can also reflect light from the xenon pumping tube if its diameter is too small. Ordinary 1 x 3-in. microscope slides are used as sample holders. About 10 drops of the liquid are applied to form a pool about in. in diameter, which gives a nearly planar sample surface a t the center of the drop where the beam impinges. Low viscosity liquids may require a wax dam to prevent excessive spreading. Fixed path length cells may also be employed but, because of added effort and possible shear effects, are recommended only when path length control becomes critical. Dimpled microscope slides must not be used; spectacular but meaningless patterns will result. Often a n outline image of the sample pool will appear on the photograph (especially if the film slide is left open longer than a
second or two) but is easily disregarded in interpreting results. All precautions m u s t be taken to avoid looking into the laser beam, either directly or by reflection. 1 shutter mechanism on the camera and the location of firing controls a t a distance from the instrument are recommended. literature Cited Mie, G., Ann. Phys., 25, 377 (1908). Pendorf, R., J . Opt. SOC.Amer., 52, 402 (1962). Rhodes, 11. B., Stein, R. S.,ASThI Special Technical Report Xo. 348, Symposium on Resinographic Methods, p 59 (1963). Willis, E., Kerker, fir., hlatijevic, E., J . Collozd Scz., 23, 182
(1967).
RECEIVED for review September 11, 1969 ACCEPTED>lay 17, 1971
Surfactant Adsorption by Pigments from Aqueous Solution Michael A, Kessick,' Ian H. McEwan,2and Henryk W. Zacharewicz Canadian Industries Ltd., Paint Research Laboratories, Toronto 19, Ontario, Canada
Surface tension measurements have been used to characterize surface active dispersant adsorption by pigments, principally rutile titanium dioxide, from aqueous solution. Comparison with the results obtained by chemical analysis indicates that saturated monolayers are formed at an equilibrium solution concentration closely approximating critical micelle concentration. Anionic surfactants containing multiple oxyethylene residues deviate from this rule. Simple dynamic rheological measurements during pigment milling are related to pigment adsorption. The dispersant demand at rheological equilibrium is less than for monolayer adsorption but instability results unless the latter is obtained. Microscopic examination of the dispersions shows that complete deaggregation is not obtained at the rheological "equilibrium" point, within the accuracy of the rheological measurements. When monolayer adsorption is attained, more or less complete deaggregation is believed to b e possible,
D i s p e r s a n t and surfactant additives, although used i n minor amounts, exert a major influence in latex paints, controlling, for example, long-term and freeze-thaw stability in the can, application properties, and t'he opacity, gloss, and scrub resistance of the dry film (Isaacs, 1966). I n such complicated multiphase systems, exact kno\vIedge of the role played by these dispersants and surfactants and their distribution is vit,al to optimum formulat'ion. F c r instance, the amount of dispersant required to ensure pigment deaggregation during "grinding," and to ensure stability of the resultant mill base, has been one of t,he principal concerns of paint forniulat'ors. Several techniques to determine this have been reported -Le., Daniels Flow Point measurements in resin or dispersant sclutions or Brookfield visccsity measurements a t various dispersant levels after dispersion (Sievard aiid Downey, 1968; Sievard and Stangs, 1964) aiid titration of pigment with dispersant solutions with a Brabender Plastigraph-type: sigma blade mixer (Liberti aiid Pierrehumbert, Present address, W. 11,Keck Laboratory of Environmental Health Engineering, California Institute of Technology, Pasadena, Calif. 91109. To whom correspondence should be addressed.
1959). Alt,hough only the last-mentioned technique allproaches grinding conditions, it suffers from lack of precision. .I more precise method has been developed specifically for aqueous systems and was compared with the more conveiitiorial t'eiisiometric and chemical means of determining surfactant adsorption from aqueous solution by use of surfactants amenable to study by all three techniques. Results and Discussion
Tensiometry. I n principle, tensiometry can be used t o determine the concentrat,icn of a n agent that depreeses the surface t,ensioii of water a t concentrations up to and including the concentration a t which saturatioii of the air-water interface occurs aiid micellar aggregation begins-the so-called critical micelle concentration (CMC). If the agent is adsorbed a t the pigment-water int,erface,addition of pigment as a disperse phase should, in general, delay the onset of micellixatioii as surfactant is added. If we assume that the additional arnouiit of surfactant required to reach the CAIC corresponds to the highest attainable coverage of the disperse-phase Purface (provided there is no competitive adsorption of impurities) a plot of the amount of surfact.ant required to reach the CMC ~
Ind. Edg. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
303
Table 1. Effective Area per Molecules Surfastmt
Method 1
Siponate DS 10
0.10
T r i t o n X 100
0.81
Pluronic L 103
19.6
Method 2
Literature
0.26 0.33 (Meader and Criddle, 1953) 0.96 NonylanalogO 5 5 4 . 6 0 (Hsiao et al., 1956) . 20.0 (Wyandotte Corp.)
