Langmuir 1993,9, 1684-1690
1684
Criteria for Effectiveness of Surfactants as Water-Moving Agents in “Unsaturated”Wet Sand Milind V. Karkare,Hoa T. La, and Tomlinson Fort* Department of Chemical Engineering, Vanderbilt University, Nashville, Tennessee 37236 Received January 25,1993. In Aha1 Form: April 19, 1993 A novel technique shows that certain water-insoluble surfactants spread at the air-water interface in “unsaturated”porous media, to move a significantamount of water from the surfactant-containingportion to the surfactant-freeportion of the system. This paper correlates properties of the surfactants with water movement in a wet but water-unsaturatedsand pack, at 23 O C . The water-moving abilities of 33surfactants (long-chain alcohols, acids, esters, and amines) were investigated. Equilibrium spreading pressures (?re) and surface pressure versw area per molecule ( P A ) isotherms were also obtained for these compounds. It is shown that for surfactants to be effective water movers they must be water-insolubleand must form a condensedsolid f i i at their ue. The amount of water moved by the effective surfactants was proportional to rewhen u, was greater than 18-20 mN/m, and the sand pack initially contained 12% water. The water-moving capacity of surfactants was independent of the polar head group, did not correlate with the rate of surfactant Spreading, and could be completely characterized by ?re and the monolayer state at ire. The pH of water did not affect water transport, thus suggestingthat the surfacecharge of the solid material was not an important variable.
Introduction Wet particulate material when packed together forms a structure with a network of pores. If the system is not *saturated”, the pores are only partially filled with water. The water coats each particle and collects in the capillary spaces between particles. The rest of the pore volume is occupied by a network of air spaces. In the work reported here, water-insoluble surfactants were applied to a portion of sucha system. The surfactants spread at the air-water interface network. The spreading led to movement of up to 90% of the water. The external energy input was small. Applications to processes of dryingldewatering, environmentalremediation, and even underground mining are obvious. The most effective surfactants tested were nontoxic biodegradable, and needed in extremely small quantities. Surfactant spreading on thin water films in “unsaturated” porous media has not been systematically studied. However, the spreading of oils on bulk water surfaces is one of the classical phenomena in surface chemistry.14 Spreadingmay occur as a “duplexfilm”or as a monolayer. The tendency toward duplex spreading is given by the spreading coefficient,718 Soil/water
-- rwater - (roil + Yoil/wateJ
where y denotes tension at the indicated interface. If the spreading coefficient is positive duplex spreading occurs. The lens becomes wider and thinner, ultimate thinness being a monomolecular layer. Filmsspreading from solids are alwaysmonolayersbecause surface solution of the solid is relatively slow, and deformation of the crystal does not
* To whom correspondence shouldbe addressedat the Department of Chemical Engineering, Box 1604, Station B,Vanderbilt University, Nashville, TN 37236. (1) Franklin, B. Philos. Tram. R. SOC.London 1774, 1774 (June 2), 144. (2) Reynolds, 0.Br. Assoc. Rep. 1881, Work 1,410. (3) Pockels, A. Nature 1891,43,437. (4) Devaux, H. J. Phys. Radium 1912,699,89. (5)Langmuir, I. J. Am. Chem. SOC.1917,39,1861. (6) Tanford, C. Ben Franklin Stilled the Waues; Duke University Prese: Durham, 1989. (7) Harkins, W. D. Chem. Reu. 1941,29, 386. (8) Harkins, W. D. Physical Chemistry of Surface Films; Reinhold Publishing Corp.: New York, 1962; Chapter 1.
