2.1 Outline of the synthesis

A modified Schramm synthesis [8] was used, starting with 1,10-dibromodecane according to the following scheme:

• 1.

  $$\mathrm{Br}{\left({\mathrm{CH}}_2\right)}_{10}\mathrm{Br}+{\mathrm{Na}}_2\left({\mathrm{SO}}_3\right)\to \mathrm{Br}{\left({\mathrm{CH}}_2\right)}_{10}{\mathrm{SO}}_3^{-}{\mathrm{Na}}^{+}+{\mathrm{Na}}^{+}{\mathrm{Br}}^{-}$$

• 2.

  $$\mathrm{Br}{\left({\mathrm{CH}}_2\right)}_{10}{\mathrm{SO}}_3^{–}{\mathrm{Na}}^{+}+{\mathrm{H}}_2\mathrm{N}–\mathrm{CS}–{\mathrm{NH}}_2\to {\mathrm{Br}}^{–}+{\left({\mathrm{H}}_2\mathrm{N}\right)}_2^{+}–\mathrm{CS}–{\left({\mathrm{CH}}_2\right)}_{10}{\mathrm{SO}}_3^{-}{\mathrm{Na}}^{+}$$

• 3.

  $${\mathrm{Br}}^{–}+{\left({\mathrm{H}}_2\mathrm{N}\right)}_2^{+}–\mathrm{CS}–{\left({\mathrm{CH}}_2\right)}_{10}{\mathrm{SO}}_3^{-}{\mathrm{Na}}^{+}\stackrel{{\mathrm{NH}}_4\mathrm{OH}}{\to }\mathrm{HS}-{\left({\mathrm{CH}}_2\right)}_{10}{\mathrm{SO}}_3^{-}{[{\mathrm{H}}_2\mathrm{N}=\mathrm{C}{\left({\mathrm{NH}}_2\right)}_2]}^{+}$$

• 4.

  $$\mathrm{HS}–{\left({\mathrm{CH}}_2\right)}_{10}{\mathrm{SO}}_3^{-}{[{\mathrm{H}}_2\mathrm{N}=\mathrm{C}{\left({\mathrm{NH}}_2\right)}_2]}^{+}\stackrel{\left[{\mathrm{H}}^{+}\right]\mathrm{resin},\mathrm{then}\mathrm{NaOH}}{\to }\mathrm{HS}–{\left({\mathrm{CH}}_2\right)}_{10}{\mathrm{SO}}_3^{-}{\mathrm{Na}}^{+}$$

In Step 1, a 1.5 × molar excess of the dibromodecane reacted with sodium sulfite in aqueous ethanol under reflux for 30 h. The monosubstituted product was isolated by solvent extraction and purified by recrystallization. The product of the reaction of this bromoalkanesulfonate with thiourea in aqueous solution under reflux for one hour in Step 2 was the thiouronium bromide salt. It separated as colorless crystals upon cooling, and was recrystallized from ethanol/water (50/50) in 79% yield. This was hydrolyzed in Step 3 to the mercaptan on one end of the molecule, while on the other, guanidinium ion replaced sodium. The sodium was reintroduced in Step 4 by means of ion exchange with H⁺, followed by titration with NaOH. Because mercaptans can be readily oxidized to disulfide in air, the last steps were carried out in an oxygen-free nitrogen atmosphere. Details are given in the experimental section.

2.2 Characterization

Two qualitative tests, ethanolic lead acetate and sodium nitroprusside, both confirmed that the product of synthesis was a mercaptan. Thin layer chromatography indicated a minor contaminant, which was confirmed to be the corresponding disulfide, no doubt formed by air oxidation of the SMDSo. Quantitatively, titration with standardized ethanolic iodine showed that the product was more than 98% pure SMDSo.

Structure proof was obtained by proton NMR and infrared spectroscopy. In the NMR, peaks at 1.29, 2.4 and 2.8 ppm shifts corresponded to known methylene protons in mid-molecule, adjacent to –SH and adjacent to –SO₃^–, respectively. In the IR spectrum, absorbances at 720, 790, 1050 (strong, sharp), 1160 (strongest, broad), 1450 (shoulder at 1400) and 1630 cm^–1 were confirmatory for SMDSo.

2.3 Surface properties of SMDSo

2.3.1 Determination of CMC

The ultimate test of importance to us was whether, and to what extent, this molecule is surface-active. It should be a micelle-former, and if so, a plot of solution conductance as a function of concentration would show a change in slope at the CMC. This was indeed the case as shown in Fig. 1 . Measurements were made at 20 °C with a Beckman Mysels-type conductivity cell and an Industrial Instruments bridge. The CMC was determined to be 6.1 × 10^–2 mol l^–1. Sodium decanesulfonate has been found to have a CMC of 4.1 × 10^–2 mol l^–1. The higher value for SMDSo is expected because of the hydrophilicity of the –SH group.

