In-Line and In Situ Monitoring of Ionic Surfactant Dynamics in Latex

Ind. Eng. Chem. Res. 2007, 46, 1465-1474

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In-Line and In Situ Monitoring of Ionic Surfactant Dynamics in Latex Reactors Using Conductivity Measurements and Ion-Selective Electrodes G. P. Santos, Jr.,† C. Martins,† M. Fortuny,†,‡ A. F. Santos,†,‡ M. Turmine,§ C. Graillat,| and T. F. L. McKenna*,| Instituto de Tecnologia e Pesquisa, AVenida Murilo Dantas 300, Aracaju 49032-490 SE, Brazil, Programa de Mestrado em Engenharia de Processos, UniVersidade Tiradentes, Aracaju 49032-490 SE, Brazil, CNRS-LECA/ENSCP, UMR 7575, UniVersite´ Pierre et Marie Curie-Paris6, Case 39, 4 Place Jussieu, 75252 Paris Cedex 05, France, and CNRS-LCPP/ESCPE, UniVersite´ Claude Bernard Lyon 1, 43 BouleVard du 11 NoVembre 1918, Baˆ t 308F, 69616 Villeurbanne Cedex, France

In an emulsion polymerization process, surfactant type and concentration play an important role in the stability, appearance, reaction kinetics, and particle size distribution of latexes. The possibility of carrying out in-line evaluation of the distribution of free surfactant molecules in the media is of interest in polymer reaction engineering, because this would provide information about the rates of nucleation and particle stabilization. Both ion-selective electrodes and conductimetry seem to be very promising techniques for this objective. Ion-specific electrodes are able to determine the concentration of several substances, in a quantitative and specific manner, and may also be a good way of monitoring ionic surfactant. Conductivity measurements offer a means of monitoring the evolution of concentrations of different species in the latex. The objective of this work is to evaluate the use of ion-selective electrodes as a tool for the on-line monitoring of an emulsion polymerization reaction and to compare this method to conductivity. Ion-selective electrodes were prepared for two ionic surfactant species: dodecyl sulfate and dodecyltrimethylammonium, coming from SDS (sodium dodecyl sulfate) and DTAB (dodecyl trimethylammonium bromide), respectively. To do so, a glass electrode was coated with polymeric membranes specific for each surfactant. It is demonstrated that ion-selective electrodes can provide insight into the evolution of the colloidal surface in the latex, even when temperature and composition drifts are present. 1. Introduction Emulsion polymerization is widely used in industry to produce a great variety of polymers with multiple uses (e.g., paints, adhesives, coatings, printing inks, and binders) that only a few years ago were prepared by other polymerization processes.1 Moreover, environmental regulations have led to the substitution of solvent-based polymerizations by water-borne latexes. As a consequence, more and more products are prepared by emulsion polymerization, and the quality and properties of such products are a subject of great importance.2 An ideal batch emulsion polymerization reaction has been generally viewed as a sequence of three intervals which involve particle nucleation, particle growth, and final monomer consumption inside swollen polymer particles.3 From a more realistic point of view, particle nucleation, growth, and coagulation can occur simultaneously at different points during the reaction, especially if one considers semibatch processes.4 The relative concentrations of the surfactant and monomer(s) can be varied to alter the number and size of polymer particles ultimately produced and the rate of polymerization.5 Since the reaction takes place in the particles (for the most part), it would clearly be an advantage to be able to predict and/or measure the evolution of the number and size of the particles in the reactorsin other words to be able to quantify the different rates * To whom correspondence should be addressed. E-mail: mckenna@ cpe.fr. Fax: (+33) (0)4 72 43 17 68. † Instituto de Tecnologia e Pesquisa. ‡ Universidade Tiradentes. § Universite´ Pierre et Marie Curie-Pariso´. | Universite´ Claude Bernard Lyon 1.

of growth, coagulation, and nucleation that can take place as a function of reaction conditions. Of these different phenomena, the particle nucleation stage can perhaps be viewed as the most “controversial”, in large part due its very rapid dynamics and difficulty in directly observing it under realistic conditions. Although, to be fair, outside of certain well-defined conditions that allow us to use the DLVO theory to predict coagulation rates, this phenomenon is not well understood either.6 One of the “stumbling blocks” on the road to the development of a description for this, and related phenomena, is measuring how the surfactant is distributed between the different phases in the reactor during the dynamic phases of the polymerization reaction. The principal job of the surfactant is of course to aid in the creation and stabilization of the polymer particles, so a major fraction of this component will be found on the particle surface during the later stages of the reaction. Surfactant can also be found in the form of micelles, on the surface of monomer droplets, as well as being dissolved in the continuous phase. In some processes, the presence of micelles is undesirable, for example in the case of a seeded emulsion polymerization,7 miniemulsion polymerizations,8 and encapsulation reactions,9 where surfactant concentration is thought to be critical. Therefore, the development of real-time techniques able to evaluate the concentration of surfactant molecules would enhance the understanding of the nucleation phenomena and the stabilization of polymer particles. The current work discusses the use of ion-selective electrodes and conductimetry for these purposes and some recent developments in how they can be applied. Ion-selective electrodes are able to determine concentrations of several substances, in a

10.1021/ie060854d CCC: $37.00 © 2007 American Chemical Society Published on Web 02/08/2007

