& (2016). The influence of the surface composition of mixed monolayer films on the evaporation coefficient of water. Physical Chemistry Chemical Physics , 18 (29), 19847-19858.

We explore the dependence of the evaporation coefficient of water from aqueous droplets on the composition of a surface film, considering in particular the influence of monolayer mixed component films on the evaporative mass flux. Measurements with binary component films formed from long chain alcohols, specifically tridecanol (C 13 H 27 OH) and pentadecanol (C 15 H 31 OH), and tetradecanol (C 14 H 29 OH) and hexadecanol (C 16 H 33 OH), show that the evaporation coefficient is dependent on the mole fractions of the two components forming the monolayer film. Immediately at the point of film formation and commensurate reduction in droplet evaporation rate, the evaporation coefficient is equal to a mole fraction weighted average of the evaporation coefficients through the equivalent single component films. As a droplet continues to diminish in surface area with continued loss of water, the more-soluble, shorter alkyl chain component preferentially partitions into the droplet bulk with the evaporation coefficient tending towards that through a single component film formed simply from the less-soluble, longer chain alcohol. We also show that the addition of a long chain alcohol to an aqueous-sucrose droplet can facilitate control over the degree of dehydration achieved during evaporation. After undergoing rapid gas-phase diffusion limited water evaporation, binary aqueous-sucrose droplets show a continued slow evaporative flux that is limited by slow diffusional mass transport within the particle bulk due to the rapidly increasing particle viscosity and strong concentration gradients that are established. The addition of a long chain alcohol to the droplet is shown to slow the initial rate of water The influence of mixed component organic surface films on the evaporation rate of water from an aqueous droplet is reported.

mole fractions of the two components forming the monolayer film. Immediately at the point of film formation and commensurate reduction in droplet evaporation rate, the evaporation coefficient is equal to a mole fraction weighted average of the evaporation coefficients through the equivalent single component films. As a droplet continues to diminish in surface area with continued loss of water, the more-soluble, shorter alkyl chain component preferentially partitions into the droplet bulk with the evaporation coefficient tending towards that through a single component film formed simply from the less-soluble, longer chain alcohol. We also show that the addition of a long chain alcohol to an aqueoussucrose droplet can facilitate control over the degree of dehydration achieved during evaporation. After undergoing rapid gas-phase diffusion limited water evaporation, binary aqueous-sucrose droplets show a continued slow evaporative flux that is limited by slow diffusional mass transport within the particle bulk due to the rapidly increasing particle viscosity and strong concentration gradients that are established. The addition of a long chain alcohol to the droplet is shown to slow the initial rate of water 2 loss, leading to a droplet composition that remains more homogeneous for a longer period of time.
When the sucrose concentration has achieved a sufficiently high value, and the diffusion constant of water has decreased accordingly so that bulk phase diffusion arrest occurs in the monolayer coated particle, the droplet is found to have lost a greater proportion of its initial water content. A greater degree of slowing in the evaporative flux can be achieved by increasing the chain length of the surface active alcohol, leading to a greater degree of dehydration.

