Segregation in like charged polyelectrolyte - surfactant mixtures can be precisely tuned via manipulation of surfactant mass ratio.
Peter W. Willsa, Sonia G.Lopezb, Jocelyn Burra, Pablo Taboadab and Stephen G. Yeatesa* a
The University of Manchester, School of Chemistry, Manchester, UK
b
Group de Física de Coloides y Polímeros, Departamento de Física de la Materia Condensada, Universidad de Santiago de Compostela, 15782-Santiago de Compostela, Spain. *Author to whom correspondence should be addressed:
[email protected]
RECEIVED DATE KEYWORDS: segregative phase separation, polyelectrolyte, cationic surfactant, p(DADMAC)
1
Abstract In this study we consider segregative phase separation in aqueous mixtures of quaternary ammonium surfactants didecyldimethylammonium chloride (DDQ) and Alkyl (C12, 70%; C14 30%)
dimethyl
benzyl
ammonium
chloride
(BAC)
upon
the
addition
of
poly(diallyldimethylammonium) chloride (pDADMAC) as a function of both concentration and molecular weight. The nature of surfactant type is dominant in determining the concentration at which separation into an upper essentially surfactant rich phase and lower polyelectrolyte rich phase is observed. However for high molecular weight pDADMAC there is clear indication of an additional depletion flocculation effect. By tuning the BAC/DDQ ratio the segregative phase separation point can be precisely controlled. We propose a phase separation mechanism for like charged quaternary ammonium polyelectrolyte/surfactant/water mixtures induced by a reduction in the ionic atmosphere around the surfactant head group and possible ion pair formation. An additional polyelectrolyte induced depletion flocculation effect was also observed.
2
Introduction
Due to their commercial importance in cosmetics, detergents, pharmaceutical and biotechnological
applications,1-6
several
polymer/surfactant/water
mixtures
have
been
investigated and many of these have been shown to undergo isothermal phase separation into a polyelectrolyte rich and surfactant rich phase. . However the area of like charged polyelectrolyte – surfactant mixture interactions remains less well understood,7-16 particularly formulations containing quaternary ammonium chloride polyelectrolytes. Comas-Rojas
et
al.7
studied
the
interaction
of
polyethyleneimine
(PEI)
and
cetyltrimethylammonium bromide (CTAB) under dilute conditions, where the presence of PEI was found to result in a reduction in the critical micelle concentration (CMC) and the formation of meso-structured thin films at the air-water interface. This was rationalised in terms of weak interactions occurring between CTAB and PEI in solution giving rise to surfactant templating leading to suppression of the electric double layer causing the counter ions to be held more tightly.17 At higher concentrations segregative phase separation of like charged polyelectrolytesurfactants into a upper surfactant rich and lower polyelectrolyte phase has been reported,11, 13-16 which could be further triggered at much lower polymer/surfactant concentrations by
the
addition of NaCl.13-15 Nilsson et al.15, 16 illustrated that in these segregative type of systems the addition of a hydrophobic cosolute can affect the polyelectrolyte/ surfactant compatibility and hence the critical phase separation concentration. The addition of octane was observed to increase compatibility while the addition of octanol decreased the compatibility. These observations were justified in terms of micelle aggregation number. Segregative phase separation
3
observed in the polymer-surfactant systems appears analogues to polymer-polymer mixtures which also segregatively phase separate and this phenomena is commonly referred to as polymer incompatibility. Polymer incompatability occurs when the effective interactions between unlike polymers is repulsive or/and have different levels of affinity to the solvent.14 Detailed evaluation of the segregative phase separation of sodium hyaluronate/sodium dodecyl sulfate/water,13
poly(sodium-4-styrenesulfonate)/sodium
poly(diallyldimethylammonium)
chloride
dodecyl
sulfate/water11
and
(pDADMAC)/cetyltrimethylammonium
bromide/water11 showed that in each case the water was found to partition so as to maintain chemical potential neutrality between the two phases. However while this explains the final equilibrium state it fails to explain what initially triggers the phase separation within these mixtures. The objective of this paper is to further study the processes triggering the isothermal phase separation in concentrated multi component aqueous mixtures of quaternary ammonium chloride polyelectrolyte and quaternary ammonium surfactant(s). In this study we focus solely on pDADMAC of varying molecular weight and its interactions with the quaternary ammonium surfactants didecyldimethylammonium chloride (DDQ) and Alkyl (C12, 70%; C14 30%) dimethyl benzyl ammonium chloride (BAC) and their mixtures.
