9834
J. Phys. Chem. 1996, 100, 9834-9841
Surfactant Interactions with HEUR Associating Polymers Kewei Zhang, Bai Xu, Mitchell A. Winnik, and Peter M. Macdonald* Department of Chemistry and Erindale College, UniVersity of Toronto, Toronto, Ontario, Canada M5S 1A2 ReceiVed: NoVember 30, 1995; In Final Form: February 23, 1996X
The interactions between a HEUR (hydrophobically-modified ethoxylated urethane) AP (associating polymer) and the anionic surfactant SDS (sodium dodecyl sulfate) or the cationic surfactant DTAB (dodecyltrimethylammonium bromide) were studied using a combination of rheology measurements plus PGSE (pulsed-gradient spin-echo) NMR self-diffusion and NMR relaxation time measurements. For comparison, parallel experiments were performed using poly(ethylene oxide) (PEO) plus surfactant mixtures. At low concentrations (4 SDS per HEUR AP hydrophobe) added surfactant enhanced HEUR AP network formation. At high concentrations (33 SDS per HEUR AP hydrophobe) added surfactant totally disrupted the HEUR AP network and the HEUR AP self-diffusion came to resemble that of PEO. Both SDS and DTAB bound with greater affinity to the HEUR AP than to PEO. SDS bound with greater affinity than DTAB to either the HEUR AP or PEO. Despite the association of SDS with the ethylene oxide backbone of the HEUR AP, the results support the hydrophobe-replacement model as a description of the effects of surfactant on network formation in solutions of HEUR APs.
Introduction HEUR (hydrophobically-modified ethoxylated urethane) APs (associating polymers) are extensively used as rheology modifiers in paints and coatings, as well as in enhanced oil-recovery and antifreeze formulations.1 They consist of a poly(ethylene oxide) (PEO) backbone, chain-extended by diisocyanates and end-capped by long-chain alkanols. Their favorable properties and widespread use have spawned an intense experimental and theoretical effort to understand the origins of their viscoelastic behavior at the molecular level. These efforts culminated in the proposal of the “transient micellar network” model to explain these properties.2,3 For HEUR APs with C16 end-caps and molecular mass greater than 10 000 g mol-1 it is generally accepted that at low concentrations the HEUR APs self-associate into discrete micelles or “rosettes” consisting of a core of hydrophobic groups surrounded by a corona of polymer chains looping back into the core. With increasing polymer concentration a number of bridging chains appear and crosslink the hydrophobic micelles together into clusters forming, eventually, a network which spans the entire solution. Network propagation results in a large increase in the macroscopic viscosity. Since the crosslinks are physical rather than chemical in origin, the network is transient. Experimentally, one finds that the linear viscoelastic response can be described in terms of a single characteristic relaxation time which Annable has ascribed to the exit rate of hydrophobic end groups from the hydrophobic micelles of the network.2 Under steady shear, shear-thinning occurs with a sharp onset at a shear rate corresponding to the inverse of this relaxation time. We note that HEUR APs with sufficiently low molecular mass (M < 6000 g mol-1) at high concentrations form liquid-crystalline structures rather than networks.4 Surfactants interact strongly with HEUR APs and influence their network-forming tendencies. It is well-known, for instance, that SDS (sodium dodecyl sulfate) increases the viscosity of HEUR AP solutions.5-8 The details depend sensitively on the polymer concentration, molecular weight, length of the hydro* To whom correspondence should be addressed. Telephone: 905-8283805. Fax: 905-828-5425. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03558-1 CCC: $12.00
phobic end-cap, surfactant structure, and concentration. Annable et al5 explained this effect in terms of the ability of one surfactant molecule to replace one HEUR AP end group within the hydrophobic micelle. That end group is then free to bridge to a neighboring micelle and, thereby, propagate the network. At higher surfactant concentrations the hydrophobic micelles are solubilized and the network disintegrates. The picture is complicated by the fact that surfactants such as SDS can bind with high affinity to the water-soluble portion of the HEUR AP (i.e., the PEO backbone) in addition to its hydrophobic end groups.9,10 Studies correlating the macroscopic viscoelastic properties of HEUR AP plus surfactant mixtures with molecular level studies of the properties of individual components within the mixture have