I. Introduction
Predicting the rates of water evaporation and condensation in aerosol is important in areas including atmospheric science, drug delivery by inhalation therapies and spray drying. The rate of mass transport of water to and from large droplets (>20 m) and flat surfaces containing dilute solutes is usually limited by the rate of gas diffusional transport and thermal conduction. However, interfacial transport across the surface and slow diffusional transport within the particle bulk can become increasingly important, especially as the particle size diminishes and/or as solute concentrations surpass their saturation/solubility limit. While it is often not feasible to gain control over the rate of mass transport by modifying thermal conductivity or gas-phase diffusion (through a change in carrier gas or total gas pressure respectively), manipulation of the surface or bulk composition may allow the rate of mass transfer to be controlled via surface transport and bulk diffusion processes. For example, condensed films formed from long-chain alcohols can slow the rate of water evaporation, reducing the evaporative mass flux by many orders of magnitude. 1,2 Further, the high solute concentrations reached in evaporating aqueous droplets containing sucrose can lead to particles of high viscosity (>10 7 Pa s), significantly enhancing water retention within a particle due to slow bulk phase diffusion, and leading to particles that continually lose water over many hours or days. [3][4][5][6][7] The mass accommodation coefficient of water at a water surface is used to characterise the proportion of condensing molecules that make the transition from the gas phase into the droplet bulk, as opposed to re-entering the gas phase. By microscopic reversibility, the evaporation coefficient for water evaporation is assumed to be equal to the mass accommodation coefficient. 8 Although widely debated for many years, the most recent measurements suggest that the mass accommodation coefficient for water accommodating at a pure water surface is close to 1, in agreement with molecular dynamics models. [9][10][11][12][13][14][15] By contrast, measurements of rates of water evaporation from an aqueous surface with a condensed monolayer film of a long-chain alcohol have shown that the evaporation coefficient can be suppressed by many orders of magnitude, depending on the length of the hydrocarbon chain, falling from a value of 2.4 × 10 -3 when the alcohol has a C12 chain to 1.7 × 10 -5 for a C17 chain. 2 Such suppressions are consistent with measurements of the impact of monolayer films on the reactive uptake coefficients of species such as N2O5 [16][17][18] and broadly consistent with the structures and permeabilities of films expected from molecular dynamics simulations. 14,[19][20][21][22] Although the transport of water and reactive species through single component monolayers on aerosol droplets has been considered in some detail, systematic studies of the structure and transport through more complex film compositions have not been so extensive. This is despite the fact that it is clear that in most real-world applications, the transport of water or reactive species will be through a complex surface film containing multiple organic components with a composition that depends on surface pressure. Molecular dynamics simulations have investigated the equilibrium ordering of multiple organic species at the surface of an aqueous phase. 23,24 For example, Habartová et al. have recently considered the film composition and packing of mixed films of palmitic acid and 1-bromoalkanes at aqueous surfaces, using molecular dynamics simulations to show that stable mixed monolayers are formed between palmitic acid (C15H31COOH) and bromohexadecane (C16H33Br), while stable mixed monolayers of palmitic acid with the much shorter chain bromodecane (C10H21Br) and bromopentane (C5H11Br) do not form. 23 Simulations indicated that, prior to film collapse, increasing surface pressure led to the "squeezing out" of the bromoalkane from the film.
For mixed films formed from organic components that are considered to be good surrogates for organic constituents of atmospheric aerosol or biological membranes, measurements of phase behaviour and surface tension have been made using conventional techniques such as surface tensiometry and Brewster angle microscopy. [25][26][27][28] Such measurements have shown the considerable complexity in film properties that can arise for mixed component films, particularly following repeated compression and expansion cycles, exactly what would be expected to occur for atmospheric aerosol when progressing through repeated evaporation and condensation cycles. 26 Surface compositions inferred from sumfrequency generation measurements have shown the greater propensity for the less-soluble component to reside at the surface. For example, Voss et al. investigated the competition between atmospherically relevant fatty acid monolayers at the air/water interface, showing that oleic acid preferentially displaced palmitic acid from the surface. 29 On exposure to ozone, the greater solubility and volatility of the products from the oleic acid ozonolysis led to an increase in partitioning of palmitic acid to the surface over time.
Gilman and Vaida studied the influence of multicomponent films on the mass transport of acetic acid across an aqueous interface. 30 In particular, they concluded that the permeabilities of mixed films containing equimolar 1-triacontanol/nonacosane and equimolar 1-triacontanol/cis-9-octadecen-1-ol were in between their single component values, demonstrating that the mass transfer rate is not governed solely by either the most or least permeable species. By contrast, Burden et al. examined the uptake of HCl into sulfuric acid doped with pentanoic acid and hexanol, concluding that the greater surface activity of hexanol led to uptake that was reminiscent of uptake onto a surface coated in hexanol alone. 31 In studies of the reactive uptake of N2O5 by aqueous sulfuric acid, Cosman and Bertram found that the addition of a small amount of a branched surfactant (phytanic acid) to a monolayer largely composed of a straight chain surfactant (octadecanol) led to a significant increase in the permeability of the film. 16 In this study, we extend our earlier work on the kinetics of water evaporation from aqueous droplets containing long-chain alcohols of varying carbon chain length to studies of the evaporation of aqueous droplets containing multiple components. In addition, we consider for the first time the interplay of slowing the mass transport rate across the interface through reducing the evaporation coefficient, and a reduction in rate of bulk diffusional transport of water within an evaporating particle that becomes increasingly viscous during drying. In Section II we present a description of the experiment and the method for simulating the evaporation kinetics of water from droplets. This is followed by a discussion of the temperature dependence of the kinetics of water evaporation from droplets coated in a single-5 component film, measurements of evaporation from droplets coated in binary component films, and then the evaporation of droplets that include both a long-chain alcohol and sucrose, a viscosity modifying constituent.

