Langmuir 1994,10, 131-134
131
Adsorption of Alkylxylenesulfonates on Alumina: A Fluorescence Probe Study A. Sivakumart and P. Somasundamn* Langmuir Center for Colloids and Interfaces, Henry Krumb School of Mines, Columbia University, New York, New York 10027 Received March 22, 1993. I n Final Form: September 18, 199P Changes in the positions of the sulfonate and the methyl groups on the aromatic ring of alkylxylenesulfonate were found to produce a marked effect on the micellization and adsorption at the solid/liquid interface of alkylxylenesulfonates. Fluorescence spectroscopy was used to probe the microstructure of the micelles and hemimicelles of the surfactants. The studies showed the p-xylenesulfonate micelles to be less polar and the p-xylenesulfoante micelles and hemimicelles to have higher aggregation number than the m-xylenesulfonatemicelles and hemimicelles,respectively. Based on results obtained, steric constraint to the packing of these molecules in the aggregateswas determined to be the main reason for the differences in the micellization and hemimicellization of the surfactants.
Introduction Adsorption of surfactants plays an important role in processes such as flotation, detergency, enhanced oil recovery, paint formulation, lubrication, and microe1ectronics.lb Also, surfactant assemblies such as micelles and hemimicelles have potential applications in novel separation and reaction schemes such as magnetic isotope separation and polymer synthesis.6J An understanding of surfactant adsorption at the solid/liquid interface and micellization is important for improving the efficiency of the above processes. Apart from measuring adsorption isotherms, techniques such as calorimetry, electrophoesis, and infrared spectroscopy have been used in the past to understand the mechanisms of adsorption.s? An important aspect of adsorption studies that is lacking is in situ characterization of the microstructure of the adsorbedlayers, even though it is recognized that the microstructure controls the interfacial properties of the solid. Experimental techniques such as fluorescence, electron spin resonance, resonance Raman, and neutron scattering, which have been used to characterize micelles in situ, have recently been extended to the study of adsorbed layers.1O-l2 These
techniques provide information on the structure of the adsorbed layers in terms of the aggregate size,the aggregate viscosity, and the orientation of the molecules and, in combination with adsorption isotherms, help in understanding the evolution of the adsorbed layer. In this work, the effect of the position of the sulfonate and the methyl groups on the aromatic ring of three alkylxylenesulfonates on adsorption a t the alumin/water interface and on micellization has been examined using fluorescence spectroscopy. Earlier work using microcalorimetry and zeta potential measurements on the same system showed steric constraints to the packing of the molecules in micelles and hemimicelles to be the main mechanisms for the differences in the adsorption of the three surfactants. Fluorescence spectroscopy has been used to investigate such steric constrints and to gain more insight into the nature of the adsorbed layers.
Experimental Section Materials. Surfactants. 5-(4-Undecyl)-2,4-xylenesulfonate (Meta), 4-(4-undecyl)-3,5-xylenesulfonate(Paral), and 444undecyl)-2,5xylenesulfonate (Para21 obtained from ARC0 Oil and Gas Co. were used in this work. All the surfactants were specified to be at least 97% isomerically pure and were wed aa received. The structures of these surfactants are shown below:
* To whom all correspondence should be addressed.
Currently at Nalco Chemical Company, Naperville, IL 60563.
e Abstract published in
Aduance ACS Abstracts, December 15, 1993. (1)Somasundaran, P., Moudgil, B. M. Eda. Reagents in Mineral Technology; M. Dekker: New York, 1987. (2) Shilliig, G.L.; Bright, G. S. Lubrication 1977,63, 13. (3)Hough, D. B.; Rendall, H. M. In Adsorption from Solution at the Solid-Liquid Interface; Parfitt, G.D., Rochester, C. H., Eds.; Academic Press: New York, 1983. (4) Schwugr,M. J. In Anionic Surfactants; Surfactant Science Series; Lucaseen-Reynders,E. H., Ed.; Marcel Dekker: New York, 1981;Vol. 11.
