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Effect of Temperature on Surfactant-Driven Water Movement in Wet “Unsaturated” Sand Milind V. Karkare† and Tomlinson Fort* Department of Chemical Engineering, Vanderbilt University, Nashville, Tennessee 37235 Received August 10, 2001. In Final Form: December 20, 2001 The effect of temperature on the surfactant-driven movement of water in wet “unsaturated” sand was investigated and correlated with the equilibrium spreading pressure of the surfactant and the character of the surfactant monolayer at πe. The previously determined criteria for surfactant effectiveness were confirmed. An approximately linear relationship between πe and the amount of water moved by effective surfactants was found. Temperature changes affect water movement by influencing the surfactant πe and the monolayer state at πe.
Introduction 1-5
we have shown how the apIn previous reports, plication of certain long-chain surfactants to part of a wet “unsaturated” porous medium can lead to movement of most of the water from the surfactant-containing to the surfactant-free part of the system. The surfactants spread as monolayers at the air-water interface to establish sharp capillary pressure gradients within the system. Water moves to reestablish system equilibrium. We have also shown that criteria for surfactant effectiveness as water movers include (1) water insolubility, (2) high equilibrium spreading pressure at the air-water interface, and (3) formation of a solid condensed monolayer at the equilibrium spreading pressure. Tests of 33 surfactants showed that long-chain alcohols with 12 or more carbon atoms best meet these criteria at room temperature. We were interested to learn the effects of temperature on surfactant behavior and water movement and, so, conducted some experiments. We report here studies of water movement in unsaturated wet sand at 3, 13, 23, 33, and 43 °C and correlate this water movement with the equilibrium spreading pressures, πe, and the character of the surface pressure-area, π-A, curves for seven longchain alcohol surfactants. Experimental Section Materials. The sand used was Fisher washed sea sand (Lot 915262B) purchased from Fisher Scientific Company, Springfield, NJ. Specifications have been given in ref 2. 1-Decanol (>99%), 1-dodecanol (98%), 1-tridecanol (97%), 1-tetradecanol (97%), 1-pentadecanol (>99%), 1-hexadecanol (99%), and 1-heptadecanol (98%) were obtained from Aldrich Chemical Co., Milwaukee, WI, and used as received. Deionized water was distilled from alkaline KMnO4 before use. The surface tension of this water was measured to be 72.5 mN/m at 23 °C. Equipment and Procedures. The equipment, experimental setup, and technique for measuring water movement, surfactant equilibrium spreading pressures, and force-area curves at room temperatures have been described previously1 and will only be summarized here. Accurate measurements of these quantities at temperatures other than room temperature required that both * To whom correspondence should be addressed. † Present address: IDEAS Technologies, 125 Clairemont Avenue, Decatur, GA 30030. (1) Karkare, M. V.; La, H. T.; Fort, T. Langmuir 1993, 9, 1684. (2) Karkare, M. V.; Fort, T. Langmuir 1993, 9, 2398. (3) Karkare, M. V.; Fort, T. Langmuir 1994, 10, 3701. (4) Karkare, M. V.; Fort, T. Langmuir 1996, 12, 2041. (5) Silverstein, D. L.; Fort, T. Langmuir 2000, 16, 835.
the air and the water temperatures be as specified. This was accomplished by placing the measuring systems inside a Precision Scientific low-temperature incubator (model 815, Precision Scientific Company, Chicago, IL) that holds temperatures constant to within ca. (1 °C. Water Movement Measurements. The middle 10-cm section of a 2.54-cm-diameter, 20-cm-long glass tube was packed with wet sand. The water content was 12% based on weight of the dry sand. The left half of the column also contained ca. 0.1% surfactant based on the weight of the dry sand. This amount of surfactant was in excess of that required to form a close-packed monolayer at the air-water interface within the column.4 The ends of the tube were capped with Teflon plugs and placed horizontally. After 24 h, the water contents of different sections along the length of the column were determined by measuring weight loss on drying and are reported as weight percentages based on the weight of dry sand. For measurements at temperatures other than 23 °C, the glass column, Teflon plugs, sand, water, and ancillary equipment were placed in the incubator at the desired temperature. The sand was weighed outside the incubator and mixed with the appropriate amount of water to obtain wet sand with the desired initial water content. The wet sand was divided into two parts and placed on two sheets of aluminum foil. One part was wrapped in the foil and placed in a sealed plastic bag. A preweighed amount of surfactant was blended with the second part of the wet sand, which was then also wrapped in aluminum foil and sealed in a plastic bag. Both samples were placed in the incubator for ca. 30 min to achieve the desired temperature. The two bags and the column were removed from the incubator and packed as previously described. The column was placed horizontally in the incubator for 24 h. The column sections were then analyzed for water content. Equilibrium Spreading Pressure Measurements. Equilibrium spreading pressures were measured by the Wilhelmy plate method using a filter paper strip (Whatman Grade 1CHR, 1 cm wide) that dipped into a quantity of water contained in a 90mm-diameter dish. The filter paper strip was suspended from a Cahn RG electrobalance (Cahn Instrument Company, Paramount, CA). The output from the electrobalance was continuously monitored with a chart recorder. When the rate of change in surface pressure was less than 0.2 (mN/m)/h, the surface pressure was taken as the equilibrium value. The precision of these measurements was ca. 0.1 mN/m, and the standard deviation of the measurements was 1 mN/m. For measurements at temperatures other than 23 °C, the entire Wilhelmy plate assembly was placed in the incubator along with the surfactant to be studied. After all materials had reached the desired temperature, the incubator door was opened very briefly, and several solid crystals or a few small drops of surfactant liquid were added to the dish. The equilibrium surface pressure was measured as at room temperature. Surface Pressure vs Molecular Area Isotherms. Force-area isotherms were determined with a circular NIMA Langmuir
10.1021/la011273q CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002
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Figure 1. Effect of temperature on the movement of water in wet sand by a homologous series of long-chain alcohol surfactants.
