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Adsorption Studies of Nonionic Surfactants on Charcoal and Alumina in Aromatic Solvents
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
179, 517–521 (1996)
0244
Adsorption Studies of Nonionic Surfactants on Charcoal and Alumina in Aromatic Solvents J. SANTHANALAKSHMI 1
AND
S. BALAJI
Department of Physical Chemistry, University of Madras, AC College Campus, Madras 600 025, India Received February 2, 1995; accepted October 31, 1995
Adsorptions of Tween 80 and Span 80 (nonionic) surfactants from benzene, toluene, and xylene solutions onto activated charcoal (AC) and neutral alumina (AN) are studied at 307C. A Langmuir fit has been found. The molecular cross-sectional surface area of the adsorbed surfactant molecules under solvated and unsolvated conditions are determined in different solvents on AC and AN. The results are rationalized, invoking horizontal adsorptions and solvent–solvation differences. The low free energy changes of adsorptions found are indicative of physisorption. q 1996 Academic Press, Inc.
Key Words: nonionic surfactant adsorption; adsorption from organic solvents; surfactant adsorption on charcoal and alumina.
INTRODUCTION
Surfactant adsorption is fundamentally important to surface properties like adhesion and wetting (1, 2). The surface activity and interactions between the surfactant and adsorbent pairs always tend to lower the free energy of the system through adsorption (3). Adsorption characteristics of ionic surfactants are well characterized in the literature (4, 5). Nonionic surfactants solubilized in nonaqueous nonpolar solvents tend to adsorb well onto hydrophobic and charged surfaces. The interaction forces between the nonionic surfactants and nonpolar solvents are disperse in nature and adsorption onto a neutral or hydrophobic surface occurs with the hydrophilic segments of the surfactant adsorbed while the hydrophobic groups remain extended in the solution through solvation (6). Hence adsorption factors such as surface excesses under monolayer coverage, molecular cross-sectional surface area, and interfacial stacking parameter are all variable with the nature of the solvent, the temperature, etc. In the present work we report the adsorption properties of nonionic surfactants like Tween 80 (polyoxyethylene(20)sorbitan monooleate) and Span 80 (sorbitan monooleate) adsorbed from benzene, toluene, and xylene solutions onto activated charcoal (AC) and neutral alumina (AN) at 307C. Corrections for concomitant solvent adsorption and pore volume corrections have been included. Solvent effects on the 1
To whom correspondence should be addressed.
molecular cross-sectional surface areas of the surfactant molecules, the effective numbers of nearest solvent molecules of the adsorbate, and the free energy changes of adsorptions are studied. THEORY
Nonionic surfactants in nonpolar solvents possess very low critical micelle concentration (CMC) values. Therefore only dilute surfactant solutions are suitable for adsorption studies. Under excess solvent conditions and in the presence of an adsorbent on which preferential and concomitant solvent adsorption takes place there is every possibility that when a surfactant adsorbs onto the adsorbent, the solvation structure in the bulk and in the adsorbed phase do not vary much. It has been shown that when the adsorbent is homogeneous and adsorption takes place from very dilute solution in only one molecular layer, the Langmuir equation holds; i.e., 1 1 1 Å / , (x/m) K Ceq (xm /m) (xm /m) and a linear fit for 1/(x/m) vs 1/Ceq will exist. The molecular cross-sectional surface area of the solute ( s2s ) is obtained from ( Sm Ms /xm Nav ), where S is the total specific surface area of the adsorbent. Ms and m represent the solute molecular weight and weight of the absorbent. But the molecular cross-sectional surface area of the surfactant molecule obtained by applying the Langmuir equation was larger than the value derived from theoretical van der Waals calculations, making no corrections for solvent adsorptions. The probability of finding the adsorbed solvent molecules would be the nearest neighbor site to the adsorbed surfactant molecule leading to the formation of the ‘‘solvated solute’’ adsorbate. Thus the s2s value evaluated from the linearized Langmuir equation may be considered as s2ss , ‘‘ss’’ refering to solvated solute. The concomitant solvent adsorption corrections are considered as follows. At any instant solvent (1) and solute (2) will be adsorbed simultaneously with different fractional surface area cover-
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ages, each maintaining an equilibrium between its adsorbed and unadsorbed components (7): Kads Unadsorbed surfactant n 2e / Adsorbed solvent n s1 ` Adsorbed surfactant n s2 / Unadsorbed solvent n 1e Thus at any instant of solute concentration (n 2e ) the total specific surface area of the adsorbent ( S ) stands occupied ˚ 2 of the solvent and (n s2 Nav s2s ) A ˚ 2 of with (n s1 Nav s1s ) A s s the solute, respectively. n 1 and n 2 represent the number of moles of solvent and solute adsorbed per gram of the adsorbent, Nav is the Avagadro number, and s1s and s2s represent the molecular cross-sectional surface area of solvent and solute, respectively: S Å n s2 Nav s2s / n s1 Nav s1s .
