Langmuir 1992,8, 403-408
403
Investigations on the Hydrolysis of Sodium n-Alkyl Sulfates in Aluminum Oxide Suspensions Klaus Lunkenheimer,' Fritz Theil, and Karl-Heinz Lerche Central Institute of Organic Chemistry, Rudower Chaussee 5,0-1199Berlin-Adlershof, Germany Received November 1,1989.I n Final Form: September 30,1991 Sodium n-alkyl sulfates (n-decyl,n-dodecyl, n-tetradecyl) have been found to be degraded when they are contained in aqueous suspensions of acidic aluminum oxide. The decomposition of the alkyl sulfate molecules in aqueous suspensions was followed by the dynamic surface tension behavior. Decomposition performance depends on the concentration and on the surface activity of the alkyl sulfates as well as on the time of treating the solutions with acidic alumina. The parent 1-alkanolscould be detected in the corresponding alkyl sulfate suspensions by thin-layer chromatography. Acidic, surface chemically pure solutions of the alkyl sulfates (4 5 pH 5 7) did not reveal decompositional effects within a few hours provided that acidic alumina was excluded. The results are interpreted in terms of a surface-enhanced hydrolysisat the acidic alumina interface. Hydrolysis has not been observed in basic alumina suspensions.
Introduction n-Alkyl sulfates belong to a group of synthetic surfactants which can be considered as "classical". Since they were synthesized for the first time, they have been widely used as standard surfactants in industry as well as in basic research.'V2 However, attempts to understand the effect of n-alkyl sulfates adsorbed at various boundaries, fluid ones in particular, in terms of scrutinized molecular models resulted in disappointing statements. No consistent experimental adsorption isotherm for the most widely used sodium n-dodecyl sulfate was founds3 It was only when one of us (K.L.) succeeded in elucidating the reasons for this failure that reliable experiments on adsorption properties of alkyl sulfates could be expected. The reasons included, above all, the problem of the sufficient degree of purity of the surfactants used. As previously shown, this problem of "surfactant chemical purity" is of crucial importance for surfactant Recently we have succeeded in deriving criteria for determining the necessary degree of surfactant purity and in elaborating a sophisticated method to produce 'surface chemically pure" surfactant solution^.^-^ It has also been possible to get rid of misleading trace impurities by using solid adsorbents in surfactant solutions.1° When we tried to apply this technique to alkyl sulfate solutions, however, unexpected effects were observed initiating the present investigations. The surface tension of aqueous sodium n-alkyl sulfate solutions containing certain amounts of aluminum oxide has been studied with regard to dependence on adsorption time, surfactant concentration, and the kind of alumina. (1) Sutherland, K. L. Reu. Pure Appl. Chem. 1961,1,35. (2) Edwards, B. E. In Anionic Surfactants; Linfield, W. M., Ed.; Marcel Dekker: New York and Basel; 1976; Part I, p 111. (3) Mysels, K. J. J. Colloid Interface Sci. 1973, 43, 577. (4) Mysels, K. J.; Florence, A. T. In Techniques and Criteria in the Purification of Aqueous Surfaces; Goldfinger, E., Ed.; Marcel Dekker: New York, 1970. (5) Lunkenheimer, K.; Miller, R. Tenside Deterg. 1979, 16, 312. (6) Miller, R.; Lunkenheimer, K. Tenside Deterg. 1980, 17, 288. (7) Miller, R.; Lunkenheimer, K. Colloid Polym. Sci. 1986, 264, 273. (8) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987,120, 176. (9) Lunkenheimer, K.; Pergande, H.-J.; Kriiger, H. Reu. Sci. Instrum. 1987.58.2313. (10) Lunkenheimer, K.; Miller, R.; Kretzschmar, G.; Lerche, K. H.; Becht, J. Colloid Polym. Sci. 1984,262, 662.
