- Email: [email protected]
Differential behavior of sodium laurylsulfate micelles in the presence of nonionic polymers
Accepted Manuscript Differential Behavior of Sodium laurylsulfate Micelles in the Presence of Nonionic Polymers Chandra Ade-Browne, Arnab Dawn, Marzieh Mirzamani, Shuo Qian, Harshita Kumari PII: DOI: Reference:
S0021-9797(19)30263-2 https://doi.org/10.1016/j.jcis.2019.02.081 YJCIS 24705
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
1 November 2018 22 February 2019 23 February 2019
Please cite this article as: C. Ade-Browne, A. Dawn, M. Mirzamani, S. Qian, H. Kumari, Differential Behavior of Sodium laurylsulfate Micelles in the Presence of Nonionic Polymers, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.02.081
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Differential Behavior of Sodium laurylsulfate Micelles in the Presence of Nonionic Polymers Chandra Ade-Browne,a Arnab Dawn,a Marzieh Mirzamani,a Shuo Qian,b Harshita Kumaria* a
James Winkle College of Pharmacy, 231 Albert Sabin Way, Cincinnati, OH 45267-0514,
USA Email: [email protected] b
Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Abstract Hypothesis: Theory and practice have proven that the cleansing properties and irritation potential of surfactants can be controlled with the addition of co-surfactants or polymers. The size of surfactant-polymer nanoassembly which differs from the pure surfactant micelle has been postulated to be the cause of the differences in a surfactant system’s ability to disrupt the skin barrier. However, a firm-structure function relationship connecting polymer and surfactant under a consumer relevant condition is yet to established. It is therefore hypothesized that apart from the size, the shape and the chemical nature of the polymer might play crucial roles.
Experiments: We used combined small-angle neutron scattering, nuclear magnetic resonance spectroscopy, tensiometry, and dye solubilization methods to investigate the shape, size, and intermolecular interactions involved in sodium laurylsulfate-based systems in the presence of two industrially important and chemically distinct polymers, polyethylene glycol and polyvinyl alcohol, adopting a consumer relevant protocol.
Findings: Apart from size, shape and inter-micellar interactions fine-tuned by the presence of the polymers are found to be the important factors. Secondly, the physicochemical property of the polymer including chemical structure, conformation, hydrophilicity, 1
presence of side groups, all can have crucial influence on polymer-surfactant interaction, micelle formation, and micelle stability.
Key Words: Micelle; Small-angle neutron scattering; Polymer; Self-assembly; Polymer– surfactant interaction
2
Introduction Surfactant self-assembly is of interest in a wide variety of fields because of the applications of surfactants in the healthcare, cosmetic, and petroleum industries, among others.1-4 At low concentrations, the surfactant molecules exist as monomers in aqueous solution. However, at the critical micelle concentration (CMC), the monomers selfassemble into micelles with hydrophobic core consisted by the hydrophobic tails facing inwards and the hydrophilic head groups facing outward. The amphiphilic nature of the assembly structure provides a chemical duality that are useful in cleansing processes or the encapsulation of organic actives. 5-8 For example, sodium laurylsulfate (SLS) is one of the most extensively used surfactants in the industrial sector because of its effective cleansing properties and low cost. SLS is a mixture, primarily consisting of sodium dodecylsulfate (SDS) with traces of other substances such as SDS with different degrees of ethoxylation, laurylsulfate, and other compounds. The properties of SDS have been well characterized with techniques such as surface tensiometry,9,10 nuclear magnetic resonance (NMR),11-14 calorimetry,10,15 and small-angle neutron scattering (SANS). In particular, SANS has been used to study the effect of temperature,16-18 salt species and concentration,11,19-22 clouding,20,23 interactions with polymers,9,10,14,15,24-30 and interactions with other (co)surfactants.12,20,23,31,32 The research interest in SLS goes beyond physical characterization and into the clinical dimension because of its ability to irritate human skin, with the degree of irritation (expressed as the irritation potential) depending on the formulation of the surfactant system.33,34 The irritation potential of a surfactant system is associated with its ability to disrupt the skin barrier.35-41 For the personal care industry, it is important to understand and correlate these two key issues, namely, the physical properties and the irritation potential of the surfactants, as perpetuated by the ever-growing demands of consumers for formulations that are milder than currently marketed products containing anionic surfactants and anionics with betaine technology. Thus, it is essential to investigate the surfactant irritation mechanism in order to mitigate it. 3
There are several protocols for modulating the mildness of a surfactant system, including increasing the degree of ethoxylation,40 adding other surfactant,33,42-44 and/or incorporating polymer(s).42-45 However, what is known in practice has not been resolved into a firm structure–function relationship. In response, the Blankschtein group proposed a size-based theory, the “micelle penetration model,” based on their 14C-radiolabelling and dynamic light scattering (DLS) studies of the penetration of pure SDS and SDS with polyethylene glycol (PEG; molecular weight 8,000 g/mol) through porcine skin. 46 According to this model, both micelles and monomers are able to penetrate porcine skin and cause irritation. However, the larger SDS+PEG assembly penetrated less than the smaller, unbound SDS micelles. As a result, they postulated that the irritation potential of a surfactant is dependent on the size of the surfactant assembly body.
SDS
PEG
PVA
Figure 1. Molecular structures of various compounds used in the study.
Later, McCardy and coworkers performed a follow-up study using human cadaver skin, adding a second polymer, polyvinyl alcohol (PVA; molecular weight 17,783 g/mol), and using SLS instead of SDS.47 This results of this study were similar to those of the Blankschtein study, as the penetration of
14
C-SDS from the SLS+PEG micellar
nanoassembly was lower than that of the SLS micellar nanoassembly. Furthermore, the SLS+PVA micellar nanoassembly penetrated less than the SLS nanoassembly but was less effective than SLS+PEG at reducing 14C-SDS penetration into human skin. In this context, we realized that comparative studies on polymer–surfactant binary systems are indispensable in assessing the contributions from various parameters to the control of the size, shape, and assembly behavior of the system. For example, the 4
contribution from the shape of an assembly system to the surfactant action remains underexplored.
In
addition,
different
polymers
have
different
conformations,
hydrophilicities, and can be further differentiated based on side chains (branching). In the present study, we investigated the size, shape, and assembly behaviors of SLS micellar systems in the presence of PEG and PVA (Figure 1). These polymers were selected for two reasons: first, both are widely used in consumer products; and second, unlike PEG, PVA possesses a polar side group that is expected to interact with polar substrates away from the polymer backbone. We have utilized techniques including SANS, 1H-NMR spectroscopy, tensiometry, and dye solubilization in a complementary manner to study SLS, SLS with PEG, and SLS with PVA. While SANS offers comprehensive information about the physical parameters (size and shape) of the various assembly structures, NMR spectroscopy addresses the interaction parameters at the molecular level. In contrast, tensiometric and dye solubilization experiments reveal differences in the aggregation behaviors. Furthermore, in the present work, we adopted a strategy in which the aggregation behavior was studied as the concentrations of the surfactant and the polymer were equally diluted, while their ratio was maintained. This approach is atypical for such studies, but it is more relevant in a consumer product scenario. Therefore, the novelty of the present work lies in the comparative assessment of the micellar systems in the presence of multiple polymeric systems under consumer relevant experimental conditions. We expect that the results of the present study will aid in the selection of appropriate combinations of surfactants and polymers for milder formulations.
