Feb., 1958
THEINTERACTION OF ANIONIC DETERGENTS
quently undergoes an intermolecular reaction. The electrostatic factor in the rate of association has been calculated above on the assumption that the effect on the rate of charges other than those in the enzymatic site may be ignored. Since the isoelectric point of fumarase is pH 7.0 at’ 0.1 ionic strength, the net charge will be rather small in the neutral pH range. However, at high pH values the electrical charge may become quite large. Application to Other Enzymatic Reactions.-If the bimolecular reaction of a low molecular weight substrate with an enzymatic site is diffusion controlled, the rate constant would be expected to fall in the range of about lo8 to 10’0 see.-’ M-l. If the substrate is a small molecule the reaction radius would not be expected to be greatly different from 5 A. Since Dll2 depends primarily upon the diffusion coefficient of the substrate and since the diffusion coefficient for spherical molecules varies with M-‘/a, Dl2 will have nearly the same magnitude for many enzymatic reactions. However, the electrostatic factors may be considerably different for different reactions. Some substrates, like adenosinetriphosphate, have high charges and some enzymatic sites may be highly charged. I n cases where the bimolecular reaction is diffusion-controlled, considerable difficulty is involved in measuring the rate by flow techniques. If the enzyme concentration is low in comparison with the initial substrate concentration, (S)o, the half-life for the transient state of the enzymatic reaction would be 0.7 X sec. for (S)o = 10-8 A4, sec. for (S)O= M for k = 109 and 0.7 X set.-' M-l. The oxidation of ferrocytochrome-c by hydrogen peroxide as catalyzed by yeast cytochrome-c
159
peroxidase has been studied by Chance.a4 He obtained a rate constant of 1.2 X 108 sec.-1 M-1 for the rate of formation of a ternary complex between ferrocytochrome-c and the peroxidase-peroxide complex. He was unable to account for this large reaction rate using collision theory and assuming only iron-iron collisions are effective in producing a reaction. Using equation 4 with a solid angle of 2a and estimating the other parameters as D12 = em., then k = 10-6 set.-' and Rlz = 5 X 2 X lo8 sec.-l M-I. The assignment of this value of the reaction radius is rather arbitrary since although only the iron atom of the substrate is involved in this reaction, steric and electrostatic effects are probably present which cannot be evaluated. However, according to this calculation, the reaction appears to be diffusion controlled. Gibson and R ~ u g h t o nhave ~ ~ obtained a rate constant of 4 X 108 sec.-l M-’for the combination of hemoglobin and nitric oxide. This is the fastest directly measured reaction rate of an oxygen-carrying pigment with any ligand. Using equation 4 with solid angle 2n, DI2 = 10-6 ems2 sec.-l and Rlz = 5 X cm., the theoretical rate constant is calculated to be 2 X 109 set.-' 214-’, which is in reasonable agreement with the experimental value. I n conclusion, the proposed method of accounting for the rate of formation of enzyme-substrate complexes gives reasonable results for the three types of complexes considered, namely, those between enzyme and charged substrate, enzyme and protein substrate, and enzyme and neutral substrate. (34) B. Chance, ”Enzymes and Enzyme Systems,” J. T. Edsall, ed., Harvard University Press, Cambridge, 1951, 1.1. 93. (35) Q. H. Gibson and F. J. W. Itoughton, J . Physiology, 136
(1957).
1NTER.ACTION OF ANIONIC DETERGENTS AND CERTAIN POLAR ALIPHATIC COMPOUNDS I N FOAMS AND MICELLES BY W. M. SAWYER AND F. M. FOWKES Shell Development Company, Emeryville, Calvomia Received July 11, 1067
The addition of non-ionic surface active compounds to solutions of a variety of anionic detergents has been shown to enhance foam stability. The effectiveness of such compounds depends strongly on the detergent. It is found that detergents increase in “susceptibility to foam stabilization” in the order (1)branched alkylbenzene sulfonates, (2) n-alkylbenzene sulfonates, (3) secondary alkyl sulfates, (4)2-n-alkane sulfonates and (5) primary alkyl sulfates. This is also the order of increasing surface tension of the detergents (without additives) a t concentrations greater than the critical micelle concentration (CMC). The olar aliphatic additives with straight hydrocarbon chains of 8-14 carbon atoms were the more effective foam-stabilizers anzamong these effectiveness increased in the order (1) primary alcohols, (2) glycerol ethers, (3) sulfolanyl ethers, (4) amides and (5) N-polar substituted amides. This order is, in eneral, the order of increasing surface activity and CMC-depress!ng activity. Monolayer com ositions calculated from surface tension and CMC measurements show that in the mixed monolayer there is negligible specid interaction between detergent and additive molecules, and that increasing foam stability in a series of detergent-additive pairs corresponds to an increasing mole fraction of additive in the adsorbed monolayer. The most stable foams were found with detergent-additive pairs having SO-SO% of additive in the adsorbed monolayers. The requirements for preferential adsorption of additives are the same as found for foam stability: solutions of the detergent (without additive) should have a high surface tension, the additive should give water a low surface tension, and should depress the CMC of the detergent. The mechanism of CMC depression by additives is discussed, and the importance of the distribution of additive between sites on the surface of a micelle and elsewhere in a detergent solution is pointed out.