[1]
˚ 2 for benzene, toluene, and Here s1s Å 30, 34, and 38 A xylene solvent molecules, respectively (8). By rearranging Eq. [1] for benzene solutions, one obtains S 30 n s1 n Å 0 . s2s Nav s2s s 2
[2]
This mass balance equation linearly relates the adsorbed solute (n s2 ) with the adsorbed solvent (n s1 ) at the solvent solute concomitant adsorption. If C0 and Ce are the concentrations (moles/liter) of the solute in solvent before and after adsorption then from (C0 0 Ce ) and m grams of adsorbent, the amount of solute adsorbed per gram of the adsorbent (x/m) may be known: n s2 Å (x/m)/Ms . From C0 and the actual amounts added, the total number of moles of solvent (n 01 ) and solute (n 02 ) per gram of the adsorbent before adsorption can be obtained. Thus the number of moles of solute in the bulk (n 2e ) can be calculated from the relation n 02 0 n s2 Å n 2e . Ce and n 2e are related as Ce Å (1000 n 2e r1 / n 1e M1 ), where r1 and M1 are the density and molecular weight of solvent. So, from experimental n 2e and Ce values, n 1e may be calculated and hence n s1 from the relation n 01 0 n 1e Å n s1 . A plot of n s2 vs n s1 results in intercept and slope values equal to S /Nav s2s and s1s / s2s , respectively. Thus in a simple manner s2s value may be evaluated. The s2s value evaluated using Eq. [2] refers to the crosssectional surface area of surfactant molecule after the solvent corrections, since exact n s1 and s1s values are considered in the above expressions. Therefore ( s2ss 0 s2s ) gives the difference in the cross-sectional surface areas between the solvated and the normal forms of adsorbate. Henceforth the n Å ( s2ss 0 s2s )/ s1s term approximately measures the ‘‘solvation number’’ (n) or the effective number of solvent molecules that solvate the adsorbed solute. n values differ with different solvents, solutes, and adsorbents. For the solute equilibrium, K2 Å n s2 /n 2e and DG2 Å 0RT ln K2 values may also be determined. Thus fundamentally speaking, the adsorbed sol-
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vent molecules associated with the adsorbed surfactant molecules may be taken as (nn s2 Nav ), and (n s1 0 nn s2 Nav ) furnishes the number of solvent molecules independently adsorbed. The associated and nonassociated adsorbed solvent molecules exist in equilibrium with the bulk solvent molecules n 1e Nav that happen to be close to the interface. EXPERIMENTAL
Materials Tween 80, Span 80, benzene, toluene, xylene (mixture of o, m, and p, as available), activated charcoal, and neutral alumina samples were from Fluka, Switzerland, A.G. as supplied by SISCO CHEM (I) Ltd. The solvents were refluxed in the presence of anhydrous sodium sulfate, freshly distilled, and collected over sodium metal wire before use. Before adsorption the adsorbents were preheated at 1007C in an air oven for 5 h with an N2 purge. Methods Since Tween 80 and Span 80 exhibit micellization in benzene, toluene, and xylene, in order to choose the concentration range of the surfactants in solution, CMC values are determined. For Tween 80 in benzene, toluene, and xylene the CMC values are 6.0 1 10 03 , 4.0 1 10 03 , and 10.5 1 10 03 M, and for Span 80 in benzene, toluene, and xylene the CMC values are 16.8 1 10 03 , 9.5 1 10 03 , and 31.2 1 10 03 M, respectively. The extent of adsorption when a known mass of the adsorbent was shaken with a solution of known concentration of the surfactant (below CMC) at a fixed temperature (307C) was determined from the concentration of the supernatant equilibrating solution. After adsorption, the mixture was centrifuged and the OD of the clear solution was determined at its lmax , from which the exact concentration is known. A Carl–Zeiss UV–visible recording-type double-beam difference spectrometer (made in Germany) was used for the spectral studies; OD at lmax was determined to be 277.8 and 284.0 nm for Tween 80 and Span 80 in benzene at 307C. For toluene and xylene, lmax shifted a few units to longer wavelengths. The total specific surface areas ( S ) of the adsorbents were determined from the BET adsorption isotherms of N2 by the usual procedure. S Å 816.940 and 38.395 m2 /g, respectively, for AC and AN were obtained after applying the mesopore corrections in the case of AC. In adsorption experiments corrections for pore volume filling by solvent alone are considered. Solvent concentration terms represent values after pore volume corrections. For aluminalike adsorbents no such corrections are needed. RESULTS AND DISCUSSION
Typical linear Langmuirian adsorption plots on AC and AN for Tween 80 and Span 80 at 307C are shown in Figs.
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0.67 0.77 0.88 — 0.46 0.63 0.78 — 0.3123 0.3073 0.3075 — 0.40 0.68 0.83 — 0.52 0.63 0.75 — 0.2817 0.2713 0.2566 — 2.49 1.24 0.83 —
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Note: n e1 , n e2 , and n s2 values are given in moles/gram of adsorbent.
