e e on e Str Sol–gel Gelation Resorcinol–formaldehyde de r from mec reactant and carbonate concentrations at a given temperature, reaching the mean hydrodynamic radius yde an rough me in materials for hydrogen storage [6]. Therefore, further development Previous research has shown that resorcinol–formaldehyde gel properties can be tailored to specific requirements, such as pore surface area and pore size distribution, by changing the conditions of their synthesis in the sol–gel process [2]. The sol–gel process has activated. Additional treatments like solvent exchange and ageing r to achieve a gel n. Althoug g the synth eir nanostr evolution is lacking, and therefore, our ability to rationally their structure for specific applications has been limited. The widely accepted reaction mechanisms for the synthesis of resorcinol–formaldehyde polymers are shown in Fig. 1. It can be divided into two steps: substitution of resorcinol with formalde- hyde to form (simply, doubly or triply) substituted hydroxymethyl resorcinol and step-growth polymerisation of substituted resor- cinol [1,3]. Although resorcinol–formaldehyde polymerisation can take place in aqueous solutions at room temperature without any added ⇑ Corresponding authors. Present address: Faculty of Chemical Engineering and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Cracow, Poland (K.Z. Gaca). E-mail addresses:
[email protected] (K.Z. Gaca),
[email protected] (J. Journal of Colloid and Interface Science 406 (2013) 51–59 Contents lists available at Journal of Colloid an r .co Sefcik). and tailoring of these materials for specific applications is of great importance for energy storage and low carbon technologies. Fur- ther important applications of carbon aerogels include catalyst supports and adsorbents [7]. may also be introduced prior to drying, in orde with properties appropriate for a given applicatio exists considerable empirical experience regardin organic gels, fundamental understanding of th 0021-9797/$ - see front matter � 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.05.062 h there esis of ucture tailor a wide range of applications [1–3]. Due to their electrical conduc- tivity, very large surface areas and internal porosity, resorcinol– formaldehyde aerogels have high potential as superior materials for electrodes in electrochemical double-layer supercapacitors and batteries [4,5], for capacitive deionisation units, as well as method, where dissolved monomers undergo series of reactions, forming primary particles or clusters at nanometre to micrometre scale, which then subsequently coalesce and/or aggregate forming a system-spanning network immersed in the solvent: a wet gel. The resulting organic gel is then dried, carbonised and chemically Organic gels Dynamic Light Scattering Primary clusters Spontaneous emulsification Nanoemulsions Thermodynamic control 1. Introduction Organic gels based on formaldeh synthesised in the late 1980s [1] th cess, and since then, they have beco of several nanometres before further changes were observed. However, more primary clusters formed at higher carbonate concentrations, and cluster numbers were steadily increasing over time. Our results indicate that the size of primary clusters appears to be thermodynamically controlled, where a solubil- ity/miscibility limit is reached due to formation of certain reaction intermediates resulting in approxi- mately monodisperse primary clusters, most likely liquid-like, similar to formation of micelles or spontaneous nanoemulsions. Primary clusters eventually form a particulate network through subsequent aggregation and/or coalescence and further polymerisation, leading to nanoscale morphologies of result- ing wet gels. Analogous formation mechanisms have been previously proposed for several polymerisation and sol–gel systems, including monodisperse silica, organosilicates and zeolites. � 2013 Elsevier Inc. All rights reserved. d resorcinol were first an organic sol–gel pro- creasingly important in been widely used in materials synthesis [8], and although it has been traditionally associated with the synthesis of inorganic mate- rials, a wide range of organically modified or purely organic mate- rials have been produced in this way [2]. The organic sol–gel process is a convenient and environmentally friendly synthesis Keywords: formation of resorcinol–formaldehyde gels in the presence of dissolved sodium carbonate. Dynamic Light Scattering measurements showed that size of freely diffusing primary clusters was independent of both Mechanism and kinetics of nanostructur of resorcinol–formaldehyde polymerisati Katarzyna Z. Gaca ⇑, Jan Sefcik ⇑ Department of Chemical and Process Engineering, University of Strathclyde, 75 Montros a r t i c l e i n f o Article history: Received 17 February 2013 Accepted 2 May 2013 Available online 3 June 2013 a b s t r a c t Resorcinol and formaldehy wide range of applications work, we investigated the www.elsevie volution during early stages eet, G1 1XJ Glasgow, UK eact in aqueous solutions to form nanoporous organic gels well suited for a supercapacitors and batteries to adsorbents and catalyst supports. In this hanism and kinetics of formation of primary clusters in the early stages of SciVerse ScienceDirect d Interface Science m/locate / jc is reso and catalyst, the overall rate is relatively low [9], and it can be expected, by analogy with a well-known phenol–formaldehyde system, that the substitution reaction is both acid or base catalysed [1,3,10]. The polymerisation step, where