Sept., 1957
ADSORPTION OF TRITIATHD SODIUM DODRCYL SULFATE
sponds according to eq. (iii) to 1.32 k . This compares well with the values quoted in literature 1.34 f 0.02k.,lS 1.30k.14and1.309.15 It will be noted that eq. (iii) holds for the CC bond irrespective of whether ’the bond is single, double, triple or intermediate. It is even possible to extrapolate below the single bond. I n graphite it is not usual t o write a bond for the energy hold(13) L. Pauling and L. 0. Brockway, J . A m . Chem. Soc., 59, 1223 (1937). (14) J. Overend and H. W. Thompson, J . Opt. Sac. Amer., 43, 1065
(1953).
(15) B. P. Stoicheff, Can. J . Phya., 33, 811 (1955).
1135
ing parallel layers together. Since the distance between them is 3.41 k. the “inter-layer bond” which might be stipulated would amount to 7.88 kcal./g.-bond. If this energy is subtracted from the heat of atomization of graphite and the difference multiplied by 2/3, the resultant bond energy gives the observed value for the in plane CC bond length in graphite within the experimental error (see Table 111). Acknowledgment.-The author wishes to express his thanks to Mr. S. Zacks for his help in the statistical evaluation of the data.
THE ADSORPTION OF TRITIATED SODIUM DODECYL SULFATE AT THE SOLUTION SURFACE MEASURED WITH A WINDOWLESS, HIGH HUMIDITY GAS, FLOW PROPORTIONAL COUNTER BY GOSTANILSSON Division of Physical Chemistry, Royal Institute of Technology, Stockholm, Sweden Received November 85,1366
Tritiated sodium dodecyl sulfate, TSDS, was prepared from tritiated dodecanol and purified. Tritiated dodecanol TD, was prepared by condensation of decyl aldehyde with malonic acid, reduction of the 2-dodecenoic acid obtained by LiAlH4 to 2-dodecen-1-01 and catalytic tritiation of the alcoholic double bond. The boiling point, melting point, refractive index and density of 2-dodecen-1-01 have been determined. A windowless flow roportional counter has been constructed and used for surface adsorption measurements at 25’. The sensitive volume o?the counter extends to the surface of the solution and the flow gas (propane) had a relative humidity of 68% in order to minimize evaporation from the solution surface. The adsorption isotherms of TSDS in water and in a buffer solution (pH 6.5) containing a constant excess of neutral salt (0.1 m ) have been determined in the concentration range 0-1 X mole/1000 g. solution. A constant surface concentration, corresponding to a monolayer with a surface area of 33 .*/molecule, was obtained when the bulk concentration in the buffer Bolution had reached a value of 2 X mole/1000 g. There is some evidence that micelles are formed below the CMC. The surface excess of TSDS in an aqueous solution has been determined a t a constant bulk concentration’of TSDS (1 X 10-3 mole/1000 g.) and different bulk concentrations of sodium tetradecyl sulfate and dodecanol. It was found that sodium tetradecyl sulfate is more effective than dodecanol in displacing TSDS from the solution surface. A qualitative measurement of the adsorption of T D at the surface of an aqueous solution of sodium dodecyl sulfate containing 1 mole % T D as “impurity” has also been performed. It is shown that the alcohol gradually disappears from the surface above the CMC and that the alcohol comprises at least 50% of the mixed film at a sodium dodecyl sulfate concentration of 1 x 10-8 mole/1000 g. The tritium method developed here for surface adsorption measurements permits an increase in accuracy and an extension .of such measurements to lower values of r/C.
