The interaction of sodium dodecyl sulphate with colloidal microgel particles

The interaction of sodium dodecyl sulphate with colloidalmicrogel particles

Nicola C. Woodwarda, Babur Z. Chowdhrya, Stephen A. Leharneb,Martin J. Snowdena,*

aDepartment of Chemical & Life Sciences, University of Greenwich, Wellington Street, London SE18 6PF, UKbSchool of Earth and Environmental Sciences, University of Greenwich, Medway Campus, Pembroke, Chatham Maritime,

Kent ME4 4AW, UK

Received 14 April 1999; accepted 9 August 1999

Abstract

The conformational transition of a 0.5% dispersion of poly(N-isopropylacrylamide) [poly-(NIPAM)] microgel, inaqueous solution, has been examined in the presence and absence of sodium dodecyl sulphate (SDS) using a variety

of physico-chemical techniques. The conformational transition temperature (Tm) of the mixed microgel-SDSdispersion, as measured by turbidimetric analysis, was found to increase as a function of SDS concentration. Thesedata were consistent with those obtained by high sensitivity di�erential scanning calorimetry (HSDSC). The same

technique revealed that the speci®c heat (DHcal) of the mixed dispersion decreased as the concentration of SDSincreased. However the DT1/2 (the width of the transition in 8C at 1/2Cp,max) and the DCp,d (the di�erence in pre-and post-transition heat capacity) both increase with SDS concentration. Viscometric data shows an increase in the

reduced viscosity under isothermal conditions at 258C, while the viscosity-temperature pro®le of each mixedmicrogel-SDS dispersion revealed that the conformational transition behaviour mirrored that obtained from theturbidity and HSDSC measurements. Isothermal viscosity data, at 258C, suggests that the structure of the microgelbecomes more extended with increasing SDS concentration. This interpretation is con®rmed by dynamic light

scattering measurements, which clearly show that the hydrodynamic diameter of the microgel particles increaseswith SDS concentration, over the range 0±12 mM, by approx. 17%. Sorption isotherm data shows that, below aSDS concentration of 4 mM, sorption is low then increases steadily until a concentration of 12 mM. The results of

conductivity measurements, carried out in the presence and absence of SDS (below and above the Tm of themicrogel dispersion), are also reported and show that the contraction of the microgel particle results in an expulsionof some surfactant from within the microgel structure. 7 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Sodium dodecyl sulphate; Colloidal microgels; High sensitivity di�erential scanning calorimetry; Viscometry; Turbidity;

Light scattering; Sorption isotherm; Conductivity

1. Introduction

The physico-chemical properties of poly(N-isopropy-

lacrylamide) [poly(NIPAM)] microgel dispersions havebeen widely investigated and recorded in the literature

European Polymer Journal 36 (2000) 1355±1364

0014-3057/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S0014-3057(99 )00207-4

* Corresponding author. Tel.: +44-181-331-9981; fax: +44-

181-331-8305.

E-mail address: [email protected] (M.J.

Snowden).

[1±4]. These microgels have been reported to undergo

a reversible conformational transition in response tochanging solvent quality, induced by a change in sol-ution temperature [1], resulting in a signi®cant change

in the hydrodynamic volume of the microgel particles[5]. The term ``conformational transition'' is usedherein to denote the complex series of events, which

occur as the microgel is heated. These include: (i) sol-vent exclusion, (ii) changes in dimensions of the par-

ticles and (iii) alterations in polymer-polymerinteractions in relation to polymer-solvent interactions(i.e. the Flory±Huggins [6] w parameter, increases as a

function of temperature). As a result of their interest-ing solution conformational properties, microgels have

been investigated with a view to a number of potentialapplications. Studies have shown that these materialshave the ability to behave as controlled uptake and

release agents where species such as dextran [7] andheavy metal ions [8] are absorbed by the microgels attemperatures below the conformational transition tem-

perature and expelled following contraction of themicrogels at elevated temperatures [7,8]. The addition

of acrylic acid to the reaction medium, during themicrogel preparation, modi®es the physical propertiesof the dispersions in such a way that the copolymer

produced is responsive to changes in temperature,ionic strength and pH [9].