the hydroxymethyl group reacts with another hydroxymethyl group, forming an ether bridge –CH2–O–CH2–, or with an unsubstituted resorcinol site, forming a methylene bridge –CH2–, can also be expected to be both acid and base catalysed [10]. Polymerisations are typically performed at ele- vated temperatures (up to 90 �C) in order to speed up the process. Moreover, previous research showed that in order to obtain very high pore surface areas of resulting gels, neutral or slightly basic conditions within a relatively narrow range of pH values are re- quired [10], and this is often achieved by adding weak bases such as sodium carbonate as reaction catalysts to resorcinol–formalde- hyde solutions. More recent research indicated that not all bases are equally effective at same pH values in promoting high surface areas of resulting gels and that both concentration and type (anion and cation) of basic catalysts added to modify solution pH affect the properties of the final gel [11], suggesting that the role of these catalysts is beyond just providing basic conditions. Electrolytes, such as sodium carbonate, are well known to destabilise colloidal suspensions due to screening repulsive electrostatic interactions between colloidal particles. Also, electrolytes are known to induce liquid–liquid phase separation in water–alcohol mixtures leading to colloidal emulsions. We note that in aqueous solutions, formaldehyde is mainly Fig. 1. Overall reaction mechanism of 52 K.Z. Gaca, J. Sefcik / Journal of Colloid present in its hydrated form as methylene glycol and its oligo- mers/polymers, and commercial formaldehyde solutions are nor- mally stabilised with methanol in order to prevent formaldehyde polymerisation, so that methylene glycols are accompanied by methoxymethylene glycol and their small oligomers, while the concentration of formaldehyde in its aldehyde form is in fact very low (usually not exceeding 100 ppm) [12]. Therefore, the initial reaction mixture in resorcinol–formaldehyde polymerisation is an aqueous solution of resorcinol and methylene glycols in the presence of strong electrolytes (such as sodium carbonate) added as catalysts. As reactions proceed, a range of products starting with variably substituted resorcinol and their condensation products appear over time, with unknown solubility/miscibility properties in respect to the background solution at relevant temperatures. While it is expected that sufficiently long polymer chains will eventually become insoluble and undergo microphase separation, it may well be possible that solubility/miscibility limits are reached earlier in the polymerisation process as relatively small molecular weight intermediates and oligomers are produced. There are two principal theories which can be found in the lit- erature aiming to explain the mechanism of the gel formation in polymerising resorcinol–formaldehyde systems: microphase sepa- ration [13,14] or aggregation of primary colloidal particles [15]. The microphase separation scenario is based on the idea of poly- meric chains growing too large and becoming insoluble and there- fore triggering demixing, resulting in two interpenetrating phases, one polymer rich and the other solvent rich, a process well known from other polymer systems. The colloidal aggregation scenario is based on the idea of formation of primary colloidal particles which then subsequently aggregate and form a space-filling network of interconnected clusters. Recent analysis of Small Angle X-ray Scat- tering (SAXS) measurements from resorcinol–formaldehyde gels suggests that these two scenarios are unlikely to be distinguishable from the resulting gel structures and in any case may well be two extremes of a more general process [16]. From published transmis- sion electron micrographs [13], it is apparent that the dry network of resorcinol–formaldehyde gels produced in the presence of car- bonates consists of interconnected dense primary particles of a few to a few tens on nanometres, with smaller primary particles for higher carbonate concentrations. However, it is not clear what is a mechanism of formation of these primary particles and how is it controlled by solution pH and the nature of cations and anions used. Formation of primary clusters was previously investigated by SAXS [17], where clusters with radius of gyration of 2–4 nm were observed before the onset of gelation at 25 �C. In a recent study, SAXS and X-ray photon correlation spectroscopy was used to monitor gel formation at 70 �C [18], where it was observed that rcinol–formaldehyde polymerisation. Interface Science 406 (2013) 51–59 primary clusters were gradually growing with their radius of gyra- tion from about 2 to about 10 nm, after which development of increasingly stiff network dynamics was observed. Sol–gel transi- tion process in resorcinol–formaldehyde solutions at 25 �C was investigated by Dynamic Light Scattering, where monomodal de- cay time distributions were observed initially with apparent clus- ter hydrodynamic diameters growing gradually up to about 5 nm. The initial formation of these primary clusters was followed by the development of much slower autocorrelation function de- cay modes, which were presented in term of steeply increasing apparent hydrodynamic diameters of growing colloidal particles [15]. The nanoscale structure of resorcinol–formaldehyde gels as ob- served by electron microscopy appears to be morphologically sim- ilar to the one of silica-based gels from silicon alkoxides through hydrolysis and subsequent step-growth polymerisation in near neutral or slightly basic aqueous solutions, where the sol–gel pro- cess is similar in some respects to synthesis of resorcinol–formal- dehyde gels. It has been previously proposed that formation of primary particles in silica sol–gel systems may include formation of transient nanoemulsions through molecular demixing driven 2. Experimental with the instrument available, they were conducted in an alterna- tive manner. Solution was divided upon filtration into several par- and Resorcinol (99 wt.