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Titrimetric characterization of amphoteric latex
F.olloids and Surfaces, 6 (1983) 271-281 Usevier Scientific Publishing Company, Amsterdam
TlTRlMETRIC CHARACTERIZATION
271 -
Printed in The Netherlands
OF AMPHOTERIC LATEX
HARUMA KAWAOUCHI+. HIROTOMO HOSHINO and YASUJI OHTEUKA Department of Applied Chembhy. Faculty ofbclence 3-14-I Hiyoshi. Kohoku-&u. Yokohama 223 (Japan) (Received
Q Tecbnohgy.
26 August 1982; accepted in final form 22 November
Keio Unit~rrify.
1982)
ABSTRACT Amphoteric latex prepared by the Hofmann reaction of slyrene-acrylamide copolymer latex was characterized by titration. The amounts of amino and carboxyl groups on the particle surface were first determined by hydrogen ion titration. The isoeleclric point (i.e.p.) of the latex was obtained by turbidimetric titration. The i.e.p. npprOxi=MelY cotncided with the pH at which an inflection was observed on the conductometric titration curve. It was confirmed that the inflection point corresponds to the point of zero charge (p.2.c.) of the latex particle and the amount of amino and carbaxyl groups can be easily determined by the conductometric titration with NaOH or KOH although the titration with Ba(OH), seem6 to result in overestimation of the amount of zarhoxyl groups.
INTRODUCTION In our previous paper [l] a new method to prepare amphoteric latices was presented and the resulting latices were characterized i~y turhidimetric titration, electrophoresis, and conductometric titration. The conductomctric titrat.ion of the amphoteric latices revealed that the more amino groups present on the particle surface, the longer the range of SP in Fig. 1 becomes. In the titration of low molecular weight analogues, an inflection appears when all the carboxyl groups have just been neutralized and tht: range of SP
corresponds to the amount of carboxyl groups, This is quifc contrary to the case of the amphoteric latex. It was also found that the isoelectric point (i.e.p.) of the amphoterlc latices determined by turbidimetric titration approximately coincided with the ~13 at the inflection point P in Fig. 1. These results, characteristic of amphotiric latices, were explained from the contribution of the Icss mobile charge carrier- latex particles [ 11. In this paper further titrations are reported and a more detailed discussion on the relation between the above-mentioned phenomena and the surface structure of amphoteric latex particles is given.
*To whom correspondence
0166-6622/83/$03.00
should be addressed.
o 1983
Elsevier Science Publishers B.V.
272
T
Fig. 1. Sketch of conductometric titration curve of amphoteric latex. S: end point of titration cf ~~~955HCI; E: end point of titration of on-surface ionic grc~ps; P: inflection point; Q: poinl so that the amount of base titrated in SQ is equal lo that fn PE. An inverse system is shown because base titrations svere mainly carried out.
EXPEIIIMENTAI.
lllateriuls ‘I’hc nm@oteric latex used in lhis study was referred to as nmphotcric latex A nnd prrpanrl ns hollows: to B 300 ml flask, 18 g of styrcne, 2 g of ncrylnmi.Jc, and 150 g of dislilled water were nddcd find nitrogen w95 bubbIcd through the mixture under stirring. Emulsifier-free uqucous copolymerizntion \YZISinitintcd nt 70°C by 0.215 g of N,N’-nzobis (isopropylamidinc hydrorhloridc) dissolved in 10 g 01 wakr. After 24 h reaction, the resulting latex w.1~ rcpralctlly ccntriru&, decanted, nnd rcdispersecl to remove water soluble polymer nnd cuntamtxu>ls from the nqucous phase. The latex sontaining 10 g pitrticlcs aT 240 nm diqmetcr was treated with 2.8 g or 8.0% NaOCl and 2.1 $ of NaOH at 35’C Ear G h to cnrry out t-he Hofm;mn rexlion of acrylnmidc units on the particle surfswe. The Hofnmnn rcactlon accompanied by hydra. lysis introhcr?s aminn zmd carboxyt groups on the particle surfar:c [ 1.1. The rcact.ion was stopped by neutralization with HCl and the lntux was purified in the sarnc method ns mentioned above. Commcrci~l titrants wcrc used for each titration cxccpt 0.1 N Ba(OHjl, which was t>repnrcd before USC by dissolving barium hydroxide in hot watt% followed by tiltration nnd dctcrtnination of the normality under nitrogen.
The amphotcric l&x, rxtrificd cxhaustivcly by rcpcatcd ccntrifugationdecant ation-rcdislwrsion zmd ion exchange, was titrated with HCI and KOli sqxwatcly untlcr nitrogen. Ion exchanged and distilled water was also tifratcd to find the relation bctwccn the amount of II l or OH’ cxisbing in aqueous
273
solution (Hb+, OH;) and the pH of the solution. The amount of H* or OHbound to the ionogenic groups on the amphotcric particle surface was calculated from the difference between the amount of H’ or OH- t.itrated to the latex and Hs or OH; at each pH.