occur. Assuming an excess of spreadingmaterial, the f i a l state of the film is given by the equilibrium spreading pressure, re,defined as *e
= Ywater - Ywater with monolayer
The classical work on monolayer spreading is that of Cary and Ridealg-l1 who carried out a detailed study of the spreading of myristic acid on water. They found an induction period before any measurable surface preeeure could be noted. Then, there was a steady rise in pressure to the equilibrium spreading pressure. The rate of the increase in pressure was proportional to the perimeter of the crystal at the three-phase interface, and to the difference between the equilibrium spreading pressure and that of the spreading film. Cary and Rideal also investigated the monolayer spreading of liquids. They found the rates of spreading of oleic acid, ricinoleic acid, and caprylic acid to be dependent on the amount of oil used for inoculation (the circumference of the lens), and that a constant velocity was attained shortly after spreading commenced. They speculated that a kind of two-dimensional pressure gradient might exist in films which spread very rapidly. Spreading of long-chain alcohols has been studied extensively,12-18a principal motivation being the utility of these materials as evaporation retarders from lakes and reservoirs. Mansfieldls reviewed three distinct monolayer spreading processes: (i) spreading of a gaseous film,(ii) spreading of a well-defined boundary across the water surface and away from the spreading source at a constant velocity, (iii) spreading of the boundary at a variable velocity. Referring to an illuminating paper by Ke~legan,’~ Mansfield associated processes ii and iii with coherent (9) Cary, A.; Rideal, E..K. Proc. R. SOC.London 1925, AlO9, 301. (10) Cary, A.; Rideal, E. K. Proc. R. Soc. London 1925, A109,318. (11) Cary, A.; Rideal, E. K. Roc. R. SOC.London 1925, A109, 331. (12) Rideal, E. K. J. Phys. Chem. 1925,29,1686. (13) Mansfield, W. W. Nature 1953,172, 1101. (14) Vines, R. G.;Meakins, R. J. A u t . J. Appl. Sci. 1959, IO, 190. (15) Mansfield, W.W. A u t . J. Chem. 1963, 16, 76. (16) Mansfield, W. W. A u t . J. Chem. 1959,12, 382. (17) Stewart, F.H. C. A u t . J. Appl. Sci. 1960,12,167. (18) La Mer, V. K. In Retardation of Evaporation by Monoluyers; La Mer, V. K., Ed.; Academic Press: New York, 1962. (19) Keulegan, G.H. J. Res. Natl. Bur. Stand., Sect. A 1951,46,368.
0743-7463/9312409-1684$04.00/0 0 1993 American Chemical Society
Langmuir, Vol. 9, No. 7,1993 1685
Surfactants as Water-Moving Agents film spreading at very low and at equilibrium spreading pressures. Brooks and Alexander20 investigated the spreading behavior of long-chain alcohols and reviewed the work of previous researchers. They correlated spreading behavior with crystalline phases of the alcohols, and measured both spreading rates and equilibrium spreading reported rates of pressures. Roylance and Jones21*22 dissolution from the crystal, and Spreading, for hexadecan01 films. G a i n e ~reviewed ~~ some peculiarities of spreading of fatty alcohols. O’Brien et al.2426investigated surfactant spreading on bulk water surfaces and related water movement. They followed the progress of the monolayers both by interferometry and by monitoring temperature transients. Spreading speeds of different materials were measured. A strong dependence of the spreading speed on the spreadingpressure was found for a given homologous series of compounds. A linear relationship was found between the spreading speed and entropy of spreading for unionized fatty acids, suggestingthat the greater the entropy change between the bulk and surface, the faster the spreading speed. They also showed that water under a spreading monolayer moves at the same speed as the monolayer to a