Hydration Promoted by a Methylene Group: A Volumetric Study on

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Hydration Promoted by a Methylene Group: A Volumetric Study on Alkynes in Water Satoshi Shibuta, and Hiroshi Imamura J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b00843 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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The Journal of Physical Chemistry

Hydration*Promoted*by*a*Methylene*Group:*A*Volumetric*Study*on* Alkynes*in*Water* Satoshi(Shibuta*,(†(and(Hiroshi(Imamura‡( †(Chiba(Institute(of(Technology,(2>1>1,(Shibazono,(Narashino,(Chiba,(275>0023,(Japan( ‡(Department(of(Applied(Chemistry,(College(of(Life(Sciences,(Ritsumeikan(University,(1>1>1(Noji>Higashi,(Kusatsu,( Shiga(525>8577,(Japan! * E>mail(address:([email protected]( ( ! ABSTRACT:(Hydrocarbons(including(a(methylene(group(are(generally(considered(a(hydrophobic(building(block,(in(the( sense(that(the(density(of(their(hydration(water(is(lower(than(that(of(bulk(water.(However,(is(the(methylene(group(always( hydrophobic?(In(this(study,(we(experimentally(determined(the(partial(molar(volume(of(a(methylene(group(in(water(as(14.01( ±(0.46(cm3(mol>1(for(1>alkyne,(9.83(±(0.35(cm3(mol>1(for(2>alkyne,(and(11.39(±(0.55(cm3(mol>1(for(3>alkyne.(These(values(are( all(unusually(small(compared(to(the(~16(cm3(mol>1(for(model(compounds(from(the(literature.(The(subsequent(volumetric( analysis(on(the(basis(of(the(Kirkwood>Buff(parameter(indicates(that(the(hydration(water(is(enriched(by(the(addition(of(a( methylene(group(for(2>alkyne,(while(it(is(depleted(for(the(reported(model(compounds(that(contain(hydrophilic(functional( groups,(1>alkyne,(and(3>alkyne.(Our(findings(suggest(the(triple(bonded(carbons(in(2>alkyne(that(reduce(hydration(water(act( as(a(hydrophobic(group(in(2>alkyne.(Thus,(the(methylene(group(should(be(called(“hydrophilic”(in(this(case,(because(it( actually(recovers(the(hydration(water(when(placed(next(to(more(hydrophobic(groups.(Therefore,(we(conclude(that(the( hydrophobicity(of(a(methylene(group(varies(depending(on(its(hydration(environment(due(to(other(functional(groups(in(the( solute.( !G(=(µ("(µoil( ( ( ( (1)(

INTRODUCTION* The( concept( of( hydrophobicity( is( used( in( chemistry(and( biology( to( describe( many( processes( in( the( aqueous( environment,( such( as( protein( folding( and( the( separation( and( extraction( of( chemicals.( However,( there( are( some( long>standing( questions( about( this( concept,( such( as( the( topic( of( hydrophobic( hydration.( Between( the( 1940s( and( 1980s,( hydrophobic( interaction( was( investigated( by( pioneering( researchers( such( as( Frank( and( Evans,1( Kauzmann,2,3( and( Ben>Naim.4( The( classic( hypothesis( by( Frank( and( Evans1( is( that( the( water( structure( around( a( hydrophobic( solute( is( ice>like.( However,( this( so>called( “iceberg( hypothesis”( is( still( being( debated.( Among( more( elaborate( studies( in( recent( years,( Baldwin( and( Rose( provided( a( new( intriguing( insight:( in( the( case( of( alkanes,( the( hydration( shells( around( the( hydrocarbon( solute( are( stabilized( by( van( der( Waals( interactions( between( the( alkane(carbon(and(water(oxygen(atoms.5( Thermodynamics( has( played( a( central( role( in( the( research( of( hydrophobicity.( Kauzmann( defined( hydrophobic( hydration( as( the( process( of( transferring( a( solute(dissolved(in(an(organic(solvent(into(water.2(Based(on( the( oil>water( relationship,( this( process( is( clearly( energetically( unfavorable.( The( free( energy( change( upon( hydrophobic(hydration,(!G,(is(given(by(

where(µ(and( µoil(are(the(chemical(potentials(of(the(solute(in( water( and( organic( solvent,( respectively.( The( pressure( derivative( of( eq( 1( provides( the( volume( change( upon( hydrophobic(hydration,(!V:( !V(=(v("(voil( ( ( ( (2)( where(v(and(voil(are(the(partial(molar(volumes(of(the(solute( in( water( and( organic( solvent,( respectively.( A( historical( controversy( about(the( magnitude( of( !V( should(be( derived( from( underestimation( of( the( effect( of( voil,( which( was( critically( reviewed( by( Imai.6,7( A( fundamental( understanding( of( hydrophobic( hydration( requires( knowledge( about( the( nature( of( µ( or( v.( Both( parameters( should( tell( us( how( the( solute( modifies( the( water,( i.e.,( the( solute>water( interactions.( In( solution( thermodynamics,( compared( to( µ,( its( derivatives( such( as( v( or( higher( derivatives( provide( more( nuanced( insights( into( the( solution( structure.8( Therefore,( in( this( study,( we( intend( to( analyze(the( hydrophobic( hydration( from( the( partial(molar( volume( of( different( solutes( in( water( (v),( which( is( experimentally(accessible.( ( Hydrocarbons( are( regarded( as( typical( hydrophobic( solutes.( Their( basic( building( block,( the( methylene( group( (>CH2>)( is( ubiquitous( in( organic( compounds.( The( contribution( of( a( single( >CH2>( to( the( partial( molar( volume(

