2324
K. BEARD,M. RIOS, D. CURRELL, AND R. REIS
with the solvent. The results in this paper suggest that the process should be considered to take place in three steps: (1) the creation of an appropriate sized cavity; (2) the entrance of the solute molecule into the cavity; and (3) interaction of the solute with the solvent. Pierot@ determined the hard-sphere diameter of water to be 2.74 at 70°, agreeing well with values of other method^.^^^^ This diameter of the water molecule is very close to the diameters of hydrogen and helium, the gases that cause least interference. Insoluble gases with larger molecules will evidently encounter a longer wait before the surface distorts sufficiently to provide a larger cavity and in the interval presumably stays adsorbed to smaller “holes” which they cannot enter. H2 and He can participate in steps (1) and (2) but only weakly in (3). Large molecules ethane and methane do not readily take any of the three steps. The soluble gases, C2H2, NzO, and COz) have very strong interactions with the water molecules which override the first two steps and cause the molecules to be pulled into the water surface. We advance the ad
hoc theory that any molecule that can readily be pushed or pulled into the water surface, e.g., by cavitation or true solution, does not cause boule hesitation but that those molecules that “have nowhere to go” do! The generalization carries no suggestion of the mechanics of causing hesitation although two alternatives may be considered. One possible mechanism would be that the temporarily adsorbed molecules block evaporation of the support water and allow the boule to approach within merging distance. The other would be that the gases line both the drop and the support permitting partial wetting. Here we must be content to leave the discussion pending further experiments.
Acknowledgment. The author wishes to thank Dr. E(. Hickman for his advice and assistance during this work, and also A. Davidhazy and E. Wright for their help in preparing the manuscript. (8) R. A. Pierotti, J . Phys. Chem., 69, 281 (1965). (9) L.Monchick and E. A. Mason, J . Chem. Phys,, 35, 1676 (1961), (10) J. S. Rowlinson, Z’rans. Faraday SOC.,45, 974 (1949).
On the Ultrasonic Cavitation Intensity of Aqueous Sodium Lauryl Sulfate Solutions’
by Kenneth Beard,2&Michael Rios, Douglas Currell,2band Richard Reis Department of Chemistry, California State College at Los Angeles, Los Angeles, California (Received March 10,1970)
The ultrasonic cavitation noise intensity as a function of the concentration of aqueous sodium lauryl sulfate solutions has been measured. The cavitation noise decreases with increasing concentration (decreasing surface tension) until a concentration of 0.0128 M is reached. Above this concentration a sharp increase in cavitation noise is observed. The possibility that micelles serve as cavitation nuclei is discussed. Introduction The results of an earlier studya showed that the rate of formation of acetylene from the ultrasonic treatment of aqueous solutions of pyridine was not affected by the addition of surfactants. This result was unexpected since the reaction is believed to take place within the cavitation bubbles which form when a liquid is treated with an intense ultrasonic beam.3*4 Hueter and Bolt6 have proposed that the higher the surface tension of the liquid the greater the amount of energy released upon collapse (or contraction) of the cavitation bubble since the energy so released should depend on the work required to form a new surface in the expanThe Journal of Physical Chemistry, Vol. 74, No. 11, 1970
sion of the bubble. Thus, the greater the surface tension of the reaction solution, the faster should be rate of the ultrasonic reaction. T o explain the lack of effect of added surfactants on the rate of acetylene formation (1) This work was supported by a National Science Foundation Institutional Research Grant. (2) (a) National Science Foundation Undergraduate Research Program Participant. (b) T o whom correspondence should be addressed. (3) D. L. Currell, G. Wilheim, and S. Nagy, J . Amer. Chem. SOC., 85,127 (1963). (4) V. Griffing, J . Chem. Phys., 20, 939 (1952); M. E. Fitzgerald, V. Griffing, and J. Sullivan, ibid., 25,926 (1956). (5) T. F. Hueter and R. H. Bolt, “Sonics,” John Wiley and Sons, New York, N. Y.,1955,p 238.