depth of 0.1 mm or less and that some motion is transferred to the water down to larger depths. Water movement in unsaturated porous media by insoluble surfactants was first called to our attention by scientists in Argentina.2731 Tschapek et aLn packed wet sand in a series of plastic ‘rings”. The sand in some rings was premixed with a small amount of surfactant. The rings were then placed in contact, and movement of water from the surfactant side to the surfactant-free side of the found the most assembly was observed. Tschapek et effective surfactants to be aliphatic alcohols with chain lengths of 12-18carbon atoms. Tschapekand Wasowski28 found some indication that surfactants spread within the packed bed and speculated that the alcohol molecules dragged water with them as they moved. However, the rate of surfactant spreading was 3 orders of magnitude lower than on a bulk water surface, and surfactant movement did not coincide with water movement. The main objective of this paper is to establish the relationship between the spreading properties of different Surfactants and movement of thin films of water in unsaturated packed beds. Water movement is correlated with the chain length of the surfactant, the equilibrium spreading pressure, and the monolayer state of the spreading molecules. The importance of specific interactions between the polar head group of the surfactant and the underlying water molecules is investigated through a comparative study of long-chain alcohols, fatty acids, esters, and amines as water-moving agents. (20)Brooke, J . H.;Alexander, A. E. Retardation of Euaporation by Monolayers: La Mer, V. K., Ed.; Academic Press: New York, 1962. (21)Roylance, A.; Jones,T. G. J. Appl. Chem. 1969,9,621. (22)Roylance, A.;Jones,T. G . J. Appl. Chem. 1961,II,329. (23)Gaines, G. L.,Jr.Zmoluble Monolayers at Liquid-GasZnterfaces; Interscience Publishers: New York, 1966,Chapter 5. (24)OBrien, R.N.;Feher, A. I.; Leja, J. J. Colloid Interface Sci. 1975, 51,366. (25)OBrien, R.N.;Feher, A. I.; Leja, J. J. Colloid Interface Sci. 1976, 56, 469. (26)OBrien, R.N.;Feher, A. I.; Leja, J. J. Colloid Interface Sci. 1976, 56,414. (27)Tschapek,M.; Wasowski, C.;TorresSanchez,R. M. Colloids Surf. 1981,3,295. (28)Tschapek, M.; Wasowski, C. Colloids Surf. 1982,5,65. (29)Tschapek, M.;Wasowski, C.; Falasca, S. Colloids Surf. 1984,II, 69. (30)Tschapek, M.; Waaowski,C.;F h ,S.J.Dispersion Sci. Technol. 1987,8,493. (31)Tschapek, M.; Wasowski, C.;Falasca,5.Colloids Surf. 1991,55,1.
Teflon Plug
I
1 1 1 Wet Sand Wet Sand with without Surfactant Surfactant
I
Teflon Plug
I I
k
10 cm
Glass Tube
Figure 1. Experimental setup for measuring the movement of water caused by surfactant spreading in wet sand.
Experimental Section Materials. 1-Decyl alcohol (99+% ), 1-dodecanol (98%), l-tridWol(W%),l - t e h h o l ( W % ) ,l-pentedeCanol(99+%), l-hexadecanol (99%), l-heptadecanol (98%), 1-acted-01 (99.6%),l-eicoeanol(98%),1-dodecanoic(lauric)acid (99.5+%), 1-tetmdecanoic(myristic)acid (99.5+ %), 1-hexadecanoic(palmitic) acid (99%), 1-dodecylamine (99+%), 1-tetradecylamine (96%),l-hexadecylamine(90%),l-octadecylamine(99%),methyl myristate (99% 1, and methyl palmitate (99+% ) were obtained fromAldrichChemicalCo.,Milwaukee,WI. l-Docosanol(98%), oleyl alcohol (99% ) ,elaidylalcohol (99% 1, oleicacid (99% 1, ethyl myristate(99% 1, and propyl stearate (99%) werepurchased from Sigma Chemical Co., St. Louis, MO. 1-Octadecanoic(stearic) acid (>99.5+%) was supplied by Alltech Associatee, Inc., Deerfield,L.l-Nonadecanol(>99%),ethylpalmitate (99+%), methyl stearate (99+% 1, ethylstearate (99+% 1, myristyl acetate (99+% ), palmityl acetate (99+% 1, stearyl acetate (99+% 1, and arachidylacetate (99+% ) were purchased from Nu Chek Prep, Inc., Elysian,MN. All these