Experimental Materials SLS, PEG, and PVA were all provided by Procter & Gamble, Co. Specifically, SLS (29 wt% in H2O) was sourced from BASF, PEG from Sigma-Aldrich, and PVA (35 wt% in H2O) from Kuraray Poval and used without further purification. D 2O with 99.9 atom% D from Cambridge Isotope Laboratories was used for SANS experiments. D 2O with 99.9 5
atom% D and 0.05 wt% 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt from Sigma-Aldrich were used for NMR experiments. SANS Studies SANS experiments were performed using the Bio-SANS CG3 instrument at the High-Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL)
48
in Oak Ridge, TN
and the NG10 beamline at the NIST Center for Neutron Research at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. At both facilities, the incident neutron wavelength was set to 6 Å with about 15% spread. The SANS data for SLS+PVA were collected at NIST using titanium cells (1 mm path length) with quartz windows at 25°C. The SANS data for SLS and SLS+PEG were collected at the Bio-SANS using quartz banjo cells (1 mm path length) at room temperature (RT, 25°C). Measurements were made at different sample-to-detector distances to cover a scattering vector (Q) range of 0.01–0.8 Å-1 for the SLS and SLS+PEG samples. For the SLS+PVA samples, the data were collected over a Q-range of 0.003–0.5 Å-1. Background corrections, accounting for empty cell contributions, sample transmission, and background scattering, were made to each spectrum by the facilities supplied reduction software.49 The data was modeled with SasView (further details can be found in the Supporting Information). 50 NMR Studies 1
H-NMR spectra were obtained using a 400 MHz NMR spectrometer (Bruker AV-400,
Coventry, UK). The samples for analysis were prepared a day in advance to replicate the procedure used for the SANS experiments. A volume of 0.5 mL of the test formulations was prepared in D2O (99.9 atom% D) with an internal standard of 0.05 wt% 3(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt. Tensiometric Studies Three 50 mL aqueous samples were made: one consisting of 50 mM SLS, one consisting of 50 mM SLS and 2 wt% PEG, and one consisting of 50 mM SLS and 2 wt% PVA. A 20 wt% PEG stock was made by weighing out dry PEG, adding water, gently heating for a few minutes until PEG was fully dissolved, and finally allowing the heated solution to cool 6
to RT. The 35 wt% PVA raw material was diluted with water to make an 18.7 wt% PVA stock. The components were added in the order of Millipore (Burlington, MA, USA) H 2O, 29% SLS stock, and then polymer stock if needed. The samples were vortexed for a few seconds after SLS addition and again after polymer addition. All samples were allowed to equilibrate for at least 48 h prior to analysis. The tensiometric data were obtained using the Wilhelmy plate method with a KRÜSS K100 force tensiometer (KRÜSS GmbH, Hamburg, Germany). The system was programmed to take surface tension measurements for 180 s to ensure that the average value reflected the equilibrium value. After the starting concentration was measured, 23.57 mL DI water was added to the solution. The solution was stirred for 2-3 min while the plate was removed and rinsed with DI water, acetone, and DI water again. The plate was then flamed with a propane flame until it turned red to remove surface-active impurities that could have accumulated on the plate during the previous measurement. The plate was reinstalled and allowed to cool, and then 23.57 mL sample was removed so the sample volume was again 50 mL before the next measurement was taken. This process was repeated for each dilution, for a total of 15 measurements. Dye Solubilization Experiments Several concentrations of each surfactant system were prepared in Millipore H2O, including the concentrations of interest: 50 mM SLS, 50 mM SLS + 2 wt% PEG, and 50 mM SLS + 2 wt% PVA. The investigated concentrations ranged from 14,418 ppm (50 mM SLS) to 1 ppm, while the same surfactant-to-polymer ratio was maintained. The same amount of Orange OT dye (0.2 wt%) was added to each solution, which was then shaken for 48 h at approximately 22 °C, followed by centrifugation and filtration. The supernatant was collected and the absorbance was measured at 470 nm and 303 K using a Molecular Devices SpectraMax 384 Plus microplate reader.51 Individual polymer samples were also analyzed using the same protocol, but the investigated concentration range was 150,000 ppm to 1,000 ppm.
Results and Discussion 7
SANS Studies SANS experiments were performed with D2O solutions of SLS alone (50 mM), and after the addition of either PEG (2 wt%) or PVA (2 wt%). The scattering cross section obtained in SANS is a measure of how large the nucleus of the particle appears to the neutron, which is determined by how strongly the neutrons scatter from it. For monodisperse interacting particles, the coherent differential scattering cross section (dΣ/dΩ) can be expressed as:19,52 (1) where n is the number density of micelles, ρs and ρm are the contrast variations between the scattering length densities of the solvent and the micelle, respectively, and V is the micellar volume. Furthermore, the shape and size of the micelle is described by the form factor P(Q), the spatial arrangement of different micelles is described by the structure factor S(Q), and B is the background of incoherent scattering. The scattering length density (SLD) was calculated as 3.37×10-7 Å-2 for SLS, whereas the scattering length densities for the SLS+PEG and SLS+PVA nanoassemblies were calculated as 6.14×10-6 Å-2 and 5.9×10-6 Å-2, respectively. All calculations were performed using the SLD calculator provided on the NIST website. The SANS data for the samples were fitted to spheroidal and ellipsoidal models. A strong correlation peak was observed for each sample (Figure 2). This correlation peak is the result of long-range Coulombic interactions. As this peak persisted in the presence of micelles, indicating that electrostatic interactions persist, the Hayter–Penfold mean spherical approximation (MSA) was used as the structure factor.53, 54
8
Figure 2. SANS data for 50 mM SLS (blue), 50 mM SLS + 2 wt% PEG (red), and 50 mM SLS + 2 wt% PVA (green). In the case of 50 mM SLS and 50 mM SLS + 2 wt% PEG, both the sphere and the ellipsoid models fit well. At a concentration of 50 mM, SLS has a shape averaged between a sphere and an ellipsoid (Figure 3).
Figure 3. From left to right, 50 mM SLS fit to the sphere model, 50 mM SLS + 2 wt% PEG fit to the ellipsoid model, and 50 mM SLS + 2 wt% PVA fit to the sphere model. This is evident from the similar charge, volume fraction, and dimensions obtained when fitted to the sphere or ellipsoid models (Supporting Information). Thus, this analyses 9
provides an average of the solution dynamics. When PEG is added to SLS, the micelles become more ellipsoidal with radii of 13.4 and 22.6 Å. A summary of the best-fit models is provided in Table 1.