%), formaldehyde (37 wt.%, aqueous solution stabilised by 13 wt.% methanol; the concentration of methanol was determined experimentally using the NMR spectroscopy [25]) and sodium carbonate (P99.5 wt.%, anhydrous ACS reagent) were all purchased from Sigma–Aldrich. Syringe filters (0.45 lm pore size, PTFE membrane and PP housing; 0.02 lm pore size, Ano- top 10; manufactured by Whatman) were purchased from Fisher Scientific. Filtration was carried out with rubber-free PVC/PP syrin- ges. Vials for DLS experiments were made from borosilicate glass, with diameter of 10 mm and height 75 mm, and were also pur- chased from Fisher Scientific. Filtration was performed in order to eliminate impurities, including dust, which could strongly affect DLS results. The reacting mixtures were filtered after the initial mixing, but before thermal treatment, where only unsubstituted and substituted resorcinol is present, but no larger oligomers are formed under these conditions [25]. The compositions of resorcinol–formaldehyde solutions used for sol–gel processes are customarily defined by ratios of amounts of reagents, carbonate and water used. The most frequently used ratios are R/C (resorcinol to catalyst; mol/mol), R/F (resorcinol to formaldehyde; mol/mol) and R/W (resorcinol to water; g/ml; W re- fers to water added as a solvent, not the water present in formal- dehyde solution). In all experiments, resorcinol to formaldehyde ratio was kept constant at 0.5 mol/mol, while resorcinol to water and overall carbonate concentrations were varied. The composi- tions of mixtures used in this work are shown in Table 1. Please note that F in the R/F ratio refers to mols of formaldehyde, while F solution refers to the volume of the added formaldehyde solution. All resorcinol–formaldehyde solutions were prepared as fol- by production of small molecular weight reaction intermediates before any substantial polymerisation commences [19]. Similar mechanisms based on formation of nanoscale liquid-like interme- diates have been proposed for several polymerisation and sol–gel systems, including monodisperse Stoeber silica particles [20], zeolites [21] and organosilicates [22,23]. We note that while small silica/organo-silica oligomers appear to be poorly miscible/soluble in water–alcohol mixtures at basic and neutral pH conditions, resulting in formation of colloidal primary particles, there appear to be no solubility issues in acidic solutions where transparent gels are formed without any apparent nanoscale phase separation phenomena. Interestingly, it appears that the mechanism of resorcinol– formaldehyde gel formation under basic conditions is also different from that in acidic conditions, where polymerisation-induced critical opalescence, as inferred from SAXS measurements [24], has been proposed to be linked to polymerisation-induced phase separation, leading to formation of micron-sized particles subsequently aggregating and forming a gel. Nevertheless, the mechanism of formation of primary clusters in resorcinol–formal- dehyde polymerisation under basic and neutral conditions and the role(s) played by basic catalysts, such as sodium carbonate, are still poorly understood. The objective of this work was to investigate mechanism and kinetics of formation of primary clusters during early stages of res- orcinol–formaldehyde polymerisation in the presence of sodium carbonate by analysing effects of carbonate and reactant concen- trations and temperature, using Dynamic Light Scattering to mon- itor formation of primary clusters and subsequent structure development towards gel formation. K.Z. Gaca, J. Sefcik / Journal of Colloid lows. The required amounts of deionised water (W) and resorcinol (R) were transferred to a beaker and stirred using a magnetic stir- rer for 5 min until resorcinol fully dissolved. Sodium carbonate (C) allel samples which were placed in an electric oven preheated to 80 �C. The vials were then taken out from the oven at certain time intervals, rapidly cooled down to ambient temperature in an ice bath to quench the reaction and taken for DLS measurements per- formed at 25 �C. In Dynamic Light Scattering experiments, autocorrelation func- tions g1(s) were measured using a digital correlator (ALV/LSE- 5004). The wavelength of incident laser light was k = 632.8 nm, and the scattering angle hwas 90�. When the autocorrelation decay was showing an exponential decay, indicating unhindered Brown- ian diffusion, we used the cumulant method [26] to estimate in the initial decay rate C [s�1]. From this, one can determine the mean diffusion coefficient D, using C = Dq2, where q = (4p/k)sin(h/2), and the mean hydrodynamic radius Rh, using the Stokes–Einstein equation, D = kBT/6plRh, where kB is the Boltzmann constant, T is the absolute temperature, and l is dynamic viscosity (taken here to be equal to that of pure water at a given temperature). For estimation of gelation times, solutions were transferred into 25 ml PP flasks (diameter 25 mm, height 100 