Article pubs.acs.org/IECR
Partitioning of 2‑Carboxyethyl Acrylate between Water and Vinyl Monomer Phases Applied to Emulsion Polymerization: Comparisons with Hydroxy Acrylate and Other Vinyl Acid Functional Monomers Amit K. Tripathi, Jenna Vossoughi, and Donald C. Sundberg* Nanostructured Polymers Research Center, Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824, United States S Supporting Information *
ABSTRACT: In an extension of our recent studies on the distribution of vinyl acid (AA and MAA) and hydroxy (meth)acrylate monomers in emulsion polymerization systems, we report the distribution behavior of 2-carboxyethyl acrylate (CEA) between water and various nonfunctional monomers. Similar to that previously reported for AA and MAA, the distribution of CEA strongly depends upon the hydrogen-bond-acceptor characteristics of the nonfunctional monomer organic phase. The logarithm of the distribution coefficients for CEA correlate linearly with the molar volume of (meth)acrylate monomers. The CEA molecule is a replica of 2-hydroxyethyl acrylate (HEA) with a different functional group. Evaluation of the distribution behaviors of CEA and HEA allows for a direct comparison of the carboxyl and hydroxyl moieties in determining the distributions. The strong dipole moment of the hydroxy group makes HEA significantly more polar than CEA. Also, comparisons of the distribution behaviors of CEA with those for AA and MAA allow us to understand the combined effect of the ester and carboxylic acid groups in CEA on the overall polarity of these vinyl acid monomers and their distribution behaviors. Recently, Fang et al.4 reported the distribution of two very different vinyl acid monomers [monocarboxylic acids of monobutyl itaconate and 2-carboxyethyl acrylate (CEA)] between the water and n-butyl methacrylate monomer phases. We became interested in CEA (as a higher-molecular-weight vinyl acid monomer (144 vs 72 and 86 g/mol for AA and MAA, respectively) and how it differed in its water/monomer distribution characteristics from the more traditional AA and MAA functional monomers. In addition to its higher molecular weight, it contains an ester group, thereby creating a vinyl acid monomer with two carbonyl groups. Beyond that, and more germane to this paper, CEA provides a direct molecular comparison of functional vinyl monomers with −COOH and −OH end groups, as can be seen in the comparison of CEA and HEA in Figure 1.
1. INTRODUCTION Functional monomers are generally used at low levels (1−10%) in emulsion polymerization, depending on specific applications. Vinyl acids (e.g., acrylic acid, AA) and hydroxy (meth)acrylates (e.g., 2-hydroxyethyl acrylate, HEA), as well as amides/amines, constitute the majority of these specialized monomers. They are generally much more hydrophilic than nonfunctional monomers (like methyl methacrylate, MMA) and provide features like enhanced colloidal stability, improved adhesion to metals, and residual functional groups for chemical modification. The overall rate of reaction in emulsion polymerization depends on the rate in the particle phase and also in the aqueous phase; they both depend on the local concentration of monomers. Thus, to understand the kinetics, it is important to know how the various monomers partition between all of the phases present in the system. We previously reported1−3 on the distribution of AA and methacrylic acid (MAA) monomers, as well as several hydroxy (meth)acrylate monomers, between water and single- and multiple-component styrene (Sty)/ (meth)acrylate monomer phases. These distribution coefficients strongly depend on the hydrogen-bond-acceptor characteristics of the organic phase and, for vinyl acids, the pH of the aqueous phase. The logarithms of the distribution coefficients for both AA and MAA correlated linearly with the molar volume of the (meth)acrylate monomers, and these values decreased as the molar volume of the monomer increased. Similar correlations were found for hydroxy (meth)acrylate monomers. Distributions to the Sty monomer are nearly completely determined by dimerization of the acid and hydroxyvinyl monomers in the Sty phase and, as such, are quite sensitive to the concentrations of these functional monomers in the water phase. © 2015 American Chemical Society
Figure 1. Comparison of the molecular structures of CEA and HEA. Received: Revised: Accepted: Published: 2447
December 23, 2014 February 17, 2015 February 19, 2015 February 19, 2015 DOI: 10.1021/ie504994d Ind. Eng. Chem. Res. 2015, 54, 2447−2452
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Article