compoundswere used as received. Chloroform(ACS certified,Spectranalyd)obtained fromFisher ScientificCo., Springfield,NJ, wasusedas the spreadingsolvent. Reagentgradehydrochloric acidand sodiumhydroxidewere uaed to adjust the pH of the aqueous subsolution. Deionized water was distilledfrom alkalineKMnO, solutionbefore use. Thesand was Washed Sea Sand, Lot No. 915262B,obtained from Fisher. Water Movement in Sand Experimental Setup and Measurements. The experimentalsetup is shown in Figure 1. It consisted of a 20-cm-long glass tube with Teflon plug^ at the ends. The plugswereloose enoughto allowatmosphericpreeeure to be maintained in the column. The glasstube had a measuring tape attached to ita surface for easy measurement of distances. The middle 10-cm section of the column was used to pack the sand. Tobeginane.periment,aquantityofsandwasweighed. Water wasmixedwiththe sand to achievea desiredinitial water content of 12%. The wet sand mixture was divided into two parts. One part was used to fill the right half (length 5 cm) of the column. The mixture was packed using a Teflon rod as a plunger. To the second half of the wet sand, a preweighed amount of surfactant wasadded to achievea deairedsurfactantconcentrationof -0.1 % based on the dry sand weight. This wet mixturewasthen packed in the left half (length 5 cm) of the column. The column was weighed to determine the amount of wet material it contained. With the plugs in place, the column was wrapped in a Saran plastic wrap (DowBrands,Inc., Indianapolis, IN). It was then placed horizontally. With the plastic wrap surrounding the column,there was practically no weight loss due to evaporation. After 24 h, the wrap was removed from the column. The column was weighed to ensure that the evaporation losses were negligible. Using a Teflon plunger, the wet sand mixture was slowly pushed out of the column. As the material came out of the right side of the assembly samples were sliced off,collected in aluminum dishes, and weighed immediately. The samples were dried to constant weight in an oven at 110 OC. The dried material was cooled and weighed. The water content of each samplewascalculatedas weightpercenton the basis of the weight ofthe dry sand. Theprecisionof the measurementswas typically better than 0.2 wt % water. All experimentswith alcohols were performed at least three timea and employed dietilled water at pH 5.8. All experimenta with other surfadante were performed at least twice. Those with acids and esters employed water acidified to pH 2. Those with amines used water with pH 12.
Karkare et al.
1686 Langmuir, Vol. 9, No. 7, 1993 20 18
8 initial Water
e
B
c
j4 3
4
Position, cm
Figure 2. Movement of water in sand cawed by a spreading monolayer of l-tetradecanol, pH -5.8.
Equilibrium Spreading Pressure (I.) Msarwements. Equilibrium spreadingpreesureswere measured by the Wilhemyplate method wing a fiiter paper strip (Whatman Grade lCHR, 1 cm wide) suspended from a Cahn RG electrobalance (Cahn
Instrument Co., Paramount, CA). The output from the electrobalance was continuously monitored by a chart recorder. A quantity of water was contained in a 90-mm-diameterglass dish. The pH of the water was adjwtad wing staadard HCl or NaOH solutions. The glass dish was enclosed in a box that was maintained at a high humidity to reduce evaporation. Small crystals of solid surfactants or small dropa of liquid surfactants were placed on the water surface, and the surface preseure was measured as a function of time. When the rate of change in the surface pressure (r)was lese than 0.2 (mN/m)/h, the surface preseure value was noted as the equilibrium spreading preseure (re). The precision of these measurements was -0.1 mN/m, and the standarddeviationof the measurementswas 1mN/m in most -8.