Figure 2. Variation of equilibrium spreading pressures, πe, with temperature for a homologous series of long-chain alcohol surfactants.
trough (NIMA 23000 System, Nima Technology Ltd., Coventry, England). The monolayer spreading solutions were prepared daily by dissolving ca. 10 mg of the surfactants in 10 mL of chloroform. Monolayers were compressed at a rate of 90-120 cm2/min, which corresponds to 0.05-0.1 (nm2/molecule)/min. For measurements at temperatures other than 23 °C, the trough was placed inside the incubator, leveled, and filled with water. The incubator temperature was set 3-4 °C above the desired temperature if the desired temperature was greater than 23 °C and 3-4 °C below the desired temperature if the desired temperature was below 23 °C. Twenty-four h was allowed for the entire assembly to reach the set temperature. The surface pressure sensor was then calibrated. A glass bead thermocouple placed in the water monitored the liquid-phase temperature, and the incubator thermocouple measured air temperature. The water temperature was confirmed with a thermometer. The incubator door was opened, and the water surface was cleaned. Opening the door caused a slight change in water temperature and a larger change in air temperature. However, the water and air temperatures remained close to the desired value. The incubator temperature was then set at the desired value. After ca. 30 min, both the water and air temperatures stabilized at this value. The monolayer spreading solution was delivered onto the water surface while the incubator door was opened for a very short time. A small change in air temperature and no measurable change in water temperature occurred. The door was closed and a period of time (20 min at 3 °C, 15 min at 13 °C, 5 min at 33 °C, and 3 min at 43 °C) was allowed for evaporation of the spreading solvent. During this time, the air temperature returned to the desired value. Just before compression began, the incubator was switched off to avoid any vibrations at the air-water interface and to stop the forced flow of air. Temperatures remained constant to within (1 °C throughout the measurement.
The temperature at which water movement was a maximum was 13 °C for 1-dodecanol, 23 °C for 1-tridecanol, and 33 °C for 1-tetradecanol. Water movement by 1-pentadecanol increased with temperature to the same maximum at 33 and 43 °C. Water movement by 1-hexadecanol and 1-heptadecanol showed a continuous increase as the temperature was raised from 3 to 43 °C. Dependence of Equilibrium Spreading Pressures on Temperature. Results of equilibrium spreading pressure measurements for six of the long-chain alcohol surfactants are shown in Figure 2. The figure shows that πe for 1-decanol decreases from ca. 50 to ca. 40 mN/m as temperature is raised from 3 to 23 °C. πe for 1-dodecanol increases to a maximum of ca. 46 mN/m at 13 °C and decreases slightly at higher temperatures. πe values for 1-pentanol and 1-hexanol are relatively low at low temperatures and increase steadily to values above 40 mN/m at the highest temperatures investigated. Some of these values can be compared with data collected by Brooks.6 See also Deo et al.7 Dependence of Monolayer Character on Temperature. Surface pressure vs area per molecule isotherms for monolayers of four of these six alcohols were obtained at the desired temperatures. Figure 3 shows π-A isotherms for 1-decanol at 3 °C and 1-dodecanol at 3, 13, and 23 °C. Clearly, 1-decanol is significantly soluble in water even at 3 °C because its molecular area at high surface pressures is much lower than the expected value of ca. 0.2 nm2. It is interesting to note that the 1-dodecanol monolayer appears to be a liquid film even at 3 °C. Determination of π-A isotherms at temperatures higher than 3 °C was impossible because of the still-greater solubility of 1-decanol. Surface pressure-area isotherms for 1-dodecanol could only be determined at 23 °C and below because of the high solubility and/or high evaporation rates of monolayer molecules at 33 and 43 °C. At 23, 13, and even 3 °C, 1-dodecanol forms an expanded monlayer at high areas per molecule and then undergoes a transition to a condensed solid film at surface pressures below 20 mN/m. The low areas per molecule for the condensed film at 23 °C indicate loss of material through dissolution or evaporation at that temperature.