0743-746319212408-0403$03.00/ 0
Experimental Section Methods. Surface tension was determined using the ring ten-
siometer method and observing the necessary precautions.1l Correction factors of Huh and Mason were employed.'* The measuring accuracy was hO.1 mN/m. For the investigation of the dynamic surfacetension behavior in the cases of adsorption and desorption, a Langmuir trough 22 X 10 x 1 cm made of Teflon was used. Before the surfacetensionadsorption time measurement was started, the solution surface was cleaned by means of a glass capillary. When the surface tension had reached a constant value ('apparent adsorptionequilibrium value"), ueA,the adsorption layer was compressed to half of ita original area and the subsequentincrease in surface tension was measured-in dependence on time until a constant surface tensionvalue, ueD('apparent desorption equilibrium value"),was observed again. This refers to the case of desorption. Substances. The sodium n-alkylsulfateswere obtained from 1-alkanols by sulfation with chlorosulfonic acid by standard laboratory methods. The equilibrium surface tension-concentration isothermsof aqueoussodium n-decyland n-dodecylsulfate solutions revealed slight minima when using the substances as received. Acidic and basic aluminaoxides were suppliedby Chemiewerk Greiz-DBhlau and were used as received. The sodium n-alkyl sulfate solutions used for investigating the effect of hydrolysis were purified by means of an automaticallyoperating apparatus described in ref 9. The state of surface-chemicalpurity of these solutions was checked by applying the criteria derived in refs 7 and 8. During these investigations sodium dodecyl sulfate samples of three different producers have been used. As these products differed in their contaminations, the procedures of preparing the state of surface chemical purity required different methods and intensities. Thus, the purification may be accompanied by a more or less pronounced loss of the main component. In addition, we did not check the content of furthersurface-inactive components in the original substances as, for example, water. Hence, the equilibrium surface tension value oe of the various 1X 10-3 M SDDSsolutionsvaries between 63.0 and 65.1 mN/m.lS The problem of absolute values of surface tension of sodium n-alkyl sulfates, however, is not important in the framework of this investigation. It is to be discussed elsewhere." The experimental pH in Figures 1-6 of the sodium n-alkyl sulfate (11) Lunkenheimer, K.; Wantke, K.-D.. Colloid Polym. Sci. 1981,259, 354. (12) Huh, C.; Mason, S. G. Colloid Polym. Sci. 1975, 253, 566. (13) Mysels discussed absolute equilibrium surface tension values of solutions of pure SDDS: Mysels, K. J. Langmuir 1986,2, 423. The u. value of the 1 mM SDDS solution at 25 O C , interpolated from Figure 5 of ref 13 is close to our highest value at 22 O C .
0 1992 American Chemical Society
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404 Langmuir, Vol. 8, No. 2, 1992
solutions was practically neutral in the absence of any alumina (cf. Table I). Addition of alumina changed the solution pH somewhat depending on the kind and amount of alumina; i.e. the pH was not held constant. Thus the pH of the suspensions containing basic alumina of Figure 2 varied between 8.10 and 8.15. Adding acidic alumina led to lower pH values (cf. Table I) but only to weakly acidic solutions. The minimum pH observed with the highest acidic alumina concentrations was 4.90. Points of zero charge (pzc) were obtained in separate experiments from potentiometric acid-base titrations of alumina suspensions at different sodium chloride concentrations N, 10-l N, 1 N) using the technique as described in ref 15. The pzc of the basic alumina was 7.5 f 0.1. The pzc of the acidic alumina was found to be 4.7 f 0.2. Analytical Detection of 1-Alkanols. Fifty milliliters of the corresponding solution of the alkyl sulfate was treated with 10 mL of a saturated solution of potassium chloride. This mixture was extracted with 15 mL of a saturated solution of bidistilled ethyl acetate. The organic extract was dried over magnesium sulfate, filtered, and evaporated to dryness. The residue was dissolved in 0.1 mL of bidistilled ethyl acetate and investigated by thin-layer chromatography (TLC). TLC was carried out on HPTLC plates (E. Merck, Darmstadt, No. 5641) using hexane diethyl ether (l:l,2 runs) (system 1)and hesane-acetone (91, 2 runs) (system 2) as mobilephases. For visualization, the plates were treated with a solution of 3.5% molybdatophosphoricacid in ethanol and heated to 120 "C. 1-Alkanols were identified by comparison with authentic samples of the alkanols. It could be shown in a semiquantitative study that 1-decanol amounts as small as 50 ng could be detected. The Rfvalues of the alkanols in systems 1and 2 are 0.50 and 0.31, respectively. The three 1-alkanols showed equal Rfvalues.
