schecter, R. S.;Wade, W. H. J.Colloid Interface Sci. 1982.85. 463. (5) Sckehorn, J. F.; Schecter, R. S.; Wade, W. H. J.Colloid Interface Sei. 1982,85, 479. (6) Scamehorn, J. F.; Schecter, R. S.; Wade, W. H. J. ColloidInterface Sci. 1982, 85, 493. (7) Somasundaran, P.; Goddard, E. D. Mod. Aspects Electrochem. 1979, 13, 207.

0743-7463/93/2409-3414$04.00/00 1993 American Chemical Society

Adsorption of Sodium Laurate on Alumina

Langmuir, Vol. 9, No. 12, 1993 3415

reflection spectroscopy (ATR) or internal reflection spectroscopy (IRS) is a method of obtaining the infrared spectrum of species located near the surface of a sample. It was first developed by Harrick,28and ita capability is attributed to the presence of an evanescent wave of light when total reflection occurs at the interface of two materials with different indices of refraction. The issue of reversibility of the adsorption process is a matter directly linked to the structure of the adsorbate and the type of interaction between the solid surface and the adsorbing layer. Except for the work by Zawadzki,32 the matter has not been studied in a systematic fashion due to the lack of kinetic studies. Results from this study indicate that the reversibility is strongly effected by the aging of the adsorbed layer. In this paper we present the resulta of our study concerning the interaction of the adsorbing surfactant and the solid surface as a function of time. The surfactant used is sodium laurate adsorbing from ita aqueous solution onto an aluminum oxide surface. For the first time we report on the kinetics of adsorption of surfactants from an aqueous solution onto a non-infrared-transparent material, such as alumina. Even in the presence of the water environment, the sensitivity achieved allowed the detection and identification of the surface structures and their evolution with time.

E = E0e-'/dp (3) Eo is the amplitude of the electric field at the interface, in the rarer medium, and can be calculated from the following equations: 28

(4)

E* =

2 sin 8 cos e (1- ~ ~ 1 2 ) ~ /v2,2)sin2 ~ [ ( 1 +e -

(6)

The distance from the interface required for the electric field to drop to e-1 of its value at the surface, is defined as the penetration depth and is given by:

= w91. The interaction of the evanescent wave with the adsorbing rarer medium causes a loss of reflection. The stronger the interaction, the greater the loss of reflectivity. Reflectivity is defiied as the ratio of the reflected light intensity to the intensity of the incident light: A1

Materials. Sodium laurate was purchased from Sigma Chemical and was used without further purification. The H2O used for the aqueous solutions was deionized and doubly distilled. ZnSe internal reflection elements were purchased from Spectra Tech and Harrick Scientific Co. and had dimensions of 50 X 10 X 3 mm and 4 5 O angle of incidence. Before every use the internal reflection elements were polished using 0.3-pm and 0.05-pm A120s suspensions (purchased from Mager Scientific) and then rinsed with acetone, water, and methanol. Following drying of the IRE, 600 A of A1203 was directly deposited on the element by sputter coating. During the coating process, the bevel surfaces of the IRE were protected by a layer of photoresist material, which was removed following the deposition using acetone. Internal Reflection Principles. Total internal reflection is a familiar phenomenon, observed in every day life. When a light beam propagating in an optical medium with a refractive index of 91 strikes a surface of another material, with a refractive index 92, then a portion of the beam will be reflected and a portion will be partially transmitted in the second medium. The transmitted beam, is refracted according to Snell's law: (1)

When the light approaches the interface from a denser medium (91 > 1 2 ) and the angle of incidence is greater than the critical angle defined by eq 1

R = I/Io (8) For small absorption losses ( P H ~ (d)~ Structure an alumina surface at pH > via a hydroxyl abstraction mechanism.

of the terminating hydroxyls at the surface. Desorption experiments, discussed in a following section, verify the strength of the electrostatic bonding. The bands a t 1472 and 1465 cm-1 are assigned to the CHz deformation. The split is caused by the presence of organization in the adsorbed layer. Following the initial 20 min, new bands at 1597,1554, and at 1412cm-1 increase in relative strength. The bands at 1554 and 1443 cm-I are assigned to the laurate ion coordinated with sodium. The two bands at 1597 and 1412cm-l are assigned to the same surface species, because their integrated absorption increases in the same fashion, as is seen in Figure 5. Thus, these bands are assigned to a new surface species formed by a "chemical reaction" of the adsorbing surfactant and the aluminum oxide surface. According to a number of investigator^^^*^*^^ carboxylate ions associated with aluminum exhibit a band corresponding to the asymmetric stretch of the COO- at 1597 cm-1. The location of the symmetric stretch is a matter of controversy.36 It is reported a t 1470 and at 1410 cm-'. The symmetric COO- stretch is very sensitive to the symmetry of the COO- ion. If the two C-0 bonds are similar, meaning that aluminum associates with both oxygen's, then the separation of the symmetric and the asymmetric stretching should be less than in the case of aluminum associating with only one of the oxygen's. This observation indicates that an aluminum carboxylate with the asymmetric COO- stretch appearing at 1412 cm-l, as in our case, has the unidentate structure. This structure does not have CzUsymmetry. In our samples, the characteristic bands in the range 1000-1380 em-l, assigned to the twisting and wagging (36)Alcock, N. W.; Tracy, V. M.; Waddington, T. C. J. Chem. SOC., Dalton Trans. 1976, 76,2243. (37) Grigor'ev, A. I.; Makeimov, V. Russ. J. Znorg. Chem. 1964,9,580.

