Oct., 1959
ADSORPTION OF A WON-IONIC DETERGENT ON QUARTZ POWDER
1613
A RADIOTRACER STUDY OF ADSORPTION OF AN ETHYLENE OXIDEPROPYLENE OXIDE CONDENSATE ON QUARTZ POWDERS BY H. R. HEYDEGGER AND H. N. DUNNING Surface Chemistry Laboratory, Petroleum Experiment Station, Bureau of Mines, u. S . Department of the Interior, Bartlesudle, Okla. Received March 8 , 1969
The adsorption isotherm of a non-ionic detergent, a ethylene oxide-propylene oxide condensate, on a standard quartz powder has been determined by radiotracer methods. Because of the high specific activity of the labeled detergent, it was possible to extend the measurements to equilibrium concentrations as low as one p.p.m. The extent of adsorption is considerably Iess than that of other non-ionic detergents tested on the same quartz Sam le. Adsorption is reversible and can be described by a Langmuir-type equation X / M = C/(60.06C 850.2), where X,&4 = pgrams detergent adsorbed per cm.2, and C = equilibrium concentration, pgrams per gram (p.p.m.).
+
Introduction The problem of detergency in petroleum production has been explored extensively in this Laboratory. A major deterrent to the use of detergents in stimulating oil production is their adsorption on the extensive surfaces of reservoir rocks. Therefore, a general study was made’ of the adsorption of series of polyoxyethylated alkyl phenols. A comparative study of non-ionic detergents adsorption also was made by radiotracer, spectrophotometric and surface tension methods.2 These studies showed that many non-ionic detergents were strongly, but reversibly, adsorbed a t the quartz-water interface. Also, detergents of higher molecular weight were less extensively adsorbed. However, in the detergent series studied, the higher molecular weight members depart from the hydrophilic-lipophilic balance of maximum detersive effectiveness. Accordingly, the adsorption of Pluronic L-64,3 a detergent of good hydrophilic-lipophilic balance but of very high molecular weight, was investigated with the standard quartz sample used previously. Unlike the alkyl phenols, the Pluronics have no groups that absorb energy in the visible or ultraviolet regions. The availability of a sample labeled with carbon-14, therefore, offered the first opportunity for precise analysis and hence for studying the adsorption characteristics of this type of detergent. Experimental Materials.-The Pluronics are a series of non-ionic detergents having the general formula4
HO(C,H,o),(C,H~o)~(C,H40),H The specific detergent studied, Pluronic L-64, is a member of this series with a molecular weight of about 2,850 and a hydrophobic base material of about 1,750 molecular weight. A stock solution of 8.092 g. of Pluronic L-64 (100% active agent) in 20 ml. of ethanol was furnished by the manufacturer. This concentration was verified in this Laboratory. The detergent molecules were labeled with C-14 in the condensed ethylene oxide portion and had a specific activity of about 0.32 mc./g. according to the manufacturer’s assay. The quartz powders used in this study have been described previously.1 The powder used in most of the experiments had a nitrogen B.E.T. surface area of 3.9 X IO4 cm.*/g. A second powder had a surface area of 1.2 X 104 cm.a/g. (1) H. N. Dunning, Ind. Eng. Chem., 2 , No. 1, 88 (1957). (2) Lun Hsiao and H. N . Dunning, THISJOURNAL, 89, 362 (1955). (3) Trademark, Wyandotte Chemical Corp., Wyandotte, Mich. (4) ”Pluronics, A New Series of Non-ionic Surface &tiye Agents,’’ Wyandotte Chemical Corp., Wyandotte, Mich,
All samples for radioactivity measurement were prepared as follows. A 100-lambda aliquot of the solution was carefully evaporated to dryness under an infrared lamp as a drop on a cleaned aluminum planchet (about 3.1 em. diameter). The micropipet was washed twice with distilled water and the wash liquid also evaporated on the.drop. Control experiments showed that no detectable activlty remained in the micropipet after this treatment. Considerable care was required to avoid splattering and spreading of the very low surface-temion solutions to the edge of the planchet. Concentration data are brtsed on at least three replicate samples. Standard statistical tests were applied before discarding anomalous results. Counting Procedure.-All radioactivity measurements were made with a thin end-window (1.7 mg./cm.2) GeigerMueller tube mounted on a vertically adjustable sample stage and attached to a scaling unit. The planchet containing a sample wan located at a fixed small distance (1.5 mm.) from the G-M tube window by use of a precision micrometer screw mounting of the sample stage, thus achieving very newly 2~ geometry. “Dead-time’’ corrections were made as required by use of a standard formula.6 The “dead-time” of the G-M tube was found to be 150 f 20 microseconds by an oscilloscopic method described by Stever.6 Since the fraction of detergent adsorbed was small a t the higher concentrations studied, the significance of selfabsorption and scattering phenomena were considered. Self-ahsorption corrections were made by use of Henriques’ formula.’ Gora and Hickey’s8 value of the mass adsor tion coefficient for C14in organic samples (0.25 It 0.01 cm.2,’mg.) was employed for these calculations. The backing provided by the planchet (about 27 mg./ emsz)and the sample stage ensured saturation backscattering for all samples.6*9J0 Because of the masking effect of the backscattered radiation and the thinness (less than 0.2 mg./cm.*) of the carrier-free samples, self-scattering corrections were not made and should be negligible.5~*0~11 Background readings were taken frequently and, if a difference was noted between the background before and after a series of sample determinations, the average value was used in correcting the radioactivity data of that ser!es for background. Standard deviations for all radioactivlty dEcta were calculated from standard formulas already given by Jarrett .l2 Adsorption Measurements.-Aliquot parts of t h e detergent-ethanol stock solution were diluted with distilled water to prepare a series of solutions of concentration varying approximately from 10 to 1,800 p.p.m. Radioactivity measurements were made on 100-lambda aliquot parts of these solutions, and a calibration curve was constructed from these data. The data yielded a straight line, which was calculated by the method of least squares. The stand(5) G. Friedlander and J. W. Kennedy, “Nuclear and Radiochemistry,” John Wiley and Sons, New York, 1955. (6) H.G. Stever, Phys. Reu., 61, 38 (1942). (7) F. C. Henriqriee, Jr., G. B. Kistiakowsky, C. Margnetti and W. G. Schneidei, Ind. Eng. Chem., Anal. Ed., 1 8 , 349 (1946). (8) E. K. Gora and F. C. Hickey, Anal. Chem.. 26, 1159 (1954). 1956. (9) G. W. Reed. Jr., ANL-5608,U,9. A.E.C., (10) L. E. Glendenin and A. K. Solomon, Science, 112, 623 (1950). ( 1 1 ) R. G. Baker and L. Kats, Nucleonics, 11, No. 2, 14 (1953). (12) A. A. Jarrett, AECU-262, U. S. A.E.C., 1956.
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Vol. 63
H. R. HEYDEGGER AND H. N. DUNNING
ard deviation of the data from the least squares line was 3.5%. All later concentrations were calculated from this calibration curve. The adsorption experiments were performed by adding an aliquot part (usually 10.0 ml.) of the prepared solution to a weighed amount of powder in a glass-stoppered Pyrex bottle. The bottle and contents were then shaken for varying periods on a "wrist-action" shaker. The concentration of the added solution was determined before and after it was shaken with the powder. The latter was determined by removing an aliquot part (usually 2.0 ml.) of the supernatant liquid after about 18 hours of sedimentation and centrifugation of the sample for 2 hours. Then the sample was evaporated and counted as above. The extent of adsorption was calculated from the observed change in concentration. Desorption experiments were performed by decanting the supernatant liquid after completion of the adsorption studies and reweighing the bottle and contents to determine the amount of solution remaining. A known volume of distilled water was added, the sample agitated as before, and concentration determinations carried out as above. The extent of desorption was calculated from the observed concentration change after shaking, corrections being made for the original detergent solution remaining. The temperature during all experiments was maintained at 23.5 i 2.0' in the air-conditioned laboratory. Control E eriments.-The adsorption of detergent by the walls of z e glass bottles used in these experiments was found to be undetectable by following the usual procedure of an adsorption experiment without quartz powder present. This would be expected since the area of the container was less than 0.05% of the area of the powder samples. A l-day shaking period was found to be sufficient to achieve equilibrium adsorption for solutions up to about 1,100 .p.m. Redetermination of several of the solutions after knger shaking periods showed negligible changes. This was also corroborated by agreement (within the experimental error) of adsorption values calculated for samples with widely varying shaking periods and weights of powder. The period of centrifugation was established by repeated determinations on the same liquid after varying centrifugation periods. No change in specific radioactivity was found after 1 hour or longer of centrifugation. The effect of the small amounts (maximum 2.5% by volume) of ethanol in the solutions was evaluated by evaporating an aliquot of the stock solution to dryness and redissolving in distilled water. Experiments with this alcoholfree solution gave results which agreed within the experimental error with those of similar concentration prepared from alcoholic stock solution. Standards were checked from time to time to assure constancy in the performance of the tube.