B
Introduction The tracer technique for measuring the surface adsorption of surface active agents was introduced by Dixon, et uZ.,l and Aniansson and Lamm.2 In the last few years a large number of such measurements have been By labelling both (1) J. K. Dixon, A. J. Weith, A. A. Argyle and D. 3. Salley, Notwe, 163, 845 (1949). (2) G . Aniansson and 0. Lamm, ibid., 165, 357 (1950). (3) C. M. Judson, A. A. Argyle, D. J. Salley and J. K. Dixon, J . Chem. Phys., 18, 1302 (1950). (4) E. Hutchinson, J . Colloid Sci., 4, 600 (1950). (5) D . J. Salley, A. J. Weith, A. A. Argyle and J. K. Dixon, Proc. Boy. Sac. (London), 8203, 42 (1950). (6) G . Aniansson, THISJOURNAL, 58, 1286 (1951). (7) G. M. Judson, A. A. Argyle, J. K. Dixon and D . J. Salley, J . Chem. Phys., 19, 378 (1951). (8) R. Loos, Mededel. Koninkl. Vlaam. Acad. Wetenschap. Belg. K l . Welenschap., 13,3 (1951). (9) G . Nilsson and 0. Lamm, Acta Chem. Scand., 6, 1175 (1952). (10) C. M. Judson, A. A. Lerew, J. K. Dixon and D. J. Salley, J . Chem. Phys., 20, 519 (1952). (11) C. M. Judson, A. A. Lerew, J. K. Dixon and D . J. Salley, THISJOURNAL, 57,916 (1953). (12) R. Ruyssen, Bull. soc. chim. BeEgee, 62, 97 (1953). (13) R. Ruyssen and J. Moebe, Mededel. Koninhl. Vlaam. Acad. Wetenschap,B d g . Kl. Wetenachap., 15, No. 4, (1953). (14) G . Aniansson and N. H. Steiger, J . Chem. Phys., 21, 1299 (1953).
the surface active ion and the gegenion with suitable &emitters, their adsorption a t the solution-air interface have been studied. The principles involved in the method are illustrated as follows. A solution of the labeled agent is prepared and a detector, usually a G-M tube, is placed close to the free surface of the solution. All measurements with the detector are carried out using the same geometry. Then two separate cases are distinguished. I. A solvent is used in which the agent is not surface active, Le., the Concentration of solute is constant right up to the interface and the activity detected is given by Ai = TCrsG
e-rrz
- - e-c&)c.p.m.
dz = TCCsG (1 u
(1)
where T is the surface area of the solution, C (mole/g.) the concentration of the solute, l(g./ (15) N. H.Steiger and G. Aniansson, THIEJOURNAL, 58, 228 (1954). (16) C. P. Roe and P. D. Brass. J . Am. Chem. Sac., 76,4703 (1954).
(17) J. K. Dixon, C. M. Judson and D. J. Salley, A m . Assoc. Advance. Sci. Monomol. Layers, 63 (1954). (18) R. Ruyssen, Compl. rend., 3 (1954); Industrie chim. belpe, PO, Spec. NO.,783 (1955).
G ~ S T NILSSON A
1136
the density of the solution, s (disintegrations/ mole) the specific activity of t6e solute, G the geometry factor, p(cm.-I) the adsorption coefficient of the @-particlesand R (em.) their range in the solution. This equation is an approximation to the real conditions but measurements of the selfabsorption of P-rays in a source have shown that experimental values fit such a relation rather well except a t very low source thicknesses. This exception is of course not valid in these measurements. For the radioisotopes used, R is so small that G has been considered independent of x within the integration range. Equation 1 is with good accuracy approximated to Ai =
fim
where ,u.l(cm.2/g.) is the mass absorption coefficient. 11. Let us now change the solvent in I in such a way that the agent becomes surface active but all other conditions (bulk concentration, geometry, etc.) are held constant. Then the concentration of solute is no longer constant right up to the interface but we have instead a concentration peak in the surface region. If the activity detected is written At = Ai Ss (3) where Ai has the same meaning as before, then A , is the activity from an excess of molecules within a distance R from the surface and approaches a measure of the total number of molecules in the surface phase with decreasing thickness of the latter. I n case I1 Ai is named “the activity from the interior of the s~lution’~, A , “the activity from the surface” anti A t “the total activity.” The “excess molecules” are so close t o the surface that &particle absorption in the solution can be neglected. Then
+
and
r
E
A
racy for high surface active agents. By choosing still weaker &emitters it would be possible to increase the accuracy and extend the measurements to agents with low surface activity and to high bulk concentrations since I’/C decreases with increasing bulk concentration. A p-emitter of this kind is tritium, applicable in principle to all organic surface active agents and having a &energy of about one tenth that of C14, the latter being the weakest p-emitter hitherto used. Data for tritium and C14are given in Table
I.
TABLE I
TCsG
-
- Ait
(5) TS where S (c.p.m./mole) is the specific activity of the solute when placed in a monolayer on the surface and I’(mole/cm.2) the surface excess of molecules, which is the quantity to be determined, usually as a function of C. This evidently involves a determination of At, Ai, T, Sand C. The determination of At, Ai and S can be performed as described or in some other principally equivalent way. For a certain I’ and the same counting time, t, the error (AI’/I’)t decreases as A , approaches At, ie., the ratio
should be as close to unity as possible. This means that it is convenient to use a low energy &emitter ( p m large) and to study agents with high surface activity (high I’/C ratio). The @-emittershitherto used have been Sss, C1*, NaZ2and Ca46and these may give sufficient accu-
Vol. 61
Tracer
T
c ‘4
Halflife, yr.
