Krafft Temperature of Cesium Dodecylsulfate Solutions at High

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Krafft Temperature of Cesium Dodecylsulfate Solutions at High Concentration Apostolos Vagias,†,‡ Wieslaw L. Suszynski,‡ H. Ted Davis,‡,§ and Alon V. McCormick*,‡ †

FORTH, Institute of Electronic Structure and Laser, Heraklion 71110, Crete, Greece Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States

‡

S Supporting Information *

ABSTRACT: Polarized transmitted light observation was used to examine the Krafft point temperature of cesium dodecylsulfate (CsDS) in water over a wide range of concentrations. CsDS shows an elevated Krafft temperature compared to that of sodium dodecyl sulfate (SDS). Unlike SDS, the CsDS solubility temperature remains virtually constant at high concentration. Also, the stable birefringent liquid crystal appears at a much lower concentration for CsDS than for SDS, and the concentration window of its coexistence with isotropic micellar solution is broader.

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dodecylsulfate (CsDS) system (Lee27 and references therein). To make the high concentration measurements, it is important to be aware of the occurrence of persistent metastable structures that complicate this measurement at high concentration; similar issues of metastability have been reported in sodium dodecyl sulfate (SDS) systems by Kekicheff23,24 and Cabral and co-workers,25 but the current report is of the metastable occurrence of a birefringent mesophase. This type of investigation of solubility at high concentration could affect the understanding of the use of such surfactants for reaction engineering and separation templates,26−29 the design1,2,30 of functional polymer−surfactant mixtures, and could shed light on the use of heavy counterion exchanges that are sometimes used to enhance scattering contrast in electron microscopy31,32 or X-ray scattering investigations.

INTRODUCTION Understanding and predicting1−3 the phase behavior of ionic surfactants in water5,6 is crucial in light of the breadth of applications, including cosmetics,4−6 drug delivery,7 and pharmaceutics.8,9 The micelle structure in water depends on the type of interactions between counterion-polar headgroups.10−14 Even just replacing the sodium counterion with the cesium counterion for anionic surfactants,15−18 the interactions among polar headgroups, counterions, and hydrocarbon chains can vary enough to change the preferred micelle structure and even to introduce micelle structure transitions as concentration increases. The Krafft point temperature of ionic surfactants is of interest in formulation because it limits the function of micellar solutions; below the Krafft point temperature, stable solid surfactant interferes with the system. The variation of the solubility temperature with concentration is of practical interest and may also lend insight into micellar structure changes with concentration, following principles described for instance by Gu and Sjoblom19 and by Hirata and co-workers.20 Hayman, Simpson, and co-workers21 provide an example of the manipulation of surfactant structure to lower the Krafft point; more recently, Prajapati and Bhagwat22 demonstrate the use of additives to achieve this. In this contribution, we report that replacing sodium with cesium as the counterion produces two unusual features: elevation of the Krafft point (reported previously by Bales and co-workers at low concentration16) and an invariance of the solubility temperature even at high concentration. These features may be associated with the ability of the micelles to change structure at high concentration in the cesium © XXXX American Chemical Society

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EXPERIMENTAL AND COMPUTATIONAL METHODS 1. Materials and Methods. Materials. CsCl (C3011, Sigma-Aldrich, cesium chloride grade I, ≥99.0 wt %; L4509) SDS (L4509, Sigma-Aldrich, certified as >98.5 wt % pure (GC)) were used for the synthesis of CsDS. A Mettler EA 100 balance was used to mass the reagents used in sample preparation. Purification of SDS Reagent. The as-received SDS was purified by solubilizing in ethanol and recrystallizing to remove Received: December 13, 2016 Accepted: April 5, 2017