l

2750

/

~

2800

,

I

,

,

,

,

1

2850 2900 Wavenumber(cm-l)

-t- 13.6 hrs

--t 26.5

,

1

~

1

~

1

(a),

,

,

2950

3000

2950

3000

,

Z

hrs

0

2750

2800

2850 2900 Wavenumber(cm-')

Figure 6. Time series of IR spectra for adsorbed sodium laurate onto AlzOs from an aqueous solution (0.01 M). Hydrocarbon stretching range shows (a) short times and (b) long times.

vibration of the (CHz), do not appear, or their intensity is below the noise level. This indicates that the surface structures are in a fluid condition. CH2 StretchingRange. In Figure 6,a series of spectra in the CH stretching range of the adsorbing sodium laurate

Langmuir, Vol. 9, No. 12,1993 3419

Adsorption of Sodium Laurate on Alumina

-A-

2850

0

1

2

3

4

5

6

(Elapsed Time)llz(hr)”z

0

5

10

15

20

25

30

Elapsed Time (hr)

Figure 7. Integrated absorbance of the u.(CH2) at 2920 cm-I and v,(CH2) at 2850 cm-1 as a function of time. Also the ratio of intensities of the two bands is shown.

can be seen. Changes in the relative intensities and positions of the different bands are taking place with time. This indicates that the composition and the structure of the film are changing with time. At the very early stages of adsorption seen in Figure 6a the asymmetric stretching of the CH2, which is located at 2920 cm-l, appears to have a broad structure bn the high frequency side, indicating the presence of a band. Differentiation of the spectra and curve fitting revealed that the shoulder is due to a band located at 2935 cm-l. The band is assigned to the weak vs(CHs),with contribution from a transition indicative of chain-chain interaction. The intensities of the two bands at the initial stages of the adsorption are very similar. This indicates that the contribution from the chain-chain interaction is large enough to change the ratio of the intensities for the v,(CH2) and vs(CHs), suggesting that there is some form of ordering or association of the aliphatic chains a t low surface coverages. With time, the band at 2920 cm-1 overwhelmsthe 2935-cm-l band, suggesting that the structure of the surface layer changes with surface coverage. This indicates that the contribution from the chain-chain interaction to the 2935-cm-I band is negligible. Also, the lack of the characteristic bands in the range 1001380 cm-I discussed previously supports our conclusion. For long contact times, the spectra of the adsorbed laurate ion in the hydrocarbon range are very similar to the spectra observed with long paraffin molecules. This suggests that the hydrocarbon tails are fluid and are in a hydrocarbon environment. In Figure 7, the integrated absorbances of the two bands at 2850 and 2920 cm-l are plotted as a function of time. This plot shows the kinetics of adsorption. The behavior of the absorbance of the two bands is similar, an expected result, as they pertain to the same functional group of the adsorbing surfactant. The ratio of the intensities of the bands and ita behavior with time suggest a similar conclusion. The adsorbed amount is proportional to the integrated absorbance of these bands. Thus, Figure 7 gives us a picture of the relative amounts adsorbed. It is very interesting that the amount adsorbed does not reach a steady value even after 25 h. When the integrated absorbance is plotted against the square root of time, as shown in Figure 8, a linear behavior is observed. This suggests that the reaction on the surface is fast compared to the diffusion processes, and the rate of adsorption is limited by the diffusion through a thin boundary layer. In such a case, the diffusion process can be described with

Figure 8. Integrated absorbance of the v.(CHa) at 2920 cm-1 and v,(CH2) at 2850 cm-’ as a function of the square root of time. The linear behavior suggests fast reaction and limitationsby the diffusion process. the following continuity equation

with the initial condition that at t = 0, C = c b (the bulk solution concentration) and the boundary conditions that a t x Q),C = c b and at x = 0, C = 0. The last boundary condition indicates that the adsorption process is so fast, compared to the diffusion, that any surfactant reaching the surface is immediately adsorbed. The non-steadystate concentration profile is then found to be

-.

