Journal of Colloid and Interface Science 317 (2008) 96–100www.elsevier.com/locate/jcis

Thermosensitive hard spheres

Matthew Clements a, Srinivasa Rao Pullela a, Andres F. Mejia a,b, Jingyi Shen a, Tieying Gong a,Zhengdong Cheng a,∗

a Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USAb Department of Chemical Engineering, Universidad Industrial de Santander, Bucaramanga, Colombia

Received 9 May 2007; accepted 5 September 2007

Available online 10 October 2007

Abstract

We demonstrate an extension of a UV–Vis spectroscopy method to determine the phase boundaries for thermosensitive colloids as an alternativeto the time-consuming sedimentation method. The Bragg attenuation peak from colloidal crystallites was monitored during the quasi-equilibriumcolloidal crystal melting. The melting and freezing boundaries of the coexistence region were determined via a blue shift of Bragg’s peak andthe disappearance of peak area. We confirm this method using poly(N -isopropylacrylamide) (PNIPAM) particles at different charge densities andtemperatures far below the lower critical solution temperature. At low pH, the particles behave as thermosensitive hard spheres.© 2007 Elsevier Inc. All rights reserved.

Keywords: Thermosensitive colloids; Crystal melting; PNIPAM-co-acrylic acid; Hard spheres

1. Introduction

Colloidal suspensions are vital model systems for the in-vestigation of not only the structure and dynamics of fluids,crystals, and glasses, but also the phase transitions betweenthem [1–5]. Colloids are directly analogous to atoms and ad-vantageous to study owing to their larger sizes and hence slowerdiffusion rates. They can be visualized by microscopy methods[6–8] or analyzed using light-scattering techniques [9–11]. Al-though hard-sphere-like colloidal suspensions were the primaryfocus of crystal nucleation studies [12–21], recently, the crystal-lization kinetics and phase behavior of aqueous dispersions ofPNIPAM particles have attracted increasing attention [22–25],partly due to their easier manipulation of volume fraction bychanging local environmental conditions [26–30].

Until now, the primary method for phase boundary analy-sis was carried out via sedimentation [2,3,22,31]. A suspensionin the fluid and crystal coexistence region is analyzed over

* Corresponding author.E-mail address: zhengdong.cheng@chemail.tamu.edu (Z. Cheng).

0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2007.09.010

a period of time. The crystals are allowed to grow and set-tle under gravity. The height of the fluid–crystal interface isused to determine the crystallinity produced by the phase tran-sition [2]. However, gravity also concentrates the suspensionsand subsequently grows a thin layer of crystals at the fluid–crystal interface [3]. Thus a major drawback of this method isthe extremely long time duration, weeks or even months, re-quired for the measurement of thin-layer growth in order toextract the effect of gravity. A more prompt method for analy-sis of phase boundaries and crystallization kinetics has beenbrought forth using UV–Vis spectroscopy, where a small den-sity difference between the PNIPAM colloidal particles and themedium elongated the sedimentation time [9,10]. However, thatstudy did not determine the melting temperatures, and thus leftthe coexistence region undefined; the hard-sphere interactionbetween particles is therefore still an assumption. Yodh and co-workers found premelting of the crystals at grain boundariesand dislocations within the colloidal crystals where they uti-lized temperature-responsive PNIPAM particles, viewed underreal time video microscopy [32]. In this work, the fluid–crystalphase boundaries were probed by UV–Vis spectroscopy in amore time-efficient manner than the aforementioned sedimen-tation method.

M. Clements et al. / Journal of Colloid and Interface Science 317 (2008) 96–100 97

2. Experimental

All chemicals were purchased from Sigma-Aldrich. N -Iso-propylacrylamide (NIPAM) (97% purity) was recrystallizedfrom a 1:5 (v:v) toluene and n-hexane mixture. All other chem-icals were used as received. The PNIPAM microgel samplescontaining 5 mol% of either acrylic acid or allylamine wereprepared by precipitation polymerization [33]. The followingreagents were mixed in a three-neck flask and stirred for 1 hat 300 rpm using a mechanical stirrer under a nitrogen purge:240 mL deionized (DI) water, 3.78 g N -isopropylacrylamide(NIPAM) monomer, and 0.12 g acrylic acid (AA) as co-polymer, 0.0665 g methylene-bis-acrylamide (MBA) as cross-linker, and 0.106 g sodium dodecyl sulfate (SDS) as a surfac-tant. The reaction was initiated by adding a 0.166 g potas-sium persulfate dissolved in 10 mL of degassed DI waterand allowed to run for 4 h at 70 ◦C. The polymer suspensionwas then purified via dialysis against DI water for a week atroom temperature, with the DI water changed twice per day.The molecular weight cutoff of the dialysis membrane was10,000 kDa. The hydrodynamic diameter measured by dy-namic light scattering (DLS) for dilute sample was 253 nmat pH 3.0 and at 23 ◦C. The pH of the particle suspensionwas first changed to an appropriate value, followed by cen-trifugation at 30,000g for 2 h at 34 ◦C. The particles werethen redispersed in DI water having same the pH as that ofconcentrated suspension. Four concentrated samples with pHvalues 2.80, 3.50, 3.90, and 4.20 were first made and crystalsformed under different concentrations. The weight percentageof each sample was obtained by drying out the solvent com-pletely.