Twbidimetrlc
titmtiolr
‘The intensity latex containing photomultiplier. half mirror and icnl variation in function of the
Simultaneous
of 1Ie -NC laser (wave length 6328 A ) passing through the 0.05% particles (I) was measured by a Hamamatsu TV 931-A The intensity of the incident beam (lo) was monitored by a another photomult;plier to eliminate the influence of pmiodthe lasct intensity. Transmittance, log I/lo, was measured as a pH which
was changed
conductonzetric
by the addition
of acid or base.
and pH titrations
Two electrodes for conductivity and pl! measurements were dipped in 59 ml of Satex containing 2% particles, The latex was titrated with three kinds of bases, that is, 0.1 N NaCW, KOH, and Ba(OH)S under nitrogen. RESULTS AND DHCUSSXON
Hydrogen
ion titmtion
The most reliable method to determine the amcunt of amino and carboxy1 groups on an nmphoteric matrix would be hydrogen ion titration which is commonly used in studies on protein structure [ 21. In the biochemical field, “hydrogen ion titration” is &fined as the titration of polyampholyte solution or dispersion which contains only the polyions and their chargcdetermining countm ions. In the present case the polyions are the ionic groups on amphoteric particle surface and the counter ions are II* or OH*, dependent on the pH of purified Iatcx. The sample for titration must be purified until other ions arc completely removed. The pH value of the amphoteric latex A, purified exhaustivcSy, was 7,55 and this is regarded as its isoionic point (i+i.p.). According to the definition of i&p., at this pH the same amount of amino groups and carboxyl groups arc ionized to form the hmcr salt and the remaining amino groups are free where the amount, of amino groups is slightly larger than that of carboxyl groups. If the difference in the amount of both ions is large, dissociation of the excess ionogenic groups should be taken into account. The correction for this can be done from the pH value of the system [ 21. Titration of this latex with HCI rmults in protonation of the excess amino groups and carboxylic ions. AS the amount of the latter is equal to the protonatcd amino groups as mentioned above, the amount of H’ bound to the particles corresponds to the amount of total amino groups. On the other
hand, the amount of carboxy g-roups ten be obtained from the amount of KOH titrated. The result of titration is shown in Fig. 2. The amount of amino and c#boxyl groups can be estimated to be about 3 X lo-’ and 1 X 10” eq/g parlick or 0.79 and 0.26 unit-jnm*,respectively,although the curve does not resemble the combination of two sigmoidal dissociat.ion curyes for amino and carboxyl groups and, in the pH range lower than 3 or higher than 10.5, the absolute values of bound hydrogen ions decrease to some degree. The reasons for this finding are unknown at present.
I
3
I
t
1
I
7
5
I
I
9
I
11
PfI
Fig. 2. liydrugcn
ion litration of amphatcric lalcx A. 0: isoionic point.
Turbidim rtric titration The result of turbidimetric titration of the sama amphotcric latex is shown in Fig. 3. The latex has low transmittancewhen it coagulates and the result in Fig, 3 sl~owvs reversible coagulation-rcdispersion, dependent on pH* According to the method of Healy ct al. 131, the i.c.p. of this latex was d&xmined to be 7.9, whtch was the mid-point of ovdappt?d caagulation ranges for forward nnd back titrations. The i-e-p. is not an inheretat value for the lrrtcx because it deperwls on the kind and amount of coexisting electrolyte, but the i.c.p. obtained here WBSalmost the same as the i.i.p. obtained by hydrogen-ion titration, The coagulation range of back titration is wider than that of forward titration. This is mainly due to a slight increase in Ionic strength in thr!course of titration. When the ioniu strength of the system increases, in general, the amphoteric latex not only shows a wider coagulation range but aIso an irrevcrs:bIc coagulated portion which results in insufficient recovery of transmittance {3,4l. The latter phenomenon is not obscrrvcdeven at the end of the back titmtion curve in Fig. 3, although the amphoteric Iatices examined previously 111 showed it.
276
f 3
I
I
I
1
5
7
I
I
9
1
IL
11
PH
Fig. 3. Turbidimctric titration of amphoteric latex A. 0: forward titration with NaOH; l : back titration with HCI; -----coagulation range. i: isoelectric point.
In a control titration of original styrene-acrylamide copolymer latex, tha latex was found to be stable in the middle and high pH ranges but to be coagulated in the pH range lower than 3.6 and never rcdisperscd by further towering of pII. Simultaneous conductometric
and pH titruzion
In our previous study 1l] it was emphasized that there appears to be an inflection point (P in Fig. 1) on the conductometric tit-ration curve of amphoteric latzx and that the point P is not the one where all of the carboxy; groups are neutralized and amino groups start to be titrated, but the point P can be rcgarderi ss the point of zero charge (p.z.c.) as suggested by Ozaki et al. 161 .(Thep.z.c. corresponds to tke pH at which the particles have no SWface chwge an6 is not necessarily ident.ical to the 1.i.p.) They pointed out in the reinvestigation of Homola’s amphoteric latex [S] that the inflection point of the conductometric titration cur-weappears at the same pH as the intersection of pH titration curves obtined at different ionic strengths, which is the p.z.c. I71 - If there is no speciCe adsorption of any ions on the particle surface, the i.e.p, should be equal to the p.z,c. [‘i’] . The fact that the pH value at the inflection point of the conductometric titration curve for amphoteric latex A approximately coincided with the i.e.p, which was obtained from turbidimctric titration is in agreement with Oxaki’s inkrprctation (Fig. 4). According to this viewpoint, in the range from S to P in Fig. 1, the particles continue to lose their cationic charge and a release of counter anions causes an increase in the slope of SP. In the range after P up to E. on the contrary, the particles continue to become more anionic and a take-up of counter cations from the aqueous phase compensates the increase in conductivity, leading to a decrease in the slope of PE.