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Ind. Eng. Chem. Res., Vol. 46, No. 5, 2007

quantitative and specific manner. They might therefore be a promising tool for the monitoring of the ionic surfactant concentration. Similarly, conductivity measurements provide estimates of the concentration of ionic species in the latex and have been shown to represent an interesting means of monitoring ionic surfactant levels. In fact, recent studies from our group have shown that conductimetry offers a means of monitoring the evolution of concentrations of ionic surfactant in the latex.10,11 It should be mentioned that a number of on-line sensors have been developed for the monitoring of emulsion polymerization reactions. These different methods, including some spectroscopic methods, have been widely discussed in the scientific literature, so we will not discuss them here.12-14 At the risk of overgeneralizing, it appears that most of the successful applications of on-line sensors are used to follow reaction kinetics through the use of different types of hardware (i.e., measuring equipment) and software sensors (e.g., calorimetry). There is still no universally accepted method for the on-line measurement of the particle size distribution (PSD), especially if we restrict ourselves to in situ measurement techniques. In the current paper we will discuss two methods for the in situ monitoring of the distribution of surfactant in the reactor as a means of monitoring the evolution of the particle phase. 1.1. Conductimetry. The conductivity meter constitutes a relatively inexpensive instrument that can be easily installed in polymerization reactors.10 It can be used to monitor the mobility of ionic species with minimum calibrations. If one has information about the conductivity signal due to initiator decomposition, then the output of the conductivity cell can be used to measure the evolution of the concentration of free ionic surfactant. This allows us to follow the surfactant dynamics in the media, which can in turn be related to particle nucleation and/or coagulation phenomena. Additionally, conductivity measurements can also give insights about the emulsion polymerization kinetics. Different studies have used a recirculation loop with an inline probe for measurements of conductance during emulsion and miniemulsion polymerizations,8 for the design of surfactant addition profiles in encapsulation studies9 to study nucleation in methyl methacrylate miniemulsions with samples withdrawn from the reactor,15 and to look at surfactant partitioning.16 With the exception of Janssen,9 most of these works relied on a sampling device, which is not always recommended in industrial situations. Santos et al.10,11 went one step further, inserting a probe directly into the reaction medium to provide in situ measurements of conductivity in real time. They demonstrated that it is possible to infer the dynamics of the evolution of the number of stabilized particles in an emulsion polymerization by following the response of the conductivity curves as a function of time. 1.2. Ion-Selective Electrodes. The term “ion-selective electrode” (or ISE) is applied to electrodes with membranes that are specifically permeable to one or a few chemically related ions. Basically, ion-selective electrodes are electrochemical halfcells in which a potential difference that is dependent on the concentration (more precisely, the activity) of a particular ion in solution arises across the electrode/solution interface. They can be used to quantitatively and specifically determine the concentration of a great variety of substances, from inorganic ions to amino acids and complex organic molecules. Many electrodes of this type have been developed in recent years and are available commercially. The design, properties, and application are reviewed elsewhere.17 Some are highly selective: examples are the lanthanum fluoride electrode, selective for

fluoride ion, and the silver sulfide electrode, which responds to silver and to sulfide. The pH glass electrode is properly considered as an ion-selective electrode. Ross18 claims that this is really the only device capable of determining ion activities which is both convenient and at the same time sufficiently free from interferences to be of general usefulness as the ion-selective membrane electrode. The sensing membrane allows only the ion of interest to penetrate the boundary between the sample solution and the membrane surface. Usually, the membrane separates two solutions of different ionic activities: an internal solution of fixed activity of the ion to which the membrane is permeable and a sample solution. There is then a momentary flux of ions across the membrane in the direction of the solution containing the lower activity of the mobile ion. The membrane being impermeable to the counterion, its flux is null. An equilibrium state is then reached.19 Changes in this membrane potential can be measured by making electrical contact to the inner solution with a suitable reference electrode and at the same time contacting the sample solution with a second reference electrode via a salt bridge. A high input impedance voltmeter connected across the two reference electrode leads will indicate a potential given by the Nernst equation:

RT log A zF

E ) cte + 2.3

(1)

Here, E is the potential, in millivolts, developed by the system. The constant term depends on the particular choice of reference electrodes used and on the choice of ion activity in the inner solution and also includes a small potential associated with the liquid-liquid junction at the salt bridge connection. RT/F is the Nernst factor, which depends on the temperature, and has usually a value about 59 mV at 25 °C. A is the activity (in the sample solution) of the ion to which the membrane is permeable, and z is its charge, including sign. In use, the electrode pair must first be calibrated with standard solutions of known activity (the calibrating buffer solutions, in the case of pH measurement). Another aspect concerning ion-selective electrodes is the presence of interfering species. It is important to be able to predict in advance if a given electrode will be subjected to interferences in a given sample. For this purpose, the following empirical equation is useful:

RT log[A + E ) cte + 2.3 zAF

∑i ki(Bi)z /z ] A i

(2)

In eq 2, A and B represent the sample activity levels of the sought-for ion and an interfering ion, respectively. The activity of each interfering ion (Bi) in the sample, raised to a power equal to the charge ratio (zA/zi), is multiplied by a weighting factor, the selectivity constant ki. If the resulting terms are negligible compared to the activity of A, then the electrode can be used with confidence.18 It should be pointed out that emulsion polymerization reactors are not the most suitable place to perform electrochemical studies. The occurrence of different phenomena in different phases, changes of viscosity, and temperature drift represent complications that can make the interpretation of electrochemical measurements difficult, but not impossible. Nevertheless, electrochemical devices are not expensive, are rapid, and are easy to use. For these last reasons, we have decided to investigate the use of ion-selective electrodes and to find means of correctly interpreting the resulting signals in order to follow the evolution