1 ACS Paragon Plus Environment

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(v∞)( of( a( molecule(at( infinite( dilution( in(water,( denoted(as( (∂v∞/∂n>CH2>)T,P,( can( be( experimentally( determined( as( the( difference( of( PMV( between( compounds( that( differ( by( one( >CH2>( unit( (e.g.( ethanol( and( propanol).( In( this( manner,( (∂v∞/∂n>CH2>)T,P(has(been(previously(examined(for(a(variety( of(alkyl(solutes((alkylbenzene,(alcohol,(alkane(diol,(ketone,( and( alkyl( amine).( The( obtained( values( of( (∂v∞/∂n>CH2>)T,P( were( all( close( to( 16( cm3( mol>1.( Therefore,( this( value( has( been(considered(as(the( standard,9–14(and( used( to( calculate( the( partial( molar( volume( of( various( compounds( on( the( basis( of( group( contribution.15–18( Although( smaller( (∂v∞/∂n>CH2>)T,P( values( of( ~14( cm3( mol>1( have( also( been( reported,19–21( such( deviations( were( duly( incorporated( by( the( use( of( a( small( correction( factor.11,14,18,22( Consider( that( the( value( of( (∂v∞/∂n>CH2>)T,P( is( related( to( the( hydration( structure(as(well(as(the(molecular(volume(of(the(methylene( group( itself,( the( similar( (∂v∞/∂n>CH2>)T,P( values( among( the( studied( compounds( mean( that( the( hydration( structure( should( not( strongly( depend( on( the( type( of( solutes.( Nevertheless,( while( this( hypothesis( is( convenient( for( the( calculation(of(partial(molar(volume,(its(validity(is(yet(to(be( proven.( ( We( suspect( that( the( previously( determined( (∂v∞/∂n>CH2>)T,P( values( are( biased( because( they( were( obtained(using(compounds(soluble(in(water,(which(is(very( different( from( the( archetypal( hydrocarbons.( Thus,( in( this( study,( we( examined( the( (∂v∞/∂n>CH2>)T,P( of( alkynes.( Unlike( the(other(compounds,(alkynes(are(considered(hydrophobic( due(to(their(very(low(solubility(in(water.(For(example,(the( solubility( of( 1>heptyne( (0.98( mol( in( 103( kg( of( water)23( is( significantly( lower( than( that( of( toluene( (an( alkylbenzene,( 5.6(mol(in(103(kg(of(water)24(under(ambient(condition.( ( In( the( present( study,( we( used( density( measurement( to( determine( the( partial( molar( volume( of( eight( alkynes( in( water:( 1>pentyne,( 1>hexyne,( 1>heptyne,( 2>pentyne,( 2>hexyne,( 2>heptyne,( 3>hexyne,( and( 3>heptyne.( We( found( anomalously( small( (∂v∞/∂n>CH2>)T,P( values( for( the( alkynes( compared( to( those( for( previously( reported( compounds.( Analysis( of( the( volumetric( data( in( terms( of( the( Kirkwood>Buff( parameter25,26( between( the( solute( and( water( indicates( that( hydration( is( promoted( by( the( methylene( group( in( 2>alkyne,( in( contrast( to( the( depletion( of( hydration( waters( around( the( solutes( examined( so( far.( This( result( suggests( that( contrary( to( previous( belief,( a( methylene(group(is(not(always(hydrophobic.( (

MATERIALS*AND*METHODS* Sample*preparation( ( ( ( The( alkynes( employed( as( solutes( in( the( present( study( are:( 1>pentyne( (>98.0%),( 1>hexyne( (>97.0%),( 1>heptyne( (>97.0%),( 2>pentyne( (>97.0%),( 2>hexyne( (>98.0%),( 2>heptyne( (>99.0%),( 3>hexyne( (>97.0%),( and( 3>heptyne( (>99.0%),( all( purchased( from( Tokyo( Chemical( Industry( in( Japan.( About( 0.5( L( of( ultrapure( water( was( accurately( weighed( by( an( analytical( balance.( A( few( drops( of( liquid( alkyne( were( weighted( in( a( small( vessel( made( of( polytetrafluoroethylene( (PTFE,( Teflon)( by( another( analytical(balance(more(suitable(for(smaller(masses.(Then,(

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the( vessel( with( liquid( alkyne( was( immersed( in( the( ultrapure(water.(By(using(this(vessel(as(a(magnetic(stirrer,( the(mixture(was(stirred(in(a(water(bath(regulator(at(25.0(±( 0.1(°C(for(24(hours,(during(which(the(alkyne(was(dissolved( in( water.( The( bottle( was( sealed( by( a( Teflon( cap( to( avoid( volatilization.( The( concentration( of( the( alkyne( was( determined(by(the(masses(of(the(water(and(the(alkyne.(

