Hydrophobicity of polymer based anion-exchange columns for ion chromatography

https://doi.org/10.1016/j.heliyon.2021.e07290

”

2.1. Ion-exchange selectivity

The pure ion-exchange process of two singly charged anions A and B taking place on solid anion-exchanger phase R can be described as:(1) $X_{R}^{+} A^{-} + B^{-} = X_{R}^{+} B^{-} + A^{-}$

At equilibrium (1) with thermodynamic activity coefficients being omitted the following concentration constant or selectivity coefficient can be expressed for the “pure” ion-exchange process as follows:(2) $K_{B / A}^{c} = \frac{{[B^{-}]}_{R} {[A^{-}]}_{e}}{{[A^{-}]}_{R} {[B^{-}]}_{e}}$

The retention factor k for anion B is defined (3) by distribution coefficient ( $K_{D}^{B, i e}$ ) and phase ratio (ω = V_R/V₀, where V₀ is the dead volume of the column and V_R is the volume of the resin in the column) which is constant for a selected chromatographic column and separation conditions such as pressure, temperature, pH and ionic strength of the eluent and some other factors [52]. While it is accepted that a large fraction of the resin particle in IC material is non-functional, thus all comprehensive IC columns will possess a surface-modified or surface-layered structure where all charged groups responsible for ion-exchange are located in a relatively narrow zone on the outer surface. Taking into account the variation in structure, the concentration of surface groups, the fact that every single ion-exchange group has its own solvent and hydrated ion environment with an unknown number of coordinated solvent molecules, counterions, etc., the calculation of the true ion-exchange phase volume becomes practically impossible. To date no single method for calculation of true volume of the stationary phase has been reported, so a possible solution is to accept the volume of the stationary phase as a volume of all insoluble pasts in the chromatographic column. Thus, the retention of ions is proportional to the amount of charged groups in the chromatographic column and these groups belong to the insoluble stationary phase.(3) $k_{i e} = K_{D}^{B, i e} ω = \frac{{[B^{-}]}_{R} V_{R}}{{[B^{-}]}_{e} V_{0}} = K_{B / A}^{c} \frac{{[A^{-}]}_{R} V_{R}}{{[A^{-}]}_{e} V_{0}}$

If anion B⁻ is present in a significantly lower concentration than the eluent concentration [A⁻]_e (mmole/L) and ion-exchange resin is saturated with A⁻ ion, then [A⁻]_R is naturally equal to the ion-exchange resin capacity Q (meq/g), which is constant under selected separation conditions. In this case, combining Eqs. (2) and (3) gives the following equation in logarithmic form (4):(4) $log k_{i e} = log K_{B / A}^{c} + log ω + log Q - log {[A^{-}]}_{e} = const (K_{B / A}^{c}, ω, Q) - log {[A^{-}]}_{e}$

For the molecules of organic acids selected as sorbates in this study the negatively charged carboxylic and sulfonic acid groups are responsible for electrostatic interactions with the functional groups of anion-exchange resins. It should be noted that the effective charge or acidity of carboxylic and sulphonic acid groups in the molecules of first homologues (n_C = 0–1) is higher than for the rest of homologues with n_C ≥ 2 due to + I inductive effect of alkyl groups linked to the acidic group. This effect is well illustrated by pK_a changes for alkanoic acids with the increase of alkyl chain length as shown in Table 1.

Table 1. Properties (acid dissociation constants and partition coefficients) of alkanoic and alkanesulfonic acids used as analytes.

Alkanoic acids	pK_a^a	logP_exp^b	logP_cal^b	n_C	Alkanesulfonic acids	pK_a^a	logP_cal^b
Formic	3.75	-0.54	-0.46	0
Acetic	4.76	-0.17	0.09	1	Methanesulfonic		-2.38
Propionic	4.86	0.33	0.58	2	Ethanesulfonic		-1.89
Butyric	4.83	0.79	1.07	3	Propanesulfonic	1.53	-1.40
Valeric	4.84	1.39	1.56	4	Butanesulfonic		-0.91
Caproic	4.85	1.92	2.05	5	Pentanesulfonic		-0.42
Heptanoic acid	4.89	2.42	2.54	6	Hexanesulfonic		0.07
Octanoic acid	4.89	3.05	3.03	7	Heptanesulfonic		0.56
				8	Octanesulfonic		1.06

a: Data adapted from NIST database.
b: both experimental (logP_exp) and calculated (logP_cal) values are obtained from EPA KOWWIN database and software.

