3640
Langmuir 1993,9,3640-3648
Dynamic Surface Tension Behavior of Hexadecanol Spread and Adsorbed Monolayers Sun Young Park, Chien-Hsiang Chang, Dong June Ahn,and Elias I. Franses* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283 Received August 2,1993. I n Final Form: September 20,1999 The spread monolayer tension behavior of hexadecanol at 25 "C was linked to the adsorption dynamics from sprinkledparticlesor from disperaions in saline. The dynamic tension was measured with the Langmuir trough (Wilhelmyplate) method, the pendant drop method, and the bubble method. The rate of adsorption was found to be proportional to the surface area of particles spinkled on the surface. Moreover, the size and concentration of dispersed particles close to the surface affected strongly the rate of tension drop for 1500 ppm dispersions. When the dispersed cryetallitee were melted, broken to smaller particles, and refrozen, the rate of adsorption increased drastically. The bubble method was used in the constant area mode and in the pulsating area mode at 1-80 cycles/min, at 25 and 37 "C. The tension amplitude during pulsation increased with increasing frequency and decreased with decreasing particle size. The constant area data were compared to the predictions of a simple diffusion-controlledadsorption model with an effective diffusion length which represents the contribution of the particles as the source of molecules available for adsorption. 1. Introduction The dynamic adsorption and surface tension behavior of soluble surfactant systems have been widely studied as a function of concentration and in developing models for diffusion-limited, adsorption-limited, or mixed-kinetics conditions.1-8 However, the dynamic behavior of insoluble surfactant systems has received little attention. There are two common ways of studying an insoluble surfactant. One is to use spread monolayers on a trough, which yields the surface pressure-area ( E A ) isotherms. The other method, which is to use sprinkled or suspended solid surfactant particles on a clean surface, gives the rate of release of molecules from the solid particles to the aqueous Insoluble surfactants have many applications. They are used in helping retardation of evaporation of lakes.1g18 Many detergents are also used above their solubility as dispersed microcrystallites, liquid microcrystallites, or even liquid emulsions (surfactanta above their cloud point). Another possible application area is the stability and rheology of soap lather. In this paper, hexadecanol is studied as a model system of a nearly insoluble surfactant. Its solubility in water at various temperatures below ita melting point of 49 "C has
* To whom correspondenceshould be addressed (Tel. (317) 4 9 4 4078; Fax (317) 494-0805). Abstract published in Advance ACS Abstracts, November 15, 1993. (1) Ward, A. F. H.; Tordai,L. J. Chem. Phys. 1946,14,453. (2) Hanaen, R. S. J. Phys. Chem. 1960,646,637. (3) Miller, R. Colloid Polym. Sci. 1981,259, 375. (4) Bomankar, R. P.; Waean, D. T.Chem. Eng. Sci. 1993,38,1637. (5) Myaela, K.J.; Friach, H.L. J. Colloid Znterfuce Sci. 1984,99,136. (6)Gumy1, R. Z.; Carbonell, R. G.; Kilpatrick, P. K. J. Colloid Interfuce SCL1986,114,636. (7) Adamczyk, Z.; Petlicki, J. J. Colloid Interface Sci. 1987,118, 20. (8) Chang, C. H.; Franaea, E. I. Colloids Surf. 1992,69, 189. (9) Nutting, G. C.; Harkins, W. D. J. Am. Chem. SOC.1939,61,1180. (10) Cary, A.; Rideal, E. K. h o c . R. Soc. London, A 1925,109,301. (11) Mansfield, W. W. Auut. J. Chem. 1959,12,382. (12) Mansfield, W. W. Auut. J. Chem. 1963,16, 76. (13) Deo, A. V.; Kulkami, S. B.; Gharpurey, M. K.; Biswaa, A. B. J. Phys. Chem. 1962,66,1361. (14) Fang,J.-P.; Jooe, P. Colloids Surf. 1992, 66, 139. (15) Gainea, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley & Sone, Inc.: New York, 1SBB; Chapter 4, p 140. (16) LaMer, V. K.; Healy, T.W.; Aylmore,L. A. G. J. Colloid Sci. 1964, 19, 676. (17) LaMer, V. K.; Healy, T. W. Science 1%6,148,36. (18) Mansfield, W. W. Nuture 1966, 176, 247.