2.3.2 Surface tension of solutions

Surface tensions, σ, of serial dilutions of SMDSo solutions of concentration C were obtained using Wilhelmy plate technique with a Rosano film balance. Because of the exposure of the surface excess to the atmosphere, we conducted all measurements in an atmosphere of oxygen-free nitrogen. Even so it became clear that some of the highly surface-active disulfide was being formed by oxidation at the interface, so that extreme care was required, and only brief spans of concentration were employed. The minimum in the Gibbs plot shown in Fig. 2 shows the effect of this contamination. Nevertheless the CMC so obtained is the same as that found conductometrically. Fig. 3 is a composite of several experiments in each of which only a few measurements were made to minimize disulfide formation. It is the best estimate of surface tension as a function of concentration, and extrapolates reasonably well to the surface tension of pure water at infinite dilution. Even so, there is some fall-off in σ at very low concentrations, no doubt attributable to disulfide.

2.4 Test for chain transfer during polymerization

A ‘quick and dirty’ experiment was carried out to compare two polystyrene latexes, one of which used sodium dodecyl sulfonate, SDSo, and the other used an equimolar amount of SMDSo. In all other respects the two recipes were the same. The initiator for the emulsion polymerization for both was 4,4′-azobis-(cyanovaleric acid) – AIBN –, which should not react significantly with the mercaptan. After polymerization, both colloids were gently shaken with highly purified, mixed bed ion-exchange resins (in the OH^– and H⁺ forms). This was repeated two more times, at the end of which the SDSo latex had coagulated completely, whereas the SMDSo latex showed no evidence of coagulum. It is well known that ion exchange will remove adsorbed surfactant, so that it is clear that in this case, when the SDSo was removed, there was insufficient surface charge to stabilize the colloid. The SMDSo apparently was grafted to the surface by chain transfer during the emulsion polymerization, and thus could not be removed by the resin. So the colloid made with it remained stable.

2.5 Making polystyrene colloids with SMDSo

A few experiments were carried out to determine whether SMDSo would act as expected for a surface-active agent in emulsion polymerization. Two sets of experiments were conducted: Set I would produce colloids containing only sulfonate surface groups; Set II would form bifunctional colloids containing both sulfonate and carboxyl (Tables 1 and 2).

Both sets were conducted under oxygen-free nitrogen after scrupulous purging of oxygen. The initiator in Set I was biacetyl (butanedione-2,3), which photolyzes readily with visible light to produce nonionic free radicals, and that in Set II was 4,4'-azobis (cyanovaleric acid) which thermolyzes to split out N₂ to form two identical free radicals. Both initiators are water-soluble The results for both sets are shown in Tables 1 and 2.

2.6 Control of surface group concentration

One of our goals in this work was to be able to control the concentration of surface functional groups on polystyrene (PS) colloidal particles independently of particle size. Ordinarily in emulsion polymerization one regulates particle size by the amount of surfactant added during the particle formation stage. But surface group concentration is affected by the concentration of initiator and by the extent of grafting of surfactant molecules onto the particles by chain-transfer, as mentioned above. Thus the two are intimately entwined.

However, by using a PS seed latex with a low surface group concentration, adsorbing the surface-active SMDSo onto it, and then doing a second stage polymerization, we should be able to regulate the concentration of chemically bound surface functional groups. The seed latex was prepared using a silver ammine/sodium sulfite redox initiator that produces only ^•SO₃^– free-radical ions, which initiate polymer chains that thereby contain a sulfonate end-group. No emulsifier was used. The product PS colloid had a bimodal particle size distribution, with the principal population having a diameter of 700 nm; these were accompanied by some very small sizes. The latter were removed by fractional sedimentation at 1700 rpm, decantation of the smaller fraction, and ultrasonic redispersion of the large particles. This was repeated four times to give a product that showed no small sized particles under the transmission electron microscope (TEM). This colloid was subsequently purified by mixed bed ion exchange, which leads to all the surface sulfonate groups having H⁺ counterions. These, upon titration with aqueous Ba(OH)₂, gave a surface charge density, σ₀, of 0.34 μC cm^–2.

The second stage, seeded emulsion polymerization, was undertaken according to the scheme below:

• • (1) adsorption of SMDSo onto PS Seed Colloid (deoxygenation of seed latex, addition of SMDSo solution, system maintained under nitrogen);
• • (2) monomer addition (addition of styrene, stirring until swelling equilibrium, or complete imbibing of styrene);
• • (3) emulsion polymerization (initiator – AIBN in dichloroethane – addition, polymerization at 55 °C);
• • (4) final work-up of the latex (solids content determined; mixed bed-ion exchange; solid content determined again, conductometric titration, TEM particle size determination).

A solution of SMDSo was added to the seed colloid in an amount calculated to give a final surface coverage of 185 Å² per molecule. This was based on an adsorption isotherm of SMDSo on PS which was admittedly approximate, but which gave, with corrections, a surface saturation value of 31 Å² per molecule. This compares to the comparable, generally accepted value for SDS on styrene of 45 Å². The calculation was not crucial since some of the transurf would probably not be involved in chain initiation, some would form by-products, and the adsorption equilibrium would shift as the SMDSo was consumed.