8001
'
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1
'
'
'
'
'
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'
I
600
3 6 .s 400 .e a
b 4
-4 200
0 100
300 500 700 900 1,100 Equiv. concn., pg./g. Fig. 1.-Adsorption isotherm of detergent on quartz (wt. basis).
0.25
3
3 OS2O .M
0.15
3P 0.10 4
0.05 0
L---A 100
Fig. 2.-Adsorption
200
300
400
Equiv. concn., p M . isotherm of detergent on quartz (molar basis).
Results and Discussion The adsorption isotherm of P h o n i c L-64 on the fine quartz powder is presented in Fig. 1. The slope of this isotherm is very steep a t the lowest concentrations, decreasing gradually until it is nearly zero a t equilibrium concentrations above 300 p.p.m. Above this concentration the extent of adsorption is essentially constant (at 640 pgrams of detergent adsorbed per gram of quartz) until a concentration of about 1,000 p.p.m. is reached. At concentrations above 1,000 p.p.m. the experimental error is quite large because of the small amount of detergent adsorbed. However, the data indicate an increase in adsorption at higher concentrations. Because of the relatively high specific activity of the detergent, it was possible to determine low concentrations more accurately than in previous work. 1.2 Accordingly, the low concentration portion of the isotherm is more clearly defined. The location of the plateau portion of the isotherm has been calculated by the method of least squares for'the data a t concentrations between 300 and 1,000 p.p.m. This straight line has 8 yalue of
I RELATIVE
CONCENTRATION.
Fig. 3.-Relative adsorption isotherms showing kinetic equilibrium data, (2-10 days shaking, 1 day effect; -, sedimentation); - -, (4hours shaking, 10 day sedimentation); ---, (4 hours shaking, 1 day sedimentation).
-
x / m equal to 640 and
641 pgrams per gram a t 300 and 1,000 p.p.m., respectively, and a slope of 0.002 between these points. The average standard deviation of the data points in this region is 35. The adsorption data are plotted on a micromolar basis in Fig. 2. Although the molar quantities have more theoretical significance, the actual value of detergent adsorbed probably is best shown by calculations on a weight basis.
Oct., 1959
A D S O R P T I O N OF A
NON-IONIC DETERGENT ON QUARTZ POWDER
1615
The adsorption of this detergent also was de- form of a chromatographic zone. Such behavior termined on the coarser powder used previously.' has been observed in the laboratory in similar sysThe determinations made indicated a slightly higher tems.Ia Adsorption isotherms provide the basic extent of adsorption, based on surface area, for the data for calculations of such zone movement accoarser powder. cording to chromatographic theories. The shapes A Langmuir plot [C/(z/m)0 e r . s ~C] ~ of the data of the adsorption isotherms of non-ionic detergents is linear to an equilibrium concentration of 1,125 indicate that, according to DeVault,14 the front pg./g. Earlier data'*2departed from such a plot at boundary of their chromatographic zone will exhibit concentrations below about 40 pg./g. As men- "self-sharpening" tendencies. That is, the front tioned above, the low concentration portion of this boundary will resist concentration decreases due to isotherm was more clearly defined. Therefore, the channeling, diffusion and non-attainment of equilibdeviations in the earlier work probably can be ac- rium. Also, the rear boundary will become sponcounted for by the larger experimental error at very taneously diffuse. Qualitatively, the low adsorplow concentrations. tion of Pluronics will lead to a much more rapid The location of the Langmuir plot was estab- zone movement than that attained by the nonlished by the method of least squares with values of ionic detergents studied previously. For this rea0.00154 for the slope and 0.0218 for the intercept. son, detergents of this type appear promising for The average standard deviation of the data points use in systems where adsorption on solid surfaces is from the calculated line was 0.0479. an economical deterrent to detergent use. Values of the Langmuir function [C/(x/m)] deterAcknowledgments.-The authors thank J. W. mined by desorption measurements lie within this Hensley, Wyandotte Chemicals Corp., for supplydeviation. Therefore, the adsorption is reversible ing the labeled detergent sample and data on its and may be described by the equation properties. The assistance of R. T. Johansen and F. E. Armstrong, of this Station, in measurements x/m = C/(O.O0154C + 0.0218) of surface area and G-M dead-time, respectively, is on a weight basis, or by the equation also acknowledged. X / M = C/(60.06C + 850.2) DISCUSSION on a surface area basis where P. BECHER(Atlas Powder Company).-I wish to make a x/m
= pg.