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any potential impurities from the slow hydrolysis to which SDS is prone.33,23,24 Initially, 40.56 g of SDS was dissolved in 400 mL of ethanol at 25 °C. The solution was heated to 50 °C and kept at that temperature for 1 min. The sample was then cooled to 0 °C and held at that temperature for 8 h to allow crystallization. Then, the crystal suspension was vacuum filtered at 0 °C and washed with water at 0 °C. The solid was collected and stored in a desiccator for at least two days at ambient temperature. Synthesis of CsDS. The purified SDS was dissolved in an aqueous CsCl solution prepared using 0.377 mol CsCl in 200 mL of water. A quantity of 0.0377 mol SDS was added to the CsCl aqueous solution (to establish an SDS:CsCl mole ratio of 1:10). To accelerate full dissolution, the solution was heated to 60 °C for 1 min. The solution was then cooled to 0 °C and kept at that temperature for 8 h for crystallization of CsDS crystals. The cold suspension of crystals was then vacuum filtered at 0 °C; the crystals were washed on the filter with four batches of 150 mL of water at 0 °C. The crystals were then redissolved in a flask containing 0.377 mol of CsCl in 200 mL of water, and the recrystallization process was repeated. At the end of the second recrystallization, elemental analysis using inductively coupled plasma (ICP) analysis confirmed that the purity of the CsDS solid product was more than 99 wt %. Sample Preparation. To ensure reproducibility in the measurements, three independent samples were examined at each concentration. The error bars in the solubility curve (Figure 2) denote the standard deviation among the three samples examined. The random uncertainty of the mass is 0.0002 g (see Table 1 for actual values of mass in sample preparation). Observation Method. For visual observation and recording, a stirred water tank (approximately 20 cm (H) by 30 cm (W)) was used with a conventional light source and a pair of polarizing filters (a polarizer and an analyzer) (Figure 1). A coiled metal tube submerged in the water tank enabled heat transfer and temperature control. A mechanical agitator ensured thermal uniformity. The polarizer was placed between the light source and the water tank, while the analyzer was attached to the camera lens (sketch, Figure 1) to produce cross-polarization of the transmitted light. A digital thermometer calibrated at 0 °C was used to measure the temperature in the water tank. The random uncertainty of the measured temperature is 0.1 K. With the Experimental Methods as described below, reproducible solubility temperature was obtained even at high concentration (where mesophases occur, as discussed below). Experimental Method. During the first heating sweep, the first apparent transition temperature was recorded. The samples were then cooled slowly to ambient temperature. A negligibly small amount of solid CsDS was added to seed any potential crystallization. As described in the Results, addition of a seed was necessary at high concentrations to obtain a reproducible transition temperature upon cooling. The tubes were then reheated.

Table 1. Masses of CsDS and Water Used for Sample Preparation at Each Nominal Concentration wCsDS before and after Solute Perturbation by Seed Additiona nominal wCsDS (%)

mCsDS, before seed addition (g)

mCsDS, after seed addition (g)

2 2 2 5 5 5 10 10 10 15 15 20 20 20 25 25 25 30 30 35 35 35 40 40 40 45 45 45 50 50 50 55 55 55

0.0131 0.0148 0.0109 0.0207 0.0225 0.0298 0.0152 0.0203 0.0231 0.0297 0.0167 0.0281 0.0248 0.0305 0.0286 0.0361 0.0329 0.0231 0.0224 0.0316 0.0333 0.0243 0.0208 0.0199 0.0327 0.0167 0.0155 0.0116 0.0092 0.0052 0.0104 0.0109 0.0127 0.0126

0.0141 0.0158 0.0119 0.0221 0.0236 0.0309 0.0163 0.0214 0.0243 0.0308 0.0176 0.0319 0.0260 0.0317 0.0297 0.0372 0.0340 0.0242 0.0235 0.0327 0.0344 0.0254 0.0219 0.0210 0.0338 0.0178 0.0166 0.0127 0.0103 0.0053 0.0114 0.0120 0.0138 0.0137

mwater (g)

wCsDS, before seed addition (g)

wCsDS, after seed addition (g)

0.649 0.722 0.537 0.386 0.429 0.576 0.424 0.181 0.289 0.170 0.097 0.118 0.097 0.122 0.086 0.110 0.0997 0.0533 0.0553 0.0713 0.0613 0.0448 0.0298 0.0283 0.0465 0.0204 0.0189 0.0138 0.0098 0.0066 0.0098 0.0095 0.0104 0.0104

0.0198 0.0201 0.0199 0.0508 0.0497 0.0491 0.035 0.101 0.074 0.148 0.147 0.192 0.203 0.2 0.251 0.247 0.248 0.302 0.288 0.307 0.352 0.352 0.411 0.413 0.413 0.450 0.451 0.457 0.484 0.441 0.515 0.534 0.550 0.548

0.0212 0.0214 0.0217 0.054 0.0522 0.0510 0.0373 0.106 0.0775 0.153 0.154 0.212 0.211 0.206 0.258 0.253 0.255 0.312 0.298 0.314 0.359 0.362 0.424 0.426 0.421 0.466 0.468 0.479 0.512 0.446 0.538 0.559 0.571 0.568

a

Three different samples for each value of wCsDS were prepared and examined.

Figure 1. (Left) Setup used for the macroscopic observations. (Right) Schematic of the beam path emitted from the conventional light bulb (a): the beam passes through a polarizer (b) before being directed through the water tank (c) with the samples (gray cylinders). The diffracted beam is guided to the camera (e) via an analyzer (d) placed on the camera and aligned vertically with respect to the polarizer.