1.00 1.73 2.25 — 0.3216 0.3217 0.3202 — 2.31 2.26 6.05 6.25 0.3029 0.2957 0.3150 0.3050 Xylene
0.85 1.11 1.21 1.31
0.34 0.30 0.91 0.58 0.33 0.13 0.49 0.27 0.3617 0.3575 0.3556 0.3516 0.23 0.80 0.41 0.19 0.35 0.50 0.30 0.29 0.3180 0.3260 0.3201 0.3474 1.55 1.56 1.60 1.14 0.04 0.60 1.16 2.19 0.3673 0.3675 0.3666 0.3667 1.70 4.20 4.86 5.63 0.2995 0.3188 0.3139 0.3374 Toluene
0.49 0.34 0.60 0.73
0.78 0.88 6.77 7.48 0.63 0.82 4.47 4.79 0.4485 0.4371 0.4249 0.4201 1.76 3.29 7.61 8.44 0.53 2.93 3.98 4.46 0.3874 0.3866 0.3725 0.4038 1.35 1.18 1.01 1.01 0.22 0.68 1.19 1.74 0.4477 0.4444 0.4441 0.4454 0.50 2.14 3.31 3.66 0.4076 0.3970 0.4113 0.4030 Benzene
1.36 1.43 1.61 1.99
n e2 1 102 n e1 n e2 1 102 n e2 1 103 n e1
Alumina
n s2 1 105
n e1
Charcoal
n s2 1 104
Span 80
n s2 1 105 n e2 1 103 n e1 Solvent
FIG. 2. Typical linear Langmuir adsorption isotherms of Span 80 on AC ( s ) and AN ( n ) from benzene at 307C.
Charcoal
1 and 2. s2ss values calculated from the intercept and slope values on the linear form of the Langmuir isotherm for Tween 80 and Span 80 and s2s , DG2 , and n values are given in Table 2. The experimental n 1e , n 2e , and n s2 values are tabulated in Table 1. FTIR spectra of toluene with and without Tween 80 on AC are shown in Fig. 3. Unlike in ionic surfactants no electrostatic interactions prevail in these systems. The surfactants being nonionic, a Langmuirian fit is seen. s2ss values are larger than s2s values, which indicate solvent influences on the solute in the adsorbed state. Utilizing the possibility of theoretically calculating the molecular cross-sectional surface areas from the atomic van der Waals radius values (9), one can compare the experimental s2s values.
Tween 80
FIG. 1. Typical linear Langmuir adsorption isotherms of Tween 80 on AC ( s ) and AN ( n ) from benzene at 307C.
TABLE 1 Experimental Data of Tween 80 and Span 80 Adsorptions onto AC and AN from Benzene, Toluene, and Xylene at 307C
Alumina
n s2 1 104
NONIONIC SURFACTANT ABSORPTION FROM ORGANIC SOLVENTS
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2.63 2.65 3.10 — — — 6.0 2.3 2.4 145.21 137.10 131.53
21.79 95.81 41.31 2.01 2.90 2.79 325.85 215.67 223.21 7.9 9.6 18.9
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˚ 2 and DG2 in kcal mol01. Note. s2ss and s2s are given in A
2.66 1.75 2.25 408.72 388.57 373.28 772.67 1088.48 1337.01 Benzene Toluene Xylene
12.1 20.6 25.4
DG2 s2s s2ss
Solvent
n
643.21 719.74 1088.98
405.95 392.16 372.55
2.67 2.94 2.82
DG2 s2ss DG2
n
Tween 80 [polyoxyethylene(20)sorbitan monooleate] is structurally written as
s2s
FIG. 3. FTIR spectra of AC–toluene solid samples in the presence (a) and absence (b) of Tween 80 in KBr pellets.