so far been few in number. The richness and complexity of these systems is such that a variety of techniques must be brought to bear to understand the origins of their behavior. Their bulk viscoelastic properties, for instance, are best characterized using rheology measurements.6 Spectroscopic techniques are the best means to extract molecular-level details. NMR self-diffusion measurements are particularly useful in complex systems because one may usually measure the diffusion of individual components independently and simultaneously, and diffusion is often uniquely sensitive to the the details of the molecular architecture.11,12 HEUR AP systems have been the subject of several extensive pulsedgradient spin-echo (PGSE) NMR diffusion studies.13-18 Studies of surfactant interactions with HEUR APs have focused primarily on HEUR AP + SDS (sodium dodecyl sulfate) mixtures and include rheological measurements plus Monte Carlo simulations,5 2H NMR relaxation measurements,18 PGSE NMR diffusion measurements,19,20 and EPR spin probe studies.21 HEUR AP + DTAB (dodecyltrimethylammonium bromide) mixtures have received less attention.19 We are not aware of any report in which bulk solution properties in HEUR AP + surfactant mixtures are correlated with molecular properties of the constituents of the mixture. In this article we describe combined rheological, PGSE NMR, and NMR relaxation time measurements on mixtures of HEUR AP with SDS or DTAB (dodecyltrimethylammonium bromide). For comparison we have performed parallel measurements on mixtures of PEO © 1996 American Chemical Society
Interactions with HEUR Associating Polymers (polyethylene oxide) with the same two anionic and cationic surfactants. The combination of these techniques allows us to correlate molecular details with macroscopic changes, to extract the surfactant/HEUR AP stoichiometries, to describe the associated structures present under given circumstances, and to evaluate the steps along the road to HEUR AP network dissolution. Experimental Section Materials and Solution Preparation. A HEUR AP, designated 46 RCHX22-3, was obtained from Dr. Richard Jenkins, Union Carbide Chemicals and Plastics Co., Inc., UCAR Emulsion Systems, Cary, NC. The general molecular structure of the HEUR AP corresponds to R-O-(DI-PEO)m-DI-OR. Here PEO is a poly(ethylene oxide) segment of nominal molecular weight 8200, DI is an isophorone diisocyanate group that links the PEO segments with a polymerization index of m ) 6, and R is a terminating hydrophobic end group (C16H33). The molecular weight is therefore approximately 51 000. This HEUR AP will be designated here as AT22-3, as in a previous report.22 The chemical characterization of this HEUR AP using 1H NMR has been reported previously,23 and shows that AT22-3 contains on average 1.7 hydrophobic groups per chain. This suggests that 70% of the AT22-3 chains are doubly end-capped while 30% are singly end-capped. Traces of residual solvent or unreacted long-chain alcohol were removed by recrystallizing the polymer from ethyl acetate and lyophilizing from benzene at room temperature. PEO of molecular weight 40 000 was obtained from Fluka, Germany, and used without further purification. Sodium dodecyl sulfate (SDS) was supplied by Merck and used as received. 2H2O (99.7 atom % 2H) was obtained from Merck. Polymer/surfactant mixtures were prepared by serial dilution of aqueous stock solutions (5 wt % polymer or surfactant in 2H O) into standard 5 mm NMR tubes. Care was taken to avoid 2 exposing the solutions to light. All samples were thoroughly mixed and allowed to equilibrate in the dark at room temperature for at least 24 h before any measurements were performed. For longer term storage samples were held at 4 °C. Rheology Measurements. Low shear viscosities were measured at 25 °C on a Rheometrics RAA with a geometry of cone and plate (50 mm diameter). The instrument is controlled by a 486 personal computer for on-line data acquisition. Steady shear viscosity was obtained by measuring the torque as a function of the shear rate. The viscosity of solutions with a high concentration of surfactant was measured using a capillary viscometer. The specific viscosity, ηsp, is defined as the solution viscosity relative to that of water. PGSE NMR Self-Diffusion Measurements. Proton selfdiffusion measurements were performed at 25 °C using an MRI (magnetic resonance imaging) probe with actively shielded gradient coils (Doty Scientific, Columbia, SC) installed in a Chemagnetics CMX 300 NMR spectrometer operating at 300 MHz for protons. A standard Stejskal-Tanner PGSE sequence (90° - τ -180° - τ), with a gradient pulse during each τ delay, was employed.24 Two levels of gradient strength, 0.796 and 1.04 T/m, respectively, were used depending on the selfdiffusion coefficient. The gradient strength was calibrated with a sample of 10 wt % PEO in 2H2O for which the self-diffusion coefficient is known. The uncertainty in the measured values of the self-diffusion coefficients was less than (5%. According to Stejskal and Tanner,24 the amplitude I of the spin-echo signal induced by the spin-echo (90° - τ -180° - τ) radio frequency pulse sequence in the presence of a pair