Table 1. Parameters calculated from the SANS results based on fits to different models. Model 50 mM SLS
Volume (Å3) 35041
Radius (Å)
Sphere
20.3 ± 0.02 13.4 ± 0.03 50 mM SLS + 2 (R) Ellipsoid 28869 wt% PEG 22.6 ± 0.06 (ωR) 50 mM SLS + 2 18.04 ± Sphere 24592 wt% PVA 0.139
Interestingly, while the SLS and SLS+PVA systems are best fitted to a sphere model, the SLS+PEG system is best fitted to an ellipsoid model. This shape variation in the presence of PEG is an indication of a slight distortion of the native micelle. Thus, instead of the radius, we calculated and compared the volumes of the three nanoassemblies (Table 1). It appears that the micellar volume decreases upon addition of the polymers, and it is the smallest in the presence of PVA. As mentioned earlier, a correlation peak is present in all systems, which is typical of ionic surfactants. This peak was defined as Qmax because it corresponds to the highest intensity Q-value. Qmax can be used to approximate the distance between neighboring micelles.26,28,55 (2) Table 2. Summary of the correlation peak positions and the calculated average intermicellar distances. Qmax (Å-1) 50 mM SLS 0.04711 50 mM SLS + 2 wt% 0.0593
dmic (Å) 133.37 105.95 10
PEG 50 mM SLS + 2 wt% 0.04297 PVA
146.22
In the presence of the different polymers, there is a distinct difference in the solution properties. When the SDS micelles are in the presence of PEG, the intermicellar distance is decreased by approximately 21% (Table 2). In contrast, in the presence of PVA, the intermicellar distance is increased by approximately 10%. These differences can be attributed to dissimilarities in how the polymers wrap around the micelles, as discussed further in the NMR results. 1
H-NMR Studies
To obtain insight into the various intramolecular and intermolecular forces involved in the self-assembly process, 1H-NMR studies were conducted on surfactant systems in D2O in the presence and absence of polymers. Although, SLS may contain traces of other substances, as mentioned earlier, the individual signals of SDS can be unambiguously assigned, where the hydrophilic and hydrophobic domains are clearly distinguishable. The effect of the presence of polymers on the proton signals of SLS is summarized in Figure 4. Upon the addition of both polymers (PEG and PVA), the protons associated with the hydrophilic part of the surfactant (head groups) experience an upfield shift, signifying a shielding effect. This shift is clear evidence that the polymers are in the proximity of the micelles and that they interact with the polar head groups. Interestingly, in the presence of PVA, the protons associated with the hydrophobic tail of SLS undergo a significant downfield shift, signifying an enhanced hydrophobic interaction within the micelle. This change could be caused by shrinkage (compaction) of the existing micelle in the presence of PVA, which is consistent with the decrease in micellar volume observed in the SANS results. A similar but smaller change is observed for the system with PEG, where only methyl protons show a downfield shift. This result is also consistent with the SANS data, which showed a small decrease of micellar volume in the presence of PEG. However, the 11
possibility of insertion of part of the polymer chain within the SLS micelle is ruled out because such a situation would result in shielding of the associated domain of the SLS molecule. In addition, the hydrophilic polymer would prefer not to interact with the hydrophobic domain of SLS. This configuration is reasonable because the polymers were added after the micelle formation. Hydrophilic
Hydrophobic
-(CH2)9-CH3
-O-CH2-O-CH2-CH2-
-CH3
(a)
(b)
(c)
Figure 4. Diagram showing hydrophilic and hydrophobic protons in SDS, and partial NMR spectra of (a) SLS + 2% PVA, (b) SLS and (c) SLS + 2%PEG, in D2O. The individual signals of SDS protons are assigned and indicated in red. Tensiometric Studies So far, we have investigated the effect of polymer addition on pre-formed SLS micelles. To obtain information about the micelle stability in the presence of added polymers, 12
tensiometric measurements were performed by diluting the SLS only, SLS with PEG, and SLS with PVA systems (Figure 5). The starting concentrations of these solutions were the same as those used in the SANS experiments. The systems were diluted in a step-wise manner until the endpoint was reached. The advantage of this experimental design is that it simulates the way in which a product is diluted when used by the consumer, as in practice both the surfactant and polymer are diluted at the same time. Another advantage is the industrial relevance of these systems, as the industrial-grade materials used contain the same kind of impurities at similar levels as would be found in commercially available personal care products. Thus, the studied systems and their results would be reflective of what occurs during consumer usage.
Figure 5. Tensiometric data for 50 mM SLS, 50 mM SLS + 2 wt% PEG, and 50 mM SLS + 2 wt% PVA. The black lines (solid for SLS, dashed for SLS + PEG, dotted dash for SLS + PVA) on each curve are logarithmic trendlines for the straight-line portions on either side of the minimum. The equations for the trendlines were used to calculate the values shown in Table 3. Figure 5 shows the surface tension data for SLS alone and with each polymer. Each curve has a minimum, caused by the presence of surface-active impurities lowering the 13
surface tension in conjunction with the SLS, which are then dissolved in micelles as more micelles form at increasing surfactant concentrations. 56 Depending on the source of the raw material in the SLS used for this work, small amounts of unreacted lauryl alcohol, other sodium alkyl sulfates, or polyvalent metal ions such as Ca 2+ could be present. Given that the SLS is industrial grade, the curves were expected to have a minimum. Polymer addition causes noticeable changes in the surface tension curves of SLS. Although the position of the minimum remains approximately the same between each curve, the slopes on either side of the dip and the minimum and final surface tension values are affected differently based on the polymer added. The change in slopes on either side of the minimum indicate that PEG and PVA affect the CMC, while the change in minimum and final surface tension values upon polymer addition suggest that both polymers interact with the impurities and SLS monomers. To determine the effects of the polymers on SLS, the CMC, the maximum surface excess concentration (Γmax), and the minimum area occupied by the surfactant at the air– water interface (Amin) were calculated for all three systems. The CMC values were obtained using the typical method of drawing trendlines on each side of the minimum, finding the intersection of the trendlines, and taking the x value of the intersection as the CMC, as shown in Figure 5. The Γ max and Amin values were calculated using the following equations: (3) where n is the number of species dependent on the surfactant concentration (in this case 2, as SLS is an ionic surfactant
57, 58
), R is the universal gas constant (8.314 J/(mol·K)), T is
the absolute temperature (293.15 K), γ is the surface tension in mN/m, C is the surfactant concentration in mol/L, and dγ/d(ln C) is the slope of the surface tension vs. ln C curve at concentrations just below the CMC; and (4) where NA is Avogadro’s number and 1023 is the unit conversion factor. 14
Table 3. CMC, surface tension at the CMC (γcmc), maximum surface excess concentration (Γmax), and minimum area/molecule (Amin) for SLS alone and in the presence of PEG and PVA. Parameter CMC (ppm) CMC (mM)
SLS 364 1.26
γcmc (mN/m) 32.12 2 10 × Γmax (mmol/m ) 2.80 Amin (Å2) 59.21 3
SLS + PEG SLS + PVA 441 386 1.53 1.34 34.35 2.09 79.36
35.56 2.01 82.56
Table 3 shows the calculated CMC, Γ max and Amin values. The CMC of SLS is lower than that of pure SDS, which is expected based on the surface-active impurities present in SLS.59 Adding PEG and PVA to SLS resulted in an increased CMC, with PEG causing the greater increase while PVA caused a more modest increase. The cause of the increased CMC in the presence of PEG and PVA is not fully clear, but it might be related to the higher solubilization of the surfactant molecules in the presence of the polymer. This could be supported by the larger minimum and final surface tension values when PEG or PVA is present compared to only SLS. The surface excess concentrations are all positive, as is expected for surfactant systems; however, compared with SLS alone the values decreased in the presence of both PEG and PVA, which also means that the minimum area per SLS molecule at the air-water interface increased with PEG and PVA. These trends could also be related to the surfactant becoming more soluble in the presence of the polymers as suggested above, but it is still difficult to identify their cause because impurities such as Ca2+, Mg2+, and alcohols are present in the systems. As had been shown by Cross & Jayson,60 Li et. al.,61 Eastoe et. al.,62 and Mysels et. al.63,
64
, multivalent metal ion
impurities can interact with the surfactant to form surface-active impurities while hydrophobic impurities like alcohols are themselves highly surface-active but adsorb slowly to the air-water interface. With these surface-active impurities present in these 15
systems and contributing to the surface tension results shown in Figure 5, the use of the Gibbs adsorption equation is limited. Additionally, it cannot be conclusively stated whether the pre-CMC surface tension trends are caused by the polymer only interacting with the SLS monomers or by the polymer interacting with some combination of SLS monomers and the surface-active impurities, using tensiometry alone. Given that the dip for the SLS + PVA curve is shallower compared to the SLS and SLS + PEG curves, it appears that the surface-active impurities have a lesser influence on the surface tension in the SLS + PVA system. Overall, the differing tensiometric trends observed for the SLS + PEG and SLS + PVA systems indicate that above the CMC SLS micelles stabilize against dilution in distinct manners depending on the polymer present, while below the CMC the SLS monomers undergo different dilution processes dependent on the polymer in the system. In other words, the nature of polymer–surfactant interactions above and below the CMC of the surfactant can be substantially different.