Max. energy, kev.
12.262 5568
17.95 155
Max. Mean energy energy Mean range, range, energy, mg./ me./ kev. cm.9 am.% 5.69
..
0.5 28
0.05
..
dva .urn, mg.7 om’./ cm.9 mg. 0.06 1 1 . 5 2.1
0.26
From eq. 6 and Table I we find that the value of A s / A t is the same for a surface active agent labeled with C14 or T if (I‘/C)T = 1/44.3(I‘/C)c~ This means that labelling with tritium will extend this tracer technique to values of I’/C 1/44 as large without increasing the error (AI’/I’)t. It was thus of interest to study the adsorption of a tritiated surface active agent. Sodium dodecyl sulfate (SDS) was chosen for this purpose mainly for three reasons: a large amount of data concerning SDS have been accumulated, the purification procedure is simple and it has a sufficiently low surface activity for ruling out the tracers hitherto. This latter can be demonstrated by comparing the value of A,/At a t the critical micelle concentration (CMC) for tritium- and C14-labeled SDS. In the former case, the measurements in this investigation give a value of 0.54 while, in the latter, a value of about 0.01 is obtained if C = 8.1 X lowsmole/ 1000 g. and I’ = 4.5 X 10-IO mole/cm.2 is introduced in eq. 6. The value of A,/At depends upon the concentration chosen but it is evident that, of the isotopes discussed, only tritium can be used to study the adsorption of SDS. Experimental (a) Preparation of Materials.-The different steps in the synthesis of tritiated 1-dodecanol (TD) and tritiated sodium dodecyl sulfate (TSDS) are as follows. ( 1) 2-Dodecenoic Acid.-Commercial decyl aldehyde was purified by distillation in a spinning band column and then added to malonic acid dissolved in pyridine. This condensation reaction was performed according to a description given by T u l u ~ ’ ~yield ; of crude product 76%. (2) 2-Dodecen-1-01.-As far as is known this alcohol has not been prepared before. It was made from the 2-dodecenoic acid following a method given by Nystrom and Browns0 for the reduction of carboxylic acids by LiAlHa. The crude 2-dodecenoic acid was distilled and the fraction (0.080 mole) with n%’ 1.4639 (a value give by Zaar2’ for 2-dodecenoic acid) was dissolved in ether (150 mi.) and slowly added to a solution of LiAlHr (0.2 mole) in ether (250 ml.). The yield of crude alcohol was 98%. The main part dlstilling between 127-128’ at 7 mm. was collected, analyzed and part of it hydrogenated to 1dodecanol. Table I1 gives the properties of the new compound, its hydrogenated form and commercial 1-dodecanol for comparison. (19) (20) (1947).
R. Tulus, Rm. facult6 8 C i . uniu. 18tanbd, 9A, 105 (1944). R. F. Nystrom and W.G. Brown, J . Am. Chem. SOC.,69, 2548
(21) B. Zaar Ber., 297 (19291.
8
1137
ADSORPTION OF TRITIATED SODIUM DODECYL SULFATE
Sept., 1957
TABLE I1 2-Dodecen-1-01
Hydrogenated 2-dodeaen-1-01 (I-dodecanol)
1-Dodecanol (Eastman Kodak)
B.p., “C.
