Impact of Surfactant Structure on NAPL Mobilization and Solubilization

Oct 25, 2016 - Micro-scale displacement of NAPL by surfactant and microemulsion in heterogeneous porous media. Gina Javanbakht , Maziar Arshadi ...
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Impact of Surfactant Structure on NAPL Mobilization and Solubilization in Porous Media Gina Javanbakht, and Lamia Goual Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03006 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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Table  of  Content  Graphic   Impact  of  Surfactant  Structure  on  NAPL  Mobilization  and  Solubilization  in   Porous  Media  (ie-­‐2016-­‐03006t)     Gina  Javanbakht  and  Lamia  Goual        

 

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Impact  of  Surfactant  Structure  on  NAPL  Mobilization  and  Solubilization   in  Porous  Media     Gina  Javanbakht  and  Lamia  Goual*   Department  of  Petroleum  Engineering,  University  of  Wyoming,  1000  E.  University  Ave.,  Laramie,   WY  82071,  USA   *E-­‐mail:  [email protected],  Phone:  307-­‐766-­‐3278     ABSTRACT     Additives  such  as  surfactants  are  commonly  used  to  enhance  the  remediation  of  oil-­‐contaminated   rocks.   Although   the   majority   of   crude   oils   are   light   non-­‐aqueous   phase   liquids   (LNAPLs),   they   often   contain  heavy  molecules  such  as  asphaltenes  that  are  classified  as  dense  non-­‐aqueous  phase  liquids   (DNAPLs).   Surfactants   are   able   to   reduce   the   interfacial   tension   between   water   and   LNAPL   and   enhance   their   mobilization.   Furthermore,   the   formation   of   microemulsions   by   surfactants   can   promote   the   solubilization   of   DNAPLs   and   restore   the   wettability   of   contaminated   surfaces.   Numerous   studies   have   indicated   that   surfactants   can   promote   the   cleanup   of   oil-­‐contaminated   rocks,  however   the   impact   of   surfactant   structure   on  solubilization   and   mobilization   is   still   unclear.   In  this  study,  we  investigated  the  remediation  of  heterogeneous  aquifer  rocks  using  four  different   nonionic   surfactants;   n-­‐dodecyl   β-­‐D-­‐maltoside,   Triton   X-­‐100,   Bio-­‐soft   N1-­‐7   and   Saponin.   The   goal   was  to  develop  an  improved  understanding  of  the  role  of  the  surfactant  molecular  structure  on  non-­‐ aqueous   phase   liquids   (NAPL)   removal   through   solubilization   and   mobilization.   Each   of   these   surfactants   has   a   unique   structural   characteristic   in   its   hydrophilic   or   hydrophobic   segment.   Dodecyl   β-­‐D-­‐maltoside   contains   hydroxyl   groups   in   its   hydrophilic   segment,   which   tend   to   form   strong   hydrogen   bonds,   while   Triton   X-­‐100   has   branched-­‐chain   alkyl   groups   in   its   hydrophobic   segment   that   are   more   soluble   in   NAPL.   Alkyl   ethoxylated   surfactants   such   as   N1-­‐7   display   the   simplest  structure,  while  saponin  with  a  heavy  and  complex  structure  cannot  solubilize  NAPLs  as   fast  as  other  surfactants.   Through  measurements  of  phase  behavior,  dynamic  interfacial  properties,   adsorption,   spontaneous   imbibition,   thin   sections   analysis,   and   high   resolution   transmission   electron   microscopy,   we   showed   that   microemulsions   formed   by   these   surfactants   are   able   to   mobilize   LNAPL,   especially   in   the   presence   of   branched-­‐chain   alkyl   groups   in   the   hydrophobic   segments   (such   as   those   in   Triton   X-­‐100),   which   promoted   higher   reduction   in   the   interfacial   tension   between   NAPL   and   brine.   Micellar   solubilization,   on   the   other   hand,   was   favored   by   the   hydroxyl   groups   in   hydrophilic   segments   (such   as   those   in   n-­‐dodecyl   β-­‐D-­‐maltoside),   which   were   able  to  form  strong  hydrogen  bonds  at  interfaces  and  favor  the  desorption  of  DNAPL  from  mineral   surfaces.  Following  a  different  trend  from  the  other  surfactants,  saponin  with  a  higher  solubility  in   brine   showed   a   tendency   to   self-­‐aggregate   and   form   micron-­‐size   clusters   of   microemulsions,   which   slowed  down  the  NAPL  remediation.  

 

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Keywords:   NAPL,   asphaltene,   crude   oil,   aquifer   remediation,   surfactant   structure,   solubilization,   mobilization     1.  INTRODUCTION   In-­‐situ  surfactant  flushing  of  porous  rocks  is  one  of  the  most  innovative  technologies  in  removing   organic  contaminants,  compared  to  other  available  remediation  methods.1  Surfactants  are  surface-­‐ active  agents  that  tend  to  adsorb  at  fluid/fluid  interfaces  and  modify  the  interfacial  energies.2  These   molecules  consist  of  a  hydrophilic  head,  which  has  affiliation  to  aqueous  phases  such  as  brine,  and  a   hydrophobic   tail,   which   is   soluble   in   organic   phases,   generally   referred   to   as   non-­‐aqueous   phase   liquids   (NAPLs).3   Based   on   their   hydrophilic   heads,   surfactants   are   categorized   into   four   main   groups:   ionic,   cationic,   amphoteric,   and   nonionic.   Among   these   groups,   nonionic   surfactants   have   the   least   tendency   to   adsorb   on   rock   surfaces,   making   them   the   most   suitable   candidates   for   soil   and  aquifer  cleanup  processes.4       The   displacement   of   NAPLs   by   low-­‐concentration   surfactant   solutions   is   the   result   of   different   mechanisms   including   mobilization   and   solubilization.   Mobilization   is   due   to   a   reduction   in   the   interfacial   tension   (IFT)   between   brine   and   NAPL,   which   often   promotes   the   immiscible   displacement   of   light   non-­‐aqueous   phase   liquids   (LNAPLs).1,5,6  Micellar   solubilization   on   the   other   hand  involves  desorption  of  contaminants  from  mineral  surfaces  and  is  desirable  in  the  presence  of   dense  non-­‐aqueous  phase  liquids  (DNAPLs).7  Although  the  majority  of  crude  oils  are  LNAPLs,  they   often   contain   heavy   organic   macromolecules   such   as   asphaltenes   that   are   classified   as   DNAPLs.   Asphaltenes   consist   of   highly   polarizable   polydisperse   surface-­‐active   components   that   tend   to   adsorb   on   rocks,   altering   their   wettability.   In   a   previous   study,   we   showed   that   microemulsions   formed  by  n-­‐dodecyl  β-­‐D-­‐maltoside  surfactant  were  able  to  mobilize  bulk  oil  in  a  porous  rock  and   solubilize  adsorbed  asphaltenes  from  its  heterogeneous  surface.  Micellar  solubilization  in  this  case   was  the  result  of  surfactant  adsorption  on  mineral  surfaces,  causing  asphaltenes  to  detach  and  form   Winsor  Type  I  microemulsions.    Spontaneous  imbibition  tests  on  the  same  rock  indicated  that  the   ratio   of   mobilized   to   solubilized   NAPL   was   about   6:1.8   Note   that   this   ratio   varies   with   the   type   of   surfactant  and  rock  properties.  Different  rocks  will  adsorb  variable  amounts  of  DNAPL.  The  amount   of   mobilized   LNAPL   also   depends   on   pore   structure   and   wettability.   In   strongly   water-­‐wet   pores,   NAPLs  that  were  occupied  in  the  center  of  pores  may  disconnect  and  trap  during  the  water  flushing   or   flooding   process.9   On   the   other   hand,   when   the   rock   is   oil-­‐wet,   the   capillary   pressure   becomes   negative   and   brine   cannot   invade   the   pores   and   mobilize   LNAPL.   Therefore,   weakly   water-­‐wet   rocks   yield   optimum   NAPL   mobilization.10   Although   wettability   is   an   important   parameter   that   controls  fluid  flow  in  porous  media,  it  has  often  been  overlooked.  A  recent  work  with  n-­‐decane  (as   the  NAPL  phase)  and  sodium  dodecyl  sulfate  in  brine  distinguished  between  the  amount  of  NAPL   mobilized  through  soil  columns  and  dissolved  in  aqueous  solutions.  A  sound  model  for  surfactant-­‐

 

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enhanced   oil   recovery   was   proposed,   however   it   did   not   account   for   micellar   solubilization   since   decane  did  not  alter  the  soil  wettability.11     The  present  work  aims  at  expending  our  former  study  on  NAPL  mobilization  and  solubilization  in   porous   media   using   other   types   of   nonionic   surfactants.   Four   well-­‐known   classes   of   nontoxic   and   biodegradable   surfactants   have   been   extensively   studied   in   the   literature   related   to   enhanced   oil   recovery   and   soil   washing   processes:   alkyl   polyglucosides,   alkyl   phenol   ethoxylates,   alkyl   ethoxylates,  and  biosurfactants.       i.