Dye Solubilization Although the surface tension method is widely used in surfactant studies, it is very sensitive to the presence of impurities within the system.59 This issue is even more crucial here because of the use of commercial-grade SLS, which contains traces of various impurities. Therefore, it was deemed necessary to use the dye solubilization method with Orange OT, a water-insoluble dye, as an alternative tool to determine the aggregation behavior of the various surfactant systems accurately. The surfactant systems were prepared at several concentrations but with the same amount of Orange OT added to the solutions. Typically, the resulting graphs exhibit initial points with an absorbance similar to that of water followed by a sharp increase in slope. This change in slope is associated with aggregate formation and subsequent solubilization of the dye. Subsequently, the absorbance increases with the concentration of the surfactant until a plateau is reached. Overall, similar trends were observed in the SLS systems in the presence and in the absence of the polymers (Supporting Information Figure S1 and S2). Here, the change in 16
slope is reflective of the collective contributions from polymer–surfactant interactions and micelle formation. The aggregation behavior of the different systems revealed from the dye solubilization experiments (Table 4) is indicative of the fact that micelle formation is favored in the presence of the polymers.51 Furthermore, the influence of PVA on SLS micelle/micellization is even greater than that of PEG.
Table 4. Comparison of the CMC values obtained using the surface tension method with the point of aggregation calculated using the dye solubilization method. CMC, Surface Tension Method (ppm) SLS SLS + PEG SLS + PVA
364 441
Aggregation Point, Dye Solubilization Method (ppm) 650 350
386
190
Next, the values reflective of SLS aggregation obtained from the tensiometric and dye solubilization experiments were compared. Usually, the values from dye solubilization experiments are higher than those from the surface tension method when the polymer concentration is kept constant and only the surfactant concentration is varied. Here, this trend was only maintained for the SLS system, whereas the polymer-containing systems exhibited the reverse trend. This anomaly is probably associated with the nontraditional experimental protocol followed here, where both the polymer and the surfactant were diluted simultaneously while maintaining a constant ratio. This protocol is more practical for consumer relevant situations, as when a personal care product is applied to the hair or skin, the surfactant and polymer are diluted at the same time during use. The tensiometric method is more sensitive to monomers, whereas the dye solubilization method is more sensitive to aggregates and therefore more accurate for determining the aggregation behavior. The polymers were also tested individually at various concentrations and the 17
results revealed that the absorbance values for PEG were never considerably higher than the absorbance of water, whereas the absorbance values for PVA began to increase at concentrations greater than 7,500 ppm (Supporting Information). This result reflects the more hydrophobic character of PVA.
Roles of PEG and PVA in Polymer–Surfactant Interactions The differences in the trends observed with PEG and PVA can be ascribed to the presence of hydroxyl side groups in the latter polymer. This structural difference has multiple implications. First, the presence of hydrophilic side groups facilitates PVA to adopt a more extended conformation in aqueous solution compared with that of PEG. Second, it is expected that the hydroxyl side groups of PVA interact with the SLS head groups via polar and/or hydrogen bonding interactions.65 Adsorption of SLS head group on PEG, on the other hand, drives SLS-PEG interaction.66, 67 Thus, it is reasonable to consider that that a hydrogen-bonding interaction in the presence of PVA would delocalize the negative charges of the SLS head group and facilitate shrinkage of the micelle, as evident from the SANS data (Supporting Information Figure S7). Such effect is absent in PEG. Therefore, the more specific nature of polymer–surfactant interaction via hydrogen bond formation in case of PVA differentiates its behavior from PEG-based system mainly governed by nonspecific adsorption mode.
Conclusion Although previous skin penetration studies have indicated that the irritation potential of SLS depends on the size of the surfactant–polymer assembly, the effect of the characteristics of the chosen polymer on the microstructure of the polymer–surfactant aggregate has not been previously determined.
46,47
The results of the present study
demonstrated that in addition to the assembly size, the assembly shape and the physicochemical properties of the polymer play a crucial role in fine-tuning the micelle behavior and polymer–surfactant interactions. The differential effects of two chemically 18
distinct polymers PEG and PVA, on existing micellar assemblies of SLS were assessed using SANS, NMR, tensiometric, and dye solubilization experiments. The SANS data revealed that the presence of PVA induced considerable shrinkage of the micellar size while maintaining the spherical shape of the SLS micelle. In contrast, an ellipsoid model was preferred to elucidate the shape and size of the micelle in the presence of PEG. The interaction of the SLS micelle with PVA appeared to be stronger than that with PEG, as shown by the NMR, and dye solubilization measurements. A hydrogen-bond-driven polymer–surfactant interaction was found to be dominant in the SLS/PVA system, in contrast to the surfactant adsorption on polymer in the SLS+PEG system. From these results, it is clear that the shape, size, and intermicellar interactions are affected by the presence of polymers. Thus, several parameters other than size are involved in determining the fate of a surfactant–polymer couple: (i) the chemical structure of the polymer, (ii) the polymer conformation, (iii) the presence of side groups, and (iv) the hydrophilicity of the polymer and its side group. Therefore, it is difficult to realize complete control over a surfactant system using a single polymer. Finally, the experimental strategy adopted in the present study closely resembles consumer relevant conditions. Therefore, our present study reveals a clearer structure–function relationship in a polymer–surfactant system that connects the macroscopic and molecular domains under consumer relevant conditions. Furthermore, the identified parameters for polymer selection provide the possibility of fine-tuning formulations to optimize property amplification.
Conflicts of Interests The authors declare no conflicts of interest.
Acknowledgements This work was primarily supported by start-up funds from UC (HK) and partially supported by the Procter and Gamble Company (Cincinnati, OH).
19
We would like to acknowledge Dr. Mike Weaver, Ryan Thompson, and Dr. Robert Glenn from P&G, Dr. Chris Garvey from ANSTO, and Dr. Vinod Aswal from Bhabha Atomic Research Center for valuable discussions. This work benefited from the use of the SasView application, originally developed under NSF award DMR-0520547. SasView contains code developed with funding from the European Union's Horizon 2020 research and innovation programme under the SINE2020 project, grant agreement No 654000. The High Flux Isotope Reactor, where Bio-SANS is located, are supported by US Department of Energy Office of Science User Facility. The Bio-SANS of the Center for Structural Molecular Biology is supported by the Office of Biological and Environmental Research of the US Department of Energy.