4
Experimental Section
Reagents and Sample Preparation
pDADMAC, Mv = 8.5 kDa was obtained from Poly Sciences Inc as a 28 wt% solution in water, and Mv = 21 kDa and 140 kDa obtained from Sigma Aldrich as 35 wt% and 20 wt% solutions respectively. DDQ was obtained from Lonza (Tradename - Lonza Bardac 2240) and received as a 40 wt% solution in water. BAC was obtained from Thor (Tradename – Acticide BAC 50 M) and received as a 50 wt% solution in water. The concentrations of the above stock solutions were verified gravimetrically by drying in a vacuum oven at 80 oC until no further mass loss was recorded. Three repeats were conducted per sample with the average result recorded. Both surfactants and polyelectrolyte were used as received without further purification unless otherwise stated, Table 1. When specifically stated the pDADMAC 140 kDa was dialysised against Millipore filtered water using a benzoylated dialysis tubing (2 kDa molecular weight cut off, Sigma Aldrich, D7884-1FT) for four days with the water changed twice every day at which point the conductivity of the water within the beaker had returned to a level comparable with Millipore filtered water. The conductivity was measured on a Jenway 4010 Conductivity meter at 25 °C. Sodium chloride was obtained from Fisher Scientific, analytical grade. Millipore filtered water was used for all solutions. Fresh solutions were always used for all experiments. Stock solutions were prepared in polypropylene bottles (Nalgene bottles, Style 200, 100 ml purchased from
5
Sigma Aldrich), with individual samples prepared in 10 ml polypropylene graduated sample tubes.
Procedures and Techniques
Size exclusion chromatography (SEC) was carried out using TSK gel columns. Two different SEC systems were used; low molecular weight (G2000PW and G3000PW) and high molecular weight (G4000PW and G5000PW) in combination with a Gilson 132 refractive index detector, at a flow rate of 0.5 ml/min of a Citric acid 0.1 M buffer at 25 °C, calibrated using polyethylene oxide standards. 1
H NMR was conducted on a Bruker Ultrashield 400 MHz spectrometer. Samples were
rehydrated in D2O (Sigma Aldrich) before measurement. Gravimetric analysis was conducted by a weighed sample being transferred into a sample pan and placed into a vacuum oven (80 oC). Every 24 hrs the sample was weighed until no further weight loss was observed. Three repeats were conducted per sample with the average result recorded. Dilute solution viscometry was carried out using an Ubblelohde (Rheotek - VIS3300) capillary viscometer at 25 °C. Individual samples were formulated of different concentrations in a 1 M NaCl, with each solution being left for 30 minutes prior to determination and the average of three measurements taken. Samples were formulated by mass with the addition of a pDADMAC stock, 2M NaCl stock and Millipore filtered water. Huggins/Kraemer Plot were produced for each polymer sample and extrapolation to zero concentration provided the Intrinsic Viscosity
.
6
Surface tension was carried out using a Kibron Delta 8 Tensiometer which is based on the DeNouy method with a platinum rod probe. Serial dilutions of 0.5 from a concentrated stock solution were performed using an Epmotion 5075 Liquid Handler (Eppendorf) into a 12 x 8 welled plate. Each well was filled with a total sample volume of 100 µl. A total of 8 replicates were measured for each concentration. A purified distilled water control was present on each plate. Photon correlation spectroscopy (PCS) particle sizing was performed using a Zetasizer-Nano from Malvern Instruments with a He-Ne laser having a wavelength of 632.8 nm and a back scattering detector set at 173 °. Solutions were filtered twice with a 0.22 µm polyethersulfone syringe filter then placed into a Polystyrene 10 x 10 x 45 mm cuvette. The results are an average of four measurements taken at 25 °C. A CONTIN program was used to deconvolute the size distribution.18 To identify a suitable concentration of NaCl to be added into the surfactant solution a series of solutions were formulated for each surfactant type at constant surfactant concentration (100 mM). NaCl concentration below the surfactant cloud points were chosen for the PCS experiment; DDQ 5 x 10-3 M and BAC 0.3 M. The samples for the phase separation studies were measured by mass from the appropriate stock solution. Samples were stirred for one hour with a magnetic stirrer then inverted ten times by hand. Visual observations were taken 48 hrs after last agitation to determine if phase separation had occurred.