II. Experimental description and analysis
Water evaporation from single droplets was studied using a concentric cylindrical electrodynamic balance (C-EDB). This technique has been described in detail in previous publications and will only be briefly recounted here. 2,9,[32][33][34][35] Individual droplets with radii ~ 25 m (± 100 nm) were produced from the solution of interest using a droplet-on-demand generator (MicroFab, MJ-ABP-01; 30 m orifice) and charged by an induction electrode. Aqueous solutions (50/50 v/v water/ethanol) containing a single fatty alcohol component, a mixture of two fatty alcohols or a fatty alcohol with sucrose were prepared with starting fatty alcohol concentrations of 0.6 mM. Ethanol was used to solubilise the fatty alcohols due to their very limited solubility in water, as described in our previous publication. 2 The ethanol component evaporates from the droplets more rapidly than water leaving a droplet that contains water alone as the solvent after < 1 s following droplet generation. The very low concentration of solute in the starting solution and the miscibility of ethanol and water ensures that the starting droplet composition can be assumed to be homogeneous. The charged droplets were injected in to the C-EDB cell, where they became trapped in the electric field generated between the upper and lower pairs of concentric cylindrical electrodes. A constant DC voltage applied to the lower electrode offset the effect of gravity and the Stokes drag force from the gas flow within the trap.
The environmental conditions in the gas phase surrounding the droplet were controlled by regulating the relative humidity (RH) and temperature of a 50 sccm nitrogen gas flow which entered the trapping cell through the lower outer electrode. The RH could be varied between fully dry conditions and > 95% RH by altering the ratio of humidified to dry nitrogen contributing to the total flow. The temperature of the gas phase was kept constant at 293. 15  The angular variation in the intensity of the elastic light scattering (the phase function) was used to infer the change in droplet radius with time. Trapped droplets were illuminated with a Gaussian 532 nm laser beam (Laser Quantum, Ventus) and the phase function, which has the appearance of a pattern of light and dark fringes, was collected over a 24° angular range by a camera (THORLABS DCC1545M) centred at 45° to the direction of laser propagation. For a droplet of known refractive index, and with estimates of camera position and angular range, the fringe spacing can be used to estimate the droplet radius with an accuracy of +100 nm/ -50 nm using the geometrical optics approximation. 36 For measurements with surfactant doped aqueous droplets, the refractive index was assumed constant at 1.335 during evaporation, the value of pure water at a wavelength of 532 nm. 37 For measurements with solute-containing aqueous droplets, the mass fraction of solute (MFS) increases as the droplet evaporates and a correction must be applied to the data to account for the resulting variation in refractive index. 33 Measurements of droplet size were recorded every 0.01 s. Droplets were trapped within the C-EDB within ~0.1 s of generation and the initial droplet size determined to within ± 50 nm by extrapolating the measurements of size at early time back to t = 0 s. The low level of charge on the trapped droplets had no measureable effect on the evaporation rate.
The mass flux of water from a trapped aqueous droplet evaporating within the C-EDB can be predicted using the semi analytical treatment of Kulmala et al. 38 Here, I is the water mass flux, Sh is the Sherwood number, r is the droplet radius, RH is the gas phase relative humidity, aw is the water activity in the droplet, R is the universal gas constant, T∞ is the gas phase temperature and M is the molecular mass of water. The Sherwood number accounts for the enhancement in mass transport that occurs due to the trapped droplet sitting within a moving gas flow.