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27
Graphical abstract
Size 20 Å
Shape
SLS 22 Å
18 Å SLS + PVA
28
13 Å SLS + PEG
S0021-9797(19)30263-2 https://doi.org/10.1016/j.jcis.2019.02.081 YJCIS 24705
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
1 November 2018 22 February 2019 23 February 2019
Please cite this article as: C. Ade-Browne, A. Dawn, M. Mirzamani, S. Qian, H. Kumari, Differential Behavior of Sodium laurylsulfate Micelles in the Presence of Nonionic Polymers, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.02.081
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Differential Behavior of Sodium laurylsulfate Micelles in the Presence of Nonionic Polymers Chandra Ade-Browne,a Arnab Dawn,a Marzieh Mirzamani,a Shuo Qian,b Harshita Kumaria* a
James Winkle College of Pharmacy, 231 Albert Sabin Way, Cincinnati, OH 45267-0514,
USA Email: [email protected] b
Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Abstract Hypothesis: Theory and practice have proven that the cleansing properties and irritation potential of surfactants can be controlled with the addition of co-surfactants or polymers. The size of surfactant-polymer nanoassembly which differs from the pure surfactant micelle has been postulated to be the cause of the differences in a surfactant system’s ability to disrupt the skin barrier. However, a firm-structure function relationship connecting polymer and surfactant under a consumer relevant condition is yet to established. It is therefore hypothesized that apart from the size, the shape and the chemical nature of the polymer might play crucial roles.
Experiments: We used combined small-angle neutron scattering, nuclear magnetic resonance spectroscopy, tensiometry, and dye solubilization methods to investigate the shape, size, and intermolecular interactions involved in sodium laurylsulfate-based systems in the presence of two industrially important and chemically distinct polymers, polyethylene glycol and polyvinyl alcohol, adopting a consumer relevant protocol.
Findings: Apart from size, shape and inter-micellar interactions fine-tuned by the presence of the polymers are found to be the important factors. Secondly, the physicochemical property of the polymer including chemical structure, conformation, hydrophilicity, 1
presence of side groups, all can have crucial influence on polymer-surfactant interaction, micelle formation, and micelle stability.
Key Words: Micelle; Small-angle neutron scattering; Polymer; Self-assembly; Polymer– surfactant interaction
2
Introduction Surfactant self-assembly is of interest in a wide variety of fields because of the applications of surfactants in the healthcare, cosmetic, and petroleum industries, among others.1-4 At low concentrations, the surfactant molecules exist as monomers in aqueous solution. However, at the critical micelle concentration (CMC), the monomers selfassemble into micelles with hydrophobic core consisted by the hydrophobic tails facing inwards and the hydrophilic head groups facing outward. The amphiphilic nature of the assembly structure provides a chemical duality that are useful in cleansing processes or the encapsulation of organic actives. 5-8 For example, sodium laurylsulfate (SLS) is one of the most extensively used surfactants in the industrial sector because of its effective cleansing properties and low cost. SLS is a mixture, primarily consisting of sodium dodecylsulfate (SDS) with traces of other substances such as SDS with different degrees of ethoxylation, laurylsulfate, and other compounds. The properties of SDS have been well characterized with techniques such as surface tensiometry,9,10 nuclear magnetic resonance (NMR),11-14 calorimetry,10,15 and small-angle neutron scattering (SANS). In particular, SANS has been used to study the effect of temperature,16-18 salt species and concentration,11,19-22 clouding,20,23 interactions with polymers,9,10,14,15,24-30 and interactions with other (co)surfactants.12,20,23,31,32 The research interest in SLS goes beyond physical characterization and into the clinical dimension because of its ability to irritate human skin, with the degree of irritation (expressed as the irritation potential) depending on the formulation of the surfactant system.33,34 The irritation potential of a surfactant system is associated with its ability to disrupt the skin barrier.35-41 For the personal care industry, it is important to understand and correlate these two key issues, namely, the physical properties and the irritation potential of the surfactants, as perpetuated by the ever-growing demands of consumers for formulations that are milder than currently marketed products containing anionic surfactants and anionics with betaine technology. Thus, it is essential to investigate the surfactant irritation mechanism in order to mitigate it. 3
There are several protocols for modulating the mildness of a surfactant system, including increasing the degree of ethoxylation,40 adding other surfactant,33,42-44 and/or incorporating polymer(s).42-45 However, what is known in practice has not been resolved into a firm structure–function relationship. In response, the Blankschtein group proposed a size-based theory, the “micelle penetration model,” based on their 14C-radiolabelling and dynamic light scattering (DLS) studies of the penetration of pure SDS and SDS with polyethylene glycol (PEG; molecular weight 8,000 g/mol) through porcine skin. 46 According to this model, both micelles and monomers are able to penetrate porcine skin and cause irritation. However, the larger SDS+PEG assembly penetrated less than the smaller, unbound SDS micelles. As a result, they postulated that the irritation potential of a surfactant is dependent on the size of the surfactant assembly body.
SDS
PEG
PVA
Figure 1. Molecular structures of various compounds used in the study.
Later, McCardy and coworkers performed a follow-up study using human cadaver skin, adding a second polymer, polyvinyl alcohol (PVA; molecular weight 17,783 g/mol), and using SLS instead of SDS.47 This results of this study were similar to those of the Blankschtein study, as the penetration of
14
C-SDS from the SLS+PEG micellar
nanoassembly was lower than that of the SLS micellar nanoassembly. Furthermore, the SLS+PVA micellar nanoassembly penetrated less than the SLS nanoassembly but was less effective than SLS+PEG at reducing 14C-SDS penetration into human skin. In this context, we realized that comparative studies on polymer–surfactant binary systems are indispensable in assessing the contributions from various parameters to the control of the size, shape, and assembly behavior of the system. For example, the 4
contribution from the shape of an assembly system to the surfactant action remains underexplored.
In
addition,
different
polymers
have
different
conformations,
hydrophilicities, and can be further differentiated based on side chains (branching). In the present study, we investigated the size, shape, and assembly behaviors of SLS micellar systems in the presence of PEG and PVA (Figure 1). These polymers were selected for two reasons: first, both are widely used in consumer products; and second, unlike PEG, PVA possesses a polar side group that is expected to interact with polar substrates away from the polymer backbone. We have utilized techniques including SANS, 1H-NMR spectroscopy, tensiometry, and dye solubilization in a complementary manner to study SLS, SLS with PEG, and SLS with PVA. While SANS offers comprehensive information about the physical parameters (size and shape) of the various assembly structures, NMR spectroscopy addresses the interaction parameters at the molecular level. In contrast, tensiometric and dye solubilization experiments reveal differences in the aggregation behaviors. Furthermore, in the present work, we adopted a strategy in which the aggregation behavior was studied as the concentrations of the surfactant and the polymer were equally diluted, while their ratio was maintained. This approach is atypical for such studies, but it is more relevant in a consumer product scenario. Therefore, the novelty of the present work lies in the comparative assessment of the micellar systems in the presence of multiple polymeric systems under consumer relevant experimental conditions. We expect that the results of the present study will aid in the selection of appropriate combinations of surfactants and polymers for milder formulations.
