J. Phys. Chem. 1996, 100, 15503-15507
15503
Effects of Dissolved Gas on Emulsions, Emulsion Polymerization, and Surfactant Aggregation M. E. Karaman,*,†,‡ B. W. Ninham,† and R. M. Pashley‡ Department of Applied Mathematics, Research School of Physical Sciences and Engineering, and Department of Chemistry, The Faculties, The Australian National UniVersity, Canberra, ACT 0200, Australia ReceiVed: March 13, 1996; In Final Form: July 8, 1996X
The presence of dissolved gas is shown to have a key role in emulsion stability and emulsion polymerization. The observations have implications for hydrophobic interactions, as well as chemical and biological reactivity. Hydrophobic and micellar surfaces may present favorable adsorption surfaces for dissolved gas as gas hydrates (i.e. clathrates) and/or submicrobubbles, and that concentrating the dissolved gas can lead ultimately to cavitation as the two such surfaces approach.
1. Introduction At normal temperature and pressure, dissolved air in water has a concentration of around 2 mM. In typical alkanes gas solubility can be 1 order of magnitude higher. In solids such a high concentration of defects, and aggregates of such impurities, is crucial in determining physical properties. Yet in liquid state physics, solutions, and colloid and surface chemistry, the role of dissolved gas has hardly been considered theoretically, and experimentally1-3 only is sporadically investigated and ignored [for example, see J. D’Arrigo4,5]. We demonstrate here that dissolved gas has a significant role in emulsion stability and emulsion polymerization. The experiments we report are so simple that the observations are difficult to dismiss, and the ramifications are of clear industrial relevance. In 1993 we demonstrated that degassing of dodecane/water emulsions increased emulsion stability, which suggested that dissolved gas may play a significant role in the balance of hydrophobic and hydrodynamic (drainage) forces responsible for the phase separation. That observation stimulated an atomic force microscopy (AFM) study which attempted to address the issue of dissolved gas in the interaction of hydrophobic polypropylene surfaces.6 The polypropylene surfaces used in that study, although very hydrophobic as measured by contact angle, generated only a relatively short-ranged attraction measurable up to about 25 nm separations. The effect of degassing was minor, but did give a tantalizing hint that these hydrophobic interactions were here indeed reduced by degassing. Here we have reexamined alkane-water emulsions in more detail, including the effects of hydrocarbon chain length and added electrolytes. In addition, we have investigated the effect of degassing on emulsion stability and emulsion polymerization. This latter process also appears to involve hydrophobic interactions in the aggregation of primary polymer particles.7 2. Materials and Methods The water used was produced from tap water purified via a Memtec Krystal Kleen unit. A prefilter removed suspended solids. Subsequent passage through a reverse osmosis membrane and an activated charcoal stage remove dissolved electrolytes and dissolved gases such as chlorine. The permeate so obtained was distilled and collected in a positive pressure †
Department of Applied Mathematics. Department of Chemistry. X Abstract published in AdVance ACS Abstracts, June 15, 1996. ‡
S0022-3654(96)00758-7 CCC: $12.00
dust-free laminar flow cabinet. Purity was checked via surface tension measured using the rod-in-free-surface method (RIFS).8 Glassware was thoroughly cleaned by first rinsing in 10% NaOH, several washes in a warm teepol solution, followed by rinses in tap water and AR grade BDH ethanol to remove any surface active material. It was then finally rinsed in the distilled water. For the emulsion stability studies, the hydrocarbons (AR) hexane, heptane, octane, decane, and dodecane were freshly distilled under reduced pressure. Azulene was obtained from Aldrich which was purified to eliminate possible surface active contaminants by zone refining and sublimation before useage. Styrene obtained from Prolabo was distilled under reduced pressure to remove the tert-butyl catechol inhibitor and any polymeric material before use. Fluorescein sodium salt was obtained from Ajax Chemicals. Potassium persulfate (analar) and sodium dodecyl sulfate (SDS) (specially pure) biochemical grade was obtained from BDH chemicals and was used without further purification. 