s2ss
Alumina Charcoal
/
144.30 137.21 131.85
s2s
n s2s
s2ss
Alumina Charcoal
Span 80 Tween 80
TABLE 2 Adsorption Characteristics of Tween 80 and Span 80 on AC and AN from Benzene, Toluene, and Xylene at 307C
n
DG2
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The polyoxyethylene chains are considered hydrophilic parts and exist in different conformational orientations. Staudinger’s model for the polyoxyethylene meander chain, the planar zig-zag (fully extended) chain, and Sauter’s meander (quasi) conformations (10) are given below:
The segmental cross-sectional areas are found to be ˚ 2 for the hydrophilic and hydrophobic 319.41 and 150.15 A parts, respectively. The cross-sectional surface area of the ˚ 2. molecule under unsolvated conditions will be 469.56 A Span 80 (Sorbitan monooleate) may be structurally represented as
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NONIONIC SURFACTANT ABSORPTION FROM ORGANIC SOLVENTS
˚ , from the Since the total length of the molecule is 54 A molecular volume, the segmental and total cross-sectional ˚ 2 (hydrophile), 112.75 A ˚2 areas are found to be 108.65 A 2 ˚ (hydrophobe), and 221.4 A , respectively:
These values are comparable with s2s in Table 2, within the allowable approximations in the molecular conformational constrictions. The values indicate a horizontal type of solute adsorption. In the presence of a charged surface, conformationally and electrostatically constrained molecular adsorptions are possible, which leads to lower s2s values (5). Lower DG2 values indicate a physisorption type of adsorption. By comparing the FTIR spectra of pure solvent adsorbed with and without solute (Tween 80) on AC as before shown in Fig. 3, the following may be noted. The major solvent peaks are reduced considerably due to solute adsorption. In the case of toluene the characteristic peaks are 2920, 2850, 1579, 1638, 1383, 1018 cm01 , etc. Tween 80 characteristic peaks in the adsorbed state are 1714, 1359, 1221, 1121, and 1018 cm01 and a cluster of medium peaks in the region 658–410 cm01 . The region of polyoxyethylene groups on AC is 3750–3414 cm01 . Even though the solvent peaks are reduced, their retention with weaker intensity along with Tween 80 peaks indicates the presence of solvent molecules being adsorbed along with the solute molecules. The FTIR spectra are recorded at the surface excesses of the solute concentrations. The effective number of nearest neighbor solvent molecules (n) determined and shown in Table 2 is found to be higher on AC than on AN surfaces. Commonly AC surface is favorable for aromatic solvent adsorptions. Because an AN surface is charged and nonporous, surfactant segmental adsorptions are possible due to the difference in the interaction of hydrophilic and the hydrophobic segments, while solvent adsorptions are improbable compared to Tween 80 and Span 80. Solvation values are lower for the AC surface, while the same are impossible to obtain on AN. Even though Tween 80 and Span 80 possess the same hydrophobic groups, the hydrophilic group is bulkier in Tween 80 than in Span 80 because of the polyoxyethylene chains. Therefore the circumference area of the horizontally adsorbed Tween 80 will be greater than that of Span 80; hence the packing of the number of nearest solvent neighbors will be greater for Tween 80 than for Span 80. Incidentally, in aqueous media, hydration of the functional group of the solute mole-
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cule in the adsorbed state on AC has been reported. This concept coincides with the present observation in nonaqueous media (11). For Tween 80, values of n increase for the solvents benzene, toluene, and xylene on AC and AN. The dipolar nature increases from benzene to toluene to xylene due to the substituent effect. Also, the ionization potential values are 9.24, 8.81, and 8.55 eV for benzene, toluene, and xylene, respectively. These indicate that xylene is associated in the solvation through easier electron polarizations more than the other two solvents. In the case of Span 80 a reverse effect, i.e., a decrease in n values on AC, is observed. Span 80 is less dipolar than Tween 80 due to the absence of polyoxyethylene units and hence has a lesser extent of interaction with more dipolar solvents. As n increases, a DG2 decreasing trend has been observed. Since DG2 refers exclusively to the surfactant adsorption when solvent associations (n) are low, DG2 will be high. CONCLUSIONS
Tween 80 and Span 80 adsorptions from benzene, toluene, and xylene onto AC and AN seem to follow a Langmuir fit. Low free energy change values for adsorptions are found and indicate physisorption type. The molecular cross-sectional surface area of the surfactant molecules in the horizontal and under solvated and unsolvated adsorbed conditions is determined. The values are compared with those calculated using the atomic van der Waals radius values. The approximate number of nearest solvent neighbors of the adsorbed surfactant molecules is found to be larger for Tween 80 than for Span 80. Evidence of concomitant solvent adsorption at the solute surface excesses is found from FTIR results. ACKNOWLEDGMENTS The authors thank UGC-INDIA for financial assistance and RSIC-IIT, Madras, for the FTIR results.
REFERENCES 1. Berg, J. C., Text. Sci. Technol. 7, 149 (1985). 2. Myers, D., in ‘‘Surfactant Science and Technology,’’ p. 303. VCH, New York, 1988. 3. Everett, D. H., in ‘‘Colloid Science,’’ Vol. 1. Specialist Periodical Reports, Chemical Society, London, 1973. 4. Esumi, K., Masuda, A., and Otsuka, H., Langmuir 9, 284 (1993). 5. Denoyel, R., and Rouquerol, J., J. Colloid Interface Sci. 143, 555 (1991). 6. Koopal, L. K., and Ralston, J., J. Colloid Interface Sci. 112, 362 (1986). 7. Adamson, A. W., in ‘‘Physical Chemistry of Surfaces,’’ p. 423. Wiley, New York, 1990. 8. Oscik, J., in ‘‘Adsorption,’’ p. 137. Ellis Horwood, Chichester, 1982. 9. Israelachvili, J. N., in ‘‘Intermolecular and Surface Forces with Applications to Colloidal and Biological Systems.’’ Academic Press, New York, 1985. 10. Schick, M. J., in ‘‘Nonionic Surfactants, Vol. 1, Surfactant Science Series,’’ Dekker, New York, 1967. 11. Abe, I., Hayashi, K., et al., Bull. Chem. Soc. Jpn. 52(7), 1899 (1979).