Alkyl   polyglucosides   such   as   n-­‐dodecyl   β-­‐D-­‐maltoside   are   sugar-­‐based   surfactants   that   are   soluble   in   polar   and   non-­‐polar   solvents.12-­‐14   Their   interfacial   properties,15   adsorption,16   and   phase  behavior17  was  studied  in  the  past.  These  surfactants  are  strongly  surface-­‐active  due  to   the   hydrogen-­‐bonding   groups   in   their   hydrophilic   heads,   enabling   them   to   significantly   reduce   the  IFT  and  adsorb  in  bilayers  on  water-­‐wet  surfaces.18  

ii.

Alkyl   phenol   ethoxylates   (or   tergitol)   contain   polyethylene   oxide   chains   that   facilitate   their   adsorption  on  NAPL.  Those  with  branched-­‐chain  alkyl  groups,  such  as  Triton  X-­‐100,  were  most   effective   at   reducing   the   IFT.3   However   regardless   of   their   alky   group   topology,   these   surfactants   showed   bilayer   adsorption   on   minerals   at   concentrations   above   their   critical   micelle   concentration   (CMC),19   and   consequently   altered   the   wettability   of   surfaces   from   oil-­‐ wet  toward  water-­‐wet.20  

iii.

Alkyl  ethoxylated  surfactants  (or  linear  alcohol  alcoxylates)  are  also  effective  in  reducing  the   IFT21  as  well  as  altering  the  wettability  of  contaminated  surfaces.22  However  with  this  type  of   surfactants;  the  ultra-­‐low  IFT  can  only  be  achieved  by  addition  of  other  agents  such  as  alkali,   co-­‐surfactants,  or  co-­‐solvents.23  The  addition  of  alkali  to  a  surfactant  solution  makes  it  possible   to  have  in-­‐situ  generation  of  surfactant  and  significant  reduction  of  surfactant  adsorption.24,  25      

iv.

Biosurfactants  are  mostly  nonionic  and  their  applications  in  NAPL  removal  from  soil  have  been   well   established   in   the   literature.26,27   Recently   they   have   also   been   successfully   applied   in   enhanced   oil   recovery.28   Saponin,   for   instance,   was   used   in   lieu   of   synthetic   surfactants   to   assist   with   emulsion   polymerization.29   Similar   to   alcohol   ethoxylates,   saponin   is   able   to   reduce   IFT  to  ultra-­‐low  values  by  addition  of  salts  or  alcohols.30  

  Despite   the   fact   that   numerous   studies   have   documented   the   application   of   surfactants   in   aquifer   and   soil   remediation,   less   attention   has   been   directed   towards   investigating   the   relationship   between  surfactant  molecular  structure  and  their  surface  activities.  As  an  example,  the  hydrophilic-­‐ lipophilic   balance   number   (HLB)   of   each   surfactant   can   influence   the   amount   of   mobilized   or   solubilized   NAPLs.   The   HLB   number   shows   the   tendency   of  hydrophilic   and   hydrophobic   segments   of   surfactants   to   dissolve   in   aqueous   or   NAPL   phases,   respectively.   For   example,   the   hydrophobic   part   of   a   surfactant   with   a   low   HLB   number   can   partition   significantly   into   the   NAPL   phase   and   form   reverse   microemulsions.31   NAPL   contamination   can   produce   different   types   of  

 

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microemulsions   with   surfactants   of   different   HLB   values.   However,   surfactants   with   HLB   close   to   10   tend   to   form   middle   phase   microemulsions,   which   promote   the   mobilization   of   NAPLs.   In   this   study,   we   explored   the   relationship   between   surfactant   molecular   structure   and   its   efficiency   in   NAPL   remediation.   One   surfactant   from   each   of   the   above-­‐mentioned   classes   (n-­‐dodecyl   β-­‐D-­‐ maltoside,  Triton  X-­‐100,  Bio-­‐soft  N1-­‐7,  and  saponin)  was  selected  and  its  ability  to  reduce  IFT  and   contact   angle   (CA)   on   contaminated   rock   surfaces   was   measured   and   interpreted   based   on   its   molecular   structure.   Novel   correlations   were   then   established   between   the   amount   of   mobilized/solubilized  NAPL  and  the  IFT/CA  of  the  surfactant  solution.       2.  MATERIALS  AND  METHODS       2.1  Materials         2.1.1  Fluids   The   NAPL   phase   used   in   this   study   is   a   medium   crude   oil   from   Milne   Point   formation   in   Alaska   and   its  properties  are  provided  in  Table  1.  This  oil  contains  about  9  wt%  of  asphaltenes  precipitated  by   n-­‐heptane   according   to   ASTM   D-­‐2007.   The   ratio   between   the   total   acid   number   (TAN)   and   the   total   base   number   (TBN)   shows   that   the   NAPL   is   fairly   neutral.   The   brine   used   in   this   study   was   prepared   by   mixing   1   M   CaCl2   in   distilled-­‐deionized   water   with   a   resistivity   of   2.75E04   Ωm.   The   procedure   used   for   the   selection   of   this   salinity   is   explained   in   our   previous   study.8   Each   surfactant   solution   contained   0.2   wt%   of   one   of   the   following   surfactants:   n-­‐dodecyl   β-­‐D-­‐maltoside   (GC   grade,   >98%,  Sigma  Aldrich),  Triton  X-­‐100  (laboratory  grade,  Sigma  Aldrich),  Bio-­‐soft  N1-­‐7  (Stepan)  and   Saponin   (molecular   biology,   Sigma   Aldrich).   These   nonionic   surfactants   are   environmentally   friendly,  biodegradable  with  low  toxicity  and  CMC.  The  structure  of  these  surfactants  can  be  found   in  Table  2.     2.1.2  Rocks   Heterogeneous   aquifer   rock   samples   were   obtained   from   the   Arkose   layer   of   Fountain   formation   located  in  east  Colorado  and  Wyoming,  which  originally  formed  from  Sherman  Granite  and  contains   various  minerals.  The  dominant  minerals  in  this  rock  are  quartz,  feldspar,  and  calcite.  Quartz  and   feldspar   constitute   more   than   80%   of   the   rock.   We   should   note   that   the   rock   also   contains   some   dolomite  in  the  form  of  cement.  Several  core  plugs  were  drilled  with  a  diameter  of  1.5  inches  (or  38   mm)   and   dried   in   an   oven   for   at   least   24   hour   before   measuring   their   permeability   and   porosity   with  an  automated  permeameter  and  porosimeter  (AP-­‐608,  Coretest  System).  The  porosity  of  the   rock  samples  was  found  to  be  in  the  range  of  12  –  20  %  and  their  permeability  varied  between  2   and  25  mD.       2.2  Methods  

 