Introduction Of Of Studies Of the surface ‘Odium dodecyl have shown that the presence of dodecaiiol reduces the Surface tenSiO11,
increases the surface viscosity, increases the foam (1) A. P. Brady, THISJOUBNAL,63, 56 (1949); ”Monomolecular Layers,” Am. Assoo. Adv. Sci., Washington, D. C., 1954, p. 33. (2) G. D. Miles and L. Shedlovsky, THISJOURNAL,48, 57 (1944).
W. M. SAWYER AND F. M. FOWKES
160
stability4and decreases the rate of liquid drainages from thin lamellae. Alcohols decrease the critical micelle concentration of long chain colloidal electrolyteso-8; those of sufficient chain length are apparently solubilized by becoming part of the surface layer of the micelle in contrast to hydrocarbons which are solubilized into the interior of the mic e l l e ~ . Solutions ~ ~ ~ ~ of mixed micelles have an increased capacity t o solubilize non-polar oi1s.l1 The analogy between the monolayer penetration studies of Schulman and collaborators12 and the formation of mixed micelles has been pointed out by Harkins6s9and recently emphasized by Stigter.21 Recently Schick and Fowkesla have shown that a large variety of amphipathic molecules reduce the critical concentration of various detergents. The adsorbed monolayers on solutions of sodium dodecyl sulfate containing dodecanol are believed t o consist almost entirely of dodecanol because the surface tension and surface viscosity of the solutions are nearly the same as for monolayers of dodecanol. It is proposed that this preferential adsorption of dodecanol is the reason for the remarkable foam stability of this detergent-additive pair, and consequently we have investigated the monolayer composition and foam stabilities of a number of other detergent additive pairs.
Vol. 62
where Nai is the partial molar area of the component i in the monolayer, p i is the activity coefficient of component i in the monolayer ut constant surface tension, and fi is the activity coefficient of component i in the solution. In order to determine xi values, the equation 1 must be integrated and appropriate reference solutions chosen
We have made the assumption that in the monolayers of detergent, additive, or their mixtures the activity coefficient of water ((PH~o) is so high that ZH,O is negligib,ly small. This appears reasonable if one considers the packing in the monolayer to be limited by the hydrocarbon groups so that the water which would penetrate between the hydrocarbon groups must have a very high pi. This assumption will be justified experimentally later. It follows that in the monolayers adsorbed on solutions of only one surface active component Xio = 1, and with two component,s xi’ = 1 - x2’. The reference solutions are chosen as solutions of detergent without additive, or additive without detergent. Equation 2 may be solved for xipi in various ways: by choosing Ciofio = ci’j’i’ (condition I) the second term is eliminated, and Theoretical by choosing 7io = y f (condition II)I6the third term Calculations of monolayer composition have been is eliminated. Condition I is used for some of the calmade by treating the monolayer as a separate phase culations presented later. It is assumed (and justiaccording to the methods of ButIer,‘4 Schucho- fied experimentally) that the heats of mixing of like witzky,16 Belton and Eva,nslBand Guggenheim.l7 hydrocarbon groups of the detergent and additive The mole fractions in the mixed monolayer of the in the monolayer are so small that pi’ = pio. With detergent (zl‘) and of the additive (22’)are related this assumption the first term reduces to kT In zi. to the surface tension 7 ’ of the solution having con- By taking advantage of the fact that y’ - Tio = centrations c1’ and c2’ of unassociated surface-ac- (yjo - Tio) 4 (7‘ - Yj”), a generalized equation tive detergent ions and unassociated additive for a number of components may be derived from molecules, respectively. By changing the activity equation 2 of either component, a change in free energy of the other is brought about which can be expressed in terms of the change in activity in solution and in the monolayer together with the change in surface tension l4-I8 where dFi = -Nuid?
+ RT d In
zi$i
= RT d In
(3) A. G. Brown, W. C. Thuman and J.
cifi
W. hfcBain,
(1)
J . CoEZoid
Sci., 8,491 (1953).