mm), sealed and placed in an electric oven preheated to a desired temperature. The process of gelation was monitored by visual observations of changes in the solution flowability, and gelation time was taken here as time measured from the moment a sample of reacting mixture is placed in an electric oven set at the desired temperature and periodically checked until a lack of flow at tilting by 45� is ob- served. Although this kind of gel time estimation is subject to sev- eral factors, including the diameter of the vessel and the total volume of the solution, it is a suitable tool for the assessment of differences in gel formation kinetics among similar samples sub- ject to different reaction conditions. It has been reported that there has been a reasonable correlation between visual and rheological determination of gel times in similar resorcinol–formaldehyde sys- tems, where visually estimated gel times were 5–45% longer than those determined rheologically [27]. With this caveat, we can use visual estimation of gel times for approximate determination of time windows for DLS measurement before networking of primary clusters and onset of gelation. 3. Results and discussion Initial gelation time observations were performed to estimate time scales appropriate for DLS measurements in order to monitor formation of primary clusters well before the gel formation itself, since previous scattering studies typically focused on later stages of the process where extensive aggregation and/or cross-linking of primary clusters resulted in complicated interplay of cluster size, structure and dynamics. Fig. 2 shows gelation times deter- mined for a range of carbonate concentrations at two different temperatures. Here, ratios of resorcinol to formaldehyde (R/F) was then added to the beaker with dissolved resorcinol, and after 10 min of further stirring to dissolve sodium carbonate, the required amount of formaldehyde solution (F solution) was added to the mixture and stirring continued for another 30 min. The solu- tions were then filtered in two steps, first using 0.45 lm and then 0.02 lm pore size syringe filter, directly into clean vials pre- washed with deionised water and filtered resorcinol–formalde- hyde solution. Vials were then sealed with a cap and parafilm, placed in the DLS measurement cell where they were thermostated at 55 �C and measurements were taken at certain time intervals. Since DLS measurements were not possible to do in situ at 80 �C Interface Science 406 (2013) 51–59 53 and resorcinol to water (R/W) were kept constant at 0.5 mol/mol and 0.10 g/ml, respectively, while concentration of carbonate, expressed as the ratio of resorcinol to catalyst (R/C), was varied between 50 and 600 mol/mol (see Table 1). As expected, increasing carbonate concentration (i.e. decreasing R/C ratio) leads to decreas- ing gel time, with an approximately linear dependence of the ob- served gel time on the R/C value (i.e. inversely proportional to the carbonate concentration). We also note that the concentration of carbonate (base) causes a systematic variation of solution pH, autocorrelation functions were measured every 3–5 min, only three representative ones are shown here for clarity. It can be seen in Fig. 3 that the autocorrelation function collected after 10 min of heating has a shape very close to that for an ideal single exponential decay, indicating that the primary clusters observed are initially approximately monodisperse and Table 1 Compositions of investigated reaction mixtures. R/W R/C (mol/mol) R/F (mol/mol) W (ml) R (g) C (g) F solution (ml) Initial pH (g/ml) (mol/mol) 0.10 0.0164 50 0.5 5.00 0.50 0.0096 0.74 7.51 0.10 0.0164 100 0.5 5.00 0.50 0.0048 0.74 7.08 0.10 0.0164 200 0.5 5.00 0.50 0.0024 0.74 6.63 0.10 0.0164 300 0.5 10.00 1.00 0.0032 0.74 6.13 0.10 0.0164 600 0.5 10.00 1.00 0.0016 0.74 6.11 0.20 0.0328 178 0.5 5.00 1.00 0.0054 1.47 – 0.30 0.0492 241 0.5 5.00 1.50 0.0060 2.21 – 0.40 0.0656 292 0.5 5.00 2.00 0.0066 2.95 – 0.50 0.0820 334 0.5 5.00 2.50 0.0072 3.69 – 54 K.Z. Gaca, J. Sefcik / Journal of Colloid and Interface Science 406 (2013) 51–59 varying from 7.5 for R/C = 50 mol/mol to 6.1 for R/C = 300 mol/ mol (these are initial solution pH values measured at ambient tem- perature before solutions were heated), related to neutralisation of slightly acidic reactants (pH of the formaldehyde stock solution was 4.3 due to small amount of formic acid present, and resor- cinol–water solution at R/W = 0.1 g/ml had pH of 4.8), see Table 1. Increasing concentration of resorcinol, corresponding to pro- portionally increasing concentration of formaldehyde concentra- tion (as the R/F ratio was kept constant at 0.5 mol/mol), also caused the gelation time to become shorter, as expected, with an approximately linear dependence of the observed gelation time on the inverse of resorcinol concentration. In terms of temperature effect, increasing the temperature by 25 �C (from 55 �C to 80 �C) caused the gelation time to decrease by a factor of about 5, what is consistent with expectations and the previous literature [27]. A series of in situ DLS experiments were performed at 55 �C where the overall gelation process is slow enough to investigate formation of primary clusters in more detail. The raw data resulting from these experiments were autocorrelation functions, which were then fitted to obtain the initial decay rates and from that the apparent mean hydrodynamic radii of primary clusters formed in the solutions. A typical example of how measured auto- correlation functions