738
SELWYN J. REHFELD
Adsorption of Sodium Dodecyl Sulfate at Various Hydrocarbon-Water Interfaces
by Selwyn J. Rehfeld Shell Development Company, Emeryrille, California
(Received September 2, 1966)
This paper presents interfacial tension data for the adsorption of highly purified sodium dodecyl sulfate from aqueous solutions at the air-water and water-n-hexane, n-octane, n-nonane, n-decane, n-heptadecane, 1-hexene, 1-octene, cyclohexane, cyclohexene, benzene, n-butylbenzene, or carbon tetrachloride interfaces. The influence of these organic liquids upon the critical micelle concentration and the adsorption isotherms is described. Also, it is demonstrated that plots of interfacial tension vs. the logarithms of surfactant concentration can be fitted by a polynomial of second degree below the critical micelle concentration. The derivative of this empirical equation is substituted into the Gibbs adsorption equation and an adsorption isotherm of the form r- = (z1/2RT) (z2/RT)In m+ is obtained for all the interfaces, where z1 and 22 are constants.
+
Introduction
Experimental Section
Although a number of investigatio~is’-~ have been reported in the literature concerning the adsorption of sodium dodecyl sulfate (NaDDS) at hydrocarbonwater interfaces, only n-decane,2 n-heptane (at 50°),3 and petroleum ether4 have been used as the hydrocarbons. The critical micelle concentration (cmc) of the NaDDS used in these experiments was 7.95,2 8.1 (at 50°),3and 8.04mM. The adsorption isotherms computed for the n-decane- and petroleum ether-water interfaces were similar, and when the bulk concentration of NaDDS reached the cmc, the area occupied per adsorbed h’aDDS molecule at these surfaces was 48 A2.214 Interfacial tension data for the adsorption of an impure S a D D S (cmc -7 mM) a t selected organic liquid-water interfaces‘ (benzene, cyclohexane, and chlorobenzene and nitrobenzene) suggested that the adsorbed film mas considerably more expanded a t benzene- and cyclohexane-water interfaces than a t the airwater interface. For the chlorobenzene-water interface the film properties were closely similar to those on water and a t the nitrobenzene-water interface the surface film appeared to be more condensed. The cmc of NaDDS in the presence of these organic liquids was not reported. The present investigation represents an extension of these earlier studies in an attempt to explore further the dependence of the cmc and adsorption isotherms upon the addition of various organic liquids.
Materials. The organic liquids were research grade and exceeded a purity of 99.5 mole %. All the organic liquids were doubly distilled before using. The nalkanes, cyclohexane, and cyclohexene were passed repeatedly through columns of silica gel until no absorption occurred in the spectral region of 220-350 mp as measured by a Cary 14 spectrophotometer. The water used in these experiments was triply distilled. Interfacial Tension Measurements. Interfacial tension measurements were made a t 25.0 0.l oemploying the drop-volume method ;5 the equilibrium drop volume was obtained within 5 min. The volumes of a t least 10-15 drops were measured for each determination. The surface tension of the various bottles of triply distilled water used was 71.8 0.1 dynes cm-I. Interfacial tensions of the purified organic liquids lis. water are given in Table I. These interfacial tension values were found to be in excellent agreement with those given in the literature, zt0.5 dyne cm-I
The Journal of Physical Chemistry
*
(1) E. J. Hutchinson, J. ColEoid Sci.,3, 531 (1948). (2) E. G.Cockbain, Trans. Faraday SOC.,50, 874 (1954). (3) W.Kling and H. Lange, Proc. Intern. Congr. Surface Activity, Znd, London, 1967, 1, 295 (1957). (4) D.A. Haydon and J. N. Phillips, Trans. Faraday SOC.,54, 698 (1958). ( 5 ) W. D. Harkins, “Physical Methods of Organic Chemistry,” Vol. I, A. Weissberger, Ed., Interscience Publishers, Inc., New York, N. Y.,1949.