3. Results 3.1. Emulsion Stability. Pairs of test tubes containing 10% (by wt) alkanes (analar grade hexane to dodecane, freshly distilled under reduced pressure) and 90% (by wt) distilled water were placed into an ice bath to avoid evaporation. One of each pair was attached to a water aspirator and evacuated for 1 h to eliminate dissolved gas and sealed by closing an air-tight gas tap. The other was not evacuated but left for 1 hour in the ice bath to ensure that both samples were treated identically apart from evacuation. Both test tubes were allowed to warm to room temperature and then shaken by hand. Emulsion stability as gauged by eye, compared, and recorded. The differences observed between the evacuated, and the aerated samples were striking. While a sample containing dissolved gas phase separated completely within minutes, its degassed partner remained slightly cloudy for hours. This procedure was repeated many times. The stability was also observed in the presence of an oil-soluble organic dye (azulene) to enhanced contrast. Exactly the same behavior was observed in the presence and the absence of the dye. Photographs of the phenomenon are shown in Figures 1 and 2. For both situations, the majority of the larger hydrocarbon droplets coalesce rapidly. But in the degassed mixture a fine mist of oil droplets remains stable for several hours, apparently independently of alkane, and therefore oil density. Differences between aerated and © 1996 American Chemical Society
15504 J. Phys. Chem., Vol. 100, No. 38, 1996
Figure 1. Two identical dodecane/water mixtures. One was evacuated using a water aspirator for 1 hour while the other was left untouched. After this time both vials were shaken by hand simultaneously and allowed to stand for comparison. For both situations, the majority of the larger hydrocarbon droplets coalesce rapidly. The outgassed mixture shown on the left denoted by “O”, 0.5 h after shaking, showed a fine, stable mist of oil droplets. By comparison the vial on the right labeled “A” is the aerated mixture, 0.5 h after shaking, showing large oil droplet coalescence (i.e. rapid phase separation) and no trace of residual fine oil dispersion.
Figure 2. Two identical dodecane/water mixtures. One was evacuated using a water aspirator for 1 hour while the other was left untouched. After this time both vials were shaken by hand simultaneously and allowed to stand for comparison. For both situations, the majority of the larger hydrocarbon droplets coalesce rapidly. The outgassed mixture shown on the left denoted by “O”, 1 h after shaking, showed a fine, stable mist of oil droplets. By comparison the vial on the right labeled “A” is the aerated mixture, 1 h after shaking, showing large oil droplet coalescence (i.e. rapid phase separation) and no trace of residual fine oil dispersion.
degassed mixtures are obvious immediately after shaking because of the substantial difference in coalescence rate. For degassed emulsions fine droplets coalesce to form larger droplets more slowly. Since degassing of aerated aqueous solutions also increases the pH value from about 5.6 to 7.0, due to the removal of dissolved carbon dioxide, we also studied nitrogen saturated mixtures. These were found to exhibit the same behavior as the aerated mixtures, so eliminating any possibility of unknown pH effects due to dissolved CO2. To determine whether emulsion stability upon degassing is affected by hydrocarbon chain length and the presence of electrolytes, above and below gas bubble coalescence concentrations,9 we repeated the procedure outlined above using a series of alkanes and electrolytes. Hydrocarbon chain length, electrolyte type, and electrolyte concentration had no observable effect on stability on degassing. Electrolyte is at 1 mM and 1 M aqueous solution levels for NaCl and NaAc; 1 M HAc and 1 M sucrose leave the stability phenonmenon untouched. These electrolytes were chosen because NaAc does not affect gas bubble interactions.9 The others totally inhibit bubble coalescence above 0.2 M.9,10
Karaman et al.
Figure 3. A scanning electron micrograph of latex particles produced under nitrogen bubbled conditions. The latex is roughly spherical but slightly angular in appearance with a broad, apparently trimodal, distribution ranging from 35-200 nm in diameter, with an average diameter of 38 nm.
