Introduction

Organic–inorganic hybrid materials constitute a class ofmaterial with properties of great technological interest.Recently, a great number of works involving hybridsbased on blends of oxides, such as silica, and organicpolymers dispersed at a molecular level have been re-ported [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. The hybridizationof organic and inorganic polymers has generally beendone with the aim of producing high-performance

thermally resistant materials, such as abrasives andcoatings [12, 13, 14, 15, 16]. The preparation of hybridsbased on silicon for several applications can be carriedout by using a sol–gel process which allows the synthesisof high-purity materials, with a wide variety of forms,such as powders, fibers and films [17, 18, 19, 20, 21, 22,23, 24, 25]. In the area of ceramic, metallic and com-posite materials, several routes of preparation of mi-crospherical precursors are described in the literature[1, 8, 17, 24, 26, 27, 28, 29].

ORIGINAL CONTRIBUTIONColloid Polym Sci (2003) 281: 19–26DOI 10.1007/s00396-002-0726-8

M.A.S. Pedroso

M.L. Dias

C. Azuma

R.A.S. San Gil

C.G. Mothe

Hydrocarbon dispersion of nanosphericalsilica by a sol–gel process.2. Alkoxysilane copolymerization

Received: 19 November 2001Accepted: 17 April 2002Published online: 1 November 2002� Springer-Verlag 2002

M.A.S. PedrosoCODAP, Universidade Federal de Sergipe,Cidade Universitaria, 49100-000,Aracaju, SE, Brazil

C.G. MotheDepartamento de Processos Organicos,Escola de Quımica, Universidade Federaldo Rio de Janeiro, C.P. 68522,21949-900, Rio de Janeiro, RJ, Brazil

M.L. Dias (&)Instituto de Macromoleculas ProfessoraEloisa Mano, Universidade Federal do Riode Janeiro, C.P. 68525,21945-970 Rio de Janeiro, RJ, BrazilE-mail: mldias@ima.ufrj.br

C. AzumaThe University of the Air, SetagayaGakushu Center, 4-1-1, Shimouma,Setagaya-Ku, Tokyo 154, Japan

R.A.S. San GilDepartamento de Quımica Organica,Instituto de Quımica, Universidade Federaldo Rio de Janeiro, C.P. 68556,21941-972 Rio de Janeiro, RJ, Brazil

Abstract The preparation of hydro-carbon dispersions of nanosphericalsilica–silicone hybrids by copoly-merization of tetraethoxysilane(TEOS) and alkylethoxysilanes suchas methyltriethoxysilane (MTEOS)and dimethyldiethoxysilane using asol–gel process was investigated.Particles completely dispersible inhexane with a diameter in the range10–24 nm were obtained by means ofTEOS and MTEOS copolymeriza-tion, using ethanol or methanol as asolvent and a terminator. The solu-bility of the nanospheres was shownto be dependent on the molar ratio ofwater to silicon, the reaction time,the terminator concentration as wellas the stirring condition. The hexane-dispersible particles have hydroxyland methyl groups on the surface. Ahigher conversion of organophilicsilica–silicon nanoparticles was at-tained using trimethylchlorosilane asa terminator and a water-to-siliconmolar ratio close to 9.

Keywords Copolymerization ÆSol–gel process Æ Hybrid ÆNanoparticle Æ Alkoxysilanes

Hybrids with controlled spherical morphology andsize in the range of nanometers can also be obtained bythe sol–gel process. The synthesis of those nanospheresby the sol–gel reaction is usually accomplished in sys-tems containing tetraethoxysilane (TEOS), methyltri-ethoxysilane (MTEOS) or dimethyldiethoxysilane(DMDEOS) in alcoholic solution, catalyzed by bases.Depending on the synthetic conditions, those nanopar-ticles can be either insoluble or homogeneously dis-persible in organic solvents [1].