Surface Pressure versu~Molecular Area (PA) Isotherms. Surfacepressure uemm moleculararea isothermswere recorded wing a Langmuir trough (NIMA 2000 System, Nima Technology Ltd., Coventry, England) made of Teflon equipped with a Wilhemy-plate-typesurfacetension sensor. A fiiter paper strip (Whatman Grade lCHR, 1 cm wide) wae used ae the Wilhemy plate. The instrument and barrier motion were e x t a d y controlled though the NIMA intelligent aerial interface by a 386-SXcomputer and NIMA controlsystemsoftware Version 4.64 (May 1992). T h e Langmuir trough was covered by a Plexigh cover during operation. T h e purity of the subsolution was checked by sweeping the barrier and measuringthe surface pressure. If any preseure rise was detected, the water surface was aspiratad following its compreesion to a small area wing a disposable glass pipet. The solvent used did not result in any detedable rise in surface pressure when spread on a clean subsolution. T h e monolayer spreading solutions were prepared daily by diesolving approximately 10 mg of surfactant in 10 mL of CHCb. A Hamilton 6O-pLgas-tightsyringewas ueed to meter the spreadingsolution onto &e water surface. The initial trough surface area was typically 700 cm2. After the evaporation of the solvent (5-10 min),monolayerswere compressedat a rate of 90cm2/minwhich corresponded to 0.05-0.1 (nm2/molecule)/min. A mom temperature of 23 O C was maintained during all
experiments.
Results and Discussion Typical Water Movement Results. Typical results of water movement experiments with l-tetradecanol as the spreading surfactant are presented in Figure 2. Initially, all of the sand in the assembly contained 12% distilled water based on the weight of dry sand. The left half of the assembly also contained 0.1 % l-tetradecanol based on the dry sand weight. The void volume fraction was estimated to be 0.3,on the basis of the weight of a packed dry sand column. The initial pore saturation by water was -39 % . The assembly contained about 90 g of
sand, which had an approximate dry geometric surface area of 130 cm2/g as determined by measuring the dimemioneof many sandgrainswit h a microscope. Figure 2 shows that after 24 h at 23 "Cthe water content of sand on the left side had been reduced from 127% to 6-7 % The water content on the right side increased an equivalent amount, from 12% to 17-18%. Thus it was found that there was a significant migration of water from the "surfactant side" to the "surfactant-free side" of the assembly. The amount of water moved was calculated as the gain in water by the surfactant-free side of the assembly. In this example, 2.4 g of water moved, resulting in a specific water movement of 0.027 g/g of sand. The new distribution of water was stable for at least 2 months. The water did not saturate the system but coated each sand grain to an average depth of -0.001 cm. This figure is based on the total surface area of the dry sand grains. The actual water f i i thickness was not constant but varied from a few nanometersto several micrometers. A network of airspaceswasleftwithin thepackedbed. Thesurfactant was insolublein water and spread at the air-water interface network. It was assumed that spreading resulted in a surfactant monolayer. Taking the water surface area to be the same as the sand surface area, and assuming that each l-tetradecanol molecule occupied -0.2 nm2 (the cross-section of a hydrocarbon chain)23a total of 4.6 X 10l8 molecules would cover 90 g of sand. This many molecules of alcohol moved 8 X loZ2 molecules of water. More dramatically, the ratio of water molecules moved to alcohol molecules effecting movement was 1.7 X This ratio is of the same order of magnitude as reported by Tschapek et al.% Equilibrium Spreading PressureData. The driving force for surfactant spreadingis the equilibriumspreading pressure, re. Equilibrium spreading pressures were measured for each surfactant at 23 OC. Table I summarizes the results along with available literature values of re. Also included are approximate times to reach 96 % of the revalue for each surfactant. These times depended on the crystal/drop perimeter. No attempt was made to control the amount of surfactant used, so the times are only a qualitative indication of spreading rates. When the rate of change in the surface pressure (r)was less than 0.2 (mN/m)/h,the surface pressure value was noted as the equilibrium spreading pressure (re).At least 2 h was allowed before any remeasurementswere recorded. It is clear from Table I that surfactants in this study develop 95% of their revalues in periods ranging from just a few minutes up to almost 1 day. All alcohols were spread on distilled deionized water (pH -5.81, and all acids and esters were spread on 0.01 N HC1 subsolution while all amines were spread on 0.01 N NaOH subsolution. The standard deviation in the re measurements was -1 mN/m for all compounds except amines for which a standard deviation of 3-5 mN/m was observed. The revalues are very sensitive to impurities and therefore sample dependent. This may explain the differences between measured and literature values. Effect of pH of the Subsolution on Water Movement. The effect of pH on water movement was studied using l-tetradecanolas the control. The amount of water moved at pH 2 was 0.0287 g/g of sand. At pH 12the water movement was 0.0277 g/g of sand. At pH 5.8 the water movement was 0.0278 g/g of sand. Differences in these values are within the limits of experimental error. It was also experimentally confirmed that pH had no significant effect on the F A isotherm and reof l-tetradecanol at 23 OC. These data confirm that water movement is inde-
.