(4) W. S. Martin, U. S. Patent No. 2,166,314, (5) G. D. Miles, J. Rosa and L. Shedlovsky, J . Am. Oil Chemists’ Sac., 87 268 (1950). (6) S. H. Herzfeld, M. L. Corrin and W. D. Harkins, T H I BJOURNAL, 54, 271 (1950). (7)H. B. Klevens, J . Chem. P h y s . , I T , 1004 (1949). (8) K. Shinodn, THISJOURNAL, 58, 1027 (1954). (9) W. D. Harkins and R. Mittlemann, J . Colloid Sci., 4, 367 (1949). (IO) H. B. Klevens, Chem. Reas., 4’7, 1 (1950). (11) M. E. L. McBain and E. Hutohinson, “Solubilization,” Academic Press, Inc., New York, N. Y., 195.5. (12) E. D. Goddard and J. €1. Schulman, J . CoUoid Sci., 8, 309 (1853). (13) M. J. Schick and F. M. Fowkes, THISJOURNAL, to be published. (14) J. A. V. Butler, Proc. Roy. Soc. (London),8 1 3 5 , 348 (1932). (15) A. Schuohowitzky, Acta Phusicochim. U.R.S.S.,19, 176, 508 (1944). (16) J. W. Belton and M. G. Evans, Trans. Faraday Soc.; 41, 1 (1945). (17) E.A. Guggenheim, Qid., 41, 150 (194.5); “Surface Chemistry,” Interscience Publishers, Inc., New York, N. Y..1949,p. 11. (18) J. H. Hildebrand and R . L. Scott, “The Solubility of Nonelectrolytes,” Reinhold Publ. Corp., New York, N. Y., 1950, pp. 406-413.
Equation 3 also may be evaluated for xi’ and x2‘ by assuming XH*O = 0 and that pif = vi: and p2’ = pzO. The reference solutions are again chosen so that q 0 = c1’ and c2O = cZf under such conditions The that one can assume fi’ = fi’ and faD = f2’. surface tension of the mixed system (7’) may now be calculated from the values of z1 or x2 by using equation 2 with condition I. Several such calculations are illustrated graphically in Fig. 1 for additives with 6 2 30 and for detergents havingoc, (sodium dodecyl sulfate) or 52 A.2 values of 37 (sodium propylene tetramer benzene sulfonate) for various values of y10 - yz0. These values of 31 are obtained by Gibbs’ adsorption equation using the surface tension data of Fig. 5. The upper graph of Fig. 1 shows the surface tension of the detergent-additive system should always , a maximum debe lower than either 710 or 7 ~ 0with crease in surface tension when yl0 = y2O. The
Feb., 1958
THE INTERACTION OF ANIONICDETERGENTS
lower graph predicts the composition of the adsorbed monolayer, It shows that if 7 2 0 (for additive) is lower than yl0 (for detergent) the monolayer is composed mainly of additive. Experimental Techniques.-Foaming properties of a given detergent solution are necessarily characterized by a t least two parameters; one specifying the ease of foam production under specified conditions and one specifying the persistence of this foam under quiescent conditions. Shaking the detergent solution in o cylinder and observing for 20 minutes the decline in foam volume is a simple procedure satisfying these conditions. The necessity of two parameters is illustrated in Figs. 2 and 3. The sodium propylene tetramer benzene sulfonate exhibits high initial foam volume and rapid decline while the sodium 2-n-hexadecylbenzene sulfonate shows low initial volume and a slow decline with time. In addition the level of foam performance with this detergent depends on the energy expended in producing the foam. These differences and corresponding variations in the response of a given detergent to various organic additives probably represent significant differences in the properties of the adsorbed films stabilizing the foam lamellae. However, for measuring the effect of various compounds in several detergents, the final (20 minutes) foam volume is most useful. The initial foam volume ( V O ) and the final foam volume ( 0 2 ~ ) are reported as O ~ / U Z Oto permit a comparison of both volume and persistence where pertinent. The experiments were performed with 25 ml. of detergent solution in 100-ml. glass-stoppered graduates along with 2 square inches of standard "soiled cloth"'9 cut into 8 equal pieces. The four cylinders used in each experiment were equilibrated at 55', removed singly from the water-bath, shaken for a standard period and immediately returned to the bath. The decline in foam volume in the open cylinders was observed for 20 minutes. The hand shaking was on the axis of the cylinder for 10 seconds and always done by the same operator in a standard manner. Shaking was provided by a mechanical agitator operating a t a rate of 5 oscillations per second along a path of seven inches. The results were more reproducible with mechanical shaking. The effect of shaking time could be readilyzassessed and results were obtained for 10, 30, 60 and 120 seconds shaking time. Except for the deviations noted, the final foam volume did not depend on shaking time, although this was often not the case for the initial foam volume. An analysis of variance of replicate data obtained at these shaking times defined the range in which a foam promoter could be considered effective. However, generally the same result was obtained using the data with shaking time of 60 seconds. The 95% confidence limits for the base detergent solution shown In the tables were obtained from the complete data. Only values from 60 seconds shaking time are reported, however. Solutions were made from a concentrated stock solution of detergent and sodium sulfate. Hard water, to give 350 p.p.m. Ca as carbonate in the h a 1 solution, and sodium tripolyphosphate were added just previous to testing. The diluted solutions were added to a weighed amount of additive to give a solution 0.4% of total solids consisting of 18% detergent, 27% sodium sulfate, 51 % sodium tripolyphosphate and 4% foam promoter. This ratio of ingredients is designated as the standard composition. For reference solutions sodium sulfate was substituted for foam promoter. Solutions were heated but not boiled to dissolve the organic additive. The majority of organic additives and detergents were synthesized in these laboratories. Others were used without purification. The surface tensions were measured at 54-55' with the Wilhelmy hanging slide technique, using a glass microscope slide. The dish and a surrounding glass shield were jacketed and water at 55' was circulated through these to allow surface tension measurements at the same temperature as the foam stability tests. The spreading pressure ( r e )of additives was measured with the same apparatus. Several additions of droplets or crystals were made to assure equilibrium values.