 M and  T are the transition correction factors for mass and heat transfer, respectively. D is the diffusion coefficient of water vapour in nitrogen, p 0 is the saturation vapour pressure of water, A is the Stefan flow, L is the latent heat of vapourisation of water and K is the gas phase thermal conductivity.

The Kulmala et al. treatment uses the Fuchs-Sutugin transition correction factors for mass and heat
transport, which have the form given in equation (2). 39 Here Kni is the Knudsen number for mass (i = M) or heat (i = T) transport, equal to the ratio of the mean free path of water molecules in the gas phase to the droplet radius, and i refers to the mass accommodation coefficient (i = M) or thermal accommodation coefficient (i = T). As discussed in the introduction, the mass accommodation coefficient describes the probability that a water molecule striking a droplet surface will be taken up in to the droplet bulk, with the thermal accommodation coefficient describing the probability that the energy of an incoming water molecule equilibrates with the droplet surface temperature upon collision. By the principle of microscopic reversibility, an equivalent term to the mass accommodation coefficient is defined for the process of water evaporation from a droplet; the evaporation coefficient, . This is assumed to have the same value as M; thus and M are often used interchangeably in the literature. It is the evaporation coefficient that is of relevance to this study, as all measurements involve the evaporative loss of water from trapped droplets.
For water accommodating on/evaporating from a water surface, molecular dynamics simulations have consistently reported values of M and  = 1 11-15 and recent experimental studies have concluded that M and  > 0.5, although the available techniques are unable to discriminate more precisely above this value. 9,10 Thus, for water evaporation from pure aqueous surfaces, the mass flux is controlled only by the rate of gas phase diffusion, as surface processes are not limiting. In measurements of water evaporation from aqueous droplets coated with fatty alcohol monolayer films, Davies et al. reported values of  as low as 1.65 x 10 -5 for mass transport through heptadecanol films, leading to droplet evaporation rates controlled by interfacial transfer kinetics rather than gas phase diffusion. 2 This demonstrates the impact that the presence of a surface film can have on mass transfer rates. The thermal accommodation coefficient for water is widely accepted to be unity, and this is the value used in the current study. [40][41][42] By comparison with predictions from the Kulmala et al. treatment in equation (1), measurements of water mass flux from a trapped droplet can be used to determine the value of the evaporation coefficient.
To do this, the gas phase relative humidity must be known with high accuracy. In this work, probe droplets containing either pure water or aqueous NaCl solutions of known concentration were injected into the C-EDB prior to each sample droplet to determine the trap RH. For water probe droplets, the Kulmala et al. treatment was used to extract the gas phase RH from the water mass flux, taking  = 1.
For aqueous NaCl droplets, the diameter growth factor of the equilibrated droplet was compared with a thermodynamic model (E-AIM) to infer the water activity, and thus determine the gas phase RH. 43,44 The uncertainties in the RH retrieved from these two fitting methods were taken as reported in Davies et al. and are on the order of 0.2 % at 95% RH, increasing to 1.5 % at 50% RH. 34

III. Results and Discussion
Recently, Langridge Figure 1). Initial rapid loss of ethanol (at t < 0.5 s) from the droplet was followed by swift water loss (0.5 < t / s < 2.3) until a condensed fatty alcohol monolayer film formed around the droplet surface (at ~ 12 m in radius and t = 2.3 s), slowing the rate of further evaporation. The droplet evaporation rate following film formation was used to determine the evaporation coefficient, , by comparing the experimental data with predictions using the semi-

III.ii Mass transport through multi-component monolayer films
To study the impact of multi-component monolayer films on mass transport, measurements were made of the evaporation kinetics of water from aqueous droplets doped with two different fatty alcohols contained within the homologous series from tridecanol (C13H27OH) to hexadecanol (C16H33OH). The previous section demonstrated a dependence of evaporation coefficient on the gas phase RH due to differences in droplet surface temperature. In order to be able to directly compare the  values for mixed component monolayer films with those of the relevant single component films, the RH must be constant within the trapping region for the duration of the measurements. Small drifts in RH at high humidities can lead to a marked change in the evaporation coefficient; a 3% increase in RH at 80% RH would increase the evaporation coefficient for tetradecanol by 26%. To identify any such drift, the trap humidity was continuously monitored during experiments through the use of probe droplets. The majority of experiments were performed at RHs in the range 50% to 70% where the value of  is less sensitive to the effect of RH drift.
