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Langmuir 1999, 15, 1556-1561
Effect of Dispersed Tetradecanol Particles or Droplets on the Dynamic Surface Tension of Aqueous Tetradecanol Systems Sabrina H. Myrick and Elias I. Franses* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283 Received June 16, 1998. In Final Form: December 3, 1998 For sparingly soluble surfactants at their solubility limit, the surface tension equilibration is usually very long, and adsorption depletes the solution considerably. By replenishing the adsorbed material and by being close to the surface, dispersed particles or droplets of the surfactant material can increase the rate of adsorption to the surface by orders of magnitude. Since the rates of particle migration or diffusion and particle dissolution can control the adsorption rates, the particle sizes and protocols of preparation can be quite important. The behavior of tetradecanol in water is reported for its importance in producing low dynamic surface tensions for potential lung surfactant applications and as a typical sparingly soluble surfactant, having a solubility of 0.3 ppm (1.4 µM) at 25 °C. The dynamic tension has been measured at 25, 37, and 41 °C (the latter is above the tetradecanol melting point) in water or saline (0.9 wt % NaCl), with a bubble method (measured from the pressure jump and the bubble radius) at constant or pulsating area (1-80 cycles/min). The dynamic surface densities Γ(t) were inferred from the dynamic surface tensions γ(t) by using the fitted data of the surface equation of state Γ(Π), where Π ) γ0 - γ, determined from pressure-area isotherms, which were obtained with a Langmuir trough and precautions to account for possible monolayer losses. At 20-80 rpm and the concentration 1500 ppm, tensions much lower than the equilibrium values (22 ( 2 mN/m) and as low as 9 mN/m were observed. These low tensions were inferred to be related to compressed monolayers rather than thicker films at the air/water interface.
1. Introduction The presence of dispersed particles including those of a long chain alcohol is important to the tension dynamics generally and in particular for a successful commercial lung surfactant formulation.1,2 The dynamic tension behavior of sparingly soluble surfactant systems has been recently studied by our group. Park et al. examined spread and adsorbed monolayers of n-hexadecanol (C16OH) and showed how the presence of dispersed particles affects dynamic adsorption.3-5 Smaller particles and a larger number density of crystallites decrease the time for tension equilibration. The present authors have examined the effect of chain length and the importance of the solubility and vapor pressure on the observed dynamic tension and adsorption behavior of spread monolayers of three alcohols, C14OH, C16OH, and C18OH.6,7 Although the solubilities are very small (Table 1), they have a large impact on tension dynamics. The shorter the chain length, the higher the solubility in water and the higher the volatility. For C14OH there are notable differences in the isotherm compared to those of C16OH and C18OH.6 The differences in the solubility of C14OH and the higher alcohols have suggested that a different mechanism controls the dynamic
Table 1. Solubilitiesa and Vapor Pressuresb of n-Alcohols in Water at 25 °Cc alcohol
melting point20 (°C)
c0 (ppb)
P° (kPa)
C14OH C16OH C18OH
38 49 59
310 41 3.0
6.33 × 10-6 5.80 × 10-7 4.59 × 10-8
a Solubility data for n ) 12-16 were used to extrapolate the value for n ) 18.6,7 b Vapor pressure and sublimation data for n ) 8-14 were used for extrapolating values for n ) 16 and 18.6,7 c The effects of temperature or salinity are discussed in the text and are expected to be minor at 37 °C and with saline.
adsorption process. For this reason, a more thorough study of the dynamic adsorption behavior of C14OH is warranted. In this paper, the bubble surfactometer was used for examining the dynamic surface tension behavior of aqueous C14OH dispersions at constant or pulsating area conditions, at 25 and 37 °C. The Π-A h isotherms of C14OH were used to determine the dynamic surface density Γ(t) from γ(t) tension data. We also report the results of some experiments which indicate that low tensions (γ < 20 mN/ m) observed during pulsating area experiments are due primarily to a compressed monolayer. 2. Experimental Section
* To whom correspondence should be addressed. Telephone: (765) 494-4078. Fax: (765) 494-0805. (1) Clements, J. A. Lung surfactant compositions. U.S. Patent 4,312,860, 1982. (2) Clements, J. A. Lung surfactant compositions. U.S. Patent 4,826,821 1989. (3) Park, S. Y.; Chang, C.-H.; Ahn, D. J.; Franses, E. I. Langmuir 1993, 9, 3640. (4) Park, S. Y.; Peck, S. C.; Chang, C.-H.; Franses, E. I. In Dynamic Properties of Interfaces and Association Structures; Shah, D. O., Ed.; American Oil Chemists Society Press: Champaign, IL, 1996; p 1. (5) Park, S. Y.; Franses, E. I. Langmuir 1995, 11, 2187. (6) Myrick, S. H.; Franses, E. I. Colloids Surf. A 1998, 143, 503. (7) Myrick, S. H. M.S. Thesis, Purdue University, West Lafayette, IN, 1996.