Results and Discussion Dynamic Surface Tension Behavior. The surface tension of surfactant solutions responds to surface-active trace impurities with extreme sensitivity. Hence it is possible to follow the effect of trace impurities by measuring the surface tension of surfactant solutions and to evaluate it in order to judge the degree of surface chemical p ~ r i t y . ~ , ~ Generally, surface-active trace impurities have two serious consequences on the adsorption behavior of a surfactant solution. Firstly, the establishment of the adsorption equilibrium is significantly retarded, usually by orders of magnitude.6 Secondly, the absolute equilibrium surface tension value of a contaminated surfactant solution is smaller than the value of the corresponding surface chemically pure solution. This is illustrated in Figure 1,where surface tension of an aqueous 1 X 10-3 M sodium n-dodecyl sulfate solution is shown as a function of sorption times both for the solution prepared from the original substance and for the corresponding surface chemically pure solution. According to the theory of diffusion-controlled sorption kinetics, the adsorption equilibrium should be established within a few millisecond^.^ If the adsorbed monolayer is compressed after having reach& a constant (apparent equilibrium)surface tension value neA,the surface tension value will then be lower and will increase as a function of time until again 5 constant (apparent equilibrium) surface tension value ueDwill be reached. As one can see from Figure 1,the difference A i , = ;eA - neD does not vanish within those time spans resulting from the theory of diffusion-controlled sorption kinetics. (14) Lunkenheimer, K.; Czichocki, G.; Hirte,R. To be submitted for publication. (15) Tadros, Th. F.; Lyklema, J. J. Electroanol. Interfacial Electrochem. 1968,17, 267.
adsorption
59
desorption
57 51
56
tr
IO
I /
q 30
20
: 20
LOmnSO
10
:
:
30
:
!
LOmm5O
Figure 1. Surface tension of an aqueous 1 X M solution of sodium dodecyl sulfate as a function of the lapse of time in the case of adsorption and desorption. The adsorption layer was compressed to 50% after haying reached the apparent equilibrium surface tension value ueA. 1,
-
M
No-n - dodecylsulfote lalkoline A1,U31
I / \ i
I 2
3
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-
5
6m%
Figure 2. Apparent adsorptionequilibriumsurfacetension value ueAof an aqueous 1 X M sodium dodecyl sulfate solution in dependence of the concentration of solid adsorbent (alkaline aluminum oxide) added. This is a phenomenon characteristic of contaminated surfactant solutions. A i e vanishes only in the case of the satisfactorily pure solution. To obtain surface chemically pure surfactant solutions, particular measures are req~ired.~JO As we have shown, this can be done by employing solid adsorbenta.1° Following the criteria of surface chemical purity, the state of sufficient purity can be obtained when the adsorbent is added successively to the solution and the purity is monitored by simultaneously measuring the corresponding surface tension. The state of surface chemical purity is reached when the surface tension value neAremains constant. The findings of such an operation are given in Figure 2 with alkaline aluminum oxide used as adsorbent. Decompositionof Sodium n-Alkyl Sulfates. When we applied acidic aluminum oxide instead of an alkaline one, we obtained an unexpected result (Figure 3). We treated a 1 X M sodium dodecyl sulfate (SDDS) solution obtained from the original product with 1.2 mg of acidic aluminum/mL of solution. After the suspension was shaken for 1 h, the resulting surface tension was measur ed. In contrast to what is characteristic of a purification effect, it turned out that the resulting curve of the dynamic surface tension behavior was below the curve of the original solution. Repetition of this experiment with an SDDS sample of different origin confirmed the trend observed.
Hydrolysis of Sodium n-Alkyl Sulfates
Langmuir, Vol. 8, No. 2, 1992 405 I
adsorption
desorption
--- --I
--.-.
r-=--'
-,-e -
I I I
d
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U:w
after rreotment with 12
Figure 5. Dynamic surface tension behavior of a 1 10-2 M 10
10
20
tA
30 mrn LO
20
30 mm Lo/
to
X
10
20
sodium decyl sulfate solution in the case of adsorption and deoriginal , solution; 0,surfacechemicallypure solution; sorption: . +, surface chemically pure solution after treatment with 1.5mg/ mL of acidic alumina.
30
min
t-
Figure 3. Apparent adsorption equilibriumsurfacetension value aeAof aqueous 1 X M sodium dodecyl sulfate solutions with dependence on time ( 0 ,product I; 0,product 11): open symbols,
original solutions; filled symbols, after treating the original solutions with 1.2 mg/mL of acidic aluminum oxide for 1h.