J. Phys. Chem., Vol. 100, No. 23, 1996 9835 of magnetic field gradient pulses of amplitude G and duration δ, separated by a time ∆, is given by
I ) I0 exp[-(γGδ)2D(∆ - δ/3)]
(1)
where I0 is a constant, γ is the magnetogyric ratio for the nuclei studied, D is the self-diffusion coefficient, and the effects of the transverse relaxation time T2 are included in the term I0. In our experiments ∆ is kept constant while δ is varied. The selfdiffusion coefficient is calculated from eq 1 using at least 10 different values of δ and the calibrated value of the gradient strength. The self-diffusion measurement directly monitors the random motions of individual molecules over the time interval (∆ - δ/3). The molecule’s mean square displacement in one dimension, 〈X2〉, and the self-diffusion coefficient D are related according to eq 2.
〈X2〉 ) 2D(∆ - δ/3)
(2)
For surfactants the self-diffusion coefficients fall typically in the range of 10-10-10-12 m2 s-1. In the present study ∆ has been set typically to 250-350 ms for polymer self-diffusion measurements and 60-100 ms for surfactant self-diffusion measurements. Consequently, during this diffusion time period the molecules diffuse over distances much larger than the size of an individual surfactant micelle or the radius of gyration of the polymer. Hence, the observed self-diffusion coefficients reflect the mobility of surfactant monomers, or entire micelles, or polymers, or surfactant-polymer complexes. Transverse Relaxation Time Measurements. The transverse relaxation time T2 was measured at 25 °C on the same NMR spectrometer using the spin-echo pulse sequence, 90° - τ -180° - τ. Values of T2 were obtained from the slope of a plot of ln I/I0 versus τ. Results and Discussion Rheology of HEUR AP + Surfactant Mixtures. Under shear stress, aqueous solutions of HEUR APs, including AT223, exhibit a combination of Newtonian behavior at low shear rates with shear-thinning at relatively high shear rates.7 This rheological behavior is modeled in terms of a “transient micellar network”, formed by linking together hydrophobic micelles into a network, thereby increasing the bulk viscosity.2,3 The association of any hydrophobic end group with a given micelle is transient and characterized by a single particular relaxation time for the entire network. When the shear rate exceeds the inverse of this characteristic relaxation time the network breaks down and shear-thinning is observed. The effects of the surfactants SDS and DTAB on the shear viscosity of an aqueous solution of 1 wt % AT22-3 are illustrated in parts A and B, respectively, of Figure 1. In the absence of surfactant the AT22-3 solution displays the expected combination of high viscosity at low shear rates plus shear-thinning at high shear rates. Adding a very low concentration of either SDS or DTAB, i.e., far below their respective cmc, results in an order-of-magnitude increase in the low-shear viscosity. Simultaneously, the shear rate at which shear-thinning is first observed shifts to lower shear rates. More DTAB than SDS is required to achieve a comparable viscosity enhancement. The rheological response of AT22-3 to low surfactant concentrations parallels precisely the response observed when the concentration of AT22-3 is increased. Higher AT22-3 concentrations encourage network formation for two reasons.22 First, the number of hydrophobic micelles, and the proportion of bridging to looping chains, increases. Second, the characteristic transient relaxation
9836 J. Phys. Chem., Vol. 100, No. 23, 1996
Zhang et al.
Figure 3. The self-diffusion coefficient of the HEUR AP and the corresponding zero shear rate viscosity as a function of the concentration of added SDS or DTAB. Either SDS (closed symbols) or DTAB (open symbols) was added to 1 wt % AT22-3 in D2O. The resulting selfdiffusion coefficient (squares) and specific viscosity (triangles) were measured as described in the text. In the absence of surfactant the selfdiffusion coefficient of 1 wt % AT22-3 is 7.5 × 10-14 m2 s-1.