ADSORPTION OF SODIUM DODECYL SULFATE
65
-
60
-
739
Air-Water Interface
-
‘E0
0 55-
u
5
This Investigation, 25%
0 Data of Brady, 29*C
A Data of Clayfield and Matthewa, 25’C 0 Data of Weil, 25OC
P
6
50-
45
-
40 -
I
>
I
o-‘
I
I
I
I
I
I l l 1
I
I
I
I
I
I
I
10”
1
Figure 1. Surface tension of sodium dodecyl sulfate at the air-water interface.
(references given in parentheses). No literature values were found for the unsaturated hydrocarbons. Preparation of Sodium Dodecyl Sulfate (NaDDS). The sodium dodecyl sulfate used in this experiment was prepared using a method described in the literature6 in which l-dodecanol was sulfated using reagent grade chlorosulfonic acid followed by neutralizing the dodecyl acid sulfate with a sodium salt. (Eastman White Label 1-dodecanol was redistilled in a column with approximately twenty theoretical plates at atmospheric pressure before using. Before distillation, the 1-dodecanol contained 3.5 wt % n-C14and after distillation the impurity was less than 0.3 wt % as determined by gas-liquid chromatographic analysis.) The surfactant was recovered from the aqueous phase by successive extractions with 1-butanol and then crystallized from 1-butanol. The recovered surfactant was then Soxhlet extracted with n-hexane to remove unreacted alcohol after which it was recrystallized from redistilled methanol followed by repeated crystallizations from triply distilled water. The purity of the sodium dodecyl sulfate was determined by measuring the crnc by three different methods: (i) the average of five electrical conductivity measurements a t 25” gave a cmc of 8.25 f 0.03 X mole/l., (ii) the average of three density measurements gave a cmc of 8.20 0.08 X a t 25”, and (iii) surface tension-log concentration curves showed
*
Table I : Interfacial Tensions of the Pure Organic Liquid vs. High-Purity Water Oil-water
interface
n-Heptadecane n-Decane n-Nonane n-Octane n-Hexane 1-Octene 1-Hexene Cyclohexane Cyclohexene Benzene n-But ylbenzene Carbon tetrachloride
Interfacial tenaion,a ergs om-’
53.2 51.7 50.9 50.7 50.5 43.7 31.3 49.6 43.5 34.4 40.1 44.8
51.gC 51.2,c50.2d 50.4,c 50.2’ 50.0d 34. I d 40. l f s u 45.0,’ 43. 7d
Dipole momentsb
0.0 0.0 0.0 0.0 0.0 0.34 0.34 0.2 0.28 0.0 0.35
...
Maximum variations f 0 . 2 erg cm-2. * A. L. McClellan, “Tables of Experimental Dipole Moments,” M. H. Freeman and Co., San Francisco, Calif., 1963. ’ R. Aveyard and D. A. HayD. J. Donahue don, Trans. Faraday SOC.,61, 2255 (1965). and F. E. Bartell, J. Phys. Chem., 56, 480 (1952). * F. Franks “International and D. J. G. Ives, J. Chem. SOC.,741 (1960). J. E. Shewmaker, C. E. Vogler, and E. R. Critical Tables.” Washburn, J . Phys. Chem., 58, 945 (1954).
’
~~
~
(6) E. E. Dreger, G. I. Keim, G . D. Miles, L. Shedlovsky, and J. Ross, I d . Eng. Chem., 36,610 (1944).
Volume 7’1, Number 9 February 1967
SELWYN J. REHFELD
740
50
-
40
-
I
8 P ii
30
b”
20
-
n-Heptadecane
0
n-Decane
0 10
-
- Water - Water n-Octane - Water
0
0 1-Octene-Water
A n-Hexane
- Water
0 1-Hexene
- Water
1
0 lo-‘
I
I
I
I
I
I
I
I 10”
I
I
I
I
I
I l l , lo-’
Concentration of Sodium Dodecyl Sulfate, moles/liter
Figure 2. Interfacial tension a t the hydrocarbon-water interface as a function of sodium dodecyl sulfate concentration.
no minimum and gave a cmc of 8.10 f 0.05 X mole/l. a t 2 5 ” . These values are in agreement with the average of the values reported in the literature for the cmc of “pure” sodium dodecyl sulfate, 8.10 f 0.10 X mole/]. a t 25”.