The dispersion behaviour and physico-chemicalproperties of the poly(NIPAM) microgel have alsobeen reported to be modi®ed by the presence of surfac-

tants. The e�ect of the addition of SDS has, forexample, been investigated by light scattering and hasbeen shown to result in an increase in the confor-

mational transition temperature and also in the hydro-dynamic diameter of the microgel particles [5]. The

addition of the cationic surfactant dodecyl pyridinebromide (DPB), however, resulted in a decrease in thetransition temperature of the dispersion and no swel-

ling of the particles was observed [5]. Other techniquesused to investigate surfactant-microgel interactionshave included small angle neutron scattering (SANS)

[10], dynamic light scattering [5,10], NMR [5] and ¯u-orescence probe studies [11].

The interaction between a surfactant and a polymercan a�ect the overall properties of the system in anumber of ways depending on the nature of the poly-

mer and surfactant involved. The interaction of non-ionic polymers with ionic surfactants for example has

been shown to enhance polymer solubility, althoughthe extent, to which the polymer and surfactant inter-act, depends on the degree of hydrophobicity of the

polymer. Surfactant aggregates are formed on thepolymer thereby reducing repulsive interactionsbetween the head groups of the surfactant molecules

when in micellar form [12]. In the case of ionic poly-mers attractive interactions between the polyelectrolyte

and surfactants of opposite charge are known to resultin precipitation [12]. These interactions are predomi-

nantly electrostatic and can be reversed by the additionof electrolyte [12]. Interestingly the repulsive inter-action between polymers and electrolytes, of like

charges, can also result in phase separation, e.g. the in-teraction between SDS and xanthan. Robb [12] hasattributed this behaviour to the rigidity of the xanthan

polymer backbone with the phase separation of rigidrods and spheres taking place to form two distinctphases, each rich in a single component. The precise

mechanism describing this process remains, however,uncertain.Poly(NIPAM) homopolymer is a linear water sol-

uble polymer which undergoes a reversible coil to glo-

bule conformational transition at 328C. Thistemperature is sometimes referred to as the lower criti-cal solution temperature (LCST). The structural prop-

erties of poly(NIPAM) have been investigated by anarray of techniques including turbidity [13], light scat-tering [14,15], ¯uorescence [16] and microcalorimetry

[16,17]. The e�ect of SDS on poly(NIPAM) was ®rstreported by Elissaf [18], who studied the increase ofthe cloud point temperature with increasing surfactant

concentration using light scattering. Further work hassince been carried out using turbidity [17,19], microca-lorimetry [17,19], ¯uorescent probe techniques [17,19],conductivity [20] and viscosity techniques [21]. The

e�ect of SDS on a number of other systems has alsobeen investigated, including gels [22] and latexes [23].These systems have also shown modi®ed confor-

mational transition temperatures in the presence ofincreasing SDS concentration.In the current report a number of physico-chemical

techniques (light scattering, sorption isotherms, con-ductivity, viscometry, turbidity and HSDSC) havebeen used to investigate the e�ect of SDS concen-tration on the swelling and conformational transition

behaviour of the same sample of colloidal poly(NI-PAM) microgel particles. The results obtained fromeach of these techniques are compared and contrasted

with each other and those reported from previouswork on the same system.

2. Experimental

2.1. Sample preparation

Colloidal poly(NIPAM) microgel particles were pro-duced, in the absence of surfactant, by a free-radicalpolymerisation reaction in water, at 708C, under a

nitrogen atmosphere 0.5 g dmÿ3 potassium persulphateinitiator (BDH Chemicals) was placed in a three-necked round-bottomed ¯ask and stirred continuously