Experimental Materials SLS, PEG, and PVA were all provided by Procter & Gamble, Co. Specifically, SLS (29 wt% in H2O) was sourced from BASF, PEG from Sigma-Aldrich, and PVA (35 wt% in H2O) from Kuraray Poval and used without further purification. D 2O with 99.9 atom% D from Cambridge Isotope Laboratories was used for SANS experiments. D 2O with 99.9 5
atom% D and 0.05 wt% 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt from Sigma-Aldrich were used for NMR experiments. SANS Studies SANS experiments were performed using the Bio-SANS CG3 instrument at the High-Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL)
48
in Oak Ridge, TN
and the NG10 beamline at the NIST Center for Neutron Research at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. At both facilities, the incident neutron wavelength was set to 6 Å with about 15% spread. The SANS data for SLS+PVA were collected at NIST using titanium cells (1 mm path length) with quartz windows at 25°C. The SANS data for SLS and SLS+PEG were collected at the Bio-SANS using quartz banjo cells (1 mm path length) at room temperature (RT, 25°C). Measurements were made at different sample-to-detector distances to cover a scattering vector (Q) range of 0.01–0.8 Å-1 for the SLS and SLS+PEG samples. For the SLS+PVA samples, the data were collected over a Q-range of 0.003–0.5 Å-1. Background corrections, accounting for empty cell contributions, sample transmission, and background scattering, were made to each spectrum by the facilities supplied reduction software.49 The data was modeled with SasView (further details can be found in the Supporting Information). 50 NMR Studies 1
H-NMR spectra were obtained using a 400 MHz NMR spectrometer (Bruker AV-400,
Coventry, UK). The samples for analysis were prepared a day in advance to replicate the procedure used for the SANS experiments. A volume of 0.5 mL of the test formulations was prepared in D2O (99.9 atom% D) with an internal standard of 0.05 wt% 3(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt. Tensiometric Studies Three 50 mL aqueous samples were made: one consisting of 50 mM SLS, one consisting of 50 mM SLS and 2 wt% PEG, and one consisting of 50 mM SLS and 2 wt% PVA. A 20 wt% PEG stock was made by weighing out dry PEG, adding water, gently heating for a few minutes until PEG was fully dissolved, and finally allowing the heated solution to cool 6
to RT. The 35 wt% PVA raw material was diluted with water to make an 18.7 wt% PVA stock. The components were added in the order of Millipore (Burlington, MA, USA) H 2O, 29% SLS stock, and then polymer stock if needed. The samples were vortexed for a few seconds after SLS addition and again after polymer addition. All samples were allowed to equilibrate for at least 48 h prior to analysis. The tensiometric data were obtained using the Wilhelmy plate method with a KRÜSS K100 force tensiometer (KRÜSS GmbH, Hamburg, Germany). The system was programmed to take surface tension measurements for 180 s to ensure that the average value reflected the equilibrium value. After the starting concentration was measured, 23.57 mL DI water was added to the solution. The solution was stirred for 2-3 min while the plate was removed and rinsed with DI water, acetone, and DI water again. The plate was then flamed with a propane flame until it turned red to remove surface-active impurities that could have accumulated on the plate during the previous measurement. The plate was reinstalled and allowed to cool, and then 23.57 mL sample was removed so the sample volume was again 50 mL before the next measurement was taken. This process was repeated for each dilution, for a total of 15 measurements. Dye Solubilization Experiments Several concentrations of each surfactant system were prepared in Millipore H2O, including the concentrations of interest: 50 mM SLS, 50 mM SLS + 2 wt% PEG, and 50 mM SLS + 2 wt% PVA. The investigated concentrations ranged from 14,418 ppm (50 mM SLS) to 1 ppm, while the same surfactant-to-polymer ratio was maintained. The same amount of Orange OT dye (0.2 wt%) was added to each solution, which was then shaken for 48 h at approximately 22 °C, followed by centrifugation and filtration. The supernatant was collected and the absorbance was measured at 470 nm and 303 K using a Molecular Devices SpectraMax 384 Plus microplate reader.51 Individual polymer samples were also analyzed using the same protocol, but the investigated concentration range was 150,000 ppm to 1,000 ppm.
Results and Discussion 7
SANS Studies SANS experiments were performed with D2O solutions of SLS alone (50 mM), and after the addition of either PEG (2 wt%) or PVA (2 wt%). The scattering cross section obtained in SANS is a measure of how large the nucleus of the particle appears to the neutron, which is determined by how strongly the neutrons scatter from it. For monodisperse interacting particles, the coherent differential scattering cross section (dΣ/dΩ) can be expressed as:19,52 (1) where n is the number density of micelles, ρs and ρm are the contrast variations between the scattering length densities of the solvent and the micelle, respectively, and V is the micellar volume. Furthermore, the shape and size of the micelle is described by the form factor P(Q), the spatial arrangement of different micelles is described by the structure factor S(Q), and B is the background of incoherent scattering. The scattering length density (SLD) was calculated as 3.37×10-7 Å-2 for SLS, whereas the scattering length densities for the SLS+PEG and SLS+PVA nanoassemblies were calculated as 6.14×10-6 Å-2 and 5.9×10-6 Å-2, respectively. All calculations were performed using the SLD calculator provided on the NIST website. The SANS data for the samples were fitted to spheroidal and ellipsoidal models. A strong correlation peak was observed for each sample (Figure 2). This correlation peak is the result of long-range Coulombic interactions. As this peak persisted in the presence of micelles, indicating that electrostatic interactions persist, the Hayter–Penfold mean spherical approximation (MSA) was used as the structure factor.53, 54
8
Figure 2. SANS data for 50 mM SLS (blue), 50 mM SLS + 2 wt% PEG (red), and 50 mM SLS + 2 wt% PVA (green). In the case of 50 mM SLS and 50 mM SLS + 2 wt% PEG, both the sphere and the ellipsoid models fit well. At a concentration of 50 mM, SLS has a shape averaged between a sphere and an ellipsoid (Figure 3).
Figure 3. From left to right, 50 mM SLS fit to the sphere model, 50 mM SLS + 2 wt% PEG fit to the ellipsoid model, and 50 mM SLS + 2 wt% PVA fit to the sphere model. This is evident from the similar charge, volume fraction, and dimensions obtained when fitted to the sphere or ellipsoid models (Supporting Information). Thus, this analyses 9
provides an average of the solution dynamics. When PEG is added to SLS, the micelles become more ellipsoidal with radii of 13.4 and 22.6 Å. A summary of the best-fit models is provided in Table 1.