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  2.2.1  Phase  Behavior     Phase  behavior  tests  were  performed  by  preparing  a  brine  solution  (0.2  wt%  surfactant  in  brine)   and  mixing  it  with  NAPL  at  a  volume  ratio  of  1:1.  The  surfactant  concentrations  were  chosen  to  be   higher   than   the   CMC   point   of   each   surfactant   to   ensure   that   surfactant   micelles   form   microemulsions   with   NAPL.   In   each   test,   the   glass   tubes   were   shaken   and   rested   at   25   °C   and   atmospheric   pressure   to   form   a   separate   microemulsion   phase   (or   rag   layer)   between   of   NAPL   and   the  brine.     2.2.2  High  Resolution  Transmission  Electron  Microscopy     We   used   Tecnai   TF20   S-­‐Twin   High   Resolution   Transmission   Electron   Microscope   (HRTEM)   from   FEI  to  image  surfactant  micelles  and  microemulsions  composed  of  surfactant  solutions  and  NAPL.   The  instrument  and  procedure  was  illustrated  in  detail  in  our  pervious  study.8  Image  J  software  was   used  for  image  processing  in  order  to  measure  the  average  size  of  microemulsions.32     2.2.3  Interfacial  Tension  and  Contact  Angle     Dynamic   interfacial   tensions   between   NAPL   and   brine   and   equilibrium   CA   of   NAPL/brine/rock   systems   were   measured   with   and   without   surfactant   by   rising/captive   bubble   tensiometry   and   a   video-­‐image   digitization   technique.   The   homemade   experimental   set-­‐up   was   described   in   a   previous   study.33  All   measurements   were   conducted   at   ambient   conditions   and   the   density   of   fluids   was  measured  at  20  °C  using  Anton  Paar  density  meter.       Each   NAPL   bubble   in   brine   was   left   for   at   least   four   hours   to   reach   equilibrium   for   IFT   measurements  with  the  rising/captive  bubble  method  during  which  images  were  captured  every  5   minutes.  The  size  of  the  needle  was  chosen  to  achieve  a  Bond  number  close  to  unity.  The  captured   images  were  analyzed  using  the  Axismetric  Drop  Shape  Analysis  (ADSA)  by  fitting  the  drop  profile   to   Young-­‐Laplace   equation.34   A   spinning   drop   tensiometer   (SITE100,   KRUSS)   was   also   used   to   measure   the   IFT   between   NAPL   and   surfactant   solution,   which   is   more   accurate   for   IFT   values   below  1  mN/m.35    

For  CA  tests,  the  rock  samples  were  first  vacuumed  at  10-­‐7  psi  for  12  hours  and  then  immersed  in   NAPL.   After   aging   in   NAPL   for   7   days   at   60   °C,   they   were   gently   placed   in   the   IFT/CA   cell.   Brine   (with   and   without   surfactants)   was   then   transferred   into   the   cell   until   the   substrates   were   fully   immersed   in   the   solution.   The   NAPL   inside   the   thin   substrates   formed   on   average   20   droplets   on   the   surface   of   the   rock,   as   it   was   released   by   spontaneous   imbibition   of   brine   (cf.   Figure  1).   Images   of   these   droplets   were   captured   every   30   seconds   and   their   contact   angle   distribution   was   determined   using   the   angle   tool   of   ImageJ   software.   These   tests   were   repeated   to   insure   reproducibility  of  the  data.    

 

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2.2.4  Adsorption     The   amount   of   surfactant   adsorption   on   crushed   Arkose   rock   was   determined   by   UV-­‐Vis   spectroscopy.   The   UV-­‐Vis   absorbance   of   the   surfactant   solutions   with   various   concentrations   was   measured  before  and  after  exposure  to  the  rock  grains.  The  diameter  of  the  rock  grains  was  sieved   to   be   between   100-­‐200   µm.   First,   1   g   of   the   grains   was   mixed   with   25   g   of   surfactant   solutions   with   different  concentrations.  The  mixtures  were  shaken  at  600  strokes/minute  for  ten  hours  to  reach   equilibrium.   The   mixtures   were   then   centrifuged   to   separate   surfactant   solution   from   the   rock   grains.   The   absorbance   of   the   separated   surfactant   solutions   were   measured   and   compared   with   the   reference   curves,   which   were   obtained   from   the   surfactant   solutions   before   they   were   mixed   with   the   rock   grains.   The   amount   of   surfactant   adsorption   on   the   rock   grain   was   calculated   and   plotted  at  different  surfactant  concentrations  using  Langmuir  isotherm.3     2.2.5  Spontaneous  Imbibition     To  perform  spontaneous  imbibition  tests,  38  mm  diameter  and  50.8  mm  length  core  samples  were   drilled   from   Arkose   rock   and   were   subsequently   polished   and   dried.  The  permeability  and  porosity   of   each   sample   were   measured   using   the   automated   permeameter   and   porosimeter   in   order   to   choose  the  core  samples  with  similar  permeability  and  porosity  for  each  set  of  tests.  The  samples   were   first   vacuumed   for   24   hours   and   then   fully   saturated   with   brine.   They   were   subsequently   placed   in   the   core   flooding   system   shown   in   Figure  2.   NAPL   was   injected   to   the   system   at   low   flow   rate  and  the  volume  of  produced  brine  was  monitored  until  an  initial  brine  saturation  of  50%  was   achieved.   After   reaching   to   50%   of   water   saturation,   contaminated   core   samples   were   placed   in   custom-­‐made   Amott   imbibition   cells   filled   with   brine   solutions   (with   and   without   0.2   wt%   surfactants).  Each  Amott  cell  included  a  bottom  sealed  glass  cell  to  hold  the  core  sample  and  a  top   graduation   tube   to   measure   the   volume   of   NAPL   expelled   from   the   core.   This   volume   was   recorded   versus   time   until   no   more   NAPL   was   produced.   All   the   tests   were   performed   at   ambient   temperature  to  better  represent  aquifer  conditions.       2.2.6  Petrographic  Thin  Section     Two   sets   of   thin   sections   were   provided   by   Wagner   petrographic   company   using   the   cores   from   spontaneous   imbibition   tests   with   brine   alone   and   surfactant   solution   (0.2   wt%   maltoside).   The   size   of   each   thin   section   was   24   ×   46   mm.   On   each   thin   section,   blue   epoxy   impregnation,   calcite   stain,  K-­‐feldspar  stain,  and  plagioclase  stain  were  applied.  A  petrographic  microscope  (Zeiss  AXIO,   Scope.A1)  equipped  with  AXIO  vision  software  were  used  for  image  analysis  of  the  thin  sections.     3.  RESUTS  AND  DISCUSSION       3.1  Surfactant  Screening    

 

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3.1.1  Phase  Behavior   The   phase   behavior   of   the   surfactants   listed   in   Table   2   was   investigated   based   on   the   procedure   explained  in  Section  2.2.1.  All  the  surfactants  were  able  to  significantly  reduce  the  brine/NAPL  IFT   and   form   Winsor   type   III   microemulsions   in   the   middle   phase   between   brine   and   NAPL,36  as   shown   in  Figure  3.  The  presence  of  these  microemulsions  can  decrease  the  capillary  drawdown  inside  the   pores,  promoting  more  NAPL  mobilization.6-­‐8     3.1.2  Microemulsion  Formation   The  HRTEM  images  of  surfactant  microemulsion  prepared  by  extracting  a  small  amount  of  the  rag   layer  between  NAPL  and  brine  and  diluted  40  times  in  the  same  brine  are  shown  in  Figure   4.  The   diameters  of  microemulsions  formed  by  n-­‐dodecyl  β-­‐D-­‐maltoside  and  Triton  X-­‐100  were  the  largest   (about   800   nm),   suggesting   that   they   can   trap   more   NAPL   than   Bio-­‐soft   N1-­‐7   and   saponin.   Saponin   has   a   heavier   and   more   complicated   molecular   structure   than   the   other   surfactants,   and   is   more   soluble  in  brine  due  to  its  higher  HLB  number.  After  imaging  the  microemulsions  formed  by  fresh   saponin   solution   and   NAPL,   we   left   the   rag   layer   for   two   weeks   and   imaged   it   again.   Figure   5   reveals  the  presence  of  spider-­‐like  shaped  micelles  that  tend  to  aggregate  between  themselves  and   trap  larger  amounts  of  NAPL.  Indeed,  the  average  size  of  microemulsions  has  increased  from  200   nm  to  about  1  µm  after  two  weeks.  The  high  tendency  of  saponin  to  self-­‐aggregate  is  likely  due  to   the   large   number   of   hydroxyl   groups   in   its   structure   that   tend   to   form   strong   hydrogen   bonds.   Consequently,   microemulsions   formed   by   saponin   are   expected   to   grow,   trap,   and   mobilize   more   NAPL  given  enough  time,  leading  to  enhanced  NAPL  removal.     3.2  Interfacial  Tension   The  effect  of  surfactant  on  the  IFT  between  NAPL  and  brine  was  examined  at  25°C  and  atmospheric   pressure.   The   surfactant   concentration   was   higher   than   their   CMC   (i.e.,   0.2   wt%).   For   measurements  with  the  pendant  drop  method,  the  IFT  varied  in  time  for  about  four  hours  before   reaching   equilibrium.   Low   IFTs   on   the   other   hand   were   measured   by   spinning   drop   tensiometry,   which   is   relatively   faster.   Figure  6  shows   that   the   addition   of   surfactant   to   brine   resulted   in   a   very   significant  decrease  in  the  IFT  between  NAPL  and  brine  from  22.5  mN/m  (IFT  of  brine  and  NAPL)   to   less   than   3   mN/m   (with   an   error   less   than   0.1mN/m).   As   expected,   the   surfactants   with   intermediate   HLB   (i.e.,   closer   to   10)   provided   the   lowest   IFTs.   At   the   NAPL/brine   interface,   the   hydrophilic  heads  of  the  surfactant  tend  to  remain  in  the  aqueous  phase  while  the  hydrophobic  tails   stretch   toward   the   NAPL   phase.   As   the   surfactant   layer   forms   at   the   interface,   the   transition   between  weak  dispersion  interactions  among  NAPL  molecules  and  strong  polar  interaction  among   water   molecules   becomes   smoother,   leading   to   an   increase   in   the   interfacial   pressure   and   a   reduction   in   the   interfacial   energy   and   IFT.37,38   The   significant   reduction   in   IFT   with   surfactant   caused  the  formation  of  Winsor  Type  III  microemulsions,  which  promoted  the  mobilization  of  NAPL   from  contaminated  aquifers.  A  closer  examinations  of  the  results  revealed  that  the  IFT  values  with    