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0244
Adsorption Studies of Nonionic Surfactants on Charcoal and Alumina in Aromatic Solvents J. SANTHANALAKSHMI 1
AND
S. BALAJI
Department of Physical Chemistry, University of Madras, AC College Campus, Madras 600 025, India Received February 2, 1995; accepted October 31, 1995
Adsorptions of Tween 80 and Span 80 (nonionic) surfactants from benzene, toluene, and xylene solutions onto activated charcoal (AC) and neutral alumina (AN) are studied at 307C. A Langmuir fit has been found. The molecular cross-sectional surface area of the adsorbed surfactant molecules under solvated and unsolvated conditions are determined in different solvents on AC and AN. The results are rationalized, invoking horizontal adsorptions and solvent–solvation differences. The low free energy changes of adsorptions found are indicative of physisorption. q 1996 Academic Press, Inc.
Key Words: nonionic surfactant adsorption; adsorption from organic solvents; surfactant adsorption on charcoal and alumina.
INTRODUCTION
Surfactant adsorption is fundamentally important to surface properties like adhesion and wetting (1, 2). The surface activity and interactions between the surfactant and adsorbent pairs always tend to lower the free energy of the system through adsorption (3). Adsorption characteristics of ionic surfactants are well characterized in the literature (4, 5). Nonionic surfactants solubilized in nonaqueous nonpolar solvents tend to adsorb well onto hydrophobic and charged surfaces. The interaction forces between the nonionic surfactants and nonpolar solvents are disperse in nature and adsorption onto a neutral or hydrophobic surface occurs with the hydrophilic segments of the surfactant adsorbed while the hydrophobic groups remain extended in the solution through solvation (6). Hence adsorption factors such as surface excesses under monolayer coverage, molecular cross-sectional surface area, and interfacial stacking parameter are all variable with the nature of the solvent, the temperature, etc. In the present work we report the adsorption properties of nonionic surfactants like Tween 80 (polyoxyethylene(20)sorbitan monooleate) and Span 80 (sorbitan monooleate) adsorbed from benzene, toluene, and xylene solutions onto activated charcoal (AC) and neutral alumina (AN) at 307C. Corrections for concomitant solvent adsorption and pore volume corrections have been included. Solvent effects on the 1
To whom correspondence should be addressed.
molecular cross-sectional surface areas of the surfactant molecules, the effective numbers of nearest solvent molecules of the adsorbate, and the free energy changes of adsorptions are studied. THEORY
Nonionic surfactants in nonpolar solvents possess very low critical micelle concentration (CMC) values. Therefore only dilute surfactant solutions are suitable for adsorption studies. Under excess solvent conditions and in the presence of an adsorbent on which preferential and concomitant solvent adsorption takes place there is every possibility that when a surfactant adsorbs onto the adsorbent, the solvation structure in the bulk and in the adsorbed phase do not vary much. It has been shown that when the adsorbent is homogeneous and adsorption takes place from very dilute solution in only one molecular layer, the Langmuir equation holds; i.e., 1 1 1 Å / , (x/m) K Ceq (xm /m) (xm /m) and a linear fit for 1/(x/m) vs 1/Ceq will exist. The molecular cross-sectional surface area of the solute ( s2s ) is obtained from ( Sm Ms /xm Nav ), where S is the total specific surface area of the adsorbent. Ms and m represent the solute molecular weight and weight of the absorbent. But the molecular cross-sectional surface area of the surfactant molecule obtained by applying the Langmuir equation was larger than the value derived from theoretical van der Waals calculations, making no corrections for solvent adsorptions. The probability of finding the adsorbed solvent molecules would be the nearest neighbor site to the adsorbed surfactant molecule leading to the formation of the ‘‘solvated solute’’ adsorbate. Thus the s2s value evaluated from the linearized Langmuir equation may be considered as s2ss , ‘‘ss’’ refering to solvated solute. The concomitant solvent adsorption corrections are considered as follows. At any instant solvent (1) and solute (2) will be adsorbed simultaneously with different fractional surface area cover-
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ages, each maintaining an equilibrium between its adsorbed and unadsorbed components (7): Kads Unadsorbed surfactant n 2e / Adsorbed solvent n s1 ` Adsorbed surfactant n s2 / Unadsorbed solvent n 1e Thus at any instant of solute concentration (n 2e ) the total specific surface area of the adsorbent ( S ) stands occupied ˚ 2 of the solvent and (n s2 Nav s2s ) A ˚ 2 of with (n s1 Nav s1s ) A s s the solute, respectively. n 1 and n 2 represent the number of moles of solvent and solute adsorbed per gram of the adsorbent, Nav is the Avagadro number, and s1s and s2s represent the molecular cross-sectional surface area of solvent and solute, respectively: S Å n s2 Nav s2s / n s1 Nav s1s .
