F. M. FOWKES, G. S. RONAYAND M. J. SCHICK
1684
Vol. 63
MONOLAYERS IN EQUILIBRIUM WITH LENSES OF OIL ON WATER. I1 DEPENDENCE OF EQUILIBRIUM PRESSURES ON pH AND ON CONCENTRATION OF SURFACTANT BY F. M. FOWKES, G. S. RONAY AND M. J. SCHICK She11 Development Company, E ~ ~ ~ y v i lCalifornia le, Received March 16,1968
The equilibrium spreading pressures ( r w )of oil solutions of surface-active substances on aqueous substrates can be measured easily and ra idly. These are found useful as analytical tools to determine type and concentration of surface-active substance in the oi!? phase. Surface active acids, bases, salts and non-ionizable substances are easily distinguished by the pH-dependence of rw. The concentration dependence of rwcan be used to determine the molecular area, and r wmeasurements of the supernatant oil can be used to measure surface areas of solids and heats of adsorption thereon.
I n the preceding paper of this series' it was shown that the equilibrium spreading pressure awof surface active substances in oil has a concentrationindependent relation to row, the equilibrium spreading pressure a t the oil-water interface
-
row rw = A
These relationships apply to systems in which there is negligible depletion from the oil phase by adsorption of the surface active substance or by its solution into the aqueous substrate. The value of A was found to be zero for 1-n-octadecanol in white oil and 5.8 dynes/cm. for l-n-tetradecanoic acid in the same oil on 0.01 N HC1 a t 25". Earlier published data of Heymann and Yoffe2 showed that for oleic acid in white oil A = 10.3 dynes/cm. Because A is independent of concentration, measurements of ?rw can be used analytically for many of the same purposes that one uses interfacial tension. Moreover, because of the greater ease and rapidity of measuring rw,it is more useful for routine measurements. Experimental Details Methods.-The surface film pressure of monolayers spread from lenses of oil solution on aqueous substrates was measured from the pull on a hanging vertical glass slide.a The glass slide was hung from one arm of a chainomatic analytical balance so that it dipped beneath the surface of water in a paraffined petri dish on the floor of the balance case. Calibration of the balance scale divisions in dynes/ cm. was accomplished by adding weight t o the opposite arm with the chainomatic device, with calculations based on the measured perimeter of the glass slide. Barriers of paraffined brass were used to sweep the surface clean before applying the oil solution. Usually five drops of oil solution were added and the film pressure rwwas read every five minutes, with two more additions of two drops each within 30 minutes. Usually the pressure rose rapidly in the first three or four minutes and gradually levelled off well before 30 minutes. Petri dishes and barriers were cleaned and rewaxed prior t o each measurement. I n the case of adsorption studies the supernatant liquid was separated from the adsorbent by centrifuging in a small clinical centrifuge. For studies of temperature coefficients of adsorption the centrifuge was operated in a large oven a t the desired temperature. Materials.-The water was twice distilled, the second time in dilute alkaline permanganate with a stream of waterpumped nitrogen to swee out GOz, and condensed in a block tin condenser. B u g r e d substrates of pH 2, 4,6, 8, 10, 12 were prepared with HCl, sodium formate, potassium ( 1 ) W. (1956).
M. Sawyer and F. hl. Fowkes, THISJOURNAL,60, 1235
(2) E. Heymann and A. Y o f f e ,Trans. Faraday Soc., 36, 999 (1940). (3) W. D. Harkins and T. F. Anderson, J . A m . Chem. Soc., 69, 2189 (1937).
phosphate, sodium borate and sodium hydroxide. A light, white oil (naphthenic), silica treated to remove oxidation products, was used as solvent for most studies, a silicatreated neutral lubricating oil in others. Silica gels or magnesia-silica gels used in adsorption studies were specially prepared in this Laboratory by L. B. Ryland. The oilsoluble amine used in adsorption studies was prepared in this Laboratory by forming the stearic amide of a dibasic amine ( NH~CHTCHOH-CH~NH~).Other surface active materials were obtained from usual sources, which are indicated in the text.
Dependence of Equilibrium Pressures on pH.Values of the equilibrium pressure awwere determined with white oil solutions containing 0.5% by weight of (a) naphthenic acids (E. K., b.p. 160198" (6 mm.) or (b) dodecylamine (Armour's Armeen 12D) or (c) dodecylamide (Armour's Armid 12) or a mixture of 0.5% each of both (a) and (b) on aqueous substrates buffered at pH 2, 4,6, 8, 10 and 12 (Fig. 1). The shape of each curve is characteristic of the surface-active substance. The naphthenic acids and the dodecylamine are most surface-active a t interfaces where dissociation of the hydrophilic group is promoted: alkaline substrates for the acids, and acidic substrates for the amine. This is interpreted to mean that the ionized molecules (soaps or salts) are much less soluble in the oil than the undissociated precursors, and back diffusion from the interface is therefore considerably reduced. On the other hand, the amide has constant awvalues over the whole pH range. The mixture of surface-active acids and amines were highly surface-active over the whole pH range but showed maximum activity over neutral substrates. Figure 2 shows how the pH-dependence of a, values can be used to follow interaction of surfaceactive substances. In this case the white oil containing 0.5% each of naphthenic acids and dodecylamine was heated at 120" for several days and the rw-pH relationship was determined intermittently. The maximum characteristic of the dodecylammonium naphthenate soap is seen to disappear until a pH-independent relationship results. This is interpreted to mean that water was split out of the soap to give an amide. Figure 3 shows the nw-pH relationship observed with three different asphalts (diluted with oil until pourable (MC-2)) in order to illustrate how this method gives information about naturally-occurring surface active substances. Here a minimum in surface-activity is observed on neutral substrates, with greatest activity on alkaline sub-
DEPENDENCE OF EQUILIBRIUM PRESSURES ON p H
Oct., 1959 50
AMINE t ACID'
AND
SURFACTANT
1685
-1
10
-
ASPHALT
I
0 10
I
,
/
,
I
,
I
,
12
PH.