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n-­‐dodecyl  β-­‐D-­‐maltoside  and  Triton  X-­‐100  are  the  lowest.  These  two  surfactants  have  similar  HLB   and  microemulsion  size  and  can  trap  more  NAPL  within  their  microemulsions,  as  discussed  in  the   previous   section.   The   presence   of   hydroxyl   groups   in   the   sugar   head   of   n-­‐dodecyl   β-­‐D-­‐maltoside   promotes  the  formation  of  strong  hydrogen  bonds.16,18  Although  Triton  X-­‐100  has  a  less  hydrogen   bonding   head   than   n-­‐dodecyl   β-­‐D-­‐maltoside,   the   branched   alkyl   group   in   its   tail   is   able   to   adsorb   into   the   brine/NAPL   interface   and   bring   down   the   IFT   significantly.   This   is   in   line   with   previous   studies  where  the  presence  of  branched  alkyl  groups  in  surfactant  tails  was  shown  to  enhance  their

efficiency in lowering the IFT,3,39,40 due to increased hydrocarbon surface area per surfactant molecule at the interface.41 It is interesting to note that microemulsions formed by Triton X-100 are different from those with other surfactants (cf. Figure 4c). They exhibit small NAPL droplets scattered throughout the microemulsion instead of a core-­‐shell  structure.   3.3  Wettability  Alteration   The  effect  of  aging  on  the  wettability  alteration  of  minerals  was  previously  investigated.8  Different   clean   minerals   (quartz,   feldspar,   and   calcite)   found   in   Arkose   rock   were   immersed   in   NAPL   for   1   to   10  days  and  the  CA  of  brine  was  measured  on  these  minerals  after  each  day.  By  increasing  the  aging   time,   the   CA   increased   until   it   remained   constant   after   3   days,   indicating   a   wettability   alteration   from   strongly   to   weakly   water-­‐wet.   Thus   in   this   study,   we   used   7   day-­‐aged   minerals   in   NAPL   to   ensure   sufficient   wettability   alteration.   The   change   in   wettability   can   be   explained   by   the   adsorption  of  asphaltene  molecules  on   mineral  surfaces.  The  thickness  of  the  adsorbed  films  is  in   the   order   of   5-­‐8   nm,   depending   on   the   type   of   minerals.42  Previous   studies   revealed   that   asphaltene   adsorption   on   calcite   is   higher   than   on   quartz.   Unlike   the   silanol   groups   of   quartz   that   usually   adsorb  asphaltene  monolayers,  the  carbonate  groups  of  calcite  interact  strongly  with  asphaltenes,   resulting   in   multilayer   adsorption.43,44   Since   Arkose   rock   contains   more   than   80%   of   quartz   and   feldspar,  the  film  thickness  is  expected  to  be  close  to  5  nm,  i.e.  monolayer  coverage.     Figure   7   shows   a   reduction   of   static   CA   on   oil-­‐contaminated   surfaces   by   addition   of   0.2   wt%   of   surfactants  with  HLB  number  close  to  13.  For  example,  n-­‐dodecyl  β-­‐D-­‐maltoside  was  able  to  reduce   the   average   CA   from   110   to   30   degrees.   This   implies   a   wettability   alteration   of   NAPL-­‐contaminated   rock   surfaces   from   weakly   water-­‐wet   back   to   water-­‐wet,   in   agreement   with   previous   work.20   The   wettability   reversal   could   be   explained   by   the   dual   adsorption   of   surfactant   molecules   via   their   hydrophobic   tails   on   the   thin   DNAPL   layer   and   via   their   hydrophilic   heads   on   the   rock   surface,   causing  DNAPL  to  detach  and  form  Winsor  Type  I  microemulsions.16  To  verify  this  mechanism,  we   measured  the  adsorption  of  these  surfactants  on  Arkose  rock  by  UV-­‐Vis  spectroscopy  according  to   the   procedure   described   in   Section   2.2.4.   Figure   8   reveals   that   hydrogen   bonding   is   the   driving   force  for  adsorption.  Indeed,  n-­‐dodecyl  β-­‐D-­‐maltoside  with  a  large  number  of  hydroxyl  groups  in  its   sugar   head   shows   a   greater   amount   of   adsorption.18   Both   Bio-­‐soft   N1-­‐7   and   Triton   X-­‐100   have   alcohol   ethoxylated   heads,   however   the   straight-­‐chain   alkyl   tails   of   Bio-­‐soft   N1-­‐7   appear   to   favor  

 

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adsorption   more   than   the   branched-­‐chain   groups   of   Triton   X-­‐100,   probably   due   to   less   steric   effects.     To   study   the   impact   of   saponin   self-­‐assembly   on   wettability   alteration,   we   measured   the   CA   on   contaminated  rock  samples  after  4  days  of  contact  with  the  surfactant  solution.  The  insert  in  Figure   7   shows   a   reduction   in   static   CA   by   10   degrees   upon   increasing   the   contact   time.   When   nonionic   surfactant   solutions   with   high   molecular   weight   and   high   HLB   such   as   saponin   are   introduced   to   the  NAPL  phase,  they  start  to  form  hydrogen  bonds  and  self-­‐associate  instead  of  quickly  adsorbing   on   NAPL   and   rock   surface.   However,   if   given   enough   time,   they   can   adsorb   at   interfaces   to   a   higher   extent  and  promote  the  micellar   solubilization  of  NAPL.  This  behavior  was  not  observed  with  the   other   surfactants,   as   their   contact   angle   remained   constant   in   time,   suggesting   minimal   self-­‐ association.         3.4  Spontaneous  Imbibition     Spontaneous  imbibition  tests  were  performed  on  Arkose  core  samples  with  similar  permeabilities   (2-­‐3   mD)   and   porosities   (12-­‐15%)   and   containing   50%   of   initial   brine   saturation.   First,   we   recorded   the   amount   of   NAPL   removal   due   to   spontaneous   imbibition   of   brine   by   placing   contaminated   core   samples   in   an   Amott   imbibition   cell   for   at   least   150   hours   until   production   of   NAPL  ceased.   The  volume  of  NAPL  removal  from  the  core  due  to  spontaneous  imbibition  of  brine  is   shown  in  Figure   9a,  as  a  percentage  of  the  original  NAPL  volume.  The  amount  of  NAPL  mobilized   by   brine   is   about   15   vol%,   which   is   due   to   the   mobilization   of   LNAPLs   in   smaller   pores   that   can   easily  be  invaded  by  brine.  Based  on  our  CA  measurements,  brine  alone  does  not  have  the  ability  to   desorb  DNAPLs  from  mineral  surfaces,  which  explains  why  no  solubilization  was  observed.       We   repeated   the   spontaneous   imbibition   tests   on   contaminated   core   samples   with   all   four   surfactants.   The   amount   of   NAPL   recovered   by   the   surfactants   versus   time   was   recorded.   After   150   hours,   all   surfactants   showed   more   recovery   compared   to   brine.   Because   of   the   lower   interfacial   tension   with   NAPL,   the   surfactant   solution   could   invade   small   pores   as   well   as   large   pores.   Contaminant   removal   starts   by   a   fast   mobilization   of   LNAPL   from   the   porous   rock.   As   the   production  curve  reaches  an  inflection  point  or  a  distinct  jump,  solubilization  of  DNAPL  occurs.  This   jump   depends   on   the   solubilization   amount   and   is   more   obvious   in   low   permeability   rocks.   Solubilization  is  indeed  slower  than  mobilization  since  it  is  a  kinetic  process  that  involves  DNAPL   desorption   by   surfactant   molecules.   The   DNAPL   desorption   can   restore   the   wettability   of   contaminated   surfaces   back   to   their   original   water-­‐wet   condition   and   reduce   the   threshold   capillary   pressure   needed   for   brine   to   invade   the   pores.   Therefore,   another   stage   of   NAPL   recovery   starts.    Assuming  that  the  imbibition  curves  due  to  mobilization  with  and  without  surfactants  have   the  same  trend,  we  can   estimate  the  amounts  of  mobilization   and   solubilization   for   each   surfactant,   see   for   instance   Reference   8   for   n-­‐dodecyl   β-­‐D-­‐maltoside.   This   surfactant,   together   with   Triton   X-­‐ 100,   showed   better   recoveries   than   the   other   surfactants     (6   and   5   vol%   more   than   brine).   The    