Table 1. Parameters calculated from the SANS results based on fits to different models. Model 50 mM SLS
Volume (Å3) 35041
Radius (Å)
Sphere
20.3 ± 0.02 13.4 ± 0.03 50 mM SLS + 2 (R) Ellipsoid 28869 wt% PEG 22.6 ± 0.06 (ωR) 50 mM SLS + 2 18.04 ± Sphere 24592 wt% PVA 0.139
Interestingly, while the SLS and SLS+PVA systems are best fitted to a sphere model, the SLS+PEG system is best fitted to an ellipsoid model. This shape variation in the presence of PEG is an indication of a slight distortion of the native micelle. Thus, instead of the radius, we calculated and compared the volumes of the three nanoassemblies (Table 1). It appears that the micellar volume decreases upon addition of the polymers, and it is the smallest in the presence of PVA. As mentioned earlier, a correlation peak is present in all systems, which is typical of ionic surfactants. This peak was defined as Qmax because it corresponds to the highest intensity Q-value. Qmax can be used to approximate the distance between neighboring micelles.26,28,55 (2) Table 2. Summary of the correlation peak positions and the calculated average intermicellar distances. Qmax (Å-1) 50 mM SLS 0.04711 50 mM SLS + 2 wt% 0.0593
dmic (Å) 133.37 105.95 10
PEG 50 mM SLS + 2 wt% 0.04297 PVA
146.22
In the presence of the different polymers, there is a distinct difference in the solution properties. When the SDS micelles are in the presence of PEG, the intermicellar distance is decreased by approximately 21% (Table 2). In contrast, in the presence of PVA, the intermicellar distance is increased by approximately 10%. These differences can be attributed to dissimilarities in how the polymers wrap around the micelles, as discussed further in the NMR results. 1
H-NMR Studies
To obtain insight into the various intramolecular and intermolecular forces involved in the self-assembly process, 1H-NMR studies were conducted on surfactant systems in D2O in the presence and absence of polymers. Although, SLS may contain traces of other substances, as mentioned earlier, the individual signals of SDS can be unambiguously assigned, where the hydrophilic and hydrophobic domains are clearly distinguishable. The effect of the presence of polymers on the proton signals of SLS is summarized in Figure 4. Upon the addition of both polymers (PEG and PVA), the protons associated with the hydrophilic part of the surfactant (head groups) experience an upfield shift, signifying a shielding effect. This shift is clear evidence that the polymers are in the proximity of the micelles and that they interact with the polar head groups. Interestingly, in the presence of PVA, the protons associated with the hydrophobic tail of SLS undergo a significant downfield shift, signifying an enhanced hydrophobic interaction within the micelle. This change could be caused by shrinkage (compaction) of the existing micelle in the presence of PVA, which is consistent with the decrease in micellar volume observed in the SANS results. A similar but smaller change is observed for the system with PEG, where only methyl protons show a downfield shift. This result is also consistent with the SANS data, which showed a small decrease of micellar volume in the presence of PEG. However, the 11
possibility of insertion of part of the polymer chain within the SLS micelle is ruled out because such a situation would result in shielding of the associated domain of the SLS molecule. In addition, the hydrophilic polymer would prefer not to interact with the hydrophobic domain of SLS. This configuration is reasonable because the polymers were added after the micelle formation. Hydrophilic
Hydrophobic
-(CH2)9-CH3
-O-CH2-O-CH2-CH2-
-CH3
(a)
(b)
(c)
Figure 4. Diagram showing hydrophilic and hydrophobic protons in SDS, and partial NMR spectra of (a) SLS + 2% PVA, (b) SLS and (c) SLS + 2%PEG, in D2O. The individual signals of SDS protons are assigned and indicated in red. Tensiometric Studies So far, we have investigated the effect of polymer addition on pre-formed SLS micelles. To obtain information about the micelle stability in the presence of added polymers, 12
tensiometric measurements were performed by diluting the SLS only, SLS with PEG, and SLS with PVA systems (Figure 5). The starting concentrations of these solutions were the same as those used in the SANS experiments. The systems were diluted in a step-wise manner until the endpoint was reached. The advantage of this experimental design is that it simulates the way in which a product is diluted when used by the consumer, as in practice both the surfactant and polymer are diluted at the same time. Another advantage is the industrial relevance of these systems, as the industrial-grade materials used contain the same kind of impurities at similar levels as would be found in commercially available personal care products. Thus, the studied systems and their results would be reflective of what occurs during consumer usage.
Figure 5. Tensiometric data for 50 mM SLS, 50 mM SLS + 2 wt% PEG, and 50 mM SLS + 2 wt% PVA. The black lines (solid for SLS, dashed for SLS + PEG, dotted dash for SLS + PVA) on each curve are logarithmic trendlines for the straight-line portions on either side of the minimum. The equations for the trendlines were used to calculate the values shown in Table 3. Figure 5 shows the surface tension data for SLS alone and with each polymer. Each curve has a minimum, caused by the presence of surface-active impurities lowering the 13
surface tension in conjunction with the SLS, which are then dissolved in micelles as more micelles form at increasing surfactant concentrations. 56 Depending on the source of the raw material in the SLS used for this work, small amounts of unreacted lauryl alcohol, other sodium alkyl sulfates, or polyvalent metal ions such as Ca 2+ could be present. Given that the SLS is industrial grade, the curves were expected to have a minimum. Polymer addition causes noticeable changes in the surface tension curves of SLS. Although the position of the minimum remains approximately the same between each curve, the slopes on either side of the dip and the minimum and final surface tension values are affected differently based on the polymer added. The change in slopes on either side of the minimum indicate that PEG and PVA affect the CMC, while the change in minimum and final surface tension values upon polymer addition suggest that both polymers interact with the impurities and SLS monomers. To determine the effects of the polymers on SLS, the CMC, the maximum surface excess concentration (Γmax), and the minimum area occupied by the surfactant at the air– water interface (Amin) were calculated for all three systems. The CMC values were obtained using the typical method of drawing trendlines on each side of the minimum, finding the intersection of the trendlines, and taking the x value of the intersection as the CMC, as shown in Figure 5. The Γ max and Amin values were calculated using the following equations: (3) where n is the number of species dependent on the surfactant concentration (in this case 2, as SLS is an ionic surfactant
57, 58
), R is the universal gas constant (8.314 J/(mol·K)), T is
the absolute temperature (293.15 K), γ is the surface tension in mN/m, C is the surfactant concentration in mol/L, and dγ/d(ln C) is the slope of the surface tension vs. ln C curve at concentrations just below the CMC; and (4) where NA is Avogadro’s number and 1023 is the unit conversion factor. 14
Table 3. CMC, surface tension at the CMC (γcmc), maximum surface excess concentration (Γmax), and minimum area/molecule (Amin) for SLS alone and in the presence of PEG and PVA. Parameter CMC (ppm) CMC (mM)
SLS 364 1.26
γcmc (mN/m) 32.12 2 10 × Γmax (mmol/m ) 2.80 Amin (Å2) 59.21 3
SLS + PEG SLS + PVA 441 386 1.53 1.34 34.35 2.09 79.36
35.56 2.01 82.56
Table 3 shows the calculated CMC, Γ max and Amin values. The CMC of SLS is lower than that of pure SDS, which is expected based on the surface-active impurities present in SLS.59 Adding PEG and PVA to SLS resulted in an increased CMC, with PEG causing the greater increase while PVA caused a more modest increase. The cause of the increased CMC in the presence of PEG and PVA is not fully clear, but it might be related to the higher solubilization of the surfactant molecules in the presence of the polymer. This could be supported by the larger minimum and final surface tension values when PEG or PVA is present compared to only SLS. The surface excess concentrations are all positive, as is expected for surfactant systems; however, compared with SLS alone the values decreased in the presence of both PEG and PVA, which also means that the minimum area per SLS molecule at the air-water interface increased with PEG and PVA. These trends could also be related to the surfactant becoming more soluble in the presence of the polymers as suggested above, but it is still difficult to identify their cause because impurities such as Ca2+, Mg2+, and alcohols are present in the systems. As had been shown by Cross & Jayson,60 Li et. al.,61 Eastoe et. al.,62 and Mysels et. al.63,
64
, multivalent metal ion
impurities can interact with the surfactant to form surface-active impurities while hydrophobic impurities like alcohols are themselves highly surface-active but adsorb slowly to the air-water interface. With these surface-active impurities present in these 15
systems and contributing to the surface tension results shown in Figure 5, the use of the Gibbs adsorption equation is limited. Additionally, it cannot be conclusively stated whether the pre-CMC surface tension trends are caused by the polymer only interacting with the SLS monomers or by the polymer interacting with some combination of SLS monomers and the surface-active impurities, using tensiometry alone. Given that the dip for the SLS + PVA curve is shallower compared to the SLS and SLS + PEG curves, it appears that the surface-active impurities have a lesser influence on the surface tension in the SLS + PVA system. Overall, the differing tensiometric trends observed for the SLS + PEG and SLS + PVA systems indicate that above the CMC SLS micelles stabilize against dilution in distinct manners depending on the polymer present, while below the CMC the SLS monomers undergo different dilution processes dependent on the polymer in the system. In other words, the nature of polymer–surfactant interactions above and below the CMC of the surfactant can be substantially different.