13-
-Qso,-
w3-
Mela
Paral
(5) Swalen,J.D.;Allara,D.L.;Andrade,J.D.;Chandross,E.A.;Garoff, S.;Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu,H. Langmuir 1987,3,932. (6) Wrightan, M. S., Ed. Interfacial Process: Energy Conversion and
soa-
Synthesis; Advances in Chemistry Series; American Chemical Society: Washington, DC, 1980,p 184.
(7)Turro,N. J.; Kraeutler,B. In Isotopes in OrganicChemistry; Buncel, E., Lee, C. C., E&.; Elsevier: Amsterdam, 1984;Vol. 6. (8)Woodbury, G.W.; Noll, L. A. Colloids Surf. 1988,33, 301. (9) Partyka, S.;Rudzinski, W.; Brun, B.; Clint, J. H. Langmurr 1989, 5,297. (IO)Chandar, P.; Somasundaran,P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31. (11)Waterman, K. C.; Turro, N. J.; Chandar, P.; Somasundaran, P. J. Phys. Chem. 1986,90,6830. (12)Somasundaran,,P.; Kunjappu, J. T.; Kumar, C. V.; Turro, N. J.; Barton, J. K. Langmurr 1989,5, 215.
Para2
Experimental Conditions. All the experimentswere carried out at 43 "C and at a constant ionic strength of 0.03 kmol/ms NaC1.
Methods. Surface Tensiometry. Surface tension was measured using a water jacketed du Nuoy ring tensiometerset to the test temperature. Adsorption. A gram of alumina was conditioned in 5 cm3of 0.03kmol/m3NaCl solution for 1h at 43 "C in a glass vial. Then
0743-7463/94/2410-0131$04.50/00 1994 American Chemical Society
Siuakumar and Somasundaran
132 Langmuir, Vol. 10, No. 1, 1994 45.0
E
1 I
\
l
I d x
E
IW.0
0
1 o Meto
Para2
1
z
0 Y
c
a 25.0' 0.50
'
'
"
1
'
1.00
l00.W 200.00
10.00
SURFACTANT CONC.
. kmo I /m3x
IK
0 KJ
0
0.1 0.5
"' 1.0
'
(r
,
'
'
10.0
"
"
"
'
L
'
103.0
RESIDUAL SULFONATE CONC..
I O4
Figure 1. Surface tension of alkylxylenesulfonate solutions.
'
' " ' ' i
1000.0
Kmo I /m3x
I O5
Figure 2. Adsorption of alkylxylenesulfonates on alumina. I .3
5 cm3 of the surfactant solution at the desired concentration was
added to the slurry and the mixture conditioned for 24 h at the set temperature. After conditioning,solid-liquid separation was achieved using centrifugation. A Beckman DU-8UV-Vis spectrophotometer was used to analyze the surfactant at low concentrations and two phase titration'* was used to analyze high surfactant concentrations. The absorbance was recorded at a wavelength of 254 nm. The equilibrium pH of adsorption was 8.2. Steady-StateFluorescence. Steady-stateemission spectra for micropolarity measurementa were obtained using a PTI LS-100 fluorescence spectrophotometer. The slurry from the adsorption samples was transferred to a 2 mm path length quartz cuvette and the solutions to a 10 mm path length cuvette and the spectra were recorded. The excitation wavelength used for pyrene was 335 nm. The ratio of surfactant to pyrene concentration in the stock solution used for adsorption atudies was 2000. FluorescenceDecay. The decay measurementawere also made using the PTI LS-100nanosecond lifetime system which consists of a thyratron gated lamp source filled with a gas mixture of nitrogen (70%)and helium (30%)and excitation and emission monochromator with mirror on the opposite side for efficient zero-order illumination. The samples were excited at 335 nm and the emissionwas recorded at 385 and 480 nm for the monomer and excimer, respectively. The fluorescence decay behavior of pyrene under excimer forming conditions was analyzed using the intramicellar kinetics model" to determinethe lifetime, rate of excimer formation, and the aggregation number of the micelles and hemimicelles of the alkylxyleneeulfonates. The equation that describes the decay behavior of the monomer is
1.1
t
0.4'
0.7
o Parol o Para2
'
'
' 1.0
l
, o-ua
I
10.0
SURFFlCTANT CONC.