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volume  of  LNAPL  mobilized  by  Triton  X-­‐100  was  slightly  higher  than  n-­‐dodecyl  β-­‐D-­‐maltoside  due   to  its  slightly  lower  IFT.  Bio-­‐soft  N1-­‐7  showed  lower  initial  NAPL  removal,  which  implied  its  weaker   ability  to  mobilize  NAPL.  This  is  in  line  with  the  higher  IFT  of  this  surfactant  with  NAPL  compared   to   other   surfactants.   On   the   other   hand,   the   volume   of   DNAPL   solubilized   with   n-­‐dodecyl   β-­‐D-­‐ maltoside  was  slightly  larger  than  Triton  X-­‐100  due  to  its  higher  ability  to  alter  wettability.  In  fact,   the   amount   of   solubilized   DNAPL   decreased   from   n-­‐dodecyl   β-­‐D-­‐maltoside   to   Bio-­‐soft   N1-­‐7   to   Triton  X-­‐100.  This  result  is  in  good  agreement  with  the  wettability  alteration  trend  in  Figure  7,  and   their  adsorption  propensity  in  Figure  8.     Since  saponin  behaves  differently  from  the  other  surfactants,  its  imbibition  curve  was  provided  in  a   separate  figure  for  two  ranges  of  rock  permeability  (cf.  Figure   9b).  In  low  permeability  cores,  the   production   curve   of   saponin   indicates   that   the   early   rate   of   NAPL   recovery   (i.e.,   mobilization)   is   smaller  than  other  surfactants  and  even  that  of  brine  alone.  This  result  was  surprising  considering   that   saponin   can   reduce   the   IFT   of   NAPL/surfactant   solution   but   not   as   much   as   the   other   surfactants.   A   possible   reason   for   this   inconsistency   is   the   self-­‐assembly   behavior   of   saponin.   As   mentioned   in   the   previous   section,   saponin   has   a   greater   aggregation   tendency   enabling   the   formation  of  large  clusters  of  microemulsions  and  hindering  their  movement  in  porous  media.  The   clustering   of   microemulsions   is   more   problematic   in   rocks   with   smaller   pore   sizes   and   permeability.   The   solubilization   of   DNAPL   for   saponin   starts   after   about   60   hours,   which   is   considerably  longer  than  the  other  surfactants  (around  20  hours).  This  delay  in  the  solubilization   process  is  related  to  the  self-­‐aggregation  of  microemulsions,  as  was  observed  in  CA  measurements   (cf.  insert  of  Figure  7).  In  this  case,  DNAPL  (i.e.,  asphaltenes)  start  to  detach  from  the  surface  after   microemulsion   clusters   are   formed,   which   explains   why   more   time   is   needed   to   reduce   CA   and   reverse  the  wettability  of  contaminated  surfaces  to  water-­‐wet.  Figure   9b   reveals  that  saponin  can   remove  5%  more  NAPL  than  Bio-­‐soft  N1-­‐7  after  120  hours.     In  addition,  similar  spontaneous  imbibition  tests  were  performed  with  saponin  and  rocks  having  a   permeability   of   20-­‐25   mD,   as   shown   in   Figure  9b.   The   imbibition   curves   in   this   case   have   different   trends   than   before.   The   larger   pore   size   and   permeability   results   in   higher   early   rate   of   NAPL   recovery   than   imbibition   with   brine   alone.   This   is   because   the   size   of   microemulsion   clusters   is   smaller   than   the   size   of   the   pores   therefore   there   are   fewer   barriers   for   their   movement   through   the  medium.             Overall,   both   mobilization   and   micellar   solubilization   occur   in   NAPL   remediation   by   all   surfactants.   Mobilization   of   LNAPL   through   formation   of   Winsor   Type   III   microemulsions   takes   place   first   followed   by   the   slow   micellar   solubilization   of   DNAPL   (i.e.,   asphaltenes)   through   formation   of   Winsor  type  I  microemulsions.    The  kinetics  of  these  mechanisms  was  similar  except  for  saponin,   which   has   a   higher   HLB.   Among   the   surfactants   with   HLB   close   to   13,   those   with   more   hydroxyl   groups  (such  as  n-­‐dodecyl  β-­‐D-­‐maltoside)  or  more  lipophilic  branched  tails  (such  as  Triton  X-­‐100)  

 

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exhibited  a  higher  solubility  in  brine  and  NAPL  phase  respectively,  and  could  bring  down  IFT  to  its   lowest  values  and  facilitate  LNAPL  mobilization  the  most.       Based  on  our  CA/IFT  measurements  and  imbibition  tests,  linear  correlations  could  be  established   between  the  amounts  of  mobilized  and  solubilized  NAPL  by  surfactants  and  their  ability  to  reduce   IFT  and  CA,  respectively.  Figure  10a  indicates  that  maltoside  is  able  to  reduce  the  CA  more  than  the   other   surfactants   by   forming   the   strongest   hydrogen   bonds   and   having   the   highest   adsorption   tendency  on  the  rock  surface;  therefore  the  amount  of  solubilized  DNAPL  with  this  surfactant  is  the   highest.   On   the   other   hand,   the   presence   of   branched   alkyl   groups   in   Triton   X-­‐100   causes   the   surfactant   to   dissolve   more   in   the   NAPL   phase,   which   reduces   IFT   and   promotes   mobilization   of   LNAPL   more   than   other   surfactants   (cf.   Figure   10b).   These   results   suggest   that   mixtures   of   surfactants  with  two  structural  types  can  promote  both  mobilization  and  micellar  solubilization  of   NAPL  in  porous  media.  Both  types  should  have  intermediate  HLB  numbers.  Type  1  contains  a  linear   tail   and   a   large   hydrogen-­‐bonding   head,   whereas   type   2   has   a   highly   branched   tail   and   a   smaller   hydrogen-­‐bonding  head.     To  visualize  wettability  alteration  of  contaminated  rock  through  micellar  solubilization  of  adsorbed   DNAPL   on   the   mineral   surfaces,   we   prepared   thin   sections   of   the   core   samples   after   spontaneous   imbibition   without   and   with   the   best   surfactant   (n-­‐dodecyl   β-­‐D-­‐maltoside)   and   compared   them   with  polarizing  petrographic  microscopy.  As  shown  in  Figure   11a,  after  imbibition  with  brine,  the   mineral   surfaces   remained   covered   with   a   layer   of   DNAPL.   After   introduction   of   surfactant   solution   to   the   contaminated   channels,   most   of   the   mineral   surfaces   became   clean   due   to   asphaltene   desorption   from   the   surface   (cf.   Figure   11b).   The   only   parts   of   the   rock   that   remained   contaminated   were   mainly   channels   that   contained   dolomite   cement.   This   illustrates   that   the   surface  roughness  plays  an  important  role  in  wettability  alteration.  The  surfactant  could  solubilize   adsorbed  asphaltenes  from  the  mineral  grains  with  smoother  surface  whereas  the  cemented  areas,   which  contain  crushed  and  small  grains  with  rough  surfaces,  retained  the  adsorbed  contaminants.   It  should  be  noted  here  that  it  was  not  possible  to  estimate  the  amount  of  mobilized  and  residual   LNAPLs   by   brine   or   surfactant   solution   in   the   rock   channels   since   fluids   in   the   channels   were   replaced  by  epoxy  before  cutting  the  samples.     4.  CONCLUSION   Systematic   experimental  analyses   were   performed   to   investigate   the   impact   of   surfactant   structure   on   mobilization   and   micellar   solubilization   of   NAPLs   in   porous   media.   All   the   surfactants   were   able   to   form   Winsor   type   III   microemulsions   with   brine   and   NAPL   that   caused   significant   IFT   reduction.   HRTEM  micrographs  revealed  that  the  size  of  these  microemulsions  was  bigger  with  n-­‐dodecyl  β-­‐D-­‐ maltoside   and   Triton   X-­‐100.   These   two   surfactants   have   distinctive   features   either   in   their   hydrophilic   head   or   hydrophobic   tail.   Hydroxyl   groups   in   the   sugar   head   of   n-­‐dodecyl   β-­‐D-­‐