Results and Discussion Interfacial Tension Measurements of Surfactant Solutions. A plot of the measured interfacial tension at the air-water interface as a function of NaDDS concentration is shown in Figure 1 ; these values are compared with those given in the literature for “pure” T\’aDDS.7-9 The literature values were -1.5 dynes cm-I lower than those obtained in this investigation, suggesting that the KaDDS used in our study was of a higher purity. Plots of the interfacial tension a t the various organic liquid-water interfaces as a function of XaDDS concentration are shown in Figures 2 and 3. (Note: The average standard deviation was 1 0 . 3 dyne cm-l.) Interfacial tension data given in the literature for the adsorption of NaDDS at the ndecane-water interface2 were also -1.5 dynes cm-’ lower, undoubtedly due to the impurities in the NaDDS. Interfacial tension/ln concentration data were fitted to the Taylor series using multiple regression analysis ; a polynomial of second degree was found to be adequate; thus CT
= xo
+ z1 In vzk + z2 ln2 mi
The Journal of Physical Chemistry
(1)
where ??a+ is the surfactant concentration in moles per liter. All calculated values of u fell within the limits of error of the measurements and the sum of the residual squares was considerably less than the values computed for the coefficients. Further terms (cubic, etc.) did not improve the fit; i.e., the sum of the squares exceeded the values computed for the coefficients. Values computed for zo, zl, and z2 and the standard error are given in Table 11. (Note: Literature data for surface or interfacial tension of aqueous nonelectrolytes, weak electrolytes, and strong electrolytes us. natural logarithm of activity also were found to be adequately fitted by a polynomial of a second degree. Examples are given in Table 111.) The interfacial tension measurements were used to determine the cmc of the aqueous NaDDS solutions as well as to compute the amounts of surfactant adsorbed a t the various interfaces. First, we will present the data on the effect of the dissolved organic liquids upon the cmc followed by a section on the computations of the adsorption isotherms. It is of some interest to note that the organic molecules which have ?r electrons gave much lower interfacial (7) A. P. Brady, J. Phys. Chem., 5 3 , 56 (1949). (8) E.J. Clayfield and J . B . Matthews, Proc. Intern. Congr. Surface Activity, Znd, London, 1967, 1, 172 (1957). (9) I. Weil, J . Phys. Chem., 70, 133 (1966).
ADSORPTION OF SODIUM DODECYL SULFATE
74 1
10
10-2
10-3
lo-'
Concentration of Sodium Dodecyl Sulfate, moles/liter
Figure 3. Interfacial tension a t various hydrocarbon-water interfaces as a function of sodium dodecyl sulfate concentration.