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of free surfactant concentration in emulsion polymerization. The use of ion-selective electrode measurements for monitoring the surfactant dynamics during emulsion polymerizations constitutes a new area of study and is still in the early stages of development. In this paper we will focus on demonstrating the feasibility of using ISE as a means of providing information on the state of emulsion polymerization reactors in real time. The results of the ISE evaluations will be compared with those obtained from in-line conductivity tests. 2. Experimental Section Ion-selective electrodes were prepared to track two types of surfactant molecules (anionic and cationic). The conductivity meter was also applied in all tests for comparison purposes. First, titration tests and calibrations were conducted in order to understand the performance of surfactant-selective electrode technique off-line. Following this, an experimental study was conducted to understand the effects of the organic phase (monomer and polymeric latex) on the conductivity and electrode signals. A simple procedure was taken in this study that consisted of measuring sensor signals (temperatures, conductivity, and ISE potential) on-line during the whole period of reactant addition and also during the polymerization reaction. Styrene, potassium persulfate (KPS, 99+%), sodium dodecyl sulfate (SDS, 99+%), V50 (2,2′-azobis(2-amidinopropane) hydrochloride, 98%, as cationic initiator), NaCl, and KCl were obtained from Acros Organics. Dodecyltrimethylammonium bromide (DTAB, 99+%) and acrylamide (98+%) were obtained from Aldrich and used as received. Surfactant and initiator used in the process define the type of stabilization of the latex during emulsion polymerization. In our case, SDS and KPS provide anionic stabilization and initiation of particles, respectively, while DTAB and V50 provide cationic stabilization and initiation. SDS-specific electrodes were employed during the anionic process, while DTABspecific electrodes were used during the cationic polymerization process. Emulsion homopolymerizations of styrene were run at 10% solids content (mass monomer with respect to total reaction mass), and the initiator concentration (KPS) was maintained at 1.00 g/L H2O for the styrene runs with SDS. In the case of styrene runs with DTAB, the initiator concentration (V50) was kept at 0.50 g/L H2O. The reaction volume (total) was 1830 mL (183 g of monomer, 1650 g of water, initiator, and surfactant). The only quantity that was varied was the mass of the surfactant; all other quantities remained constant. Polymerizations were carried out in a 3 L calorimetric reactor. The reactor vessel and lid were jacketed, and water was circulated at a constant temperature of 60 °C. There are four inlets in the vessel wall to allow for the insertion of sensors into the reaction mixture. A platinum sensor connected to a conductivity meter (TACUSSEL CD 810) was inserted into the reaction mass through one of these ports. ISE potential measurements were performed by inserting the electrodes through two of these ports (detailed description about the electrode measurement setup is given in the next section). The measurement frequency was set at 1 kHz. Unpublished results from our laboratory suggest that the time scale at which a micelle of SDS exists is on the order of 5 µs (this was found by adjusting the sampling time of a Malvern 4700 goniometer). Therefore a measurement frequency of 1 kHz should be sufficient to provide a representative value of the free ion concentration. Temperature, conductivity, and potential measurements were recorded by means of a data acquisition unit.

Samples were regularly collected during the reaction for offline analysis of conversion (x) by gravimetry and average particle size (dp, in nanometers) by dynamic laser light scattering (Malvern Instruments, model S7032). Note that the value of dp used here is for the unswollen (i.e., monomer-free) particles. These data were used to compute important latex properties, such as the number of particles (Np) from eq 3, the latex surface (SL, given in square centimeters) from eq 4, and surface coverage by surfactant θ, given as a percentage), given by eq 5, where mS (in grams) is the amount of surfactant added in the system, aS ( expressed in units of Å2) is the specific area occupied by a single surfactant molecule (45 Å2 for SDS 10 and 35 Å2 for DTAB), and MWs stands for the surfactant molecular weights 288.38 g/mol for SDS and 308.35 g/mol for DTAB.

NP )

6mm × 1021 πFpoldp3

SL ) θ)

NPπdp2 1014

NAVmsas 1014SLMWs

(3)

(4)

(5)

Here, NAV is Avogadro’s number, Fpol is the density of the polymer, and mm is the mass of monomer consumed by the reaction. Note that we have made a certain number of limiting assumptions in the use of eqs 3-5. First of all, it is assumed that the particles are monodisperse in terms of the average diameter. Second, in writing eq 5 we have used the approximation that all surfactant molecules available in the reactor will adsorb on the surface of the particles so long as this is possible, and only then they will be found in the aqueous phase. This is obviously a rough approximation but is adequate for the analysis of the results. 2.1. Surfactant-Selective Electrodes for DTAB and SDS: Experimental Setup. A detailed description to the design of specific electrodes and membranes is given elsewhere.20-22 In the case of the DTA+-specific electrode, the sensitive membrane was prepared using a cross-linked polymer made of a mixing of silicone gel (Rhodorsil, Rhodia) and polymethylhydromethylcyanopropylsiloxane (Petrarch system). The selectivity of the membrane is due to a “carrier” of the cationic amphiphile, dodecyl trimethylammonium tetraphenylborate (DTABΦ4), which was prepared by precipitation in water from sodium tetraphenylborate (NaBΦ4) and DTAB mixture in stoichiometric quantities. To construct the body of the electrode, a glass tube (physical support for the membrane, usually a glass electrode) was dipped in the membrane solution two times, allowing 15 min air-drying between each immersion.22 The preparation of the dodecyl sulfate (DS- from SDS) specific electrode followed the same steps as those of the DTA+ case, except for the membrane. The carrier used to ensure the selectivity of the membrane was cetyl trimethylammonium dodecyl sulfate (CTADS). CTADS is an amphiphile, prepared by precipitation in water from cetyltrimethylammonium bromide and sodium dodecyl sulfate. Figure 1 illustrates the electrochemical cell applied in the measurements of this work. The ion-selective electrode is formed by the glass electrode coupled with a selective membrane, prepared as described above. The reference employed in our case was KCl-saturated calomel electrode protected from

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Figure 2. Scheme of the electrochemical cell.