Aliphatic hydrocarbons dispersions of silica nano-particles obtained by homopolymerization of TEOS inan alcoholic medium using a base-catalyzed sol–gelprocess in the presence of a terminator were recentlydescribed in the literature [25]. The process of the for-mation of those nanoparticles involves a first stage,where by means of the hydrolysis of the alkoxide thehomopolymerization occurs with the growth of theparticle (growth step). In a second stage (terminationstep), the introduction of the terminator based on tri-alkylchlorosilane or trialkylalkoxysilane promotes thereaction with the surface of the nascent particle whichcontains hydroxyl groups, stopping the growth and in-troducing apolar groups which allow the homogeneousdispersion of the nanospheres in hydrocarbons. Thereaction time in the stage of particle growth is decisivein the establishment of the particle size, allowing anincrease in the diameter with an increase in time. Theconditions employed in the homopolymerization re-sulted in a maximum of 62% hexane-dispersible frac-tion [25]. Low temperatures in the termination stagefavored an increase in the content of hexane-dispersibleparticles, which also increases when the terminatorconcentration is increased. Chlorinated terminators aremore efficient in increasing the fraction dispersible inhydrocarbons.

In this work, organophilic polymeric hybrid nano-spheres of silica–silicone were synthesized by the copo-lymerization of TEOS with MTEOS, DMDEOS anddimethoxydiphenylsilane (DMODPS) in similar condi-tions to those described previously [25]. The results ofthe characterization by light scattering and NMR arepresented.

Experimental

Materials

TEOS, MTEOS, DMDEOS, 28% aqueous ammonium hydroxide,ethanol, methanol, tetrahydrofurane (THF), n-hexane, trimethyl-chlorosilane (TMCS), dichlorodimethylsilane (DCDMS), trime-thyletoxysilane (TMES) and dichlorodiphenylsilane (DCDPS) werepurchased from Wako Pure Chemical Industries (Japan) and wereused as received. Commercial silica Aerosil was supplied by AerosilNippon.

Nanoparticle preparation

The syntheses of organophilic hybrid nanospheres of silica–siliconeby a sol–gel reaction using TEOS, MTEOS and/or DMDEOS asmonomers were carried out according to the following procedure.

Monomers, solvent (ethanol, methanol or THF), aqueous am-monium hydroxide and water were added to a 100- or 1000-mlreactor in this order. The system equipped with a magnetic or amechanical stirrer and a thermometer was placed in a silicon oilbath at 50 �C. The reaction was carried out at atmospheric pressureand the time was varied for each reaction. After the prescribedreaction time, the heating bath was removed and an n-hexane so-lution of the terminator was added to the reaction medium. Fourtypes of terminators were used in the termination stage: TMCS,TMES, DCDMS and DCDPS. The resulting reaction mixture wasthen stirred vigorously for 16 h at room temperature. After this, thereaction medium contained two phases: one rich in ethanol ormethanol and the other rich in n-hexane. The alcohol-rich phase,containing a large amount of chloride formed in the reaction andthe insoluble silica particles, was then separated by decantation.The n-hexane rich phase containing the silica–silicone hybrid waswashed with water to remove possible salts and the solid powder ofthe organophilic nanospheres was recovered by evaporating then-hexane under vacuum. The powder was kept under a controlledatmosphere until it was analyzed.

Nanoparticle characterization

Light scattering analysis was made using an Otsuka, DL S-7000Tapparatus. Ethanol was used as the solvent. In this alcohol thenanospheres were easily dispersed homogeneously.

Solid-state NMR experiments were carried out using a BrukerAvance DRX-300 at 59.63 MHz (29Si) with a supersonic probe(Bruker CP/MAS 4 mm) and 4-mm zirconium oxide rotors. Theblock decay pulse sequence with spinning of the sample at 4 kHz(magic-angle spinning, MAS, NMR) was used to obtain the rela-tive quantitative evaluation of SiOH sites. The acquisition condi-tions for MAS NMR were P1 (p/2) 5 ms, AQ 12.041 ms, D1 10 s,1,000 acquisitions. It is known that the rate of polarization transferin a cross-polarization experiment (CP MAS NMR) are differentfor the sites Q2 [Si(OH)2], Q3 [Si(OH)] and Q4 (SiO). This meansthat the contact times were also different so that these sites wereobserved at values of maximum sign intensities [30]. In this way, theexperimental conditions used for CP MAS NMR were contact time4 ms, P1 (p/2) 5 ms, AQ 12.041 ms, D1 5 s, 600 acquisitions. Akaolin sample (d=–91.5 ppm) was used as a secondary reference.

