ENGINEERING, DESIGN, AND EQUIPMENT
Effects of Surface Active Agents on Extraction from Droplets F. H.
GARNER AND A. H. P. SKELLAND
The University, Edgbaston, Birmingham 15, England
P
REVIOUS papers (8-11)have shown that it is of some importance to discriminate between a change of the interfacial properties produced by the presence of the solute undergoing transfer and that resulting from the addition of specifically surface active agents. According to the sign of the polarity of the adsorbed surface active molecule, the orienting forces already acting on the solvent molecules near the surface may be either strengthened or weakened and may even undergo reversal with increasing concentration of surface active molecules. Johnstone (14) found that addition of “a very small amount” of Terpitol decreased the extraction of acetic acid from isopropyl ether by water droplets to one third of that obtained in the absence of Tergitol. Chu, Taylor, and Levy ( 4 ) found that the rate of extraction in a packed column increased linearly with decrease in interfacial tension owing to the addition of small amounts of highly surface active agents. They show, however, that this is merely because the packing is able to break up the disperse phase into smaller drops when the interfacial tension becomes low. With increasing concentrations of weakly surface active agent, the rate of extraction goes through a maximum and they consider that, at higher concentrations, the diffusional resistance offered by oriented molecules of agent at the interface outbalances the advantages of increased interfacial area. Farmer (6) increased extraction of acetic acid from carbon tetrachloride drops by water, from a given nozzle, by addition of Tergitol Nos. 4 and 7, but concluded that this was entirely due to reduction in drop size. In extraction of acetic acid from benzene drops by water, West, IIerrman, Chong, and Thomas (18) added various alcohols (Cl to Cl6) to the benzene phase, finding substantial improvement in extraction with the lower alcohols (C1 to Ca), but an actual slight decrease in extraction for CIOand Cls alcohols. Thus 2.5% decyl alcohol, with no beneficial effect on extraction, decreased the drop size to the same degree as 2.5% hexyl alcohol, which improved the extraction coefficient sevenfold, Results obtained by Sherwood, Evans, and Longcor (17) for aqueous extraction of acetic acid from benzene drops and those obtained for the same system by West, Robinson, Morgenthaler, Beck, and McGregor (19) and by Farmer (6) showed discrepancies which West, Herrman, Chong, and Thomas (18) later explained in terms of contamination of the benzene-acetic acid solution due to its passage through Tygon tubing. Garner and Hale (’7) showed that trace quantities of Teepol inhibited circulation within nitrobenzene droplets containing aluminum particles and falling in water, while for the aqueous extraction of diethylamine from toluene droplets-a system in which the droplets are internally stagnant-the addition of 0.015% of Teepol to the water phase reduced the extraction rate to 45% of its original value. Two factors, however, limit the conclusions to be drawn from results obtained with commercially prepared wetting agents. First, such agents may contain substantial quantities of inorganic “builders” which may have been ‘added to improve deJanuary 1956
tergency or eliminate corrosion, and, secondly, the formulas assigned to the organic molecules are often only approximate, isomers are present, and there is often a range of molecular weights. These factors obscure the specific effects due to structure and ion sign of the surface active molecules and it was therefore considered desirable to investigate effects of specific agents of known structure and of anionic, cationic, and nonionic types for the system involving transfer of the anionic solute, acetic acid, from nitrobenzene droplets to water. Effects of anionic, cationic, and nonionic compounds were studied
Material Used, Acetic acid contained not less than 99.5% by weight of CHaCOOH. Birmingham tap water was used; analysis showed it to have negligible acidity and a total solids content of 44 p.p.m. The nitrobenzene used had a maximum acidity of 0.05 ml. of IN acid per 100 grams. It was used as supplied, without redistillation. SURFACEACTIVB COMPOUNDS. Anionic. Dodecyl sodium sulfate 1 (sulfated on first carbon atom). Dodecyl sodium sulfate 6 (sulfated on sixth carbon atom). Cationic. Hexadecyl trimethyl ammonium bromide. Dodecy 1 pyridinium bromide. Nonionic. Texofor B1 of representative structure Cl2HZs(CnH0)iaOH. Apparatus. To provide a constant head, a 25-cc. buret was fitted with a tightly fitting rubber stopper through which passed a thin-walled glass tube to admit a slow stream of air to a point within the nitrobenzene-acetic acid solution in the buret. A pointer was attached to the buret tap and a variation of about 9’ on a stationary protractor behind the tap covered a range of drop formation times from 0.5 to 50 seconds. Polythene tubing, which is not noticeably attacked by the solution, was used to attach a fire-polished glass nozzle to the buret. (A 25-ml. portion of a 6% solution of acetic acid in nitrobenzene showed no detectable change in refractive index, determined by an Abbe refractometer, after a 12-day contact with 0.9 gram of tubing.) This is not conclusive-it has been shown (8) that circulation within droplets can be stopped by impurities too dilute to have a measurable effect on interfacial tension, at least when the drop-weight method is used. However, any contamination caused by the polythene should be common to all runs reported here. Similar-sized nozzles were used throughout the work with surface active agents.