--______/__-
62
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surface c h e d / b pure solution
,
f
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-
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-
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Figure 6. Dynamic surface tension behavior of a 5 X 10"' M solution of sodium tetradecyl sulfate in the case of adsorption
-
Figure 4. Surface tension of aqueous 1X 10-3 M SDDSsolutions in dependence on the lapse of time in the case of adsorption
0 ) surface (open symbols) and desorption (filled symbols): (0, chemically pure solutions; (0,m) after treating the surface chemicallypure solution for 1h with 1.5mg/mL of acidicalumina.
In a blind test we applied the same concentration of acidic alumina to pure water and repeated this experiment. No indication of surface-active impurity was found. These drastic changes in surface tension suggest that they are due to a strong contamination of the surfactant solutions. From our experience with aqueous surface chemically pure SDDS solutions we known, however, that they are stable over days. Consequently, the said effects could only be assigned to some degradation of sodium dodecyl sulfate at the acidic aluminum surface. To obtain reliable evidence for this suggestion, we repeated this experiment again. This time we started with an SDDS solution prepurified by the procedure described in ref 9 and with its degree of purity judged by employing convenient criteria.'~~The result of this experiment is given in Figure 4. The dynamic surface tension behavior of the alumina-treated solution resembles that of a contaminated solution. Analogous experiments were carried out using sodium n-decyl and sodium n-tetradecyl sulfate solutions. The dynamic surface tension behavior of the solutions for three different states of purity was monitored here, i.e. (i) for the state of the original
and desorption: 0 , original solution; 0,surface chemically pure solution; X, surfacechemicallypure solution after treatment with 1.5 mg/mL of acidic alumina. (contaminated) solution, (ii) for the state of the purified (surface chemically pure) solutions, and (iii) for the state of the surface-chemicallypure solutions which were treated for 1 h with 1.5 mg of acidic aluminum oxidelml of solution. The results are given in Figures 5 and 6. The comparison of the behavior of the solutions in states i and iii shows that the correspondingcurvesof the dynamic surface tension are very much alike and reveal very similar characteristic effects of comparativelystrong surface-active impurities. The solutions, including a 1 X M SDDS solution, were analyzed to identify the corresponding 1alkanols which were thought to be responsible for the impurity effects. We succeded in detecting 1-tetradecanol and l-dodecan01 according to the procedure described in the experimental part. 1-Decanol, however, could not be detected in the extract by this method. These findings suggest that alkyl sulfate molecules are degraded via adsorption at the acidic alumina surface. Mysels and Princen observed the hydrolysis of SDDS on fritted glass after it had been acid washed.16 Muramatau and Inoue investigated the possibility of hydrolysis of sodium dodecyl sulfate in aqueous s01utions.l~ They observed hydrolysis, although a very slow one, at elevated temperatures (34.5 and 40 "C) in strong acidic medium (pH 1-4) only. (16) Mysels, K. J.; Princen, L. H. J. Phys. Chem. 1959, 63,1696. (17)Muramatsu, M.; Inoue, M. J.Colloid Interface Sci. 1976,55,80.
4%
Lunkenheimer et al.