Results and discussion

It is known that sol–gel reactions with alkoxysilanesresult in linear polymeric products with acidic catalystsor result in microspherical polymeric products whenbasic catalysts are used [31, 32, 33].

The reactions taking place in the growth and termi-nation stages of particles prepared by the sol–gel processwith TEOS were presented previously [25]. The copoly-merization process of TEOS with MTEOS or DMDEOSinvolves similar reactions. The hydrolysis of the mono-mers produces molecules with hydroxyl groups bondedto Si atoms by the reaction with water. By condensationof these silanol groups, progressively larger particles aregenerated. These particles may contain predominately

20

Si–O bonds in the internal structure and hydroxyl andethoxy groups on the external part. Addition of func-tional compounds such as TMCS or TMES, capable ofreacting with surface hydroxyl groups, can interrupt thegrowth of the particles (termination stage) by the in-troduction of –Si(CH3)3 groups on the surface of theparticles.

Effect of the reaction time

In the copolymerization reactions described in thiswork, H2O/Si molar ratios, r, from medium to high(r=5.2–16.2) were used. This procedure was done withthe aim of investigating the effect of the reaction time onthe size of the particle. The increase in r promotes theacceleration of the hydrolysis reaction, decreasing thereaction time.

The effect of the reaction time of the growth stage onthe structure of the nanospherical copolymers formed ispresented in Table 1. The percentage of hydrocarbon-dispersible silica–silicone particles changed from 15.9 to100%. The values of the number-average diameter of theparticles, Dn, changed from 11 to 100 nm with pre-dominance around 10 nm. Size dispersion indexes, Dw/Dn, from 1.03 to 1.32 were obtained, indicating that anarrow particle size distribution is attained in all thecondition used. In the synthesis performed in methanol,Dw/Dn increased as Dn increased.

The best reaction time was 2 hours where 99.8% oforganophilic silica with Dn=11 nm and Dw/Dn=1.05was attained. After 2 h, it is possible that these particlesinteract with each other, forming particles of larger di-ameters. This observation could justify the increase inthe fraction insoluble in hexane with the increase in thereaction time. If the aggregation process takes place inthe growth stage, the termination stage has little influ-ence on the organophilicity of the particles.

When ethanol was used as a solvent, a gel was formedindependent of the reaction time that was varied from 1to 24 h. In all cases, magnetic stirring was used. It isimportant to mention that the gel was formed during theprocess of solvent elimination which was accomplishedunder vacuum at 40 �C. This gel is a network structurewith low solubility. When methanol was used as thesolvent, gel formation was not observed.

Effect of the terminator

The role of the terminator in the reaction is to interruptthe growth of the particles and to provide the intro-duction of apolar groups such as –Si(CH3)3 on thesurface of the nanospheres, making them hydrocarbon-dispersible. The reaction conditions can allow fastgrowth of the spheres, forming structures with largerdiameters close to a critical value for soluble products,but highly organophilic owing to a large number ofapolar groups on its surface.

The effects of the terminator concentration on theorganophilicity of the silica–silicon particles obtainedfrom TEOS/MTEOS copolymerization terminated byTMCS are summarized in Table 2. The results showthat when the concentration of TMCS increases, thepercentage of the organophilic nanospheres as well asthe particle size increase. In the conditions used, itwas possible to obtain a high amount of hexane-dis-persible product when the TMCS concentration was0.80 mol/l.

A performance comparison of the three terminatorsused in this work (TMCS, DCDMS and DCDPS) ispresented in Table 3. The highest value of the hexane-dispersible fraction was obtained when TMCS was used.Such a result is similar to that discussed in our previousreport for homopolymerization of TEOS in ethanol[25]. The efficiency of the terminator decreases in the

Reactionsolvent

Reactiontime (h)