Dye Solubilization Although the surface tension method is widely used in surfactant studies, it is very sensitive to the presence of impurities within the system.59 This issue is even more crucial here because of the use of commercial-grade SLS, which contains traces of various impurities. Therefore, it was deemed necessary to use the dye solubilization method with Orange OT, a water-insoluble dye, as an alternative tool to determine the aggregation behavior of the various surfactant systems accurately. The surfactant systems were prepared at several concentrations but with the same amount of Orange OT added to the solutions. Typically, the resulting graphs exhibit initial points with an absorbance similar to that of water followed by a sharp increase in slope. This change in slope is associated with aggregate formation and subsequent solubilization of the dye. Subsequently, the absorbance increases with the concentration of the surfactant until a plateau is reached. Overall, similar trends were observed in the SLS systems in the presence and in the absence of the polymers (Supporting Information Figure S1 and S2). Here, the change in 16
slope is reflective of the collective contributions from polymer–surfactant interactions and micelle formation. The aggregation behavior of the different systems revealed from the dye solubilization experiments (Table 4) is indicative of the fact that micelle formation is favored in the presence of the polymers.51 Furthermore, the influence of PVA on SLS micelle/micellization is even greater than that of PEG.
Table 4. Comparison of the CMC values obtained using the surface tension method with the point of aggregation calculated using the dye solubilization method. CMC, Surface Tension Method (ppm) SLS SLS + PEG SLS + PVA
364 441
Aggregation Point, Dye Solubilization Method (ppm) 650 350
386
190
Next, the values reflective of SLS aggregation obtained from the tensiometric and dye solubilization experiments were compared. Usually, the values from dye solubilization experiments are higher than those from the surface tension method when the polymer concentration is kept constant and only the surfactant concentration is varied. Here, this trend was only maintained for the SLS system, whereas the polymer-containing systems exhibited the reverse trend. This anomaly is probably associated with the nontraditional experimental protocol followed here, where both the polymer and the surfactant were diluted simultaneously while maintaining a constant ratio. This protocol is more practical for consumer relevant situations, as when a personal care product is applied to the hair or skin, the surfactant and polymer are diluted at the same time during use. The tensiometric method is more sensitive to monomers, whereas the dye solubilization method is more sensitive to aggregates and therefore more accurate for determining the aggregation behavior. The polymers were also tested individually at various concentrations and the 17
results revealed that the absorbance values for PEG were never considerably higher than the absorbance of water, whereas the absorbance values for PVA began to increase at concentrations greater than 7,500 ppm (Supporting Information). This result reflects the more hydrophobic character of PVA.
Roles of PEG and PVA in Polymer–Surfactant Interactions The differences in the trends observed with PEG and PVA can be ascribed to the presence of hydroxyl side groups in the latter polymer. This structural difference has multiple implications. First, the presence of hydrophilic side groups facilitates PVA to adopt a more extended conformation in aqueous solution compared with that of PEG. Second, it is expected that the hydroxyl side groups of PVA interact with the SLS head groups via polar and/or hydrogen bonding interactions.65 Adsorption of SLS head group on PEG, on the other hand, drives SLS-PEG interaction.66, 67 Thus, it is reasonable to consider that that a hydrogen-bonding interaction in the presence of PVA would delocalize the negative charges of the SLS head group and facilitate shrinkage of the micelle, as evident from the SANS data (Supporting Information Figure S7). Such effect is absent in PEG. Therefore, the more specific nature of polymer–surfactant interaction via hydrogen bond formation in case of PVA differentiates its behavior from PEG-based system mainly governed by nonspecific adsorption mode.
Conclusion Although previous skin penetration studies have indicated that the irritation potential of SLS depends on the size of the surfactant–polymer assembly, the effect of the characteristics of the chosen polymer on the microstructure of the polymer–surfactant aggregate has not been previously determined.
46,47
The results of the present study
demonstrated that in addition to the assembly size, the assembly shape and the physicochemical properties of the polymer play a crucial role in fine-tuning the micelle behavior and polymer–surfactant interactions. The differential effects of two chemically 18
distinct polymers PEG and PVA, on existing micellar assemblies of SLS were assessed using SANS, NMR, tensiometric, and dye solubilization experiments. The SANS data revealed that the presence of PVA induced considerable shrinkage of the micellar size while maintaining the spherical shape of the SLS micelle. In contrast, an ellipsoid model was preferred to elucidate the shape and size of the micelle in the presence of PEG. The interaction of the SLS micelle with PVA appeared to be stronger than that with PEG, as shown by the NMR, and dye solubilization measurements. A hydrogen-bond-driven polymer–surfactant interaction was found to be dominant in the SLS/PVA system, in contrast to the surfactant adsorption on polymer in the SLS+PEG system. From these results, it is clear that the shape, size, and intermicellar interactions are affected by the presence of polymers. Thus, several parameters other than size are involved in determining the fate of a surfactant–polymer couple: (i) the chemical structure of the polymer, (ii) the polymer conformation, (iii) the presence of side groups, and (iv) the hydrophilicity of the polymer and its side group. Therefore, it is difficult to realize complete control over a surfactant system using a single polymer. Finally, the experimental strategy adopted in the present study closely resembles consumer relevant conditions. Therefore, our present study reveals a clearer structure–function relationship in a polymer–surfactant system that connects the macroscopic and molecular domains under consumer relevant conditions. Furthermore, the identified parameters for polymer selection provide the possibility of fine-tuning formulations to optimize property amplification.
Conflicts of Interests The authors declare no conflicts of interest.
Acknowledgements This work was primarily supported by start-up funds from UC (HK) and partially supported by the Procter and Gamble Company (Cincinnati, OH).
19
We would like to acknowledge Dr. Mike Weaver, Ryan Thompson, and Dr. Robert Glenn from P&G, Dr. Chris Garvey from ANSTO, and Dr. Vinod Aswal from Bhabha Atomic Research Center for valuable discussions. This work benefited from the use of the SasView application, originally developed under NSF award DMR-0520547. SasView contains code developed with funding from the European Union's Horizon 2020 research and innovation programme under the SINE2020 project, grant agreement No 654000. The High Flux Isotope Reactor, where Bio-SANS is located, are supported by US Department of Energy Office of Science User Facility. The Bio-SANS of the Center for Structural Molecular Biology is supported by the Office of Biological and Environmental Research of the US Department of Energy.
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27
Graphical abstract
Size 20 Å
Shape
SLS 22 Å
18 Å SLS + PVA
28
13 Å SLS + PEG