2325
ULTRASONIC CAVITATION INTENSITY
mediately preceded by, and compared with, measurements on distilled water. A minimum of three determinations was made a t each concentration and the results were averaged and expressed as per cent increase = in cavitation noise over water, N , where N H ~ O
I
z I
$
z
-50-
cavitation noise of water in millivolts and N s ~ s= cavitation noise of surfactant solution in millivolts.
-eo--
Results and Discussion
J
z
I
-3.7
I
-3.4
I -31
I -2.8
I -2.5
I
-2.2
I -1.9
I -1.6
I
log conc. SLS
Figure 1. Per cent increase of cavitation noise over water us. log concentration aqueous sodium lauryl sulfate solutions.
in the ultrasonic treatment of aqueous solutions of pyridine, it was suggested3 that a greater number of cavitation bubbles formed in the solutions of lower surface tension since a greater number of bubbles leading to a greater production of acetylene per unit time would, perhaps, compensate for the lower yield of acetylene per bubble. To gain insight into the effect of surfactants on the cavitation process, we undertook a study of the ultrasonic cavitation intensity of aqueous sodium lauryl sulfate solutions. Results reported by other laboratories indicate that increased chemical activity can be accompanied by a decrease in cavitation noiseV6 Experimental Section The ultrasonic generator (McKenna Laboratories Model EB240, 1 RiIcps) was coupled to a cup-shaped barium titanate-ceramic transducer (9 cm in diameter) which was covered with 350 ml of the sodium lauryl sulfate (Eastman Kodak) solution. The cavitation noise was measured with a barium titanate ceramic hydrophone disk (diameter, 0.750 in., thickness, 0.100 in.) mounted on the side of the flask perpendicular to the plane of the transducer. An electronic 1-Mc shunt was used to filter the driving frequency. The solution was aged for 1 hr a t an intensity of 0.2 W/cm2 (measured by the method of Herrey') a t the surface of the transducer after which the intensity was reduced and the cavitation noise measured through small intervals to zero. The results reported are for one intensity, 0.09 W/cm2. The average error at this intensity was approximately 5%. Results a t three other intensities (0.05, 0.07, 0.10 W/cm2) were qualitatively the same. The temperature was maintained constant at 40.0" by circulation of water from a constant-temperature bath through cooling coils surrounding the flask. The determinations of the surfactant solutions were im-
The results expressed in Figure 1 indicate a decrease in cavitation noise for the surfactant solution relative to water with increasing concentration below a concentration of 0.0128 M . This result can be explained as due to a reduction in surface tension over this range with a resulting lessening of the amount of energy released upon collapse (or contraction) of the cavitation bubble. Above this concentration the cavitation noise increases with an increase in concentration of surfactant. This increase in noise coincides approximately with an increase in surface tension for solutions of sodium lauryl sulfate containing small amounts of lauryl alcohol as an impurity which occurs at 0.0079 M S s Above 0.0079 M the situation may be complicated by micelle formation. The critical micelle concentration of sodium lauryl sulfate a t 40" is reported to be 0.0089 M.9 Support for this idea is indicated by the observation that the surface tension is the same for the two highest concentrations measured (0.016 and 0.020 M ) while the cavitation noise continues ta increase with increasing concentration. Pease and Blinks'O have shown that cavitation occurs relatively easily at a water-glass interface if the glass walls are coated with surfactant. We suggest that an analogous situation may exist in the presence of micelles of surfactant and that the micelles can thereby serve as cavitation nuclei with a resulting increase in cavitation noise above the critical micelle concentration. This suggests that cavitation noise measurements could be used to study the aggregation of association colloids. These results are not consistent with the previously reported observation that added surfactants had no effect on the rate of production of acetylene in the ultrasonic treatment of aqueous solutions of pyridine. It is (6) M . Degrois and B. Badilian, C, R.Acad. Sci., 254, 837 (1962). Amer. 27, 891 (1955). (7) E. M. J. Herrey, J . Acoust. SOC. (8) 5. P. Harrold, J . Colloid Sci., 15, 280 (1960): lauryl alcohol is produced by the hydrolysis of sodium lauryl sulfate. (9) K. Meguro, T. Kondo, N. Ohba, T. Ino, and 0. Yoda, Bull. Chem. Soc. Jup., 30,760 (1957). (10) D. C. Pease and L. R. Blinks, J. Phys. Colloid Chem., 51, 556 (1947). (11) B. E. Noltingk and E. A . Neppiras, Proc. Phys. SOC. London Sect. E., 63, 674 (1950).