le,
Langmuir, Vol. 9, No. 7, 1993 1687
Surfactants as Water-Moving Agents Table I. Equilibrium Spreading Prerrum of Surfactantr
surfactant
*.(red) at approximate *.(abed) temperature, time required O C , shown in to reach 95% of a t 23 OC, mN/m parenthesb, mN/m the rovalue
Alcohols 40.7 42.7 48.3 45.4 46 (25)" 42.5 43 (25)" 37.7 40 (25)" 43.5 41 (25)" 33.4 35 (25)" 34.5 34.1 30.1 28.4 33 (25)" 28.2 28 (25)" Acids (Spread on 0.01 N HC1) 1-dodecanoicacid 23.9 1-tetradecauoic acid 24.7 17.5 (23)% 1-hexadecanoicacid 6.0 12.5 (23)% 1-octadecanoicacid 4.8 3.5 (23)'O oleic acid 31.2 30 (25)" Eatera (Spread on 0.01 N HC1) methyl myristate 22.3 ethyl myristate 17.9 20 ( 2 5 P methyl palmitate 24.5 14 (20)'O ethyl palmitate 26.1 15 (20)'O methyl stearate 16.3 ethyl stearate 23.5 propyl stearate 11.0 myristyl acetate 19.5 palmityl acetate 33.6 31 (15)'O stearyl acetate 24.1 23 (15)'O arachidyl acetate 25.1 Amines (Spread on 0.01 N NaOH)" 1-dodecylamiie 28 1-tetradecylamine 37 1-hexadecylamine 37 1-octadecylamine 38 1-d-01 1-dod-01 l-tridecanol 1-tetradecanol 1-pentadecanol 1-hexadecanol 1-heptadecanol 1-octadecanol elaidyl alcohol oleyl alcohol 1-nonadecanol 1-eicoeaaol l-docmanol
C1 min C1 min
s
ti t;j
z
a,
20
e
0.010
10
0
-
Parent Alcohol Acetate Esters
Amines
Amines
Acetate
Esters
--*--
Amines
Figure 9. Movement of water in sand using acetate esters of long-chainalcohols and amines as spreading surfactants,pH -2 for esters and pH -12 for amines. The bars indicate water movement, and the line and point graphs show ?r,'s.
1 k-,
"" I 50
E
I Arachidyl Acetate
40
2
E
30
g
20
0.1
0.2
0.4
0.3
0.5
O.O1
t
0.00
0
. .
0
e , , 10
20
30
40
50
ne, mN/m
Figure 11. Correlationbetween reand water-movingcapacities of different surfactants (temperature23 "C, surfactant concentration 0.1% ,initial water content 12% ,pH -2 for acids and esters, 6.8 for alcohols, and 12 for amines, time 24 h, column length 10 cm, porous medium is sand).
3
0.000
c
s
0.6
*
Area, nm /molecule Figure 10. Surface pressurearea isotherms for esters of longchain alcohols (temperature 23 "C, pH -2, compression rate 0.05-0.1 (nm2/molecule)/min).