Density*measurement* * ( ( The( densities( of( the( alkyne( aqueous( solutions( were( measured(using(an(oscillation>type(density(meter(made(by( Prof.( K.( Sueoka( (Josai( University,( Japan).( Before( and( after( each(solution(measurement,(the(instrument(was(calibrated( using( dry( air( and( water.( The( temperature( of( the( solution( was( controlled( at( 25.000( ±( 0.003( °C( by( two( temperature>controlled( water( baths,( one( strictly( for( temperature( regulation( and( the( other( for( preheating/precooling(the(water.(Repeated(measurements( (565( times)( of( water( gave( a( density( uncertainty( of( ±( 0.000008( g( cm>3( at( 25.000( ±( 0.003( °C( and( atmospheric( pressure.(The(partial(molar(volume(of(benzene(in(water(at( infinite( dilution( determined( by( the( present( measurement( system( was( 82.47( ±( 0.06( cm3( mol>1,( which( is( in( good( agreement( with( previously( reported( value( (82.6( cm3( mol>1).27( Partial*molar*volume( ( The(volume(of(a(binary(aqueous(solution,(Vsolution,(is(given( by( wwater + nsolute(Msolute Vsolution = # (3)( ρ where( wwater( is( the( mass( of( water.( nsolute,( Msolute,( and( ρ( are( the(mole(number(of(solute,(the(molecular(weight(of(solute,( and( the( density( of( the( solution,( respectively.( The( partial( molar( volume( of( the( solute,( v,( is( the( first( derivative( of( Vsolution(in(eq(3(with(respect(to(nsolute:( ∂Vsolution #v = $ % ∂nsolute T,!P 1 = ' (ρ!Msolute ρ ∂ρ − (wwater + nsolute(Msolute) $ (4)( % ,# ∂nsolute T,!P where( T( and( P( are( absolute( temperature( and( pressure,( respectively.( At( infinite( dilution( (nsolute( #( 0),( the( partial( molar(volume(of(the(solute,(v∞,(is(given(by( 1 #v∞ = ' -ρwater!Msolute ρwater ∂ρ − wwater $ (5)( % .# ∂nsolute T,!P where( ρwater( is( the( density( of( water.( The( superscript( ∞( indicates( infinite( dilution( of( the( solute.( Assuming( wwater( =( 1000(g,(eq(5(is(rewritten(as( 1 #v∞ = ' -ρwater!Msolute ρwater ∂ρ − wwater $ (6)( % .# ∂msolute T,!P (


RESULTS* Partial*molar*volume*of*alkynes* * Figure( 1( shows( the( density( difference( (!ρ)(between(the( alkyne( solution( (ρ)( and( water( (ρwater)( at( 25.000( °C,( as( a( function( of( molality( of( the( solute( (msolute).( All( measured( density( values( are( tabulated( in( Supporting( Information.( For( all( the( alkyne( solutions,( !ρ( decreases( linearly( with( increasing( msolute.( In( other(words,( the(alkyne( solutions(are( less( dense( than( water( at( any( concentrations.( In( Figure( 1,( the( slope( (%!ρ/%msolute)T,P( that( is( equivalent( to(

-6 0 1 2 3 4 5 -3 -1 m1-hexyne / 10 mol kg

-3

CH≡C(CH2)4CH3

0

-6 -9 0 3 6 9 -3 -1 m2-hexyne / 10 mol kg 0

CH3C≡C(CH2)3CH3

-15 0 2 4 6 8 10 -4 -1 m1-heptyne / 10 mol kg

-8

-12 0 2 4 6 8 10 -3 -1 m3-hexyne / 10 mol kg

-1

-5

-5

-10

-4

-5

-3

-5

-4

CH3CH2C≡CCH2CH3 0

-3

CH3C≡C(CH2)2CH3

-5

-2

0

∆ρ / 10 g cm

∆ρ / 10 g cm

-3

CH≡C(CH2)3CH3

0

-3

-3 0 5 10 15 20 25 -3 -1 m1-pentyne / 10 mol kg

CH3C≡CCH2CH3 0 -1 -2 -3 -4 -5 0 1 2 3 -2 -1 m2-pentyne / 10 mol kg

∆ρ / 10 g cm

-3 -4

-2

∆ρ / 10 g cm

-3 -3

∆ρ / 10 g cm

-1

-4

∆ρ / 10 g cm

CH≡C(CH2)2CH3

0

-5

∆ρ / 10 g cm

-3

-2 -3 0 1 2 3 4 -3 -1 m2-heptyne / 10 mol kg

CH3CH2C≡C(CH2)2CH3 0 -1 -2 -3 -4 0 1 2 3 4 5 -3 -1 m3-heptyne / 10 mol kg

(

Figure(1.(Molarity(dependence(of(the(density(difference((∆ρ!=( ρ(–(ρwater)(for(the(alkyne(solution(at(25.000( °C.(The(symbols(of( open( circle,( square,( and( triangle( indicate( 1>alkyne,( 2>alkyne,( and(3>alkyne(aqueous(solutions,(respectively.(