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RESULTS AND DISCUSSION The Krafft point temperature observed in the CsDS/water system is shown in Figure 2 (blue triangles) and compared against the published solubility temperature for the SDS/water system. Solution Regime: w CsDS = 2−30 wt %. In this concentration range, we observe typical Krafft point behavior.

Above 32 °C (wCsDS ∼ 2 wt %) to 33 °C (wCsDS > 5 wt %), an isotropic single-phase solution is found. The values recorded here are consistent with those reported to about 9 wt % by Bales et al.,16 though slightly offset. B

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Figure 3. Evidence of transient birefringence when the heating rate exceeded 0.2 °C/min, starting from T = 25 °C. (a) CsDS/water samples at T = 35 °C for wCsDS = 30 wt % (left), 20 wt % (middle), and 10 wt % (right). (b) Transient birefringence at T = 48 °C for wCsDS = 15 wt % (left) and 30 wt % (right). Cross−polarized light was used. Size scale: tube diameter is 15 mm.

temperature for extended periods. For instance, when heated 6 °C above the Krafft temperature, the time required for full disappearance was 2.5 h at 20 wt % and 4 h at 5 wt %. Lower heating rates could avoid the formation of this metastable material; we conclude this birefringent material does not represent a thermodynamically stable phase at wCsDS < 30 wt %. Coexistence Regime: (wCsDS = 30−45 wt %). The results of the macroscopic observations with cross-polarized light at T > 34 °C suggest some type of two-phase region where micelles coexist with some birefringent crystal phase, presumably liquid crystal analogous to that obtained in the SDS/water system. At these concentrations, slowing the heating and cooling rate does not change the amount of the observed birefringent material. The observed birefringence points to the presence of anisotropic crystals. Above 37 °C and 45 wt %, we observe only birefringent material. Superheating and Subcooling in the Coexistence Regime: (wCsDS = 30−45 wt %). Two samples at wCsDS = 40 wt % were each treated with a different thermal treatment: a superheated (middle panel/top in Figure 4) and a subcooled sample (middle panel/bottom in Figure 4). The superheated sample appeared viscous upon tilting. Afterward, a small solid CsDS seed was added. An interfacial recrystallization at the seed surface boundary was quickly observed (Figure 4; middle panel, top). In many cases, initially

Figure 2. A schematic of the CsDS/water phase diagram (blue triangles, present work) compared with that of SDS/water (black squares, from Fontell et al.40 and Kekicheff et al.23), where T is temperature and wsurfactant is the surfactant weight fraction. Vertical lines denote our estimated (dashed, CsDS) and previously reported (solid, SDS) phase boundaries.

The invariance of the solubility temperature with the concentration in this range and its higher value compared to the SDS/water system are both consistent with the micellar structures and transitions shown by Lee et al. using cryotransmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS).34 The most important aspect of our data is the invariance of the solubility temperature with concentration above about 10 wt %. At wCsDS > 5%, the CsDS solubility curve is concentration-independent with TKrafft ∼ 33 °C, until wCsDS > 30%. TKrafft slightly increases from T = 33 ± 1 °C to T = 34 ± 1 °C, as wsurfactant increases from 30 to 40 wt %. This remarkably concentration-invariant Krafft temperature behavior at high concentration contrasts with the trend for the SDS/water system (black squares),23 where the solubility temperature increases by about 12 °C as wSDS increases from 2 to 30 wt %. Moreover, it also contrasts with the solubility behavior at lower concentration, carefully monitored by Bales et al.16 up to ∼9 wt %.16 Furthermore, recent experimental evidence in the CsDS system for the increase in both aggregation number and degree of bound counterions upon increasing wCsDS from 2 to 8 wt %34 and observation of unusual micelle structure34 contrast with the SDS/water system. We speculate the unusual CsDS micellar nanostructures and transitions at high concentration create the conditions that maintain the plateau in Krafft temperature. It is intriguing to consider that the solubility temperature behavior might yield insight into the difference between the micellar structure and behavior of CsDS and SDS. It is worthwhile to point out that even the simple compact spherical SDS micelle structure has detailed structure of interest35−37 and that changes with ionic strength38 and concentration.39 Another important feature of concentrations above 10 wt % was the appearance of persistent metastable mesophases. The heating rate needed to ensure equilibrium in our system was found to be less than or equal to 0.2 °C/min (cooling rates at 0.1 °C/min). For heating rates higher than 0.2 °C/min at 34 < T < 50 °C (concentration less than 30 wt %; Figure 3), long-lasting metastable birefringent phases occurred. In these cases, these birefringent structures disappear when held at constant