2.1. Materials and Sample Preparation. Tetradecanol, n-CH3(CH2)13OH (97% pure), was purchased from Aldrich, Inc., and was used as received. Dispersions (1500, 600, 150, and 10 ppm) were prepared in 0.9 wt % saline or in Millipore water (which was prepared by passing distilled water through a prefilter, a carbon cartridge, two mixed-bed ion-exchange cartridges, and then a 0.2 µm Millipore microfiltration membrane). NaCl (99.3% pure) was purchased from Mallinckrodt. Most dispersions were prepared by weight in Teflon bottles to avoid possible contamination by ions leaching from glass and were analyzed within a day. Three different protocols of preparation were used for varying the size and number density of particles in dispersions. The
10.1021/la980708f CCC: $18.00 © 1999 American Chemical Society Published on Web 01/28/1999
Dynamic Surface Tension of Aqueous Tetradecanol Systems protocols are similar to those used by Park et al.3-7 In protocol 1 (P1), the dispersions were examined after they were shaken vigorously for 1 min at constant temperature, to break up agglomerates of irregularly shaped crystallites (average dimension d > 100 µm). For protocol 2 (P2), dispersions were first heated to a temperature (50-70 °C) above the melting point of the alcohol, and the resulting emulsion was shaken by hand for 1-6 min or magnetically stirred vigorously for 30 min. The samples were allowed to cool in a water bath at room temperature or were quenched in an ice/water bath. The average diameter was from 3 to 63 µm. In protocol 2S (P2S), the sizes of dispersed emulsion droplets were further reduced by sonication (d ) 2-41 µm). Two methods of sonication were used. For small samples (∼20 mL), the emulsions, contained in small glass vials, were placed in a sonicator bath which has a fixed power output. For larger samples, the heated emulsions were sonicated by placing a sonicator horn tip into the sample for 15, 60, or 90 min. The sonicator ultrasonic processor was Model W-370, from Heat Systems-Ultrasonic, Inc., Plainview, New York. The latter samples were maintained at temperatures above the melting point of the alcohol and were magnetically stirred both before and during sonication. Immediately after sonication, the samples were quenched in an ice/water or water bath as in protocol 2 to minimize droplet coalescence during cooling. 2.2. Apparatus and Procedures. A Leitz polarizing microscope was used for measuring particle sizes and for detecting birefringence in particles. A Langmuir minitrough with a platinum Wilhelmy plate connected to an electronic microbalance, from KSV Instruments, Finland, was used for measuring surface pressures (Π ≡ γ0 - γ) of aqueous alcohol systems in a Class 100 clean room. The surface tension was measured with a nominal precision of 0.01 mN/m but with a reproducibility of 1-2 mN/m. The pulsating bubble surfactometer (PBS), purchased from Electronetics Co. (Amherst, NY), uses a sensitive pressure transducer for measuring the pressure drop ∆P(t) across a bubble, which is then used to calculate a surface tension from the Laplace-Young equation applied to nonequilibrium systems. Constant-area tension measurements are recorded every 50 ms after an initial 1 s delay upon forming a new bubble. Measurements were taken until the surface tension reached equilibrium, or a constant value, (2 mN/m, for constant-area experiments. The area of the bubble was changed nearly sinusoidally at frequencies from 1 to 100 cycles/min for pulsating area experiments. At least 10 cycles were recorded for frequencies over 10 cycles/min, and at least three cycles, for shorter frequencies, before a steady-state oscillation was observed. Rheological and other dynamic effects are not significant at the conditions used.8-11 The Model 500 spinning drop interfacial tensiometer (SBT), from the University of Texas, was also used for obtaining equilibrium and dynamic tension data for dispersions against air or for liquid/liquid interfaces.12 The speed of rotation of the sample was varied from 7 to 20 ms/rev. A glass microliter syringe or a glass disposable pipet (with a wide tip to allow large particles to pass) was used to fill the tube. With the syringe, one 10 µL air bubble (at standard conditions) was introduced into the sample tube. There was usually a delay of 3-5 min from the time the bubble was introduced until the first measurement could be taken. The minitrough was used for obtaining Π-A h , or Π-Γ, data. Before each experiment, the trough was filled with water or saline solution, which remained undisturbed for 1 min or more. The surface barriers were manually controlled to compress the surface to the minimum area. The compressed surface layer, which may have contained some unwanted impurities, was then removed with an aspirator, and the balance was zeroed. This procedure was repeated if needed (if Π was more than 1 mN/m). (8) Hall, S. B.; Bermel, M. S.; Ko, Y. T.; Palmer, H. J.; Enhorning, G.; Notter, R. H. J. Appl. Physiol. 1993, 75, 468. (9) Chang, C.-H.; Franses, E. I. Chem. Eng. Sci. 1994, 49, 313. (10) Chang, C.-H.; Franses, E. I. J. Colloid Interface Sci. 1994, 164, 107. (11) Chang, C.-H.; Coltharp, K. A.; Park, S. Y.; Franses, E. I. Colloids Surf. A 1996, 114, 185. (12) Franses, E. I.; Chang, C.-H.; Chung, J. B.; McGinnis, K. C.; Park, S. Y.; Ahn, D. J. In Micelles, Microemulsions, and Monolayers; Shah, D. O., Ed.; Marcel Dekker: New York, 1998; p 417.