127-128 (7 mm.) 1.5-2.5 23.0-23.6 23.3-23.9 nmD 1.4510 d5D 1.4490 1.4412 1.4410 d“r 0.8417 Calcd. for 2-dodecen-1-01: C, 78.19; H, 13.13; mol. wt., 184.3. Found: C,77.4; H, 13.43; mol. wt., 183. (3) Tritiated I-Dodecano1.-The 2-dodecen-1-01 was converted to TD by catalytic tritiation in the apparatus shown in Fig. 1. Water-free diisopropyl ether (10 ml.) and PtOTH20 (0.03 g.) was introduced into the reaction vessel G. The apparatus was flushed with hydrogen and the catalyst reduced to an almost black color with hydrogen because sufficient tritium was not available. The ether was distilled off and the reaction vessel placed in a desiccator, which was evacuated to remove water and hydrogen adsorbed on the catalyst and then filled with carbon dioxide. Water-free diisopropyl ether (15 ml.) and 2-dodecen-1-01 (0.195 g.) were added to the reaction vessel. The vessel was attached to the hydrogenation apparatus and cooled in liquid air. The whole system was then flushed with hydrogen and evacuated through V. A certain amount of tritium was extracted from the tritium ampoule C by placing the mercury in D at a certain level and then distributing the tritium-hydrogen mixture (180 mC. with 1% T obtained from A. E. R. E. Harwell) over both C and D. Knowing the volumes of the different parts of the system, a known amount of tritium could be left in D when the mercury level was raised over stopcock 2. The T-H mixture in D (99 mC.) was then introduced into the evacuated space between stopcocks 4 and 7 by raising the mercury level in D to stopcock 5. Capillary tubing (heavy line in fig.) was used to reduce the “dead space” (15 ml.) and thus increase the consumption of tritium. The solution was now at room temperature and the magnetic stirrer H was started immediately. The reaction was allowed to proceed for about 10 min. and then pure hydrogen was introduced from B and the reaction continued a t atmospheric pressure as long as there was any consumption of hydrogen in order to ensure complete saturation of the alcohol. The chemical yield was 96%. The specific activity was determined a t the Nobel Institute by Dr. L. Melander, who has developed a procedure for tritium analysis.22 It was found to be 14 C./mole corresponding to a yield of 22oJ, when the effect of “dead space” is corrected for. Part of the activity was located at the hydroxyl group but, from the adsorption measurements, it was found t h a t , y amount >4.8 C./mole was situated a t the “double bond. The amount of tritium in the hydroxyl group was reduced, by a factor of lo5, by repeated dissolution of the substance in methanol and evaporation of the solvent. T o minimize radiation decomposition, the alcohol was stored in methanol solution until used. (4) Tritiated Sodium Dodecyl Sulfate.-The tritiated alcohol was sulfated with chlorosulfonic acid according to Dreger ,et a1 .es; yield 77 % The soap was purified by extraction of the solid substance in a micro-extraction device with dry diethyl ether. This method has previously given successful results with SDS prepared in this Laboratoryg as is seen from the surface tension curve D in Fig. 2 but, in the present case, it failed as can also be seen from Fig. 2, where curve A was obtained before and curve B after purification. It is possible that foam or emulsion extraction would have given better results but the loss of substance would have been too great when only 90 mg. of TSDS was available. From the measurements by Miles and Shedlovsky,a4 it can be estimated that the amount of dodecanol required to give theminimumincurveBisaboutO.l~o. Itis, however, most likely that the impurity is not dodecanol, because it is so hard to remove, but some other compound possibly produced by radioactive decomposition of TSDS.
M.P., “C.
.
(22) L. Melander. Acta Chem. Scand., 2 ‘ , 440 (1948). (23) E. E. Dreger, et al., Ind. Eng. Chsm., 86, 610 (1944). (24) G. D. Miles and L. Shedlovsky, THISJ O V ~ N A48,57 L, (1944).
Fig. 1.-The
48 46
r-
242 a g40 $44
\
-
.d
$ 38 8 36
d
434 32 30
-
t
tritiation apparatus.
t-
\
D
Y+: C
hd
0 2 4 6 8 1012 Concn. x 10-8, mole/l. Fig. 2.-Surface tension curves: A, impure TSDS; B, TSDS after purification; C, impure SDS and D, pure SDS. The surface tension measurements were made with a D u Nouy tensiometer a t 25”. The water used in the surface tension measurements and in all other experiments was purified using a mixed bed ion-exchange column. It had a surface tension of 71.9 dyne/cm. a t 25”, a specific conductivity of about 8 X 10-7 ohm-lcrn.-l and a p H of 6.1. (b) Description of Flow Counter.-Detection of the weak tritium P-radiation from the surface of a water solution presents counting problems which are not usually encountered. A low signal to noise ratio makes scintillation counting questionable and water vapor in a windowless ion chamber is expected to give a high back-ground current. There remains a windowless flow proportional or G.M. counter but (1) water vapor would impair the counting characteristics all the more as the flow gas must be moist before entering the counter in order to avoid evaporation which effectsthe adsorption9 and (2) for any detector the distance between its sensitive volume and the surface of the solution is critical. A variation of 0.1 mm. would introduce an error of 20% if the space is filled with moist propane. The principles of a proportional flow counter with a high humidity gas have, however, been given in a previous paper,a5 thus eliminating (1). This counter was constructed for an external source and was not suitable for the tritium measurements. A new counter was therefore built and is shown in Fig. 3. The counter D is made of stainless steel. The high tension electrode W is a tungsten wire loop 10 mm. in diameter. The wire diameter is 0.01 mm. The electrode is insulated from ground potential by the Teflon insulator I. The heating coil E and the thermoregulator TI keep the (25) G . Nilsson and G. Anisnsson. Nucleonics, 18, No.2, 38 (1955).