been reported by several researchers using different techniques.1s21 Krause and Lange19 used isotopically labeled hexadecanol at 33 "Cto measure the solubility of 3.3 X 1 V M (0.008 ppm) with *lo% uncertainty. Radioactive hexadecanol was spread on the water surface and was equilibrated for 1-2 weeks. Then, ita concentration in the bulk water was determined. Hoffman and Anackerm analyzed by gas chromatography saturated solutions prepared by a procedurebased on that of Krause and Lange. They reported the solubility at 43 "C to be 6.4 X W0 M (O.OOO155 ppm). Robb21 prepared a saturated solution by stirring hexadecanol powder with water for 24 h at 25 "C and filtering twice to remove particles (filter pore size 350-1000 A). He determined the solubility to be 1.7 X le7 0.2 X le7M (0.04 ppm) by comparingthe surface properties of recovered hexadecanol spread monolayer to the standard II-A isotherm. The discrepanciesare probably due to the solubility being quite small. Resolving the discrepancies is clearly a nontrivial task. Some material can be lost due to the adsorption on n overestimated the container surfaces. The solubilityc ~ be if small submicrometer suspended particles are present. Nonetheless, the solubility of hexadecanol in saline at 25 "C can be safely taken to be smaller than 1.7 X le7M (0.04ppm). Thus, hexadecanol is nearly insoluble in water at 25 "C. The solubility is expected to be of the same order of magnitude at 37 "C, either in water or in saline. No information is available on whether it forms micelles. Because of the small solubility,we consider this possibility unlikely. The suspended phase is crystalline. Hexadecanol has been chosen for study because it is sparingly soluble in water, is chemically stable without the hydrolysis problem of ester-type lipids, and is used in many practical household and pharmaceutical producta. Moreover, hexadecanol (or other higher alcohols) is used as a key ingredient in controlling the respreadability and dynamic surface tension behavior of a successful commercial lung surfactant replacement drug called ExosurF"N and marketed by Burroughs Wellcome, Co., (19) Krauee, F. P.; Lange, W. J. Phys. Chem. ISM, 69,3171. (20) Hoffman, C. S.; Anacker, E. W. J. Chromatogr. 1967,30, S90. (21) Robb, I. D. A u t . J. Chem.1966,19, 2281. (22) Durand, D. J.; Clay", R. I.; Heymann, M. A.; Clementa, J. A. J. Pediatr. 1986,107,775. (23) Clementa, J. A. United Statea Patent 1982, 4,312,860.
0743-7463/93/2409-3640$04.00/00 1993 American Chemical Society
Langmuir, Vol. 9, No. 12, 1993 3641
Spread Monolayer Tension Behavior of Hexadecanol
Research Triangle, NC. The natural lung surfactant is a complex mixture of phospholipids and proteins, most of which are insoluble in water. The lung surfactant is highly surface active and can yield low equilibrium tensions and very low (less than 10 mN/m) nonequilibrium surface tensions. It helps decrease the pressure difference across the alveolar membrane and stabilize the alveoli from collapsing. Lack of lung surfactants is a direct cause of the respiratory distress syndrome (RDS) of neonates. Exogenous surfactant replacement drugs have been used to treat the RDS with success.26126 The key ingredient of lung surfactant is DPPC (dipalmitoylphosphatidylcholine), and the other components such as proteins and unsaturated phospholipids play supporting roles in controlling the dynamic surface t e n ~ i o n . ~ ' In * ~ Exosurf, ~ hexadecanolapparently plays a role similar to that of the secondary components in lung surfactant.22-24Moreover, a soluble polymericnonionic surfactant (tyloxapol)is used for improving the dispersibility of DPPC-hexadecanol mixtures.24 Before the action of Exosurf can be elucidated in clear fundamental physical terms, one needs to understand the behavior of pure hexadecanol. The key questions addressed here about the behavior of hexadecanoland more generallyof aqueous monolayers or dispersions of insoluble surfactants are the following. First, how can the spread monolayer behavior (II-A isotherms) be linked to the behavior of monolayers adsorbed from aqueousdispersion (or solution)? Secondly, when monolayers are formed from particles, what are the effects of particle microstructure, morphology, size, and surface area (or perimeter) on dynamic surface tension behavior? Thirdly, how comparable are the results of different measurement techniques? Lastly, for the application to lung surfactants where the aidwater surface area varies periodically, what is the relation of dynamic tension at constant area conditions to the tension at pulsating area conditions? We have obtained data useful for answering these questions, and we have attempted to relate the behavior of hexadecanol to that of soluble surfactants. Although the solubility of hexadecanol is small, it seems to play an important role in the overall adsorption mechanism. 