Table 11: Coefficients Computed from Fitting Eq 3 Type of interface u s . water
n-Hept sdecane n-Decane n-Nonane n-Octane n-Hexane Cyclohexane Cy clohexene 0ctene- 1 Hexene-1 Benzene n-Butylbenzene Carbon tetrachlo'ride Air
221
dynes om-'
-102.3 13.7 -108.6 15 . 2 -120.413.8 -114.813.3 -139.3 i 5 . 0 -121.3 1 6 . 2 -124.2i4.9 -115.9 1 4 . 2 -133.3 k 7 . 1 -100.013.3 -115.8f6.5 -113.3-1.2.6 -118.0 i 6 . 7
tensions than the n-alkanes whereas molecules with dipoles had only slightly lower interfacial tensions. The decrease in the interfacial tension is probably due t o the strong interaction of the water molecules with the r-electron orbitals,lON1lLe., an induced dipole by polarization of the organic molecule by the water molecule. Critical Micelle Concentrations. The concentrations a t the inflection points in the interfacial tension curves, the cmc, in the presence of the various organic liquids are given in Table IV, column 1. (Note:
-31.5 -32.5 -36.4 -34.1 -41.7 -35.5 -36.1 -34.3 -40.0 -28.4 -34.8 -32.3 -46.6
i1.2 f1.8 i1 . 2 f 1.1 11 . 7 12 . 1 11 . 6
i1.4 i2 . 3 zt 1 . 0 f 2.2
10.8
f2.3
-1.62i0.09 -1.6510.15 -1.99 + 0.09 -1.78=kOo.08 -2.37i0.14 -1.90 1 0 . 1 8 -2.0110.14 -1.85iO.12 -2.5110.19 -1.54iO.08 -2.02i0.18 -1.69iO.06 -2.92i0.20
Substituting the minimum value of u into the empirical expression 1 gave the cmc; see column 2 in Table IV.) For the case of dissolved n-alkanes, the cmc decreases as the chain length is decreased. The introduction of unsaturation into n-Cs and n-Cs reduces the cmc relative to the saturated compounds. A decrease in the cmc due t o the presence of unsaturation in the hydrocarbon is illustrated also by the lower cmc (10) I. M. Goldman and R. 0. Crisler, J . Org. Chem., 23,751 (1958). (11) M.OkiandH. Iwamura, Bull. Chem. SOC.Japan., 33,717(1960).
Volume 71,Number J
February 1967
SELWYNJ. REHFELD
742
Table 111: Coefficients and Standard Errors for the Fit of Surface or Interfacial Tension us. Natural Logarithm of Activity for Various Bqueous Nonelectrolytes and 1: 1 Electrolyte Solutions
Xonelectrolyte (25") 1-Butanol" \Teak 1;1 electrolyte (25") n-Propionic acidb n-Butanoic acidb Strong 1 :1 electrolyte (25") Sodium n-butyrateb Surfactant at petroleum etherwater interface (20') Sodium dodecyl sulfate"
20,
21,
22 9
dynes/cm
dynes/cm
dynes/cm
23.3 f 0 . 5
-18.2 f 0 . 4
-1.7 f 0.1
47.6 4 0 . 3 30.1 f 0 . 4
-12.0 f 0.3 -15.7 f 0 . 4
-1.6 i:0 . 1 -1.5 4 0 . 1
66.4 Ilt 0.4
-3.2 f 0 . 4
-98.6 4 2 . 9
-29.4 f 0 . 9
a W. D. Harkins and R. W.{Wampler, J. Am. Chem. Soc., 53,850(1931). See ref 4.
-0.44 f 0.08 -1.5 4 0.1
* F. M. Fowkes and W. D. Harkins, ibid., 62,3377 (1940).
Table IV : Critical -1Iicelle Concentration, Organic Liquid Solubility in Water, Values of A( -AGs0), n / R T , r-omo, and A'omo Soly
22/RT,O
r-omo,g
A2crnc,'
moles X 1010 cm-l
moles X
area/
1010 cm-8
molecule
20 90
0.65 0.66 0.80 0.71 0.95
3.25 3.39 3.49 3.45 3.67
51.0 49.0 47.6 48.3 45.2
0.04' 0.67'
40 350
0.74 1.01
3.32 2.91
50.0 57.1
7.6 7.1
0. 65b 1.94d
90 170
0.76 0.81
3.43 3.27
48.3 50.8
6.8 6.0
6.8 6.0
0.37" 23. 81e
220 370
0.81 0.62
3.08 2.56
54.1 64.9
6.8 8.2
6.9 8.2"
5.0'
210
0.68 1.17
3.12 3.78
53.1 43.9
(md,
Cmo, m M
A(-AGmo),a
(1)
(2)
mM
oal/mole
n-Heptadecane n-Decane n-Nonane n-Octane n-Hexane
8.5 8.2 8.2 8.1 7.6
8.6 8.2 8.2 8.0 7.7
...