Figure 1. (A) Steady shear viscosity profiles of 1 wt % AT22-3 in D2O in the presence of various concentrations of SDS. (a) No SDS, (b) 0.79 mM SDS, and (c) 1.44 mM SDS. (B) Steady shear viscosity profiles of 1 wt % AT22-3 in D2O in the presence of various concentrations of DTAB. (a) No DTAB, (b) 1.88 mM DTAB, and (c) 5.17 mM DTAB.
Figure 2. Zero shear rate viscosities of 1 wt % AT22-3 in D2O as a function of surfactant concentration. Circles, SDS. Squares, DTAB. The solid and dashed lines serve to guide the eye.
time increases, an effect which is likewise manifest in the observation that the onset of shear-thinning shifts to lower shear rates with increasing polymer concentration. By analogy, it seems that low SDS concentrations, and to a lesser degree DTAB, increase the number of hydrophobic micelles, the proportion of bridging to looping chains, and the characteristic transient relaxation time of the system. At surfactant concentrations only slightly higher than those shown in Figure 1 there is a sudden and catastrophic two-ordersof-magnitude decrease in the viscosity of a 1 wt % AT22-3 solution. Consequently, the specific viscosity, ηsp, is a better basis for comparing across a wider range of surfactant concentrations, as shown in Figure 2 for 1 wt % AT22-3 mixed with either SDS or DTAB. In the absence of surfactants ηsp is about 35 cP, which is typical of the viscosity expected for 1.0 wt % AT22-3.22 With increasing surfactant concentration, both SDS and DTAB first increase ηsp by a factor of 10 and then decrease ηsp at higher surfactant concentrations, eventually reaching a
viscosity an order-of-magnitude lower than that of the surfactantfree 1 wt % AT22-3 solution. For SDS the viscosity maximum occurs at about 1.5 mM SDS (equivalent to 4 SDS per AT22-3 hydrophobe), while the low-viscosity region is reached at about 13 mM SDS (equivalent to 33 SDS per AT22-3 hydrophobe). For DTAB the viscosity maximum is achieved with about 5 mM DTAB (equivalent to 12 DTAB per AT22-3 hydrophobe), while the low-viscosity regime is reached upon adding just over 20 mM DTAB (equivalent to 50 DTAB per AT22-3 hydrophobe). The hydrophobic domains which form when HEUR APs, such as AT22-3, are mixed with water contain about 20 hydrophobic groups.3,13 Adding surfactant molecules permits the formation of greater numbers of hydrophobic domains and decreases the average separation between domains. This permits the conversion of looping chains to bridging chains and propagation of the network with its attendant viscosity changes. When the number of surfactant molecules equals or exceeds the number of HEUR AP hydrophobes in a typical cluster, there can be at most one polymer hydrophobe per cluster, and the network will have dissipated. In effect the polymer hydrophobes will have been solublized by the surfactant. DTAB is much less effective at doing so than SDS, and the origin of this difference must be sought at the molecular level. HEUR AP Self-Diffusion in Mixtures with Surfactant. In the 1H NMR spectrum of an aqueous mixture of AT22-3 plus surfactant three well-resolved resonances are generally apparent: the HDO resonance from residual protons in D2O, the ethylene oxide resonance of the AT22-3 backbone, and the methylene resonance of the surfactant hydrocarbon chain. The latter will overlap with the methylene resonance of the hydrophobic end-caps of the HEUR AP. However, for the situations of concern here the surfactant concentration generally far exceeds that of the HEUR AP hydrophobic end-caps. Consequently, the decay of the intensity of the methylene resonance line in the PGSE NMR experiment is taken to reflect the diffusion of the surfactant molecules alone. Thus, it is possible to measure self-diffusion coefficients for the surfactant and the polymer separately and independently. Other expected resonance lines are visible, but their intensities relative to these three are much smaller and, therefore, less useful for PGSE NMR diffusion coefficient measurements. In Figure 3A the self-diffusion coefficient of AT22-3 in the presence of either SDS or DTAB is plotted as a function of the surfactant concentration. Shown as well is the corresponding specific viscosity of the particular AT22-3 plus surfactant