change over time is shown in Fig. 3. Although Fig. 2. Gelation time as a function of composition for solutions kept at 55 �C (squares) an ml and 0.5 mol/mol, respectively, (b) R/C and R/F ratios equal to 100 mol/mol and 0.5 m subject to unhindered Brownian motion. As the time proceeds, the shape of the autocorrelation function changes in two respects, as seen in the shape of the autocorrelation function collected after 75 min of heating: the initial decay rate is lower and there is a sec- ondary decay significantly deviating from an ideal monodisperse exponential decay. This is most likely due to aggregation of pri- mary particles forming larger clusters; however, most of them still remain free to move around and appear to be subject to Brownian motion, since the measured autocorrelation function fully decays within about 1 ms. Finally, the shape of the autocorrelation func- tion measured after 103 min of heating is consistent with one that is expected for a gel, consistently with the macroscopic gel time estimated by tilting a vial containing the gelling solution (see Fig. 2). At this point, the shape of the autocorrelation function de- cay shows clear power law features, typical for cases where mobil- ity of clusters is severely restricted as would be expected for a gel network consisting of a three-dimensional structure of intercon- nected clusters, while unconnected clusters can diffuse within the pores of the gel structure, as indicated by the initial decay still visible. A standard analysis of autocorrelation functions can be used to estimate the mean hydrodynamic radius of clusters in the solution, assuming that the clusters are freely diffusing, and they are not yet d 80 �C (circles). Lines are guides to eye only. (a) R/W and R/F ratios equal to 0.10 g/ ol/mol, respectively, and sodium carbonate concentration equal to 7.9 mmol/dm3. ution s ca ncti K.Z. Gaca, J. Sefcik / Journal of Colloid and Interface Science 406 (2013) 51–59 55 interconnected, as evidenced by the autocorrelation function following an exponential decay rather than stretched exponential or power law decays characteristic of gel networks. We have there- fore restricted our attention to the early times where monomodal primary cluster populations are present, so that true mean hydro- dynamic radii of primary clusters can be determined. The initial decays of measured autocorrelation functions were fitted as de- scribed in the experimental section, and the resulting initial decay rates (C) and the corresponding mean hydrodynamic radii are shown in Fig. 4. Fig. 4b shows clearly that the (true) mean hydrodynamic radius of primary clusters during the initial stage of their formation (up to about 50 min of heating at 55 �C) is essentially independent of the R/C ratio: it grows from about 1.5 nm at 10 min to about 3.5 nm at 50 min, following a power law dependence of size on time. We note that the hydrodynamic radius of a hydrated resorcinol mole- cule is about 0.4 nm, which we were able to directly measure in an Fig. 3. DLS autocorrelation functions (log-linear and log–log plots shown) for sol respectively, kept at 55 �C for times as indicated. Dashed lines show theoretical decay equal to the mean value determined from the initial decay of the autocorrelation fu aqueous solution of resorcinol using our DLS instrument, while variously substituted hydroxymethyl derivatives of resorcinol are Fig. 4. Time evolution of the initial decay rate (a) and the mean hydrodynamic radius (b) and R/C as shown in legends. expected to be somewhat larger, so it is clear that primary clusters found after the first 10 min are several times larger than single res- orcinol–formaldehyde molecules. Further change of the initial decay is very slow, but the second decay starts developing at longer times, indicating subsequent aggregation of primary clusters. During this stage, the apparent mean hydrodynamic radius grows only slightly. Eventually, the secondary decay increases in relative magnitude and assumes a power law scaling indicative of a gel network (see Figs. 3 and 4). Although one could still technically fit the initial decay rate and determine the corresponding apparent mean hydrodynamic radius during the next stage of the gel formation process, it would not be a physically meaningful quantity describing a real size of growing clusters. These observations are consistent with previously published re- sults [15] for experiments performed at 25 �C, although those authors presented the apparent hydrodynamic diameter deter- s with R/C, R/W and R/F ratios equal to 50 mol/mol, 0.10 g/ml and 0.5 mol/mol, lculated assuming a monodisperse cluster population with the hydrodynamic radius on. mined by DLS for times extending well into the stage of cluster aggregation and network formation, where their reported decay measured at 55 �C. Ratios R/F and R/W were 0.5 mol/mol and 0.10 g/ml, respectively ns w and time spectrum became bimodal and very long decay times were observed. However, in the early stage where monomodal decay spectra were observed (see their Fig. 1), initial sizes of primary clusters appear to be quite similar for various values of the R/C ra- tio, although there are only few data points reported for hydrody- namic diameters below 5 nm (see their Fig. 2). Similarly, the growth of primary clusters was investigated at 25 �C by SAXS [17], and from the data reported in that work, it can be seen that the radius of gyration of primary clusters appears to be around 2 nm (see their Fig. 4) for all conditions investigated at the same time of 500 min, which is well before any increase in viscosity indi- cating approaching gelation. The same authors also showed scaling exponents for their SAXS intensity data increasing over time from their initial values around 2 to plateau at values around 4 at longer times. The power law scaling exponent of 4 is indicative of smooth Fig. 5. DLS autocorrelation functions (log-linear and log–log plots shown) for solutio carbonate concentration 7.9 mmol/dm, kept at 55 �C for times as indicated. 