Figure 1. Representation of the titration cell. Ion-selective electrode and a reference electrode protected by a saline bridge are connected to a voltmeter.

amphiphile diffusion by a saline bridge (a microtube containing 2 M NaCl in a gel). Because our major interest concerns measurements in a reactor at temperatures greater than ambient, special attention was paid in the design of the saline bridge. In this type of electrode, saline bridges generally constitute an agar-agar gel containing the saline solution. Agar-agar gels resist temperatures up to 40 °C but lose their gel properties at higher temperature, characteristic of emulsion polymerizations (60-80 °C). Thus, another gel with higher temperature resistance, with permeability, and without affinity to surfactant molecules was used. This gel was specially fabricated for the current studies as follows.23 2.2. Acrylamide Gel Syntheses for Designing Saline Bridges. Acrylamide gels have the interesting property of temperature resistance. In fact, polyacrylamide is a hydrophilic polymer that does not interact with SDS,24 is soluble in water, and may be used in the design of temperature-resistant saline bridges. The design of saline bridges for electrochemical cells involves the ionic charge introduction into the polymeric matrix. This can easily be made through the polymerization of the highly water soluble acrylamide monomer in a diluted saline solution. In this work, acrylamide polymerization reactions in an aqueous medium were carried out inside Teflon microtubes of 1.68 mm average diameter (Bioblock, France). The aqueous medium was an NaCl solution (2 M). Tests with other salts were also performed, but, NaCl offered the most consistent results. Bis-acrylamide (N,N-methylene bisacrylamide, ACROS, France) was used as cross-linking agent and KPS as the free radical initiator. All the reactants are initially introduced into a beaker, stirred at approximately 200 rpm, and heated to a constant temperature of 40 °C. The system was purged by nitrogen for several minutes. The mixture is then introduced into the microtube by means of a syringe, paying attention to avoid the formation of bubbles inside the microtube. This task is quite easy when microtubes are disposed horizontally. The microtubes are then placed in hot water (60 °C) containing a small amount of NaCl (2 M) with nitrogen purging in order to initiate the reaction. Finally, they are transferred to an oven at 70 °C during 5 h, to finish the reaction. 2.3. Surfactant-Selective Electrodes for DTAB and SDS: Calibration. According to the electrode measurement setup displayed in Figure 1, the electrochemical cell can be schematized as follows (Figure 2):

The ISE and the external reference are connected onto a pH meter (Tacussel LPH330T), used in the millivoltmeter mode. During calibration tests, small amounts of concentrated surfactant solution are added into the titration cell by means of a burette. Measurements of potential are made for each concentration of surfactant in the media. The temperature is kept constant by immersing the titration cell in a thermostatic bath. If the temperature is constant and if electrodes follow the Nernst equation, a simple plot of the measured potential versus the logarithm of surfactant concentration should be linear up to the critical micelle concentration (CMC). Little change of potential (no change, in the ideal case) is expected in the system above the CMC. However, temperature fluctuations are commonly found during polymerization reactions. According to Bates,17 the potential of the saturated calomel electrode becomes about 0.2 mV more positive when the temperature is elevated 1°; furthermore, the author states that errors greater than 1.0 mV are likely to be incurred only immediately following a change of 10 °C or more. Similar errors are expected for a selective electrode, if no auxiliary chemical reactions are involved in concentration determinations using ion-selective electrodes, as in the case of our application. 3. Results and Discussions 3.1. Determination of the CMC. Titration tests were performed with DS-- and DTA+-specific electrodes in order to determine the CMC of each surfactant in aqueous media. In both cases, the ISE potential was evaluated using the KClsaturated calomel electrode as external reference. During these tests, conductivity measurements were also performed. Average potential and conductivity results are displayed in Figure 3. When surfactant is slowly added into pure water, conductivity increases until the CMC is reached. From this point on, surfactant molecules start to form micelles. Due to the larger aggregate size and lower mobility of the micelles, the rate of change of the conductivity signal is much slower than it is below this point. A plot of the conductivity signal for increasing surfactant concentrations at constant temperature produces two straight lines that intersect at the CMC. Similar behavior occurs for the potential measured by surfactant-specific electrodes (if one considers that, for SDS electrodes, potential measurements become more negative). The results in Figure 3 show that the CMC values obtained from ISE measurements for both surfactants indicated by the arrows (CMCSDS ) 2.6 g/L; CMCDTAB ) 4.5 g/L) are in very good agreement with the CMC values obtained through conductivity measurements in previous studies (CMCSDS ) 2.5 g/L;10,25 CMCDTAB ) 4.5 g/L 25) at 25 °C. Note that the CMC will depend on a number of factors, including the temperature, presence of other species, and traces in impurities or other products in the original surfactant. Thus, one can find different values for the CMC of different species in the literature. For example see data in Santos et al.10 for SDS. 3.2. ISE Measurements during Emulsion Polymerizations. Styrene polymerization runs are summarized in Table 1. The CMC of the SDS employed on these experiments is 0.01 M at 60 °C.10 For DTAB, the CMC measured using similar conditions and procedures of those used for SDS is 0.02 M at 60 °C. Before