3. EXPERIMENTAL DETAILS The monomers used in this study were styrene (Sty), methyl acrylate (MA), methyl methacrylate (MMA), and n-butyl methacrylate (nBMA) (all from Acros Organics) and 2ethylhexyl acrylate (2EHA) and 2-carboxyethyl acrylate (CEA) (both from Aldrich). HPLC-grade water (Pharmco), acetonitrile (JT Baker), and a 85% phosphoric acid solution in water (Acros Organic) were used as received. The experiments were done in 14 mL glass vials. Various amounts of CEA monomer along with water and nonfunctional monomer were added to the vials and capped. The amounts of all components were randomized while keeping in mind that in common emulsion polymerization recipes the monomer-towater ratio is less than 0.6. In general, we studied functional monomer levels of less than 20% of the total monomer. In a few instances, the functional monomer level exceeded this limit to obtain better insight into the distribution characteristics. The vials were shaken vigorously and kept in a water bath maintained at the desired temperature for at least 6 h. In all of the experiments, a number of precautionary steps were taken during extraction of ∼1 mL of the water phase (by a glass pipet or a syringe needle) to avoid contamination with the organic phase. For example, the glass pipettes were wiped thoroughly after extraction, and some of the sample fluid was wasted prior to transferring into sample vials. The extracted aqueous phase was analyzed using a highpressure liquid chromatograph (Agilent 1100) equipped with a UV detector (Hewlett-Packard series 1100, model G1314A). The column Agilent ZORBAX Eclipse XDB-C8 (150 × 4.6 mm, 5 μm particle size) was maintained at 30 °C, and the UV detector was set at a wavelength of 254 nm. The mobile phase was a 30:70 (v/v) mixture of acetonitrile and an aqueous solution of phosphoric acid [0.1% (v/v)] with a flow rate of 1 mL/min. This mobile phase was adapted from Kossen6 and has been used successfully for the distribution behavior of a number of other functional monomers.1−3 The high-pressure liquid chromatograph (HPLC) signal for CEA solutions (in water) alone results in multiple peaks with elution times ranging from 2 to 6 min, as shown in Figure 2a. The reason for such behavior is unclear at this time. We used the sum of all peak areas to quantify the amount of CEA in the aqueous phase during the experiments. For the cases where the nonfunctional monomers were Sty, nBMA, and 2EHA, the peak due to the nonfunctional monomer does not interfere with any CEA peaks (an example HPLC curve for the Sty case is shown in Figure 2c), and all peaks associated with CEA were used for quantification. However, for MA and MMA, some of the CEA peaks are interfered with by the nonfunctional monomer signal (an example HPLC for the MA case is shown in Figure 2b), and only the first few CEA peaks were used. The amount of CEA monomer in the organic phase was calculated by a mass balance. The densities used in this work for monomer and water at the experimental temperatures are shown in Appendix A in the Supporting Information (SI). Because of the small sample size, the pH of the aqueous phase was not measured directly. However, as we have reported previously,1,2 the natural pH of the aqueous phase can be determined by using eq 4 (also derived in Appendix B in the SI).
The goal of this phase of our work was to develop a fundamental understanding of the distribution of CEA between aqueous phases typically encountered in emulsion polymerization and a variety of organic phases comprised of singlecomponent Sty, acrylate, or methacrylate monomers. In particular, we sought to quantify the impact of replacing the −OH end group with a −COOH end group in an otherwise identical vinyl monomer.
2. THEORETICAL ASPECTS The equilibria involved in the distribution of carboxylic acids between water and an organic phase are shown in Scheme 1. Scheme 1. Distribution of Carboxylic Acid between Water and an Organic Phase
Considering the above equilibria between the aqueous and organic phases, the distribution coefficient for the carboxylic acid between water and an organic phase can be expressed as1,2,5 D=
Corg,tot Caqu,tot
=
Kd + 2Kd 2Kdim,o[HA W] 1 + K a /[H+]
(1)
where D is the partition coefficient for the acid [total measurable concentration of acid in the organic phase (Corg,tot)/total measurable concentration of acid in the aqueous phase (Caqu,tot)], Kd is the equilibrium distribution coefficient describing the transfer of carboxylic acid monomer from the aqueous phase to the organic phase, Kdim,o is the dimerization coefficient of the acid in the organic phase, Ka is the acid dissociation constant in the aqueous phase, and [H+] is the concentration of hydrogen ions in the aqueous phase. The value of [HAW] can be obtained from the total concentration of acid (dissociated + undissociated; Caqu,tot) in the aqueous phase using eq 2. Any dimerization of the acid in the aqueous phase is neglected. This is a valid approach considering the low amount of acid monomers used in emulsion polymerization ( nBMA > 2EHA. In other words, with an increase in the (meth)acrylate monomer polarity, the affinity of the polar CEA monomer will increase toward the nonfunctional organic phase, and this will result in an increase in the value of Kd. Table 1 also shows that, with increases in the nonfunctional monomer polarity (and the number of carbon atoms), the value for Kdim,o increases (except for the 2EHA case). In previous distribution studies for AA1 and MAA1, we have shown that, with increases in the nonfunctional monomer size, the value of K dim,o increases. This is due to the lowering of the concentration of the carbonyl units in the nonfunctional monomer as the number of carbon atoms in the monomer increases. Expecting similar trends for the CEA monomer, the
where C is the total concentration of the acid compound in the aqueous phase (mol/L). The pKa value for CEA is taken as 4.994 (at 25 °C). It should be noted that, without the addition of a buffer or base, the ionized portion of CEA is less than 2% when the acid level in the water is >0.05 mol/L.
