Langmuir 2008, 24, 10133-10137
10133
Ultrasonic Nebulization in Aqueous Solutions and the Role of Interfacial Adsorption Dynamics in Surfactant Enrichment Beenamma Jimmy, Sandra Kentish,* Franz Grieser, and Muthupandian Ashokkumar* School of Chemistry and Department of Chemical and Biomolecular Engineering, UniVersity of Melbourne, Victoria 3010, Australia ReceiVed June 15, 2008. ReVised Manuscript ReceiVed July 3, 2008 High-density micron-sized aerosols from liquid surfaces were generated using an ultrasonic (frequency ) 1056 kHz) nebulization technique in the absence and presence of a number of surfactants. The surfactants included cationic surfactants, cetylpyridinium chloride and dodecylpyridinium chloride, and anionic surfactants, sodium dodecylbenzenesulfonate and sodium benzenesulfonate. The nebulization process generated aerosols of a narrow size distribution with a number mean diameter of about 3.4 µm, which is close to the theoretical value suggested by the Lang Equation. The aerosol droplets are enriched in surfactant as a consequence of the large interfacial area. The enrichment factor varied for different surfactants, depending on their surface activity. The extent of enrichment can be related to the rate of mass transfer of surfactant to the liquid surface. Surface concentrations of between 15 and 30% of the equilibrium value are observed, indicating turbulent mass transfer is the rate limiting step.
Introduction Ultrasound induced nebulization of liquids has long been recognized as a technique for producing fine liquid droplets with a narrow size distribution.1 The application of high frequency ultrasound below an air/aqueous interface results in a complex and dynamic interface characterized by surface waves, a central fountain jet and a surrounding mist of nebulized droplets. Lang2 proposed a capillary wave theory whereby the nebulization is a result of the instability of the surface waves under perturbation and the instantaneous break-off of the wave tips from the surface of the liquid.2 For sonically generated capillary waves, the surface wave frequency has been shown to be equal to one-half of the exciting sound frequencies.3 The equation derived by Lang shows the dependence of the number mean diameter of droplets (dn, m) on ultrasound frequency (F, Hz) as well as on solution density (F, kg/m3) and surface tension (γ, kg/m2):
( )
8πγ dn ) 0.34 FF2
1 3
and surface activity of a surface active solute in the bulk solution, the interfacial concentration and thereby the mass transferred to the droplets can vary. Previous studies have shown a strong correlation between the enrichment factor and the concentration of the surface active solutes.4-7 Rassokhin6 shows qualitatively that this enrichment is limited by the rate of mass transfer of surfactant to the interface. That is, the droplet formation rate is so fast that there is no time for surfactant to saturate the interface to the level of the equilibrium surface excess (Γeq). The time required for the surfactant molecules to reach equilibrium depends upon the concentration and structure of the surfactant. Rassokhin does not record aerosol droplet size and so cannot quantitatively determine the extent of these effects. Ferri and Stebe8 show that for diffusion controlled absorption with a surfactant diffusion coefficient (D), the characteristic time scale for diffusion to the interface is
τD )
(1)
During the development of a capillary wave tip into a droplet, surfactant molecules will be preferentially adsorbed at the droplet/ air interface, whereas the bulk solution will occupy the droplet volume. The large surface area of the resulting aerosol relative to the bulk solution means that this aerosol will be enriched in the surface active species. Indeed it has recently been reported that ethanol is preferentially enriched in the droplets that break off from an aqueous ethanol solution during ultrasonic nebulization.4,5 The accumulation of surface active solutes in the aerosol droplets has also been reported.6,7 Depending on the concentration * To whom correspondence should be addressed. E-mail: sandraek@ unimelb.edu.au (S.K.);
[email protected] (M.A.). (1) Morgan, A. Ultrasonic Atomization: AdVances in Sonochemistry; JAI Press Ltd: London, 1993; Vol. 3, p 145-164. (2) Lang, R. J. J. Acoust. Soc. Am. 1962, 34, 6–9. (3) Eisenmenrger, W. Acustica 1959, 9, 328–340. (4) Nii, S.; Matsuura, K.; Fukazu, T.; Toki, M.; Kawaizumi, F. Chem. Eng. Res. Des. 2006, 84, 412–415. (5) Sato, M.; Matsuura, K.; Fujii, T. J. Chem. Phys. 2001, 114, 1–5. (6) Rassokhin, D. N. J. Phy. Chem. B 1998, 102, 4337–4341. (7) Takaya, H.; Nii, S.; Kawaizumi, F.; Takahashi, K. Ultrason. Sonochem. 2005, 12, 483–487.
Γ2eq
(2)
DCB2
with all surfactants equilibrating within 1-10 times of this diffusion time. More generally, Ward and Tordai9 show that in a stagnant solution the dynamic adsorption density (Γ) is given as a function of time (t) by
Dtπ + 2 Dπ ∫ C (t)d (√t - τ)
Γ(t) ) 2CB
t
0
s
(3)
where CB is the bulk surfactant concentration and Cs that in the subsurface layer. The integration variable is τ. Chang et al.10 show that, when the aqueous phase is not stagnant and convection occurs, the diffusion time reduces. In the present case, ultrasonic nebulization provides a complex interface, which will be highly turbulent as a result of the dynamic nature of the surface waves, and also the underlying acoustic cavitation events and acoustic streaming that occurs. Consequently, surfactant mass (8) Ferri, J. K.; Stebe, K. J. AdV. Colloid Interface Sci. 2000, 85, 61–97. (9) Ward, A. F. H.; Tordai, L. J. Chem. Phys. 1946, 14, 453–461. (10) Chang, C. H.; Wang, N. H. L.; Franses, E. I. Colloids Surf. 1992, 62, 321–332.