N.C. Woodward et al. / European Polymer Journal 36 (2000) 1355±13641356

at 120 rpm. Pre-dissolved N-isopropylacrylamide(NIPAM; Aldrich) and the cross-linking agent N,N-

methylenebisacrylamide (Aldrich) were added to thereaction vessel, at concentrations of 5.0 and 0.5 gdmÿ3 respectively. The reaction was allowed to pro-

ceed for 6 h. The crude microgel was then cooled, ®l-tered through glass wool and repeatedly dialysedagainst deionised water (to remove unreacted mono-

mer and ionic species) until a constant conductivityvalue (1 mS) was obtained. Typical reaction yields wereof the order of 95%. Dry weight analysis of the micro-

gel showed the dispersion to be 0.48 (20.02) %w/w.Samples for subsequent experiments were preparedusing analytical grade sodium dodecyl sulphate(Sigma) in 0.48% poly(NIPAM) microgel dispersion

over a range of SDS concentrations (0±12 mM).

2.2. Electron microscopy

Transmission electron micrographs of the microgel

particles were obtained using a electron microscopeoperated at 50 kV. The particles were supported oncolloidian copper grids.

2.3. High sensitivity di�erential scanning calorimetry

(HSDC)

Thermodynamic analysis of the microgel dispersionswas carried out using a Microcal MC-2D ultrasensitiveDSC (Microcal Inc. Northampton, MA, USA). Data

acquisition and analysis were carried out using theDA2 software package, supplied by the manufacturer.Baselines were obtained over the appropriate tempera-

ture range, using the appropriate solvent system (i.e.H2O or H2O+SDS) in both reference and sample cellsand were subtracted from subsequent HSDSC phase

transition pro®les of the microgel dispersions. HSDSCdeterminations used scan rates of 60 K hÿ1 and all theconformational transitions examined were reversible in

that the ®rst and second heating scans were identical.

2.4. Viscometry

Viscosity determinations were carried out on all ofthe microgel samples in the absence and presence of

SDS using an Ostwald viscometer submerged in a con-trolled temperature water bath. Reduced viscosity (Zr)was calculated according to the equation:

Zr ��tS ÿ t0� ÿ 1

�=c

where tS is the ¯ow time of the microgel dispersion (in

the absence and presence of SDS), t0 is the ¯ow timeof the solvent (H2O or H2O+SDS) and c is the con-centration of the microgel dispersion. The samples

were analysed over a temperature range of 25±808C atintervals of 5 or 108C and 18C around the transition

temperature.

2.5. Turbidimetric analysis

The temperature dependence of the poly(NIPAM)

microgel dispersion particle diameter was determinedturbidimetrically. Analysis was carried out using a Per-kin±Elmer Lamda 2 spectrophotometer with a pro-

grammable temperature scanning water bath. Thetemperature of the sample was measured using a plati-num thermocouple temperature probe. The transmit-

tance of each of the microgel dispersions wasmeasured against the appropriate reference and thedata collected using an Epson AX2 PC. The transmit-tance was measured as a function of temperature (8C)at a wavelength of 547 nm and both heating and cool-ing scans were obtained for each sample.

2.6. Light scattering

The hydrodynamic diameter of the microgel particleswas determined using a `Malvern Instruments' Zetasi-zer 3 instrument ®tted with a 5 mW He±Ne laser

(l=632.8 nm) and a detector placed at 908. Sampleswere prepared at particle concentrations of the orderof 0.07% in clean, stoppered vials. All samples were

tumbled overnight to ensure thorough mixing. Typi-cally ®ve, 2-min measurements were made and theaverage value used. The temperature of the sampleswas maintained at 258C using a Grant LTD6 program-

mable water bath.

2.7. Sorption isotherms

The SDS-microgel mixed dispersions were centri-

fuged (after 24 h incubation) using a Sorvall RC-5B re-frigerated superspeed centrifuge set at 258C. Sampleswere left in the centrifuge until they reached equili-

brium temperature and centrifuged at 12 000 rpm for1 h. Epton titrations [24] were used to determine theconcentration of SDS present in the supernatant. Anappropriate volume of supernatant was titrated in the

presence of dichloromethane (AnalR) against hyamine1622 (0.004 M; BDH Chemicals) using acidi®ed dimi-dium bromine disulphide blue (BDH Chemicals) as the

indicator.