2. Experimental Section 2.1. Materials and Sample Preparations. Hexadecanol (-99% pure) waspurchasedfromSigmaChemicalCo.,St.Louis, MO, and was used without purification. Sodium chloride (AR grade) was purchased from Mallinckrodt,Paris, KY. HPLC grade hexane was purchased from Aldrich, Milwaukee, WI. All experimentswere done with Milliporewater with a Milli-Q4-bowl system, which uses distilled water as input. The water had an initial resistivity of 18 M h m . Saline solution was 0.9 wt % NaCl dissolved in Millipore water. For the spread monolayer studies, hexadecanol was dissolved in hexane to make a 1mg/mL solution. The originalhexadecanol crystals were ground to make smaller globular particles for some of the sprinkled-particles experiments. Their sizes ranged from 10 to 200 pm, as determined by optical microscopy, and the average diameter was about 100 pm. In order for the effects of dispersion state on the dynamic surface behavior to be elucidated, hexadecanol dispersions were prepared with various protocols. We do not imply that all these protocols should be considered for lung surfactant applications, (24) Clements, J. A. United States Patent 1989,4,826,821. (26) Shapiro,D. L. Surfactant Replacement Therapy;Shapiro,D . L., Notter, R. H., Ede.; Alan R. Lisa, Inc.: New York, 1989; Chapter 1,p 1. (26) Sehgal, S.; Taeusch, W. Pediat. Res. 1991,29,233A. (27) Notter, R. H. Surjactant Replacement Therapy; Shapiro, D. L., Notter, R. H., E&.; Alan R. Lies, Inc.: New York, 1989; Chapter 2, p 19. (28) Chug, J. B.; Hannemann, R. E.; Franses, E. I. Langmuir 1990, 6, 1647.
but they provide useful insights. In protocol 1,a dispersion of 1500ppm of hexadecanolin saline solutionwas shaken vigorously at room temperature. This dispersion was quite inhomogeneous. The crystallites of hexadecanol were large and particles floated to the air/water interface within minutes. The density of solid hexadecanol crystal below its melting point is 0.982 0.019 g/cm3 29 and that of liquid hexadecanol at 50 OC is 0.8176 g/cm*. Since saline has a higher density than hexadecanol(1.004g/cm*), the particles float. The dimensions of the nonspherical,platelike rough-edged original particles varied in a wide range from 60 to 2000 pm. Their crystallinity was detected by using a polarizing microscope. In protocol 2, a sample was prepared by first heating a protocol 1dispersion to 52 f 2 OC, which is above the melting point of hexadecanol(49 "C), and then by shaking the resulting emulsion vigorously to break up the droplets to smaller sizes. Then, the sample was cooled to room temperature. At cross polarizers in the microscope, maltese crosses were evident after cooling, indicating that the fluid particles had crystallized. This dispersion was more turbid than the previous protocol 1dispersion. The particles were spherical, with most diameters less than 20 pm and also some larger (500 pin) particles. Certain protocol 2 samples were melted again and sonicated in a bath. These "sonicated" preparations produced the most turbid and stable dispersions with smaller spherical particles, also with maltese crosses, of diameters less than ca. 3 pm. Certain protocol 1 samples were filtered through 0.2-pm Nuclepore ultrafiltration membranes to remove the dispersed crystallites and to obtain samples with total hexadecanol concentration at the solubility l i t or a little above the solubility. In this filtrate sample, referred to as protocol 3 sample, there were no visible particles, and the filtrate looked as a clear solution. 