+40
0. 006b 0. llb
1-Octene 1-Hexene
7.9 6.1
7.8 6.1
Cyclohexane Cyclohexene
7.5 7.1
n-Butylbenzene Benzene Carbon tetrachloride Air
...
..,
...
...
...
...
L. Bastin and M. A. C. McAuliffe, Nature, 200, 1092 (1963); also, J . Phys. Chem., 70, 1267 (1966). mo+taken as 8.2 mM. E. J. Farkas, Anal. Chem., 37, 1173 (1965). e R. Bahari and W. F. Claussen, J. Am. Chem. SOC.,73, 1571 Muhs, to be published. H. Stephen and T. Stephen, "Solubilities of Inorganic and Organic Compounds," Vol. I, Macmillan and Co., New York, (1951). N. Y., p 369. These values were computed using the coefficients given in Table 11; therefore, the variations in these computed values could be as large as 6-870 depending upon the standard error.
'
found for cyclohexene. Benzene, which is, of course, the most unsaturated molecule, reduced the crnc to its lowest value. The increase in the cmc found in the presence of n-heptadecam was reproducible, but the cause of this increase has not been resolved. (Note: Hydrocarbon solubilities in aqueous surfactant solutions below the crnc have been shown to be equal to their water s0lubilities.~~-~4 The decrease in the cmc was found to be related to the water solubilities of the hydrocarbons (m,) (see Table IV) ; the larger the water The Journal of Physical Chembtry
solubilities the greater t,he decrease in the cmc with the exception of CCl,.) To our knowledge, these are the first data to demonstrate the effect of dissolved organic liquids upon the crnc of NaDDS. Various investigators, using either (12) M. E. L. McBain and E. Hutchinson, "Solubilization," Academic Press Inc., New York, N. Y.,1955,Chapter 3. (13) G. K. Brashier, Thesis, Louisiana State University, 1964. (14) A. S. Arambulo, Thesis, University of Illinois, 1964.
ADSORPTIONOF SODIUM DODECYL SULFATE
743
the electrical conductivity or dye method, have found that the presence of dissolved benzene, toluene, cyclohexane, n-hexane, n-octane, n-dodecane, n-heptadecane, and n-octadecane in the aqueous solutions of dodecylammonium chloride, l5 potassium carboxylate," sodium alkyl sulfonates,** and potassium dodecyl sulfonates'* caused a decrease in the cmc with the exception of n-heptadecane and n-octadecane. The presence of these two hydrocarbons had no detectable effect upon the rmc. The critical micelle concentrations of these surfactants mere also reduced to their lowest values in the presence of benzene. Recently Nakagawa and Tori, l9 using nuclear magnetic resonance spectroscopy, measured the chemical shift of the benzene signal as a function of X'aDDS concentration in deuterium oxide solutions of NaDDS saturated with benzene. These investigators found that no shift of the benzene proton position occurred below the cmc, but when the surfactant concentration reached the cmc, some of the benzene molecules were incorporated into the micelles; thus the benzene signal shifted.lg The cmc found by this measurement, -6.9 m X , was d s o lower than the cmc in the absence of benzene. Two different models have been used in attempts to describe the micellization process n(monomer)
(micelle).
(2)
One is an equilibrium process obeying the laws of mass actionz0 and the other is a pseudo-phase separation.15 Both of these models were reported to give the same semiempirical thermodynamic expression for the standard free energies of m i c e l l i ~ a t i o n , ~namely, ~-~~ at constant temperature and pressure
AG,'
=
2RT In X*Y*
(3)
efficient of the monomer just a t the cmc (as defined by Phillips).25 As previously pointed out by Herrmann and Benjamin,z1 the standard free energies of micellization obtained according to expression 3 depend on the concentration units used and on the choice of standard state. The solvated monomer standard state was taken as the hypothetical state of mole fraction unit; the micelle standard state was taken as the solvated micelle at mole fraction unit.z1 This process was presented diagrammatically by Herrmann and Benj aminZ1as hydrated monomer (hypothetical)
micelle (hydrated) z(micel1e) = 1
1 //..-.