2.8. Conductivity measurements

These measurements were carried out using a Corn-

ing PTI-8 digital conductivity meter calibrated to 1000O with a 1000 O resistor cell immersed in 0.01 M KClgiving a reading of 1285 mS. The conductivity cell was

N.C. Woodward et al. / European Polymer Journal 36 (2000) 1355±1364 1357

also calibrated using 0.01 M KCl at 258C giving avalue of 1409 mS. The cell constant is therefore 0.91.

Conductivity measurements were carried out in theabsence and presence of di�erent concentrations ofSDS (but at a constant microgel concentration of

0.44%, w/w) at 25 and 708C; thermal equilibriumbeing attained by immersing the samples in a thermo-stat tank. The di�erence in conductivity (DC ), at both

25 and 708C, is equal to the conductivity of the micro-gel+SDS minus the conductivity of SDS in H2O.

3. Results and discussion

Transmission electron micrographs (not shown, seee.g. Ref. [9]) showed the poly(NIPAM) microgels to be

monodisperse spheres. The mean particle diameter ofthe samples was measured and found to be 430 215 nm. These values are consistent with the values

obtained for the hydrodynamic diameters of similarmicrogels prepared under equivalent conditions andreported previously [25]. In addition the TEM results

are also consistent with the data obtained by light scat-tering (see below) which also indicated monodispersityand an average hydrodynamic diameter of 450 (25)

nm.The poly(NIPAM) microgel dispersions undergo a

reversible conformational transition, at 35.18C. Fig. 1shows the conformational transitions, as observed by

HSDSC, for the microgel dispersion both in theabsence and in the presence of the surfactant SDS.The corresponding thermodynamic parameters associ-

ated with the transitions are shown in Table 1. It canbe seen [Fig. 2(a)] that the conformational transitiontemperature (Tm) of the microgel increases with a cor-

responding increase in SDS concentration. The ther-mograms obtained at all concentrations of SDSexamined are quite broad [Fig. 2(b)] indicating that the

transition is a continuous process rather than instan-taneous. For a transition under thermodynamic con-trol with a ®nite enthalpy change the breadth of thesignal represents the changing composition (controlled

by the van't Ho� isochore) of the system with tem-perature.Initial addition of SDS (2 mM) reduces the enthalpy

of the conformational transition (Fig. 1) but there is asmall reproducible increase of 0.48C in the Tm of thetransition. For SDS concentrations of 4 and 6 mM the

CMC is not temperature dependent because the calori-metrically observed enthalpy (DHmin) is too small. Athigher SDS concentrations it can be seen that the

microgel conformational transitions are changingshape (relative to microgels in the absence or at lowconcentrations of SDS). Initially, in the absence ofSDS, the di�erence in excess heat capacity between the

pre- and post transition baseline (DCp,d) is negativeand small, however it becomes large and positive withincreasing SDS concentration. DCp,d may have several

components. One contribution could arise from thechange in enthalpy of the microgel transition with tem-perature, this is negative, although there must be some

positive contribution arising from the SDS or SDS-microgel interaction. When the microgel shrinks SDSmust be expelled and it micellizes if the bulk concen-tration is above the CMC. This may make a positive

Table 1

Microcalorimetrically recorded thermodynamic parameters for

the poly(NIPAM) microgel dispersions (in water at a concen-

tration of 0.48%) in the presence and absence of SDS using a

scan rate of 60 Khÿ1

Thermodynamic parameters

SDS concentration Tm Speci®c heat DT1/2

(mM) (8C) (cal/g) (8C)

0 35.0 10.5 4.6

2 35.8 5.5 4.8

4 39.4 5.3 4.6

6 45.6 4.3 5.7

9 54.8 4.0 6.4

12 64.0 3.6 6.7

Fig. 1. HSDSC output for poly(NIPAM) microgels in the

absence and presence of SDS (the concentration of SDS is

given in mM).