7
Results and Discussion
Poly(diallyldimethylammonium) chloride (pDADMAC)
Molecular weight characterisation of the pDADMAC samples used in the study is given in Table 2. SEC chromatograms in all cases were monomodal of varying polydispersity. Viscosity average molecular weights Mv, used subsequently to define the polymers, were obtained using the Kuhn-Mark-Houwink-Sakurada relationship
, the constant K = 4.71 x 10-3 and
exponent α = 0.83 being an average from a range of references with similar experimental conditions.19-21
Quaternary Ammonium Chloride Surfactant
Surface tension profiles of the pure surfactants and BAC/DDQ (2:3 mol/mol), chosen for particular attention due its recently reported beneficial performance in antimicrobial formulations22 were determined, Figure 1a. CMC and calculated surface area per head group were determined using the Gibbs adsorption isotherm17, Table 3, with both BAC and DDQ having comparable head group surface areas. For BAC this arises from the phenyl group contained within BAC orientating itself next to the quaternary ammonium head group instead of inside the micellar interior thus creating a more bulky head group,23-27 whilst in DDQ the two decyl tails of the surfactant cause greater tail packing, constraining the surfactants ability to adsorb efficiently at the air-water interface.
8
Photon correlation spectroscopy (PCS) was used to determine the hydrodynamic diameter of the micelles. The PCS profiles of BAC and BAQ/DDQ 2:3 mol/mol, Figure 1b, show a monomodal distribution over the concentration range of interest, with the mean Z-Average and the monomodal distribution suggestive of ellipsoidal or spherical micelle geometry. DDQ shows bimodal / trimodal distribution suggesting micellar structures of large aggregation number. This can be explained either by a cylindrical geometry which has a radial length and axial length or spherical micelles in equilibrium with unimellar or multimellar vesicles.28 Both scenarios are consistent with the findings of an earlier fluorescence probe study of DDQ which observed an increase in micelle aggregation number from 20 to 86 when increasing concentration from 10 to 81 mM.29
Phase separation in mixtures of p(DADMAC) with DDQ and BAC
In an attempt to quantify the critical ionic strength required for phase separation within each systems a series of samples were formulated containing 100 mM (≈ 3.6 wt%) of surfactant plus differing concentrations of polyelectrolyte, including by way of comparison the effect of NaCl and the first sample within the series to phase separate identified. A phase separation boundary diagram was constructed to illustrate the phase boundary between non-phase separating and phase separating mixtures for pDADMAC 8.5 kDa/DDQ, Figure 2a, with an example of the phase separating behaviour shown in Figure 2b.