.le nm'.
in the presence of pigment vs. pigment concentration should enable determination of either mean particle size or the effective area per molecule of adsorbed surfactant. Use of surfactant adsorption to determine latex particle size is well known (Maron and Elder, 1954; Maron e t al., 1954a,h). The molecular area of the surfactant a t a hexane-water interface has been used in such measurements (Brodnyan and Brown, 1960). For adsorption from distilled water onto rutile TiO, of known surface area, the effective areas per molecule (As) of Siponate DS 10 (sodium dodecylhensene sulfonate, American Alcolac Chemicals, Inc.), Triton X 100 Ian octylphenylpolyethoxyethanol, CsHnCsHs(OCH2CHz).0H, where n is approximately 10, Rohm and Haas Inc.], and Pluronic L 103 (a polyethylene oxide-polypropylene oxide block copolymer, Wyandotte Chemical Corp.) were determined by application of this technique, with a Du Nuoy Tensiometer (Table I, Method 1). The poor agreement of these molecular areas for adsorption onto TiO, with previously reported values for molecular areas obtained by measurement on surface films at, the air-water interface was possibly the result of the rliffirulties in obtaining consistent results for surface tension e presence of pigment a t various degrees of dispersion. idified technique was thus developed in which the pigwas washed, dried, and dispersed in excess aqueons ctant solution, by use of a wrist-action shaker over long ds of time. The surface tension was then measured on :ssive dilutions with distilled water under temperature
Figure 2.
Dispersion rheology apparatus
controlled conditions. From Figure 1it is seen that the surface tension (uncorrected) rose dramatically a t a certain concentration-the CMC. The process was repeated a t various pigment concentrations and since
@ . M + VCM x 10- 8 As N where S = surfactant, A p = area of pigment, A s = effectivt mea per molecule of surfactant, A4 = molecular weight o surfactant, N is Avogadro's no., C = CMC grams/liter, V = total volume a t CMC. The value of As could be obtained from multiple deter minations (Table 1, Method 2), provided C was assumed t(1 romn:-. n n n o i - n t L a m -nn rl:.naninn +m +ho no-+ N T LIY.Yrt ohvnn ~ .yl,,~ll. l"..uwlly "" rise in surface tension was observed in this method for Pluronic L 103, and the results for Siponate DS 10 and Triton X 100 appeared to be in no better agreement with the literature values than those from Method 1,but were more reproducible. No adsorption of Triton X 200 or X 202 [octylphenylpolyethoxy sulfonates, CIH&H5(OCHzCHz).SOsNa, where n is believed to be 7 and 10, respectively, Rohm and Haas Inc.1 could he detected by eith er method.-The results would seem to indicate that adsorptioin of organic surfactants to the rutile I^ ^:+L"" UII=LII ~~ w ~ A Y L L ~ L Tiorwater interface was MUGU"L U L U L ~ ,u..."":pc"' r o ~ t+L"" the air-water or the pclymer-water interface. The Du Nuoy method was found too imprecise to determine complete isotherms from concentration and surface tension values a t sub-CMC values. Rheological Measurements. A photograph of equipment used appears in Figure 2. It consists of the base of an IC1 Rotothinner (Monk, 1958) which supports a pint can used as a sand grinding chamber. The base is surine . Iloaded and its deflection duririg dispersion is a measure'of the torque transmitted. The 1ieriphery is calibrated into "poise" with ~.~~11/1.... T,T:ll reference to the normal measuremenx PUILULIUIIJ. v v i l i i i a. "sand grind" single blade paddle and high-speed air motor, a charge of pigment of 200 grams, 20- to 40-mesh Ottawa sand (100 grams) and water (100 grams) was used, giving a
l."..l"..-
".."..>..".
y.up"."."LA
---"
1
~
~~~~~~~
~~~1
\
\
\
-
1
0.25 1
%
Figure 3.
DISPE~SANT
AMMONIA
0.25
0-so
075
I .oo
io
I,I5
ON PIGMENT
Rheological behavior of Ti02 pigments A B C
D E
Tioxide Titanox Titanox Titanox Titanox
R HD 2 RA 45 A 160 LO RA N C RA 46
Nopcosant 1 is o water-soluble anionic polymer (Nopca Chemical Co.).