[1]
˚ 2 for benzene, toluene, and Here s1s Å 30, 34, and 38 A xylene solvent molecules, respectively (8). By rearranging Eq. [1] for benzene solutions, one obtains S 30 n s1 n Å 0 . s2s Nav s2s s 2
[2]
This mass balance equation linearly relates the adsorbed solute (n s2 ) with the adsorbed solvent (n s1 ) at the solvent solute concomitant adsorption. If C0 and Ce are the concentrations (moles/liter) of the solute in solvent before and after adsorption then from (C0 0 Ce ) and m grams of adsorbent, the amount of solute adsorbed per gram of the adsorbent (x/m) may be known: n s2 Å (x/m)/Ms . From C0 and the actual amounts added, the total number of moles of solvent (n 01 ) and solute (n 02 ) per gram of the adsorbent before adsorption can be obtained. Thus the number of moles of solute in the bulk (n 2e ) can be calculated from the relation n 02 0 n s2 Å n 2e . Ce and n 2e are related as Ce Å (1000 n 2e r1 / n 1e M1 ), where r1 and M1 are the density and molecular weight of solvent. So, from experimental n 2e and Ce values, n 1e may be calculated and hence n s1 from the relation n 01 0 n 1e Å n s1 . A plot of n s2 vs n s1 results in intercept and slope values equal to S /Nav s2s and s1s / s2s , respectively. Thus in a simple manner s2s value may be evaluated. The s2s value evaluated using Eq. [2] refers to the crosssectional surface area of surfactant molecule after the solvent corrections, since exact n s1 and s1s values are considered in the above expressions. Therefore ( s2ss 0 s2s ) gives the difference in the cross-sectional surface areas between the solvated and the normal forms of adsorbate. Henceforth the n Å ( s2ss 0 s2s )/ s1s term approximately measures the ‘‘solvation number’’ (n) or the effective number of solvent molecules that solvate the adsorbed solute. n values differ with different solvents, solutes, and adsorbents. For the solute equilibrium, K2 Å n s2 /n 2e and DG2 Å 0RT ln K2 values may also be determined. Thus fundamentally speaking, the adsorbed sol-
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vent molecules associated with the adsorbed surfactant molecules may be taken as (nn s2 Nav ), and (n s1 0 nn s2 Nav ) furnishes the number of solvent molecules independently adsorbed. The associated and nonassociated adsorbed solvent molecules exist in equilibrium with the bulk solvent molecules n 1e Nav that happen to be close to the interface. EXPERIMENTAL
Materials Tween 80, Span 80, benzene, toluene, xylene (mixture of o, m, and p, as available), activated charcoal, and neutral alumina samples were from Fluka, Switzerland, A.G. as supplied by SISCO CHEM (I) Ltd. The solvents were refluxed in the presence of anhydrous sodium sulfate, freshly distilled, and collected over sodium metal wire before use. Before adsorption the adsorbents were preheated at 1007C in an air oven for 5 h with an N2 purge. Methods Since Tween 80 and Span 80 exhibit micellization in benzene, toluene, and xylene, in order to choose the concentration range of the surfactants in solution, CMC values are determined. For Tween 80 in benzene, toluene, and xylene the CMC values are 6.0 1 10 03 , 4.0 1 10 03 , and 10.5 1 10 03 M, and for Span 80 in benzene, toluene, and xylene the CMC values are 16.8 1 10 03 , 9.5 1 10 03 , and 31.2 1 10 03 M, respectively. The extent of adsorption when a known mass of the adsorbent was shaken with a solution of known concentration of the surfactant (below CMC) at a fixed temperature (307C) was determined from the concentration of the supernatant equilibrating solution. After adsorption, the mixture was centrifuged and the OD of the clear solution was determined at its lmax , from which the exact concentration is known. A Carl–Zeiss UV–visible recording-type double-beam difference spectrometer (made in Germany) was used for the spectral studies; OD at lmax was determined to be 277.8 and 284.0 nm for Tween 80 and Span 80 in benzene at 307C. For toluene and xylene, lmax shifted a few units to longer wavelengths. The total specific surface areas ( S ) of the adsorbents were determined from the BET adsorption isotherms of N2 by the usual procedure. S Å 816.940 and 38.395 m2 /g, respectively, for AC and AN were obtained after applying the mesopore corrections in the case of AC. In adsorption experiments corrections for pore volume filling by solvent alone are considered. Solvent concentration terms represent values after pore volume corrections. For aluminalike adsorbents no such corrections are needed. RESULTS AND DISCUSSION
Typical linear Langmuirian adsorption plots on AC and AN for Tween 80 and Span 80 at 307C are shown in Figs.
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0.67 0.77 0.88 — 0.46 0.63 0.78 — 0.3123 0.3073 0.3075 — 0.40 0.68 0.83 — 0.52 0.63 0.75 — 0.2817 0.2713 0.2566 — 2.49 1.24 0.83 —
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Note: n e1 , n e2 , and n s2 values are given in moles/gram of adsorbent.