Runs 14-17 18-30 31-85
Inside Diameter of Nozzle, Cm. 0.300 0.294 0.284 (0.d. 0.469 cm.)
.
.Runs 1 to 14 were with distilled water as continuous phase and glass nozzles of different inside diameters were used. The
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
51
ENGINEERING, DESIGN, AND EQUIPMENT nozzles were immersed in the continuous phase throughout the work. Drop sizes were known from the volume of disperse phase required to form a given number of drops. Two borosilicate glass extraction columns of about 5-em. diameter were used, with effective heights of 60 and 7.5 cm. At the foot of each column was a 60" conical section, from which drops were immediately withdrawn through a capillary exit via a tap, and collected in a 25-cc. measuring cylinder. Droplet fall velocities were measured by a stop watch and about 5 cc. of disperse phase were passed through the column during each run. For the runs using distilled water and those using solutions of dodecyl sodium sulfate 6, the continuous phase temperature was 19" i:2' C. The solubility of dodecyl sodium sulfate 1 however, increases drastically with only slight temperature increase from a low value (ca. 0.2%) at around 18" C. (Schwartz and Perry, 1 6 ) and to obtain reasonably clear solutions it was necessary to carry out these runs a t a temperature between 19" and 22" C. For similar reasons runs using hexadecyl trimethyl ammonium bromide were carried out between 25" and 28' C. When surface active agent was present it was in the continuous phase, which contained 0.00035, 0.00174, 0.00358, 0.00695, and 0.01736 gram-mole of agent per liter of distilled water. All acid concentrations were estimated with aqueous potassium hydroxide, sufficient ethyl alcohol being added to the nitrobenzene samples to maintain a single phase. Calculations were based on the concentration of the organic phases only. Droplets of the organic phase containing traces of aluminum powder were formed from an open tube. These aluminum particles, when viewed by reflected light, showed clearly whether the droplets were internally circuIating or stagnant for all concentrations of each surface active agent. Experimental Results and Visual Observations, The fractional extraction during free fall was calculated as the difference between droplet concentration at the foot of the 7.5-em. column and that a t the foot of the GO-em. column, divided by the concentration at the foot of the 7.5-cm. column, thus eliminating end effects. Fractional extraction during free fall is plotted against droplet diameter in Figure 1,where distilled water formed the continuous phase. The points are combined with a cross to denote that internal circulation was present as shown by separate experiments with aluminum particles. I n all experiments, both with distilled water and with water containing surface active agents as the outer phase, the disperse phase initially contained about G% volume of acetic acid in nitro-
benzene, and the effects of various aqueous concentrations of agents on the interfacial tension are shown in Figure 2. Interfacial tensions were measured by the drop-weight method, using the correction factors of Harkins and B r o m ( l a ) , and in the determinations the drop formation time was about 5 secondsLe., similar to the period of free fall in the mass transfer experiments. The relation between fractional extraction and aqueous concentration of the surface active agents is shown in Figures 3 t o 7. Only at the lower concentrations of agent n-ere the droplets circulating, as denoted by points with crosses. Drop diameters throughout Figures 3 to 7 lie between 0 3 6 and 0.55 cm.; consideration of Figure 1 shows therefore that variation in fractional extraction due to change in drop rize alone is negligible here, The experimental curves (circles) in Figures 3 to 7 are compared with theoretically predicted curves for evtraction from rigid spheres of the same diameter (triangles). The curves, drawn through the experimental points of fractional extraction vs. concentration of the five surface active agents, are replotted on the same graph in Figure 8 for comparison purposes. The over-all transfer coefficient, Kd,is plotted against aqueous concentrations of the five additives in Figure 9. Kd was obtained from the following mass balance on a droplet of diameter d, whose concentration changes from Co to CF in time t:
whence
lid =
d
-
Gt
X In
where CO = concentration at foot of 7.5-em. column CF = concentration at foot of 60-cm. column t = difference betn-een times spent by a drop in 60-cni. and 7.5-em. columns
1 I. DODECYL WRID'NIUM BROMIDE
X DODECYL SODIUM SULFATE 6
(.t low c o n c u
em0.00584 3 > 2 . 3 6 X 10-10
56
5.4
...