"mill base loading" of 662/&0 by weight with reference to pigm,ent. The torque response a t approximately const'ant grinding speed was then noted on the addition of (10%) dispersant solution from a hypodermic syringe. I n practice, the response to the addition of 0.025 mole per liter of a dispersant could be detected. From Figures 3 and 4 it can be seen t h a t the addition of dispersant caused a dramatic drop in torque response initially, leading to an equilibrium point a t which further addition caused little or no drop-Le., the hlinimum Effective Dispersant Level (MEDL). Usually further additions showed no response, although in some cases small decreases or abrupt increases were noted. This determination of the M E D L under dynamic conditions seemed more expedient than the more usual static measurements, allowing, for instance, a suitable dispersant to be selected quickly for a given pigment, and also indicating the amount needed for the formation of a stable dispersion. The suitability of a given dispersant for a pigment was considered to be reflected in t,he ext,eiit to which the rheology index could be depressed by addition of the dispersant before a n equilibrium point was reached, and the amount added t o reach that point'. Hence, from Figure 4, Tamol 731 would be considered a more suitable dispersant for Softex Red 1445 than Tamol 850 (Tamol 850 and Tamol 731 are both polymeric carbosylatet'ype dispersants, Rohm and Haas Inc.). I n some cases, the surface characteristics of a pigment could be evaluated by this method. The effect of surface treatment in particular was demonstrated by examination of a rutile pigment, especially intended for latex paints, whose surface had been modified grossly with silica (7%). Small amounts of potassium tripolyphosphate were required to disperse this pigment in water only after silicate ion had
Ammonia is 28% ommonium hydroxide
been added, presumably to depress solution of the silica from the surface (Table 11). The action of strong bases (Figure 4) as dispersants for Ti02 recalled previous work (Bowman and Hughes, 1951). Undoubtedly adsorption of the OH- ions governing surface potential was a t least partially responsible for dispersion, as indicated by the apparent dependence of the torque response on final pH. For compariso~i,potassium tripolyphosphat'e, a well known dispersant for TiOs, is included in the figure. The surface area of a pigment may also be measured by applicabion of this technique, provided t>heeffective molecular area of a dispersant for adsorption to the pigment is known. Wit.h phthalocyanin blue, dispersant addition gave an init'ial depression of t,ransmitted tcrque, followed by a n increase. The end point &-as taken when no subsequent increase occurred on prolonged grinding (a slight modification of the original method). Results for this pigment with various surfactant's appear in Table 111. ll'ith an effective surface area per molecule a t 0.60 nm2 for X 100 (see Table I ) , the
Table II. Effect of Sodium Silicate on MEDL of Potassium Tripolyphosphate (Silica Modified Ti02 Pigment) 40' Baum; sodium silicate
MEDL
(% on pigment)
(% on pigment)
0 0.5 1.0 1.5
1.8i 0.80 1.185 0.90
Ind. Eng. Chem. Prod. Res. Develop., Vol. .lo, No. 3, 1971
305
14
I
2i I.!
0
0.05
0 10
Figure 4.
0.15
0-20
050
015
% A V 0 0
Potassium tripolyphosphate MeaNOH NaOH Ethylene diamine NHlOH (2/3 neutralized with HCI)
surface area for this pigment calculated on the basis of the result above agreed well with published data (Crowl, 1963) for this type of pigment indicating monolayer adsorption. The relationship between the h'IEDL and the surface coverage of a particular TiOn pigment, as measured by the chemical methods below is shown for the surfactants DS 10 and X 202 in Figure 5. AIicroscopic examination of samples taken during dispersion was also used to evaluate the degree of dispersion a t the pcint's indicated in Figure 5 (Table IV). Dilution for examination was carried out with surfactant solutions of the same surface tension as the sample. The results show that the MEDL need not correspond to complete deagreggation. The small decrease in rheology index between
Table 111. Phthalocyanine Blue. Dispersion Dispersant
MEDL, gram
%
Tamol S S b 4.1 Igepal CO 897. 4.2 Hyamine 1622d 4.6 Triton X 100. 5.0 Renex 20' 5.2 Tetronic 7018 6.0 Pluronic L l O l * 9.2 a Blue pigment is Nonastral Blue BT-383-D ( I l u Pont of Canada Ltd). Sodium salt of condensed naphthalene sulfonic acid (Rohm & Haas). Nonyl phenoxypolyethoxy (40) ethanol, (GAF). d Disobutylphenoxyethoxyethyl dimethylbenzyl ammonium chloride (Rohm & Haas). e Octylphenoxypolyethoxy (10) ethanol (Rohm & Haas). Polpoxyethylene (16) esters of mixed fatty and resin acids (Atlas Chemicals). 8 Block polyethoxypropoxy derivative of ethylene diamine. h Block polyethoxypropoxy (10/90) compound of about 3500 mol wt (Wyandotte).