Experimental Results The influence of a variety of simple alcohols on the foam performance of several detergents is shown (ID) General Dyestuff Corporation.
Q,
161
-37
R
a"
-10
-20
-30
-10
D
t 10
0
O
Yt-Yi
a
Fig. 1.-Relation between monolayer composition and surface tensions of detergent-additive solutions, 55". A -10. 100
90
L
I
5
I
10
I
I
I
15 5 T i m e , Minutes,
1
10
I 15
I
20
Fig. 2.-Foam volume vs. time for propylene tetramer benzene sodium sulfonate in standard composition a t 55'; shaking time seconds, 0 , 120; 0,60; 0 , 30; A, 10.
in Table I. With the exception of the highly branched C16-alcohol,all increase the final foam volume of the sodium dodecyl sulfate solutions in standard composition. These data are not sufficiently precise to establish a quantitative order of effectiveness for the various compounds. Apparently alcohols which improve the final foam volume are not limited strictly to those containing a straight hydrocarbon chain. The alcohol containing a cyclohexane ring, 4-n-octy1cyclohexano1, is essentially as effective as dodecanol. However, the introduction of branching in the hydrocarbon destroys the foam promoting property of the compound. Several of the simple alcohols are also effective when a mixture of sodium secondary CB-C18alkyl sulfates (SAS) is substituted for sodium dodecyl sulfate in the detergent composition.
W. M. SAWYER AND F. M. FOWKES
162 A-10.
I 5
!O
II
1 5 Time, Minutes, I
15
I 10
I 15
I
Fig. 3.-Foam volume us. time for 2-n-hexadecylbenzene sodium sulfonate in standard composition at 55'; shaking time, seconds, 0, 120; 0, 60; 0,30; A, 10.
Vol. 62
in foam performance. Among the glycerol ethers, a-(p-t-octyl phenyl) glycerol ether increases the final foam volume for sodium dodecyl sulfate solutions but not for SAS solutions, although a-(ndecyl) glycerol ether causes an appreciable increase in foam volume for each of the three detergents. The foam performance of SAS with hexadecyl sulfolanyl ether is exceptional; the level of foam performance depends strongly on shaking time and the decline in foam volume i s unusually small. It is clear from Tables I and I1 that the improvement in foam performance in PTBS, particularly as measured by V Z O , brought about by the additives is much less than for the other detergents. Indeed, several of the compounds which greatly increase the v i 0 for sodium dodecyl sulfate or SAS solutions have no influence on the vz0 for PTBS. This detergent places a greater restriction on the nature of the additive required to increase the final foam volume of comparable detergent compositions.
TABLE I TABLE11 INFLUENCE OF ALCOHOLS ON THE FOAM PERFORMANCE OF JNFLUENCE OF ETHERS AND ESTERSO N THE FOAM SOLUTIONS OF VARIOUS DETERGENTS PERFORMANCE OF SOLUTIONS OF VARIOUS DETERGENTS Standard detergent composition, 0.40/,total solids, 55" Standard detergent composition, 0.47,solids, 55", DDSDDS-sodium dodecyl sulfate, SAS-sodium secondary alkyl sodium dodecyl sulfate, SAS-sodium secondary alkyl sulfates sulfates, PTBS-sodium propylene tetramer benzene sulfo- PTBS-sodium propylene tetramer benzene sulfonate nate Foam vol., V Q / V ~ O ml. , Alcohol
None n-Decanol n-Dodecanol n-Hexadecanol Dodecanol-2 Tetradecanol-2 Hexadecanol-2 4-n-0cty~cyclohcxanol Highly branched Cisalcohol 95% Confidence limit ?I20 plus
Foam vol. V O / V Z O , ml. DDS SAS PTBS MaMaMaHand chine chine chine
40/1
53/44 40/33 29/22 44/35 45/28 45/33 40/33
73/10
63/26
75/59
73/45
80/18 75/24 70/19 67/12 67/12
65/55
73/18
29/2
9
* 1
With sodium propylene tetramer benzene sulfonate (PTBS), on the other hand, these alcohols do iiot increase the final foam volume and in some cases even reduce it. Insofar as foam stability can be related to drainage rate in single foam lamellae, this result agrees with those of Miles, Ross and Shedlovskyl who found that alcohols which impart slow draining characteristics to foam lamellae of sodium dodecyl sulfate and sodium 1auroyl-Nmethyl taurine solutions did not similarly influence sodium alkyl aryl sulfonate solutions. A variety of compounds containing more than a single polar linkage with oxygen were tested to det,ermine their influence on the foam performance of various detergent solutions. As shown in Table I1 the final foam volume of solutions of sodium dodecyl sulfate and SAS was increased by a large variety of compounds. Further, as in the case of the simple alcohols, compounds which increase the final foam volume of solutione of these detergents do not have a comparable effect on solutions of PTBS. However, sodium dodecyl sulfate and SAS are not equivalent detergents with respect to improvement