The rate of adsorption is then given by

Following through the initial condition that for t = 0, I’i = 0 we have ri = 2Cb(Dt/T)”2 (19) indicating the proportionality of the adsorbed amount with the square root of the elapsed time.

Desorption Experiment Desorption experiments were also conducted to determine if the laurate ion was irreversibly adsorbed on the alumina surface. The solvent was pure water at a pH = 7.0. Figure 9a shows a typical spectra series for the adsorbed layer as it is exposed to water. The bands at 1595 and 1411 cm-l show a drastic decrease in intensity with time. Within 9 min, the bands have almost completely vanished. The bands at 1540 cm-l and the structure a t 1460 cm-l show almost no evidence of change, indicating that even after 19 h of flowing pure water there is no desorption of these surface species. After 24 h of continuous flowing of water the only carboxylate bands observed are at 1540 cm-l and a broad structure centered at 1466 cm-l. These spectra resemble very closely the spectra of the adsorbed carboxylate at the early stages of adsorption. The intensity of these bands does not change significantly over a period of 24 h. This indicates that the electrostatically bonded carboxylates are irreversibly adsorbed on the alumina surface. The bands assigned to the CH2 stretching modes also exhibit a decrease in intensity and also the band a t 2920 cm-l shows a distinct

Couzis and Culari

3420 Langmuir, Vol. 9, No. 12, 1993

also conclude that the electrostatically adsorbed laurate is permanently attached to the surface and adopts its final configuration almost immediately.

-

Y .#

3

Discussion

e,

B e s: D d

z

Ln

3 9

2750

rat^'''(a) 2800

2850

I

'

4

2900

Wavenumber

'

2950

J

3000

(cm.')

Figure 9. Time series of IR spectrafor adsorbed sodium laurate onto AlzOs, as pure water flows over the established layer: (a)

carboxylvibration range shown; (b)hydrocarbon stretchingrange shown. l

"

"

l

'

"

'

l

'

'

'

~

I ' " ' / ' " ' I " "

18 6 ius of desorption

0 38 hrs of adsorpuon (expanded 3X)

T

1300

1350

1400

1450

1500

1550

1600

1650

Wavenumber (cm'l)

Figure 10. Comparison of the film structure after desorption of the aluminum carboxylates with the film at the initial stages of adsorption.

shoulder on the high frequency side. This effect is shown in Figure 9b. This decrease is due to the loss of the aluminum carboxylates. Comparison of the structure of the adsorbed laurate after 18.6 h of flowing pure water to the structure of the film after 0.38 h of adsorption indicates a very strong similarity. The two spectra are shown in Figure 10. From this observation, it is concluded that the aluminum carboxylate is a completely reversible structure. We can

Results deduced from inelastic electron tunneling spectroscopy studies16 of adsorbed small acids on aluminum oxide surfaces suggest that the carboxylates adsorbed form bridge bonded structures, i.e. the carboxylate is symmetrically attached to two A1 sites of the oxide layer. The band assignment by the investigators indicate that the aluminum carboxylate asymmetric stretch is located at 1613 cm-1 and the symmetric stretch at 1406 and 1420 cm-1. Such a structure because of it CzVsymmetry (the similarity of the two C-0 bonds) will give rise to an IR spectrum very similar to that of a coordinated carboxylate ion, and in such a case the symmetric COO-stretch should be located in the range of 1450-1470cm-l. This conclusion gives rise to doubts concerning the reality of a symmetric bridged structure of the carboxylates on the surface. In addition a bidentating, chelating structure is not considered possible as there are only two known cases of carboxylates forming this stru~ture.3~ Allara and Nuzzo15 have also studied the adsorption of carboxylateson alumina from hexadecane solutions and have concluded that the carboxylate forms a bridged structure that does not have full CzVsymmetry. The symmetric and asymmetric bands of COO- in their study appear at 1475 and 1608 cm-l, respectively. Based on the above, we conclude that the aluminum and the carboxylate form a unidentate structure that does not have C% symmetry and that this structure is reversibly adsorbed. In addition, carboxylates adsorb on the surface irreversibly if they are electrostatically bonded to the surface. From these observations and the reported studies the following question arises: What is responsible for the differences in the structure of the adsorbed carboxylates? The answer to this question lies in the properties of the alumina surface used. In the case of the IETS experiments and the Allara and Nuzzo paper, the alumina surface is hydroxylated only. There is no net positive surface charge that can be used to electrostatically adsorb the carboxylate. In this case, there are two possible adsorption mechanisms. Hydrogen bonding between the carboxylateand the surface OH groups or ion exchange between the surface hydroxyls and the carboxylate (hydroxyl abstraction). The two mechanism are schematically shown in parts b and c of Figure 4. The ion exchange mechanisms explains very well the small tilt exhibited by the hydrocarbon chains, which is proposed in Allara's work.14J5 In the case of adsorption taking place from an aqueous solution, the pH of the solution determines the surface properties of the alumina. In our studies for the 0.01M solution of sodium laurate the pH is 8. A t this pH the alumina surface is protonated, giving rise to a net positive charge on the surface. Under these conditions, there are two possible mechanisms of adsorption. First, electrostatic interaction of the carboxylate ion due to the dissociation of the sodium laurate with the positive charged sites on the surface gives rise to a coordinated carboxylate structure with complete Ca symmetry. The molecules adsorbed in this configuration have the ability to align the aliphatic chains and exhibit some type of ordering as seen from the spectroscopic evidence for low surface coverages. Second, for the same pH range, and at higher surface coverages,the carboxylate reacts with the hydrated alumina surface. During this reaction, water is abstracted from the surface and an