Fig. 1.-Dependence of spreading pressure of oil solutions of surface-active substances on pH of the substrate.
0 DAYS AT 120'
20
10
0.001
12
pH OF SUBSTRATE.
I
I
O H H H H
Fig. 2.-Effect of heating at 120' on rw-pH relationship of white oil solution containing 0.5% of dodecylamine and of naphthenic acid. 50
0.01 0.1 CONCENTRATION OF AMINE IN OIL, %.
C11H8L&-I&-b-b--b-"Z
A AI A
H Fig. 6.-Concentratioii-dependence of spreading pressure r mof neutral oil solution of an amine.
A CALIFORNIA ASPHALT
0 , O MIDCONTINENT ASPHALTS
40
f'
5 30 W
r 20
3
e
LO
o
h
10
I2
J
pH OF SUBSTRATE.
Fig. 3.-Relation
of rmto pH of substrate for oil-diluted asphalts. Fig. 7.-Concentration of amine in oil phase of silicamagnesia oleogel determined by rwmeasurements.
P TEXAS WAXY CRUDE OIL
pH O F SUBSTRATE.
Fig. 4.-Characterization of surface-active substances in crude oils by the r,-pH relation.
strates. It appears that asphalts contain both surface-active acids and bases, the acids being the more surface active. The minimum is quite the opposite of the maximum shown in Fig. 1 for a mixture of surface-active acids and bases. The minimum could very well result from molecules having both acidic and basic groups, such as in porphyrins or their residue^.^ These contain carboxylic groups (which over alkaline substrates give these substances the properties of surface active acids) and nitrogen-containing rings which often chelate metal ions. A ferric ion in such a ring has a (4) 11.
N. Dunning, J . Coolbid Sci., 8 , 279 (1953).
1686
F. M. FOWKES, G. S. RONAYANI) M. J. SCHICK
single remaining positive charge in acidic media, but becomes a neutral hydroxide in alkaline media; it could thus act as a surface-active base. Similar results were obtained with various crude oils (Fig. 4). Here it is seen that the aw-pH relntionship is characteristic of the type of crude oils; the greatest surface-activity is observed with asphaltic crudes. The presence of added surface-sctive agents in oil products can sometimes be determined by the awpH relationship. Figure 5 shows the effect of adding 1% each of oleic acid (Merck) or octadecylamine (Armour's Armeen 18), or O.5y0of both to an (oildiluted) California asphalt. The amines raise the activity of the naturally-present bases, the acids raise the activity of the naturally-present acids, and the mixture eliminates the minimum over neutral substrates. Dependence of Equilibrium Pressures of Concentration.-Figure 6 shows the dependence of a, on a substrate buffered at pH 8 of a neutral oil containing various concentrations of an oil-soluble amine derived by forming the mono-stearamide of tJhe amine H2NCH2-CHOH-CH2NH~. The area per molecule can be determined from these data with the Gibbs adsorption equation6
Vol. 63
Measurements of rw are useful for determining the concentration of surface active agents. An application of this nature is illustrated in Fig. 7, showing adsorption equilibria of the above amine on a silica-magnesia gel in a neutral oil. The original aquagel had been transferred into the oil phase with a minimum of the amine used as a transfer agent and the remaining water cooked out. To several aliquots of the oleogel were added increasing increments of the amine, the aliquots equilibrated and centrifuged at a desired temperature and a, of the gel-free oil measured a t 25" on a substrate buffered a t pH 8. By reference to Fig. 6 the concentration of amine in equilibrium with the adsorbed monolayer can be determined. By means of the temperature coefficient of the concentration necessary in solution to give a monolayer of any desired packing, one can calculate the heat of adsorption, which in this case was 12 kcal. per mole. To determine the specific area of the gel, we must know the area per molecule in the adsorbed monolayer on the gel; Figure 7 shows this monolayer to be fairly compressible. We make the assumption that a t aw = 0 the area per molecule is 49 A.2 as was found for the oil/water monolayer under moderate to high f lm pressures; this value may be too small. At 49 per molecule, 0.7 g. of amine per gram of solid corresponds to 600 m.2/g. if one assumes that the activity coefficient for tfhe for the surface area of the gel. amine is constant in this concentration range. The Acknowledgments.-The authors thank Miss for the area per molecule of this Helen Robbins for making most of these measureresults give 49 amine. ments and Dr. M. W. Tamele for encouragement in these studies. ( 5 ) W. A. Zisman. J . Chem. Phys., 9, 789 (1941).