2.2. Apparatus and Procedures. A computer-controlled minitrough from KSV Instruments, Finland, with a platinum Wilhelmy plate was used to study the monolayer behavior of hexadecanol. The accuracyof the surface pressure measurement was *0.004 mN/m, and that for the surface area was *l%, accordingto the manufacturer. The surface could be repeatedly compressed and expanded at a desired rate with two barrier bars made of Teflon. The change in the surface area with time was linear. Barrier moving speeds of 0.1, 10, 27, and 108 mm/min were used. The surface area could also be oscillated between 22.5 and 42.5 cm2, in order for the surface area to change by 89% between its minimum and maximum areas (as in the PBS instrument below). During a pulsation, no visible disturbance on the surface could be discerned. Area pulsation was repeated for five cycles, after which a l i t cycle in the surface pressure was achieved. Experiments with the trough were done at room temperature and saline solution was used as the subphase. A commercial, thermostated, computer-controlled pulsating bubble surfactometer (PBS) from Electronetics Co. Amherst, NY, was used to measure dynamic surface tensions (r(t)) at 25 and 37 OC. This instrument is based on Enhorning's design.* The surface tension can be measured aa a function of time either under constant or pulsating area conditions. Under pulsating area conditions, the radius of the bubble is changed from r = 0.40 mm to r = 0.55 mm (area ratio 1.89) by using a liquid volume displacer. The bubble can be pulsated at various pulsating rates (1-100 cycles/min), with the area varying nearly sinusoidally.*' The instrument measures the pressure difference across the bubble surface with a pressure transducer and gives a tracing of surface tension calculated from the LaplaceYoung equation, hP(t) = 2y(t)/r(t). Since the bubble is quite small, it is nearly spherical. The surface tension is measured every 50 ms after a 1-s initial delay or "dead" time. Most of the experiments were done within a day after the dispersions were prepared, to eliminate possible effects of particle coagulation, especiallywith protocol 2 samples. A pendant drop apparatus (RambHart Co.) was coupled with a video camera to a Q570 image analyzer (Cambridge Instru-
*
~
~~~
(29) Abrahamsson, S.; Lareson, G.; von Sydow, E. Acta Crystalbgr. 1960. 13. 770.
(3'0)Enhoming, G. J. Appl. Physiol.: Respirat. Enuiron. Ererciae Physiol. 1977, 43, 198. (31) Chang, C. H.; Franses, E. I. Chem. Eng. Sei., in press. ~
3642 Langmuir, Vol. 9, No.12, 1993
Park et al.
SurfaceArea (cm')
distinguished with the sensitivity used from that of a pure saline solution. Hence, the monolayer at large molecular areas, where the liquid expanded or gaseous state exists, affects the surface pressure to a negligible extent. The hydrophilic head group of hexadecanol is fairly small, and the molecules may interact little with each other and may not form a coherent monolayer. The molecules should be quite close to each other before they could sense the presence of neighboring molecules and affect the surface pressure significantly. With further compression, when the molecular area was decreased to 21.4 A2/molecule a t ll = 10mN/m, a significantincrease in the slope occurred, indicating a transition to a solid monolayer state with a! = 0.0019m/mN. The collapse of the monolayer occurred at A = 19.6 A2/moleculeat ll = 47 mN/m (y = 25 mN/m). When compressed at 2 (A2/molecule)/min,the phase transition to the liquid condensed state occurred at a slightly smaller molecular area, 22.6 A2/molecule. The transition to the solid state occurred around 21 A2/ molecule, which is close to that a t the low compression rate. The phase change to the solid state occurred at the same surface pressure of 10 mN/m. The solid monolayer region extended until the monolayer collapse when the surface pressure reached 58 mN/m (y = 14 mN/m) at 20.5 A2/molecule. The collapse pressure was higher than in the slower compression rate (near-equilibrium)isotherm, but the molecular area a t the transition was substantially the same. After collapse,there was a minor drop in surface pressure. The II-A isotherm reported by Nutting and Harkinsg at 20 "Cfor hexadecanol spread on 0.01 N aqueoussulfuric acid had the transition to the liquid condensed phase occurring at -21.8 A2/molecule. The transition to the solid phase occurred at ca. 20.9 A2/molecule with a corresponding surface pressure of 10.2mN/m. They used a slow compression rate of 0.035 (A2/molecule)/min. The monolayer compressibility a t II = 0 mN/m was 0.0055 m/mN, and at ll = 20 mN/m it was 0.0010 m/mN. Once the monolayer was changed to the liquid condensed or the solid state,the surface pressure increased drastically with a small reduction