1138
G ~ S T NILSSON A
~
Fig. 3.-The
flow counter.
6 -
p5x4-
a
63 -
u
21-
011 21
I
I
I
I
1
I
I
I
I
22 23 24 25 26 27 28 29 30 Counter potential x lo*, v. Fig. 4.-Plateau curves and bias curve for the flow counter. counter a t 27 f 0.5". The flow gas was commercial propane ("gasol") containing 87% propane, 10% butane, 3% ethane and 0.003% S-compounds. To obtain a high humidity, the gas was passed through two wash-bottles containing distilled water a t 25 & 0.1" before entering the counter. The relative humidity of the gas at the inlet tube was 68%. The rateof gas flow was 14-17ml./min., the counter being stable after about 20 min. of flushing. The stable counter had a background counting rate of about 30 c.p.m. . The four symmetrically arranged cuvettes in the Lucite plate C were filled to the brim with the solutions to be tested. The distance between the surface of the solution and D was 0.5 mm. while that between the surface and the top of the loop was 8 mm. The cuvettes were 2 mm. deep and 18 mm. in diameter. The opening of the counter tube had a diameter of 10 mm. Beta particles coming from the edges of the cuvettes, where the adsorption conditions are less defined, are in this geometry largely screened off. The solution in each cuvette was connected by a platinum pin P to the aluminum plate B, L e . , to ground potential. This will make the sensitive volume of the tube extend to the surface of the solution, thus eliminating (2). The plate B could be rotated relative to D and A. Thus, any
Vol. 61
desired solution could be brought under the opening of the counter. I n order to maintain the solutions at 25', the plate B was thermostated a t 25 f 0.5" by the heater H and the thermoregulator Tz. The counting equipment consisted of an EKCO H, F. head amplifier ty e 1008, pulse amplifier type 1008, scaler type 1009, and tabgear E.H.T. supply unit type D 4019/P. Integration and differentiation time constants were 0.6 psec., bias level 5 volts, gain 16 X lo3 and E.H.T. 2500 volts. In Fig. 4, a bias curve and plateau curves for different concentrations of TSDS in water are shown. With the experimental conditions described, except that a cuvette with a diameter of 35 mm. was used, the diameter of the opening of the counter was varied and the effect on the plateau slope observed. Slopes of 9.4,6.7,5.6 and 4.4% were obtained a t diameters of 30,20, 15 and 10 mm. When the cuvette was filled with an aqueous solution of Ca46C12, the slope was 0.9% a t a diameter of 30 mm. The slopes are given in per cent. per 100 volt, measured at 2500 volt. This experiment shows that in order to obtain a good plateau it is necessary to have a small opening since the tritium betas are absorbed close to the surface of the solution (max. and mean ranges in moist propane 3 and 0.3 mm., respectively) where the sensitive volume expands from the center to the periphery with increasing counter potential. This is not the case for beta radiation from Ca46, which is able to reach the more constant sensitive volume in the interior of the counter. When the distance between the top of the loop and the surface had been reduced to 3 mm., the plateau curve was not reproducible. With increasing distance the tube was stable and parallel plateau curves with a decrease of about 3%/ mm. in the countin rate a t 2500 volt were obtained. A consequence of the short range of the @-particlesfrom tritium is that the part of the sensitive volume utilized is mainly situated between D and the solution surface. Moreover, if the surface of the solution is convex, the activity is decreased while if the surface is concave, the activity is increased due to a corresponding geometry variation in articular for betas coming from the edges of the cuvette. ?! ! or a deviation of 0.1 mm. in the center of the surface, the activity changed by about 2%. When the rate of gas flow was increased, parallel plateau curves with increasing counting rate at 2500 volt were obtained. The increase was about 2% if the gas flow was changed from 9 (min. value for stable counter) to 18 ml./ mm. In the experiments, the rate of gas flow was constant to within 20% and this variation had a negligible effect on the counting rate. (c) Measurement of Activity.-Three of the cuvettes (1,.2 and 3) in the Lucite plate were filled with the radioactive solutions and the fourth with solvent. The tube was placed in position and the gas flow started with the solvent-filled cuvette in counting position. The measurements were begun after 35-45 min. of flushing with propane. The tube was f i s t checked with an external standard source of Cs'37. Then the measurements were carried out as follows in order to see if there was any change in activity with time or any contamination of the tube; 0, 1, 2, 3, 0, 1, 2, 3, and 0 (0 = background, i.e., solvent-filled cuvette in counting position). This order of measurements and the same time scheme was followed in all measurements described unless otherwise stated. The counting time was 5-10 mm., giving standard deviations of about 0.5% for .most measurements. The activities are corrected for the tritium decay. All concentrations have been determined by weighing the solutions and are given in mo1./1000 g. solution. Each solution has been used in only one measurement. The I?-values are calculated from eq. 5 and all measurements refer to a temperature of 25 f 0.5".