306
O b 0 g/lOC
Effect of buses on dispersions of Ti02 loading,
X
OjS
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
82 80 81 79 71
MEDL,
% 0.075 0.087 0.125 0.375 0.20
samples 1 to 5 indicates that small clusters may break dovin under high shear conditions, but reform quickly, in this case prior to microscopic examination. Extended high shear action contributes only marginally to deagglomeration a t a given dispersant concentration, by comparison of samples 3 and 4. Similar results to those in Table IV were obtained for X 202. I n addition samples taken during dispersion were centrifuged and the surface tension of the clear solution measured. In contrast to the DS 10 samples, which showed decreasing surface tension with increasing surfactant concentration as the N E D L was approached, the X 202 samples showed a constant value of 33 dyne/cm, corresponding to the value a t the ChlC. For TiOz (Titanox RA 45) a mill base prepared with D S 10 a t the RlEDL developed structure slowly on storage. This could be avoided only if additional DS 10 was added to give the equivalent of complete surface coverage as determined by surface tension or chemical methods. Similarly, pigment
Table IV. Microscopic Evaluation of TiOn Dispersion Sample
Magnification
1 2 3 4a 5
4 50 450 970 970 970
Observation
Many large clusters Few large clusters Some particles Few clusters, Bromnian motion Xot resolvable, rapid Brownian motion a This sample was milled for an extended time, ca. 20 min, to determine if changes could be detected. No actual change in rheology occurred; the point is displaced in Figure 5 for clarity only.
:
1
;i
$
b.
5-
h
9i ’ .
O*
W
3
t.
3
-5
w
2
3.
MEDL SI d
(I
v
NATE DS-IO
3k EQUILLBRIUM
~
CONCENTOATION
4-0 (HOLES/LlTPE
!
L IO+)
Figure 6.
Adsorption isotherm of dispersants on Ti02 pigment (Titanox RA 45) 0 Siponote DS 10 A Triton X 200 X
(Igepal CO 530)-sulfonate
derivative [ C Q H I ~ C ~ H ~ ( O C H Z C H ~ ) ~ O C H ~ C H Z C H ~ S O ~ N ~ ]
dispersed with ammonia solution could be stabilized by t.he addition of nonionic surfactant equivalent to complete coverage. As pigment loading in a mill base was increased, the MEDL tended toward complete surface coverage, as indicated in Table V. Adsorption Measurements by Chemical Analysis. Adsorption isotherms were constructed for DS 10 and X 200 by use of chemical analysis. The pigment was dispersed by shaking in a standard sulfonate solution. At regular time intervals, samples were centrifuged and filtered through 0.2micron Rlillipore filters. Sulfonate concentrations were then determined by the “Antara” method (Siggia, 1963).
DS 10 gave a two-stage adsorption curve similar to that observed (Greenwood e t al., 1968; Zettlemoyer, 1968) for sodium dodecyl sulfate on Graphon (Figure 6). This may explain the observation that the MEDL was found to occur at 60% coverage, since the ratio of inflection point to complete coverage is 0.57. According to Greenwood, the inflection for Graphon may have involved a change in orientation of adsorbed groups from horizontal to vertical with respect to the surface. The greatest possible coverage was probably obtained just before the onset of micelle formation (Figure 6). With X 200, however, such coverage occurred only with equilibrium solution concentrations considerably larger than Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
307
literature Cited
Table V. Effect of Pigment loading MEDL, % of complete coverage
60.4 66.5
79.0
on MEDL
Loading of mill b a r e pigment, wt %
66’/3
71 75
the apparent CMC, as measured by tensiometry in water alone. This would indicate that the surfactant affinity for the pigment-water interface was less than that for the air-water interface, leading to saturation of the latter before the former. Similar results were obtained fcr the known product C9H19C~Hs(OCH2CH~)70CH2CHZCH2SO3Na made from the sodium alcoholate of the nonionic polyether alcohol and propane sultone (partial curve in Figure 6). Adsorption only after the apparent CRSC has been attained explains the uniform surface tension values of 33 dyne/cm obtained from samples taken during the grind with X 202 (similar to X 200) referred to earlier. Unsuccessful attempts to characterize adsorption of this surfactant on pigment by the tensiometric method can also be explained by similar argument.