276
N/10 1.5
I
HCl 1.0 I
lmll
OS
?
1.0
CL5 WlO
NaOH (ml)
Fig. 4. Conductometric titration of amphoteric latex A with NaQIi conductivity x 10’ (SI-‘a”), A: 0.32; 0: 3.44; 0: 6.84;-----calcut.ted. pH values at inflection points.
and HCI. Initial specific Numbers Etrethe
B
277
Van der Put and Bijstirbosch gave an equation expressing the relation between the observed conductivity of latex (I&b) containing a low fraction of particles and the conductivity of bulk aqueous phase of the latex (K) at relatively high na (K is the reciprocal Debye length and u is the average radius of particles) [ 81, &br
= {I
- (3p/2)II-
3R./(l + 2R)j
}K + 3poula
(1)
where R was defined by R = p/Ku
(2)
and K was expressed as K = E Xi cj Zi /lOOO
(3)
p is the volume fraction of part.icles, o t.he elect.rokinetic charge density, u the electrophoretic mobility, ~1the surface conductivity, and Xi, ci, and Zi are the molar concenfration, charge, and equivalent conductance of free ion i, respectively. The contribut.ion of the last term in eq. (1) to K,,h was found to be negligible in all cases. K was calculated using Eq. (3) on the assumption of the take-up and release of counter ions in the course of the titratiofi of amphoteric latex as mentioned above. A comparison of an experimental titration CU~VCand a calculated K curve is shown in the bottom of Fig, 4. Both curves tolerably roincided with each other although the discrepancy ir.creased in the later stage of titration. The discrepancy can not be attributed to the term in the parentheses of Eq. (1) because the ionic strength Increased during the titration and this must result in a decrease in R and, therefore, should rather cause a decrease in Km&K. For all the latices t.itrated, p was 0.02 and u was 120 nm. The ionic st.rength changed from 2.8 X 10” at the starting of the lowest titration CUNC in Fig, 4 to 2.6 X 10’) at the end point. MCIwas less than 20 which might be too low to discuss this interesting curve by reference to Eq. (1). At relatively small KU, the change of ionic strength may affect the conductivity of the latex In a different manner. The binding force of counter ions to the surface polymeric ions would be weakened with increasing ionic strength and, therefore, the amount of counter ions bound to the particles might be less than that estimated in the later stage of titration, although the exact amount is hard to determine, In Fig. 4 ticre are three conductometric titration curves obtained at three different concentrations of coexisting electrclytes. The ratios of 1 ..- length of Mration range before and after the inflection points seem to coincide for three curves with an error of several percent. it is worth mentioning that, when back t.itrations with HCI were carried out after each forward titration (Fig. 4). almost the same results were obtained with respect to the length of two titration ranges before and after ths inflection point. (Only the last back titration curve is shown in Fig. 4.) Inflection points similar to the one in Fig. 4 appear occasionally even on the conductometric titration curves of non-amphoteric latices, for example, partially hydrolyzed poly(metbyl acrylate) latex [9] and styrene-methacrylic
278
acid copolymer latex 1 lo]. In the former system, the inflection is considered to be due to some con$ormational change of carboxyl group-containing polymer chains in the surfpxe layer resulting from the increasing ionization during base titration 19). The conformatibn of carboxy] group-containing chains is affected by ionic strength and, therefore, the back titration and further forward titration are expected to result in some drift of inflection point on the titration curves it the inflection is due to conformational change. The inRection points in Fig. 4 are independent of the ionic strength as m*eIlas the dir=tion of titration and, therefore, arc distinguishable from the inflection points for non-amphobric latices. The conduct.ometric titration curves of the amphoteric latex A with three kinds of Lib-ants, that is, NaOH, KOH, and Ba(OH),, are shown in Fig. 5, in
which the pH values at each character&tic point on the curyes are also presented. The pH values at the inflection points are almost the same among the three titrations but the length of titration range after the inflection point for Ba(OH), titration was extraordinarily large and this was a common result independent of the ionic strecgtb of latex.
N/ICI base (ml) Pig. 5. Conductometric A: KOH; a: Ba(OH),.
titration of ampboteric latex A with different titrmts. Numbers are :he pH values at each characteristic point.
o: N&H;
SP in Fig. 1 should be larger than PE when the latex has more amino groups groups, because in the range of SP not only all the carboxy groups but also a part of the ammonium Ions arc deprotunated up to the point where the amount of remaining ammonium ions becomes equal to the amount than carboxy
279
~Ptx243’&
&s $?sk& P. ~~~~t~~, PE, vih?m -Sk eJzsFx&~m-h ions are depmtanated, corresponds to the amount of carLuxyl ~TOII~S, The ratio of the length of SP to PE (in Fig. 1) for NaOH and KOH tikations was alwU% 3jI M-&I9 w85i*IX5rs%iDGf aminr, groqx+ to ra&SX4yI gSS%qXS on #I#? particle surface determined by hydrogen-ion titration in the previous section. Therefore, it was concluded that the amounts of ionogenic groups on the amphoteric latex particles can be determined by the conductometric titration with NaOH or KOH but not by the titration with Ba(OH).. Use of Ba(OH)l ‘~-‘Ulrj~~‘CLl~‘L~#~i~~~~J~‘hItztnNmj,-J1T~~.~~ulr;mR.