Organophilicnanospheres (%)a

Dn (nm) Dw/Dn Gel formationb

Ethanol 1 100 14 1.17 Yes2 100 11 1.114 100 59 1.046 100 – –

24 84.5 11 1.07Methanol 1 99.3 11 1.03 No

2 99.8 11 1.054 15.9 – –6 16.4 – –

24 19.9 100 1.32

aDispersed in hexanebDuring the drying step

Table 1 Effect of reaction time of the growth step on the nano-spherical copolymer. Tetraethoxysilane (TEOS) and methyltrieth-oxysilane (MTEOS) concentration= 0.3 mol/l; 50 �C; H2O:Si

molar ratio r=9.25 mol H2O/mol Si; 0.1 mol/l NH4OH; termina-tor: 0.67 mol/l trimethylchlorosilane (TMCS); number-averageparticle diameter: Dn; particle size dispersion index: Dw/Dn

21

following order: TMCS � DCDMS � DCDPS.Bifunctional chlorinated terminators seem to promoteinterparticles reactions, reducing the solubility of thereaction product. A possible reaction for this cross-linking process which results in aggregation of the par-ticles takes place according to the following scheme:

2 particle�OHþ Cl� SiR2 � Cl! particle�O

� SiR2 �O� particleþ 2 HCl

where R represents a methyl or a phenyl group.The DCDMS terminator was used in concentra-

tions from 0.375 to 0.670 mol/l, producing a maxi-mum of 61% of dispersible particles with gelformation during drying under vacuum. The presence

of two chlorine atoms for each terminator moleculealso results in considerable formation of HCl in thereaction medium and consequently a pH decrease.These conditions may result in the cross-linking ofparticles. In the termination stage with chlorinatedcompounds, the formation of HCl is neutralized withNH4OH and NH4Cl is formed; this is removed fromthe system after the end of the process by washingwith water.

Table 3 also shows that the highest amount of theinsoluble hexane fraction is attained when two types ofterminators (DCDMS+TMCS) was used together. Inthis case, 85.5% of the particles precipitate, but no gelwas formed after the solvent elimination under vacuum.This means that not only the concentration but also thetype of terminator is important in this process. Althoughan increase in the concentration of the terminator shouldresult in an increase in the fraction of organophilic co-polymer, a small amount of bifunctional terminator iscapable of increasing the number of cross-linking par-ticles.

DCDPS was used in concentrations of 0.50 and0.67 mol/l resulting in about 50% of insoluble particlesin both cases. The increase in the terminator concen-tration also resulted in an increase in Dn and Dw/Dn

from 13 to 25 nm and 1.05 to 2.47, respectively.The effect of the termination stage conditions on

TEOS/MTEOS copolymerization is summarized inTable 4. A long reaction time reduces the number ofsoluble particles and increases the particle size owing tocross-linking of the particles even at high terminatorconcentration.

Effect of water-to-silicon molar ratio

The influence of the water-to-silicon molar ratio, r,on the synthesis of organophilic silica particles in twodifferent solvents is shown in Table 5. For 2 h reac-tion time, when r is around 9.2, a high conversion toorganophilic silica was obtained without any gelformation. A smaller r resulted in a larger particlesize.

Table 2 Effect of terminator concentration on the dispersion of thenanospheres. Reaction time (growth step): 4 h; reaction solvent:ethanol; 50 �C; termination step of 16 h; 0.3 mol/l TEOS and0.3 mol/l MTEOS; r=9.3 mol H2O/mol Si; 0.1 mol/l NH4OH

TMCS(mol/l)

Organophilicnanospheres (%)a

Dn (nm) Dw/Dn Gelformationb

0.50 89.4 52 1.23 Yes0.60 92.5 77 1.22 Yes0.80 99.6 198 1.09 Yes

aDispersed in hexanebDuring the drying step

Table 4 Effect of reaction time and terminator type on the dispersion of the nanospheres. Reaction solvent: ethanol; 50 �C; r=9.3;trimethylethoxysilane (TMES); 0.3 mol/l TEOS and 0.3 mol/l MTEOS; 0.1 mol/l NH4OH

Reaction time (h) Terminator Organophilicnanospheres (%)a

Dn (nm) Dw/Dn Gel formationb

Type Concentration (mol/l)