Langmuir, Vol. 8, No. 2, 1992
Q
pH3.92
c
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0 10 20 nnn 10 20 30 10 20 3 0 m i n 70 80 90minl00
-f Figure 7. Surface tension of a 1 X M, surface chemically pure sodium dodecyl sulfate solution as a function of time at various pH of the solution. Table I ~
although the pzc was found to be slightly below the pH of the solutions. As the difference between the pzc and the pH is small, perhaps very small or even negligible, the net surface charge will be very small. Hence it follows that the repulsive effect of the aluminum oxide surface with like charge as the surfactant ion, if any, does not contribute significantly to the overall adsorption. As the surface charge would be very small and surface heterogeneity would exhibit a wide distribution of surface charge densities,25it is possible that there are fractions of the surface being capable of adsorbing alkyl sulfate anions by an ion exchange mechanism. (The adsorption of divalent Cu2+ions on a Ti02 surface with like charge was reported in ref 24.) Following this assumption the adsorption will take place at acidic surface groups with hydroxyl ions as counterions, according to
~~~
system
PH 6.94 6.89 5.02
>A10H,+-40SR + Na+ + OH+ Thus, while being adsorbed, the alkyl sulfate displaces 5.20 the previouslyadsorbed hydroxyls which are released into To exclude acid catalyzed alkyl sulfate hydrolysis in the bulk water phase from the inner Helmholtz layer.26 the bulk, we followed the dynamic surface tension behavior This, in its turn, will result in an increase of pH. of a surface chemically pure 1X M SDDS solution in It is well-known that acid-base pair sites at metal oxide acidic media a t fixed pH. The results of these experiments surfaces may be involved in heterolytic bond cleavage or are given in Figure 7. It appears that there is no detectable can a t least lead to very strong polarizations. There are change in the constant surface tension value during the some oxides, preferably mixed oxides such as silicatime spans relevant for this investigation. alumina, that posses an appreciable number of acidic On the Mechanism of Alkyl Sulfate Hydrolysis. protons. These provide the active sites for typical acidicAdsorption on solid adsorbents, alumina in particular, catalyzed rea~ti0ns.l~ As the alkyl sulfates represent esters must be thought of as occurring on energetically hetercontaining the chemically not very stable C-0-S bond, its ogeneous surfaces.18 Among the various adsorption sites acid-catalyzed cleavage will be achieved easier as the available there may be a large number permitting the polarization by the electrostatic attraction by adsorption adsorption of surface active molecules. Acidic and basic is stronger. The dependence of the C-0 bond distance sites may exist in many different configurations leading and/or C-0 charge polarization on the acidity of active to a distribution of their strength. Only very few sites in centers can also be derived from quantum chemical agiven distribution can be considered as catalyticallyactive calculations.26 sites.lg Additionally, evidence has been provided that As already mentioned above, the possibility of hydrolyalumina like most solid oxides contains different types of sis of sodium dodecyl sulfate in acidic aqueous solutions spectroscopically distinguishable surface OH groupswhich has been reported in ref 17. The hydrolysis rates observed are chemically inequivalent and have different reactiviin the acidic aluminum oxide suspensions exceed those in ties.20 homogeneous solutions by orders of magnitude. This The adsorption of anionic surfactants on alumina is usually brought about by electrostatic i n t e r a c t i ~ n . The ' ~ ~ ~ ~ follows from kinetic experiments with SDS solutions of different concentrations. adsorption of the charged species is controlled by the We started with surface chemically pure solutions and conditions of the solution with respect to the point of zero treated them with 10 mg of acidic alumina/mL in the charge (pzc). This means that adsorption of anionic surmanner described above, but with times for shaking the factants is favored at pH < pzc and that of cationic sursuspensions between 5 and 5000 min. We monitored the factants a t pH > pzc. When we measured the pH of a beginning of hydrolysis by surface tension measurements. surface chemically pure (scp) alkyl sulfate suspension Already after 5 min of shaking, a distinct effect of hycontaining acidic alumina and compared it with the pH drolysis could be established which increased continuously of the corresponding alumina suspension without alkyl with the time of contact. As we did not observe hydrolysulfate, we always observed a small but significant sis of the said solutions with excluded acidic alumina for difference between the pH of the two suspensions. The a period of several days (cf. also Figures 4 and 7 for SDDS), pH of the surfactant-containing suspension was slightly the rate enhancement of hydrolysis as catalyzed by the increased compared with that of the surfactant-free aluminum oxide surface becomes evident. suspension. (The pH of the scp alkyl sulfate solutions After we had finished this investigation,a paper by Stone was practically identical to that of bidistilled water.) The appeared where he reports on base-catalyzed hydrolysis findings of such experiment are compiled in Table I. of monophenyl terephthalate in aluminum oxide suspenThe increase of pH upon addition of SDDS supports an anionic exchange mechanism at the alumina ~ u r f a c e , ~ ~ ~ ~ ~ bidistilled water 1 X 10-3M SDDS (surface chemically pure) bidistilled water 10 mg/mL acidic A1203 1 X 10-3M SDDS (scp) + 10 mg/mL acidic A1203
(18) Derylo-Marczewska, A.; Jaroniec, M. In Surface and Colloid Science; Matijevic, E., Ed.;Plenum Press: New York and London, 1983; Vol. 14, pp 301. (19) Knozinger, H. In Aduances in Catalysis; Weisz, P. B., Ed.; Academic Press: New York, 1976; Vol. 25, p 184. (20) Van Veen, J. A. R. J. Colloid Interface Sci. 1988, 121, 214. (21) Goddard, E. D.; Somasundaran, P. Croat. Chem. Acta 1976,48, 451.