Since the rate of coalescence of small droplets is determined by a competition between hydrodynamic drainage forces and some attractive (hydrophobic ) interaction we are forced to the conclusion that one or the other or both are affected by degassing. 3.2. Emulsion Polymerization. Emulsion mixtures containing 0.90 g of SDS (0.003 mol), 55 mL of distilled water, 5 mL of styrene (0.044 mol), and 1.095 g of potassium persulfate (0.004 mol) thermal initiator were poured into two 150 mL round bottomed flasks. Both flasks were placed in an ice bath to reduce evaporation. One was nitrogen bubbled and the other evacuated with a water jet pump for 1 h. The evacuated sampled formed a very stable emulsion, whereas the nitrogen-bubbled sample easily separated into two distinct layers. Evidently, degassing enhances stability even in the presence of high levels of surfactant. Thereafter both the evacuated and the nitrogenbubbled flasks were sealed and allowed to react for the next 2.5 h in a shaking water bath set at 69 ( 1 °C. This procedure was repeated several times. Latex particles produced by the two methods were examined with both a Cambridge S2000 and a Hitachi S900 FESEM scanning electron microscope. To do this the latex samples were diluted 1000 and 10000 times in distilled water. Droplets of these diluted latex suspensions were allowed to air dry on plastic slips coated with 2 nm of chromium. Latex particles from the nitrogen-bubbled samples were roughly spherical but slightly angular in appearance with a broad, apparently trimodal, distribution ranging from 35-200 nm in diameter and an average diameter of 38 nm (cf. Figure 3). By contrast, the evacuated emulsions gave latex particles that appear to be much more monodisperse spherical particles with a mean diameter of 46 nm (Figure 4). At higher magnification (74.2 K) both the evacuated and nonevacuated latex particle surfaces appeared to be made up of conglomerates of smaller aggregates, indicating that they had been formed via a heterocoagulation process. The emulsion polymerization procedure was repeated with the addition of 20 mg fluorescein dye, a procedure used previously7 to explore the polymerization mechainsm i.e. via a micellar or heterocoagulation process. The bulk latex solutions produced in the presence of the fluorescein dye, under gas evacuation and nitrogen bubbling, appeared to be different in
Effects of Dissolved Gas on Emulsion Stability
Figure 4. A scanning electron micrograph of latex particles produced under evacuated conditions. The latex was spherical and quite monodisperse with a mean diameter of 46 nm.
color. The nitrogen-bubbled and -evacuated latices containing dye were dialyzed against distilled water to observe dye leaching characteristics and so provide information on possible mechanisms of polymerisation. Both samples showed no evidence of dye leaching, suggesting that both were produced via heterocoagulation. Regardless, of the mechanism of formation there is no question on our main point: that dissolved gas affects the nature of the latex formed. 4. Discussion The first measurements of the hydrophobic interaction between surfactant-coated surfaces11 formed a starting point in the search for an explanation based on the effect of a nonpolar surface on adjacent water layers. The early measurements were obtained with only a weakly hydrophobic surface, but later studies have produced a wide range of attractive interaction forces, some of which extend up to separations of 200-300 nm. This extraordinary range and magnitude is orders above that expected from van der Waals forces and has produced a major problem for theory. So far, no theory has been able to explain the range of interactions observed for the various surfaces and electrolyte conditions reported, and many mechanisms are almost certainly involved depending on situation. In 1985 the first observation of spontaneous cavitation of water held between two hydrophobic surfaces was reported.12 In these experiments cavitation was observed after separation of the surfaces from strongly adhesive contact. Later studies indicated that cavitation may actually occur prior to surfacesurface contact of two hydrophobic surfaces.13 This possibility may offer a way forward in the search for an explanation of the range of the hydrophobic interaction. It has been suggested that hydrophobic surfaces may present a favorable adsorption surface for dissolved gas.10 Work by Miller et al. on eight hydrocarbon gases in lipid bilayers suggested that the relative magnitude of gas solubilities in a given bilayer was closely related to the strength of their intermolecular forces.14 A 1 L amount of water at room temperature and atmospheric pressure contains about 20 mL of dissolved gas, close to its saturation level. The unfavorable interaction of water layers adjacent to a hydrophobic surface might act to concentrate dissolved gas, and this could lead to cavitation when two such surfaces approach. This possibility has been considered with regard to the unusual effect of electrolytes on the inhibition of bubble coalescence.9 A suggestion has been made that the attractive