The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
2326
P. URONE,Y. TAKAHASHI, AND G. H. KENNEDY
conceivable that at the extremely high temperatures possible within the cavitation bubblell that the amount of energy available so far exceeds that necessary to
bring about the chemical reaction that any change in the energy available brought about by the addition of surfactantsis negligible.
Sorption Isotherms of Polar-Nonpolar Systems on Liquid-Coated Adsorbents by Paul Urone, Yoshihiro Takahashi, Department of Chemistry, University of Colorado, Boulder, Colorado 80602
and George H. Kennedy Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401
(Received August 5 , 1960)
Experimental studies of polar-nonpolar liquid-coated adsorbent systemsat low liquid coatingsreveal basic sorption relationshipsnot previously understood. Sorption isothermswere obtained for the bulk liquids, the uncoated adsorbents,and adsorbents coated with 0.1-20% liquid phase. Relative pressures ( P / P o )were varied from 0,001 to 0.30, and temperature studies were made for thermodynamic evaluation. Bulk liquid partition coefficients and Henry's law constants at 30' are given. Sorption of polar solutes on squalane-coated adsorbents equal the sum of the contributions of the squalane and the adsorbent, and nonpolar solutes show slight sorption from solution equilibrium effects. Tri-o-tolyl phosphate (TOTP) and tris(cyanoethoxypr0pane) (TCEP) interact with the adsorbent to form a modified surface which gives reduced, and sometimes linear, isotherms. Polar solutes show isotherms that are a sum of the contributions of the liquid-modified surface and the bulk liquid. The isotherms of hexane, as a nonpolar solute, on TOTP-coated adsorbents show the possibility of a small contribution from adsorption on the liquid surface in addition to the contributions of the bulk liquid and the liquid-modified surface. The demonstrated modified surface and the additivity of the sorption terms have long-range potentials for the measurements of thermodynamic properties and the development of specialized surfaces.
An earlier study of sorption isotherms of acetone on liquid-coated adsorbentsl showed that the amount of solute sorbed at a given partial pressure (Qtota1) was equal to the sum of: (1) the amount adsorbed by the surface of the adsorbent modified by a thin layer of coating liquid (Qmod) and ( 2 ) the amount sorbed by the remainder of the coating liquid (&liquid). &total
=
&mod
+
(1)
&liquid
The amounts sorbed in each case depended upon the partial pressure of the solute and its isotherm on each of the respective phases; i e . , a t constant temperature Qt
= W,
lp
dP
+ WL
sop
dP dP
(2)
ivhere Qp is the amount of solute sorbed a t a given partial pressure, P; dQ,,d/dP and d&L/dP describe the isotherms of the modified surface of the adsorbent and of the bulk liquid, respectively. W , is the weight of the modified adsorbent and WL is the weight of liquid coating above that needed to modify the surface. When, as is common at low conThe Journal of Physical Chemistry, Vola74,No. 11, 1070
centrations, the isotherm for the solute in the liquid is linear, d&L/dP is constant and eq 2 becomes Qt =
W,
sop
d&mod d P d P + WLKLP
(3)
where K L , the partition coefficient, is the ratio of the concentration of the solute in the liquid to that in the gas phase. When squalane was used as the coating liquid, the adsorbent surface was only slightly modified, and the amount sorbed was essentially the same as that sorbed by the uncoated adsorbent. This report extends the studies to cover in more detail examples of polarnonpolar solutes, solvents, and modified adsorbent surfaces.
Experimental Section The sorption isotherms were determined gravimetrically using the apparatus and techniques described previously1 except that an electrotorsion Bourdon gauge was added to the vacuum system to measure (1) P. Urone, Y. Takahashi, and G. H. Kennedy, Anal. Chem., 40, 1130 (1968).