(4)
hydrated monomer x(monomer) = just at the cmc At the cmc in the ternary system-1 : 1 electrolyte, dissolved hydrocarbon, and water-the hydrocarbons are solubilized in the micelle-water interfacez6and the hydrocarbon core of the micelle.l2 This process may be described by the diagram shown in Figure 4. This model assumes that the hydrocarbon liquid in the micelle interior has the same activity as the excess hydrocarbon liquid at the oil-water interface and that the hydrocarbon dissolved in water is in the same state as the hydrocarbon dissolved in the surface of the hydrated micelle. Thus the standard free energy changes for these processes are zero. The standard free energy of micellization in the presence of organic liquids, AG,', represents in part the change in free energy due to the incorporation of the organic liquid into the micelle. The difference between the standard free energies of micellization in the presence of hydrocarbons
where xi and yi are the mole fraction and activity coHydrated Monomer Hypothetical (X = 1.0)
A G : , ~ . =o
(15) E. Hutchinson, A. Inaba, and L. G. Bailey, 2. Physlk. Chem (Frankfurt), 5 , 344 (1955). (16) A . W. Ralston and R. N. Eggenberger, J . Am. Chem. Soc., 70, 983 (1948). (17) H. B. Klevens, J . Phys. Colloid Chem., 54, 1012 (1950). (18) W. Lim, Bull. Chem. Soc. Japan, 28, 227 (1955). (19) T. Nakagawa and K. Tori, Kolloid-Z., 194, 143 (1964). (20) R. C. Murray and G. S.Hartley, Trans. Faraday Soc., 31, 183 (1935). (21) K. W. Herrmann, J . Phys. Chem., 6 6 , 2 9 5 (1962); L. Benjamin, ibid., 68, 3575 (1964). (22) J. M. Corkill, J. F. Goodman, and S. P . Harrold, Trans. Faraday SOC.,60, 202 (1964). (23) P. Molyneux, C. T. Rhodes, and J. Swarbrick, ibid , 61, 1043 (1965). (24) H. F. Huisman, Verhandel. Koninkl. S e d . A k a d . Wetenschap. Afdel. Natuurk., Proc. Ser. B , 67, 407 (1964). (25) J. N. Phillips, Trans. Faraday Soc., 51, 561 (1955). (26) J. E. Eriksson, Acta Chem. Scand., 17, 1478 (1963).
y wK":'
4
Hydrocarboo ( 1 4.) (Separate P 9 a r e a t Oil-i'iater
Hydrated M o n o m e r
Interface)
-
=
O
Hydrocarbon (rat. a q . )
AG = 0
Figure 4, Diagram of micellization process in presence of organic liquids.
Volume 7 1 , Number 9
February 1967
744
SELWYN J. REHFELD
(AG,') and in the absence of hydrocarbons (AG,'), the reference state in this case, was computed using the expression A(-AG,">
= AGG'
- AG,"
=
2RT In x ~ y ~ / x o * y o i ( 5 ) where A(-AG,") is the change in free energy due to solubilizing the organic liquid in the micelle and x'* and yoi are the niole fraction and activity coefficient a t the cmc in the absence of organic liquids (see Table IV). (Kote: The ratio of the activity coefficients, y*/y'*, since the change in concentration is small, was assumc5d to be unity.) The values of A(-AG,") increased as the n-alkane chain length decreased. Solubilities of the n-alkanes in micelles increase as the n-allcane chain length decreasesI2 (or as the hydrocarbon solubility in water increases) ; thus, the greater the n-alkane solubility in the micelle the larger the decrease in the value of A(-AG,") (see Table IV). This was also true of the other homologous series; e . g . , benzene has the largest solubility in micelles'" and had the largest value for A(-AG,"); whereas n-butylbenzene is less soluble in rnicellesl2 and had a corresponding smaller value for A(AG,"). JTe believe that these changes in the standard free energy arise from structural changes when these organic liquids are solubilized in the hydrated micelle surface and in the micelle interior. Before making any further specdations the adsorption isotherms were computed to determine if a correlation existed between the cmc and the concentration of KaDDS at the organic liquid--water interface when the bulk concentration is at the cmc. Calculated Adsorption Isothenns. The surface "concentrations" of KaDDS as a function of the equilibrium concentration of surfactant in the bulk phase are calculated from interfacial tension data using the Gibbs adsorption equation. The equation for variations in composition in the surface of a 1: 1 aqueous electrolyte solution, considering the surface layer as having a thickness of molecular dimensions a t constant temperature, was derived by Guggenheim*' as