In Figure 3 we report the change in droplet radius with time for aqueous droplets doped with only tridecanol, only pentadecanol, and with a mixture of both fatty alcohol species. The measurements were performed at an RH of 60% and at 293. 15 K. As before, all droplets show an initial rapid loss of ethanol followed by swift water loss until a condensed monolayer film is formed around the droplet surface, inhibiting further mass loss (at ~ 15 m in radius, t = 3 s). As expected, the droplets doped only with tridecanol and pentadecanol show a linear change in droplet radius with time following film formation, corresponding to a constant value for the evaporation coefficient of the homogeneous monolayer film.
By contrast, droplets doped with both fatty alcohol species, and thus exhibiting multi-component monolayer films, show a highly non-linear dependence of droplet radius on time following film formation, with the precise evaporation profile depending on the molar ratio of the alcohols present.
This change in droplet evaporation rate with time following film formation indicates that the evaporation coefficient of the mixed monolayer film must be evolving as the droplet continues to evaporate, something which can only be caused by a change in the composition or morphology of the monolayer film with time.
As seen earlier, the droplet evaporation rate following film formation can be used to determine the evaporation coefficient for the monolayer film using the model of Kulmala et al. The following procedure was adopted in order to use the time-dependence of the evaporation rate of water through the multi-component monolayer film to determine a time-dependent value of the evaporation coefficient.
First, a polynomial expression chosen to most closely represent the experimental radius versus time data following film formation was fitted to the data (Figure 4(a)), and then differentiated with respect to time, giving the dependence of the droplet evaporation rate (dr/dt) on time at each point during the evaporation event (Figure 4(b)). Using the semi analytical model of Kulmala et al. and the gas phase RH determined from the probe droplets, droplet evaporation profiles were then simulated for a range of  values using an initial droplet radius representative of the measurements (Figure 4(c)). The correlation between the gradient (dr/dt) of each modelled evaporation profile and the evaporation coefficient itself was used to create a calibration plot (Figure 4(d)). This was used to transform the experimentally measured evaporation rate to a γ value, and thus obtain the variation in γ with time.  formation. This is possibly due to the difficulty in accurately fitting the first few seconds of film formation (where the rate of change in droplet radius is most rapid) with a higher order polynomial which also encompasses the rest of the radial data.

III.iii. Combining surface and bulk phase influences on water evaporation: Monolayer coated viscous aerosol
As discussed in the introduction, mass transport to and from a droplet may be limited either by the rate of gas phase diffusion, interfacial transfer or bulk phase diffusion. In the previous two sections we have considered the impact of slow interfacial transport; in this section we consider mass transport from a monolayer-coated viscous droplet, which can experience both reduced interfacial transport and bulk phase diffusion.
Several studies have reported that secondary organic aerosol (SOA) can exist as ultra-viscous droplets, leading to bulk phase diffusion limitations in mass transport. [46][47][48][49] As secondary organic aerosol contain a wide range of chemical species, it is highly likely that some of these will be surface active, leading to the possibility that SOA droplets could also contain a surface film which may inhibit mass transport.
Here we study the water mass transport from a simplified surrogate system consisting of aqueous sucrose aerosol doped with different surface active fatty alcohols.
The evaporation rates of droplets formed from four aqueous solutions (50/50 v/v water/ethanol) were measured, one containing 60 g/l sucrose and three containing 60 g/l sucrose doped with ~0.6 mM concentrations of either tridecanol, tetradecanol or pentadecanol. Droplets from each solution were injected in to the C-EDB trap at 293.15 K and ~30% RH, conditions under which significant bulk phase diffusion limited mass transport would be expected for aqueous sucrose droplets. 3 The change in droplet radius with time observed for droplets from each solution is shown in Figure 7(a).