56 K.Z. Gaca, J. Sefcik / Journal of Colloid surfaces (Porod regime), while value below 3 is considered to be indicative of mass fractal structures. However, in the reported data (see their Fig. 3), there is an extremely narrow range of q values over which a power law scaling is fitted, and it can be argued that such data cannot be used reliably to extract structural information from scattered intensities. In fact, it has been shown previously that for small fractal-like aggregates, the apparent power law scal- ing determined from scattering structure factors can be signifi- cantly lower than the actual mass fractal scaling exponent and only approached the actual value when aggregates are sufficiently large [28]. A series of experiments with varied R/W ratios were also per- formed at 55 �C in order to determine the influence of overall reac- tants concentration on the evolution of autocorrelation function and the apparent hydrodynamic radius. Ratio of resorcinol to form- aldehyde was kept at 0.5 mol/mol, and instead of keeping R/C ratio constant, the concentration of sodium carbonate was kept constant at and equal to 7.9 mmol/dm, while the ratios of resorcinol to water were changing from 0.10 g/ml to 0.50 g/ml. Fig. 5 shows typ- ical autocorrelation functions for a sample with R/W 0.50 g/ml. Interestingly, the measured autocorrelation functions are very sim- ilar to those shown in Fig. 3, and the decay times are comparable, despite the fivefold increase in the resorcinol concentration (cf. curves for 10 min in Figs. 3 and 5). Mean hydrodynamic radii were calculated from the autocorre- lation functions for samples with varied R/W ratios for a range of times where decays were still following a single exponential pat- tern, and so, there was no hindrance on free diffusion of primary clusters (in the same manner as it was done for samples with var- ied R/C ratios). The results of this analysis are shown in Fig. 6. One can clearly see that the mean hydrodynamic radius is again very similar for all values of R/W ratios investigated here, indicating that the overall concentration of reactants does not control the sizes of primary clusters. This is also consistent with previous observations of Yamamoto et al. [15] at 25 �C, where reported hydrodynamic diameters of primary clusters were similar for a range of R/W ratios at early times (see their Fig. 2), while they be- came very different at later times (where these can only be seen as apparent hydrodynamic diameters which do not reflect real cluster sizes anymore, as discussed above). From the data reported in Figs. 4 and 6 above, we can see that ith R/W and R/F ratios equal to 0.50 g/ml and 0.5 mol/mol, respectively, and sodium Interface Science 406 (2013) 51–59 the size of primary clusters does not appear to vary significantly with the concentration of carbonate or reactants. This might seem surprising, because carbonate is a catalyst for the substitution of resorcinol with formaldehyde, and therefore, it is expected that there is more resorcinol–formaldehyde present and hence faster polymerisation at higher carbonate concentration. However, there is a clear effect of the carbonate concentration as well as the reac- tants concentration on the measured values of scattered intensities as shown in Fig. 7. In Fig. 7a, we can see that scattered intensities (expressed here in terms of mean count rates measured at the scattering angle of 90�) stay approximately constant during the first 5–10 min and then start increasing steadily over time. However, unlike mean hydrodynamic radii, there is a clear dependence of scattered inten- sities on the R/C ratio, with higher intensities for smaller R/C ratios (i.e. higher carbonate concentrations). Since hydrodynamic radii were very similar at all three R/C ratios (see Fig. 4), while the scattered intensity is higher for higher carbonate concentration at a given time, it follows that the scatterers (i.e. primary clusters) are either present at higher concentrations or they possess higher optical contrast (difference in refractive index respective to sur- rounding solvent) at higher carbonate concentrations. Since the surrounding solvent is very similar in all cases (same reactant con- centrations in water, with carbonate being present at comparably low concentrations), it is most likely that in the case of various R/C ratios, the differences in scattered intensity are due to more (b) K.Z. Gaca, J. Sefcik / Journal of Colloid and Interface Science 406 (2013) 51–59 57 primary clusters formed at higher carbonate concentrations. This is reasonable, since higher carbonate concentration should imply fas- ter substitution reaction and so faster production of substituted resorcinol and presumably also faster subsequent condensation reaction. In Fig. 7b, we can see that scattered intensities are lower for higher concentrations of resorcinol (and formaldehyde, since the R/F ratio was kept constant equal to 0.5). By the same argument as above, the primary clusters are either