this seems to be atkibuted to some extraordinarily strong adsorption of Ba” onto the anionic particle surface because it may retard the increase in the conductivity, or to a non-stoichiometrical adsorption of Ba2+ to COO‘ because the carboxyl groups in the polymer chain are not necessarily located in a manner favorable for neutralization by divatent cations. Actually B$’ as well as other higher valence cations show stronger interaction with negatively or uncharged polymer particles than monovalent cations 111 J and this sometimes causes abnormal shape of the conductomctric titration curves in Ba(OH), titrations. Titration with Ba(OH), resulted in bending titration curves and the bend was attributed to stepwise titration of innermost and outermost H’ in the eiectric double layer [ 12, 13 J - But the end points in the titration with Ba(OH)l v.+erethe same as those in the titrations with other bases (12,131 in studies which dealt with latices having sulfate end groups on the particle surface. Therefore, the extension of the titration range for the amphoteric latex is not a common phenomenon in titrations with Ba(Ofl)t and more data are necessary for further discussion. The results of conductometric titration are summarized in Table I in which the data from three titrations with each titrant at different ionic strength were
TABLB
I
hmcnmts of ionogcnlc groups *n amphoteric mctrir? titrations with different titrank +-_---.-
-_.--_ Titrant
latex particles
nminD
---
0.34
group x 10. (q/g
by conducta__-..-
Carhoxyl patticle)
group ---
0.14
NaOtl t1CI
0.33
0.15
KOtI HCI
0.34 tJ.33
0.13 0.15
Ba(OFI), HCt
0.34 0.32
0.21 0.26
IIM
cbermined
was used for back k~Str&lonalter each ba5e
-
likra’tion.
280
averaged. It- could be confirmed that similar results were obtained from the titrations with NaOH and KOH and their back titrations. Theoretically, bhe end point of titration for carboxy groups should be Q in Fig. f where SQ = PE. No recognizable discontinuity was observed at the rorrcsponding position ou the conductomet-ric titration curves in Figs. 4 and 5. This is also the case for the pH and dpH/dV curves in Fig. 6. Another simultaneous conductometric and pH titration was carried out to a highly carboxylated but s?igbtly aminated amphotcric latex, but again the point Q
Fig. G. p’1 titration CUITCS with a condccto:nctric tion: +low; - - - middle; high.
titration
curve.
Ele:trolyte
concentra.
could not be detected on the bitration curves (not shown here). It was concluded that the shape of the pH curve as well as the conductometric one are governed by the total charge of each particle, but not by each of the ionogenie groups on the Farticle surface. Below the p.z.c. the particles donate hydrogen ions and counter anions to the aqueous phase. Above the p.z.c. the particles are regarded as an acceptor of hydroxyl ions and counter cations. In other words, below the p.z.c., deprotonation of carboxyl groups and ammonium ions has the same meaning against the aqueous phase with respect to the release of counter ions, and this leads to no remarkable change at point Q on the pH and conductometric titration curves. Another interpretation might be possible for the absence of discontinuity at point Q. The dissociation of both ionogenic groups might occur at a very wide and overlapped pH range because of different circumstances for each ionogenic group exkting in various chair- sequences and microstructures. It would, in any event, be impossible to obtain information about the dissociation constants for each ionogenic group on the particle surface from the shape of the pH titration curve. CONCLUSIONS
The conductomotric titration curve of an amphoteric latex has a characteristic inflection point. It was concluded from the results of hydrogen ion titration and turbidimcttic titration that the pH at the inflection point could be regarded as the p.z,c. of the latex particles and that the amount of amino and carboxyl g-roupa on the particle surfFtce could be determined by conductometric titration with NaOH or KOH.
REFERENCES 1 2 3 4
H. Kawaguchi, H. Hoshino, and Y. Ohisuka, J. Appl. Polym. sici.,26 (1931) 2015. in C,B. Anfinm, Jr., M-L. Anson, K. Bettey, and J-T. Edsall (Eds.). Advances In Protein Chemktry, Vol. 17, Academic Press, New York, 1962, p. 69. T.W. Healy, A. Homoht, and R,O. James, Faradey Disc. Chem. Sot., 65 (1978) 156. R.O. James, A. HomoIa, and T.W. Healy, J. Chem. Sot. Faraday Tram. I, 73 (1977) 1436. M+ Ozaki, K. Kutlta, and Y. Kowata, J. Chsm. Sot. Jpn., 1980 (1980) 1295. A. Homola and R.O. James, J. Co!told Interhe Set., 69 (1977) 123. B.H. Bijrterbosch and J. Lykiems, Adr Colloid Interfaci! Sci., 0 (1978) 147. A.G. Van der Put and B.H. BijLferbowh, J. CoIlold Interface Sri., 76 (lI)80) 625. R.M. Fitch, personal communicattan. 1982. H. Kawaguchi and Y. Ohtruka. unpubttshed. M-H. Wrtght and AM. James, Koltoid Z.Z. Polym., 251 (1973) 745. ILL Van den Hul and J-W. Vanderhoff, Etectroanat. Chem.. .37 (1972) 161. fC. Purusawa, Butt. Chem. Sot. Jpn., SB (1982) 48.