In the uncoated aqueous sucrose droplets, rapid loss of water is observed until the droplet has decreased in radius to ~ 10 m, at which point an abrupt arrest in the evaporation rate is observed, corresponding to the beginning of a regime in which water mass transport is limited by slow diffusion within the bulk of the droplet. In this regime, the droplet continues to lose water very slowly over a period of several thousand seconds. The behaviour of the aqueous sucrose droplets doped with the fatty alcohols is different, exhibiting three distinct mass transport regimes. Droplets show an initial rapid loss of water controlled by the rate of gas phase diffusion, followed by slowing of the evaporation rate at t ~ 0.7 s when the fatty alcohol forms a monolayer film around the droplet surface. At this point, water mass transport is controlled by interfacial transfer as reported previously in sections III.i and III.ii. When the droplets reach a radius of around 9 m, a further change in the evaporative flux is observed corresponding to the onset of bulk diffusion limitations on mass transport within the viscous sucrose bulk. As expected, the droplets containing tridecanol, tetradecanol and pentadecanol show different evaporation rates during the period where mass transport is controlled by interfacial transport due to their different  values.
It is difficult to compare the behaviour of the four different droplet systems directly by examining the radius versus time data as small changes in the initial size of the droplets generated by the droplet-ondemand generator for each solution can mask fine changes in behaviour. In order to compare the droplets directly, the bulk averaged mass fraction of sucrose in each of the partially dry particles was estimated as a function of time (Figure 7(b)) using the initial mass fraction of solute in the droplet and an aqueous sucrose density parameterisation based on a third order polynomial fit to bulk MFS data. 50 This provides a way of comparing the total amount of water that has been removed from each of the particles by any given time point. It is important to note that the bulk-averaged mass fraction of sucrose reveals no information regarding concentration gradients that exist within the particle; previous studies have shown that slow bulk phase diffusion during evaporation leads to the formation of a solute rich shell and a water rich core. 3,6 As such, the bulk-averaged mass fraction of sucrose at the point at which the particle begins to display bulk phase diffusion limited behaviour is not expected to match the sucrose mass fraction at 30% RH predicted by thermodynamic models. can be interpreted as the uncoated aqueous sucrose particles displaying a greater non-uniformity in droplet composition with greater surface enrichment of sucrose than their monolayer coated counterparts, leading to a greater degree of water retention with the particle bulk once bulk diffusion through a viscous sucrose surface becomes the rate limiting step. While the difference in Péclet number between the uncoated and monolayer coated aqueous sucrose particles is most pronounced, a clear decrease in the Pe value and a shift in peak position to longer times is also observable as the chain length of the monolayer fatty alcohol is increased. This suggests that a more uniform droplet composition results from the slower mass transport prior to bulk phase arrest, allowing the removal of more water from the particle. Figure 8 shows a schematic that compares the limiting processes that determine the mass flux of water from an evaporating droplet based on our previous measurements of evaporative flux from droplets in the typical size range of these measurements (8 -12 m radius). In decreasing order of the resulting total mass flux (I) from the droplet these are: gas-diffusion limited evaporation of water, Ig (~10 -12 Kg.s -1 ) 34 ; surface limited evaporation through formation of a monolayer film, Is (~10 -14 Kg.s -1 ) 2 ; and bulk limited evaporation by slow diffusion of water from within the particle to the particle surface, Ib (~10 - 16 Kg.s -1 ) 32 . For the droplet containing only aqueous sucrose, the rate of the initial loss of water is controlled by the mass flux in the gas phase as water transport within the droplet bulk and across the interface is rapid. Thus, the mass flux is determined by the value of Ig, i.e. gas phase diffusion is initially the limiting rate determining mass transport. As more water is removed from the particle and its mass fraction of sucrose increases, the diffusion coefficient of water in the droplet bulk decreases by many orders of magnitude. Then, the mass flux of water from the particle is determined by the value of Ib and the slow release of water from within the droplet core. Without the presence of a monolayer film, transport across the surface is facile and the evaporation rate is either determined by the gas diffusional transport or the slow release of water from within the droplet bulk, depending on the relative rates of the two processes.