4. RESULTS AND DISCUSSION We first treated the data obtained for use of the nBMA monomer as the organic phase. In Figure 3, we have plotted our
Figure 3. Concentration of the CEA monomer in the nBMA phase (Corg,tot) versus its undissociated form in the aqueous phase ([HAW]) at equilibrium: (●) this work, 30 °C; (□) Fang et al.,4 25 °C. The dashed line is the fit from eq 3. 2449
DOI: 10.1021/ie504994d Ind. Eng. Chem. Res. 2015, 54, 2447−2452
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tional monomer phase. The Sty monomer does not contain any such hydrogen-bond-acceptor groups. As a result, the values obtained for Kd for CEA distributions for Sty are dramatically lower than those for the (meth)acrylate monomers reported in Table 1. Also Kdim,o (or the dimerization tendency) for CEA in Sty is, as expected, dramatically higher than that for the (meth)acrylate monomers. Comparisons between the Vinyl Acid Monomers. Tripathi and Sundberg1 demonstrated a linear relationship between log(Kd) and the molar volume of the nonfunctional (meth)acrylate monomers for the distribution of MAA and AA (independently) vinyl acid monomers. A similar attempt is shown in Figure 6 for the CEA distribution between water and
Figure 4. Concentration of the CEA monomer in the nonfunctional monomer phase [(●) MA, (◆) MMA, and (○) 2EHA] (Corg,tot) versus its undissociated form in the aqueous phase ([HAW]) at equilibrium at 30 °C. The dashed line is the fit from eq 3.
smaller K dim,o value for 2EHA is likely attributed to experimental errors. Variation in the dimerization coefficient, Kdim,o, with the monomer polarity is a reflection of the hydrogen-bond-acceptor characteristics of the >CO group present in the (meth)acrylates. These groups can form hydrogen bonds with CEA, which will decrease the tendency of CEA dimerization. With an increase in the carbon number of the (meth)acrylate monomer, the concentration of the >CO groups in the organic phase will decrease, which will lead to more CEA dimerization with attendant higher values for Kdim,o. Similar to (meth)acrylate monomers, eq 3 effectively represents the distribution of CEA between the aqueous and Sty phases, as shown in Figure 5. Comparing these results with
Figure 6. log(Kd) versus nonfunctional monomer molar volume for distributions of (●) CEA, (○) MAA1, and (◆) AA1 between water and (meth)acrylate monomers at 30 °C.
different (meth)acrylate monomers. As can be seen, a linear relationship is obtained. The (meth)acrylate monomers studied here range from MA to 2EHA (two very different monomers in terms of polarity), and thus this linear correlation can be used to interpolate values of the distribution to other (meth)acrylate monomers with significant confidence. Monomers that are of special interest in emulsion polymerization reactions will include n-butyl acrylate and ethyl acrylate, among others. In the same figure, we also compare the plot obtained for CEA with those for AA1 and MAA1. The parallel feature of these three log(Kd) versus molar volume curves indicates the general nature of the effect of dilution of the carbonyl concentration in the nonfunctional monomers on the manner in which this class of vinyl acid monomers distributes between water and the organic phase. From Figure 6, we also find that CEA distributions are very much closer to those for AA than to those for MAA. It is obvious that the extra methyl group in MAA makes it significantly more nonpolar than AA, despite the fact that each is completely water-soluble. When the molecular structures of CEA and AA are compared (Figure 7), it is clear that CEA has two additional nonpolar methylene groups along with a polar ester [−C(O)O−] group. The similar Kd values for AA and CEA (Figure 6) suggest that the polarity effect of the two methylene groups is nearly balanced by the polar ester [−C(O)O−] group in CEA. So, despite the higher molecular weight of CEA, it appears to have a polarity similar to that of AA.