5.6
21.3 j 21.3
t . .
5.6
... ... ...
6.6
7.4 7.4
...
13.7 13.7 10.2 1 10.2
Stagnant scant oscillation
+
6 ,3 6.5 J
17.36 17.36 17.36 17.36 17.36 17.36 17.36
obstruction due to the nitrobenzene and water molecules assuming an orientation induced by the adsorbate molecules, and to some form of interaction, such as adsorption, between the diffusing acetic acid and the adsorbed film. The minima in the experimental curves of Figures 3, 4, and 5 occur a t a surface active agent concentration of about 0.00174 gram-mole per liter, Figure 2 implies that the interface is far from saturated with agent at this bulk concentration, as the interfacial tension curves do not become horizontal until much higher bulk concentrations. If a dilute film in equilibrium with the bulk concentration is sufficient to stop circulation, the mechanical obstruction offered by adsorbed surface active molecules should increase steadily with film and bulk concentrations well beyond those correspond-
Table I.
7.99 5.14 6.29 5.35 7.8 6.07 6.71 6.43 4.95 5.62 5.69 5.27
ing to the minima of curves in Figures 3 , 4 >and 5. These minima, however, denote maximum transfer resistances, so that, assuming the dilute film mechanism, it is concluded that mechanical obstruction constitutes only a minor part of the interfacial barrier, which is due mainly to adsorption of the solute on the film. [Hutchinson (IS)came to a similar conclusion after his work on plane interfaces.] =It concentrations beyond those corresponding to the minima in the curves, the rates of adsorption and desorption of acetic acid from the film become equal and further additions of adsorbate to the film have little effect. However, some more concentrated or coherent film may be required to stop the circulation. At very low concentrations of adsorbate the circulating motion of the interface may cause the adsorbed molecules to accumulate at the rear of the drop surface, giving a coherent film which progressively extends over the interface until circulation is stopped. This is in accordance with ohservations using Texofor B1, in which circulation ceased after 15- to 20-em. fall; motion ceased first in the rear of the drop, and stagnancy then gradually progressed to the leading pole. The rate of accumulation must then exceed the rate of desorption from the film, whose concentration will be above the equilibrium value for the bulk of the solution. In this way the minima in the curves of Figures 3 to 7 may cor-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
ENGINEERING, DESIGN, AND EQUIPMENT Table II.
Experimental Data (Confinued)
Run
Drop Diam., Cm.
Concn. of Surface Active Agent in Water, G.-Mole/ Liter X 103
44 45 46 47 48 49 50 51 52
0.505 0.505 0.505 0.475 0.475 0.475 0.460 0.460 0.460
0,348 0,348 0.348 1.736 1.736 1.736 3.48 3.48 3.48
53 54 55
0.430 0.430 0.430
6.95 6.95 6.95
1.039 0.935 0.928
7.5 00 60
5.5 5.8 6.06
56 57 58 59
0.390 0.390 0,390 0,390
17 36 17.36 17.36 17.36
0.686 0.972
60
6.02 6.16 5.71
Acid in Extraqted Or anlo P%ase G.-Molk/ Cc. X 108
Column Height, Cm.
Volume of Disperse Phase, Co.
Time of Fall, Sec.
Continuous Phase Contained Dodecyl Pyridinium Bromide 0.784 60 7.3 5.2 60 6.46 5.2 0.754 ... 1.092 7.5 4.75 60 6.56 5.3 0.713 ... 1.092 7.5 5.46 1.071 7.5 5.6 ... 0.703 60 5.95 5.4 60 7.01 5.4 0.69 ... 1.08 7.5 5.95
1.000 0.676
7.5 7.5 60
...
Interfacial Tension Dynes/C&. 19.75 19.75 19.75 16.8 16.8 } 15.05 15.05 15.05
...
...