It should be noted that some of the common anion-exchangers can be considered as zwitterionic ion-exchangers [53]. For example, a popular class of, so called, agglomerated anion-exchangers possesses a core-shell structure with sulfonated PS-DVB microspherical particles representing a core coated with an electrostatically retained layer of positively charged latex nanoparticles. The diameter of latex particles is varied from 60 nm (OmniPac PAX-500) to 530 nm (IonPac AS7) as presented in Table 2, so the space between these particles is easily accessible for ions that can influence anion-exchange selectivity. For example, IonPac AS7 column has the cation-exchange capacity of 43 μequiv/col, which is about a half of its anion-exchange capacity of 100 μequiv/col and such a zwitterionic nature of this column was used for simultaneous separation of anions and cations [37, 54].

Table 2. Properties of anion-exchange columns and pre-column as specified by producers [15, 53, 65, 69, 70, 71].

Columns	Structure of ion-exchange group	Column properties
		Size, mm	D_p, μm	Matrix and morphology	Capacity
		Size, mm	D_p, μm	Matrix and morphology	μeq/col	μeq/g
PRP-X100	–CH₂N⁺(CH₃)₃	150 × 4.1	5	PS-DVB, 415 m²/g, d_pore 10 nm, V_pore 0.79 cm³/g	75	190
ICSep AN1^c	–CH₂N⁺(CH₃)₂CH₂OH	250 × 4.6	9	PS-DVB, 415 m²/g, d_pore 8 nm	35	50
Metrosep A Supp 8	–N⁺R₃	150 × 4.0	5	PS-DVB	700^d
Metrosep A Supp 5	–N⁺R₃	100 × 4.0	5	Polyvinylalcohol	34^d	94–107
TSK-Gel IC-Anion-PW	–N⁺(C₂H₅)₂CH₃	150 × 3.0	10	PMMA	>32^a	30 ± 3 μeq/ml
Separon HEMA-S 1000 Q-L^b	–CH(OH)CH₂N⁺(CH₃)₃	80 × 8.0	10	HEMA-EDMA, 200 m²/g, d_pore 100 nm	32	100–150
IonPac AS4A	–CH₂NR₂R’OH	250 × 4.0	15	sulphonated PS – DVB (4%, nonporous) with layer of 180 nm PVBC-DVB (0.5%) beads	13–20/~10∗	10–50
IonPac AS4A SC	–CH₂NR₂R’OH	250 × 4.0	13	sulphonated PEVB – DVB (55%, microporous) with layer of 160 nm PVBC-DVB (0.5%) beads	20–24/~10∗
IonPac AS5	–CH₂NR₂R’OH	250 × 4.0	15	sulphonated PS – DVB (2%, nonporous) with layer of 120 nm PVBC-DVB (1.0%) beads	20/~10∗
IonPac AS5A-5u	–CH₂NR(R’OH)₂	150 × 4.0	5	sulphonated PS-DVB (2%, nonporus) with layer of 60 nm PVBC-DVB (4.0%) beads	35/~10∗
IonPac AG7 (same as AS7)	–CH₂N⁺R₃	50 × 4.0	10	sulphonated PS-DVB (2%, nonporous) with layer of 530 nm PVBC-DVB (5%) beads	20	50/8.6∗
IonPac AS9	–CH(OH)CH₂N⁺R₃	250 × 4.0	15	sulfonated PS-DVB (2%, nonporous) with layer of 140 nm PGDMA(20%) beads	20–26
IonPac AS9-SC	–CH(OH)CH₂N⁺R₃	250 × 4.0	13	sulphonated PEVB-DVB (55%, microporous) with layer of 110 nm aminated PGDMA (20%) beads	30–35/residual∗
IonPac AS9-HC	–CH(OH)CH₂N⁺R₃	250 × 4.0	9	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) with layer of 90 nm aminated PGDMA (18%) beads	190
IonPac AG10 (same as AS10)	–N⁺R₂R’OH	50 × 4.0	8.5	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) with layer of 65 nm microporous PVBA-DVB beads with 5% cross-linking	34/~10∗