Figure 3. Potentiometric and conductometric titrations of SDS and DTAB in aqueous media (25 °C). Table 1. Styrene Emulsion Polymerization Batches exp

[SDS] (M)

final dp (nm)

final θ (%)

exp

[DTAB] (M)

final dp (nm)

final θ (%)

R2 R3 R1 R4

0.02 0.03 0.04 0.05

52 51 46 45

45 67 75 93

C1 C2 C3

0.060 0.020 0.014

50 71 112

101 46 52

treating what happens in the reaction stage, we first analyze the signals obtained during the preparation phase. Typical results of sensor measurements during the reactant addition period and the reaction stage are seen in Figure 4 and provide information about sensor response when concentration and temperature change in the media. Reaction (Tr) and jacket (Tj) temperatures are plotted in the same graph, showing that the temperature of the jacket is raised from 40 to 60 °C and kept at this value throughout the test. The figure shows that when the reactor

Figure 4. On-line sensor measurements during addition of reactants and polymerization reaction (R2). Labels indicating reactor content: (A) water; (B) aqueous solution of SDS ) 0.02 M; (C) styrene emulsified in the surfactant solution; (D) addition of KPS and the beginning of the reaction stage.

contains only water (interval A in Figure 4), the conductivity values are very low even with heating (as expected) and that the measurement of potential is unstable since no SDS is present in the mixture. The conductivity increases upon the addition of the SDS aqueous solution (interval B) and exhibits the same general evolution as a function of the temperature during heating. At this point, the SDS concentration in the media is 0.02 M, which represents 2 × CMC. The potential drops quickly, and as expected, when the SDS is added at the beginning of the interval, and then increases slowly as the temperature increases. At this SDS concentration and in the presence of water, it appears that the ISE potential curve is sensitive to temperature changes, which does not seem to be the case for the conductivity measurements. At the start of interval C, the monomer (styrene) is added to the surfactant mixture, and both conductivity and potential signals respond to this change in the composition of the medium. According to Graillat et al.,26 the reduction of conductivity following the introduction of monomer is likely due to the decrease in the concentration of free surfactant in the aqueous phase as some of it is absorbed on the surface of the monomer droplets that are formed in the medium. Concerning the ISE signal at this stage, the main effect is a shift of the curve toward higher free surfactant concentrations. This observation disagrees with the conductivity response and is not what one would expect to find intuitively since the potential shift is generally linked to the change of the reactor medium.20 It is possible that a solvent effect may be responsible for the potential change into a new stable level. This solvent effect is supported by Bates17 and attributed to the redistribution of the organic phase in the medium, which produces changes in the reference values of the electrodes. The addition of initiator (KPS) increases both signals, principally in the case of the conductivity signal. ISE measurements should also be influenced by the added persulfate basically due to the presence of additional ions which may affect the

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Figure 5. On-line sensor measurements during addition of reactants and polymerization reaction (C1). Labels indicating reactor content: (A) aqueous solution of DTAB ) 0.06 M; (B) styrene emulsified in the surfactant solution; (C) addition of V50 and the beginning of the reaction stage.

reference value measured by the reference electrode. Since the persulfate decomposes in the water, this result is to be expected. Concerning polymerization runs carried out with DTAB as surfactant and V50 as initiator, typical results of sensor measurements during the reactant addition period and the reaction stage are seen in Figure 5. When reactor content is DTAB aqueous solution (interval A in Figure 5) and the temperature is about 60 °C, it can be observed in the figure that both the conductivity and potential values are stable. At this point, the DTAB concentration in the media is 0.06 M, which represents 3 × CMC. At the start of interval B, the monomer (styrene) is added to the surfactant mixture, and both conductivity and potential signals respond to this change in the composition of the medium. Again, the main effect on the ISE signal is a shift of the curve toward higher free surfactant concentrations, which can be related to the solvent effect discussed before. The addition of initiator (V50) in the beginning of interval C produces very small changes of both signals, since its concentration is low (0.5 g/L). Comparing the sensor responses upon the addition of V50 with those responses found when the KPS was added (SDS system), one can conclude the greater selectivity of the DTAB-specific electrode and the lower mobility of the radicals released during the decomposition of the azo initiator (V50). As a consequence, DTAB systems will probably exhibit more easily interpreted signals. Typical results of sensor measurements during styrene emulsion polymerization batches using DTAB as surfactant are shown in Figure 6. Off-line data obtained for conversion, surface coverage, and the number of particles per liter are also presented. It can be seen in Figure 6A (experiment C2) that after a short inhibition time particles are nucleated and, both conductivity meter and ISE respond to the consumption of DTAB in the media during nucleation. After the nucleation period, one can observe that the temperature ceases to increase and the surfactant content available in the aqueous phase is almost zero. From this point, the conductivity signal tends to stabilize at low values, meaning that little DTAB is available in the aqueous media, as