( 180

180

30 0

2 4 6 n-CH2- of 1-alkyne

-1

130

(∂v /∂n-CH2-)T,P 3 -1 = 11.39 cm mol

3

80 30 0

∞

∞

80

∞

v / cm mol

130

(∂v /∂n-CH2-)T,P 3 -1 = 9.83 cm mol

3

130

180 ∞

-1

∞

(∂v /∂n-CH2-)T,P 3 -1 = 14.01 cm mol

v / cm mol

− vsolute∞( (7)( where(gsw(r)(is(the(pair(distribution(function(of(water,(i.e.,( the(probability(of(finding(water(molecules(at(a(distance(of(r( from( the( solute,( R( is( the( gas( constant,( and( κT,w( is( the( isothermal( compressibility( of( water( (4.516$10>4( MPa>1( at( 25( °C).35,36( The( contribution( of( water( molecules( excluded( by(an(immersed(solute(to(Gsw∞(is(given(by(Gsw,molecular∞:( Gsw,moleculer∞(≡(RTκT,w − Vmolecular (8)( where( Vmolecular( is( the( molecular( volume( of( the( solute,( i.e.,( the( volume( inside( the( molecular( surface.37( We( calculated( Vmolecular(of(the(present(solutes(using(the(theoretical(method( developed(by(Prof.(Nagao((National(Institute(of(Technology,( Hakodate(College,(Japan)38(and(implemented(in(Winmostar( software( (X>Ability( Co.,( Ltd.( Japan).39( In( the( calculation,( a( water( molecule( is( presumed(to( be(a( hard( sphere(with( the( radius( of( 1.4( Å.( To( calculate( the( volumes( of( the( solutes,( their( structures( were( optimized( by( semi>empirical( QM( method( of( MOPAC740( in( Winmostar( software.( Combining( eqs( 7( and( 8,( we( define( Gsw,hyd∞( (the( contribution( of( hydration(waters(to(Gsw∞)(by(eq(9:( Gsw,hyd∞(≡Gsw∞ − Gsw,molecular∞( (9)( ∞ Gsw,hyd (is(a(good(indicator(of(the(enrichment(or(depletion( of(water(around(the(solute:( #Gsw,hyd∞(>(0(→(ρlocal(>(ρbulk( (10)( #Gsw,hyd∞(arrangements( in( the( solvent.”( ( (

-6

0

∆ρ / 10 g cm

∞

#Gsw∞(≡(4π / [gsw(r) − 1]r2dr =RTκT,w

-1

KirkwoodJBuff*parameter* * ( ( The( Kirkwood>Buff( theory( and( analysis( have( been( widely( used( to( obtain( insights( into( solution( structures,( such(as(the(aggregation( state( of( solute( in( solution.25,26,28–34( The( Kirkwood>Buff( parameter( between( an( infinitely( diluted( solute( and( water( solvent,( Gsw∞,( is( defined( by( the( following(equation:29,33,34( (

(%ρ/%msolute)T,P(gives(the(partial(molar(volume(of(alkynes(at( infinite( dilution( (v∞),( according( to( eq( 6.! The( v∞( values( for( all( the( alkynes( are( presented( in( Table( 1.( According( to( Figure( 2,( v∞( increases( linearly( with( the( number( of( methylene(groups((n>CH2>)(in(the(alkynes.(The(slope,(namely( (∂v∞/∂n>CH2>)T,P( (depicted( by( the( fitted( lines),( is( the( partial( molar(volume(of(>CH2>(of(the(alkyne(in(water.(As(shown(in( Figure( 2(and(Table( 1,( the( value( of( (∂v∞/∂n>CH2>)T,P( depends( on( the( position( of( the( triple( bond:( 1>alkyne,( 14.01( ±( 0.46( cm3( mol>1;( 3>alkyne,( 11.39( ±( 0.55(cm3( mol>1;(and( 2>alkyne,( 9.83( ±( 0.35( cm3( mol>1,( as( arranged( in( descending( order.( These( smaller( (∂v∞/∂n>CH2>)T,P( values( and( their( position( dependence( seem( anomalous,( in( terms( of( the( well>accepted( value( (∂v∞/∂n>CH2>)T,P( of( ~16( cm3( mol>1( previously( determined( for( a( wide( variety( of( more( soluble( model(compounds.9–14( ( (

3

where(msolute(is(the(molality(of(the(solute((mol/kg).( ( (

∞

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


v / cm mol

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80 30 0


(

Figure(2.(Partial(molar(volume(of(alkynes(in(water(at(infinite( dilution( (v∞)( and( 25.000( °C( as( a( function( of( n>CH2>.( The( solid( lines( are( fitted( by( a( linear( function.( The( error( bars( representing( the( uncertainties( in( the( fitted( values( are( much( smaller(than(the(marker(size.( (

(

KirkwoodJBuff*parameter*of*hydration*water*for*Alkynes* * To( further( understand( the( hydration( water,( we( determined(Gsw,hyd∞(for(the(alkyne(aqueous(solutions(by(the( partial( molar( volumes( and( the( molecular( volumes( according(to(eq(9.(Gsw,hyd∞(is(plotted(as(a(function(of(n>CH2>(in( Figure( 3.( According( to( Table( 1,( all( Gsw,hyd∞( values( for( the(