A fiber optic UV–Vis spectrometer EPP2000 (StellarNetInc., Tampa, FL), a halogen light source with an SL1 filter, anda temperature-controlled cuvette holder from Quantum North-west (QNW, Spokane, WA) were used for sample analysis.SpectraWiz software was used for spectra acquisition. To avoidwater condensation inside the cuvette holder at lower tempera-tures, a nitrogen gas purge setup, equipped with a Rego twin-gauge regulator and a ChemGlass air-free bubbler for visualflow rate observation, was employed.

The laboratory temperature was maintained at approxi-mately 22 ◦C. Visual observation of the samples provided agood basis for the freezing point estimation. To avoid shearmelting, each sample was heated to a temperature at which onlythe fluid phase was seen, whereupon a reference spectrum wastaken. Then, the cuvette temperature was maintained well be-low the estimated freezing temperature for several hours so thatthe sample fully crystallized and equilibrated. Beginning at thislow temperature, the cuvette temperature was slowly increasedin increments of 0.25 ◦C, allowing 30 min to equilibrate aftereach step. A UV–Vis transmission spectrum was recorded ateach temperature. With the fluid references taken well abovethe freezing temperatures for each sample, an intense Braggattenuation peak evolved as the crystallites began to form. Astemperature was increased in measured increments, the Braggpeak started to shrink until it fully disappeared at or below thefluid reference temperature.

3. Results and discussion

We compiled spectra gathered at different temperatures fora specific sample and overlayed them on a single plot of trans-mittance versus wavelength using OriginLab 7.5 (Northampton,MA). Fig. 1 plots the data for charged PNIPAM-co-allylaminespheres at pH 7.4. Most relevant to the determination of themelting and freezing points for the suspensions are the decreasein the peak integration and the blue shift of the Bragg diffractionas temperature increases. Determination of the freezing pointexploits the change in peak area (integration) as a function oftemperature, which is plotted in Fig. 2a. The Bragg peak areawas calculated by fitting two Gaussian peaks to each spectrumand the value for the integration of the sharp, narrow peak isused in Fig. 2a, which indicates that the freezing point is lo-cated between 20 and 20.5 ◦C. By fitting the data on both sidesto a linear and third-order polynomial line, the freezing point iscalculated to be at their intersection, 20.2 ± 0.1 ◦C. Note thatthe area is not linearly dependent on temperature in the coexis-tence regime because the volume fraction of the suspension isnot linearly dependent on temperature.

The melting point determination utilizes the temperature-responsive shrinkage of the PNIPAM particles and the result-ing blue shift of Bragg scattering as the particles move closer.The peak position is described by the dynamic diffraction the-ory [34],

(1)λb = 2nwater sin θ

(h2 + k2 + l2)

[1 + ψ0

2 sin2 θ

][2π

3φ

]1/3

D,

where n is the refractive index, and ψ0 is photonic scatteringstrength of the particles, which is almost zero due to the highporosity of the hydrogels. D is the diameter of the colloidalparticles, φ is the volume fraction of the particles inside thecrystallites, θ is the averaged Bragg angle, and h, k, l are the in-dices of the scattering plane. Considering the quasi-equilibriumnature of the melting and the random orientation of the crys-tallites, the average scattering angle can be assumed to be thesame as the temperature increases. Fig. 2b shows that the melt-

Fig. 1. UV–Vis transmission spectra of the PNIPAM-co-allylamine microgeldispersions of pH 7.4 at different temperatures. With a fluid reference at 22 ◦C,the Bragg peak blue-shifts and disappears with the melting of crystallites. (Forinterpretation of the references to color in this figure legend, the reader is re-ferred to the web version of this article.)

98 M. Clements et al. / Journal of Colloid and Interface Science 317 (2008) 96–100

Fig. 2. Determination of phase boundaries using PNIPAM-co-allylamine mi-crogels by UV–Vis spectroscopy. (a) Area of the Bragg peak. The Bragg peakdisappears completely when the last of the crystallites has melted. Intersectionof the two trend lines yields the precise freezing temperature. (b) Wavelengthof the peak position. The Bragg peak begins to shift to a lower wavelength asthe crystallites begin to melt. The precise melting point is shown to be at the in-tersection of the two trend lines. The particles move closer when the crystallitesmelt as illustrated by the inset diagram.

ing point separates two distinct behaviors of the crystallites, i.e.,the retaining of a constant Bragg peak position at lower tem-peratures and the strong blue shifting right above the meltingpoint.