... 9.2
Circulation
+
oscillation
+
Stagnant oscillation
6.5 6.5 9.2
Conditions within Droplet, Showa bv A1 Particles
9.7
Staqnant
+
vigorous
9.7
oscillation
Continuous Phase Contained Hexadecyl Trimethyl Ammonium Bromide
60 61 62 63
0,580 0.550 0.550 0.550
0.348 0.348 0.348 0 348
0.993 1.01 1.095 1.09
60 60 7.5 7.5
7 74 6.46 7 76 6.7
5.8 5 8
64 65 66 67 68 69 70 71 72 73 74 75
0.495 0.495 0.495 0.460 0.460 0.460 0.460 0.400 0.400 0.365 0.365 0.365
1.736 1.736 1.736 3.48 3.48 3.48 3.48 6.95 6.95 17.36 17.36 17.36
0.974 1.08 1.116 0.95 0.966 1.081 1.103 0.95 1.082 0.95 0.957 1.059
60 7.5 7.5
7.07 6.36 4.66 5.55 6.5 6.05 5.83 6.16 5.64 5.98 5.19 5.04
7.2
0.348 0.348 1.736 1.736 3.48 3.48 6.95 6.95
60 60
7.5 7.5 60 7.5 60 60 7.5
... ...
7.7 7.7
...
... ...
7.9
8.0 8.0
~ . .
Continuous 0.991 1.10 0.922 1.073 0.873 1.053 0.803 1.105
Phase Contained Texofor Bi 6.2 60 6.32 ... 7.5 5.57 60 7.56 6.4 7.5 6.11 ... 6.8 60 6.75 ... 7.5 5.41 6.8 60 5.8 ... 7.5 5.58
60
76 77 78 79 80 81 82 83
0.510 0.510 0.485 0.485 0.470 0.470 0,440 0.440
84
0.400
17.36
0.87
85
0.400
17.36
1.045
7.5
respond to films approaching saturation, despite the low bulk concentration of the continuous phase. Mechanical obstruction may then, be an important part of the interfacipl barrier. Aqueous bulk concentrations of sodium dodecyl sulfate 6 in equilibrium with various concentrations a t a static interface are deduced below for a tentative analogy with conditions a t the droplet interface. The dodecyl chain is assumed to lie flat near the plane of the interface and to be a cylinder (3) about 13.8 A. in length and 20.5 sq. A. in cross section (1). The Avogadro number is taken to be 6.023 X loza and the Gibbs adsorption c dr equation for dilute solutions as -8 = iRT X dC where, according to Addison (W), S approximates the amount of solute in a layer one molecule deep on unit surface, y is interfacial tension, and c is bulk concentratlion. i lies between 1 and 2, depending upon the degree of ionization of adsorbate (6) and upon the relative interfacial concentrations of anion and cation (16). Types of Static Film Structure. Three types of static film structure may then be postulated: 1. A dilute film, for which the maximum interfacial concentration possible without mutual interference between adsorbed molecules corresponds to their forming the diameters of hexagonally close-packed circles (3). 2. Higher concentrations, leading to increasing interference January 1956
... ...
5.06
7.0
5.1
...
12.75’ 12.75 12.75 10.22 10.22 10.22 i 10.22 7.04 7.04 5.3 5.3 5.3 ;
13.84 13.84 11.86 11.86 11.05 11.05 9.2
7.05
Circulation stopped after 15-20 om. of fall, oscillation present
+
Stagnant oscillation
+
Circulation oscillation
Circulation st.opped after 15-20 om. of fall no tionoscilla-’
until a layer of tlghtly packed dodecyl chains lying flat near the interface is formed. 3. Further adsorption, accomplished by the two hydrophobic portions of the molecule assuming an orientation more and more nearly normal to the interface, leading to a “condensed monolayer” a t saturation. The calculated film and bulk concentrations corresponding to these three types of structure appear in Table I. Comparing the values, assuming i = 1, with Figure 4 implies that internal circulation was stopped a t a bulk concentration near the upper limit of the concentration range predicted for Type 1 film structure. The minimum in the curve occurs within the range calculated for Type 2 films, while concentrations above this coincide with predictions for Type 3 structures, in which the percentage of interfacial area occupied by adsorbate molecules remains roughly constant and there is little further increase in transfer resistance per unit interface, as shown by Figure 9. While values derived using i = 2 give poorer correspondence with the curve of Figure 4, the indications may be fortuitous in view of the assumptions made. The other four agents had their hydrophilic groups at one end of a hydrophobic chain which would probably be oriented nearly normal to the interface in-. stead of parallel to it, at all concentrations. Accordingly, similar. calculations were not attempted for these compounds,
INDUSTRIAL AND ENGINEERING CHEMISTRY
57
ENGINEERING, DESIGN, AND EQUIPMENT A
DODECYL SODIUM SULFATE I 0 DODECYL SODIUM SULFATE 6 DOMCYL PYRlDlNlUMBROMIDE TEXOFOR B @ HEXADECYLTRIMEMYL AMMONIUM
Table 111.