Results and Discussion Dependence of Water Movement on Temperature. The water movement caused by seven long-chain alcohol surfactants was measured at five temperatures. The experiments at 43 °C were repeated three times, and the experiments at 33 °C were repeated two times. Excellent reproducibility was obtained: the maximum standard deviation was 0.002 (g of water moved)/(g of sand). The results, summarized in Figure 1, show that the abilities of all of the alcohols to move water varied with temperature. However, the trends were different for different alcohols. Water movement by 1-decanol, which was only studied at room temperature and below, was greatest at 3 °C and was almost zero at 23 °C. Water movement by 1-dodecanol, 1-tridecanol, and 1-tetradecanol first increased and then decreased as the temperature was raised.
(6) Brooks, J. H.; Alexander, A. E. In Retardation of Evaporation by Monolayers; La Mer, V. K., Ed.; Academic Press: New York, 1962. (7) Deo, A. V.; Kulkarni, S. B.; Gharpurey, M. K.; Biswas, A. B. J. Colloid Sci. 1964, 19, 813.
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Figure 3. Monolayer compression characteristics of 1-decanol and 1-dodecanol at several temperatures.
Figure 4. Monolayer compression characteristics of 1-tetradecanol and 1-hexadecanol at several temperatures.
Figure 4 shows π-A isotherms for 1-tetradecanol at 3, 13, 23, 33, and 43 °C and for 1-hexadecanol at 43 °C. Up to 23 °C, 1-tetradecanol forms condensed solid monolayer films that yield almost identical π-A isotherms. At 33 and 43 °C, it was very difficult to obtain reliable π-A isotherms for 1-tetradecanol. In addition, the molecular areas were low at higher surface pressures. Brooks and Alexander8 found that 1-tetradecanol monolayers lose 11% of film molecules per minute at 40 °C and concluded that the loss mechanism was evaporation and not dissolution in the subphase. Despite these difficulties, 1-tetradecanol monolayers at 33 and 43 °C appear to undergo a transition to condensed solid films at surface pressures above 2530 mN/m. A π-A isotherm for 1-hexadecanol at 43 °C was obtained. The evaporation rate for 1-hexadecanol monolayers is reported6 to be only 0.9% of film molecules per minute at 40 °C. This isotherm, also shown in Figure 4, was obtained easily and shows that 1-hexadecanol forms a condensed solid monolayer film at 43 °C and, presumably, also at lower temperatures. Discussion. The monolayer compression and equilibrium spreading pressure measurements show that alcohols with 14 or more carbon atoms form condensed solid (8) Brooks, J. H.; Alexander, A. E. In 3rd International Congress on Surface Activity; Verlag der Universita¨ts Druckerei: Mainz GmbH, Vol. 2, p 196.
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Figure 5. Dependence of water movement on equilibrium spreading pressure, πe, of long-chain alcohol surfactants at several temperatures. The water movement data for these alcohols is taken from Figure 1. The πe values are taken from Figure 2.
monolayer films at their πe values at temperatures equal to or less than 43 °C. 1-Decanol at all temperatures and 1-dodecanol at temperatures greater than 23 °C form liquid or gaseous monolayer films at their πe’s. Our earlier studies at room temperature showed2 a linear dependence of water movement on equilibrium spreading pressure, if the monolayer formed a condensed solid film at πe. Figure 5 summarizes the correlation for systems studied in this work. The data points taken from monolayers that were condensed solid films are indicated by open symbols. The data points that do not represent condensed solid films are indicated by closed symbols. The condensed solid film data were fit with a least-squares line, which suggests that a minimum πe value of 25 mN/m is necessary for long-chain surfactants to be effective water movers in sand that contains 12 wt % of water. A similar plot based on experiments at 23 °C only indicated2 this lower limit to be a comparable 20 mN/m. It is interesting to note that the only reason found for the failure of long-chain surfactants that have a high πe to move significant amounts of water (for example, 1-decanol at 3 °C where π e is 50 mN/m) is their inability to form condensed solid monolayer films. Conclusions The results of this study strongly support the criteria for surfactant effectiveness proposed earlier. An effective long-chain water-moving surfactant must be waterinsoluble, must have a high equilibrium spreading pressure, and must form a condensed solid monolayer film at its equilibrium spreading pressure and operating temperature. Above a critical lower limit, there is an approximately linear relationship between equilibrium spreading pressure and the amount of water moved. Temperature variations affect water movement by affecting the value of the equilibrium spreading pressure and the monolayer state. Acknowledgment. We thank Ms. Darcy E. Fleming and Ms. Xueli Zhang for checking some of the surface pressure vs area isotherms and equilibrium spreading pressure data presented in this work. Financial support for our work was provided by the National Science Foundation, Grant CTS-9703574. LA011273Q