Interactions with HEUR Associating Polymers solution. Near the surfactant concentration yielding the maximum viscosity, diffusion is so slow that the AT22-3 selfdiffusion coefficient falls below the practical lower limit measureable with our current field gradient strength (1 × 10-14 m2 s-1). Only upon further addition of SDS, when the viscosity begins to decrease due to dissolution of the network, does the AT22-3 self-diffusion coefficient increase to the point that it is readily measured. For instance, at approximately 13 mM SDS the AT22-3 self-diffusion coefficient equals 7.5 × 10-12 m2 s-1. This may be compared with the value of 7.5 × 10-14 m2 s-1 measured for the self-diffusion coefficient of 1 wt % AT22-3 in the absence of surfactant.13,17 From approximately 13 to 50 mM SDS the specific viscosity and the AT22-3 self-diffusion coefficient are virtually independent of surfactant concentration. However, at higher SDS concentrations the specific viscosity begins to increase once again and the AT22-3 self-diffusion coefficient decreases. DTAB evidences similar, but not identical, effects on the specific viscosity and the AT22-3 self-diffusion coefficient, but only at higher concentrations than required by SDS. Thus, low DTAB concentrations cause the specific viscosity to increase so that the AT22-3 self-diffusion coefficient falls below our lower measuring limit of 1 × 10-14 m2 s-1, but the maximum effect occurs at about 5 mM DTAB, versus about 1.5 mM SDS. Likewise, higher DTAB concentrations decrease the specific viscosity so that the AT22-3 self-diffusion coefficient increases, but complete dissolution of the HEUR AP network requires about 30 mM DTAB, versus about 10 mM SDS. Eventually, the specific viscosity and the AT22-3 self-diffusion coefficient achieve values similar to those measured with SDS, but it requires about 50 mM DTAB to attain this state. A major difference in the response to the two surfactants is that with DTAB one never observes that the viscosity begins to increase again, or the AT22-3 self-diffusion coefficient to decrease again, in the fashion seen at the higher SDS concentrations. One further point regarding the diffusion of AT22-3 is that the self-diffusion coefficients were highly monodisperse in every case in which significant concentrations of surfactant were present. The same observation has been reported previously by Persson et al.20 In the absence of surfactant the self-diffusion coefficient of 1 wt % AT22-3 is highly polydisperse,13,17 which is manifest as a distinct nonlinearity in the intensity decay with increasing gradient pulse duration in a plot of ln(I/I0) versus δ2(∆ - δ/3). In these HEUR AP systems at these concentrations the polydispersity is attributed to a distribution of sizes of the diffusing unit within the nascent network. Presumably, one is observing the diffusion of oligomeric clusters of the original “rosettes”. When the HEUR AP network is disrupted by surfactants the remaining source of polydispersity in the selfdiffusion coefficient is the molecular weight distribution of the polymer itself, and this is relatively narrow. Consequently, one effect of surfactant is to homogenize the distribution of selfdiffusion coefficients. PEO Self-Diffusion in Mixtures with Surfactant. It is informative to compare the response of AT22-3 with that of a non-hydrophobically-modified analogue of similar molecular weight such as PEO (Mn 40 000). Figure 4 shows the selfdiffusion coefficient for PEO in a 1 wt % aqueous solution in the presence of either SDS or DTAB as a function of the surfactant concentration. Shown as well are the corresponding specific viscosities of the PEO-surfactant mixtures. Since it lacks hydrophobic modifications PEO does not form a network. Consequently its specific viscosity is very low, and its selfdiffusion coefficient is very high, relative to AT22-3. DTAB has little or no effect on either the PEO self-diffusion coefficient
J. Phys. Chem., Vol. 100, No. 23, 1996 9837
Figure 4. The self-diffusion coefficient of PEO and the corresponding zero shear rate viscosity as a function of the concentration of added SDS or DTAB. Either SDS (closed symbols) or DTAB (open symbols) was added to 1 wt % PEO in D2O. The resulting self-diffusion coefficient (squares) and specific viscosity (triangles) were measured as described in the text.