DDS
Compound None a-(n-Octyl) glycerol ether a-(n-Decyl) glycerol ether a-(n-Dodecyl) glycerol ether a-(p-t-Octyl phenyl) ether Glycerol monochlorohydrin octyl ether Glycerol monochlorohydrin dodrcyl ether Decyl 3-sulfolanyl etlier Hexadecyl 3-sulfolanyl ctlier Glycerol inonocaprate Glycerol monolaurate Pontaerythritol monocapratc Pentaerythritol nionolaurate Dodecanediol-l,2 95% Confidence limit, u z o plus
Hand 40/1
60/43
Machine 73/10
SAS Machine 63/20
100/70
95/57
90/26
73/28
45/30 40/22 55/38 25/25 50/37 37/30
PTBS Machine
SO/ 18 90/31 90/32 78/30 83/25 73/27
59/32 72/55 59/53 72/50 79/58 100/65 79/63 9
68/23 72/27 64/19 100/28 95/27 69/15 85/28 85/26 7
The results presented in Table 111 indicate the improvement in foam performance of various detergent solutions upon the addition of amides. With both SAS and PTBS the value of v20 increases with increasing chain length. The effect of chain length on CMC lowering has been discussed by Schick and FowkesI3 and the results here suggest that the amides with longer hydrocarbon chains are more effective foam stabilizers as well as more effective in lowering the CMC. Polar substituents on the amide nitrogen increase the v20 for PTBS over that of the unsubstituted amides. This effect is less pronounced with SAS as the detergent. The results with N-(3-sulfolanyl) lauramide show clearly that the large increase in final foam volume found with sodium dodecyl sulfate and PTBS is also obtained with other detergents. Although the results reported in Tables 1-111 are based on a single concentration of organic additive, similar results are obtained a t reduced concentration of foam promoting additive as shown in Fig. 4. The general level of foam performance depends on
THEINTERACTION OF ANIONIC DETERGENTS
Peb., 1958
163
TABLE I11 FOAM PERFORMANCE OF SOLUTIONS OF VARIOUS DETERGENTS Standard detergent composition, 0.4% total solids, 55’, DDS, sodium dodecyl sulfate, SAS, sodium secondary alkyl sulfates, PTBS, sodium propylene tetramer benzene sulfonate, OBS, sodium 2-n-octylbenzene sulfonate, DDBS, sodium 2-ndodecylbenzene sulfonate, HDBS, sodium 2-n-hexadecylbenzene sulfonate, TDS, sodium tetradecane-2 sulfonate.
INFLUENCE OF AMIDESO N
THE
Foam vol., V D / V D Y , ml. TDS
Amide
DDS
SAS
PTBS
None Octanamidc Decanamide Dodecanamide N-( 3-Sulfolanyl) lauramide N-( 2-Hydroxyethyl) lauramide
73/10
63/26 72/22 90/60 90/70 95/64 77/58
80/18 85/15 75/28 82/32 83/48 87/48
87/72
the concentration of active material. A t the same ratio of components as present in the standard composition, but using 0.2% total solids in solution, the results were quite comparable to those obtained at 0.4% total solids. The data of Tables 1-111 indicate that detergents vary markedly in “susceptibility to foam promotion.” This is shown more clearly in Table IV where, because the nature of the foaming test requires a statistical treatment, the organic additives have been classified according to their effect on vzo. Compounds which appreciably increase u20 for PTBS always result in a large increase in vzo for SAS and sodium dodecyl sulfate. Similarly compounds which increase v20 for SAS only slightly may have a large influence on v20 for sodium dodecyl sulfate. Thus the order of increasing “susceptibility to foam promotion” for detergents is propylene tetramer benzene sulfonate, secondary alkyl sulfates, and primary alkyl sulfates. Similarly, the order of increasing effect on z120 for various polar groups attached to straight hydrocarbon chains is established in Table IV as primary alcohols, 1,2-diols, glycerol and sulfolanyl ethers, amides, and N-polar substituted amides.
OBS
DDBS
HDBS
73/16
11/0
77/35
27/25
88/68 80/57
83/53
81/70 95/58
52/42 46/38
In Table IV are also listed some surface tension measurements (obtained at 54-55’) pertinent to calculations of the composition of the monolayer. It can be seen that the detergents increase in surface tension (n)with increasing “susceptibility to foam promotion”. (Here y1 is the surface tension of detergent solutions without additive at concentrations in excess of the CMC.) The effectiveness of additives is also seen to increase with decreasing values of yz0(satd.), the surface tension of saturated solutions of additives without detergent.