C. Tanford,
TlTRlMETRIC CHARACTERIZATION
271 -
Printed in The Netherlands
OF AMPHOTERIC LATEX
HARUMA KAWAOUCHI+. HIROTOMO HOSHINO and YASUJI OHTEUKA Department of Applied Chembhy. Faculty ofbclence 3-14-I Hiyoshi. Kohoku-&u. Yokohama 223 (Japan) (Received
Q Tecbnohgy.
26 August 1982; accepted in final form 22 November
Keio Unit~rrify.
1982)
ABSTRACT Amphoteric latex prepared by the Hofmann reaction of slyrene-acrylamide copolymer latex was characterized by titration. The amounts of amino and carboxyl groups on the particle surface were first determined by hydrogen ion titration. The isoeleclric point (i.e.p.) of the latex was obtained by turbidimetric titration. The i.e.p. npprOxi=MelY cotncided with the pH at which an inflection was observed on the conductometric titration curve. It was confirmed that the inflection point corresponds to the point of zero charge (p.2.c.) of the latex particle and the amount of amino and carbaxyl groups can be easily determined by the conductometric titration with NaOH or KOH although the titration with Ba(OH), seem6 to result in overestimation of the amount of zarhoxyl groups.
INTRODUCTION In our previous paper [l] a new method to prepare amphoteric latices was presented and the resulting latices were characterized i~y turhidimetric titration, electrophoresis, and conductometric titration. The conductomctric titrat.ion of the amphoteric latices revealed that the more amino groups present on the particle surface, the longer the range of SP in Fig. 1 becomes. In the titration of low molecular weight analogues, an inflection appears when all the carboxyl groups have just been neutralized and tht: range of SP
corresponds to the amount of carboxyl groups, This is quifc contrary to the case of the amphoteric latex. It was also found that the isoelectric point (i.e.p.) of the amphoterlc latices determined by turbidimetric titration approximately coincided with the ~13 at the inflection point P in Fig. 1. These results, characteristic of amphotiric latices, were explained from the contribution of the Icss mobile charge carrier- latex particles [ 11. In this paper further titrations are reported and a more detailed discussion on the relation between the above-mentioned phenomena and the surface structure of amphoteric latex particles is given.
*To whom correspondence
0166-6622/83/$03.00
should be addressed.
o 1983
Elsevier Science Publishers B.V.
272
T
Fig. 1. Sketch of conductometric titration curve of amphoteric latex. S: end point of titration cf ~~~955HCI; E: end point of titration of on-surface ionic grc~ps; P: inflection point; Q: poinl so that the amount of base titrated in SQ is equal lo that fn PE. An inverse system is shown because base titrations svere mainly carried out.
EXPEIIIMENTAI.
lllateriuls ‘I’hc nm@oteric latex used in lhis study was referred to as nmphotcric latex A nnd prrpanrl ns hollows: to B 300 ml flask, 18 g of styrcne, 2 g of ncrylnmi.Jc, and 150 g of dislilled water were nddcd find nitrogen w95 bubbIcd through the mixture under stirring. Emulsifier-free uqucous copolymerizntion \YZISinitintcd nt 70°C by 0.215 g of N,N’-nzobis (isopropylamidinc hydrorhloridc) dissolved in 10 g 01 wakr. After 24 h reaction, the resulting latex w.1~ rcpralctlly ccntriru&, decanted, nnd rcdispersecl to remove water soluble polymer nnd cuntamtxu>ls from the nqucous phase. The latex sontaining 10 g pitrticlcs aT 240 nm diqmetcr was treated with 2.8 g or 8.0% NaOCl and 2.1 $ of NaOH at 35’C Ear G h to cnrry out t-he Hofm;mn rexlion of acrylnmidc units on the particle surfswe. The Hofnmnn rcactlon accompanied by hydra. lysis introhcr?s aminn zmd carboxyt groups on the particle surfar:c [ 1.1. The rcact.ion was stopped by neutralization with HCl and the lntux was purified in the sarnc method ns mentioned above. Commcrci~l titrants wcrc used for each titration cxccpt 0.1 N Ba(OHjl, which was t>repnrcd before USC by dissolving barium hydroxide in hot watt% followed by tiltration nnd dctcrtnination of the normality under nitrogen.
The amphotcric l&x, rxtrificd cxhaustivcly by rcpcatcd ccntrifugationdecant ation-rcdislwrsion zmd ion exchange, was titrated with HCI and KOli sqxwatcly untlcr nitrogen. Ion exchanged and distilled water was also tifratcd to find the relation bctwccn the amount of II l or OH’ cxisbing in aqueous
273
solution (Hb+, OH;) and the pH of the solution. The amount of H* or OHbound to the ionogenic groups on the amphotcric particle surface was calculated from the difference between the amount of H’ or OH- t.itrated to the latex and Hs or OH; at each pH.