present at lower concen- trations or they have lower optical contrast. Since it would be ex- pected that the primary clusters would become more (not less) numerous at higher concentration of reactants (while the carbon- ate concentration was held constant at all R/W ratios), it follows that they ought to have decreasing optical contrast with increasing reactant concentration. This is sensible, since composition of solvent surrounding primary clusters becomes more similar to clusters as concentration of reactants increases and water concen- Fig. 6. Time evolution of the initial decay rate (a) and the mean hydrodynamic radius dm3, and R/W was as shown in the legend. tration decreases in the reaction mixture with increasing R/W ra- tios. Refractive indices of reaction mixtures used here were Fig. 7. Time evolution of the scattered intensity measured at 55 �C. (a) Ratios R/F and R/W (b) Ratio R/F was 0.5 mol/mol, sodium carbonate concentration was 7.9 mmol/dm3, and estimated to vary between 1.35 for R/W = 0.1 g/ml and 1.40 for R/W = 0.5 g/ml [29], while the typical literature values for dense li- quid phase systems chemically similar to the clusters of reaction intermediates have been reported between 1.44 (saturated aque- ous solution of phenol; [30]) and 1.48 (phenol–formaldehyde resin at 50% w/w solids; [31]). Therefore, it can be expected that primary clusters composed of resorcinol–formaldehyde reaction intermedi- ates have higher optical density than surrounding bulk solution, and thus indeed, their optical contrast will be lower in solutions with higher reactant concentrations, i.e. at higher R/W ratios, which have higher refractive index. A series of DLS experiments were also performed on solutions reacting at 80 �C, while quenching them at specified times to 20 �Cwhere reactions are expected to bemuch slower andDLSmea- surements could be performed. An example of results from these experiments is shown in Fig. 8. Comparison of Figs. 3 and 8 shows that the evolution of the autocorrelation function indicates the at 55 �C. Ratio R/F was 0.5 mol/mol, sodium carbonate concentration was 7.9 mmol/ same overall pattern at both temperatures – from an initiallymono- disperse population of primary clusters, through a polydisperse were 0.5 mol/mol and 0.10 g/ml, respectively, and R/C was as shown in the legend. R/W was as shown in the legend. tion asu and Interface Science 406 (2013) 51–59 collection of aggregated clusters to a gel network of interconnected clusters. However, there is a significant difference in time when the autocorrelation function falls into one of these three stages. For example, at the composition corresponding to results shown in Fig. 3, the aggregation stage was observed only after more than 50 min at 55 �C, while it appeared in just 10 min at 80 �C. Similarly, the solution gelled about five times later at 55 �C than at 80 �C (see Fig. 3). These results indicate that the temperature influences the overall rate of the gel formation, but it does not appear to influence an overall mechanism of its formation, at least within the range of temperatures investigated. In order to assess the effect of temperature on the mean hydro- dynamic radius of primary clusters, in Fig. 9, we show a compari- son of mean hydrodynamic radii of primary clusters obtained by heating at 55 �C and 80 �C. It can be that the primary clusters ob- tained at 80 �C are larger than those at 55 �C, while the time evolu- Fig. 8. DLS autocorrelation functions (log-linear and log–log plots shown) for solu respectively, kept at 80 �C for times as indicated and then rapidly quenched and me 58 K.Z. Gaca, J. Sefcik / Journal of Colloid tion is following the same overall trend irrespective of the R/C ratio. We have seen that under the range of conditions investigated here, the size of primary clusters varies only little when changing concentrations of reactants or carbonate at a given temperature. Furthermore, primary clusters are more numerous, and overall gelation process is faster at higher carbonate concentrations. If pri- mary clusters are more numerous, then it is sensible that their aggregation is faster, and space-filling by growing aggregates leads to gel formation at shorter times. It may seem intriguing that the primary cluster size is only little sensitive to the reaction mixture composition. This appears to be inconsistent with the scenario where growth of primary clusters is driven by homogeneous polymerisation of substituted resor- cinol, as proposed previously [17], since faster reactions should lead to faster growth of cluster size, but this is not the case at early stage of the process before the onset of primary cluster aggrega- tion. However, our results are consistent with a process of nano- scale demixing, controlled by solubility/miscibility equilibria, similar to that involved in micellisation or spontaneous emulsifica- tion, such as in Ouzo effect. Unlike the classical Ouzo effect in non-reacting mixtures, though, resorcinol–formaldehyde species in primary clusters undergo further reactions, and hence, chemical and physical nature of clusters evolves over time. We propose that certain intermediates produced in resorcinol– formaldehyde polymerisation (either multiply substituted resorcinol or its oligomers) reach a limit of their solubility/misci- bility in the solution mixture at a given temperature, and the solution phase undergoes a process similar to spontaneous nano- emulsification or micelle formation. This leads to primary clusters observed in this work as