in the molecular area. The increase was much more pronounced in the fast compression case. The surfacepressure change from -0 to 58mN/m occurred over only a 15% decrease in the molecular area. This large change in the surface pressure with a small change in the surface area has significant consequence when the surface is regularly pulsated with a 90% change between the maximum and the minimum areas, as in the lung alveoli. To be effective, a lung surfactant should have more moderate changes in the surface tension with large changes in the surface area. The changes in the surface pressure (or surface tension) of hexadecanol under oscillating surface area conditions will be discussed in section 3.7. 3.2. Properties of Monolayer Spread from Particles. We now consider the monolayer behavior in the presence of particles on or near the surface. After a monolayer is spread from a solvent as previously, with the trough barriers fully open, 1 mg of ground hexadecanol crystals was sprinkledon the surface. The surface pressure quickly ( 1.7 pm, and the adsorption rate will be smaller. Some predictions of the model discussed above are shown in Figure 7 for 1 = 0.01 to loo0 pm. The data for protocol 2 are close to the model predictions for 1 I 0.01 pm. Apparently, the particles dissolve sufficientlyfast to replenish the molecules lost due to adsorption. If the particles size is larger, as in protocol 1, the dissolution is (35) Lin,S.-Y.;McKeigue, K.;Maldarelli, C. AlCHE J. lSSo,36,1785. (36)Chang,C. H.; Wang, N.-H. L.; Frames, E. I. Colloids Surf. 1992,
62,321. (37) h e n , M. J.Surfactantaandlnterfacial Phenomena;John Wiley EQ Sone, Inc.: New York, 1989,Chapter 2, p 33. (98) Tsonopoulon, C.; Newman, J.; Prausnitz, J. M. Chem. Eng. Sci. 1971,26,817.
80
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100
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Figure 7. Comparisons of dynamic tension data for 1600 ppm
hexadecanol in saline by different protocola at 26 'C (dashed linea 1,2, and 3;see Figure 3a) to predictions of a simpleW o n controlled model for various values of the diffusion length: a, 0.01 pm; b, 0.1 pm; c, 1 pm; d, 10 pm; e, 100 pm; f, lo00 pm. For parameters used and further detaile, see text.
slower and the data are close to the model prediction for 1 = 1pm. If now the particles are filtered (protocol 3), their concentration is much smaller and they cannot replenish the molecules lost to adsorption. Then, the surface never gets saturated, and the tension does not reach the equilibrium value. 4. Conclusions 1. The concentration, size, and possibly morphology of dispersed particles affect greatly the dynamic tension behavior of hexadecanol dispersions. Dissolution (or redeposition) of particles, the rate of which is proportional to the surface area of the particles, is a key factor in adsorption dynamics. 2. Adsorption dynamics can be fairly represented by a simple diffusion-controlled model of adsorption, desorption, and molecular diffusion with a variable diffusionlayer thickness. The effective thickness represents the contribution of particle dissolution to the rate of molecular diffusion. 3. Different tension measurement techniques for prob ing adsorption dynamics from hexadecanol dispersions are affected by the direction of particles floating due to gravity. 4. The behavior of monolayer adsorbed from dispersion has been linked to the spread monolayer behavior and adsorption dynamics. 5. The tension behavior under pulsating area conditions is close to the insoluble monolayer behavior when the time scale of adsorption is much larger than the period of area pulsation. 6. Hexadecanol monolayers in saline have a steep surface pressurehurface area isotherm (Figure 1). This produces a large variation in the dynamic surface tension amplitude under pulsating area conditions (Figures 5 and 6)and precludes the use of hexadecanol alone as a lung surfactant replacement. Nonetheless, an improvement in performancecan be achieved by the proper preparation protocol, namely if the particles have been previously melted, broken to small droplets, and refrozen. Acknowledgment. This research was supported in part by a grant from the Showalter trust and by the National Science Foundation equipment grant (BCS91121541,which allowed the purchase of the PBS instrument. We thank Professor N.-H. L. Wang and Dr. G.Enhoming for helpful comments.