>AlOH,+OH- + RSOLNa'
F?
(22) Rosen, M. J. Surfactantsand InterfacialPhenomem; John Wiley
& Sons: New York, Chichester, Brisbane, Toronto, 1978; Chapter 2.
(23) Harwell,J. H.; Schechter, R.; Wade, W. H. Collect.Colloq. Semin. (Imt. Fr. Pet.) 1985, 42, 371. (24) Ottaviani,M. F.;Cereaa, E. M.; Visca, M. J. Colloid Interface Sci. 1985, 108, 114. (25) Partyka, S.;Rudzinski, W.; Brun, B.; Clint, J. H. Langmuir 1989, 5, 297. (26) Broclawik, E. Mater. Sci. Forum 1988,2546.
Langmuir, Vol. 8, No. 2, 1992 407
Hydrolysis of Sodium n-Alkyl Sulfates
1 ' lo-k SDDS
I
Codr
10
-a
12 & q 2 0 c Cads
'
20
Figure 8. Apparent equilibrium surface tension value aEAand compressioneffect ueA- (reD of an aqueous 1.2 X 10-2M suspension of sodium decyl sulfate as a function of the amount of acidic aluminum oxide added. ~ i o n s There . ~ ~ he was able to explain the alumina surfaceenhanced hydrolysis rate by a mechanism involving an attack of specifically adsorbed monophenyl terephthalate anions by hydroxide ions in the diffuse double layer. This may additionally support the understanding of the alkyl sulfate decompositiondiscussedin this paper in terms of an acid-catalyzed hydrolysis in the neutral or weakly acidic pH range. The production of 1-alkanols from the corresponding alkyl sulfates in aqueous suspensions of acidic alumina is also indicated by the investigation of the concentration dependence of the contaminationeffect. This is illustrated in Figure 8. There the difference beA - beDtogether with the apparent equilibrium surface tension value ;eA is plotted versus the concentration of acidic aluminum oxide contained in a 1.2 X 10-2M SDS solution. This experiment was carried out in the following manner. The said SDS solution was purified until the state of surface chemical purity was reached. Then a certain amount of solid adsorbent was added and the solution shaken for 10 min. Subsequently, the dynamic surface tension behavior was observed in the case of adsorption and desorption. Thereafter, additional amounts of adsorbent were added to the same solution and the same procedure and measurements were repeated successively. According to the findings presented in Figurs 8, there is an exponentially rising compression effect (ueA - ueD) with increasing concentration of the solid adsorben! Cads. However, it seems to conflict with the steep rise of ueAat high Cads values. According to the princbles of surface chemical purification, a positive slope of daeA/dcabmeans that part of the surface-active material involved in the adsorption process is removed from the adsorption competition between bulk and surface.*JOAs long as the solutes are chemically stable, this in turn would result in a decreasing compression effect with increasing Cads in contrast to what was observed in the experiment. However, this behavior can be explained by assuming two simultaneously occurring processes. One of them is the process of decomposition of decyl sulfate molecules at (27) Stone, A.
T.J . Colloid Interface Sei. 1989, 127, 429.