. kmo I /m3x
30.0
1 O4
Figure 3. Polarity of alkylxylenesulfonate solutions in terms of 13/11.
Results and Discussion Surface Tension and Adsorption. Surface tensions of the three surfactant solutions and their adsorption isotherms are shown in Figures 1and 2,respectively. The p-xylenesulfonates are more surface active and adsorb to
a greater extent than the m-xylenesulfonate. By use of a combination of microcalorimetry, electrophoresis, and high-performance liquid chromatography (HPLC),the lower surface activity and adsorption of the m-xylenesulfonate were explained in terms of its lower hydrophobicity and higher steric constraints to the packing of its molecules in micelles and hemimicelle~.'~The results of the fluorescent probe characterization of the micelles and the hemimicelles of the three surfactants are reported below. Micropolarity Studies. Micelles. The micropolarity of the surfactant solutions was measured in terms of the ratio of the intensities of the third to first peak, 13/11.The 13/11values for the three surfactant solutions as a function of concentration are shown in Figure 3. At low surfactant concentrations, the value of 13/11is 0.55, which is the same as that observed in water. With the onset of cmc, the value increases slowly and reaches a value of 1.1€or the p-xylenesulfonates and 0.95 for the m-xylenesulfonate. The slow increase in 13/11 is in contrast to the abrupt increase above cmc observed for dodecyl sulfate.lO The slow increase can be attributed to the fact that due to their bulky nature, these surfactant aggregates evolve slowly, i.e., the aggregate size increases slowly with the increase in concentration and finally becomes constant above a certain concentration. The plateau 13/11value of 1 is in between that for water (-0.6) and for organic solvents such as hexane (- 1.6). This is attributed to the
(13) Li, 2.;h e n , M. J. A d . Chem. 1981,53, 516. (14) Atik, S.;Nam, A.; Singer, L. Chem. Phys. Lett. 1979,67, 75.
(15) Sivakumar, A. D.E.S. Thesis, Columbia University, New York, 1991.
Z,(O) = I,(t) exp[-k,t + a(exp(-k,t - 1))l (1) where ko is the reciprocal lifetime of the monomer in the excited state, k. is the rate of intramicellar excimer formation, and is the average Poisson occupancy number of the probe in the aggregates. The aggregation number N was calculated from n using the following equations:
[PlN/[C - cmcl for micelles (2) a = [P]N/[C - C,] for adsorbed layer (3) where [PI is the pyrene concentration, C is the total surfactant concentration, cmc is the critical micelle concentration, and C, is the residual surfactant concentration. il =
Adsorption of Alkylxylenesulfonates on Alumina 1.1
Langmuir, Vol. 10, No. 1, 1994 133
I I
o Parol
I,o.
O.'-
A
Meta
D
par02
as 04
a4 a7
-
0.8
2
0.7
u
-
aa
Y
a4
0.6 -
t
as
I
aa 0
la
0.5
I
m
9D
TIME, mec 1.0
0.5
10.0
RESIDUAL SULFONRTE CONC..
40.0
kmol/m 3 x IO5
Figure 4. Polarity of adsorbed layers of alkylxylenesulfonates in terms of Ia/Zlas a function of equilibrium surfactant concentration.
Figure 6. Pyrene monomer mdexcimer decay profiles in micellar solutions of alkylxylenesulfonates, surfactant concentration = 0.01 M: (A) monomer emission for surf/py = 2089; (B) monomer emission or surf/py = 108; (C)excimer emission for surf/py = A
108.