2 TMCS 0.67 90.3 11 1.08 No3 TMCS 0.67 50.2 123 1.15 Yes3 TMES 0.67 18.1 – – Yes3 – – 0 12 1.38 No

aDispersed in hexanebDuring the drying step

Table 3 Terminator efficiency in the TEOS/MTEOS copolymer-ization. Reaction time (growth step): 2 h; reaction solvent: meth-anol; 50 �C; termination step of 16 h; terminator concentration:0.67 mol/l; dichlorodimethylsilane (DCDMS); dichlorodiphenyl-silane (DCDPS); 0.3 mol/l TEOS and 0.3 mol/l MTEOS;r=9.25 mol H2O/mol Si; 0.1 mol/l NH4OH

TerminatorType

Organophilicnanospheres (%)a

Dn

(nm)Dw/Dn Gel

formationb

TMCS 99.8 11 1.05 NoDCDMS 40.0 17 1.21 YesDCDPS 57.3 25 2.47 YesDCDMS+TMCSc 14.5 18 1.08 No

aDispersed in hexanebDuring the drying stepcDCDMS=0.375 mol/l and TMCS=0.50 mol/l

22

The sample prepared at a high ratio (r=11.7) alsohad a high content of organophilic silica (99.8%) with-out gel formation; however, the particles were larger indiameter (24 nm). In this case, vigorous mechanicalstirring was used. This condition resulted in gel forma-tion being avoided. When the water-to-silicon molarratio was similar and magnetic stirring was used, a gelwas produced.

Effect of the polymerization solvent

Polar protic, polar aprotic and nonpolar aprotic can beused as solvents in sol–gel polymerizaton. A comparisonbetween three solvents, THF, ethanol and methanol, ispresented in Table 6. The table shows that when THFwas used as the solvent, a high amount of gel wasformed during solvent evaporation under a vacuum. Theapolar solvent may decrease the hydrolysis, enhancingpolymerization and even cross-linking. Methanol andethanol are the recommended solvents, since a gel is notformed during the drying process. In these polar sol-vents, a high hydrolysis rate is favorable for the for-mation of small particles. Under some conditions, gelformation was observed when either ethanol or metha-nol was used. Nevertheless, the gel was easily eliminatedby additional treatment. Gel formation during the dry-

ing could be eliminated with the addition of anothersolvent followed by stirring or resolubilization followedby a new drying stage.

Effect of the stirring speed

The stirring speed also influences the formation of thenanospheres. Two types of stirring were used in thiswork: magnetic stirring and mechanical stirring. Thequantity of the reaction medium also influences thesolubility of the material obtained. In TEOS homopo-lymerization, large volumes of reactants resulted in in-soluble products, depending on the stirring system. Theinfluence of the stirring speed on the formation of hy-drocarbon-dispersible silica–silicone hybrids is presentedin Table 7.

The best result was obtained when homogenizationwas carried out with mechanical stirring (sam-ple 101=99.8% of organophilic fraction). Anothersynthesis carried out in similar conditions, but with ahigher amount of the reaction medium (sample 102)resulted in only 9.6% of hydrocarbon-dispersible ma-terial. Thus, it is probable that for that volume usedthe stirring was not adequate, resulting in a high pro-portion of precipitated silica. Sample 104, prepared insimilar conditions to sample 101, also had a low

Table 5 Effect of r on the dispersion of the nanospheres. Reaction time: 2 h; 50 �C; terminator: 0.67 mol/l TMCS; 0.3 mol/l TEOS and0.3 mol/l MTEOS; 0.1 mol/l NH4OH

Solvent r (mol H2O/mol Si) Organophilicnanospheres (%)a

Dn (nm) Dw/Dn Gel formationb

Ethanol 8.3 90.4 35 1.09 Yes9.2 100 11 1.11 Yes9.3c 90.3 11 1.08 No

Methanol 5.2 88.6 163 1.27 Yes9.2 99.8 11 1.05 No

11.7c 99.8 24 1.12 No12.5 82.4 – – Yes

aDispersed in hexanebDuring the drying stepcMechanical stirring

Table 6 Effect of solvent and comonomer on the dispersion of the nanospheres. 50 �C; terminator: 0.67 mol/l TMCS; concentration ofmonomers A=0.3 mol/l TEOS and 0.02 mol/l dimethoxydiphenylsilane (DMODPS); B=0.2 mol/l TEOS: and 0.02 mol/l DMODPS;C=0.3 mol/l TEOS and 0.3 mol/l MTEOS; 0.1 mol/l NH4OH