IonPac AS11	–CH₂N⁺CH₃(CH₂OH)₂	250 × 4.0	13	sulphonated PEVB-DVB (55%, microporous) with layer of 85 nm aminated PVBA-DVB (6%) beads	45/~10∗
IonPac AS11 HC	–CH(OH)CH₂N⁺CH₃(CH₂CH₂OH)₂	250 × 4.0	9	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) with layer of 70 nm aminated PVBGE-DVB (6%) beads	290
IonPac AS12A	–CH₂N⁺(C₂H₅)₃	200 × 4.0	9	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) with layer of 140 nm microporous PVBC-DVB (0.15%) beads	52
IonPac AS14	–N⁺R₃	250 × 4.0	9	PEVB-DVB (55%, d_pore 10 nm, 450 m²/g) grafted layer	65
IonPac AS14A	–N⁺R₃	250 × 4.0	7	PEVB-DVB (55%, d_pore 10 nm, 450 m²/g), grafted layer	120
IonPac AS15–5 um	–CH₂N⁺(CH₃) (CH₂CH₂OH)₂	150 × 3.0	5	PEVB-DVB (55%, d_pore 10 nm, 450 m²/g) modified with PVBC and methyldiethanolamine	70
IonPac AS16	CH(OH)CH₂N⁺(CH₃) (CH₂CH₂OH)₂	250 × 4.0	9	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) with layer of 80 nm PVBGE-DVB (1%) beads functionalized with methyldiethanolamine	170
IonPac AS17	–N⁺R₂R’OH	250 × 4.0	10.5	sulphonated PEVB-DVB (55%, microporous), layer of 75 nm PVBA-DVB (6%) beads	30
IonPac AS-18 Fast	–N⁺R₂R’OH	150 × 2.0	7.5	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) with layer of 65 nm PVBA-DVB (8%) beads	45
IonPac AS19	N⁺(CH₃) (CH(CH₂OH)O-)₃	250 × 4.0	7.5	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g), layer of hyperbranched BDDE-methylamine polymer	240	160 μeq/ml
IonPac AS20	–N⁺R₂R’OH	250 × 4.0	7.5	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g), layer of hyperbranched BDDE-amine polymer	310
IonPac AS21	–N⁺R₂R’OH	250 × 2.0	7	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g), layer of hyperbranched BDDE-methylamine polymer	45
IonPac AS22	–CH(OH)CH₂N⁺R₂R’OH	250 × 4.0	6.5	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g), layer of hyperbranched BDDE-amine polymer	210
IonPac AG22	–N⁺R₂R’OH	50 × 4.0	11	sulphonated PEVB-DVB (55%, nonporous), layer of hyperbranched BDDE-amine polymer	6
IonPac AS23	–CH(OH)CH₂N⁺R₂R’OH	250 × 4.0	6	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) layer of hyperbranched BDDE-amine polymer	320
IonPac AS25	–CH(OH)CH₂N⁺R₂R’OH	250 × 4.0	7.5	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) layer of hyperbranched polymer	350
IonPac AG25	–N⁺R₂R’OH	50 × 4.0	13	sulphonated PEVB-DVB (55%, d_pore <0.1 nm) layer of hyperbranched polymer	3.5
IonPac AS26	–CH(OH)CH₂N⁺R₂R’OH	250 × 4.0	7.5	sulphonated PEVB-DVB (55%, d_pore 200 nm, 20 m²/g) layer of hyperbranched polymer	250
OmniPac PAX 100	–CH₂N⁺R₂R’OH	250 × 4.0	8.5	sulphonated PEVB-DVB (55%, microporous, < 1 m²/g) with layer of 60 nm PVBC-DVB (4%) beads functionalized with tertiary amine	40	25
OmniPac PAX500	–CH₂N⁺R₃	250 × 4.0	8.5	sulphonated PEVB-DVB (55%, d_pore 6 nm, 300 m²/g) with layer of 60 nm PVBC-DVB (4%) beads functionalized with tertiary amine	40/<1∗	16
ProSwift SAX 1S	–N⁺R₃	50 × 4.6	n/a	PS-DVB based monolith, transport pore size 1600–2700 nm	no data