Twbidimetrlc
titmtiolr
‘The intensity latex containing photomultiplier. half mirror and icnl variation in function of the
Simultaneous
of 1Ie -NC laser (wave length 6328 A ) passing through the 0.05% particles (I) was measured by a Hamamatsu TV 931-A The intensity of the incident beam (lo) was monitored by a another photomult;plier to eliminate the influence of pmiodthe lasct intensity. Transmittance, log I/lo, was measured as a pH which
was changed
conductonzetric
by the addition
of acid or base.
and pH titrations
Two electrodes for conductivity and pl! measurements were dipped in 59 ml of Satex containing 2% particles, The latex was titrated with three kinds of bases, that is, 0.1 N NaCW, KOH, and Ba(OH)S under nitrogen. RESULTS AND DHCUSSXON
Hydrogen
ion titmtion
The most reliable method to determine the amcunt of amino and carboxy1 groups on an nmphoteric matrix would be hydrogen ion titration which is commonly used in studies on protein structure [ 21. In the biochemical field, “hydrogen ion titration” is &fined as the titration of polyampholyte solution or dispersion which contains only the polyions and their chargcdetermining countm ions. In the present case the polyions are the ionic groups on amphoteric particle surface and the counter ions are II* or OH*, dependent on the pH of purified Iatcx. The sample for titration must be purified until other ions arc completely removed. The pH value of the amphoteric latex A, purified exhaustivcSy, was 7,55 and this is regarded as its isoionic point (i+i.p.). According to the definition of i&p., at this pH the same amount of amino groups and carboxyl groups arc ionized to form the hmcr salt and the remaining amino groups are free where the amount, of amino groups is slightly larger than that of carboxyl groups. If the difference in the amount of both ions is large, dissociation of the excess ionogenic groups should be taken into account. The correction for this can be done from the pH value of the system [ 21. Titration of this latex with HCI rmults in protonation of the excess amino groups and carboxylic ions. AS the amount of the latter is equal to the protonatcd amino groups as mentioned above, the amount of H’ bound to the particles corresponds to the amount of total amino groups. On the other
hand, the amount of carboxy g-roups ten be obtained from the amount of KOH titrated. The result of titration is shown in Fig. 2. The amount of amino and c#boxyl groups can be estimated to be about 3 X lo-’ and 1 X 10” eq/g parlick or 0.79 and 0.26 unit-jnm*,respectively,although the curve does not resemble the combination of two sigmoidal dissociat.ion curyes for amino and carboxyl groups and, in the pH range lower than 3 or higher than 10.5, the absolute values of bound hydrogen ions decrease to some degree. The reasons for this finding are unknown at present.
I
3
I
t
1
I
7
5
I
I
9
I
11
PfI
Fig. 2. liydrugcn
ion litration of amphatcric lalcx A. 0: isoionic point.
Turbidim rtric titration The result of turbidimetric titration of the sama amphotcric latex is shown in Fig. 3. The latex has low transmittancewhen it coagulates and the result in Fig, 3 sl~owvs reversible coagulation-rcdispersion, dependent on pH* According to the method of Healy ct al. 131, the i.c.p. of this latex was d&xmined to be 7.9, whtch was the mid-point of ovdappt?d caagulation ranges for forward nnd back titrations. The i-e-p. is not an inheretat value for the lrrtcx because it deperwls on the kind and amount of coexisting electrolyte, but the i.c.p. obtained here WBSalmost the same as the i.i.p. obtained by hydrogen-ion titration, The coagulation range of back titration is wider than that of forward titration. This is mainly due to a slight increase in Ionic strength in thr!course of titration. When the ioniu strength of the system increases, in general, the amphoteric latex not only shows a wider coagulation range but aIso an irrevcrs:bIc coagulated portion which results in insufficient recovery of transmittance {3,4l. The latter phenomenon is not obscrrvcdeven at the end of the back titmtion curve in Fig. 3, although the amphoteric Iatices examined previously 111 showed it.
276
f 3
I
I
I
1
5
7
I
I
9
1
IL
11
PH
Fig. 3. Turbidimctric titration of amphoteric latex A. 0: forward titration with NaOH; l : back titration with HCI; -----coagulation range. i: isoelectric point.
In a control titration of original styrene-acrylamide copolymer latex, tha latex was found to be stable in the middle and high pH ranges but to be coagulated in the pH range lower than 3.6 and never rcdisperscd by further towering of pII. Simultaneous conductometric
and pH titruzion
In our previous study 1l] it was emphasized that there appears to be an inflection point (P in Fig. 1) on the conductometric tit-ration curve of amphoteric latzx and that the point P is not the one where all of the carboxy; groups are neutralized and amino groups start to be titrated, but the point P can be rcgarderi ss the point of zero charge (p.z.c.) as suggested by Ozaki et al. 161 .(Thep.z.c. corresponds to tke pH at which the particles have no SWface chwge an6 is not necessarily ident.ical to the 1.i.p.) They pointed out in the reinvestigation of Homola’s amphoteric latex [S] that the inflection point of the conductometric titration cur-weappears at the same pH as the intersection of pH titration curves obtined at different ionic strengths, which is the p.z.c. I71 - If there is no speciCe adsorption of any ions on the particle surface, the i.e.p, should be equal to the p.z,c. [‘i’] . The fact that the pH value at the inflection point of the conductometric titration curve for amphoteric latex A approximately coincided with the i.e.p, which was obtained from turbidimctric titration is in agreement with Oxaki’s inkrprctation (Fig. 4). According to this viewpoint, in the range from S to P in Fig. 1, the particles continue to lose their cationic charge and a release of counter anions causes an increase in the slope of SP. In the range after P up to E. on the contrary, the particles continue to become more anionic and a take-up of counter cations from the aqueous phase compensates the increase in conductivity, leading to a decrease in the slope of PE.