The Journal of Physical Chemistry alkynes( were( negative( (Gsw,hyd∞( ( in( Figure( 3.( The( slope( (∂Gsw,hyd∞/∂n>CH2>)T,P( represents( how( hydration( waters( are( enriched/depleted( by( inserting( one( methylene( group( (>CH2>)( into( the( alkyne( chain,( i.e.,( the( difference( of( the( local( density( of( hydration( water( (ρlocal)( before( and( after( inserting( the( >CH2>.( Accordingly,( (∂Gsw,hyd∞/∂n>CH2>)T,P( >( 0( indicates( an( increase( of( ρlocal,( and( (∂Gsw,hyd∞/∂n>CH2>)T,P( CH2>.(A(hydrophobic(group(is(assumed(to(fall(into( the( second( category( because( of( its( “hydrophobicity”,( i.e.,( hydration( waters( are( depleted( near( the( solute.( Indeed,( such(depletion(of(hydration(waters(by(inserting(>CH2>(is(the( case(for(1>alkyne(and(3>alkyne(in(this(study(on(the(basis(of( their( negative( (∂Gsw,hyd∞/∂n>CH2>)T,P.( However,( the( values( of( (∂Gsw,hyd∞/∂n>CH2>)T,P( are( positive( for( 2>alkynes,( and( the( reason(will(be(discussed(later.( ( ( -20

-1

-25

∞

3

Gsw,hyd / cm mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-30

2-alkyne

3-alkyne

-35 -40 -45 0

((((((

1-alkyne

2 4 n-CH2-: -CH2- number

6

(

Figure( 3.( Gsw,hyd∞( of( the( alkynes( as( a( function( of( n>CH2>.( The( open( circle,( square,( and( triangle( correspond( to( 1>akyne,( 2>akyne,(and(3>alkyne,(respectively.(The( solid( lines(are(fitted( by(a(linear(function.(The(error(bars(indicate(the(uncertainties( in(the(fitted(values.(

(

DISCUSSIONS* According(to(our(measurement,(the(partial(molar(volume( of( a( methylene( group( in( the( alkynes,( (∂v∞/∂n>CH2>)T,P,( is( smaller( than( ~16( cm3( mol>1,( the( assumed( constant( value( used( for( volume( calculation( according( to( an( additive( scheme.13,15–18,43,44(To(understand(the(reason,(we(calculated( Gsw,hyd∞( and( (∂Gsw,hyd∞/∂n>CH2>)T,P( of( the( previous( model( compounds9–12,14( using( their( partial( molar( volumes( in( water,(as(shown(in(Figure(4(and(Table(2.(Note(that(there(is( a(variation(for(the(values(of(n>alkane((ethane(and(propane)( in( Table( 2,45–48(which( is( ignored( in(the( discussions.( Unlike( the(alkynes,(previously(studied(model(compounds,(namely( alkylbenzene,(alcohol,(alkane(diol,(ketone,(and(alkyl(amine,( are(all(regarded(as(hydrophilic(in(terms(of(their(solubilities(