In this experiment, azobis-iso-butyronitrile, AIBN, was used as the polymerization initiator because of its hydrophobic nature (it would concentrate in the particles thereby reducing the probability of new particle formation), and because it was not expected to oxidize the SMDSo (as would the very commonly used persulfate). All operations were carried out under O₂-free nitrogen purging. An aqueous solution of the transurf was pipetted into the reaction flask containing the seed latex, and allowed to reach equilibrium adsorption under stirring for 45 min. Styrene monomer was added, and the mixture was stirred overnight to ensure complete absorption. Fifty microliters of 0.0064 M AIBN solution in 1,2-dichloroethane was added and stirred for another 3 h, time enough to totally diffuse into the polymer/monomer particles [9]. Polymerization was then carried out at 55 °C for 18 h. The results are summarized in Table 3.

Particle sizes were estimated by Higher Order Tyndall Spectra (HOTS), which is useful for monodisperse colloids [10]. The final colloid, under careful TEM examination, showed no new particle formation. The surface coverage/surface charge density values were obtained as before after mixed bed ion exchange with highly purified resins [11] followed by titration with Ba(OH)₂ reagent solution. This procedure produced a dramatic 16-fold increase in the surface functional group density, not as high as the theoretical value, no doubt because of the other fates to which this transurf is subject, as mentioned above.

3.1 Synthesis of sodium 10-bromo-1-decane sulfonate

A 3-l, round bottom flask containing a magnetic stirring bar was charged with 1660 ml ethanol (95%) and 200 g 1,10-dibromodecane (Aldrich, 97%). The dibromodecane was dissolved, and a solution containing 56.0 g of Na₂SO₃ in 660 ml of distilled water was added to the flask. The contents were heated under reflux with stirring for 30 h. The flask was cooled to room temperature, followed by cooling at 0 °C. The contents were decanted from the excess solid dibromodecane. The reaction mixture was evaporated to dryness at 60 °C on a rotary evaporator. The resulting solid was washed with petroleum ether (boiling range 30–60 °C). Further drying of the solid, to remove trace amounts of water, was often required to facilitate petroleum ether washings. Washings were continued until the dry solid was powdery in appearance. This indicated complete removal of the last traces of dibromodecane. The solid was washed with ice-cold acetone to remove the color. The product, sodium 10-bromo-1-decanesulfonate, was extracted from the dry solid with ethanol (100%), leaving a residue of NaBr.

3.2 Synthesis of 10-S-thiouronium-1-decanesulfonate (10-S-T)

Both 18 g of sodium 10-bromodecanesulfonate and 4.8 g of thiourea (Aldrich 98%) were dissolved in 150 ml water. The solution was heated under gentle reflux for one hour. After cooling the flask in ice, the resulting crystals were isolated by filtration, and washed several times with cold water. The 10-S-T could be recrystallized from 50% ethanol/water but it did not have a sharp melting point. After one recrystallization, 13.1 g (79% of theory) of purified product was obtained.

3.3 Guanidinium 10-mercapto-1-decanesulfonate (GMDSo)

Thirteen grams of the 10-S-thiuroniumdecanesulfonate and 400 ml of concentrated aqueous ammonia were placed in a 1-l round-bottom flask containing a magnetic stirring bar, and fitted with a reflux condenser. The contents were stirred and slowly warmed to 85 °C. Two 100-ml portions of concentrated aqueous ammonia were added over a period of about one half hour. After the 10-S-T had dissolved, the solution was stirred for 10 min. The stoppered flask was then cooled to room temperature and held there for 2 h.

The flask was placed on a rotary evaporator at 35 °C to remove ammonia and water. The product, GMDSo, was waxy in appearance, and water-soluble. In some instances, a white, water-insoluble precipitate formed as the ammonia was being removed. When this occurred, the yield of GMDSo could be improved by evaporating the mixture to dryness and redissolving the solid in 200 ml of aqueous ammonia at 85 °C. Rotary evaporation at 35 °C was then performed.

3.4 Sodium 10-mercapto-1-decanesulfonate (SMDSo)

A chromatographic column (63 cm × 2.5 cm diameter) was filled with 250 ml (wet volume) of cation exchange resin in its acid form. The resin had been cleaned previously by washing in 3 N NaOH, 3 N HCl and hot distilled water repeatedly, in that sequence up to five times. The GMDSo from the previous step was dissolved in 150 ml of deoxygenated, distilled water and filtered under nitrogen to remove any insoluble impurities. The solution was added under nitrogen to the top of the column, and the product was collected at the bottom. The resin was rinsed with several portions of distilled, deionized water. The combined eluates were brought to pH 7.0 with 1 N NaOH solution. The neutralized solution was evaporated to dryness, and the product, SMDSo, was extracted from its disulfide impurity with absolute ethanol. The product was recrystallized under nitrogen from absolute ethanol. Maximum recovery was achieved by cooling to about –10 °C in an ice-salt bath before filtration. After three recrystallizations, 3.25 g SMDSo were obtained, for an overall yield based on GMDSo of 27%. The melting point was 185.5–186.5 °C with decomposition.

3.5 Characterization of SMDSo

3.6 Synthesis and characterization of polystyrene colloids