276
N/10 1.5
I
HCl 1.0 I
lmll
OS
?
1.0
CL5 WlO
NaOH (ml)
Fig. 4. Conductometric titration of amphoteric latex A with NaQIi conductivity x 10’ (SI-‘a”), A: 0.32; 0: 3.44; 0: 6.84;-----calcut.ted. pH values at inflection points.
and HCI. Initial specific Numbers Etrethe
B
277
Van der Put and Bijstirbosch gave an equation expressing the relation between the observed conductivity of latex (I&b) containing a low fraction of particles and the conductivity of bulk aqueous phase of the latex (K) at relatively high na (K is the reciprocal Debye length and u is the average radius of particles) [ 81, &br
= {I
- (3p/2)II-
3R./(l + 2R)j
}K + 3poula
(1)
where R was defined by R = p/Ku
(2)
and K was expressed as K = E Xi cj Zi /lOOO
(3)
p is the volume fraction of part.icles, o t.he elect.rokinetic charge density, u the electrophoretic mobility, ~1the surface conductivity, and Xi, ci, and Zi are the molar concenfration, charge, and equivalent conductance of free ion i, respectively. The contribut.ion of the last term in eq. (1) to K,,h was found to be negligible in all cases. K was calculated using Eq. (3) on the assumption of the take-up and release of counter ions in the course of the titratiofi of amphoteric latex as mentioned above. A comparison of an experimental titration CU~VCand a calculated K curve is shown in the bottom of Fig, 4. Both curves tolerably roincided with each other although the discrepancy ir.creased in the later stage of titration. The discrepancy can not be attributed to the term in the parentheses of Eq. (1) because the ionic strength Increased during the titration and this must result in a decrease in R and, therefore, should rather cause a decrease in Km&K. For all the latices t.itrated, p was 0.02 and u was 120 nm. The ionic st.rength changed from 2.8 X 10” at the starting of the lowest titration CUNC in Fig, 4 to 2.6 X 10’) at the end point. MCIwas less than 20 which might be too low to discuss this interesting curve by reference to Eq. (1). At relatively small KU, the change of ionic strength may affect the conductivity of the latex In a different manner. The binding force of counter ions to the surface polymeric ions would be weakened with increasing ionic strength and, therefore, the amount of counter ions bound to the particles might be less than that estimated in the later stage of titration, although the exact amount is hard to determine, In Fig. 4 ticre are three conductometric titration curves obtained at three different concentrations of coexisting electrclytes. The ratios of 1 ..- length of Mration range before and after the inflection points seem to coincide for three curves with an error of several percent. it is worth mentioning that, when back t.itrations with HCI were carried out after each forward titration (Fig. 4). almost the same results were obtained with respect to the length of two titration ranges before and after ths inflection point. (Only the last back titration curve is shown in Fig. 4.) Inflection points similar to the one in Fig. 4 appear occasionally even on the conductometric titration curves of non-amphoteric latices, for example, partially hydrolyzed poly(metbyl acrylate) latex [9] and styrene-methacrylic
278
acid copolymer latex 1 lo]. In the former system, the inflection is considered to be due to some con$ormational change of carboxyl group-containing polymer chains in the surfpxe layer resulting from the increasing ionization during base titration 19). The conformatibn of carboxy] group-containing chains is affected by ionic strength and, therefore, the back titration and further forward titration are expected to result in some drift of inflection point on the titration curves it the inflection is due to conformational change. The inRection points in Fig. 4 are independent of the ionic strength as m*eIlas the dir=tion of titration and, therefore, arc distinguishable from the inflection points for non-amphobric latices. The conduct.ometric titration curves of the amphoteric latex A with three kinds of Lib-ants, that is, NaOH, KOH, and Ba(OH),, are shown in Fig. 5, in
which the pH values at each character&tic point on the curyes are also presented. The pH values at the inflection points are almost the same among the three titrations but the length of titration range after the inflection point for Ba(OH), titration was extraordinarily large and this was a common result independent of the ionic strecgtb of latex.
N/ICI base (ml) Pig. 5. Conductometric A: KOH; a: Ba(OH),.
titration of ampboteric latex A with different titrmts. Numbers are :he pH values at each characteristic point.
o: N&H;
SP in Fig. 1 should be larger than PE when the latex has more amino groups groups, because in the range of SP not only all the carboxy groups but also a part of the ammonium Ions arc deprotunated up to the point where the amount of remaining ammonium ions becomes equal to the amount than carboxy
279
~Ptx243’&
&s $?sk& P. ~~~~t~~, PE, vih?m -Sk eJzsFx&~m-h ions are depmtanated, corresponds to the amount of carLuxyl ~TOII~S, The ratio of the length of SP to PE (in Fig. 1) for NaOH and KOH tikations was alwU% 3jI M-&I9 w85i*IX5rs%iDGf aminr, groqx+ to ra&SX4yI gSS%qXS on #I#? particle surface determined by hydrogen-ion titration in the previous section. Therefore, it was concluded that the amounts of ionogenic groups on the amphoteric latex particles can be determined by the conductometric titration with NaOH or KOH but not by the titration with Ba(OH).. Use of Ba(OH)l ‘~-‘Ulrj~~‘CLl~‘L~#~i~~~~J~‘hItztnNmj,-J1T~~.~~ulr;mR.