Figure 4. Metastable states observed at wCsDS = 40 wt %. (Left) Unheated stable sample at T = 25 °C. (Middle, top) Superheated (from T = 25−55 °C with a heating rate higher than 0.2 °C/min and then slowly cooled to T = 25 °C). (Middle, bottom) Subcooled sample at T = 25 °C (heated from T = 25−55 °C with a heating rate higher than 0.2 °C/min and then cooled quickly to 0 °C, held for 1 h, and finally slowly warmed to T = 25 °C). (Right) Sample after recovering its initial state at T = 25 °C. No cross−polarized light was used; all images shown were taken at T = 25 °C. Tube diameter: 15 mm. C

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hydrated radius of the cation, directly affecting the associated intra- and interparticle interactions, and must be associated with the distinctive micelle behavior in the CsDS system revealed in a prior work using cryoTEM and SAXS.34

subcooled or superheated samples without a seed could be left days or even weeks at T = 25 °C before again appearing as a dispersion of solid crystals in water (Figure 4; middle panel, bottom). Hence, this transient crystal growth behavior most likely41 results from metastability stemming from the fast (>0.2 °C/min) cooling rates, but this metastability can be disrupted easily with a small seed crystal. Further compelling evidence that our methods have achieved equilibration is the consistency of the solubility temperature through the highest concentrations we explore, as shown in Figure 2. Comparison between SDS/Water and CsDS/Water Systems. Cesium produces an elevated but strikingly concentration-invariant Krafft temperature, a lower concentration onset of the stable liquid crystal, and a broader concentration window for coexistence of isotropic micellar solution and liquid crystal. The observed differences in the aqueous phase behavior between the two anionic surfactant systems are attributed to the influence of the difference in alkali metal size. Specifically, the hydrated Cs+ radius being smaller than Na+ suggests a difference in counterion attraction and charge compensation and screening in the vicinity of anionic headgroups in micelles and solid phases. This difference may cause for a corresponding change in the surfactant’s effective critical packing parameter,42,17 and this might cause both (i) the increased (and concentration-independent) Krafft temperature and (ii) the broader concentration width of birefringent crystals/micellar solution two-phase region in the CsDS/ water system16,43 compared to that in SDS/water. Due to the reduced headgroup repulsions (from shielding by complexation with the cation), the corresponding net CsDS micellar charge becomes lower,16 and this may lead to stronger attractive forces between CsDS micelles and aggregation of CsDS micelles into mesophases. To be able to measure these effects with the phase diagram, though, requires using slow heating and either slow or seeded cooling; otherwise, transient but persistent appearance of metastable mesophases obscures these trends. The only method used in this paper is macroscopic observation with polarized light, but we find this to be adequate to observe the effect of Cs on the docecylsulfate solubility and the onset of stable coexistence of liquid crystals. Future work employing additional methods to measure solubility might become necessary in related systems if the use of polarized light is not adequate or to reveal more evidence for changes in micelle structure. Beyond the methods we reported in our earlier work (cryoTEM and SAXS in Lee et al.34), Lindblom and co-workers44 show the combined use of conductimetry, surface tension, calorimetry, and solution NMR methods to detect solution behavior in a study of counterion effects on mesophase structure (in that work, with alkylpyridinium octane-1-sulfonates). Moreover, the structure of mesophases can be further investigated with SAXS (e.g., Angelov et al.45) and by deuterium NMR (e.g., Capitani et al.46).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b01034. Heating protocol and table with CsDS amounts in each sample (PDF) Video of heating treatment for a sample at wCsDS = 40 wt% (MPG)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: 612-625-1822. ORCID

Alon V. McCormick: 0000-0002-8885-1330 Funding

This work was supported in part by the Industrial Partnership for Interfacial and Materials Engineering (IPrime), University of Minnesota. Notes

The authors declare no competing financial interest. § Deceased.

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ACKNOWLEDGMENTS We thank Mr. Russell Anderson of the ICP Lab of the University of Minnesota, Department of Soil, Water, and Climate, Soil Testing and Research Analytical Laboratory for inductive coupled plasma analysis of the CsDS used in this study. We are indebted to Professors Yeshayahu Talmon and Thomas Sottmann for fruitful discussions.

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CONCLUSIONS We report the solubility curves and the onset of birefringent crystal for the CsDS/water system, showing distinct quantitative differences in the phase behavior compared to that of SDS. The CsDS/water system shows higher solubility, lower concentration onset of stable birefringent liquid crystal, and broader coexistence of that crystal with isotropic micellar solution. These differences can be ascribed to the change in the D

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