0;
E
70
10
0 N- (3-Sulfolanyl) Lauramide 0 N- (2-Hydroxyethyl) Lauramide 0 a ( n-Decyl) Glycerol E t h e r
-
L
TABLE IV 1 2 3 4 STATISTICALCLASSIFICATION OF THE IMPROVEMENT IN Additive Concentration, % Total Soltds. FOAMPERFORMANCE OF SOLUTIONS OF DETERGENTS BY Fig. 4.-Final foam volume vs. additive concentration VARIOUS COMPOUNDS Standard detergent composition, 0.4% total solids, 55”, for propylene tetramer benzene sodium sulfonate in standard DDS, sodium dodecyl sulfate, SAS, sodium secondary alkyl composition, 55’. sulfates, PTBS, sodium propylene tetramer benzene sulfoTABLE V nate. A, B, C indicate large, moderate and slight improvement in foam performance: SURFACE TENSION OF DETERGENT SOLUTIONS CONTAINING DDS SAS PTBS FOAM PROMOTING COMPOUNDS (7‘) Yl = YZO Yl = 71 = Compound (Sat’d.) 35.5 30-31 28.4 Standard detergent composition, 0.4% total solids, 5455O, DDS, sodium dodecyl sulfate, PTBS, sodium propylene N-( 3-sulfolanyl) lauramide 25.7 .. A A tetramer benzene sulfonate A A A N-(2-hydroxyethyl) lauramide 22.8 DDS PTBS Decanamide 27.0 .. A B I = 35.5 yi = 28.4, &nes/cm: dynes/om. Dodecanamide .. .. A A Compound Surface tension, dynes/cm. a-(n-Decyl) glycerol ether 21.9 A A A N-( 2-Hydroxyethyl) lauraiiiide .. 24.8 A A B Decyl-3-sulfolanyl ether 22.9 a-(n-Decyl) glycerol ether 24.1 24.5 Glycerol monolaurate 21.4 .. A B Decanol 24.2 24.7 Dodecanediol-1,2 .. A A C Dodecanol 24.1 24.4 Decanol 35 4 A .. c Glycerol monolaurate 29.0 25.1 A Dodecanol 31.4 B C Decyl 3-sulfolanyl ether 24.0 24.6 A .. 4-n-Octylcyclohexanol B C Tetradecanol-2 24.7 .. Glycerol monochlorohydrin octyl ether 37.4 B c c I n Fig. 1 it is predicted that solutions of deterGlycerol monochlorohydrin gents and additives together will have lower surface dodecyl ether 35.9 A c c tensions ( 7 ’ ) than solutions of either component B Tetradecanol-2 35.7 alone (yIo or yzo), and that the composition of the .. c .. B Pentaerythritol nionocnprtlte .. c mixed monolayer may be deterniined from knowl-
W.,M. SAWYER AND F. M. FOWKES
164
54 50
t'
.$
-
-
46-
E
42-
eE
2
v1
3834-
\ ( Na+) = 0. 1 M o l a r
DDS
- PTBS Co= 1. 55 x 26 -
30
Co
1 . 3 5 ~lo-' M o l a r
IO-' M o l a r
I
I
A N- (3-Sulfolanyl) L a u r a m i d e
e J 800 p; 70
26
' E
Fr,
-
60-
-
a ( n - D e c y l ) G l y c e r o l Ether 0 Dodecanol 0 D o d e c a n e d i o l - 1, 2
A --8-- --- - 4
B ~
\
0
50-
30-
E
2010
.
\
\
A
'L0
-
40
A
n
40-
3
t.
ST1
TDS
DDS
I
I
38
36
I
0
DDBS PTBS
I
I
I
Vol. 62
of 90% in the monolayer, when computed by equation 3 for ideal solution behavior in the monolayer. This is closely checked by the 92% calculated with equation 2 which makes no assumptions regarding the ideality of behavior of the additive. Both figures agree well with the experimental value of 95%020obtained by radio tracer study. The calculated surface tension (22.9 dynes/cm.) also agrees with the experimental value (23.0 dynes/cm.). Thus, for this well known system the equations for calculating monolayer composition from surface tensions give very satisfactory results. This appears to justify the assumptions that the water content of the monolayer may be ignored and that
54'. Some approximations are involved; the CMC lowering shown for PTBS was actually measured with a single chemical species (sodium 2-ndodecylbenzene sulfonate), and 7.2 (satd.) values were measured on distilled water rather than on hard water with added builders. Nevertheless, with the alcohols and PTBS good agreement is found between calculated and experimental surface tensions and between surface area fractions calculated by the two equations. I n the case of the alcohols with sodium dodecyl sulfate the experimental surface tensions are lower than calculated. Although this might mean that the activity coefficients in the mixed monolayer are 0.7-0.8, or that 01 is less when 1c1 is small, this seems unlikely
Feb., 1958
THEINTERACTION OF ANIONIC DETERGENTS
165
tensions for the mixed monolayers were higher than y#(satd.), indicating that the solutions Sodium Dodecyl Sulfate and Dodecanol were not saturated with additive, and thus A g g r e g a t i o n N u m b e r = 80 equation 3 could not be used. However, values for the detergent content of the monolayer can still be calculated by equation 2 and are shown in the table. A comparison of the values of x’1 by equa- 5 tion 2 from Table VI with foam stabilities of Table IV shows good correlation; the mono60 layers in the more stable foams always have a smaller detergent content (and, consequently, a greater additive content). Accepting this 80 as a criterion for foam stability, more stable foams should be found with detergent-additive loo 01 pairs in which (1) the detergent solution has 0 0.2 0.4 0. 