Bowman, A., Hughes, W., J . 021 Colour Chem. Ass., 46, 412 (1951). Brodnyan, J. G., Brown, G. L., J . Colloid Sei., 15, 76 (1960). Crowl, V. T., J . Oil Colour Chem. Ass., 4b, 169 (1963). Greenwood, F. G., Parfitt, G. D., Picton, 5 . H., Whatson, D. G., A d v . Chern. Ser., 79, 135 (1968). Hsiao, L., Dunning, H. N., Lorenz, P. B., J . Phys. Chem., 60, 657 (1956). Isaacs,‘P. K , J . Jlacromo/. Chewa., 1, 163 (1966). Liberti, F. P., Pierrehumbert, R. C., Of. Dtg., 31, 252 (1959). Maron, - - S. H., Elder, 31.E., J . Co2lozd Scz, 9, pp 263, 347, 353 (1Y 54 ).
RIaron, S.H., Elder, 11.E., Moore, C., ibid.,9, 104 (1954a). Maron, S.H., Elder, X. E., Ulevitch, 0. N., ibid., 9, 89 (1954b). Meader, A. L., Criddle, D. W., ibid.,8, 170 (1953). , (1958). Monk, C. J. H., J . Oil Colour Cherrz. ~ s s . 41,599 Sievard, L. L., Downey, W. W., J . Paint Techno/., 40, 293 (1968). Sievard, L. L., Stangs, R. W., Of. Dig., 36,862 (1964). Siggia, S., “Quantitative Analysis via Functional Groups,” 3rd ed., p 635, Wiley, New York, N.Y. (1963). Zettlemoyer, A. C., J . ColloidZnterfuce Sci., 28,342 (1968). RECEIVED for review June 29, 1970 A 4March 28, ~ 1971 ~
Presented at the Symposium on Behavior of Pigments in Organic Coatings sponsored by the Chemical Institute of Canada jointly with the American Chemical Society, Toronto, May 1970. The work was performed with the partial support of the Canadian Govt. under the Industrial Research Assistance Plan.
Adsorption of Hydrocarbons by Synthetic Zeolites Gordon R. Youngquist,l James 1. Allen, and Joseph Eisenberg Clarkson College of Technology, Potsdam, X . Y . lS676
Rate and equilibrium data for adsorption of ethane, ethylene, propane, propylene, butane, and butylene on synthetic zeolite, Davison 5A calcium microtraps were obtained. Equilibrium isotherms were of the Langmuir type. Analysis of the rate data indicates that macropore transport is limiting.
T h e attractiveness of zeolites as selective adsorbents has long been recognized. The general properties and action of zeolites as adsorbents have been well described elsewhere by Hirsch (1961), Breck (1964), and Barrer (1959). The structure of crystalline zeolites consists of a three-dimensional framework of Si04 and X104 tetrahedra in which the size of the opening (micropores) to adsorption cavities is controlled by the presence of various cations. Rates and equilibria for sorption of various gases and vapors on such crystalline zeolites, both natural and synthetic, have been studied frequently, as by Satterfield and Frabetti (1967), Barrer and co-workers (1953, 1961, 1964), Habgood (1958), Brandt and Rudloff (1964, 1965)) as well as others. Sorption rates generally have been observed to be limited by intracrystalline diffusion and isotherms frequently have been of the Langmuir type. The zeolites often encountered in industrial uqe, however, are made from a mixture of synthetic zeolite crystals and inert clay. These zeolites may be obtained in the form of beads or pellets of various sizes and have intercrystalline pores (macropores) which may range in diameter from 1
To whom correspondence should be addressed.
308
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
several hundred to several thousand angstroms. Thus, during sorption on this type of zeolite, sorbate molecules must first diffuse through the macropores, then through the micropores, and either or both processes may be rate limiting. =intonson and Dranoff (1969), for example, in a fised-bed study of ethane adsorption on Linde type-4A and 5-1 molecular sieve pellets, indicate that micropore diffusion controls t,he rate for 4h pellets, while both micropore and macropore diffusion are involved with 5 d pellets. Timofeev and Erashko (1964) found micropore diffusion rate limiting for water vapor sorption on Linde 5-1 pellets. In the present study, rates and equilibria were studied for the adsorption of several light hydrocarbons, including ethylene, ethane, propylene, propane, butane, and butylene, on Davison calcium microtraps. Zeolite samples were exposed to pure hydrocarbon in a closed system and t,he rate of uptake to equilibrium was determined gravimetrically. Experimental
The adsorption rate and equilibrium measurements were made with a recording electrobalance tem used to detect the change in weight which occurred as adsorption took place
~