Below the melting temperature, no crystallites are meltedeven though the particle diameter is reduced. Because all par-ticles shrink simultaneously, the relative positions among theparticles remain constant. Hence, φ ∝ D3, which leads to

(2)λb,below melting = AD

φ1/3= constant,

where

A = 2nwater sin θ

(h2 + k2 + l2)

[1 + ψ0

2 sin2 θ

][2π

3

]1/3

,

which is a constant for the same sample.On the contrary, the particle volume fraction of the crystal-

lites inside the coexistence regime is constant, φcrystal, coexistence= φcrystal, melting. Therefore,

(3)λb,above melting = AD

φ1/3= A

D

ϕ1/3melting

.

As illustrated by the inset of Fig. 2b, when crystallitesmelt, the further shrunken particles (red positions) need tomove closer (blue positions) to maintain the proper osmoticpressure of the crystallites. This closer-packing motion cre-ates tighter lattice planes, hence shorter Bragg scattering wave-length. When the crystallites begin to melt, the peak begins toshift; this point is taken as the melting point. In a manner sim-ilar to that of the freezing point determination, the intersectionof a linear trend line to the left and a second degree polyno-mial trend line to the right of these temperatures yields a precisemelting temperature of 15.4 ± 0.1 ◦C.

Fig. 3a plots the determined transition temperatures versuspolymer concentration for PNIPAM-co-acrylic acid particles at

Fig. 3. Phase diagram of the PNIPAM-co-acrylic acid microgel dispersions at pH 2.8 measured via UV–Vis spectroscopy. (a) Temperature–concentration represen-tation. At room temperature, the fluid–crystal transition width normalized to the freezing transition concentration is approximately 7%, consistent with hard-spheresuspensions with 5% polydispersity. Inset: illustration of the phase diagram in a broader concentration and temperature range. (b) Temperature–particle volumefraction φ representation. Inset: Particle diameter with temperature (open circles are dynamic light-scattering measurement). Red solid line indicates the diameterof the particles used to transfer from temperature–concentration representation to temperature–volume fraction representation. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of this article.)

M. Clements et al. / Journal of Colloid and Interface Science 317 (2008) 96–100 99

Fig. 4. Acrylic acid dissociation (—) and relative width of coexistence region(- - -) normalized to the freezing concentration as a function of pH at roomtemperature.

pH 2.8. Using the precise melting and freezing temperatures, anaccurate depiction of the phase diagram for PNIPAM particleswas provided far below their lower critical solution tempera-ture, which is around 34 ◦C. Following a horizontal path acrossthe diagram, we determined the width of the coexistence regionto be approximately 7% relative to the freezing concentrationat room temperature. This value is consistent with hard-sphereboundaries with 5% polydispersity in size [35,36]. The poly-mer concentration was converted into the corresponding vol-ume fraction for PNIPAM hard spheres at room temperature bycalibrating the particle volume fractions at the freezing transi-tion to be 49.4% (Fig. 3b). The corresponding diameters of theparticles with temperature are shown in the inset. The diametersof the particles used were in agreement with the dynamic light-scattering measurement of the particles. To investigate the inter-action between the particles, we systematically changed the pHof the PNIPAM-co-acrylic acid suspensions. Fig. 4 plots the rel-ative width at room temperature as a function of pH. The widthincreases from pH 2.8 to 3.9 and then decreases up to pH 6.5for PNIPAM-co-acrylic acid spheres. When pH increases, theparticles are physically charged. We anticipate that the initialincrease in the width is due to the increase in the Debye lengthof the particles whereas and the decrease in the width after 3.9is a result of the increase in the charge of the particles.

4. Conclusions

In conclusion, We developed a critical technique to char-acterize the melting and freezing temperatures for PNIPAMmicrogel dispersions using UV–Vis transmission spectroscopy.This method provides an accurate and time-efficient method ofphase boundary analysis for temperature-sensitive colloids, anda valuable technique for interparticle potential characterization.At low pH, the PNIPAM-co-acrylic acid microgel particles be-have as thermosensitive hard spheres.

We envision that our method can be broadly applicableto “intelligent” colloidal systems in which the effective hard-sphere particle diameter during phase transition is sensitive tocontrol variables such as temperature, pH, and salt or otherchemical concentrations. This method can also offer an ana-

lytical tool for direct size measurement for the particles in thecoexistence regime if φmelting and the dominant Bragg scatter-ing angle are determined. For example, we can measure thedeswelling of the hydrogels under osmotic pressure at the fluid–crystal phase transition.

Acknowledgments

M. Clements acknowledges support from the Texas Engi-neering Experiment Station (TEES) Undergraduate ResearchSummer Grants Program. Z.D.C. acknowledges support fromTEES and Texas A&M University Start-up Funds. We grate-fully thank Dr. Zhibing Hu for providing PNIPAM microgelsuspensions for preliminary investigation.

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