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in( water.49(Hereafter,( we( will(refer(to(them(as( hydrophilic( compounds.( All( the( compounds( in( Figure( 4A( indicate( Gsw,hyd∞( CH2>( =( 0,( would( depend( on( the( size( of( the( functional( group;( because( the( larger( the( solute,( the( more( waters(are(hydrated(around(it.(Indeed,(the(intercepts(gave( good( correlation( with( the( van( der( Waals( volumes( of( the( functional(groups.( The( difference( in( (∂Gsw,hyd∞/∂n>CH2>)T,P( between( the( hydrophilic( compounds( and( the( alkynes( (as( shown( in( Figure( 4B)( is( the( most( prominent( result( in( this( paper.( Specifically,( (1)( The( (∂Gsw,hyd∞/∂n>CH2>)T,P( values( for( the( hydrophilic(compounds(are(similar((ca.(‒5(cm3(mol>1),(with( only( weak( dependence( on( the( functional( groups;( (2)( in( comparison,( the( alkynes( have( larger( (∂Gsw,hyd∞/∂n>CH2>)T,P( values;( and( (3)( the( (∂Gsw,hyd∞/∂n>CH2>)T,P( value( for( 2>alkyne( is( positive,( which( is( the( most( interesting( finding.( Prior( to( further( discussion,( we( first( note( that( the( partial( molar( quantities( v∞( and( Gsw,hyd∞( use( bulk( water( as( reference.( On( the( other( hand,( their( derivatives,( (∂v∞/∂n>CH2>)T,P( and( (∂Gsw,hyd∞/∂n>CH2>)T,P,( take( waters( modified( by( the( solute( before( inserting( >CH2>( as( the( reference.( This( could( be( represented(as( (∂Gsw,hyd∞(/∂n4CH24)T,P#>(0(→(ρlocal,+CH2(>(ρlocal,ini( (12)( where( ρlocal,+CH2( and( ρlocal,ini( are( the( local( densities( of( water( molecules( around( the( solute( with( and( without( inserted( >CH2>( (e.g.,( propanol( and( ethanol).( dV( indicates( a( volume( increment.(Thereby,((∂Gsw,hyd∞/∂n>CH2>)T,P(tells(us(the(change( in( the( density( of( hydration( water( upon( inserting( a( >CH2>( group(into(the(solute(carbon(chain.((∂Gsw,hyd∞/∂n>CH2>)T,P(CH2>( (e.g.,( propanol)( are( less( dense(than( those( of(the(solute(without(>CH2>((e.g.,(ethanol).( Functional( groups( such( as( hydroxyl,( amino,( carbonyl( groups( and( benzene( rings( would( affect( the( surrounding( water( according( to( their( respective( ρlocal,ini.( Nonetheless,( their(difference(in(ρlocal,ini(is(rather(small(or(cancelled(out(by( ρlocal,+CH2.( As( a( result,( an( almost( constant( (∂Gsw,hyd∞/∂n>CH2>)T,P(value(of(ca.(‒5(cm3(mol>1(was(obtained( for( these( compounds.( For( the( alkynes,( in( contrast,( their( (∂Gsw,hyd∞/∂n>CH2>)T,P( values( are( notably( larger.( This( indicates( that( the( density( difference( |ρlocal,+CH2( "( ρlocal,ini|( is( smaller( for( the( alkynes( than( for( the( hydrophilic( compounds.( According(to( the( solubility( data,( ρlocal,ini( of( the( alkynes( should( be( smaller( than( ρlocal,ini( of( the( hydrophilic( compounds,42(since(hydration(waters(are(more(depleted(by( a( triple( bond( than( by( the( other( functional( groups.( This( is( one(of(the(reasons(why((∂Gsw,hyd∞/∂n>CH2>)T,P(is(larger(for(the( alkynes.( The( hydration( waters( are( further( depleted( by( an( inserted( >CH2>( for( 1>alkyne( and( 3>alkyne(as(well(as( by( the( hydrophilic(compounds((see(Figure(5A),(but(this(is(not(the( case(for(2>alkynes.(Since(they(have((∂Gsw,hyd∞/∂n>CH2>)T,P(>(0,( hydration( waters( are( enriched( by( an( inserted( >CH2>( in( 2>alkyne,( as( illustrated( in( Figure( 5B.( The( sign( of( (∂Gsw,hyd∞/∂n>CH2>)T,P( is( the( indicator( of( “relative( hydrophobicity”,( i.e.,( the( difference( in( hydrophobicity( between(the(functional(group(initially(present(in(the(solute( molecule( and( the( inserted( >CH2>.( Although( >CH2>( is( typically(called(“hydrophobic”,(it(can(act(as(a(“hydrophilic”(


in( Figure( 5,( the( addition( of( a( methylene( group( into( a( hydration( layer,( in( which( fewer( water( molecules( are( populated( around( the( chains( with( the( triple( bond( in( 2>alkyne,( effectively( increases( the( hydration( water.( The( present( finding( indicates( that( a( methylene( group( can( potentially( enhance( the( hydration( in( regions( with( preexisting( depleted( hydration( water,( due( to( environmental( factors( such( as( other( functional( groups( in( the(solute.( ( 3 Model Alkynes 2 compounds 1 2-alkyne -20 0 -1 3-alkyne -2 -40 -3 1-alkyne -4 Alkane diol -5 -60 Alcohol 3 -1 Alkyl amine -5 cm mol -6 Ketone on average Alkyl benzene -7 Alkane -80 -8 l 0 2 4 6 8 10 12 o e ol z. e e e e e di min oh en ton kan kyn kyn kyn ne l a Alc yl b Ke Al -al -al -al a y 1 2 3 k k k n-CH2-: -CH2- number Al Al Al

0

(B)

∞

3

(∂Gsw,hyd /∂n-CH2-)T,P / cm mol

-1

-1

(A)