Meta
Table 1. Kinetic Analysis of Pyrene Decay in Micelles of Alkylxylenesulfonate surfactant surf/Py ko(ne-1) k. (ne-') N
'
0.4 0.I
Para1 Para2
Meta I .o
J 10.0
AOSORPTION DENSITY. MoI/cm2x
80.0
10l2
Figure 5. Polarity of adsorbed layers of alkylxylenesulfonates in terms of 13/11as a function of surfactant adsorption density.
possible location of the probe in the palisade layer of the micelles which contains some penetrated water. The difference in the 13/11 plateau values for the p-xylenesulfonates and m-xylenesulfonate is proposed to be due to the different degree of water penetration. The lesser water penetration in the p-xylenesulfonate micelles indicates a tighter packing of the moelcules in this case. Adsorbed Layers. The 13/11 is plotted in Figure 4 as a function of the equilibrium surfactant concentration of the three surfactants. At low adsorption densities, prior to the onset of hemimicellization, the 13/11is -0.6. Once the hemimicelles form, the value increases to 1,indicating a behavior similar to that of micellization. In order to compare the polarities of the adsorbed layers, &/I1 is plotted as a function of the adsorption density in Figure 5. As seen from the figure, there is no difference in the polarities of the adsorbed layers unlike in the case of micelles. This lack of difference could be due to the fact that the adsorbed layers are more compact than the micelles and hence the water penetration is the same for all the surfactants. Fluoresence Decay and Excimer Formation. Micelles. Typical monomer and excimer decay profiles obtained for one of the surfactants are shown in Figure 6. At low pyrene concentrations, there is no excimer formation and the monomer decay (curve A) is single exponential. At high pyrene concentrations there is excimer formation and as per the intramicellar kinetic
105 110 108
0.0056 0.0054 0.0057
0.015 0.016 0.018
0.33 0.32 0.24
35 35 26
model, the monomer decay (curve B)is multiexponential. The excimer decay profile (curve C) shows the typical growth and decay behavior. The aggregation number for the three surfactants was calculated by fitting the decay profiles and the results obtained are tabulated in Table 1. The aggregation numbers of the micelles of p-xylenesulfonate are similar and higher than that of the m-xylenesulfonate micelles. The values obtained are comparable to those obtained for other alkylbenzeneeulfonates in literature.l6J7 The lower aggregation number of the m-xylenesulfonate micelles suggests that fewer molecules are able to pack into the micelles which in turn reflects the higher steric constraints to such packing. ko, which is the reciprocal lifetime of the monomer in the excited state, is the same for all the surfactants suggesting a similar environment around the three surfactants. The excimer formation rate constant, ke, is also similar for the three surfactants. The Ke depends on the size, shape, and the rigidity of the aggregates and evidently the difference in these parameters is not large enough to produce measurable changes in ke. Adsorbed Layers. Monomer and excimer decay profiies of pyrene in the adsorbed layers were obtained at different adsorption densities. The decay profiles below an adsorption density of 2.5 X 10-l' mol/cm2 could not be obtained due to the low pyrene concentration in the adsorbed layer. Typical monomer and excimer decay profiles for one of the alkylxylenesulfonates are shown in Figure 7. These profiles are similar t o those obtained for the micelles, indicating the presence of fragmented ag(16) Cheng, D. C. H.; Gulari, E. J. Colloid Interface Sci. 1982,90,410. (17) Sinton, S. W.; Huff,S . L.J. Colloid Interface Sci. 1987,120,358.
134 Langmuir, Vol. 10, No. 1, 1994
Sivakumar and Somasundaran
I
OB
aa a7
f
ao aa
o Heta
a4
o Para2
a3
ai 1
ai 0
im
SO
1
1 s Y
TIME, nsec
0 0
U
O.IO'.'..' 0.50
1.00
'
'
"
'
"
10.0)
'
'
"""'
'
100.00
RESIDUAL SULFONATE CONC..
'"....' 1000.00
kmal/m3x
IO5
Figure 8. Aggregation numbers determined at different adsorption densities shown along the isotherm.