indicated by the estimates of surface coverage. Similarly, ISE measurements tend toward low potential values (although measurements are noisy, trends are clearly seen), which is clearly correlated with the free surfactant consumption by growing polymer particles. The nucleation period is very rapid here, and the final surface coverage of the particles is on the order of 40% if we accept the approximations discussed earlier to calculate this value. At close to 80% conversion, conductivity shows a slight increase. According to Santos et al.,10 the increase generally observed in the conductivity signal after the nucleation stage can be associated with the depletion of monomer in the aqueous phase, the depletion of monomers inside the polymer particles with particle shrinking and a related release of surfactant, and finally the decomposition of the initiator. Unlike the conductivity signal, the potential signal shows a slight, but continuous, decrease after 20 min (the conductivity value actually increases slightly after being stable for a certain time). This is most likely due to the change in the temperature. Figure 6B exhibits results for the test performed with a DTAB concentration of 0.7 × CMC (C3). When reaction starts, both conductivity and ISE signals drop as a result of consumption of residual surfactant due to the nucleation of particles. Once again, at such low levels of surfactant, the nucleation period is extremely rapid, and we see very similar behavior for both C2 and C3. Finally, when conversion is almost 100%, sensor signals present slight changes in the direction of the consumption of surfactant, meaning that the particles are still being stabilized. To more clearly see the evolution of the surfactant concentration in the aqueous phase during the reaction, this was calculated from the surface coverage data, expressed as a fraction of the CMC (CMCDTAB ) 6.1 g/L, at 60 °C) and plotted as a function of time (N × CMC in Figures 6 and 7). According to these curves, the micelles disappear when the fraction of the CMC drops below 1.0. The results for runs C2 and C3 are shown in Figure 6. Again, once the reaction begins, the ISE signal responds clearly to the surfactant consumption as confirmed by the calculated surfactant concentration at aqueous phase. Note that the electrode signals follow a curve profile quite similar to the calculated surfactant concentration for both C2 and C3. When reaction is performed at DTAB concentration of 3 × CMC (C1), the electrode measurements are quite different from the cases analyzed before. Real-time sensor signals, off-line data obtained for conversion, surface coverage, and the number of particles for C1 are shown in Figure 7. It can be seen for C1 that after the addition of initiator (V50), a short inhibition time is followed by particle nucleation for a few minutes. At this point the conductivity signal drastically increases, ceasing when the nucleation period is finished. In this case, it seems that the conductivity curve responds to the monomer consumption. This behavior is expected when the monomer droplets are very small since small droplets will have a huge specific surface which absorbs a large fraction of surfactant molecules (note in Table 1 that the final latex yielded a dp of 50 nm for this run). Since the reaction occurs with a surfactant concentration well above the CMC, it is possible that enhanced micellization exists during monomer consumption, resulting in the increase of the conductivity signal. Note that in the middle of the conductivity curve, around the inflection point, micelles are still present. Considering the surfactant concentration in the aqueous phase as a fraction of the CMC (N × CMC), the DTAB-specific electrode response seems to be directly related to the consumption of free surfactant species, once the micelles have disappeared. As can be seen from Figure 7, there is a well-defined decrease of signal once the surfactant concentration hits 1 × CMC. This result indicates


Figure 6. Real-time sensor outputs and off-line measurements of latex properties for DTAB tests (where, for example, 5E+17 represents 5 × 1017): (A) C2 and (B) C3.

Figure 7. Real-time sensor outputs and off-line measurements of latex properties for DTAB test C1 (where, for example, 1.6E+18 represents 1.6 × 1018).

that the DTAB-specific electrode is sensitive to the free surfactant species and not to the micelles (cf. Figure 3). Note that just before the consumption of micelles, the potential signal tends to increase, which is likely due to the temperature increase. From this point, the electrode signal goes toward low free surfactant concentrations, but with a curve profile quite similar to both the calculated surfactant concentration and the temperature signal. Besides, as shown by the Nernst equation, eq 1, the temperature plays a role in the signal; therefore the signal should be corrected if we are to estimate the surfactant concentration from potential measurements. Finally, it is interesting to point out that the potential signal is much less noisy in this last run. In this case the final surface coverage of

the particles is close to 100%. Recall that partitioning of surfactant is not taken into account in our rough analysis presented here. It is nevertheless likely that this is not entirely true (and a process model based on the input of the potential signal should take this into account). The values of the potential signal show clearly that there is a higher surfactant concentration for run C1 than for the others, which is probably the reason for the higher signal-to-noise ratio in this last figure. The possibility of taking the conductivity signal and making its association with the latex proprieties in a quantitative manner has been investigated in previous papers from our groups.10 It was demonstrated that a semiempirical model can be built to describe the conductivity signal as a function of the latex composition and of the reactor temperature. The model was inverted and combined with the available conductivity signal, conversion, and temperature measurements, being able to accurately predict the number of polymer particles in the latex (polystyrene stabilized by SDS). Another interesting output provided by that model was the surfactant concentration in the many phases. However, it should be considered that these outputs were not compared to experimental data of partitioning; hence, they should be regarded as rough estimates of what is going on during the reaction in order to explain the conductivity results. Once those polymerization runs were also followed by SDS-specific electrodes, it is possible to relate the potential measurements with the model estimates for surfactant partitioning. Results for batches carried out with SDS at concentrations of 2 × CMC, 3 × CMC, 4 × CMC, and 5 × CMC are exhibited in Figure 8. A sharp evolution of the potential measurements is observed for both reactions R1 (4 × CMC) and R4 (5 × CMC). After the reaction begins, the ISE signal tends to increase, assuming less negative values during the course of both reactions, which indicates the consumption of the anionic surfactant due to nucleation/stabilization of growing particles. The trends for the increase of the ISE signal can be divided into two steps: a first one that is strong and stops at a short plateau, followed by a second increase that is less significant than the first one. The delay between the decrease observed with the conductivity signal and the first increase in the ISE