3

group(relative(to(the(2>alkyne((C>C&C)(moiety.(The(context( (i.e.,( location)( dependence( of( the( partial( molar( volume( of( >CH2>( is(generally(accepted.(Indeed,(this( value( on( nitrogen( is( known( to( be( smaller( than( that( on( carbon.50( Hitherto,( >CH2>( has( been( regarded( as( “hydrophobic”( or( excluding( hydration( waters.( For( the( first( time,( the( present( study( found(that(the(hydration(can(be(actually(promoted(by(>CH2>( in( the( context( of( the( 2>alkyne( (C>C&C)( moiety,( which( is( a( relatively(strong(hydrophobic(group((i.e.(strongly(depleting( the(hydration(waters).( ( ( The( discussion( above( focused( on( ρlocal,ini.( Another( possible( explanation( for( (∂Gsw,hyd∞/∂n>CH2>)T,P( >( 0( is( an( increase(of(ρlocal,+CH2(by(an(attractive(force(of(>CH2>(to(water.( When( two( hydrophobic( units( (>CH2>( and( a( stronger( hydrophobic( group)( in( a( molecule( both( exclude( waters,( and(the( exclusion( overlaps( (i.e.,( being( non>additive),(there( would( remain( a( net( attractive( force( between( >CH2>( and( water.( Similarly,( Baldwin( and( Rose( proposed( that( the( hydrophilic( nature( of( alkyl( chains( stems( from( van( der( Waals( interactions( between( the( alkane( carbon( and( water( oxygen( atoms,5( which( is( supported( by( the( results( from( Gallicchio(et(al.51( ( ( There( also( remains( the( dependence( of( v∞( and( its( derivatives(on(the(position(of(the(triple(bond(among(1>,(2>,( and(3>alkynes.(We(hypothesize(that(the(effect(of(the(triple( bond( on( hydration( depends( on(whether(the( triple(bond( is( adjacent( to( a( hydrogen,( a( carbon,( or( carbons.( While( theoretical( analyses( using( e.g.( molecular( dynamics( simulation( or( liquid( theory( may( shed( light( on( the( details,( the(theoretical(treatment( of(alkynes( is(also(challenging.( In( theoretical( chemistry,( assigning( force( fields( to( atoms( is( a( critical(procedure.(Mobley(et(al.(modified(the(force(field(of( the( triple>bonded( carbons( to( reproduce( the( experimental( hydration( free( energy,( because( the( initially( guessed( force( filed( failed.52( However,( successful( prediction( of( the( hydration( free(energy(would( not(be( sufficient( to(establish( the( force( field( for( the( triple>bonded( carbons.( That( is,( derivatives( of( the( free( energy,( such( as( the( partial( molar( volume( should( also( be( correctly( predicted.( Consequently,( the( present( volumetric( study( poses( an( interesting( challenge:( can( the( current( force( field( for( triple>bonded( carbons( reproduce( the( anomalously( small( partial( molar( volume( of( the( methylene( group( in( alkynes?( An( accurate( force( field( could( help( to( explain( the( observed( difference( among(1>,(2>,(and(3>alkynes.( ( ( In( summary,( the( present( density( measurement( determined( the( partial( molar( volumes( of( 1>,( 2>,( and( 3>alkynes,( and( the( results( showed( much( smaller( partial( molar( volumes( of( the( methylene( group( (14.01( ±( 0.46( cm3( mol>1(for(1>alkyne,(9.83(±(0.35(cm3(mol>1(for(2>alkyne,(and( 11.39( ±( 0.55( cm3( mol>1( for( 3>alkyne)( compared( to( the( well>accepted( value( of( ~16( cm3( mol>1( previously( determined( for( more( hydrophilic( model( compounds.( We( analyzed( how( the( volume( changes( by( the( hydration( of( a( methylene( group.( For( all( the( compounds( examined( in( the( previous( studies( and( the(current( one,( except( for( 2>alkyne,( the( addition( of( a( methylene( group( to( the( chain( depletes( hydration( water( molecules.( In( the( case( of( 2>alkyne,( however,( the( hydration( waters( are( enriched( by( the( addition( of(a(methylene(group( in(the( solute.(As( illustrated(

∞

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Gsw,hyd / cm mol

Page 5 of 10

(

Figure( 4.( (A)( Gsw,hyd∞( and( (B)( (∂Gsw,hyd∞/∂n>CH2>)T,P( of( the( hydrophilic( model( compounds( (alcohol,( ketone,( alkane( diol,( alkyl( amine,( alkylbenzene,( and( alkane)( and( the( alkynes( at( 25(°C.(Gsw,hyd∞(are(shown(as(a(function(of( n>CH2>.(The(dash(and( solid( lines( are( the( linear( fitting( results.( When( n>CH2>( =( 0,( the( hydrophilic(compounds(are(methanol,(methyl(amine,(acetone,( toluene,(and(ethane.(Regarding(the(alkanes,(the(average(of(the( multiple( values( from( the( literature45–48( is( depicted( with( the( standard(error.( (

(

( Figure( 5.( Schematic( illustration( of( (de)hydration( caused( by( inserting( a( >CH2>( group( into( molecules( containing( (A)( hydrophilic(functional(group(and((B)(2>alkyne(with(the(triple( bond.(Note(that(the(number(of(water(molecules(shown(is(not( quantitative.( (

( (

(


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 6 of 10

! Table&1.&Volumetric¶meters&(including&standard&error)&of&alkyne&aqueous&solutions&at&25.000&°C.&Symbols&are&indicated&in&the&main&text.& Vmoleculara! (cm3!mol.1)"

(cm3!mol.1)!

1.Pentyne! 1.Hexyne! 1.Heptyne!

52.58! 63.60! 74.63!

80.47±0.10! 93.79±0.19! 108.48±0.57!

2.Pentyne! 2.Hexyne! 2.Heptyne!

51.22! 62.12! 73.03!

3.Hexyne! 3.Heptyne!

61.66! 72.56!

Solute!

v∞!

(∂v∞/∂n.CH2.)T,P! (cm3!mol.1)!

Gsw∞! (cm3!mol.1)!

Gsw,molecular∞! (cm3!mol.1)"

Gsw,hyd∞! (cm3!mol.1)"

(∂Gsw,hyd∞/∂n.CH2.)T,P" (cm3!mol.1)"

14.01±0.46!

.80.47±!0.10! .93.79±!0.19! .108.48±!0.57!