BDDE – 1,4-Butanediol diglycidyl ether.

a: Calculated by using ion-exchange capacity (30 ± 3 μeq/mL) and empty column volume.
b: 150 × 3.0 mm ID column is also known as Metrosep Dual 1 column.
c: This is analogue of IonPac AS9-HC according to the producer.
d: Measured for Cl⁻.
∗: Residual cation-exchange capacity from sulphonated core of the particle.

Electrostatic repulsion of anions from alkanoldialkylammonium functional groups (Type II anion-exchangers) is also possible, when a zwitterionic structure is formed due to dissociation of hydroxyls in the concentrated alkaline eluent. The effect of negatively charged groups’ presence in anion-exchange on selectivity is quite strong, especially for bulky anions [55]. This, however, is applicable to only rather short linkers (e.g., one methylene group) and an internal salt cannot be formed.

2.2. Hydrophobic selectivity

Certainly, an n-alkyl chain of the sorbate molecule is also contributing to its retention due to hydrophobic interactions with the ion-exchanger. Hydrophobic properties of anion-exchange resins depend on multiple factors including the polarity of a polymer matrix, the configuration of ion-exchange layer (classic neutral carrier with bonded functional groups, a grafted polymer layer, etc.) or shell (agglomerated ion-exchangers), porosity, and related surface area, the surface density of ion-exchange groups, structures of functional groups and linkers and others. Therefore, the hydrophobicity of anion-exchangers only can be evaluated by using empirical methods. One of the methods that is widely used in reversed phase HPLC is the calculation of methylene selectivity or methylene increment α(CH₂) through the measurement of chromatographic retention for a series of homologues with a linear alkyl chain –(CH₂)_nH with n_C methylene groups [56]. The retention of homologues can be expressed in the form of Eq. (5):(5)log k = α(CH₂)n_C + constwhere α(CH₂) is the retention increment for one methylene group, and the constant reflects the ion-exchange contribution of electrostatic interactions between charged functional group of the sorbate and the ion-exchanger surface. The slope of the dependence (eqn. 5) α(CH₂) can be regarded as a measure of the hydrophobic selectivity or affinity of anion-exchange resins towards alkyl chains in homologues. The linearity of the logk vs. n_C plot has been validated for various polymer based adsorbents [50, 57] including PS-DVB based anion-exchange resin Dowex 1 × 8. The α(CH₂) values measured for adsorption of alkanols from water on Dowex 1 × 8 in sulphate and chloride forms were 0.470 [28] and 0.375 [29], respectively. The α(CH₂) of 0.167 for alkanols adsorbed from water on Dowex 1 × 2 in chloride form was reported by Small et al. [11]. As discussed earlier, data points for ionogenic homologues with n_C ≤ 3 are normally outside the linear range of logk – n_C plots for ion-exchange resins due to difference in the charge of the terminal functional group following changes in pK_a values (Table 1).

According to Eq. (5), α(CH₂) value reflects the separation selectivity (α_n+1/n) of two homologues with n and n+1 carbon atoms in alkyl chain according to the following expression:(6)log α_n+1/n = log (k_n+1/k_n) = log k_n+1 – log k_n = α(CH₂)

This Eq. (6) is correct only for neutral or ionogenic homologues with equal contribution of electrostatic interactions into retention. This requirement is met for alkanoic acid homologues having practically equal pK_a values of carboxylic groups at n_C > 3 (see Table 1).

2.3. Mixed-mode retention mechanism

In a real IC separation of anions, it is practically impossible to observe a pure single mode retention mechanism, especially for complex anions like alkanoates and alkanesulfonates. Apart from the most dominant electrostatic and hydrophobic interactions, a certain contribution in the retention can be provided by hydrogen bonding between anions and hydroxyls- in functional groups, linkers, and residual polar groups at the surface of the ion-exchanger matrix. Various steric hindrance effects related to the different structures of complex multi- element anions can influence retention and separation selectivity. Thus, the carboxylate group with the trigonal planar structure and the sulfonate group with trigonal pyramidal structure are expected to interact differently with an ionic polymer-grafted type of ion-exchangers or microporous anion-exchangers, where pore structure can influence the accessibility of functional groups.

Since its formal introduction by Cox and Stout in 1987 [58] the combination of reversed phase and ion-exchange interaction became the most popular variant of mixed-mode HPLC. The authors suggested that the resulting retention factor is determined not only by “the sum of individual distribution coefficients but also by the addition of the products of interacting distribution coefficients and phase ratios” [58]. So, in the case of mixed ion-exchange and hydrophobic interactions we have the following dependence:(7) $k_{t o t a l} = K_{D}^{B, i e} φ_{1} + K_{D}^{B, h y d} φ_{2} + K_{D}^{B, i e} K_{D}^{B, h y d} φ_{1} φ_{3}$