 

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maltoside   form   strong   hydrogen   bonds   and   branched-­‐chain   alkyl   groups   in   Triton   X-­‐100   cause   strong  interactions  with  the  NAPL  phase.  Therefore,  at  the  interface  between  brine  and  NAPL,  these   two  surfactants  decrease  the  interfacial  tension  to  a  greater  extent  resulting  in  better  mobilization   of   LNAPL.   Furthermore,   CA   measurements   and   spontaneous   imbibition   tests   revealed   that   the   presence   of   hydroxyl   head   groups   and   straight-­‐chain   alkyl   tails   in   both   n-­‐dodecyl   β-­‐D-­‐maltoside   and   Bio-­‐soft   N1-­‐7   facilitated   the   desorption   of   DNAPL   from   rock   surfaces   and   consequently   promoted   micellar   solubilization,   as   shown   in   the   schematic   of   Figure   12.   Thus   two   correlations   have  been  established  based  on  this  work;  one  between  the  amount  of  solubilized  DNAPL  and  the   extent  of  CA  reduction  (or  adsorption  propensity  of  surfactant),  which  is  favored  by  the  hydrogen   bonding  ability  of  its  head,  and  one  between  the  amount  of  mobilized  LNAPL  and  the  extent  of  IFT   reduction   by   the   surfactant,   which   is  promoted   by   the   branching   of   its   tail.   Results   with   a   high   HLB   surfactant   such   as   saponin   were   also   provided   to   show   the   impact   of   surfactant   self-­‐assembly   on   remediation  efficiency,  especially  in  low  permeability  rocks.     5.  ACKNOWLEDGEMENT   The   authors   would   like   to   thank   the   National   Science   Foundation   (Career   Award   #1351296)   and   Hess   Corporation   for   financial   support.   The   authors   are   also   grateful   to   Dr.   Erwin   Sabio   for   HRTEM   imaging,  Dr.  Fred  McLaughlin  for  his  help  in  collecting  rock  samples  and  thin  section  analyses,  Dr.   Mohammad  Sedghi  and  Dr.  William  Welch  for   insightful   discussions,  and  Deraldo  Andrade  for   his   help  with  IFT  and  CA  measurements.     6.  REFERENCES     1. Mulligan,  C.  N.;  Yong,  R.  N.;  Gibbs,  B.  F.  Surfactant-­‐Enhanced  Remediation  of  Contaminated  Soil:   A  Review.  Engineering  Geology  2001,  60  (1–4),  371–380.   2. Laha,   S.;   Tansel,   B.;   Ussawarujikulchai,   A.   Surfactant–soil   Interactions   during   Surfactant-­‐ Amended   Remediation   of   Contaminated   Soils   by   Hydrophobic   Organic   Compounds:   A   Review.   Journal  of  Environmental  Management  2009,  90  (1),  95–100.   3. Rosen,  M.  J.;  Wang,  H.;  Shen,  P.;  Zhu,  Y.  Ultralow  Interfacial  Tension  for  Enhanced  Oil  Recovery   at  Very  Low  Surfactant  Concentrations.  Langmuir  2005,  21  (9),  3749–3756.   4. Wagner,   J.;   Chen,   H.;   Brownawell,   B.   J.;   Westall,   J.   C.   Use   of   Cationic   Surfactants   to   Modify   Soil   Surfaces   to   Promote   Sorption   and   Retard   Migration   of   Hydrophobic   Organic   Compounds.   Environ.  Sci.  Technol.  1994,  28  (2),  231–237.   5. Abdul,  A.  S.;  Ang,  C.  C.  In  Situ  Surfactant  Washing  of  Polychlorinated  Biphenyls  and  Oils  from  a   Contaminated  Field  Site:  Phase  II  Pilot  Study.  Ground  Water  1994,  32  (5),  727–734.   6. Harwell,  J.  H.;  Sabatini,  D.  A.;  Knox,  R.  C.  Surfactants  for  Ground  Water  Remediation.  Colloids  and   Surfaces  A:  Physicochemical  and  Engineering  Aspects  1999,  151  (1–2),  255–268.     7. Soerens,  T.,  D.  Sabatini,  and  J.  Harwell.  Surfactant  Enhanced  Solubilization  of  Residual  DNAPL:   Column  Studies.  Subsurface  Restoration  Conference,  Dallas,  TX,  June  1992.    