was better correlated to their monolayer states than to their values. Acetic acid esters of c14,c16,c18,and C20 alcohols were also tested. Results shown in Figure 9 demonstrate that the c16 acetate is more effective than either the c18 or the C20 acetate. More striking is the complete inability of the C14 acetate to move water as compared with the c 1 6 acetate even though its r e value (20 mN/m) is only slightly lower than that for the Cis ( r e = 24 mN/m) and C20 ( r e = 25 mN/m) acetates. This difference can be explained by the T-A isotherms of the acetate esters shown in Figure 10. Myristyl acetate, the only ineffective ester among the acetate esters tested, was also the only surfactant in the group that did not form a condensed solid film at 23 "C. Amines as Spreading Surfactants. Figure 9 also shows results from water movement experimentswith longchain amines. The c14, c16, and c18 amines with similar values displace approximately the same amount of water. l-Dodecylamine,with a lower is not as effective
as the others in the group. The observationsof Adam32,33 that the c14, c16, and Cla amines form condensed solid films whose F A isotherms are very similar to the correspondingalcohols were confirmed(resultsnot shown). The T-A isotherm for l-dodecylamine was difficult to obtain due to its high solubility. However, on a subsolution saturated with C12 amine, the film formed may be a condensed solid at its r e . Summary of all Spreading Surfactants. Figure 11 shows the correlation between water movement abilities and equilibriumspreadingpressures for all the surfactants studied. In this figure,filled symbolsrepresent surfactant molecules that do not form condensed solid films at their values while open symbolsrepresent the moleculesthat do. The least-squares line obtained for the alcohols is shown for comparison. It is clear that, at values higher than about 18-20 mN/m, the relationship between water movement and can be described by a straight line. It is also very clear that the data for molecules that do not form condensed solid films at their r e values as well as data for molecules with < 20 mN/m do not lie on this line. It can be concluded that the water movement is independent of the polar head group of the surfactant molecule. Specific interactions between the polar head group and water molecules are not important as long as the spreading occurs as a condensed solid film at A high r e results in a high driving force, and therefore a greater amount of water is moved. Spreading Speed and Water Movement. An attempt was made to correlate surfactant spreadingspeedson bulk water reported by O'Brien et aZ.26with their ability to move water. Water movement was plotted against the spreading speeds of even-numbered C12-C22 alcohols and even-numbered c12%18 fatty acids, and their methyl and ethyl esters. Results are shown in Figure 12. A linear fit to the plot of water movement versus spreading speed resulted in a correlation coefficient of only 0.33. Water movement was independent of the initial spreading speed of the monolayer. T e e
Conclusions Water-insoluble surfactants spread at the air-water interface in unsaturated porous media and move water into the surfactant-free region of the media. For surfac(32) Adam, N. K. The Physics and Chemistry of Surfaces, 3rd ed.; Oxford University Press: New York, 1941; Chapter 2. (33) Adam, N. K.h o c . R. SOC.London 1931, A126,526. (34) Deo, A. V.; Kulkami, S. B.; Gharpurey, M. K.;Biswae, A. B. J. Phys. Chem. 1962,66,1361. (35) Boyd, G. E. J. Phys. Chem. 1958,62,536.
Karkare et al.
1690 Longmuit, Vol. 9, No. 7, 1993 0.030 0
Alcohols
0
Esters
0 u)
m .
0.020
m
0
100
200
300
Spreading Speed, mmlsec Figure12. Correlationbetween the spreadingspeed of different surfactante on bulk water and water movement caused by them in an unsaturated sand column. The spreading speed data were used from O'Brien et al.% tanta to be effective water movers they must form a condensed solid f h at their equilibrium spreading pressure. Water moved in sand initially containing 12%
water is directly proportional to rewhen the r e values are greater than 18-20 mN/m. The amount of water moved by a given surfactant can be completely characterized by ita r e and monolayer state at r e . Specific interactions between the polar head group and the underlying water molecules are not important. Themoet effective surfactant at 23 "Cis l-tridecanol which forms a condensed solid film at ita equilibrium spreading pressure and has the maximum r e among the surfactants studied. l-Tetradecanolis almost equally effective. The rate of surfactant spreading on bulk water does not correlate with the amount of water moved. The pH of water does not affect water transport, thus suggestingthat the surface charge of the solid material is not an important variable.
Acknowledgment. We thank Ms. Alice May and Ms. Denvia Laugel for their help in obtaining some of the surface pressure versus area isotherms presented in this work. Financial support was provided by the National Science Foundation, Grant No.CTS-9213478.