If $K_{D}^{B, i e}$ is related to the retention of the first homologue in a series, which is a hydrophilic inorganic anion (formate for alkanoic acids or bisulfate for alkanesulfonic acids), and $K_{D}^{B, h y d}$ is related to the retention of neutral hydrophobic alkane, the phase ratios for these two separate interactions can be accepted as equal ( $φ_{1} = φ_{2}$ ) for the same stationary phase. In the case of cooperative interaction the phase ratio $φ_{3}$ in (7) is different as alkyl radical from organic acid anion coordinated around the functional group has a limited possibility for interaction with hydrophobic sites. Then the following Eq. (8) is true:(8) $k_{t o t a l} = k_{i e} + k_{h y d} + p k_{i e} k_{h y d}$ where p is a probability of cooperative ionic and hydrophobic interactions for a selected sorbate. If ion-exchange is a primary and dominant interaction for alkanoic and alkanesulfonic acids, the cooperative interaction coefficient p in (8) reflects the accessibility of hydrophobic sites around anion-exchange groups for interaction with alkyl radicals, as shown in Figure 1. In this case, factor p is proportional to the surface area, free of charged groups, or reciprocal to the surface density of charged groups. It also depends on the hydrophobicity of the polymeric core and the linker connecting the surface and the functional group, the length and flexibility of the linker, as well as the alkyl chain length in sorbates.

The ion-exchange capacity of commercially available IC columns varies in a broad range from 13 (IonPac AS4) to 700 μeq/col (Metrohm A Supp 8) and so can their hydrophobicity as shown by calculated methylene selectivities which range from 0.050 (IonPac AS21) to 0.646 (ICSep AN1). As such, for practical use, it is important to know the ratio of electrostatic and hydrophobic interactions to understand the separation selectivity of organic anions.

Relative hydrophobicity of ion-exchangers and its contribution to the retention of organic ions under defined separation conditions (constant temperature, eluent composition, etc.) can be expressed by using the ratio of methylene selectivity α(CH₂) and distribution coefficient ( $K_{D}^{B, i e}$ , see eqn. 3) for a small spherical and singly charged anion such as chloride, which is retained exclusively due to electrostatic interactions. If phase ratio ω is assumed as an insignificant parameter for the comparison of the ion-exchange columns’ selectivity, the retention factor k_Cl can be used instead of $K_{D}^{C l, i e}$ and the following factor F can be introduced:(9) $F = \frac{i o n e x c h a n g e s e l e c t i v i t y}{m e t h y l e n e s e l e c t i v i t y} = \frac{K_{D}^{C l, i e}}{α ({CH}_{2})} = \frac{k_{C l}}{α ({CH}_{2})}$

An alternative approach was suggested by Carr et al. [59] who used the slope of a plot k_ie vs. 1/ ${[A^{-}]}_{e}$ (eqn. 3) as a measure of the ion-exchange interaction strength for the characterisation of mixed-mode stationary phases instead of using k_Cl in Eq. (9). However, this slope is proportional to the ion-exchange equilibrium constant $K_{B / A}^{c}$ .only under the condition of constant ion-exchange capacity Q at a varied concentration of the eluent ${[A^{-}]}_{e}$ used for plotting k_ie vs. 1/ ${[A^{-}]}_{e}$ . Notably, this condition is not valid for Type II resins in sodium hydroxide eluent of varied concentration, as hydroxyl group in alkanolalkylammonium ion-exchange groups can dissociate with the formation of zwitterionic sites that changes the effective ion-exchange capacity of the resins. It should be noted that the possibility of dissociation of hydroxyl groups in choline type substances mimicking the structure of ion-exchange groups in Type II resin is well known from the literature. For example, pK_a values of the hydroxyl group in formocholine (CH₃)₃N⁺CH₂OH and in choline (CH₃)₃N⁺(CH₂)OH are equal to 10.1 ± 0.02 and 12.10 ± 0.02, respectively [60]. The pK_a value reported for choline hydroxide is 11.2, which is significantly less than that of chloride form [61]. Undoubtedly, the possibility of formation bipolar ion-exchange cites is high for Type II anion-exchange resins.

“

Cookie	Duration	Description
cookielawinfo-checkbox-analytics	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Analytics".
cookielawinfo-checkbox-functional	11 months	The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional".
cookielawinfo-checkbox-necessary	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookies is used to store the user consent for the cookies in the category "Necessary".
cookielawinfo-checkbox-others	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Other.
cookielawinfo-checkbox-performance	11 months	This cookie is set by GDPR Cookie Consent plugin. The cookie is used to store the user consent for the cookies in the category "Performance".
viewed_cookie_policy	11 months	The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. It does not store any personal data.

Chunfei Wu

Follow:

Hydrophobicity of polymer based anion-exchange columns for ion chromatography

2.1. Ion-exchange selectivity

2.2. Hydrophobic selectivity

2.3. Mixed-mode retention mechanism

Leave a Comment Cancel Reply

Useful Links

Contact Us

Phone: +44 (0)28 9097 5573

Email: c.wu@qub.ac.uk