Figure 7. Pyrene monomer and excimer decay profiles in adsorbed layers of alkylxylenesulfonates,adsorption density = 3.1 X 1611 mol/cm2: (A) monomer emission for surf/py = 545; (B)monomer emission or suf/py = 102;(C)excimer emission for surf/py = 102. Table 2. Kinetic Analysis of Pyrene Decay in pXylenesulfonate (Paral)/Alumina Adsorbed Layer ads. density (mol/cm2) surf/Py ko (ns-1) k. (ns-1) R N 3.3 x 10-11 105 0.0053 0.015 0.16 17 6.2 X 10-" 1.9 x 10-10 3.8 X 10-10
108 110 116
0.0058 0.0053 0.0054
0.015 0.009 0.006
0.32 34 0.5 55 0.64 74
Table 3. Kinetic Analysis of Pyrene Decay in pXylenesulfonate (Paraa)/Alumina Adsorbed Layer ads density (mol/cm*) surf/Py ko (ns-9 k. (ns-l) il N 3.1 X 6.7 X 10-" 3.1 X 10-lo 3.9 x 10-10
102 107 111 112
0.0056 0.0057 0.0054 0.0054
0.015 0.015 0.008 0.0062
0.2 0.31 0.58 0.7
20 33 64 78
Table 4. Kinetic Analysis of Pyrene Decay in m-Xylenesulfonate (Meta)/Alumina Adsorbed Layer N ads density (moVcm2) surf/Py ko (ns-l) k, (ns-1) 3.6 X 10-" 5.9 x 10-11 1.0x 10-10 3.8 X 10-l0
82 87 90 94
0.0058 0.0055 0.0053 0.0056
0.018 0.0092 0.0082 0.0075
0.2 17 0.4 35 0.55 49 0.66 62
gregates a t the solid/liquid interface. The results of the kinetic analysis of pyrene decay for the three surfactants are given in Tables 2-4 for selected adsorption densities. The error in the calculated value of the aggregation numbers was -25% at adsorption densities below 5 X 10-l' mol/cm2 and -10% above that. The ko values are the same as a function of adsorption density with the experimental error for the three surfactants. Also, similar, ko vlaues in the micelles and the adsorbed layers indicate a similar environment in both the aggregates. Average aggregation numbers of the three surfactant hemimicelles increase with increase in adsorption from 17 to 76 (Figure 8). The aggregation numbers of the two p-xylenesulfonates are similar throughout the range mea-
sured. However, at higher adsorption densities, the aggreation numbers of the m-xylenesulfonate are lower than that of the p-xylenesulfonates. This suggests higher steric hindrance to the packing of the surfactant moelcules in the hemimicelles of the m-xylenesulfonate. k, values for the three surfactants decrease gradually with an increase in the amount adsorbed. The value decreases from 0.015 (same as that of the micelles) to 0.006 ns-l a t the highest adsorption density studied. This decrease in k, is attributed to increase in the aggregate size or due to increased rigidity of the adsorbed layers. Summary Fluorescence spectroscopy was used to probe the microstructure of the micelles and adsorbed layers of alkylxylenesulfonates. The studies show that the microstructure of the hemimicelles of these surfactants is similar to that of these micelles. Micropolarity studies showed the polarity in the interior of the p-xylenesulfonate micelles to be lower than that of the m-xylenesulfonate micelles due to lesser water penetration in the case of the former. The reduced penetration suggests tighter packing of the molecules in the p-xylenesulfonate micelles. No difference was observed between the polarities of the adsorbed layers of the three surfactants. Fluorescence decay studies showed the aggregation number of the p-xylenesulfonate micelles (-35) was higher than that of the m-xylenesulfonate (-26). Also, a t higher adsorption densities the aggregation number of the p-xylenesulfonate hemimicelles (-76) was higher than that of the m-xylenesulfonate (-63). The lower aggregation number of the m-xylenesulfonate micelles and hemimicelles confirm the postulate of increased steric hindrance to the packing of the surfactant molecules in the case of m-xylenesulfonate micelles and hemimicelles. Based on the evidence provided here, the effect of change in the position of the functional groups on the aromatic ring of alkylxylenesulfonates on micellization and hemimicellization can be explained in terms of steric constraints to the packing of the surfactant molecules in their aggregates.
Acknowledgment. The authors acknowledge the National Science Foundation, Department of Energy, ARCO, and BP America for support of this work and S. Thach for supplying the surfactant samples.