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Figure 8. Experimental off-line latex properties, on-line measurements, and model results for surfactant masses in the phases (ADS ) adsorbed on the particles; AQ ) aqueous phase; MIC ) as micelles) obtained by Santos et al.,10 including SDS-specific electrodes measurements: from A to D corresponding to 2-5 × CMC, respectively (CMC ) critical micelle concentration).

signal indicates that the conductivity meter is measuring the dynamics of surfactant aggregates due to the nucleation period, which could not be seen by the ISE measurements. During this micellar nucleation, the potential signal seems to respond rather to the temperature increase. When micelles are consumed, the ISE signal responds indicating the disappearance of free surfactant available in the aqueous phase. At this point, the temperature ceases to increase and drops quickly. After this, a

competitive mechanism of adsorption and desorption of surfactant over the growing particles plays a role, as suggested by the conductivity and ISE signals. In both cases R2 (2 × CMC) and R3 (3 × CMC), the noisy measurements observed after the nucleation stage are probably due to the lack of surfactant in a medium that likely contains monomer droplets. In the region of noisy ISE data, surface coverage values are too small, meaning that little SDS is available in the aqueous media.


Figure 8 also shows the electrical potential measurements provided by SDS-specific electrodes compared with the results obtained by the conductivity model developed by Santos et al.10 Model estimates of Np are also retrieved and compared with experimental values for each run. Note that the evolution of the potential signal seems to agree with the estimates of the surfactant masses in the phases. Basically, for experiments carried out with lower SDS content (R2 and R3), the model estimates indicate fast consumption of both micelles and free surfactant molecules, resulting in a lack of surfactant species in the media. The lack of surfactant species is also felt by the specific electrodes, and even though electrode responses indicate a continuous consumption of surfactant species during the reaction (instead of a quick consumption), the potential signal may be considered as a further effort into the comprehension of the surfactant partitioning. Another important point is that when there is a lack of the surfactant species in the aqueous phase, the specific electrode signal seems to be more sensitive to the presence of monomer droplets, which delay the signal. Otherwise, when higher surfactant concentrations are employed, the specific electrode measurements are cleaner, indicating the consumption of the free surfactant species available in the aqueous phase. Again, some delay between model estimates of surfactant masses and potential results does exist, but at a lower extent. In addition, as mentioned above, the monomer conversion should also play a role in the potential results. As a consequence, if we are to model the potential measured by these electrodes in the matter of the latex properties, one should also consider the influence of both monomer conversion and the temperature. To finish, reproducibility tests were performed for SDS runs and showed very good results for the conductivity sensor and reasonable results for the ISE sensors (as is the case in the data shown here, the ISE signal is noisier than the conductivity). The uncertainties in the measurements of the ISE sensors can be minimized if tight control of the sensing membrane characteristics is applied, which includes for instance the insertion of the specific electrode in diluted surfactant solution between tests. 4. Conclusions Several efforts were made to implement surfactant-specific electrodes in the monitoring of free surfactant molecules during anionic (SDS) and cationic (DTAB) stabilized emulsion polymerization. To the best of our knowledge, real-time monitoring of surfactant molecules during emulsion polymerization process by ion-selective electrodes has never been reported in the open literature. Some difficulties were found in adapting the technique to the conditions of a polymerization reactor, and successful solutions were proposed. Noisy measurements were observed when little surfactant was available in the reactor. Therefore, for surfactant dynamics monitoring purposes, these noisy measurements can be useful once they may serve as an alert about a forthcoming lost of stability of the latex. However, the potential measured by the electrodes can be more sensitive to low surfactant contents, thus being smoother, if one employs a higher impedance pH meter onto which electrodes may be connected. The ISE signal seems to agree with the conductivity results, with some delay most likely due to the existence of micelles. This highlights the idea that conductivity measurements and ISE signal may be suitable for the inference of surface coverage, which is a critical parameter and of great interest during industrial-scale production. Indeed, usual determinations of