6 0.8 a high surface tension y1 (at concentrations XA, Mole F r a c t i o n Alcohol in M i c e l l e . greater than the CMC); (2) the additive is Fig. 7.-Calculated reduction in CMC by mixed micelle effective in lowering the CMC (thereby raising formation at constant aggregation number. ylo) ; and (3) the additive must be highly surface active and have low water-solubility so that 7 2 0 has more difficult. a low value. Such conclusions are further supThe work presented in this paper confirms emported by the foam stability studies of the previous pirically the original notion that the foam stability section. It is shown in Fig. 6 and Table IV that of detergent solutions containing long chain nonthe foam stability (as measured by V Z O ) of detergent- ionic polar additives results from having a preadditive pairs has the predicted dependency on the ponderance of additive in the adsorbed monolayer. surface tension y1 of the pure detergent above the However, we have no proven physical chemical CMC, becoming less stable with detergents having basis for this correlation. One may well wonder low values of yl; of the detergents tested PTBS has why a high proportion of such additives in the monthe lowest y1 and lowest foam stability. This fig- olayers results in more stable foam. Since 8 2 is ure and table also show that as we shift to deter- less than al, films with a high proportion of additive gents of lower y1 values there develops a wide have the hydrocarbon chains more closely packed spread in the foam stabilizing ability of additives, together, and this should result in lower compressiwith the predicted better performance of additives bilities and higher surface viscosity. At 25O, for having low yz0values. (Though y1 values in Fig. 6 instance, the surface viscosity of solutions of sowere obtained a t 25O, we have found these to be in- dium dodecyl sulfate with dodecanol was found by dependent of temperature and salt concentration, Brown, Thuman and McBainS to be 41 to 80 surso they may be used to predict foam performance a t face millipoise as compared with 2 surface milli55’ in the standard composition.) poise for solutions of detergent alone. I n these The relation of surface tension of detergents to laboratories Dr. M. J. Schick, using similar apparatheir “susceptibility to foam stabilization” has tus, obtained values of 101 and 2 surface millibeen explored for a wider variety of detergents with poise, respectively. However, a t 55’, the surface a few additives. Detergents may be referred to by viscosity of pure monolayers of dodecanol, and of classes, since the surface tension above the CMC solutions of sodium dodecyl sulfate with dodecanol is nearly independent of chain length. Straight were the same as for solutions of the detergent chain alkylbenzene sulfonates, with surface ten- alone, 2 to 3 surface rnillipoise. This confirmed the sions of 30-30.2 dynes/cm., are found to be inter- film drainage experiments of Miles, Ross and Shedmediate in susceptibility between PTBS (a highly lovsky6 which showed that the effect of dodecanol branched chain alkylbenzene sulfonate) and SAS in retarding film drainage of sodium dodecyl sul(mixed secondary alkyl sulfates). Some data for fate solutions disappears above 45 ’. While some sodium 2-n-dodecylbenzene sulfonate (DDBS) are effect of the closer packing in the additive-rich shown in Fig. 6. Similar results were obtained monolayers a t 55’ may be expected, it is not maniwith two different commercial kerylbenzene sulfo- fested by an increase in surface viscosity measurnates, in which the alkyl groups are essentially un- able with our surface viscometer. branched. The same effect of branching of the alThe foam stabilization for detergents of low surkyl group has been illustrated with mixed secondary face tension is dependent on an additive’s ability to alkyl sulfates. An essentially unbranched crystal- lower the CMC (cf. equation 2). Since the compoline fraction was found to have higher surface ten- sition of the surface film has been calculated, it is sion than the whole mixture (34.5 us. 30.5 dynes/ relevant to attempt a calculation of the composicm.) and to be nearly as susceptible to foam stabili- tion of the mixed micelles in equilibrium with this zation as sodium dodecyl sulfate. It is remarkable monolayer. This has been done using the theory of that the surface tension y1 of detergent solutions Overb,eek and StigterZ1to calculate the electrical (above the CMC and in the absence of other sur- part of the free energy of micellization, Fe1. The face active agents) appears to be a function of area charge density is assumed to be reduced a t constant per molecule (81) so that increased branching (21) J. Th. G. Overbeek and D. Stjgter, nec. trau. chim., 76, 1263 increases 81, lowers 71, and makes foam promotion (1956).