-
Figure 9. Apparett eqEilibrium surface tension value aeAand compression effect uEA- uEDof an aqueous 1 X 10-3M suspension of sodium dodecyl sulfate in dependence on the amount of acidic aluminum oxide added. the acidic alumina surface. The other refers to the adsorption of 1-decanol produced during decomposition a t the alumina surface. From the results given in Figure 8 it follows that 1decanol produced by catalytic decomposition a t the acidic alumina surface is released into the adjacent aqueous bulk phase. This is in line with the assumption that the decyl sulfate anion was adsorbed at the acidic surface group by an ion exchange mechanism. After the decyl sulfate anion concerned is decomposed at the acidic adsorption site, there is no reason for the resulting nonionic 1-decanol to remain adsorbed a t this surface group. Consequently it is released into the bulk water phase. Hence the effect of 1-decanol becomes apparent at the air/water interface by an increase in the compression effect. At higher concentrations of acidic alumina the loss of n-decyl sulfate by adsorption is already remarkable and leads to a decrease of the SDS bulk concentration which in turn results in an increase of the apparent equilibrium surface tension value of adsorption, The more SDS is adsorbed at the alumina surface, the greater is the part which is decomposed, i.e. the greater the amount of 1-decanolwhich is released into the aqueous phase. This corresponds with the increase of the compression effect observed at increasing alumina concentrations. If the produced 1-decanolhad not been desorbed into the aqueous phase, the compression effect would have remained negligible over the entire concentration interval. At higher 1-decanol concentrations, i.e. at higher alumina concentrations, however, the possibility for 1-decanol to be adsorbed at convenient surface sites at the alumina surface must be taken into account additionally. This results in a mechanism of competition between decomposition and adsorption. Following this mechanism of competition between decomposition and adsorption, we can conclude that the result of this process depends both on the solid adsorbent concentration and on the concentration and surface activity of the n-alkyl sulfates as indicated by the course of the corresponding surface tension values. Therefore, we additionally investigated sodium dodecyl sulfate solutions by applying the same procedure as for decyl sulfate. The results of these experiments are given in Figures 9 M SDDS solution, a decrease of and 10. For the 1X together with an increase of the compression effect has been found with rising Cads at lower adsorbent concentrations (Figure 9). The left branches of the corresponding two curves in Figure 9 (and Figure 10) could be read as a sort of "reversen purification characteristics.
aeA
408 Langmuir, Vol. 8, No. 2, 1992 3.10-4 SDDS
Lunkenheimer et al.
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Starting from the corresponding extreme values of Figure 9 and turning toward Cads 0 leads to decreasingly contaminated states of the surfactant solution. This behavior is well-known from our investigations of the problem of surface chemical p ~ r i t y . ~ *It~means J ~ that already at low alumina concentrations the amount of 1dodecanol produced from the parent SDDS was large enough to be reflected not only in th_eapparent equilibrium surface tension of desorption, beD, but also in the apparent equilibrium surface tension value which is less sensitive to the “impurity” 1-dodecanol. Beyond the extreme values the concentration of l-dodecan01 in the aqueous phase becomes so high that part of it is removed again from the latter by adsorpti_on at the alumina surface. Thus there is an increase in the ueAvalues together with an decrease of the corresponding compression effect a t the highest alumina concentrations. When the SDDS concentration is raised by a factor of 3, daeA/dcad,is always positive (cf. Figure lo), but the compression effect is passing through a maximum as in Figure 9. These findings suggest an adsorption of dodecy1 sulfate as well as 1-dodecanol resulting from decomposition that starts already a t low adsorbent concentrations. Evidence for the adsorption of 1-alkanol at acidic and basic alumina has been provided by applying the adsorbents to surface chemically pure 1-decanolsolutions. This
aeA,
75
IO cads
Figure 10. Apparent equjlibrium surface tension value aeAand compression effect neA- ueD of an aqueous 3 X 10-3 M sodium dodecyl sulfate suspension as a function of the amount of acidic aluminum oxide added.
alkaline ac’d’c
mc*
20
Figure 11. Equilibrium surface tension of an aqueous 5 X 10-5 M 1-decanolsolution in dependenceon the amount of aluminum oxide added 0,basic alumina; +, acidic alumina.
follows from the increase of equilibrium surface tension ;eA with rising alumina concentration as demonstrated in Figure 11.
Conclusions Adsorption of organic solutes on solid adsorbenta from solutions is a very important process and frequently used in industry. Therefore, a great number of papers has already aimed at the elucidation of the underlying mechanism so far. However, to get reliable evidence on the adsorption mechanism, one has to comply not only with the requirements of surface chemical purity of the solutions i n ~ e s t i g a t e d but ~ - ~also with the pitfall of transformation of the adsorbed molecules. From gas-phase adsorption we know that many oxide surfaces possess oxidative or hydrolyzing properties which may lead to additional surface compounds upon reaction at the surface. The findings of this paper show that investigating solid adsorption from solutions surface reactions have to be considered too. Thus, by use of surfactants which contain labile chemical bonds, such as esters, the possibility of degradation or transformation has to be excluded beforehand. As far as surfactant solutions are concerned, pitfalls can be excluded by monitoring the surfacetension changes in connection with appropriate criteria of surface chemical purity. Registry No. SDDS, 151-21-3;SDS, 142-87-0; STDS, 119150-0; Al203, 1344-28-1.