10.1021/la801876s CCC: $40.75 2008 American Chemical Society Published on Web 08/20/2008
10134 Langmuir, Vol. 24, No. 18, 2008
Jimmy et al.
Table 1. Surfactants Used in This Study and Some Solution Properties
transfer rates well above those in a stagnant solution might be expected. In the present work, we have analyzed the composition and size distribution of droplets produced by ultrasonic nebulization in aqueous solutions containing various concentrations of surface active solutes. The efficiency of solute accumulation in the droplets has been correlated with the surface activity of the solutes. A detailed discussion on the dynamics involved during the ultrasonic nebulization process has been provided. This discussion compares the kinetics of surfactant adsorption with respect to the timescales of droplet formation from capillary waves.
Experimental Details As shown in Table 1, the surfactants selected for this study were cetylpyridinium chloride (CPC; UNILAB), dodecylpyridinium chloride (DPC; TCI), sodium dodecylbenzene sulfonate (SDBS; Sigma - Aldrich) and sodium benzene sulfonate (SBS; Sigma Aldrich). Aqueous solutions of surfactants were made using Milli-Q water. A schematic diagram of the experimental set up is shown in Figure 1. Ultrasound frequency of 1062 kHz was generated by an ELAC RF generator and delivered through an Allied Signal transducer. A water jacketed pyrex cell with 220 mL volume capacity, mounted on the stainless steel top of the transducer was used as the sonication cell. The volume sonicated was 170 mL. The temperature was maintained at 20 ( 5 °C. The electrical power delivered from the power generator was 100 W. The mist droplets generated for a total sonication time of 5 min were carried by vacuum suction into a collection flask, which initially contained 300 µL of the feed sample. The change in mass of the flask due to the collection of droplets (about 800 µg) was recorded. The concentrations of the surfactants in the bulk solution and the collection vessel were analyzed using a Varian UV-visible spectrophotometer. The concentration in the aerosol droplets was then determined by mass balance using eq 4
CA )
CABVAB - CBVB VAB - VB
Results and Discussion The air/solution surface tension (γ) measured for the surfactants studied is presented in Figure 2. These results indicate that CPC is the most surface active species and SBS the least surface active. The equilibrium surface excess (Γeq) at any bulk concentration (CB) was then determined in the usual manner using the Gibbs-Duhem eq 5.
Γeq ) -
1 ∂γ 2RT ∂ln CB
(5)
Figure 3 shows the percentage enrichment E, of the surfactant in the aerosol produced during nebulization as a function of its concentration in the bulk solution (CB). The value of E is defined by eq 6, where CA is the concentration in the aerosol.
E )
CA - CB × 100 CB
(6)
Control experiments, where the concentrations of the surfactants were monitored for the duration of nebulization ensured
Figure 1. Schematic representation of the aerosol collection process.
(4)
where CA, CAB and CB are the concentrations of the surfactant in the aerosol, collection vessel (after the droplet collection) and bulk, respectively. VB and VAB are the volumes of solution before and after droplet collection, in the collection vessel. The equilibrium surface tensions were measured using the Wilhelmy plate technique. The size distribution of droplets produced by ultrasound was measured using a laser light scattering technique (Malvern Instruments, SpraytecSTP2000v300).
Figure 2. Experimentally measured surface tension values versus bulk concentration of the solutes.
Ultrasonic Nebulization in Aqueous Solutions
Figure 3. Enrichment of surfactants in the ultrasonically generated droplets.
Langmuir, Vol. 24, No. 18, 2008 10135
Figure 5. Average CPC concentrations in aerosol droplets as a function of droplet size distribution and bulk CPC concentration. These calculations assume that equilibrium is attained at the droplet interface.
small; sizes of between 3.3 µm (for 2 mM CPC) and 4.0 µm (for water) are predicted. The small changes in the droplet size with surfactant concentration are too small to be detected using the experimental technique used in this study. However, the experimental values are consistent with the observed average diameter of 3.4 µm, supporting the capillary wave theory of ultrasonic nebulization. Figure 4 indicates that most droplets are less than 10 µm in diameter with a relatively narrow size distribution. Following the approach of Rassokhin6 a model for the aerosol droplet formation can be considered to explain the enrichment process. Based on this model, the average concentration of the solute in a droplet of radius ri, if at equilibrium (Ceq, i), can be estimated by eq 7:
4 C πr + Γ 4πr ) ( 3 ) ( 34 πr ) 3 i
B
Ceq,i
Figure 4. Size distribution of aerosol droplets based on their percentage number: (a) Effect of surface activity of the surfactant; all solutions contained 0.2 mM surfactant. (b) Effect of solute concentration for CPC surfactant.
that none of the surfactants degraded during sonication. Enrichment is observed for all surfactants with a peak value observed at lower concentrations. The observation of surfactant enrichment is consistent with other studies,4,6,7 even though the frequency of ultrasound and the surfactants used were different in our study. As has been reported in these studies, aerosol droplets are produced from the surface of the liquid during ultrasound induced nebulization. In aqueous surfactant solutions, the interface differs in composition relative to the rest of the bulk due to the adsorption characteristic of the solutes. The total surface area provided by the aerosol droplets is significantly larger than the interfacial area of the solution undergoing nebulization. The surfactant, CPC, showed the highest enrichment (100-115%) followed by SDBS, DPC and SBS (