J. Phys. Chem., Vol. 100, No. 38, 1996 15505 force involved in the coalescence of bubbles is involved in hydrophobic interaction.10 The involvement of cavitation would be crucial here because it would lead to coalescence directly. Taken together, our observations on emulsion stability and latex polymerization have clear practical applications. But much more is involved. It is possible that dissolved gas plays a significant role in surfactant or self-assembly. Serra et al.15 found that argon solubility increases significantly with increasing SDS concentration. A comparison of argon solubility in water, dodecane and micellar solutions was interpreted to indicate that the micellar interior provides a hydrocarbonlike environment to enhance gas solubility. (The solubility of argon in water is two orders of magnitude lower than in dodecane.) Premicellar aggregates do not affect gas solubility, but above the cmc the solubility increases linearly with surfactant concentration.15-17 Both hydrocarbon core volume and total area of micelles increase linearly with total surfactant concentration. Hence the increased solubility could be due to either enhanced solubility in the core or increased nucleation of microbubbles at the micellar surface. There is a considerable body of work18 devoted to bubble nucleation and growth of CO2 bubbles, in supersaturated solutions, and on the influence of adsorbed SDS on nucleation at both hydrophobic and hydrophilic surfaces. The essential conclusion is that above the cmc bubble nucleation is enhanced (i.e. micelles provide additional nucleation sites). We remark in passing that the known lowering of surfactant cmcs in D2O,19,20 suggested by Kresheck21 as due to stronger hydrophobic bonding in D2O, may be related to the increased gas solubility in D2O as compared to water.22 Other researchers19,21 have observed differences in optical density and conductivity for surfactant solutions in D2O and H2O, only above their critical micelle concentrations (cmc). Therefore, if dissolved gas is central to surfactant self-assembly, then the phase behavior of ionic surfactants and surfactant-water-oilsalt microemulsions must depend on dissolved gas. (The cloud point for coexisting lamellar phases of the water-didodecyldimethylammonium bromide system is unaffected by degassing.23) The situation must be further complicated for low water content systems because gas solubility itself reduces exponentially at high electrolyte (∼1-2 M), typical of most ionic microemulsions. Hence quantitative attempts so far to explain phase behavior must be viewed with a somewhat jaundiced eyesinteractions between aggregates, besides intrinsic curvature at the surfactant-water and surfactant-oil interface, may also be affected by dissolved gas. We can put our results within a wider perspective: Evidence from acousto luminescence and sonar chemistry generally over more than 50 years has pointed to the existence in water, and in electrolyte solutions, of some substructure in water in the form of submicrobubbles. (These act as nuclei for optical cavitation, which is otherwise unexplained.24,25) The careful work of S. M. Bezrukov et al.26 on large scale conductance fluctuations in solutions of strong electrolytes provides evidence for such substructure. So too recent work on laser-induced cavitation near hydrophobic and hydrophilic walls27 and on fractal clusters of submicrobubbles in aqueous solutions28 reconfirms that there is much we do not know about the organization of microstructure of water containing dissolved gas, with or without surfactants that may or may not provide the principal source of such self organization. It is known in sonar chemistry that free radical production is vastly enhanced within such submicrobubbles. Consequently it may be reasonable to expect that chemical reactivity as dictated by free radicals in emulsion polymerization should be affected by dissolved
15506 J. Phys. Chem., Vol. 100, No. 38, 1996 gasspresumably surfactant assists microbubble organization containing gas in specific ways. A further and related issue is the role of dissolved gas in hydrophobic interactions. Cavitation of liquid between two solid surfaces for which the dynamic receding contact angle is greater than 90° is expected thermodynamically. First observed by Lord Rayleigh for mercury between glass29 and later by Laskowski and Kitchener30 for methylated silcia-bubble interaction and others,31,32 the distance typically at which films rupture of the order of 100 nm has remained a mystery. (Ordinary heteronucleation theory would predict a distance of the order of tens of nanometers based on the maximum strength of liquids.)33 Between solid hydrophobic surfaces with contact angle