For selected pDADMAC/surfactant/water mixtures which exhibited segregative phase separation aliquots from the top and bottom phases were taken 24 hrs from last agitation. The
9
volume of each phase was measured and a predicted wt% calculated which assumed complete phase separation between the two phases. Gravimetric analysis was performed which indicated a concentrated top phase and a more dilute bottom phase. In Table 5 for pDADMAC 8.5 kDa (3 wt%)/DDQ (3 wt-%)/water the predicted and actual wt% whilst in close agreement are not statistically comparable. 1H NMR (Figure 3) showed that the upper phase was surfactant rich whilst the lower phase was found to be polyelectrolyte rich which is consistent with earlier studies.11 As such the terms surfactant rich and polyelectrolyte rich layers will be used to describe the different phases. For the surfactants and surfactant mixtures studied increasing the molecular weight of pDADMAC from 8.5 to 140 kDa has a negligible effect on the phase separation boundary, Figure 4a. The effect on the phase separation boundary of surfactant type at constant polyelectrolyte molecular weight was more dramatic with DDQ phase separating at a much lower concentration compared to BAC, with mixtures, exemplified by BAC/DDQ (2:3 mol/mol), lying systematically between the two extremes, Figure 4b. It should be noted that the tensiometry profile of BAC showed a minimum around the CMC which is indicative of impurities present, he common explanation being that hydrophobic co-solutes are present namely long chain alcohols. As discussed previously long chain alcohols can increase polyelectrolyte/surfactant compatibility, but are present at concentrations where this will have a small effect. To fully investigate the effect of the surfactant ratio BAC/DDQ on the phase boundary a series of mixtures were formulated at constant surfactant concentration (100 mM ≈ 3.6 wt-%) with differing pDADMAC (21 kDa) concentration and as a comparison differing NaCl concentrations. The first sample within the series to phase separate was identified as the phase separation concentration for that system, Figure 5. The NaCl/surfactant/water mixtures which
10
showed segregative phase separation were observed to give a surfactant rich upper phase analogous to the phase separation within the polyelectrolyte/surfactant/water mixtures. This illustrates that the phase boundary can be manipulated by altering the ratio of BAC/DDQ for both the pDADMAC (21 kDa) and NaCl systems.
Discussion So a direct comparison could be made between the critical pDADMAC and NaCl phase separating concentrations the number of moles of electrolyte and consequently the number of chloride ions ( Figure 5b we compare the
required for phase separation was calculated, Table 6. In for the pDADMAC and NaCl systems as a
function of DDQ mass fraction. We see that for a surfactant mixture where the mass fraction of DDQ is greater than 0.4 the ionic strength required for segregative phase separation is the same irrespective of whether it is in the form of NaCl or polyelectrolyte. We ascribe this electrolyte driving force to the ionic atmosphere around the surfactants quaternary ammonium head group reducing in size resulting in condensing of the counter ion. Upon phase separation the surfactant becomes insoluble in water forming immiscible oil which due to its lower density will cream to the top of the solution and coalesce to form a surfactant rich phase. The surfactant molecules become soluble within this surfactant rich phase due to the absence/reduction in polyelectrolyte concentration hence the ionic atmosphere around the head group increases. From the data presented it is not possible to determine whether the reduced ionic atmosphere around the quaternary nitrogen was enough to induce phase separation on its own or if an actual intimate ion pair is formed. An intimate ion pair is when the charged moiety, in this case the quaternary ammonium group, is in direct contact with its corresponding counter ion subsequently
11
neutralizing the ionic charge and further investigation is required on this aspect to fully elucidate the mechanism. We believe the reason that surfactant mixtures containing a greater mass fraction of DDQ require a lower ionic strength to induce phase separation is due to the different micellar properties of the surfactants DDQ and BAC. It has been noted previously within the literature that the degree of counter ion dissociation within a micellar structure is smaller for cylindrical micelles compared to spherical micelles, the reasoning behind this is that the distance between the surfactant head groups is larger within a spherical geometry compared to a cylindrical geometry.30-32 PCS results suggest that over the concentration regime studied BAC micelles are ellipsoidal/spherical in shape with nominal change in micelle size compared to DDQ which are either cylindrical in geometry or a combination of spherical micellar and vesicle structures. A difference in the micelle counter ion dissociation constant of the two surfactants