Figure 5. Concentration of the CEA monomer in the Sty phase (Corg,tot) versus its undissociated form in the aqueous phase ([HAW]) at equilibrium. The dashed line is the fit from eq 3.
those for CEA distributions using (meth)acrylate monomers (Figures 3 and 4), we see that CEA distributes much more strongly to polar (meth)acrylate monomers compared to Sty monomers. As noted above, the (meth)acrylate monomers contain carbonyl groups (>CO), which can form hydrogen bonds with the carboxylic acid groups. This can disrupt the dimerization tendency of the CEA monomer in the nonfunc2450
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that the distribution behavior of CEA will be quite similar to that of AA, as corroborated by our experiments. Comparison of the Vinyl Acid and Hydroxy Acrylate Monomers. As seen in Figure 1, CEA and HEA are different only in that the end groups on the ester are −COOH or −OH. This made it interesting to us to directly compare the distribution behaviors of the CEA and HEA monomers. The distribution of HEA between water and various nonfunctional monomers has been studied in detail by Tripathi et al.3 In Figure 8, we compare the experimental log(Kd) values for CEA
Figure 7. Comparison of the molecular structures of CEA, AA, and MAA.
A more quantifiable comparison of the polarity characteristics of all three vinyl acid monomers can be done by using their octanol/water distribution coefficients (KOW). The predicted log(KOW) values, more commonly indicated as log P, for these functional monomers were obtained by using KOWWIN, version 1.68 (this software is available in the EPI Suite, version 4.117). KOWWIN utilizes the group-contribution approach suggested by Meylan and Howard8 to predict the log(KOW) value for a compound. The group-contribution values used in KOWWIN for a few key chemical groups are reported in Table 2, and the resultant estimated log(KOW) values for the vinyl Table 2. Group Contribution toward the log(KOW) (also log P) Values Estimated Using KOWWIN7 group
coefficient
>CH2 (aliphatic carbon) −CH3 (aliphatic carbon) CH2 (olefinic carbon) CH− or C< (olefinic carbon) −C(O)O− (ester, aliphatic attachment) −COOH (acid, aliphatic attachment) −OH (hydroxy, aliphatic attachment) >CO (carbonyl, aliphatic attachment)
0.4911 0.5473 0.5184 0.3836 −0.9505 −0.6895 −1.4086 −1.5586
Figure 8. log(Kd) versus nonfunctional monomer molar volume for distributions of (●) CEA and (○) HEA3 between water and (meth)acrylate monomers at 30 °C.
with those reported for HEA3 for different nonfunctional monomer systems. From this figure, we find that the Kd value for CEA is significantly higher than that for HEA for a wide range of nonfunctional (meth)acrylic monomers, suggesting that CEA is quite a bit less polar than HEA. In the carboxylic acid group, two highly electronegative oxygen atoms are covalently bonded to the same carbon atom. The dipole moment of each of the C−O bonds will be directionally oriented toward the oxygen group. However, the overall dipole moment for the −COOH group will be the combined effect of the individual dipole moments of the C−O bonds and will be somewhat self-compensating because of vector addition. Compared to the dipole moment characteristic of the −OH group on HEA, that from the −COOH in CEA is judged to be lower in magnitude, thus suggesting that CEA is less polar than HEA. This is consistent with the experimental data in Figure 8. This comparative behavior between compounds containing −COOH and −OH groups (given that rest of the molecular structure remains the same) can also be seen from the groupcontribution values for log(KOW) for −COOH and −OH (Table 2). The group-contribution value (higher numbers indicating more nonpolar contributions) for −COOH is, in fact, higher than that for the −OH group. The estimated log(KOW) for CEA and HEA (Table 3) follow a trend similar to that observed here for their experimental distributions. Another interesting set of comparisons from Table 2 can be made for a simple carbonyl group (>CO), an −OH group, and a −COOH group. Here we find that the group contributions of >CO and −OH alone are similar in value but are significantly lower than that for −COOH. Thus, it is the
Table 3. Logarithm of the Octanol/Water Distribution Coefficient (KOW) predicted using KOWWIN.7 functional monomer
log (KOW) (also log P)
functional monomer
log (KOW) (also log P)
CEA AA
0.47 0.44
MAA HEA
0.99 −0.25
acids are reported in Table 3. It should be pointed out that the group-contribution approach8 used in KOWWIN7 is not a simple addition of the various coefficients associated with the groups forming the molecule but also includes different correction factors to improve the log(KOW) prediction. From Table 3, we find that the predicted values for KOW are in the order AA < CEA < MAA and that the values for AA and CEA are quite close. The water/organic phase distribution expectations from these predicted KOW values are in excellent agreement with the experimental data shown in Figure 6. Further insight regarding the similarities of the distribution behaviors of AA and CEA can be obtained from Table 2. Here it is seen that the arithmetic combination of the group contributions of the two nonpolar >CH2 groups and a single polar ester [−C(O)O−] group is nearly zero. This suggests 2451
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vector addition of the dipole moments in the −COOH group that causes the molecule to have less overall polarity than an isomer with separated >CO and −OH groups.