contribution to DCp,d. The change in shape of the ther-

mograms with increasing SDS concentration is import-ant as the asymmetry suggests that, overall, the

conformational transition could consist of two or more

component transitions, each of which may have di�er-ent DCp,d values. Interestingly Wu [5] has suggested

that the expulsion of the SDS from the microgel net-work and the collapse of the microgel particle might

consist of two separate processes [5]. Another possi-bility is that there may be kinetic limitations, although

this appears unlikely because the transitions are revers-

ible and therefore not kinetically limited (at least at themacroscopic level). Thirdly the asymmetry could indi-

cate that dissociation of the SDS from the microgelparticles is occurring as the temperature increases. The

speci®c heat vs temperature graph [Fig. 2(c)] has a

negative gradient indicating a negative DCp,d. There-fore the reason why the calorimetric enthalpy decreases

with temperature maybe related to changes in waterstructure. There is a large decrease in DH when SDS is

added. The relationship between Tm and [SDS][Fig. 2(a)] indicates that the SDS favours the swollen

form of the microgel, i.e. the diameter of the microgel

increases with SDS concentration. This results in adecrease in Laplace pressure which in turn means that

the fugacity of water is also reduced and results in

water being released from the microgel at a higher

temperature in the presence of SDS compared to itsabsence. In addition the release of water from the

microgel, at temperatures above the conformationaltemperature, will result in the reduction of cation ac-

tivity which will stabilise the dispersion. In the pre-

sence of SDS there will be an increase in surfacenegative charge on the microgel; thus the microgel will

expand to reduce this e�ect. The increase in Tm withSDS concentration is believed to be due to an increase

in the intramolecular solubility of the microgel dis-persion brought about by the presence of SDS mol-

ecules. As has already been stated the microgel

particles undergo a conformational change collapsingin response to decreasing solvent quality; with increas-

ing temperature the hydrophobic microgel will collapsein upon itself to reduce its interactions with the sur-

rounding water. It has been suggested that surfactantmolecules interact with the polymer chains of the

microgel particle so that the chains are shielded from

the surrounding water thereby minimising polymer±surfactant interactions.

Viscometry (Fig. 3) and turbidity (Fig. 4) data also

show that the Tm of the microgel increases withincreasing SDS concentration. First derivative analysis

of the data obtained from these analyses, by viscome-

Fig. 2. Graphs showing changes in (a) Tm (8C) vs [SDS], (b) DT1/2 (8C) vs Tm and (c) speci®c heat vs Tm (8C) for microgel-SDS

mixtures obtained from HSDSC data.


try and turbidity measurements, show a strong corre-

lation between the Tm values obtained by each of thesemethods (Table 2). Analysis by turbidity indicates thatin the presence of increasing SDS concentration the

cooling curves shift to the left (lower temperature) in-dicating it takes longer for the microgel particles toreturn to their original size.Analysis by viscometry indicates that above a SDS

concentration of 4 mM an increase in the reduced vis-cosity of the system is observed. By comparing this ob-

servation to the light scattering data and adsorptionisotherms obtained (see below) it can be seen that thisincrease in the reduced viscosity may be attributable to

the increased amount of SDS adsorbed onto the micro-gel and the increasing hydrodynamic diameter of theparticle. As a result of this increasing particle size

there will be a corresponding increase in the resistanceto ¯ow resulting in higher viscosity's being observedfor each microgel-SDS system. The observation, from

viscosity data, that the reduced viscosity's for themicrogel dispersions below an SDS concentration of4 mM are similar might be attributed to the amount ofSDS adsorbed not being enough to produce a substan-

tial change in the particles hydrodynamic diameter. Asa result the particles would be of a similar size to themicrogel in the absence of SDS i.e. a similar size to the

absolute volume of the microgel. On going from a con-centration of 0±12 mM SDS, light scattering reveals a17% increase in particle diameter corresponding to an

increase in volume of approx. 1.6-fold which clearlywould lead to a sharp increase in viscosity. It shouldbe noted that at temperatures above the phase tran-

sition, of the microgel/SDS mixed dispersions, thereduced viscosity's observed are very similar indicatingthat the particle size of the collapsed particles are simi-lar even in the presence of increased SDS concen-

tration.Turbidimetric analyses of the dispersions are

shown in Fig. 4. The samples were continuously

scanned in both heating and cooling modes severaltimes to ascertain the reversibility of the transition.Analysis by turbidity indicates that with increasing