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8. Javanbakht,   G.;   Goual,   L.   Mobilization   and   Micellar   Solubilization   of   NAPL   Contaminants   in   Aquifer  Rocks.  Journal  of  Contaminant  Hydrology  2016,  185–186,  61–73.     9. Landry,  C.  J.;  Karpyn,  Z.  T.;  Piri,  M.  Pore-­‐Scale  Analysis  of  Trapped  Immiscible  Fluid  Structures   and  Fluid  Interfacial  Areas  in  Oil-­‐Wet  and  Water-­‐Wet  Bead  Packs.  Geofluids  2011,  11  (2),  209– 227.   10. Suicmez,   V.   S.;   Piri,   M.;   Blunt,   M.   J.   Effects   of   Wettability   and   Pore-­‐Level   Displacement   on   Hydrocarbon  Trapping.  Advances  in  Water  Resources  2008,  31  (3),  503–512.   11. Tsakiroglou,   C.   D.;   Aggelopoulos,   C.   A.;   Tzovolou,   D.   N.;   Theodoropoulou,   M.   A.;   Avraam,   D.   G.   Dynamics   of   Surfactant-­‐Enhanced   Oil   Mobilization   and   Solubilization   in   Porous   Media:   Experiments   and   Numerical   Modeling.   International  Journal  of  Multiphase  Flow   2013,   55,   11– 23.   12. Pilakowska-­‐Pietras,   D.;   Lunkenheimer,   K.;   Piasecki,   A.   Synthesis   of   Novel   N,N-­‐Di-­‐N-­‐ Alkylaldonamides   and   Properties   of   Their   Surface   Chemically   Pure   Adsorption   Layers   at   the   Air/water  Interface.  Journal  of  Colloid  and  Interface  Science  2004,  271  (1),  192–200.   13. Griffin,   W.   C.   Classification   of   Surface-­‐active   Agents   by   HLB.   Journal   of   the   Society   of   Cosmetic   Chemists  1946,  1,  311-­‐26.   14. Aramaki,   K.;   Ozawa,   K.;   Kunieda,   H.   Effect   of   Temperature   on   the   Phase   Behavior   of   Ionic– Nonionic  Microemulsions.  Journal  of  Colloid  and  Interface  Science  1997,  196  (1),  74–78.   15. Niraula,  B.  B.;  Chun,  T.  K.;  Othman,  H.;  Misran,  M.  Dynamic-­‐Interfacial  Properties  of  Dodecyl-­‐Β-­‐ D-­‐Maltoside   and   Dodecyl-­‐Β-­‐D-­‐Fructofuranosyl-­‐Α-­‐D-­‐Glucopyranoside   at   Dodecane/water   Interface.   Colloids   and   Surfaces   A:   Physicochemical   and   Engineering   Aspects   2004,   248   (1–3),   157–166.   16. Somasundaran,   P.;   Zhang,   L.   Adsorption   of   Surfactants   on   Minerals   for   Wettability   Control   in   Improved   Oil   Recovery   Processes.  Journal  of  Petroleum  Science  and  Engineering   2006,   52   (1–4),   198–212.   17. Kahlweit,   M.;   Busse,   G.;   Faulhaber,   B.   Preparing   Nontoxic   Microemulsions   with   Alkyl   Monoglucosides  and  the  Role  of  Alkanediols  as  Cosolvents.  Langmuir  1996,  12  (4),  861–862.   18. Zhang,   L.;   Somasundaran,   P.;   Maltesh,   C.   Adsorption   of   n-­‐Dodecyl-­‐Β-­‐D-­‐Maltoside   on   Solids.   Journal  of  Colloid  and  Interface  Science  1997,  191  (1),  202–208.   19. Li,   W.;   Gu,   T.   Equilibrium   Contact   Angles   as   a   Function   of   the   Concentration   of   Nonionic   Surfactants  on  Quartz  Plate.  Colloid  &  Polymer  Sci.  1985,  263  (12),  1041–1043.   20. Golabi,   E.;   Seyedeyn   Azad,   F.;   Ayatollahi,   S.;   Hosseini,   N.;   Akhlaghi,   N.   Experimental   Study   of   Wettability   Alteration   of   Limestone   Rock   from   Oil   Wet   to   Water   Wet   by   Applying   Various   Surfactants.  Society  of  Petroleum  Engineers  Heavy  Oil  Conference  (SPE-­‐157801),  Calgary,  Alberta,   Canada,  12-­‐14  June  2012.   21. Galindo,   T.   A.;   Rimassa,   S.   M.   Evaluation   of   Environmentally   Acceptable   Surfactants   for   Application   as   Flowback   Aids.   Society   of   Petroleum   Engineers   International   Symposium   on   Oilfield  Chemistry  (SPE-­‐164122),  The  Woodlands,  TX,  8-­‐10  April  2013.  

 

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22. Mohan,  K.;  Gupta,  R.;  Mohanty,  K.  K.  Wettability  Altering  Secondary  Oil  Recovery  in  Carbonate   Rocks.  Energy  &  Fuels  2011,  25  (9),  3966-­‐3973.   23. Yang,   H.   T.;   Britton,   C.;   Liyanage,   P.   J.;   Solairaj,   S.;   Kim,   D.   H.;   Nguyen,   Q.   P.;   Weerasooriya,   U.;   Pope,   G.   A.   Low-­‐Cost,   High-­‐Performance   Chemicals   for   Enhanced   Oil   Recovery.   Society   of   Petroleum   Engineers   Improved   Oil   Recovery   Symposium   (SPE-­‐129978),   Tulsa,   OK,   24-­‐28   April   2010.   24. Chu,   W.;   Kwan,   C.   Y.   Remediation   of   Contaminated   Soil   by   a   Solvent/Surfactant   System.   Chemosphere  2003,  53  (1),  9–15.   25. Hirasaki,  G.  J.;  Miller,  A.;  Pope,  G.  A.;  Jackson,  R.  E.  Surfactant  Based  Enhanced  Oil  Recovery  and   Foam   Mobility   Control.   Annual   report   of   DE-­‐FC26-­‐03NT15406   grant,   Rice   University,   2004,   http://www.owlnet.rice.edu/~gjh/Consortium/resources/DOE-­‐Surfactant-­‐2004-­‐Annual.pdf   26. Banat,   I.   M.;   Makkar,   R.   S.;   Cameotra,   S.   S.   Potential   Commercial   Applications   of   Microbial   Surfactants.  Appl.  Microbiol.  Biotechnol.  2000,  53  (5),  495–508.   27. Makkar,   R.   S.;   Cameotra,   S.   S.   Synthesis   of   Enhanced   Biosurfactant   by   Bacillus   Subtilis   MTCC   2423  at  45°C  by  Foam  Fractionation.  J.  Surfact.  and  Deterg.  2001,  4  (4),  355–357.   28. Shahri,   M.   P.;   Shadizadeh,   S.   R.;   Jamialahmadi,   M.   A   New   Type   of   Surfactant   for   Enhanced   Oil   Recovery.  Petroleum  Science  and  Technology  2012,  30  (6),  585–593.   29. Schmitt,  C.;  Grassl,  B.;  Lespes,  G.;  Desbrières,  J.;  Pellerin,  V.;  Reynaud,  S.;  Gigault,  J.;  Hackley,  V.  A.   Saponins:  A  Renewable  and  Biodegradable  Surfactant  From  Its  Microwave-­‐Assisted  Extraction   to  the  Synthesis  of  Monodisperse  Lattices.  Biomacromolecules  2014,  15  (3),  856–862.   30. Shahri,   M.   P.;   Shadizadeh,   S.   R.;   Jamialahmadi,   M.   Applicability   Test   of   New   Surfactant   Produced   from  Zizyphus  Spina-­‐Christi  Leaves  for  Enhanced  Oil  Recovery  in  Carbonate  Reservoirs.   Journal   of  the  Japan  Petroleum  Institute  2012,  55  (1),  27–32.   31. Griffin,  W.  C.  Calculation  of  HLB  Values  of  Non-­‐Ionic  Surfactants.   Am.  Perfumer.  Essent.  Oil  Rev.   1955,  65,  26–29.   32. Vega,  C.;  Miguel,  E.  de.  Surface  Tension  of  the  Most  Popular  Models  of  Water  by  Using  the  Test-­‐ Area  Simulation  Method.  The  Journal  of  Chemical  Physics  2007,  126  (15),  154707.   33. Saraji,   S.;   Goual,   L.;   Piri,   M.;   Plancher,   H.   Wettability   of   Supercritical   Carbon   Dioxide/Water/Quartz   Systems:   Simultaneous   Measurement   of   Contact   Angle   and   Interfacial   Tension  at  Reservoir  Conditions.  Langmuir  2013,  29  (23),  6856–6866.   34. Neumann,  A.  W.;  David,  R.;  Zuo,  Y.  Applied   Surface   Thermodynamics,  Second  Edition,  CRC  Press,   2011.   35. Vonnegut,  B.  Rotating  Bubble  Method  for  the  Determination  of  Surface  and  Interfacial  Tensions.   Review  of  Scientific  Instruments  1942,  13  (1),  6–9.   36. Israelachvili,   J.   N.   Intermolecular   and   Surface   Forces:   Revised   Third   Edition,   Academic   Press,   2011.   37. Howard,   J.   D.   Patterns   of   Sediment   Dispersal   in   the   Fountain   Formation   of   Colorado.   The   Mountain  Geologist  1966,  3  (4),  147–153.  