.52.58! .63.60! .74.63!

.27.89±0.10! .30.19±0.19! .33.86±0.57!

.2.98±0.39!

83.25±0.13! 92.81±0.21! 102.90±0.29!

9.83±0.35!

.83.25±!0.13! .92.81±!0.21! .102.90±!0.29!

.51.22! .62.12! .73.03!

.32.02±0.13! .30.69±0.21! .29.87±0.29!

1.08±0.15!

92.98±0.29! 104.37±0.26!

11.39±0.55!

.92.98±!0.29! .104.37±!0.26!

.61.66! .72.56!

.31.32±0.29! .31.81±0.26!

.0.49±0.39!

!For!calculation!of!Vmolecular,!the!radius!of!the!water!molecule!was!set!to!1.4!Å.!Varying!this!radius!by!±0.2!Å!did!not!change!our!conclusion.!For!example,!for!1.pentyne,!using!the! radii!of!1.2!and!1.6!Å!changed!Vmolecular!by!only!.0.35!and!+0.35!%,!respectively.! a.


Page 7 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table& 2.&The& KirkwoodFBuff¶meter&between& solute& and& water& solvent& (Gsw∞),&molecular& volume& (Gsw,molecular∞),& and&hydration& (Gsw,hyd∞)& in& alkylbenzene,& alcohol,& alkane& diol,& ketone,& alkyl& amine,& and& alkane& aqueous& solutions& at& 25& °C& calculated& by& the& partial& molar& volume& from& previous& studies.& nFCH2F& is& the& number& of& methylene&groups&(FCH2F).& Solute! Toluene13! Ethyl!benzene13! Propyl!benzene13!

v∞! (cm3!mol.1)" 100.5! 114.6! 131.6!

Gsw∞! (cm3!mol.1)!

Gsw,molecular∞! (cm3!mol.1)"

Gsw,hyd∞! (cm3!mol.1)"

.100.5! .114.6! .131.6!

.61.3! .72.7! .82.6!

.36.7! .43.3! .47.4!

(∂Gsw,hyd∞/∂n.CH2.)T,P" (cm3!mol.1)" .4.94±!0.68!

Methanol9! Ethanol9! Propanol9! Butanol9! Pentanol9! Hexanol9! Heptanol9!

38.0! 54.9! 70.4! 86.12! 101.8! 117.5! 133.4!

.38.0! .54.9! .70.4! .86.12! .101.8! .117.5! .133.4!

.22.3! .32.7! .43.7! .54.6! .65.5! .76.4! .87.3!

.15.7! .22.2! .26.7! .31.5! .36.3! .41.1! .46.1!

.4.95±!0.10!

1,2.Ethanedilol10! 1,3.Propanediol10! 1,4.Butanediol10! 1,5.Pentanediol10! 1,6.Hexanediol10! 1,7.Heptanediol10! 1,8.Octanediol10! 1,10.Decanediol10!

54.6! 71.2! 88.3! 104.4! 120.4! 136.4! 152.6! 184.4!

.54.6! .71.2! .88.3! .104.4! .120.4! .136.4! .152.6! .184.4!

.37.7! .48.6! .59.5! .70.4! .81.3! .92.2! .103.1! .124.9!

.16.9! .22.6! .28.8! .34.0! .39.1! .44.2! .49.5! .59.5!

.5.30±!0.07!

Propanone14! Butanone14! Pentanone14!

66.9! 82.5! 98.0!

.66.9! .82.5! .98.0!

.40.0! .50.6! .61.5!

.27.1! .31.9! .36.5!

.4.70±!0.00!

Methyl!amine11,12! Ethyl!amine11,12! 1.Heptyl!amine11,12!

41.9! 58.6! 137.6!

.41.9! .58.6! .137.6!

.24.3! .35.3! .89.7!

.17.6! .23.3! .47.9!

.5.01±!0.01!

.27.8! .38.8!

.24.7±!1.1! .29.9±!2.9!

.5.2±!3.1!

Ethane! Propane!

52.9,48!53.3,47!51.246! 70.7,48!66.645!

.52.9,!.53.3,!.51.2! .70.7,!.66.6!

!



ASSOCIATED)CONTENT) Supporting*Information.*The$Supporting$Information$is$ available$free$of$charge$on$the$ACS$Publications$website$at$ DOI:$ 10.1021/acs.jpcb.xxxxxxx.$ Tabulated$ densities$ of$ alkyne$aqueous$solutions,$and$their$difference$from$that$of$ bulk$ water$ at$ various$ molarities$ under$ 25.000$ ±$ 0.003$ °C$ and$atmospheric$pressure.$(PDF)$

ACKNOWLEDGEMENTS) The$ authors$ are$ grateful$ to$ Prof.$ K.$ Sueoka$ (Josai$ University,$Japan)$for$his$experimental$supports.$We$thank$ K.$ Yamanaka,$ Dr.$ H.$ Mizuguchi$ (Chiba$ Institute$ of$ Technology,$ Japan),$ and$ K.$ Shibuta$ (Tokai$ University,$ Japan)$for$fruitful$discussion.$

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Hydration Promoted by a Methylene Group: A Volumetric Study on

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