Results I. Determination of 8.-The cuvettes were filled with an almost saturated aqueous solution of (NH4)zS04(0.7 g./ml.) and TSDS was spread over the surface from an Agla microburet containing an alcohol-benzene Bolution (1:40) of TSDS. The spreading solution was prepared by first dissolving TSDS in alcohol and then diluting the solution with benzene treated with active charcoal. The sub-
.
c
Sept., 1957
ADSORPTION OF TRITIATED SODIUM DODECYL SULFATE
strate was treated in the same way to avoid surface active impurities. The activity versus surface concentration curve shown in Fig. 5 , was obtained from measurements made a t about the same time interval from the moment of spreading. When the area per TSDS molecule had been reduced to about 50 they began to dissolve in the substrate. This appeared as a decreased slope of the curve and also as a slow decrease in activity with time. The straight line portion of the curve where no change in activity with time was found indicates that the surface film is stable and equally distributed over the surface within these concentrations. Therefore, only this part of the curve was used for the calculation of S. The first eight points give a mean value of 430 X 1O1O c.p.m./mole. Hence TS = 1096 X 1O1O c.p.m. cm.2/mole and this figure has been used to calculate all I?-values. An attempt was made to determine S by spreading T D on pure water, 0.02 m HC1 and also 0.2 m NaOH. This would have given conditions which are more like the real ones from a back-scattering point of view. It was impossible, however, to determine S by this method because T D evaporated from the surface and contaminated the counter. 11. Determination of &-The activity from the interior of the solution was determined by measurements on solutions of T D in dodecanol (D), Such a determination satisfies the condition that no surface adsorption of T D takes place but, on the other hand, a correction has to be made for the fact that the solvent is dodecanol and not water. The corresponding values of Ai for a water solution are obtained from eq. 2 which gives
1139
87 6 -
*'5
0
*4
E 43
-
2 I -
0
? 10xlO-'Om~'M..' 0 2 4 6 8 t u 1 1
I
I
I
50 40 30 h0 : 8 A.ymolecule 100 Surface concn o f T S D S a n d area occupied b y TSDS molecules
Fig. 5.-The activity from TSDS spread on an aqueous solution of (NH&S04 (0.7 g./ml.) a t different surface concentrations.
14
12
-
-
10 -
5
8 -
Then, for equal concentrations (mole/1000 g.) of TD in both solvents
where pm is nearly independent of the nature of the absorber, L e . Ai(Hz0) = Ai(D)
(9)
The Ai(H20) curve shown in Fig. 6 was obtained from the measurements and eq. 9. The mean value of Ai(HzO)/concentration for the experimental points is 524 c.p.m. 1000 g./mmole. This figure has been used to calculate all I'-values. 111. Adsorption of TSDS in Water and in Buffer Solution.-The different aqueous solutions (pH 6.1) were prepared by diluting a concentrated solution which was kept stored in a quartz container at -15". At is shown in Fig. 6 and I' is given by curve A in Fig. 7. The squares in Fig. 7 are corresponding values obtained from surface tension measurements at 23" by Hutchinson.26 The NaH2P04-NaOH buffer solution of pH 6.5 contained in addition sufficient NaCl to give a total sodium ion concentration of 0.1 m. The different solutions were prepared in the same way a.s the aq. solutions and I' is given by curve B in Fig. 7. IV. Displacement of TSDS from the Solution Surface by Sodium Tetradecylsulfate and Dodeca(2.6)
E.Hutchinson, J . CoZZoid Sci., 8, 413 (1948).