SDS concentration the microgel returns to itsexpanded con®guration more slowly as indicated bythe cooling curves observed to be shifting to theleft. Previous studies have used ¯uorescence spec-

troscopy and molecular probes to investigate the in-teraction of the microgel and SDS above and belowthe Tm of the dispersion [11] and shown that SDS con-

tinues to interact with the microgel particles above the

Fig. 3. Viscometry analysis of a 0.48% poly(NIPAM) dis-

persion in the presence of SDS (0±12 mM) as a function of

temperature. The concentrations of SDS are as follows: Q(0 mM), * (2 mM), R (3 mM), T (4 mM), W (6 mM), q(9 mM) and w (12 mM).

Fig. 4. Turbidimetric results for a 0.48% dispersion of poly(-

NIPAM) in the absence and presence of SDS (0±12 mM) at

547 nm as a function of temperature at 60 and ÿ60 K hÿ1

(heating and cooling scan rates respectively). The symbols

represent the following: Q, q (0 mM); *, w (2 mM); R, r,

(4 mM); t, T (6 mM); r, W, (9 mM) ,$ (12 mM) of SDS

(the ®rst symbol represents the heating scan and the second

symbol the cooling scan).

Table 2

Transition temperatures (Tm in 8C) observed by HSDSC and

®rst derivative analysis of data obtained by viscometry and

turbidity analysis

SDS concentration (mM)

Method 0 2 4 6 9 12

HSDSC 35.0 35.8 39.4 45.6 54.8 64.0

Turbidity 35.6 36.6 39.0 44±45 54.0 63±64

Viscometry 35.0 36.0 40.0 44.0 54.0 62.0


phase transition temperature. However it has also been

suggested [5] that not all of the SDS might be expelled

from the particles when they shrink. It is possible that

the SDS helps to resolubilise the microgel when return-

ing to its original conformation. If this was not the

case then the particle would not be expected to regain

its original conformation until a lower temperature

was reached. However it should also be noted that this

could be due to a change in the optical properties of

the microgel changing with increasing SDS concen-

tration. Analysis by turbidity has also shown that the

conformational transition is gradual indicating that the

transition is a continuous process. This has previously

been attributed to the microgel particle being com-

posed of a network of chains of varying lengths each

believed to undergo their own phase transition at an

individual temperature. The overall phase transition

can then be visualised as a composite of these tran-

sition temperatures [5]. Alternatively it may be postu-

lated that the partition coe�cient for the system, Q is

given by the expression 1+eÿDG/RT. The fractional

population of the initial and ®nal states are then given

by 1/Q and 1ÿ1/Q respectively. The rate of change of

the population distribution between the initial and

®nal states with temperature is then governed by the

enthalpic and entropic contribution to the system.

Essentially the greater the enthalpy change the nar-

rower the temperature range over which the transition

occurs.

Fig. 5 shows the change in hydrodynamic diameter,

as observed by light scattering, both in the absence

and in the presence of increasing SDS concentration.

The results show that the particle size of the microgel

in the presence of SDS, but below the CMC of the sur-

factant (8.3 mM), is not very di�erent from that of the

microgel in the absence of SDS. It should be noted

that the particles observed by light scattering were

monodisperse corroborating the data obtained by elec-tron microscopy. However at a SDS concentrationabove that of the CMC a large increase in the particle

size is observed. In addition Fig. 5 also shows that thereduced viscosity, at 258C, increases with increasingconcentration of SDS until 4 mM but then remains

fairly constant thereafter. This could be attributed tothe observation, from the sorption isotherm data, thatbelow a concentration of 4 mM, the sorption of SDSto the microgel particle is low. Similarly the increase in

hydrodynamic diameter of the microgel particles couldbe attributed to the increase in SDS sorbed onto themicrogel particle. This can be explained by comparison

with the sorption of SDS observed from the bindingisotherms shown in Fig. 6. The isotherm indicates thatsorption of SDS at a concentration of 2 mM is low,

only slowly increasing until a concentration of 4 mMis reached. As a result of the small amounts of SDS

Fig. 5. Plot showing the hydrodynamic diameter (nm) of the microgel dispersion as a function of [SDS] (mM).