(4) Fang, C.; Wang, Y.; Lin, Z.; Daniels, E. S.; Klein, A. Partitioning of monobutyl itaconate and β-carboxyethyl acrylate between organic and water phases. J. Appl. Polym. Sci. 2014, 131, 40868. (5) Al-Modhaf, H. F.; Hegazi, M. F.; Amer, S. A.; Abu-Shady, A. I. Partition data of propionic and butyric acids between aqueous NaCl solutions and cyclohexane. Sep. Purif. Technol. 2001, 24, 329−335. (6) Kossen, S. P. HPLC Determination of Free Amounts of (Meth)acrylic Monomers in Solvent and Water-borne Poly(meth)acrylates. LC·GC Eur. 2001, 14, 679−686. (7) Estimation Programs Interface Suite (EPI Suite), version 4.11; United States Environmental Protection Agency: Washington, DC, accessed Dec 16, 2014. (8) Meylan, W. M.; Howard, P. H. Atom/Fragment Contribution Method for Estimating Octanol−Water Partition Coefficients. J. Pharm. Sci. 1995, 84, 83−92.
5. CONCLUSIONS The distribution of CEA between water and a wide variety of Sty/acrylate monomers can be well described by the same equilibrium expressions as those used for AA and MAA. Separately determined values for the monomeric distribution coefficient, Kd, and the dimerization coefficient, Kdim,o, allow one to predict the overall partition coefficient, D, for a wide variety of conditions. Despite the large molecular weight differences between CEA and AA, these vinyl acids distribute very similarly and in many different ways than MAA does. Comparing the octanol/water distribution coefficients (log P, either calculated or measured) and/or the assessment of the overall dipole moments for these three acid-based monomers provides a very useful gauge to predict the relative distribution coefficients. Using hydroxy acrylate monomers to provide contrast to vinyl acid monomers has shown that there are significant differences in the expected water/organic phase distribution coefficients. When added to otherwise similar vinyl monomer molecular structures, the carboxylic acid group contributes significantly less overall polarity to the monomer than does the hydroxy group. This is seen by the estimation of log P values by group-contribution methods and by experiment. This translates into CEA distributing much more strongly to the nonfunctional Sty/(meth)acrylate monomer phase than its analogue HEA does.
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ASSOCIATED CONTENT
* Supporting Information S
Appendices A and B and coefficients A, B, and C for density by eq S1. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for critical discussions with Professors John Tsavalas and Glen Miller. The authors are also grateful for financial support provided by the Latex Particle Morphology Control Industrial Consortium at the University of New Hampshire.
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REFERENCES
(1) Tripathi, A. K.; Sundberg, D. C. Partitioning of Functional Monomers in Emulsion Polymerization: Distribution of Carboxylic Acid Monomers between Water and Monomer Phases. Ind. Eng. Chem. Res. 2013, 52, 3306−3314. (2) Tripathi, A. K.; Sundberg, D. C. Partitioning of Functional Monomers in Emulsion Polymerization: Distribution of Carboxylic Acid Monomers between Water and Multimonomer Systems. Ind. Eng. Chem. Res. 2013, 52, 9763−9769. (3) Tripathi, A. K.; Sundberg, D. C. Partitioning of Functional Monomers in Emulsion Polymerization: Distribution of Hydroxy (Meth)acrylate Monomers between Water and Single and Multimonomer Systems. Ind. Eng. Chem. Res. 2013, 52, 17047−17056. 2452
DOI: 10.1021/ie504994d Ind. Eng. Chem. Res. 2015, 54, 2447−2452