seems likely to be a major factor in the differing critical ionic strengths required to induce phase separation within the above mixtures. When the mass fraction of DDQ is less than 0.4 a deviation is observed between the NaCl and pDADMAC systems, with the polyelectrolyte system inducing phase separation at a lower ionic strength, with the effect being most pronounced the higher the pDADMAC molecular weight. To further explore this effect a range of different molecular weight pDADMAC molecules were studied a over a range of BAC/DDQ ratios at constant overall surfactant concentration. The critical phase separation concentration was identified and subsequently the calculated. This can be explained as a complimentary depletion flocculation effect, where in the case of a polyelectrolyte which does not adsorb onto the surfactant micelle entropic depletion interactions have been known to induce phase separation. Entropic depletion interactions results
12
from changes in the conformational entropy of the polymer chains which prevents polymers from getting too close to a micelle. As a result of this, when two micelles are close enough together to prevent the polyelectrolyte from separating them the region between the micelles is said to be depleted of polymer. The polyelectrolyte outside the depletion zone between micelles induces an osmotic pressure pushing micelles together and encouraging phase separation within the solution mixture.11, 33 Numbers of studies have shown that in colloid-polymer mixtures of like charge and low salinity an enhanced depletion interaction is observed at much lower concentrations compared to neutral systems. Long-range repulsive electrostatic forces were found to be behind this enhanced depletion interaction, with the Debye length of the solvent reported to be a major factor. It was also noted that at higher electrolyte concentrations the radius of gyration of the polymer became an important factor increasing the range of the depletion force.34-37 For the pDADMAC mixtures studied here the proposed depletion effect is more significant for the mixtures containing a lower mass fraction of DDQ. The depletion effect is also more pronounced for the higher molecular weight, with pDADMAC 8.8 kDa in 1M NaCl reported to have a radius of gyration of 8.6 nm38 while pDADMAC 114 kDa is reported to have a radius of gyration of 64 nm.19 We speculate this is because these mixtures are more concentrated. In more concentrated mixtures a smaller Debye length would be expected but also entropic interactions will become more dominant, increased depletion interactions would be expected.
Conclusion Within this paper we propose a segregative phase separation mechanism for mixtures of like charged quaternary ammonium polyelectrolyte/surfactant/water induced by a reduction in the
13
ionic atmosphere between the quaternary nitrogen and chloride counter ion of the ionic surfactant and an additional polyelectrolyte depletion flocculation effect. The nature of surfactant type is dominant in determining the onset of segregative phase separation. For mixtures having a high mass fraction of BAC there is indication of an additional depletion flocculation effect particularly for high molecular weight pDADMACs. To our knowledge this paper is the first to report the ability to tune the phase separation point of like-charged polyelectrolyte-surfactant mixtures by varying surfactant ratio (BAC/DDQ).
Acknowledgements This Project was funded via an EPSRC CASE award grant involving the collaboration of The Organic Materials Innovation Centre, University of Manchester and Byotrol Plc. We are also grateful to Dr. B Carter for his help with the Epmotion 5075 Liquid Handler (Eppendorf) and Kibron Tensiometer at The Centre of Materials Discovery, University of Liverpool.
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19
Figure 1. 1 (a) Surfaace tension profiles in water at 255 oC of ( ) BAC, (O)) DDQ, andd (Δ) BAC/DD DQ (2:3 mol/mol). (b) PCS P intensityy profiles at 25 oC as a function of concentratioon (i) BAC (0.33 M NaCl) (ii)BAC/DDQ Q 2:3 mol/m mol (0.05 M NaCl) N (iii) DDQ D (5 x10-3 M NaCl).
20
Figure 2. 2 (a) Phase separation s booundary for pDADMAC C (8.5 kDa)/D DDQ/water at 25 oC; ( ) one phase, (■ ■) two phase. (b) Phasee separation image for sample pDA ADMAC (3 wt%) / DD DQ (3 wt%) / water w (t = tim me since last agitation).
21
(a)
(b)
Figure 3. 3 1H NMR of o the top phaase and the bottom b phase for the sam mple (a) pDA ADMAC 8.55 kDa (3 wt%) / BAC (3 wt%)/water w s sample, b) pDADMAC p 8.5 kDa (3 wt%) / DDQ (3 wt%)/w water sample, 24 hrs sin nce last aggitation. (reed) top phaase (surfacttant), (blue) bottom phase p (pDADM MAC).
22
ADMAC/DD DQ/water ass a functioon of Figure 4. (a) Phase separatioon boundarry for pDA pDADM MAC molecular weight; ( ) 8.5 kDaa, (o) 21 kD Da and (Δ) 140 1 kDa at 25 oC. (b) Phase P separatioon boundary for pDADM MAC (21 kD Da)/surfactannt/water for (o) DDQ, ( ) BAC and (Δ) BAC/DD DQ (2:3 mo ol/mol) at 25 2 oC. Dashhed lines inndicate appproximate loocation of phase p boundaryy.