Solvent Reactiontime (h)

Monomers(mol/l)

r (mol H2O/mol Si) Organophilicnanospheres (%)a

Dn (nm) Dw/Dn Gelformationb

Tetrahydrofuran 1 A 16.0 68.5 17 1.44 High2 B 15.3 50.2 28 1.03 High

Ethanol 2 C 9.2 100 11 1.11 No4 C 8.3 90.2 61 1.31 No

Methanol 2 C 9.2 99.8 11 1.05 No

aDispersed in hexanebDuring the drying step

23

number of organophilic particles, probably owing topoor stirring.

Effect of comonomer type

The influence of the comonomer type on the nano-spheres of silica–silicon is illustrated in Table 8. Thecopolymerization of TEOS with DMDEOS resulted ina high conversion of organophilic silica; however, awaxlike gel was formed during the vacuum-dryingprocess. In the copolymerization of TEOS withDMODPS, a low conversion in organophilic silica wasobserved, even at long reaction time. A possible ex-planation for this may be the high reactivity differencebetween TEOS and DMODPS. For these alkoxysil-anes, the hydrolysis rate decreases in the follow-ing order: TEOS � MTEOS>DMDEOS>DMODPS.TEOS, which is more reactive, homopolymerizes fast,creating a compact silica nucleus without incorporationof the other comonomer. DMODPS, which is less re-active, may be incorporated at the end of the growthprocess, generating a core–shell structure. So, at theend of the growth stage the number of available hy-droxyl groups that react with the chlorinated termina-tor may be insignificant. During the drying process,residual ethoxy groups are then hydrolyzed, promotinginterparticles reactions that generate aggregation. The

copolymerization of TEOS with MTEOS gave the bestresult. In this case, in low-reaction-time (2 h) gel is notformed and high conversion in organophilic silica dis-persible in hexane is obtained.

Table 8 Influence of comonomers on the dispersion of the nanospheres obtained by copolymerization of TEOS and alkylalkoxysilanes.Reaction solvent: ethanol; 50 �C; terminator: 0.67 mol/l TMCS; DMDEOS; 0.1 mol/l NH4OH concentration

Comonomer Reactiontime (h)

r (mol H2O/mol Si) Organophilicnanospheres (%)a

Dn (nm) Dw/Dn Gel fomationb

Type Concentration (mol/l)

DMDEOS 0.3 2 9.3 99.7 – – Yes (Wax)DMODPS 0.02 72 16.2 8.5 11 1.04 NoMTEOS 0.3 2 9.3 90.3 11 1.08 No

aDispersed in hexanebDuring the drying step

Fig. 1 29Si magic-angle-spinning NMR spectra of sample 104:a organophilic and b nonorganophilic

Table 7 Influence of the stirring on the dispersion of the nanospheres obtained by TEOS and MTEOS copolymerization. 50 �C

Stirring TEOS+MTEOS(mol/l)a

r TMCS(mol/l)

NH4OH(mol/l)

Solvent OrganophilicNanospheres(%)b

Dn (nm) Dw/Dn Gelformationc

Remarks(samplenumber)

Magnetic 0.3 9.3 0.67 0.1 Methanol 74.9 11 1.08 Yes 950.3 11.3 0.58 0.05 Methanol 20.9 11 1.03 No 104

Mechanical 0.3 9.3 0.67 0.1 Methanol 93.0 27 1.03 Yes 970.3 11.7 0.67 0.1 Methanol 99.8 24 1.12 No 1010.6 9.3 0.63 0.2 Methanol 9.6 11 1.04 No 1020.3 9.3 0.67 0.1 Ethanol 90.3 11 1.08 No 100

aTotal concentration of comonomersbDispersed in hexanecDuring the drying step

24

Nanosphere structure

The nanospheres samples 101, 102 (organophilic frac-tion) and 104 (organophilic and nonorganophilic frac-tion) obtained by TEOS/MTEOS copolymerizationwere characterized by 29Si NMR. Through the chemicalshifts of the Si nuclei, some structural information wasobtained.