1.00 1.73 2.25 — 0.3216 0.3217 0.3202 — 2.31 2.26 6.05 6.25 0.3029 0.2957 0.3150 0.3050 Xylene
0.85 1.11 1.21 1.31
0.34 0.30 0.91 0.58 0.33 0.13 0.49 0.27 0.3617 0.3575 0.3556 0.3516 0.23 0.80 0.41 0.19 0.35 0.50 0.30 0.29 0.3180 0.3260 0.3201 0.3474 1.55 1.56 1.60 1.14 0.04 0.60 1.16 2.19 0.3673 0.3675 0.3666 0.3667 1.70 4.20 4.86 5.63 0.2995 0.3188 0.3139 0.3374 Toluene
0.49 0.34 0.60 0.73
0.78 0.88 6.77 7.48 0.63 0.82 4.47 4.79 0.4485 0.4371 0.4249 0.4201 1.76 3.29 7.61 8.44 0.53 2.93 3.98 4.46 0.3874 0.3866 0.3725 0.4038 1.35 1.18 1.01 1.01 0.22 0.68 1.19 1.74 0.4477 0.4444 0.4441 0.4454 0.50 2.14 3.31 3.66 0.4076 0.3970 0.4113 0.4030 Benzene
1.36 1.43 1.61 1.99
n e2 1 102 n e1 n e2 1 102 n e2 1 103 n e1
Alumina
n s2 1 105
n e1
Charcoal
n s2 1 104
Span 80
n s2 1 105 n e2 1 103 n e1 Solvent
FIG. 2. Typical linear Langmuir adsorption isotherms of Span 80 on AC ( s ) and AN ( n ) from benzene at 307C.
Charcoal
1 and 2. s2ss values calculated from the intercept and slope values on the linear form of the Langmuir isotherm for Tween 80 and Span 80 and s2s , DG2 , and n values are given in Table 2. The experimental n 1e , n 2e , and n s2 values are tabulated in Table 1. FTIR spectra of toluene with and without Tween 80 on AC are shown in Fig. 3. Unlike in ionic surfactants no electrostatic interactions prevail in these systems. The surfactants being nonionic, a Langmuirian fit is seen. s2ss values are larger than s2s values, which indicate solvent influences on the solute in the adsorbed state. Utilizing the possibility of theoretically calculating the molecular cross-sectional surface areas from the atomic van der Waals radius values (9), one can compare the experimental s2s values.
Tween 80
FIG. 1. Typical linear Langmuir adsorption isotherms of Tween 80 on AC ( s ) and AN ( n ) from benzene at 307C.
TABLE 1 Experimental Data of Tween 80 and Span 80 Adsorptions onto AC and AN from Benzene, Toluene, and Xylene at 307C
Alumina
n s2 1 104
NONIONIC SURFACTANT ABSORPTION FROM ORGANIC SOLVENTS
520
AID
JCIS 4117
2.63 2.65 3.10 — — — 6.0 2.3 2.4 145.21 137.10 131.53
21.79 95.81 41.31 2.01 2.90 2.79 325.85 215.67 223.21 7.9 9.6 18.9
6G0F$$$$43
˚ 2 and DG2 in kcal mol01. Note. s2ss and s2s are given in A
2.66 1.75 2.25 408.72 388.57 373.28 772.67 1088.48 1337.01 Benzene Toluene Xylene
12.1 20.6 25.4
DG2 s2s s2ss
Solvent
n
643.21 719.74 1088.98
405.95 392.16 372.55
2.67 2.94 2.82
DG2 s2ss DG2
n
Tween 80 [polyoxyethylene(20)sorbitan monooleate] is structurally written as
s2s
FIG. 3. FTIR spectra of AC–toluene solid samples in the presence (a) and absence (b) of Tween 80 in KBr pellets.