or the specific viscosity. SDS, on the other hand, produces a distinct decrease in the PEO self-diffusion coefficient while causing the specific viscosity to increase. Clearly, SDS interacts strongly with PEO while DTAB does not, a distinction long recognized.25,26 The response of PEO to SDS versus DTAB is highly reminiscent of the behavior of AT22-3 at higher SDS or DTAB concentrations (i.e., above 13 mM for SDS and above 50 mM for DTAB). This indicates that once the HEUR AP network is disintegrated the AT22-3 responds to surfactant in a manner essentially identical to that of a linear water-soluble polymer. One notes that the self-diffusion coefficients of AT22-3 and PEO at high surfactant concentration are not identical in the cases reported here. Instead, D0 for AT22-3 (∼8 × 10-12 m2 s-1) is significantly lower than D0 for PEO (∼1.7 × 10-11 m2 s-1), but this factor of 2 difference can be accounted for entirely in terms of the different specific viscosities of the two solutions combined with the lower molecular weight of PEO (Mn 40 000) versus AT22-3 (Mn 51 000). Surfactant Self-Diffusion in Mixtures with HEUR AP or PEO. The self-diffusion coefficients of SDS alone, and in the presence of either PEO or AT22-3, are shown as a function of SDS concentration in part A of Figure 5. Below an SDS concentration of about 4 mM, signal-to-noise considerations render it difficult to accurately measure surfactant self-diffusion and the diffusion data in Figure 5A are for higher concentrations only. Note again that in the 1H NMR spectrum of such mixtures the methylene resonances of the surfactant overlap with those of the AT22-3 hydrophobic end-caps. However, at surfactant concentrations of 4 mM and greater, corresponding to greater than 10 surfactant per AT22-3 hydrophobe, the surfactant contribution to this resonance is dominant. We note, further, that monoexponential echo attenuations are always observed for both SDS and DTAB. Moreover, in no case could we observe any dependence of the self-diffusion coefficient on the diffusion time. Therefore, exchange of the surfactant molecules between different sites is rapid relative to the diffusion time (∆ - δ/3), so that the apparent self-diffusion coefficient is a weighted-average of the self-diffusion coefficients in different sites. For SDS alone in aqueous solution the self-diffusion coefficient decreases progressively with increasing SDS concentration. Two effects are at work here. First, micellization of SDS occurs at its cmc of 8 mM27 and the large micelle diffuses more slowly than surfactant monomers. Thus the weighted-average self-diffusion coefficient is expected to decrease. Second, obstruction effects increase with increasing number of surfactant micelles28 which tends to decrease diffusion.
9838 J. Phys. Chem., Vol. 100, No. 23, 1996
Figure 5. (A) Self-diffusion coefficient of SDS as a function of SDS concentration in the presence of various polymers. Closed circles, no polymer. Triangles, 1 wt % PEO. Squares, 1 wt % AT22-3. (B) Selfdiffusion coefficient of DTAB as a function of DTAB concentration in the presence of various polymers. Closed circles, no polymer. Triangles, 1 wt % PEO. Squares, 1 wt % AT22-3.
In the presence of 1 wt % PEO, SDS diffusion is slower at all SDS concentrations. Two effects contribute. First, SDS is known to associate with PEO,20,29 and the center-of-mass diffusion of the large PEO chains should be slower than that of SDS in either its monomeric or micellized states. Second, the solution viscosity increases with SDS concentration, as shown in Figure 4, so that PEO diffusion decreases even further. Note that at the polymer concentration employed here (1.0 wt %) the polymer contribution to any obstruction effect is expected to be small.28,30,31 In the presence of 1 wt % AT22-3, at low SDS concentrations, there is an additional decrease in SDS self-diffusion, even relative to that observed with PEO. However, above 20 mM SDS, AT22-3 and PEO have nearly identical effects on SDS self-diffusion. The very real differences in the observed SDS self-diffusion coefficients in the presence of AT22-3 versus PEO at low SDS concentrations have two possible origins. First, the diffusion of AT22-3 is slower than that of PEO. For instance, a comparison of the diffusion data in Figures 3 and 4 reveal that for SDS concentrations less than 20 mM the AT22-3 self-diffusion coefficients are far smaller than those of PEO. Second, there is more SDS bound to AT22-3 versus PEO at these low SDS concentrations. The surfactant-polymer binding isotherms to be discussed below demonstrate that this is the case. The self-diffusion coefficients of DTAB alone, and in the presence of either PEO or AT22-3, are shown as a function of DTAB concentration in part B of Figure 5. In the absence of polymer, DTAB diffusion decreases with increasing surfactant concentration in a manner similar to that for SDS. However, at a given surfactant concentration the observed DTAB selfdiffusion coefficient is always greater than that of SDS. This reflects the higher cmc of DTAB (16 mM27) and the consequent difference in the populations of monomeric versus micellar surfactant. Adding PEO has no effect on DTAB diffusion at
Zhang et al.