this seems to be atkibuted to some extraordinarily strong adsorption of Ba” onto the anionic particle surface because it may retard the increase in the conductivity, or to a non-stoichiometrical adsorption of Ba2+ to COO‘ because the carboxyl groups in the polymer chain are not necessarily located in a manner favorable for neutralization by divatent cations. Actually B$’ as well as other higher valence cations show stronger interaction with negatively or uncharged polymer particles than monovalent cations 111 J and this sometimes causes abnormal shape of the conductomctric titration curves in Ba(OH), titrations. Titration with Ba(OH), resulted in bending titration curves and the bend was attributed to stepwise titration of innermost and outermost H’ in the eiectric double layer [ 12, 13 J - But the end points in the titration with Ba(OH)l v.+erethe same as those in the titrations with other bases (12,131 in studies which dealt with latices having sulfate end groups on the particle surface. Therefore, the extension of the titration range for the amphoteric latex is not a common phenomenon in titrations with Ba(Ofl)t and more data are necessary for further discussion. The results of conductometric titration are summarized in Table I in which the data from three titrations with each titrant at different ionic strength were
TABLB
I
hmcnmts of ionogcnlc groups *n amphoteric mctrir? titrations with different titrank +-_---.-
-_.--_ Titrant
latex particles
nminD
---
0.34
group x 10. (q/g
by conducta__-..-
Carhoxyl patticle)
group ---
0.14
NaOtl t1CI
0.33
0.15
KOtI HCI
0.34 tJ.33
0.13 0.15
Ba(OFI), HCt
0.34 0.32
0.21 0.26
IIM
cbermined
was used for back k~Str&lonalter each ba5e
-
likra’tion.
280
averaged. It- could be confirmed that similar results were obtained from the titrations with NaOH and KOH and their back titrations. Theoretically, bhe end point of titration for carboxy groups should be Q in Fig. f where SQ = PE. No recognizable discontinuity was observed at the rorrcsponding position ou the conductomet-ric titration curves in Figs. 4 and 5. This is also the case for the pH and dpH/dV curves in Fig. 6. Another simultaneous conductometric and pH titration was carried out to a highly carboxylated but s?igbtly aminated amphotcric latex, but again the point Q
Fig. G. p’1 titration CUITCS with a condccto:nctric tion: +low; - - - middle; high.
titration
curve.
Ele:trolyte
concentra.
could not be detected on the bitration curves (not shown here). It was concluded that the shape of the pH curve as well as the conductometric one are governed by the total charge of each particle, but not by each of the ionogenie groups on the Farticle surface. Below the p.z.c. the particles donate hydrogen ions and counter anions to the aqueous phase. Above the p.z.c. the particles are regarded as an acceptor of hydroxyl ions and counter cations. In other words, below the p.z.c., deprotonation of carboxyl groups and ammonium ions has the same meaning against the aqueous phase with respect to the release of counter ions, and this leads to no remarkable change at point Q on the pH and conductometric titration curves. Another interpretation might be possible for the absence of discontinuity at point Q. The dissociation of both ionogenic groups might occur at a very wide and overlapped pH range because of different circumstances for each ionogenic group exkting in various chair- sequences and microstructures. It would, in any event, be impossible to obtain information about the dissociation constants for each ionogenic group on the particle surface from the shape of the pH titration curve. CONCLUSIONS
The conductomotric titration curve of an amphoteric latex has a characteristic inflection point. It was concluded from the results of hydrogen ion titration and turbidimcttic titration that the pH at the inflection point could be regarded as the p.z,c. of the latex particles and that the amount of amino and carboxyl g-roupa on the particle surfFtce could be determined by conductometric titration with NaOH or KOH.
REFERENCES 1 2 3 4
H. Kawaguchi, H. Hoshino, and Y. Ohisuka, J. Appl. Polym. sici.,26 (1931) 2015. in C,B. Anfinm, Jr., M-L. Anson, K. Bettey, and J-T. Edsall (Eds.). Advances In Protein Chemktry, Vol. 17, Academic Press, New York, 1962, p. 69. T.W. Healy, A. Homoht, and R,O. James, Faradey Disc. Chem. Sot., 65 (1978) 156. R.O. James, A. HomoIa, and T.W. Healy, J. Chem. Sot. Faraday Tram. I, 73 (1977) 1436. M+ Ozaki, K. Kutlta, and Y. Kowata, J. Chsm. Sot. Jpn., 1980 (1980) 1295. A. Homola and R.O. James, J. Co!told Interhe Set., 69 (1977) 123. B.H. Bijrterbosch and J. Lykiems, Adr Colloid Interfaci! Sci., 0 (1978) 147. A.G. Van der Put and B.H. BijLferbowh, J. CoIlold Interface Sri., 76 (lI)80) 625. R.M. Fitch, personal communicattan. 1982. H. Kawaguchi and Y. Ohtruka. unpubttshed. M-H. Wrtght and AM. James, Koltoid Z.Z. Polym., 251 (1973) 745. ILL Van den Hul and J-W. Vanderhoff, Etectroanat. Chem.. .37 (1972) 161. fC. Purusawa, Butt. Chem. Sot. Jpn., SB (1982) 48.
C. Tanford,