s2ss
Alumina Charcoal
/
144.30 137.21 131.85
s2s
n s2s
s2ss
Alumina Charcoal
Span 80 Tween 80
TABLE 2 Adsorption Characteristics of Tween 80 and Span 80 on AC and AN from Benzene, Toluene, and Xylene at 307C
n
DG2
SANTHANALAKSHMI AND BALAJI
04-18-96 10:43:27
The polyoxyethylene chains are considered hydrophilic parts and exist in different conformational orientations. Staudinger’s model for the polyoxyethylene meander chain, the planar zig-zag (fully extended) chain, and Sauter’s meander (quasi) conformations (10) are given below:
The segmental cross-sectional areas are found to be ˚ 2 for the hydrophilic and hydrophobic 319.41 and 150.15 A parts, respectively. The cross-sectional surface area of the ˚ 2. molecule under unsolvated conditions will be 469.56 A Span 80 (Sorbitan monooleate) may be structurally represented as
coidal
AP: Colloid
NONIONIC SURFACTANT ABSORPTION FROM ORGANIC SOLVENTS
˚ , from the Since the total length of the molecule is 54 A molecular volume, the segmental and total cross-sectional ˚ 2 (hydrophile), 112.75 A ˚2 areas are found to be 108.65 A 2 ˚ (hydrophobe), and 221.4 A , respectively:
These values are comparable with s2s in Table 2, within the allowable approximations in the molecular conformational constrictions. The values indicate a horizontal type of solute adsorption. In the presence of a charged surface, conformationally and electrostatically constrained molecular adsorptions are possible, which leads to lower s2s values (5). Lower DG2 values indicate a physisorption type of adsorption. By comparing the FTIR spectra of pure solvent adsorbed with and without solute (Tween 80) on AC as before shown in Fig. 3, the following may be noted. The major solvent peaks are reduced considerably due to solute adsorption. In the case of toluene the characteristic peaks are 2920, 2850, 1579, 1638, 1383, 1018 cm01 , etc. Tween 80 characteristic peaks in the adsorbed state are 1714, 1359, 1221, 1121, and 1018 cm01 and a cluster of medium peaks in the region 658–410 cm01 . The region of polyoxyethylene groups on AC is 3750–3414 cm01 . Even though the solvent peaks are reduced, their retention with weaker intensity along with Tween 80 peaks indicates the presence of solvent molecules being adsorbed along with the solute molecules. The FTIR spectra are recorded at the surface excesses of the solute concentrations. The effective number of nearest neighbor solvent molecules (n) determined and shown in Table 2 is found to be higher on AC than on AN surfaces. Commonly AC surface is favorable for aromatic solvent adsorptions. Because an AN surface is charged and nonporous, surfactant segmental adsorptions are possible due to the difference in the interaction of hydrophilic and the hydrophobic segments, while solvent adsorptions are improbable compared to Tween 80 and Span 80. Solvation values are lower for the AC surface, while the same are impossible to obtain on AN. Even though Tween 80 and Span 80 possess the same hydrophobic groups, the hydrophilic group is bulkier in Tween 80 than in Span 80 because of the polyoxyethylene chains. Therefore the circumference area of the horizontally adsorbed Tween 80 will be greater than that of Span 80; hence the packing of the number of nearest solvent neighbors will be greater for Tween 80 than for Span 80. Incidentally, in aqueous media, hydration of the functional group of the solute mole-
AID
JCIS 4117
/
6G0F$$$$43
04-18-96 10:43:27
521
cule in the adsorbed state on AC has been reported. This concept coincides with the present observation in nonaqueous media (11). For Tween 80, values of n increase for the solvents benzene, toluene, and xylene on AC and AN. The dipolar nature increases from benzene to toluene to xylene due to the substituent effect. Also, the ionization potential values are 9.24, 8.81, and 8.55 eV for benzene, toluene, and xylene, respectively. These indicate that xylene is associated in the solvation through easier electron polarizations more than the other two solvents. In the case of Span 80 a reverse effect, i.e., a decrease in n values on AC, is observed. Span 80 is less dipolar than Tween 80 due to the absence of polyoxyethylene units and hence has a lesser extent of interaction with more dipolar solvents. As n increases, a DG2 decreasing trend has been observed. Since DG2 refers exclusively to the surfactant adsorption when solvent associations (n) are low, DG2 will be high. CONCLUSIONS
Tween 80 and Span 80 adsorptions from benzene, toluene, and xylene onto AC and AN seem to follow a Langmuir fit. Low free energy change values for adsorptions are found and indicate physisorption type. The molecular cross-sectional surface area of the surfactant molecules in the horizontal and under solvated and unsolvated adsorbed conditions is determined. The values are compared with those calculated using the atomic van der Waals radius values. The approximate number of nearest solvent neighbors of the adsorbed surfactant molecules is found to be larger for Tween 80 than for Span 80. Evidence of concomitant solvent adsorption at the solute surface excesses is found from FTIR results. ACKNOWLEDGMENTS The authors thank UGC-INDIA for financial assistance and RSIC-IIT, Madras, for the FTIR results.
REFERENCES 1. Berg, J. C., Text. Sci. Technol. 7, 149 (1985). 2. Myers, D., in ‘‘Surfactant Science and Technology,’’ p. 303. VCH, New York, 1988. 3. Everett, D. H., in ‘‘Colloid Science,’’ Vol. 1. Specialist Periodical Reports, Chemical Society, London, 1973. 4. Esumi, K., Masuda, A., and Otsuka, H., Langmuir 9, 284 (1993). 5. Denoyel, R., and Rouquerol, J., J. Colloid Interface Sci. 143, 555 (1991). 6. Koopal, L. K., and Ralston, J., J. Colloid Interface Sci. 112, 362 (1986). 7. Adamson, A. W., in ‘‘Physical Chemistry of Surfaces,’’ p. 423. Wiley, New York, 1990. 8. Oscik, J., in ‘‘Adsorption,’’ p. 137. Ellis Horwood, Chichester, 1982. 9. Israelachvili, J. N., in ‘‘Intermolecular and Surface Forces with Applications to Colloidal and Biological Systems.’’ Academic Press, New York, 1985. 10. Schick, M. J., in ‘‘Nonionic Surfactants, Vol. 1, Surfactant Science Series,’’ Dekker, New York, 1967. 11. Abe, I., Hayashi, K., et al., Bull. Chem. Soc. Jpn. 52(7), 1899 (1979).
coidal
AP: Colloid