In my bibliography I have the following two entries:

```
Hosseini, S.A., Shah, N., 2011a. Enzymatic hydrolysis of cellulose part ii: Population balance modelling of hydrolysis by exoglucanase and universal kinetic model. Biomass Bioenerg. 35,
3820–3840.
Hosseini, S.A., Shah, N., 2011b. Modelling enzymatic hydrolysis of cellulose part i: Population balance modelling of hydrolysis by endoglucanase. Biomass Bioenerg. 35, 3841–3848.

```

The problem is that they appear in reverse order from the point of view of logical sequence (part i should be before part ii). The bibliography style that I use (model2-names from elsevier) sorts the entries alphabetically but for these two entries I would like to force part i to appear before part ii.

Here are the `.bib` entries:

```
@ARTICLE{Hosseini2011,
  author = {Seyed Ali Hosseini and Nilay Shah},
  title = {Modelling enzymatic hydrolysis of cellulose part I: Population balance
    modelling of hydrolysis by endoglucanase},
  journal = {Biomass Bioenerg.},
  year = {2011},
  volume = {35},
  pages = {3841-3848},
  file = {:Hosseini and Shah 2011 - Enzymatic hydrolysis of cellulose part I.pdf:PDF},
  journaltitle = {Biomass and Bioenergy}
}

@ARTICLE{Hosseini2011a,
  author = {Seyed Ali Hosseini and Nilay Shah},
  title = {Enzymatic hydrolysis of cellulose part II: Population balance modelling
    of hydrolysis by exoglucanase and universal kinetic model},
  journal = {Biomass Bioenerg.},
  year = {2011},
  volume = {35},
  pages = {3820-3840},
  file = {:Hosseini and Shah 2011 - Enzymatic hydrolysis of cellulose part II universal kinetic model.pdf:PDF},
  journaltitle = {Biomass and Bioenergy}
}

```

Is there a way to hack it? Part II comes before Part I but the titles are arranged alphabetically (which is normal since 'Enzymatic' comes before 'Modelling').