Fig. 6. Sorption isotherm of SDS to a 0.48% poly(NIPAM)

dispersion at 258C.


adsorbed the particle size of the microgel would not be

expected to increase substantially. However sorption ofthe SDS to the microgel particle becomes greater at anoriginal bulk concentration of 9 mM, i.e. above the

CMC of SDS in pure water (8.3 mM). This increase inthe amount of surfactant adsorbed can be consideredto be related to the corresponding increase in the hy-

drodynamic diameter of the particles in the presence ofa SDS concentration above the CMC as observed by

light scattering. It should also be noted that the par-ticles observed by light scattering even in the presenceof SDS were monodisperse in accordance with the elec-

tron microscopy data. At a SDS concentration abovethat of the CMC, a large increase in the particle size is

observed.It is not fully understood exactly how the microgel-

SDS interaction takes place. Other authors [5,10] have

suggested this phenomenon is attributable to the inter-action of the hydrophobic alkyl chain of the surfactantassociating with the polymer backbone of the microgel

particle thereby enhancing the solubility of the micro-gel in the solvent. Analysis, by ¯uorescence spec-

troscopy using molecular probes has been used toinvestigate the microenvironment of the microgel par-ticles and it has been shown that hydrophobic sites do

exist within the particles' infrastructure. These sitesexist both above and below the phase transition tem-

perature of the microgel particle, however with increas-ing temperature the microenvironment within themicrogel particle becomes increasingly hydrophobic [2].

It has also been suggested that with increasing tem-perature the microgel particle rearranges it's confor-mation so that the hydrophilic sites (sulphate) are

located mainly on the outside of the microgel particleand the hydrophobic sites (isopropyl-) are mainly

located within the microgel structure [2].Other workers [5,23] have investigated the e�ect of

the addition of surfactant with di�erent head groups

on the physical properties of microgels. This work hasbeen carried out using surfactants with the same non-

polar chain length but with head groups of di�erentfunctionality, i.e. anionic [5,23], cationic [5,23] andneutral [23]. Their ®ndings have shown that the ad-

dition of anionic SDS and cationic dodecyl pyridinebromide (DPB) [5] and dodecyl trimethylammoniumbromide (DTAB) [23], both change the transition tem-

perature of the microgel dispersion. SDS increases thetemperature at which the microgel undergoes a confor-

mational change whereas DPB and DTAB decreasethe temperature at which the conformational changeoccurrs but the neutral surfactant Triton X-100 has lit-

tle e�ect on the transition temperature. Similarly SDSwas shown to swell the microgel particles with increas-ing concentrations and also to increase the solubility

of the microgel in aqueous solutions at elevated tem-peratures. The cationic surfactant DPB causes some

swelling whereas DTAB does not. However the micro-

gels collapses due to attractive interactions between thecationic head groups of the surfactants and the anionicmicrogel particle [5]. Again Triton X-100, has no e�ect

on the swelling of the microgel particle [23].Previous workers [5] have attributed the swelling of

the poly(NIPAM) microgel as being due to the for-mation of micelles within the microgel particle. In arecent investigation Cosgrove and co-workers used

SANS to investigate the microgel-surfactant interaction[10]. They have suggested that not only does SDS as-sociate with the polymer chains but that aggregates of

several SDS molecules are formed within the microgelparticle. The formation of aggregates is believed to

contribute to the swelling of the microgel particles as aresult of the anionic head groups of the surfactantmolecules repelling the residual sulphate groups left on

the polymer backbone by the potassium persulphateinitiator [10].In the case of the interaction of DPB with microgel

particles it was suggested that it was more feasible thatthe formation of any aggregates within the microgel

particle would be hindered by attractive interactionsbetween the cationic surfactant and the anionic micro-gel particle [5].