0
E$
CIRCULATING. CIRCULATION DIES OUT
,002 ,001
0
Figure 9.
,0025 ,005 ‘0075 01 ,0125 ,015 -GMMOLE SURFACEACTIVEAGENT/LlTER WATER-
,On5
Relation of aqueous concentration of additives to over-all transfer coefficient
Nomenclature
d
D
CF
= 2r = diameter of a spherical drop, cm. = solute diffusivity sq. cm. per second =
Ci =
GO
=
Kd = n = 8
=
solute concentratjion in a rigid sphere after time t, grammole per cc. constant (lower) solute concentration at the interface of a rigid sphere, gram-moles per cc. solute concentration throughout a rigid sphere when diffusion begins, gram-moles per cc. over-all mass t r a d e r coefficient, cm. per second an integer number time for diBusion, seconds.
Acknowledgment
Thanks are due to T. K. Ross for synthesizing sodium dodecyl sulfates 1 and 6. The authors are grateful for a research scholarship from a grant made by the Esso Development Co., Ltd. literature cited
(1) Adam, N. K., “Physics and Chemistry of Surfaces,” 3rd ed., p. 67, Oxford Univ. Presa, London, 1941. (2) Addison, C. C., J. C h m . SOC.1944, p. 477. (3) Addison, C. C., and Litherland, D., Ibid., 1953, p. 1146. (4) Chu, J. C., Taylor, C . C., and Levy, D. J., IND.EXG.C m x . 42, 1157 (1950). (5) Donnan, F. G., and Barker, J. T., Proc. Rou. SOC.A85, 557 (1911). (6) Farmer, W. S.,Oak Ridge National Laboratory unclassified report, ORNL 635 (1950). (7) Garner, F. H., and Hale, A. R., Chem. Eng. Sci. 2, 157 (1953). (8) Garner, F. H., and Skelland, A. H. P., Ibid., 4, 149 (1955). (9) Garner, F. H., and Skelland, A. H. P., IND.ENG.CHEM.46, 1255 (1954). (IO) Garner, F. H., and Skelland, A. H. P., Trans. Inst. Chem. Engre. 29, 315 (1951).
Interpreted Experimental and Theoretical Data during 52.5 Cm. of Free Fall
[Solute (acetic acid) diffusivity in nitrobenzene = 0.665 X 10-6 sq. om./ second ( 9 )1 Conon. of Theoretical Surface Active Fraction Exptl. Exptl. Extracted Drop Agent, G.Kd Fraction for Rigid Diameter, RIole/Liter Cm. x 108 Cm./Sec. Extracted Spheres Continuous Phase, Distilled Water 0.540 0 0.0115 0,437 0.075 0 0.0102 0 . 4 3 , 0.416 ... 0.515 0.490 0 0 00948 0.417 ... 0.415 0 0.00843 0.438 ... 0,420 0 0.00848 0.435 ... Continuous Phase Contained Sodium Dodecyl Sulfate 6 0.535 0.348 0.00474 0.218 0.076 1.736 0.525 0.00037 0,019 0.078 0.625 1:736 0 . 00065 0 030 0.078 0.485 1.736 0.00067 0,038 0.078 0.460 3.48 0.00073 0.049 0.09 0.425 6.95 0.00098 0.07 0.1 0.360 17.36 0,00123 0,119 0.124 Continuous Phase Contained Sodium Dodecyl Sulfate 1 0.348 0.00158 0.00074 0.510 0.084 0.08 1.736 0,440 0.049 0,095 1 736 0,00096 0.440 0.061 0.095 0.00129 3.48 0.107 0.395 0.114 6.95 0.336 0.122 0.14 o ooi 0 098 6.95 0.335 0.14 0 00133 0 . 15.5 6.95 0.336 0.14 6.95 0.132 0.335 0.14 17.36 0.195 0.310 0.148 0.00