For the monolayer coated sucrose droplets, the relative magnitudes of all three mass fluxes (gas-phase, surface and bulk) need to be considered in order to explain the observed behaviour. At the very beginning of the droplet evaporation, the evaporative flux is again controlled by the rate of gas phase diffusion. Once the monolayer is formed around the surface of the droplet, water loss is characteristic of that expected for evaporation limited by mass transport through the surface film, Is. It is this point at which the evaporative mass flux becomes dominated by the rate of surface exchange. For each of the fatty alcohol doped droplets, the transition from a surface inhibited mass flux to a bulk phase diffusion limited mass flux occurs when the near surface concentration of water in the droplet becomes significantly lower than that remaining within the particle core due to the reduced rate of bulk phase diffusion. The slower the rate of loss of water from a particle surface (for example due to a pentadecanol monolayer film as opposed to a tridecanol monolayer film), the slower the diffusional transport of water from the core of the particle to the surface can be whilst still maintaining the evaporative flux limited by the transport across the surface. This allows particles coated in longer chain alcohols to lose water for longer before the bulk content of water (and, thus, the bulk diffusion constant of water) has diminished to such an extent that the bulk can no longer supply water to the surface to even sustain the low flux demanded by the alcohol film.

IV. Conclusions
Quantifying the mass transport of water, and volatile components more generally, through organic monolayer films on droplet surfaces and from amorphous particles is of importance to fields as diverse as atmospheric chemistry, spray drying and drug delivery to the lungs. We have considered here the competition between mass transport limited by interfacial transport and bulk phase diffusion. For droplets coated in a monolayer condensed film of a single long chain alcohol, the evaporation coefficient has been shown to depend on the relative humidity of the gas phase. This is a consequence of the effect that the magnitude of the diffusional gradient in the gas phase has on the mass and heat flux from an evaporating droplet, with evaporation in to lower RHs leading to a larger suppression in the droplet surface temperature. This effects the coherence of the monolayer film, influencing the packing density and thereby impacting on the rate of interfacial transport. Lower surface temperatures lead to higher packing density and lower evaporation coefficients. The evaporation coefficient through monolayers of mixed composition is shown to depend on the mole fractions of the different organic components forming the monolayer, consistent with previous work. 16,[29][30][31] As an evaporating droplet diminishes in surface area, the more soluble, shorter alkyl chain component is found to preferentially partition into the bulk of the droplet, leading to a temporal dependence in evaporation coefficient with the value tending towards that observed through a monolayer containing only the least soluble, longer alkyl chain component.
The drying kinetics of binary sucrose-water droplets show the onset of suppressed water loss within 2 s for the droplet sizes considered here. After this time, the evaporation rate of water is governed by slow diffusional transport of water from the particle core through a surface region strongly enriched in sucrose and characterised by considerably diminished diffusion constants. 5,6 When a long chain alcohol is added to the droplet, the surface propensity of the alcohol leads to the formation of a surface film that slows the rate of water loss through a reduction in the evaporation coefficient. This leads to the droplet composition remaining more homogeneous through diffusional mixing for a longer period of time, even in excess of 100 s. By the time the sucrose concentration has reached a sufficiently high value, and the diffusion constant of water has decreased accordingly, a larger fraction of water has been removed from the monolayer coated droplet than in the binary aqueous sucrose droplet case. A greater degree of slowing in the evaporative flux achieved by increasing the chain length of the alcohol leads to a greater degree of dehydration.
Through detailed measurements of the evaporation rate of water from droplets of increasingly complex chemical composition, it is possible to resolve the factors that govern droplet drying rates and the potential morphology of the particles that are formed. In future studies, we will focus on expanding the operating range of droplet and gas phase temperature that can be accessed in such studies, the capability to probe evolving size and composition at much shorter evaporation times, and the additional complexity that will be apparent when studying drying droplets containing dispersions of insoluble inclusions.  Correlation between the measured evaporation coefficient and the calculated droplet surface temperature.

Table of Contents Graphic
The influence of mixed component organic surface films on the evaporation rate of water from an aqueous droplet is reported.