 

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38. Mazor,   E.   Understanding   Groundwater   Systems   of   the   Southern   Laramie   Basin,   Albany   County,   Wyoming  through  Applied  Chemical  and   Physical  Data,  Report  WWWRC-­‐90-­‐19,  Wyoming  Water   Research  Center,  1990.   39. Wu,   Y.;   Iglauer,   S.;   Shuler,   P.;   Tang,   Y.;   Goddard,   W.   A.   Branched   Alkyl   Alcohol   Propoxylated   Sulfate   Surfactants   for   Improved   Oil   Recovery.   Tenside   Surfactants   Detergents   2010,   47   (3),   152–161.   40. Phan,  T.  T.;  Attaphong,  C.;  Sabatini,  D.  A.  Effect  of  Extended  Surfactant  Structure  on  Interfacial   Tension   and   Microemulsion   Formation   with   Triglycerides.   J.   Am.   Oil   Chem.   Soc.   2011,   88   (8),   1223–1228.     41. Varadaraj,   R.;   Bock,   J.;   Valint,   P.;   Zushma,   S.;   Thomas,   R.   Fundamental   Interfacial   Properties   of   Alkyl-­‐Branched   Sulfate   and   Ethoxy   Sulfate   Surfactants   Derived   from   Guerbet   Alcohols.   1.   Surface  and  Instantaneous  Interfacial  Tensions.  J.  Phys.  Chem.  1991,  95  (4),  1671–1676.   42. Dudášová,   D.;   Simon,   S.;   Hemmingsen,   P.   V.;   Sjöblom,   J.   Study   of   Asphaltenes   Adsorption   onto   Different   Minerals   and   Clays:   Part   1.   Experimental   Adsorption   with   UV   Depletion   Detection.   Colloids  and  Surfaces  A:  Physicochemical  and  Engineering  Aspects  2008,  317  (1–3),  1–9.   43. Keleşoğlu,   S.;   Volden,   S.;   Kes,   M.;   Sjöblom,   J.   Adsorption   of   Naphthenic   Acids   onto   Mineral   Surfaces  Studied  by  Quartz  Crystal  Microbalance  with  Dissipation  Monitoring  (QCM-­‐D).  Energy   Fuels  2012,  26  (8),  5060–5068.   44. Rudrake,   A.;   Karan,   K.;   Horton,   J.   H.   A   Combined   QCM   and   XPS   Investigation   of   Asphaltene   Adsorption  on  Metal  Surfaces.  Journal  of  Colloid  and  Interface  Science  2009,  332  (1),  22–31.                                      

 

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  Table  1.  Properties  of  NAPL  used  in  this  study     ρ20C    (g/mL)  

0.9214  

Reflective  Index  at  20°C  

1.5222  

Viscosity  (mPa.s)  

112.0  

 

 

C  (%)  

85.07  

H  (%)  

7.75  

N  (%)  

1.09  

O  (%)  

1.61  

S  (%)  

4.63  

H/C  

1.1  

 

 

Asphaltenes  (wt%)  

9.03  

TAN  (mg  of  KOH/g)  

1.69  

TBN  (mg  of  KOH/g)  

2.25  

TBN/TAN  

1.33  

                             

 

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  Table  2.  Properties  of  selected  surfactants       Name  

                             Structure  

Chemical    

MW   (g/mol)  

HLB  

CMC   (wt%)  

 

 

 

 

C24H46O11  

511  

13.35      

0.02  

H3(CH2)10-­‐

647  

12.9  

0.01  

 

 

formula    

 

n-­‐Dodecyl  β-­‐ D-­‐Maltoside    

      Bio-­‐soft  N1-­‐7  

O(C2H4O)7H  

 

  Triton  X-­‐100  

 

 

 

 

C14H22O(C2H4O)10H  

625  

13.5  

0.02  

 

 

 

 

 

 

 

 

Saponin  

C45H73NO15  

1650  

36.3  

0.01  

 

 

   

 

                   

 

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  (a)  

   

 

 

 

 

 

(b)  

    Figure   1.   Examples   of   images   used   for   CA   measurement   on   contaminated   rock   surfaces   during   spontaneous  imbibition  a)  with  brine,  b)  with  surfactant  (maltoside)  solution.  NAPL  droplets  have   higher  CA  in  the  presence  of  surfactant          

 

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    Figure  2.  Schematic  of  the  core  flooding  system  used  to  obtain  an  initial  water  saturation  of  50   wt%  in  Arkose  core  samples                                            

 

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  (a)  

(b)  

 

(c)  

 

(d)  

 

 

  Figure   3.  Winsor  type  III  microemulsion  phase  between  NAPL  and  0.2  wt%  surfactant  solution  in   brine,  a)  n-­‐Dodecyl  β-­‐D-­‐maltoside,  b)  bio-­‐soft  N1-­‐7,  c)  Triton  X-­‐100,  d)  Saponin                                          

 

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  (a)  

(b)  

  800  nm  

(c)  

     400  nm    

(d)  

 

  800  nm  

200  nm    

  Figure   4.  HRTEM  micrographs  of  o/w  microemulsions  with:  (a)  n-­‐Dodecyl  β-­‐D-­‐maltoside,  (b)  Bio-­‐ soft  N1-­‐7,  (c)  Triton  X-­‐100,  (d)  Saponin.  The  average  microemulsion  size  is  provided  at  the  bottom   of  each  image                                  

 

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  Figure  5.  HRTEM  micrographs  o/w  microemulsions  with  saponin  solution  aged  for  two  weeks.  The   average  microemulsion  size  is  about  1  µm.                            

 

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3   Maltoside   Interfacial  Tension  (mN/m)  

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2.5   2  

Triton  X-­‐100L   N1-­‐7   Saponin  

1.5   1   0.5   0  

 

Figure  6.  Effect  of  surfactants  on  the  interfacial  tension  between  NAPL  and  brine  (IFT  without   surfactants  is  22.5  mN/m)              

 

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  Figure   7.   Effect   of   surfactants   on   equilibrium   contact   angle   of   brine   on   NAPL-­‐contaminated   minerals.   Insert   represents   the   effect   of   aging   with   saponin   on   wettability   alteration   of   contaminated  surfaces      

 

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4.0   3.5   3.0   Adsorption  (mg/g)  

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2.5   2.0   1.5   Maltoside  

1.0  

N1-­‐7   Triton  

0.5   0.0   0  

0.05  

0.1  

0.15  

0.2  

0.25  

0.3  

0.35  

0.4  

Surfactant  Concentration  (wt%)  

  Figure  8.  Adsorption  of  surfactants  on  rock  grains  using  UV-­‐vis  spectroscopy                            

 

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(a)  Maltoside,  Triton,  N1-­‐7 25  

NAPL  Removal  (vol  %)  

20  

k  =  2-­‐5  mD  

Start  of     solubilization  

15  

10   Brine   N1-­‐7   Triton   Maltoside  

5  

0   0  

20  

40  

60  

80  

100  

120  

140  

160  

180  

Time  (Hours)  

 

(b)  Saponin     40  

40  

k  =  2-­‐5  mD  

30   25   20   15   10   Brine   Saponin  

5   30  

60  

90   120   Time  (Hours)  

150  

30   25   20   15   10   Brine   Saponin  

5  

0   0  

k  =  20-­‐50  mD  

35   NAPL  Removal  (vol  %)  

35   NAPL  Removal  (vol%)  

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0  

180  

0  

30  

 

60   90   120   Time  (Hours)  

150  

180  

  Figure   9.   a)  Effect  of  non-­‐associating  surfactants  on  NAPL  removal  from  low  permeable  (2-­‐5  mD)   Arkose   cores   with   Swi=50%,   and   (b)   impact   of   rock   permeability   on   saponin-­‐enhanced   NAPL   removal  from  Arkose  cores  with  Swi=50%      

 

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(a)  Solubilization   70   Triton  

CA  (Degree)  

60   50  

N1-­‐7  

40   30  

Maltoside  

20   10   0   1  

2  

3  

4  

5  

Solubilized  DNAPL  (%)  

 

  (b)  Mobilization   1.8   1.6   IFT  (mN/m)  

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N1-­‐7  

1.4   1.2  

Maltoside  

1  

Triton    

0.8   0.6   12  

13  

14  

15  

16  

Mobilized  LNAPL  (%)  

17  

18  

 

  Figure  10.  a)  Amount  of  solubilized  DNAPL  vs.  contact  angle  with  each  surfactant.  b)  Amount  of   mobilized  LNAPL  vs.  interfacial  tension  with  each  surfactant          

 

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(a) Without  surfactant  

  (b) With  n-­‐dodecyl  β-­‐D-­‐maltoside  

  Figure  11.  Thin  sections  of  rock  samples  after  spontaneous  imbibition  tests  (a)  without  surfactant   and  (b)  with  n-­‐dodecyl  β-­‐D-­‐maltoside  surfactant    

 

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    Figure   12.   Schematic  of  NAPL  removal  from  contaminated  porous  rocks.  Surfactants  can  mobilize   LNAPL,   especially   in   the   presence   of   branched-­‐chain   alkyl   tails.   Micellar   solubilization,   on   the   other   hand,  is  promoted  by  surfactants  with  large  hydrogen-­‐bonding  heads  and  straight-­‐chain  alkyl  tails,   which  are  able  to  arrange  at  interfaces  and  favor  DNAPL  desorption  from  rock  surfaces.    

 

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