Langmuir, Vol. 15, No. 4, 1999 1557
Figure 1. Effect of concentration of the dynamic surface tension of tetradecanol dispersions in saline obtained with the bubble (PBS) method at 37 °C and the following concentrations: (1) [, 150 ppm prepared with protocol 2; (2) 9, 600 ppm, protocol 2; (3) 2, 1500 ppm, protocol 2. For reference, data obtained with the KSV Wilhelmy plate method at 25 °C are shown; (4) 10 ppm, protocol 2; (5) 600 ppm, protocol 1. Measurements were obtained for times more than 1 s. It was found necessary to implement a procedure for preventing or minimizing monolayer losses during experiments where monolayers were spread from an organic solvent (hexane). After a monolayer was spread and the solvent had evaporated, the surface area was compressed until the monolayer reached its collapse pressure. Then the monolayer was removed with an aspirator. The surface area was then expanded to its original area and compressed and aspirated again, so that possibly remaining alcohol molecules would be mostly removed. Then a second monolayer was spread as before, and the aforementioned removal procedure was implemented. In most cases for tetradecanol, the procedure was considered adequate after adding the monolayer for the third time. The monolayer formation, compression, and aspiration may have the effect of depositing some material on the edges of the surface or “priming” the trough.13 The procedure was developed to address problems of possible monolayer dissolution or evaporation for the first or second monolayer but to help minimize dissolution, evaporation, or priming losses in the third monolayer. To compensate for some evaporation losses at 37 °C, each point of the isotherm curve was slightly shifted to the right by 1 or 2 Å2. This correction is small and is expected to affect the surface density results by less than 10%.
3. Results and Discussion 3.1. Dynamic Tensions and Surface Densities at Constant Area. In sparingly soluble alcohol systems, the dynamic surface tension behavior can be greatly affected by the presence of the dispersed alcohol particles. The dynamic tension data in Figure 1 were obtained with the bubble method. The equilibrium tension was also measured with the Wilhelmy plate method in a Langmuir trough. Curves 1, 2, and 3 are for dispersions prepared with protocol 2 from the same stock dispersion; see section 2.1 for details about the preparation protocols. The tension for a concentration of 150 ppm was initially close to that of water and decreased to a plateau at γ ) 53 mN/m. The tension of a 600 ppm dispersion (curve 2) had dropped to 64 mN/m within 1 s after the bubble had formed, before (13) Schu¨rch, S.; Bachofen, H. In Surfactant Therapy for Lung Disease; Robertson, B., Taeusch, H. W., Eds.; Marcel Dekker: New York, 1995; p 3.
1558 Langmuir, Vol. 15, No. 4, 1999
Myrick and Franses
Table 2. Tension Data for 1500 ppm Aqueous Tetradecanol Dispersions in Saline Τ νb δγ ) γmax - γmin γmin γeq t95c protocola (°C) (rpm) (mN/m) (mN/m) (mN/m) (s) P1
37
P2
37
P2S-15
37
P2S-60
37
P2
25
P2
37
P2
41
1 20 40 80 1 5 20 40 60 80 20 80 1 20 40 80 1 20 40 80 1 20 40 80 1 20 40 80
35 46 44 45 7 10 15 14 19 20 8 12 11 11 23 26 6 23 19 29 2 13 10 20 2 8 6 9
22 16 21 22 21 20 18 21 15 14 18 16 21 23 20 15 19 12 16 9 18 13 16 10 20 17 19 16
26 ( 2
75
24 ( 2
3
23 ( 2