Typical 29Si NMR spectra are shown in Fig. 1 (sam-ple 104, organophilic and nonorganophilic fractions). Inthese spectra, the absence of signs at –81.5 ppm isobserved in the samples, indicating that TEOS was to-tally consumed.

The results obtained with simple pulse technique (SP,5 kHz) are summarized in Table 9. Although a termi-nator was introduced in order to react with the hydroxylgroups, a significant amount of silanol is present in thestructure. The percentage of species containing Si–OH isaround 19%. Practically, there is no marked differencein the composition of the hexane-soluble and hexane-insoluble fractions, suggesting that the differences insolubility are due to the size only. Considering the rel-ative intensities of the signs corresponding to (CH3)3Si–O–, the relative concentration of this type of fragmentfollows the order 102<101<104. Nevertheless, the dif-ferences are small and the comparison is not adequate,specially for organophilic and nonorganophilic fractionsof sample 102.

The results obtained by 29Si MAS NMR are co-herent. Sample 101 had the highest percentage of or-ganophilic material (99.8%). That sample was preparedusing mechanical stirring and a small amount of reac-tants. On the basis of this result and with the objectiveof obtaining a higher number of organophilic nano-spheres, the amount of reactants was increased in thepreparation of sample 102, however without changes inthe stirring conditions. The 29Si MAS NMR analysisconfirmed that the homogenization conditions duringthe sol–gel process have a significant influence on thestructure of the particle. Sample 102 had a smaller

content of (CH3)3SiO– groups (14.6%) than sam-ple 101 (19.2%) prepared under conditions of betterhomogenization.

Two different types of hydroxyl groups on the surfaceof the nanospheres were confirmed by NMR. The or-ganophilic fraction of the samples had a lower contentof hydroxyl groups of the (OH)SiO3 type and highercontent of the (OH)(C)SiO2 type. It was also possible toconfirm that the structure of the nanoparticles is formedpredominantly by SiO, as demonstrated by the per-centage of (C)SiO3 and SiO4 between 50 and 65%.However, the particles obtained by copolymerizationhave a significant percentage of silicon linked to carbon(35–50%).

The organophilic nanospheres had 19–40% of siliconof the SiO4 type, very different from the commercialAerosil silica that has about 100% SiO4.

Conclusions

Organophilic nanospheres homogeneously dispersible inhexane with diameters in the range 10–198 nm wereobtained by means of copolymerization of TEOS andalkylalkoxysilanes, using base catalysis, ethanol ormethanol as a solvent and a terminator. The solubility ofthe nanospheres in a hydrocarbon such as hexane wasshown to be highly dependent on the reaction condi-tions. For high conversion to organophilic silica–siliconhybrids, the use of MTEOS as a comonomer, TMCS asa terminator, short reaction times, a high stirring speedand a water-to-silicon molar ratio around 9 were foundbe the best conditions. Hexane-dispersible particles havehydroxyl and methyl groups on the surface and a largepercentage of SiO4 species.

Acknowledgements The authors thanks the Conselho Nacional deDesenvolvimento Cientıfico e Tecnologico, Coordenacao doAperfeicoamento de Pessoal de Nıvel Superior and The Universityof the Air, Japan, for partial support of this work.

Table 9 Composition of silicon species by 29Si magic-angle-spinning NMR of silica–silicon nanosphere samples 101, 102 and 104

Si type Chemical shift (d, ppm/ %)a

101 organophilic 102 organophilic 102 insolubleb 104 organophilic 104 insolubleb

(CH3)3SiO 8.5/19.2 7.8/14.6 8.7/16.5 8.3/26.5 9.0/23.1(OH)(C)SiO2 –57.3/6.0 –59.3/9.5 –53.5 and –57.1/5.8 –56.9/11.7 –59.0/7.9(C)SiO3 –64.9/24.7 –65.4/ 31.5 –65.0/27.1 –65.5/33.4 –65.2/39.8(OH)SiO3 –102.5/11.0 –101.9/9.4 –100.0 and –103.1/12.0 –100.2/5.9 –101.6/9.9SiO4 –110.5/39.1 –110.3/35.1 –110.5/38.7 –110.0/22.5 –119.3/19.3

aKaolin, d (OH)Si–O=– 91.5 ppm (100%) was used as a secondary referencebHexane-insoluble

25

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