well as by others using in situ DLS or SAXS measurements (see above). Such a process is controlled by solubil- ity/miscibility equilibria with demixing resulting in formation of discrete non-interconnected primary clusters freely diffusing in surrounding solution matrix, in contrast to two interpenetrating phases resulting from a classical polymerisation driven microphase separation. As indicated in the overall reaction mechanism shown in Fig. 1, there are a series of reactions starting with the first resorcinol sub- stitution with formaldehyde, followed by the second and possibly further ones. Then, the substituted resorcinol species undergo sub- sequent condensation reactions to form dimers and further oligo- mers. It is likely that a certain substituted and/or condensed s with R/C, R/W and R/F ratios equal to 200 mol/mol, 0.10 g/ml and 0.5 mol/mol, red at 20 �C. species has limited solubility/miscibility in the background reac- tion mixture, and as it accumulates over time, it reaches a critical concentration at which primary clusters are formed. As can be seen in Fig. 7a, there is an earlier onset of an increase in scattered inten- sity at higher carbonate concentrations indicating that such a crit- ical concentration of reaction intermediates is reached earlier, as Fig. 9. Early evolution of mean hydrodynamic radius for solutions kept at 55 �C (squares) and 80 �C (circles). (R/C ratios 50, 100 and 200 mol/mol�1; R/F is 0.5 mol/ mol�1; R/W is 0.10 g/ml�1). expected for a series of reactions catalysed by bases, such as car- bonates. At higher carbonate or reactant concentrations, reaction rates are higher, more intermediates are produced, and therefore, more primary clusters (of approximately the same size) are formed within the same solution matrix, eventually resulting in faster aggregation and gelation. Since intermediate species in primary clusters undergo further reactions, chemical and physical nature of clusters continually evolves, which may help to explain why their size is changing over time even though their formation is thermodynamically controlled. Primary clusters may be initially li- intermediates. This leads to a competition between polymerisa- tion, phase separation, coalescence and aggregation resulting in nanoscale morphologies of wet gels. Analogous formation mecha- nisms have been previously proposed for several polymerisation and sol–gel systems, including monodisperse silica, organosilicates and zeolites. There are likely to be multiple roles played by carbonates in the process of resorcinol–formaldehyde gelation. In addition to car- bonates being catalysts for reactions of resorcinol and formalde- K.Z. Gaca, J. Sefcik / Journal of Colloid and Interface Science 406 (2013) 51–59 59 quid-like and subject to subsequent coalescence as well as aggre- gation. Further polymerisation may proceed faster within the clusters than in the bulk solution due to locally higher concentra- tions of intermediates, eventually leading to solidification and colloidal destabilisation or polymerisation driven microphase separation. The final size of particles observed to form the gel network then results from a delicate interplay of polymerisation, phase equilib- ria, coalescence and aggregation phenomena. Furthermore, car- bonates might play role in colloidal stability of primary clusters or their aggregates, resulting in faster aggregation and subsequent gelation at higher carbonate concentrations, which could also con- tribute to the observed effect of carbonates on gel times. 4. Conclusions Formation of primary clusters and their subsequent aggregation and gelation during resorcinol formaldehyde polymerisation was monitored by DLS at two different temperatures, 55 �C and 80 �C. We found that the kinetics of growth of primary clusters, before they were subject to further aggregation, changed only very little when the carbonate concentration was varied. Although more pri- mary clusters were formed at higher carbonate concentrations, as deduced from scattered light intensities, and corresponding gela- tion times were shorter, the size of primary clusters was very sim- ilar at all carbonate concentrations. We also found that the growth kinetics of primary clusters was only little dependent as the con- centration of reactants (resorcinol and formaldehyde) was varied. Resulting gelation times were again shorter at higher reactant con- centrations, indicating that more primary clusters were produced, although this could not be unequivocally established from scatter- ing intensities. Our results indicate that size of primary clusters appears to be thermodynamically controlled, where a miscibility limit is reached due to formation of certain reaction intermediates, most likely with relatively small molecular weight, resulting in nanoscale molecular demixing leading to formation of approximately mono- disperse primary clusters, similar to formation of micelles or spon- taneous nanoemulsions. At higher carbonate concentration or reactant concentrations, more primary clusters were produced due to faster reactions. Primary clusters may be initially liquid-like, and further polymerisation may proceed faster within them than in the bulk solution due to locally higher concentrations of hyde, resulting in faster reactions and higher numbers of primary clusters formed, they may also influence solubility/miscibility equilibria in complex multicomponent reaction mixtures responsi- ble for formation of primary clusters. 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