-duI2RT
= r-d In a,
(6)
where r- is the surface "concentration" of NaDDS (anion), c is the interfacial or surface tension, and a& is the mean activity of the ions S a + and DDS-. The term du/d In a* was evaluated by differentiating the empirical expression I, assuming a* % mi, and substituting the derivative into expression 6; thus an expression for the adsorption isotherm is obtained, namely The Journal of Physical Chemistry
-du/dp*
= du/2RT d In a* =
The area per molecule is defined as ( I ' J , J - l ; thus expression 8 has the following form for changes in area per molecule vs. In m& -du/2kT d In a, =
Values of the slope, x2/RT, the amounts adsorbed at the cmc, and the corresponding areas occupied per molecule of KaDDS, A2,,,, are given in Table IV. The value of A 2 c m c computed for the air-water interface of 43.9 A2 is within the range of values reported in the literature of 45 h 5 A2.7-g The value obtained for the n-decane-water interface of 49.3 A2 was in good agreement with the literature value of 48.8 A2.2 The surface films at the benzene-water interface are more expanded (65 A2) suggesting penetration or dissolving of benzene in the surface film.' Surface films of YaDDS were more closely packed a t the n-alkanewater interface and the area per niolecule at the cmc increased as a function of n-alkane chain length, 4551 A2. The surface films of S a D D S a t the unsaturated hydrocarbon-water interface mere more expanded than those at the saturated hydrocarbonwater interfaces. Also, note the expanded surface film in the presence of carbon tetrachloride. Variations of the area per adsorbed NaDDS molecule at the various interfaces a t the cmc are not easily explained without independent data concerning interactions between KaDDS, water, and the organic liquids. The fact that the presence of n-C6 hardly changes the area per adsorbed S a D D S molecule at the interface while the benzene expands the area from 44 to 65 A2 per molecule is interpreted as indicating far larger amounts of benzene are dissolved in the surface film as compared with n-C6. A correlation between the concentration of XaDDS at the oil-water interface and the cmc is observed for each homologous series (see Table IV). For example, as the solubility of the n-alkane decreased, the cmc increased and the concentration of NaDDS at the oilwater interface decreased. I n the presence of the other organic liquids, the cmc increased with decreasing solubility, but the concentration of S a D D S at the oilwater interface increased. This implies that the surfactant film at the micelle-mater interface becomes (27) E. A. Guggenheim, "Thermodynamics," Interscience Publishers, Inc., New York, N. Y . , 1957.
745
ACID-BASEPROPERTIES OF QUARTZ SUSPENSIONS
more expanded as the n-alkane solubility in the micelle decreases, whereas the direct opposite is implied for the other organic liquids. As the organic liquid solubilities in the micelle decrease, the micelle-water interface would be less expanded.
Acknowledgments. The author profoundly expresses his gratitude to Drs. J. W. Otvos, W. C. Simpson, and J. N. Wilson for helpful discussions and encouragement, and to G. V. Seastrom, Jr., for assistance in making duplicate interfacial tension measurements.
Acid-Base Properties of Quartz Suspensions1
by S. Storgaard Jprrgensen and A. Tovborg Jenseii Chemical Laboratory, Royal Veterinary and Agricultural College, Copenhagen, Denmark (Received October 17, 1966)
Narrow particle-size fractions of quartz, vitreous silica, and flint