23
(a)
(b)
Figure 5. 5 (a) Criticaal phase sepparation conncentration as a a functioon of BAC/D DDQ mass ratio. Surfactannt (BAC/DD DQ) concentrration was kept k constantt (100 mM ≈ 3.6 wt%), (○) NaCl annd (□) pDADM MAC (21 kDaa). (b) Numbber of chlorride ions from m electrolytte to induce phase separration (
as a funcction of BAC C/DDQ ratioo. Surfactannt (BAC/DDQ) concentrration
100 mM/ ≈ 3.6 wt%). Electrolyte E t type; (○) NaaCl and (□) pDADMAC C (21 was keptt constant (1 kDa).
24
Table 1. Chemical Structures of the polymerr and surfacttants used in the study. Chem mical Name
Struccture
Polyy(diallyldimetthylammoniuum) chloride (pD DADMAC)
Dideccyldimethylam mmonium chlloride (DDQ))
Alkyl (C12 70%; C14 C 30%) Dim methyl benzyl ammonium m chloride (B BAC)
25
Table 2. Molecular weight determination of pDADMAC polymers. SEC
Viscometry
Supplied Mw (kDa)
Mn (kDa)
Mw/Mn (PDI)
Mv (kDa)
Poly Sciences Inc
12.3
4.6
2.7
8.5
Sigma Aldrich
37.6
4.1
9
21
Sigma Aldrich
403
19
21
140
pDADMAC
26
Table 3. Tabulated CMC and surface area per head group values for BAC, DDQ and BAC/DDQ 2:3 mol/mol at the air-water interface at 25 oC
Surfactant
Experimental
Literature
CMC (mM)
CMC (mM) C12 – 8.125, 8.824
Experimental Area of Head group (Ǻ2+/- 1) 83
BAC
3.5
DDQ
1.6
1.228 1.929
79
BAC/DDQ (2:3 mol/mol)
1.9
N/A
78
C14 – 1.927, 2.024
27
Table 4. Tabulated PCS Z-Average values of the respective surfactants 25 oC. Intensity - Z-Average (nm)
Surfactant Concentration (mM)
BAC
BAC/DDQ (2:3 mol/mol)
10
N/A
N/A
20
7.8
8.0
50
6.8
9.2
100
7.1
9.1
DDQ (1) 3.3 (2) 235.0 (1) 3.1 (2) 13.0 (3) 263.4 N/A (1) 3.1 (2) 48.1
28
Table 5. Gravimetric analysis of top and bottom layer of pDADMAC 8.5 kDa (3 wt%)/DDQ (3 wt%)/water, 24 hrs after last agitation.
Layer
Volume (ml) (± 0.05)
Predicted (wt%)
Actual (wt%)
Top
0.60
24.0 ± 2.0
22.7 ± 0.1
Bottom
4.40
3.3 ± 0.1
4.4 ± 0.1
29
Table 6. Tabulated results showing the number of chloride ions (
) added into the ternary
mixtures by electrolyte to induce phase separation. Surfactant concentrations kept constant at 100 mM (≈ 3.6 wt%).Surfactant mixture BAC/DDQ ratio = (2:3 mol/mol).
a
Surfactant Type
Electrolyte Type
Wt-% of polyelectrolyte to induce Phase Separation
DDQ
NaCl
0.04 M
2.4
DDQ
pDADMAC (8.5 kDa)
0.7
2.6
DDQ
pDADMAC (21 kDa)
0.7
2.6
DDQ
pDADMAC (140 kDa)
0.6
2.2
BAC/DDQ
NaCl
0.20 M
12
BAC/DDQ
pDADMAC (8.5 kDa)
3.0
11
BAC/DDQ
pDADMAC (21 kDa)
2.9
11
BAC/DDQ
pDADMAC (140 kDa)
2.2
8
BAC/DDQ
pDADMAC (140 kDa)a
2.5
9
BAC
NaCl
1.10 M
66
BAC
pDADMAC (21 kDa)
12.5
47
BAC
pDADMAC (140 kDa)
10.0
37
(x1020)
pDADMAC was dialyised against water to remove low molecular weight impurities.
30