detergent adsorbed per g. of quartz
C = equilibrium concn., pg. per g. (p.p.m.) X / M = pg. detergent adsorbed per cm.2 of surface
Another series of experiments was conducted in which the powder and solution were shaken for only 4 hours. The relative adsorption isotherm is shown above in Fig, 3. The preliminary data generally indicated lower specific adsorption than that for longer shaking periods as might be expected. However, there was a definite minimum in the isotherm a t a concentration of about GOO p.p.m. Upon redetermination after about 10 days of standing, the minimum was considerably less pronounced, as shown in Fig. 3, indicating a kinetic effect in the region from 370 to 770 p.p.m. A similar minimum in the equilibrium adsorption of Igepal CO-710 on Tip Top sand in approximately the same conceiitration range was reported previously2 but refuted by later data.' The present study indicates that this minimum is a kinetic effect rather than an equilibrium condition. The extent of maximum adsorption (640 pg./g. or 0.225 pM/g.) is unusually low in comparison to the non-ionic detergents studied previously. The adherence to the Langmuir formula indicates that the plateau of the isotherm represents a monomolecular !nyer. The area per molecule for this layer is 2,880 A.2. Similar areas calculated for polyoxyethylated alkyl phenols range from about 35 to 200 A.z. The unusually low adsorption of this detergent may be due to the large size of the hydrophobic polypropylene group located in the center of the molecule compared to the terminal hydrophobic alkyl phenol of the detergents already studied. Because non-ionic detergents are reversibly adsorbed at the quartz-water interface, they would be expected to move through porous media in the
comment on the shapes of these curves. We were working with this identical Pluronic a short time ago. We found that the critical micelle concentration was about 260 gamma per gram. This is the concentration a t which your isotherm flattens out. Now, if detergent monomer is the species which is being absorbed, you will initially get a strong dependence on concentration. However, above the CMC there is no increase in monomer concentration. Thus, you get something that looks like a Langmuir isotherm, but in which the mechanism is quite different. H. N. Dun;vIw.-Some of our previous work with polyoxyethylated detergents has shown that their critical micelle concentrations and critical adsorption concentrations are rather directly related. In fact, one can be calculated from the other. Therefore, I would expect a relationship for these properties with this detergent. The data here, below the critical micelle concentration, fit the Langmuir equation closely. This leads to the conclusion that the adsorption is reversible. I would agree that adsorption of such detergents is influenced by the micellar phenomenon. This would cause a relatively constant adsorption above the critical micelle concentration, not too different from the Langmuir sha e since such adsorption increases very slowly in the higi'eer concentration ranges. A. C. ZETTLEMOYER(Lehigh University ).-I'd like to comment about your slow rate of attainment of adsorption equilibrium. Some work we did on alkyl aryl sufonates showed the rete of breakdown of the micelles was slowed by the presence of small amounts of hydrocarbon produced by hydrolysis. The rate of de-micellization is usually extremely rapid, but could it be that the slow approach to equilibrium you observed was due to a similar cause? H. N. nTJNNTNG.-we did have some alcohol present from the original stock solution. This we would have preferred to avoid but felt that the solution could be more accurately preparcd this way. The detergent probably also contained hydrocarbons, such as those you mentioned, and your observations may be a good explanation. The difference in attainment of equilibrium results from shaking for four hours as opposed to one day. In the finely divided powder, and with the very large molecular weight adsorbate, diffusion of course would be slow anyway.
(13) H. N . Dunning and Lun Haiao, Producere Monthly, 18, No. 1, 24 (1953). (14) Don DeVault, J . Am. Chem. SOC.,66, 532 (1943).