Langmuir, Vol. 9,No. 12, 1993 3421

Adsorption of Sodium Laurate on Alumina

aluminum carboxylate is formed. This aluminum carboxylate is of unidentate structure, giving rise to an infrared spectrum that exhibits a large wavenumber separation between the symmetric and asymmetric COOstretch. This structure does not have CzVsymmetry, and because the laurate aliphatic chain is not long enoughs ordering is not observed. Figure 4d shows a schematic of this mechanism. Results from experiments with alumina powder a t pH = 9 verified our conclusions. At this pH, the alumina surface is not positively charged (the isosteric point for alumina is pH = -9.1). This negates the possibility for water abstraction reaction taking place. Also, the highly hydroxylated surface results in an OH- abstraction reaction, that leads to a laurate which is unsymmetrically chelated to an aluminum site, as seen in Figure 4d. This structure does not exhibit an intense symmetric COO- stretch, a t 1411 cm-1, and this was observed with the powder sample at pH = 9.2. In addition a t high pH values, because of the negatively charged surface, electrostatic interaction of the laurate ion is less probable. Also since a hydroxyl abstraction reaction is not favored, the adsorbed amount decreases, as does the rate of adsorption. From the above results, it is also evident that the two reported symmetric band assignments for aluminum carboxylates are associated with two different structures, a chelating and a unidentate structure. The desorption experiments performed allowed us to distinguish the reversibly adsorbed structures from the irreversible ones. The results indicate that the aluminum laurate formed on the surface is reversibly adsorbed on the surface. This observation is in agreement with our conclusion that the aluminum laurate is formed by water abstraction. This reaction mechanism is the only possible pathway to a reversible adsorbed structure. In addition the stability of the structure assigned to the electrostatically adsorbed laurate ion, under the desorption conditions, pH = 7, shows that this structure is irreversibly adsorbed on the surface.

The aluminum laurate formed is a unidentate species. Experiments conducted a t pH values above the isosteric point of alumina indicate the formation of a chelating structure. This observation is in accordance with results from previous studies. The results of this study clearly show that the adsorption process is not elementary and that the complexity of the mechanism is unique to the system studied. We have also shown that the structure of the adsorbed film and reversibility of the adsorption process are strongly linked. The structure of the chemically adsorbed layer dictates the extent of the reversibility of the process itself. These findings are very important in the area of surfactant adsorption, because for the first time we have tackled the basic question of the dynamics of formation of the adsorbed surfactant layer.

Conclusions We have, for the first time shown evidencefor the various steps involved in the dynamics of adsorption of sodium laurate on a protonated alumina surface. The data suggest that the initial step of the process is the irreversible electrostatic bonding of the dissociated laurate ion with the surface. The laurate ion then reacts with the alumina surface leading to the formation of an aluminum salt of the carboxylate. This reaction involves a water abstraction reaction, and it leads to a reversibly adsorbed structure.

121

A a cb

Ci de d,

El E

Eo Eoi El ri

1 11

(c

k 1 1

x N 9

R

T

Nomenclature integrated absorption absorption coefficient bulk solution concentration interfacial concentration effective thickness penetration depth electric field amplitude of the perpendicular polarization molecular emissivity electricfield amplitude at the interface,in the rarer medium electric field amplitude at the interface, in the i-direction electric field amplitude of the parallel polarization surface excess refractive index index of refraction of the ZnSe element, equal to 2.42 ratio of indices of refraction of A1203 and ZnSe angle of refraction Boltzmann coefficient film thickness thickness of the adsorbed layer [AI wavelength [pml number of reflections angle of incidence reflectivity temperature [KI