0 2 4 6 8 10 12 14 16 Concn. of TSDS X IO-a, mole/1000 g. Fig. 6.-The total activity, At, from an aqueous solution of TSDS and the activity from the interior of the solution, Ai, obtained by measurements on solutions of tritiated dodecanol in dodecanol.
no1.-The solvent was water (pH 6.1) and the solutions were prepared as described in section 111. The concentration of TSDS was constant and equal to 1.0 X mole/1000 g. The concentration of sodium tetradecylsulfate (STS) was varied. The adsorption of TSDS is given by curve A in Fig. 8 as a function of the STS concentration. The experiment with dodecanol is identical with the preceding one except that dodecanol (Eastman Kodak, m.p. 23.3-23.9") was used instead of STS. The different solutions were prepared by pouring into a small vessel the required amount of dodecano1 in benzene solution. The solvent was evaporated off and TSDS added as a 14.52 mM solution which was then diluted with water (pH 6.1). At a dodecanol concentration of about 0.4 X mole/
1140
G&TA NILSSON
Vol. 61
solution surface. Only one cuvette was filled with the SDS solution and the others were filled with water. The first measurement was made 20 min. and the second 80 min. after the cuvette had been filled, upper and lower curve in Fig. 9. The activity and background were each measured over a period of one minute-the background was measured immediately after the activity since the contamination of the tube with TD was found t o almost disappear within a few minutes after the water-filled cuvette had been placed in counting position. 0 2 4 6 8 10 12 14 16 Discussion Concn. of TSDS X 10-2, mole/1000 g. Systematic errors in 'I may arise from the methFig. 7.-The adsorption isotherm of TSDS a t 25' in ods of determining S and Ai from the influence of water, curve A, and in a buffer solution of p H 6.5 containing in addition an excess of neutral salt, [Na+] = 0.1 m, the electric field or the flow gas on the adsorption, curve B. The squares are values obtained from surface from evaporation from the surface, slow adsorption tension measurements a t 23' by Hutchinson.26 and impurities. I n the determination of S , the back-scattering power of the (NH4)2S04 3r solution differs from that of water because the mean atomic number of water is 7.2 while that of the (NH4)2S04solution is 8.0. The difference in back-scattering power cannot be accurately determined but, from measurements of the Z-dependence of the back-scattering it is estimated that the activity from the ("4)~SO4solution surface will be no more than 1% higher than that from a water surface. Ai was determined indirectly and therefore had to be calculated from eq. 8 using the approximation pm(H20) = pm(D). This is a drawback but the method has the advantage that one can be sure that there is no adsorption of solute a t the solution 0 1 2 3 surface. Concn. X 10-8, mole/1000 g. It is generally stated that pm is almost indcpendFig. 8.-The adsorption of TSDS at constant bulk concentration (1 X 10-3 mole/1000 6 . ) and different bulk ent of the nature of the absorber but there is in concentrations of sodium tetradecylsulfate, curve A, and reality a slight variation of pm with the atomic dodecanol, curve 13. n~mber.30-3~Dorfmana2 has measured the absorption of tritium P-rays in hydrogen, helium and oxygen and, from his measurements, the ratio p m . (D)/pm(€120)is estimated to be 1.03. The errors in the Ai and S measurements evidently have an opposite effect on r. Tn any case, the systematic error in I? due to errors in S and Ai cannot be accurately determined but may be no more than a few per cent. 0 To see if the electr'c field had any influence on 0 2 4 6 8 10 12 14 16 the adsorption of TSI3S some measurements of At, Concn. of SDS X 10-8, mole/1000 g. Fig. 9.-The adsorption of tritiated dodecanol at the sur- A; and S were made with the solutions not at face of an aqueous solution of sodium dodecylsulfate con- ground potential. The activities were found in all taining in addition 1 mole yo tritiated dodecanol as "im- three cases to be 70 f 1% lower. Since the elecpurity." There is a time interval of 60 min. between the tric field has no influence on the surface concentrafirst measurement, upper curve, and the second measuretion in the determination of .4i and S, the change in ment, lower curve. activity can in all three cases he attributed to a 1000 g., the solutions began to show a slight opales- change in the sensitive volume. Thus, any error incence which increased with increasing dodecanol troduced by the electric field can be neglected. concentration. The adsorption of TSDS is given Water-soluble surface active impurities in the by curve B in Fig. 8 as a function of the dodecanol flow gas are certainly absorbed in the wash bottles. concentration. The effect of the hydrocarbons in the flow gas on V. The Adsomtion of TD in a Solution of SDS the adsorption is hard t o predict. The adsorption Containing 1 mole % TD.