Figure 6. (A) T2 relaxation time of SDS as a function of SDS concentration in the presence of various polymers. Closed circles, no polymer. Triangles, 1 wt % PEO. Squares, 1 wt % AT22-3. (B) T2 relaxation time of DTAB as a function of DTAB concentration in the presence of various polymers. Closed circles, no polymer. Triangles, 1 wt % PEO. Squares, 1 wt % AT22-3.
DTAB concentrations below 10 mM but results in a small decrease at higher concentrations. Adding AT22-3 results in a further small decrease in the DTAB self-diffusion coefficient. Clearly, the reduction of the surfactant diffusion induced by adding PEO or AT22-3 is smaller in the case of DTAB compared to SDS. Surfactant T2 Relaxation Times. A different perspective on surfactant-polymer interactions is provided by NMR relaxation time measurements.18 In particular the spin-spin or transverse (T2) relaxation time is sensitive to changes in the spectral density of slow motions, such as those which accompany changes in the aggregation state of surfactants. Part A of Figure 6 shows the results of 1H NMR T2 relaxation time measurements on the methylene resonances of SDS alone and in the presence of either AT22-3 or PEO. Part B of Figure 6 shows the corresponding data for DTAB. We note that the intensity decay versus the interpulse delay τ from which the T2 is measured was always monoexponential in every case investigated here. Consequently, the measured T2 in a given situation will always be a weighted-average over the relaxation times of the different environments sampled by the surfactant. When there is no polymer present the T2 of SDS or DTAB decreases progressively with increasing surfactant concentration. Relaxation times in surfactant micelles are often interpreted in terms of a two-step model32,33 involving a combination of fast motions (segmental fluctuations and rotations) and slow motions (rotational tumbling of the entire micelle and lateral diffusion of the surfactant over the surface of the micelle). Since the fast motions are relatively unaffected by the aggregation state, the concentration dependence of the T2 for SDS or DTAB can be attributed to a decrease in the proportion of monomeric to micellar SDS, probably combined with an increase in the micelle size plus obstruction effects.
Interactions with HEUR Associating Polymers In the presence of 1 wt % PEO there is an abrupt increase in the T2 of SDS at an SDS concentration of about 4.5 mM. Thereafter, T2 for SDS decreases progressively with increasing SDS concentration in a manner paralleling the behavior in the absence of PEO. We identify the T2 maximum at 4.5 mM SDS with the critical aggregation concentration (cac) for the interaction between PEO and SDS, in agreement with previous reports.34,35 Since the cac is lower than the cmc for SDS in aqueous solution (8 mM), SDS-PEO interactions are thermodynamically favoured. Such an effect is rationalized in terms of the reduction in micellar core-water contacts when micelles are “wrapped-up” by polymer and the increased entropy of the counterions due to lowering of the micellar surface charge density when “masked” by the polymer.36,37 The fact that T2 initially increases upon SDS binding to PEO suggests that rates of slow motions have increased relative to free SDS micelles. Since whole micellar tumbling for an SDS micelle bound to PEO is unlikely to be faster than that of a free SDS micelle, it is logical to look to the lateral diffusion of SDS within the PEObound micelle for an explanation of the observed T2 effects. At low surfactant-polymer ratios one expects the aggregation number of polymer-bound SDS to be about half that of free SDS micelles,10 so that lateral diffusion within the polymerbound micelle would be enhanced, leading to a longer T2. The progressive decrease in the SDS T2 at higher SDS concentrations is readily attributed to a change in the proportion of nonbound versus polymer-bound SDS. Compared to PEO, AT22-3 has a less profound effect on the T2 of SDS. At low SDS concentrations (