Another explanation for the increase in solubil-isation of the microgel with increasing SDS concen-

tration is that the addition of surfactant is similar tothe grafting of acrylic acid monomers into the microgelstructure [9]. In the case of the grafting of acrylic acid

chains the hydrophilicity of the microgel is increasedleading to changes in the overall physical properties ofthe microgel making them responsive to changes in pH

and ionic strength. The surfactant is enhancing thesolubility of the microgel as a result of its attachment,

presumably to the hydrophobic sites in the structureenabling the microgel to remain solubilised at highertemperatures leading to an increase in the Tm.

The sorption isotherm describing the interaction ofSDS with the microgels is shown in Fig. 6 and indi-cates that below a SDS concentration of 4 mM sorp-

tion is low, then increases steadily until aconcentration of 12 mM. Binding isotherms have pre-

viously been obtained by surface tension techniques[10] and a depletion from solution technique [10]. Thephysical sorption isotherms reported in the current

study (Fig. 6) were obtained by the depletion from sol-ution method and the results reported are in agreementwith those obtained by Cosgrove et al. [10]. The results

reported here also agree with previous work in that thesorption of SDS, at concentrations below approx.

2 mM, is low. Other authors have suggested that thisis a result of there being a minimal concentration atwhich SDS begins to bind to the microgel, this concen-

tration is referred to as the critical aggregation concen-tration (CAC) and these CAC values have been


reported as being approx. 0.75±1 mM of SDS [11].The binding isotherms obtained by surface tension

measurements begin to plateau at a concentration of6 mM [11]. Such a value seems low when compared tothe sorption isotherms reported in this study and pre-

viously reported by Cosgrove et al. [10]. This could beattributed to the authors' assumption, that the micro-gel-SDS species is not surface-active [11]. The potential

existence of a synergistic e�ect between surfactant andmicrogel upon the surface tension should not be disre-garded and does not appear to have been taken into

account.The di�erence in the conductivity between the

microgel/SDS mixture and aqueous dispersions of SDSat an equivalent concentration at 258C decreases with

an increase in SDS concentration (Fig. 7) up to a con-centration approximately equivalent to the CMC ofSDS in water at room temperature. This suggests that

a large proportion of the surfactant is absorbed intothe microgel structure and thus not contributing to theoverall conductivity of the microgel/SDS dispersion.

This data is entirely consistent with the sorption data,which clearly indicates SDS is removed from the bulksolution phase. At 708C the conductivity di�erence is

less than at 258C indicating less association betweenthe microgel and the SDS. This is not surprising, asthe accessible volume of the microgel is much lessabove its conformational transition temperature.

These data also show that on going from 25 to 708Cexpulsion of SDS takes place from the microgel as thenow free SDS molecules contribute to the bulk con-

ductivity of the dispersion. This once again, is a directconsequence of any surfactant molecules residingwithin the interstitial spaces of the microgel being

squeezed out as the conformation collapses followingheating.

4. Conclusions

Viscometric, turbidity and HSDSC analysis has beenused to show that the addition of the anionic surfac-

tant SDS to an anionic colloidal microgel having term-inal SOÿ3 results in an increase in the conformationaltransition temperature of the particles. Furthermore atSDS concentrations of 4 mM and above, swelling of

the particle takes place under isothermal conditions.HSDSC studies of microgel-SDS mixtures shows that(a) the Tm of the conformational transition increases

with increasing SDS concentration but (b) the speci®cheat associated with the microgel conformational tran-sition decreases with increasing SDS concentration

whilst (c) the DT1/2 (8C) of the transitions increaseswith Tm (8C). From conductimetric measurements itcan be seen that an expulsion of a portion of thebound surfactant takes place on heating the microgel.

This is a result of the microgel particles undergoing aconformational change with a corresponding reductionin particle volume and the elimination of a high pro-

portion of water.

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The interaction of sodium dodecyl sulphate with colloidal microgel particles

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