-This experiment was (27) 9. Eklund, Ark. Mat. Astr. F y s . , 331, No. 14 (1946). made to study the effect of 1 mole % ' T D as an (28) J. G.Balfour. J . Sci.Instr.. 31,395 (1Q54). (29) K. Siegbahn, "Beta and Gamma, Ray Spectroscopy,'' North"impurity" in SDS. The solvent was water (pH 6.1) and the solutions were prepared as described in Holland Publ. Comp.. Amsterdam, 195.5,p. 7. (30) T. Wecterinark, Trans. Instr. and Measurements Coni. Stocksection 111. holm, 1949,p. 308. The measurements were not ca,rried out in the (31) V. N. Smith and J. W. Otvos, Anal. Chem., 26,359 (1954). (32) L. M. Dorfman, Phys. Rev., 96, 393 (1954). usual way because TD rapidly evaporated from the
" I
Sept., 1957
ADSORPTION OF TRITIATED SODIUM DODECYL SULFATE
of hexane on water and on stearic acid monolayers has been studied33 but, as far as is known, the effect of organic vapors on soluble monolayers has not been investigated. As seen below, a large amount of T D in the surface has almost no effect on the adsorption of SDS. The effect of a hydrocarbon is expected to be much smaller. Evaporation from the surface and slow adsorption would make At time-dependent. If there was any time effect in the first 35 min., it could not be observed due to the reasons already given. I n the following 25-30 min., the time interval between the first and the second measurement, no time effect was found. Due to solubilization, surface active impurities would not affect r a t concentrations above the CMC. At lower concentrations, the effect cannot be predicted because the nature of the small amount of impurity present is not known. The relative mean errors in I' due t o standard deviations in At, Ai and S are in experiments I11 1.0-0.8%, in IV 7.8-0.9% (STS) and 1.549% (D) and in V about 3%. To this we have t o add about 2% from variations in the level of the surface. Random errors from other sources are difficult to estimate. A best value of the CMC of SDS in water obmolejl. tained by different methods is 8.1 X a t 25°.a4 The adsorption isotherm, Fig. 7, curve A, however, changes from and becomes linear at about 6X mole/1000 g. This may be an indication that ion association or micelle formation takes place below the CMC. That this possibility exists already has been pointed out by several author^.'^,^' There was not sufficient material t o follow the adsorption t o higher concentrations but curve A will probably show the same constant surface excess of about 5 X lO-"J mole/cm.2 as curve B at slightly higher concentrations. If this is true then r a t high concentrations would correspond t o a monolayer with a surface area of 33 a.z/molecule. Pethica36has found that, for SDS adsorbed at the airwater interface, the surface tension data a t 20" fit the equation z ( A - 31) = 1.2kT where A is the area per molecule and T the surface pressure. The area would then be close to the limiting area in Pethica's equation. Recently D a ~ i e has s ~ ~found that inohis surface equations of state a limiting area of 33 A.2/molecule gives the best fit with experimental points for the adsorption of SDS in water. Ruyssenlz and Ruyssen and R40ebe,~~ have found a surprisingly rapid increase in r for Sa5-labeled SDS at concentrations above 3 X mole/]. One reason may be that, by labelling with S35,the A,/At ratio is very small at high bulk concentrations which makes the I'-values uncertain. The adsorption isotherm B in the same figure was measured in the buffer solution where the only variable components are the Na+ and DS- ions. Gibbs' equation for this simple system is dy = -ET [ r N a + d In UNS+ 4-I'm- d 1n ans-] (10) K.E. Hayes and R.B. Dean, THISJOURNAL, 67, 80 (1953). (34) R. J. Williams, J. N. Phillips and K. J. Mysels, Trans. Faraday (381
Soc., 61, 728 (1955).
(35) P. Ekwall, SIJ.Kern. T i d s h , 63, 277 (1951). (36) B. A. Pethica, Trans. Faradav Sac., 60, 413 (1954). (37) J. T,Davies, J. Colloid Scz., 11, 377 (1956).
1141
where y is the surface tension and a the molar activity of the components. There was, however, a constant excess of sodium ions in the solution and thus d In UNst