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HANSJ. BORCHARDT
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where K is a distribution coefficient such that KC#zO = C A =~ moles alcohol in micelles per liter of solution, C:Hpo, moles alcohol in water per liter of solution, and C'd is the CMC in moles detergent per liter of solution. I n the range where the CMC lowering is approximately linear with %A, C'd is 1 1 (AFO + AF~") - 5 linear with KC,"' as is observed experimentally. - ln,fnx,, - - In n - Infizl = When the aqueous phase is saturated with alcohol n n kT kT a CMC, the data of Schick and Fowkes13 can The results in Table VI1 were obtained with aggre- bet the used to estimate the CMC to be roughly 0.4 gat,ion number n = 80 ( 2 5 O ) , f1 = f n = 1, AFOjkT times the CMC of pure detergent. This gives rise = -12.21, and AFm/kT = -In (1 - ZA), where to a distribution coefficient of about 1.8at the CMC ZA is the molecule fraction of alcohol in a micelle, (as defined), taking the solubility of dodecanol in x1 the mole fraction of free detergent, and z ~ the , water as 0.9 X moIe/liter. If the logarithmic mole fraction of micellized detergent. The critical relation between x1 and X A is used directly the exconcentration is defined such that XI = 0.98xT perimentally observed linearity of CMC and alcowhere XT is the mole fraction of total detergent. The hol concentration is not predicted except by linear reduction in critical micelle concentration with in- approximation of (1 - XA)3.56. A similar result was creasing 2.4 is shown in Fig. 7. This relation is obtained by ShinodaZ2using the less exact treatment roughly linear to a CMC lowering of about 60%. of the electrical contribution to the micellar free The data are represented over a greater range by energy as indicated by Hobbs.2a log 21 = log XIo 3.55 log (1 - 2.4). This procedure for obtaining F,1 is admittedly approximate insofar as the model for decreasing TABLE VI1 the change density may be an oversimplification. A N D ENTROPY CONTRIBUTIONS TO THE FRFE ELECTRICAL ENERGY OF MIXEDMICELLES OF SODIUM DODECYL SULFATE However, the calculations clearly indicate that the mole fraction of alcohol in the micelle is considerA N D DODECANOL AT CONSTANT AGGREGATION ably smaller than that in the mixed monolayer, as CMC, mole fraction, x 0 x 104 is found experimentally. To a large extent, this *Frn = Mole fraction difference is duo to the fact that the additive reduces aloohol in -_ AFT* kT micelle, X A Fel/kT" kT A F ~= 0 In(1 - X A ) the free energy of micelle formation much more by 3.44 0 1.55 1.55 0 decreasing the electrical work than by increasing 3.43 0.0127 1.54 1.52 0.0125 the entropy of mixing in the micelle (see Table VII). micelle diameter and aggregation number; this process can be imagined to occur by simple replacement of a lauryl sulfate ion with a lauryl alcohol molecule; a further decrease in free energy of mixing AFmof the detergentz2is included. The equation of Overbeek and Stigter then becomes
+
I
.0625 .I25 .3125 .50 -75 a Corrected 21).
3.28 .0644 1.32 1.23 3 . Io .1334 1.10 0.96 2.48 .3742 0.58 .40 1.72 ,6914 .27 0.40 1.3848 .07 ,017 for smooth charge distribution (see reference
The relation between the mole fraction alcohol in the micelles and the alcohol concentration in the mater is given by
After this paper was submitted for publication, B pertinent article by A. Wilson, M. B. Epstein and J. Ross appeared in J. ColZoid Sci., 12, 345-355 (1957). Using solutions of sodium dodecyl sulfate containing 2.5% of dodecano1 a t 25-28", they measured x1 to be 0.49-0.57. We calculate yIo = 39.5 dynes/cm. and from reference ( I ) y' = 32.5 dynes/cm. in 1 second, 29.0 dynes/cm. in 5 minutes. Using equation (Z), we find xlql = 0.54 and 0.40, respectively.
Acknowledgment.-The authors wish to express their appreciation to Miss Helen L. Robbins, Miss June R. Hughes and Mr. S. J. Rehfeld for assistance %t,h measurements and calculations. (23) M. E. Hobbs, ibid., 65, 675 (1951).
(22) I(.Shinoda, THIXJ o w n ~ a 68, ~ , 1136 (1986).
A NEW HYDRATE OF SODIUM CHROMATE BY HANSJ. BORCHARDT Contiibz~tionfrom the General Engineering Laboratory, General Electric Company, Schenectady, New York Received July 16, 1967
The dehydration of NazCr04.4H~Owas studied with differential thermal analysis, thermogravimetry and X-ray analysis. Evidence for the existence of a thermodynamically stable intermediate hydrate having the composition Na2Cr0~.1.5Hz0is presented. Its position in the phase diagram of the Na2CrO4-hydrate system is discussed.
Introduction Inorganic hydrates were investigated intensely at the turn of the century. Studies of the Na2Cr04-hydrate system led t o the finding that hydrates with ten, six and four moles of water per mole of NazCr04 occur. A dihydrate was reported by Wyrouboff ,I but the existence of this compound
was disputed by Traube2 and R e t g e r ~ . ~At present, no text or reference work recognizes the existence of a NazCr04-hydrate with less than four moles of water. (1) E. N. Wyrooboff, Bull. 80c. franc. miner., 18, 50 (1879). (2) H. Traube, 2.Krist., 22, 138 (1894). (3) J. W. Retgers, Z . phyaik. Chem., 8 , 47 (1891),
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