surface coverage require time-consuming analysis of particle size distribution and conversion. However, it is shown here that a conductivity meter and the specific electrodes may be suitable to provide on-line information about surface coverage, without time-consuming analysis nor expensive devices. As a final remark, it may be said that conductivity results combined with the surfactant-specific electrodes signal allow the understanding of different phenomena during emulsion polymerization, providing insight about the surface coverage of latex by surfactant, in a qualitative way. Further studies and efforts will be made in order to predict surface coverage and also to quantify surfactant concentration in the media. Acknowledgment The authors thank FAP-SE and CNPq (Brazilian Agencies) for the financial support for the development of this research project. Literature Cited (1) Lamb, D.; Anstey, J. F.; Lee, D.-Y.; Fellows, C. M.; Monteiro, M. J.; Gilbert, R. G. Rational Design of Polymer Colloids. Macromol. Symp. 2001, 174, 13. (2) Zhao, C. L.; Roser, J.; Heckmann, W.; Zosel, A.; Wistuba, E. Structured Latex Particles with Improved Mechanical Properties. Prog. Org. Coat. 1999, 35, 265. (3) El-Aasser, M. S.; Sudol, E. D. Features of Emulsion Polymerization. In Emulsion Polymerization and Emulsion Polymers; Lovell, P. A., ElAasser, M. S., Eds.; John Wiley & Sons: Chichester, U.K., 1997; pp 3858. (4) Gilbert, R. G. Emulsion Polymerization: A Mechanistic Approach, 1st ed.; Academic Press: London, 1995. (5) Tauer, K.; Kuhn, I. Modeling Particle Formation in Emulsion Polymerization: An Approach by Means of the Classical Nucleation Theory. Macromolecules 1995, 28, 2236. (6) Vale, H.; McKenna, T. F. Modeling Particle Size Distribution in Emulsion Polymerization Reactors. Prog. Polym. Sci. 2005, 30, 1019. (7) Urretabizkaia, A.; Asua, J. M. High Solids Content Emulsion Terpolymerization of Vinyl Acetate, Methyl Methacrylate, and Butyl Acrylate. I. Kinetics. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1761. (8) Fontenot, K.; Schork, F. J. Batch Polymerization of Methyl Methacrylate in Mini/Macroemulsions. J. Appl. Polym. Sci. 1993, 49, 633. (9) Janssen, R. Q. F. Polymer Encapsulation of Titanium Dioxide: Efficiency, Stability and Compatibility. Ph.D. Dissertation, Eindhoven University of Technology, Eindhoven, The Netherlands, 1994. (10) Santos, A. F.; Lima, E. L.; Pinto, J. C.; Graillat, C.; McKenna, T. F. On-line Monitoring of the Evolution of Number of Particles in Emulsion Polymerization by Conductivity Measurements. Part I. Model Formulation. J. Appl. Polym. Sci. 2003, 90, 1213. (11) Santos, A. F.; Lima, E. L.; Pinto, J. C.; Graillat, C.; McKenna, T. F. On-line Monitoring of the Evolution of Number of Particles in Emulsion Polymerization by Conductivity Measurements. Part II. Model Validation. J. Appl. Polym. Sci. 2004, 91, 941. (12) Chien, D. C. H.; Penlidis, A. On-line Sensors for Polymerization Reactors. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1990, C30, 1. (13) Kammona, O.; Chatzi, E. G.; Kiparissides, C. Recent Developments in Hardware Sensors for the On-Line monitoring of Polymerization Reactors. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1999, C39, 57. (14) McKenna, T. F.; Othman, N. Suivi et Commande de Reácteurs de Polyme´risation en EÄ mulsion. In Les Latex Synthe´ tiques; Pichot, C., Daniel, J. C., Eds.; Editions TEC & DOC, Lavoisier: Paris, 2006; Chapter 27, pp 727-758. (15) Reimers, J. L.; Schork, F. J. Predominant Droplet Nucleation in Emulsion Polymerization. J. Appl. Polym. Sci. 1996, 60, 251. (16) Stubbs, J. M.; Durant, Y. G.; Sundberg, D. C. Competitive Adsorption of Sodium Dodecyl Sulfate on Two Polymer Surfaces within Latex Blends. Langmuir 1999, 15, 3250. (17) Bates, R. G. Electrode Potentials. In Treatise on Analytical Chemistry. Part I, Theory and Practice; Kolthoff, I. M., Elving, P. J., Eds.; John Wiley & Sons: New York, 1978; Vol. 1, pp 773-820.

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(18) Ross, J. W. Solid-State and Liquid Membrane Ion-Selective Electrodes. In Ion-SelectiVe Electrodes; Durst, R. A., Ed.; National Bureau of Standards (Special Publication): Gaithersburg, MD, 1969; Vol. 314, pp 1-56. (19) Sirieix-Plenet, J.; Turmine, M.; Letellier, P. Membrane Electrodes Sensitive to Doubly Charged Surfactants. Application to a Cationic Gemini Surfactant. Talanta 2003, 60, 1071. (20) Martin, J. V.; Turmine, M.; Letellier, P.; Hemery, P. Study of β-Cyclodextrin/Dodecyltrimethylammonium Bromide Complex into WaterIsopropanol Mixtures. Electrochim. Acta 1995, 40, 2749. (21) Mokus, M. Realisation d’Electrodes Membranaires Indicatrices d’Anions Amphiphiles: Application a la Determination de Grandeurs Thermodynamiques et aux Dosages d’Agents Tensioatcifs, Ph.D. Dissertation, Universite´ Pierre et Marie Curie, Paris, France, 1996. (22) Turmine, M.; Mayaffre, A.; Letellier, P. A Potentiometric Study of the Adsorption of a Cationic Surfactant onto Laponite in Water-Methanol and Water-Dimethylsulfoxide Mixtures. J. Colloid Interfaces Sci. 2003, 264, 7.

(23) Santos, A. F. Acompanhamento em Tempo Real de Propriedades de Sistemas de Polimerizaç aõ, Ph.D. Dissertation, Universidade Federal do Rio de Janeiro, Brazil, 2003. (24) Jean, B.; Lee, L.-T.; Cabane, B. Interactions of Sodium Dodecyl Sulfate with AcrylamidesN-Isopropylacrylamide Statistical Copolymer. Colloid Polym. Sci. 2000, 278, 764. (25) Pe´rez-Rodriguez, M.; Varela, L. M.; Garcıá, M.; Mosquera, V.; Sarmiento, F. Conductivity and Relative Permittivity of Sodium n-Dodecyl Sulfate and n-Dodecyl Trimethylammonium Bromide. J. Chem. Eng. Data 1999, 44, 944. (26) Graillat, C.; Santos, A. F.; Pinto, J. C.; McKenna, T. F. On-Line Monitoring of Emulsion Polymerization by Conductivity Measurements. Macromol. Symp. 2004, 206, 433.

ReceiVed for reView July 3, 2006 ReVised manuscript receiVed December 1, 2006 Accepted December 19, 2006 IE060854D

In-Line and In Situ Monitoring of Ionic Surfactant Dynamics in Latex

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